ACGIJ - Industrial Ventilation A Manual of Recommended Practice For Design PDF [PDF]
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INDUSTRIAL VENTILATI ON
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A Manual of Recommended Practice
for Design 27th Edition
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Copyright © 201 O by ACGllf® Previous Editions Copyright© 1951, 1952, 1954, 1956, 1958, 1960, 1962, 1964,1966, 1968, 1970, 1972,1974,1976,1978,1980,1982,1984,1986,1988,1992,1995,1998,2001,2004,2007 by ACGffi®Jndustrial Ventilation Committee 1st Edition - 1951 2nd Edition- 1952 3rd Edition- 1954 4th Edition- 1956 5th Edition- 1958 6th Edition- 1960 7th Edition - 1962 8th Edition - 1964 9th Edition- 1966 1Oth Edition - 1968 11th Edition - 1970 12th Edition- 1972 13th Edition- 1974
14th Edition- 1976 15th Edition- 1978 16th Edition - 1980 17th Edition - 1982 18th Edition- 1984 19th Edition - 1986 20th Edition- 1988 21st Edition- 1992 22nd Edition- 1995 23rd Edition- Metric- 1998 24th Edition - 2001 25th Edition- 2004 26th Edition - 2007
ISBN: 978-1-607260-13-4
All rights reserved. Printed in the United States of Arnerica. Except as perrnitted under the United States Copyright Act of 1976, no part ofthis publication may be reproduced or distributed in any form or by any means or stored in a database or retrieval system, without prior written permission from the publisher. ACGffi® Kemper Woods Center 1330 Kemper Meadow Drive Cincinnati, Ohio 45240-4148 Telephone: 513-742-2020 Fax: 513-742-3355 Email: [email protected] http://www.acgih.org
.........____________________
CONTENTS FOREWORD ..................................................................................................vii DEDICATION .................................................................................................viii ACKNOWLEDGMENTS ........................................................................................ .ix DEFINITIONS ..................................................................................................x ABBREVIATIONS ..............................................................................................xii CHAPTER 1 EXPOSURE ASSESSMENT ..................................................................... 1-1 1.1 Introduction ............................................................................ .1-2 1.2 Hazards of the Operation .................................................................. 1-2 1.3 Identify the Inherent Hazards .............................................................. .1-2 1.4 Potential Exposure During Normal Equipment Operation ........................................ 1-3 1.5 Potential Exposure Other Than During Normal Operation ....................................... .1-6 1.6 Potential Source Identification .............................................................. 1-7 l. 7 Assessing the Exposure .................................................................. .1-7 1.8 Hierarchy of Exposure Control Options ..................................................... .1-7 1.9 Common Airborne Hazards ............................................................... .1-9 1.10 Airborne Contaminants .................................................................... 1-9 1.11 Indoor Air Quality Assessment lssues ....................................................... 1-13 1.12 Exposure Monitoring .................................................................... 1-13 1.13 Legal and Code Requirements ............................................................. 1-15 1.14 Setting an Exposure Control Strategy ....................................................... 1-16 1.15 Ventilation System Worker Safety and Health Issues .......................................... .1-18 REFERENCES .............................................................................. .1-18 PRELIMINARY DESIGN ....................................................................... 2-1 CHAPTER2 2.1 Introduction ............................................................................. 2-2 2.2 Project Goals and Success Criteria ........................................................... 2-2 2.3 Large Project Team Organization ............................................................ 2-4 Team Responsibility Matrix (TRM) .......................................................... 2-4 2.4 2.5 Project Team Safety ...................................................................... 2-5 2.6 Document Control ...................................................... , ................ 2-5 2.7 Project Team Organization, Selection and Skills ................................................ 2-5 2.8 Responsibility for Final Approval ofBudget, Technical Merit and Regulatory Issues .................. 2-6 2.9 Communication ofPlant (and Project) Requirements ............................................ 2-6 2.10 Design/Build, In-House Design or Outside Consultant ........................................... 2-8 2.11 Design-Construct Method (Separate Responsibilities for Engineering and lnstallation) ................. 2-8 2.12 Design/Build (Turnkey) Method- Single Source ofResponsibility ................................ 2-9 2.13 Project Team and System Evaluation ......................................................... 2-9 2.14 Project Risk and Non-Performance ........................................................ .2-10 2.15 Using Plant Personnel as Project Resources .................................................. 2-11 2.16 Interface Between the Plant and Project ..................................................... 2-11 2.17 Impact ofNew Systems on Plant Operation .................................................. 2-12 REFERENCE ................................................................................ 2-12 PRINCIPLES OF VENTILATION ............................................................... .3-1 CHAPTER3 3.1 Introduction ............................................................................ .3-2 3.2 Conservation ofMass ..................................................................... 3-5 3.3 Conservation ofEnergy .................................................................. .3-6 3.4 System Pressures (Static, Velocity, Total) ..................................................... 3-7 3.5 System Loss Coefficients .................................................................. 3-8 3.6 The Fan in the System .................................................................. .3-11 3.7 Applying the Fan to the System (System Curve) ............................................. .3-11
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iv
Industrial Ventilation
CHAPTER4
CHAPTER5
CHAPTER6
3.8 Tracking Pressure Variations Through a Simple System ......................................... 3-12 3.9 Assumed Conditions (StandardAir) ................................................. · ..... .3-13 3.10 Assumed Conditions (Non-StandardAir) .................................................... 3-14 3.11 Density and Density Factor .............................................................. .3-14 REFERENCES .............................................................................. .3-16 GENERAL INDUSTRIAL VENTILATION ........................................................ .4-1 4.1 Introduction .............................................................................4-2 4.2 Dilution Ventilation Principies ............................................................. .4-2 4.3 Dilution Ventilation for Health ............................................................. .4-2 4.4 Mixtures- Dilution Ventilation for Health ................................................... .4-7 4.5 Dilution Ventilation for Fire and Explosion ................................................... .4-8 4.6 Fire Dilution Ventilation for Mixtures ....................................................... .4-9 4.7 Ventilation for Heat Control ............................................................... .4-9 4.8 Heat Balance and Exchange ............................................................... .4-9 4.9 Adaptive Mechanism ofthe Body ......................................................... .4-10 4.10 Acclimatization ........................................................................ .4-11 4.11 Acute Heat Disorders ................................................................... .4-11 4.12 Assessment ofHeat Stress and Heat Strain .................................................. .4-12 4.13 Worker Protection ...................................................................... .4-13 4.14 Ventilation Control ..................................................................... .4-14 4.15 Ventilation Systems .................................................................... .4-14 4.16 Velocity Cooling ....................................................................... .4-15 4.17 Radiant Heat Control ................................................................... .4-15 4.18 Protective Suits for Short Exposures ....................................................... .4-16 4.19 Respiratory Heat Exchangers ............................................................. .4-16 4.20 Refrigerated Suits ...................................................................... .4-16 4.21 Enclosures ............................................................................ .4-17 4.22 lnsulation ............................................................................. .4-17 REFERENCES ............................................................................... 4-17 DESIGN ISSUES- SYSTEMS .................................................................. .5-1 5.1 Administration oflndustrial Ventilation System Design .......................................... 5-2 5.2 Design Options for Industrial Ventilation Systems ............................................. .5-4 5.3 Design Procedures ....................................................................... 5-6 5.4 Distribution of Airflow In Duct Systems ..................................................... 5-19 5.5 Local Exhaust Ventilation System Types .................................................... .5-11 5.6 System Redesign ....................................................................... .5-13 5.7 System Components .................................................................... .5-13 5.8 Hoods ............................................................................... .5-13 5.9 Duct Systems ......................................................................... .5-15 5.10 Fans and Blowers ....................................................................... 5-15 5.11 Air-Cleaning Devices ................................................................... .5-15 5.12 Discharge Stacks ....................................................................... .5-16 5.13 Duct Construction Considerations ......................................................... .5-20 5.14 Testing and Balancing (Tab) ofLocal Exhaust Ventilation Systems ................................ 5-24 REFERENCES .............................................................................. .5-24 DESIGN ISSUES- HOODS ..................................................................... 6-1 6.1 Introduction ............................................................................. 6-3 6.2 Enclosing Hoods - Introduction ............................................................. 6-5 6.3 Totally Enclosing Hoods .................................................................. 6-6 6.4 Enclosing Hoods That Rely On Plug Flow To Protect Users ...................................... 6-8 6.5 Downdraft Occupied Hoods ("Rooms") ..................................................... 6-13 6.6 Hot Processes In Enclosing Hoods ......................................................... 6-16 6.7 Capturing Hoods ........................................................................ 6-16 6.8 Choosing Between Capturing and Enclosing Hoods................................ . ........ 6-29 6.9 Ergonomic Design ofHoods Used by Workers .............................................. 6-29
Contents
6010 Work Practices 6011 Material Handling In and Near Hood Workstations 6012 Maintenance and Cleaning for All Hoods 6013 Man-Cooling Fans 6014 Ventilation ofRadioactive and High Toxicity Processes 6015 Laboratory Operations 6016 Hood Pressure Losses REFERENCES APPENDIX A6 LOCAL EXHAUST HOOD CENTERLINE VELOCITY FANS 701 Introduction 702 Basic Defmitions 703 Fan Selection 7.4 Fan Motors 705 Fan lnstallation and Maintenance REFERENCES AIR CLEANING DEVICES 8.1 Introduction 802 Selection ofDust Collection Equipment 803 Dust Collector Types 8.4 Additional Aids in Dust Collector Selection 8.5 Control of Mist, Gas and Vapor Contaminants 806 Gaseous Contaminant Collectors 807 Unit Collectors 808 Dust Collecting Equipment Cost 809 Selection of Air Filtration Equipment 8.10 Radioactive and High Toxicity Operations 8.11 Explosion Venting/Deflagration Venting REFERENCES LOCAL EXHAUST VENTILATION SYSTEM DESIGN CALCULATION PROCEDURES 901 lntroduction 902 Preliminary Steps to Begin Calculations 903 Design Method and Use ofLoss Coefficients Basic Calculations and Procedures Required for System Design 9.4 Calculation Sheet Design Procedure 905 906 Sample System Design #1 (Single Branch System/StandardAir Conditions) Distribution of Airflow in a Mu1ti-Branch Duct System 907 908 Increasing Velocity Through a Junction (WeightedAverage Velocity Pressure) 909 Fan and System Pressure Calculations 9010 System Curve/Fan Curve Relationship 9011 Sample System Design #2 (Multi Branch System/Standard Air Conditions) 9012 Calculation Methods and Non-StandardAir Density 9013 Psychrometric Principies 9014 Mixing Gases ofDifferent Conditions Considering Temperature and Moisture 9015 Sample System Design #3 (Multi-Branch System/Non-StandardAir Conditions) 9.16 Sample System Design #4 (Adding a Branch to Existing System/Non-Standard Air Conditions) 9.17 Air Bleed Design REFERENCE SUPPLY AIR SYSTEMS 1001 Introduction 10.2 Purpose of Supply Air Systems 10.3 Supply Air System Design for Industrial Spaces 10.4 Supply Air Equipment 1005 Supply Air Distribution 1006 Airflow Rate o
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06-32 06-33 06-34 06-34 06-35 06-35 06-35 06-39 6-40 07-1 07-2 07-2 07-6 07-23 07-26 07-29 08-1 08-2 08-2 08-3 08-23 08-26 08-26 08-31 08-31 08-35 08-35 08-37 08-37 09-1 09-3 09-3 09-4 09-9 09-11 09-14 09-17 09-19 09-20 09-21 09-22 09-26 09-27 09-29 09-30 09-35 09-38 09-38 010-1 010-3 010-3 010-7 .10-9 010-19 .10-23 o
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vi
Industrial Ventilation
1007 Heating, Cooling and Other Operating Costs ooooooooooooooooooooooooooooooooooooooooooooooool0-23 1008 Industrial Exhaust Recirculation ooooooooooooooooooooooooooooooooooooooooooooooooooooooooool0-25 1009 System Control ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.10-30 10.10 System Noise oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo010-30 REFERENCES ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.10-30 CHAPTER 11 ENERGY CONSIDERATIONS ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.11-1 11.1 Introduction ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.11-2 11.2 Exhaust System Energy Use oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.11-2 1103 Recirculation of Exhaust Air ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo011-7 11.4 Energy Conservation Opportunities oooooooooooooooooooooooooooooooooooooooooooooooooooooooo011-7 REFERENCES ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo011-14 CHAPTER 12 COST ESTIMATING oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo. .12-1 1201 Introduction ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.12-2 1202 Capital Costs oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo012-2 1203 Total Annual Costs and Operating Cost Methods ooooooooooooooooooooooooooooooooooooooooooooo.12-4 12.4 Cost Comparison Methods ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo012-6 REFERENCES ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo.12-10 CHAPTER 13 SPECIFIC OPERATIONS ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo013-1 APPENDICES ooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo014-1 A Threshold Limit Va1ues for Chemica1 Substances in the Work Environment with Intended Changes for 2006 ooooooooooooooooooooooooooooooooooooooooooooooooooo014-3 B Physical Constants/Conversion Factors oooooooooooooooooooooooooooooooooooooooooooooooooooooooo014-25 C Testing and Measurement ofVentilation Systems ooooooooooooooooooooooooooooooooooooooooooooooo014-33 INDEXo oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooool5-l
FOREWORD Since its first edition in 1951, Industrial Ventilation: A Manual ofRecommended Practice has been used by engineers and industrial hygienists to design and evaluate industrial ventilation systems. The 27th edition of this Manual continues to be a basic reference.
Detailed Design
In developing the 26th Edition, the Industrial Ventilation Committee considered several new chapters for this Manual. As the chapters developed, it became apparent that a reorganization of the Manual would be desirable. Consequently, two Manuals were proposed and have been published: Industrial Ventilation: A Manual of Recommended Practice for Design (referred to as the Design Manual) addresses design of an industrial ventilation system and Industrial Ventilation: A Manual ofRecommended Practice for Operation and Maintenance (referred toas the O&M Manual), was published as a separate manual and addresses operation and maintenance of ventilation systems. Clearly, the two are intertwined and the materials could rightfully be placed in either Manual. The Committee decided to reduce redundancy and to have each Manual freely refer to the other Manual.
• Design Issues - Systems - Design ManualChapter 5
• Principies ofVentilation- Design ManualChapter 3 • General Ventilation - Design Manual- Chapter 4
• Design Issues - Hoods - Design ManualChapter 6 • Design Issues- Fans- Design Manual- Chapter 7 • Design Issues - Air Cleaners - Design Manual Chapter 8 • System Design Calculations - Design ManualChapter 9 • Supply Air- Design Manual- Chapter 1O • Energy Issues - Design Manual - Chapter 11 • Specific Operations - Design Manual - Chapter 13 lnstallation
Four new chapters were added to the 26th Edition of the Design Manual providing information on exposure assessment, prelirninary ventilation system design considerations, ventilation system costs and energy considerations. The Principies ofVentilation chapter was rewritten to provide most of the basics for the development of calculation and basic methods as well as examples of how the Laws of Physics are derived for easier use in later chapters. Chapter 5 was expanded into two chapters, 5 and 9. Chapter 5 expanded the basic information on the issues and basic methods involved in a ventilation system design. Chapter 9 provides expanded calculation Industrial Ventilation System design procedures for both standard and non-standard operating conditions.
• Construction - O&M Manual- Chapter 1 Commissioning • Commissioning- O&M Manual- Chapter 2
• Air System Testing - O&M Manual- Chapter 3 • Balancing - O&M Manual- Chapter 4 Monitoring and Maintenance of a Ventilation System • M&M Ventilation Systerns - O&M ManualChapter 5 • M&M Air Cleaning Devices - O&M ManualChapter 6
In this 27th Edition, Chapter 6, Design Issues - Hoods has been rewritten to provide broader hood type coverage. A new section on Nanoparticles as well as a section on Exothermic Heated Process Ventilation has been added.
Managing Ventilation Systems • Troubleshooting - O&M Manual - Chapter 7 • Change Management- O&M Manual- Chapter 8 Operator Training - O&M Manual- Chapter 9
To facilitate navigation between the two Manuals, an insert on the front, inside cover shows how the chapters are related. The two Manuals are divided into several topics, which generally follow the timeline for the development of an industrial ventilation system.
lnformation provided as a guideline can be influenced by other factors in an industrial environment (material handling techniques, cross-drafts and replacement air, work practices, and housekeeping, etc.); therefore formulae developed in the laboratory and at other sites may need to be altered further for actual field conditions. In many cases, ranges of values are shown, leaving final selection to be based on the experience of the practitioner and appropriate field conditions. Hence, the practitioner should always evaluate the effectiveness of hoods and other parts of the system after installation and be prepared to make changes as needed. Indeed, due to process changes,
Concept Design Exposure Assessment - Design ManualChapter 1 • Prelirninary Design - Design Manual - Chapter 2 • Ventilation Systems Costs - Design Manual Chapter 12 vii
vili
Industrial Ventilation
work-practice changes, and to the effects of the aging of the system, practitioners should continually evaluate and modify systems throughout their life cycles. This Manual is intended to be used as a guide, notas an official standard. It is designed to present current information with regard to the subject matter covered. It is distributed with the understanding that the Industrial Ventilation Committee and its members, collectively or individually, assume no responsibility for any inadvertent rnisinformation, for inadvertent ornissions, or for the results in the use ofthis publication.
INDUSTRIAL VENTILATION COMMITTEE
GS. Rajhans, GSR & Associates, Canada, Chair GA. Lanham, CECO Environmental, Inc., Ohio, Vice Chair R. Dayringer, MIOSHA, Michigan D.L. Edwards, KBD/Technic, Ohio G Grubb, MIOSHA, Michigan S.E. Guffey, West Virginia University, West Virginia J.F. Hale, Air Systems Corporation, North Carolina R.L. Herring, North Carolina Department ofHealth and Human Services, North Carolina R. T. Hughes, Retired, Ohio G W. Knutson, Knutson Ventilation Consulting, Minnesota J.L. McKeman, CDC, NIOSH, Ohio K.M. Paulson, NFESC, California J.L. Topmiller, NIOSH, Ohio A.W. Woody, Ventilation/Energy Applications, Michigan
DEDICATION With this new 27th Edition of Industrial Ventilation: A Manual ofRecommended Practice for Design, the Committee has undertaken the task of updating and modernizing technical information and putting more emphasis on the energy aspects of the powered ventilation system.
It is only fitting that we stand to dedícate this edition to our colleague, Robert T. Hughes, MSME, PE. Bob has served on the committee since 1976 including 11 years as its Chair. During that time he has been a steadfast advocate for health and safety in the workplace and has used his engineering education and background to irnprove the efficiency of ventilation systems
worldwide. In addition, he has been a leader on improved hood design and push-pull systems. He has served as United States representative at severa! Intemational Ventilation Conferences as well as being a staff member at the Industrial Ventilation Conference at North Carolina State University. He has authored numerous papers documented in conference proceedings, as well as papers and reports published by NIOSH and in professional journals, including the AIHA Journal and Applied Occupational and Environmental Hygiene. But more irnportant than these credentials are the sense of humor, the leadership, the integrity and intelligence that Bob has shown to us as we have published this new 27th Edition. It is with sincere appreciation that we dedicate this endeavor to our friend, Bob Hughes.
f ACKNOWLEDGMENTS nal contributors listed at the end of the F oreword for their contributions to the sixth chapter of the O&M Manual.
Industrial Ventilation is a true Committee effort. It brings into focus useful practica! ventilation data from all parts of the world in one source. The Committee membership of industrial ventilation engineers and industrial hygienists represents a diversity of experience and interests that ensures a well-rounded cooperative effort.
We are also grateful for the faith and fmn foundation provided by past Committees and members listed below. Special acknowledgment is made to the Division of Occupational Health, Michigan Department of Health, for contributing their original field manual, which was the basis ofthe First Edition, and to Mr. Knowlton J. Caplan who supervised the preparation of the Manual.
From the First Edition in 1951, this effort has been successful as witnessed by the acceptance ofthe "Ventilation Manual" throughout industry, by governmental agencies, and as a worldwide reference and text.
To many other individuals and agencies who have made specific contributions and have provided support, suggestions, and constructive criticism, our special thanks.
As indicated in the Foreword, we now have two volumes of the Manual; the Operation and Maintenance (O&M) Manual and the Design Manual. We are extremely grateful to the exter-
INDUSTRIAL VENTILATION COMMITTEE
Previous Members H.S. Jordan, 1960-1962 J. Kane, Consultant, 1950-1952 J. Kayse, Consultant, 1956-1958 J.F. Keppler, 1950-1954; 1958-1960 G W. Knutson, 1986-present G Lanham, 1998-present, Vice Chair, 2008-present J.J. Loefller, 1980-1995; Chair, 1984-1989 J. Lumsden, 1962-1968 J.R. Lynch, 1966-1976 K.R. Mead, 1995-2001 G Michaelson, 1958-1960 K.M. Morse, 1950-1951; Chair, 1950-1951 R.T.Page, 1954-1956 K.M. Paulson, 1991-present; Vice Chair, 1996-2008 O.P. Petrey, Consu1tant, 1978-1999 GS. Rajhans, 1976-1995; Vice Chair, 1994-1995; Chair, 2002-present K.E. Robinson, 1950-1954; Chair, 1952-1954 A Salazar, 1952-1954 E.L. Schall, 1956-1958 M.M. Schuman, 1962-1964; Chair, 1968-1978 J.C. Soet, 1950-1960 J.L. Topmiller, 2004-present AL. Twombly, 1987-2001 J. Willis, Consultant, 1952-1956 R. Wolle, 1966-1974 AW. Woody, 1998-present J.A. Wunderle, 1960-1964
GM. Adams, 2004-2008 AG Apol, 1984-2002 H. Ayer, 1962-1966 R.E. Bales, 1954-1960 J. Baliff, 1950-1956; Chair, 1954-1956 J.C. Barrett, 1956-1976; Chair 1960-1968 J.L. Beltran, 1964-1966 D. Bonn, Consultant, 1958-1968 D.J. Burton, 1988-1990 K.J. Caplan, 1974-1978; Consultant, 1980-1986 AB. Cecala, 1998-1999 G Carlton, 1999-2002 W.M. Cleary, 1976-present; Chair, 1978-1984 M. Davidson, 1995-1998 R. Dayringer, 2004-present L. Dickie, 1984-1994; Consultant, 1968-1984 T.N. Do, 1995-2000 N. Donovan, Editorial Consultant, 1950-2008 D.L. Edwards, 2003-present B. Feiner, 1956-1968 M. Flynn, 1989-1995 M. Franklin, 1991-1994; 1998-2001 S.E. Guffey, 1984-present · J.F. Hale, 2004-present GM. Hama, 1950-1984; Chair, 1956-1960 R.P. Hibbard, 1968-1994 R.T. Hughes, 1976-present; Chair, 1989-2001 GQ. Johnson, 2001-2008
ix
DEFINITIONS Aerosol: An assemblage of small particles, solid or liquid, sus-
Dejlagration: A propagation of a combustion zone that occurs
pended in air. The diameter of the partíeles rnay vary from lOO rnicrons down to 0.01 rnicron or less, e.g., dust, fog, smoke.
at a velocity that is less than the speed of sound in the unreacted medium.
Density: The ratio of the mass of a specimen of a substance to
A ir Cleaner: A device designed for the purpose of removing
the volume of the specimen. The mass of a unit volume of a substance. When weight can be used without confusion, as synonymous with mass, density is the weight of a unit volume of a substance.
atmospheric airbome impurities such as dusts, gases, mists, vapors, fumes, and smoke. (Air cleaners include air washers, air filters, electrostatic precipitators, and charcoal filters.)
Density Factor: The ratio of actual air density to density of stan-
Air Filter: An air-cleaning device that removes light particu-
dard air. The product of the density factor and the density of standard air (0.075 lb/fl?) will give the actual air density in 3 pounds per cubic foot; Density = df x 0.075 lb/ft (the density of standard air).
late loadings from normal atmospheric air before introduction into the building. Usual range: loadings up to 3 grains per thousand cubic feet (0.003 grains per cubic foot). Note: Atmospheric air in heavy industrial areas and in-plant air in many industries have higher loadings than this, and dust collectors are then indicated for proper air cleaning.
Dust: Small solid particles created by the breaking up oflarger particles by processes, i.e., crushing, grinding, drilling, explosions, etc. Dust particles already in existence in a mixture of materials may escape into the air through such operations as shoveling, conveying, screening, sweeping, etc.
Air Horsepower: The theoretical horsepower required to drive a fan if there were no losses in the fan; that is, if its efficiency were l 00 percent.
Dust Ca/lector: An air-cleaning device to remove heavy partic-
Aspect Ratio: The ratio ofthe width to the length; AR = W/L.
ulate loadings from exhaust systems. Usual range of particulate loading: 0.003 grains per cubic foot or higher.
Aspect Ratio ofan Elbow: The width (W) along the axis of the
Entry Loss: Loss in pressure caused by air flowing into a duct
bend divided by depth (D) in the plane of the bend; AR = W/D.
or hood (inches H 20).
Blast Gate: Sliding darnper.
Fumes: Small, solid particles formed by the condensation of vapors of solid materials.
Blow (throw): In air distribution, the distance an air stream
Gases: Formless fluids that tend to occupy an entire space uni-
travels from an outlet to a position at which air motion along the axis reduces to a velocity of 50 fpm. For unit heaters, the distance an air stream travels from a heater without a perceptible rise dueto temperature difference and loss of velocity.
formly at ordinary temperatures and pressures.
Hood: A shaped inlet designed to capture contarninated air and conduct it into the exhaust duct system.
Hood Flow Coefficient: The ratio of flow caused by a given
Brake Horsepower: The horsepower actually required to drive
hood static pressure compared to the theoretical flow that would result if the static pressure could be converted to velocity pressure with l 00 percent efficiency. NOTE: This
a fan. This includes the energy losses in the fan and can be deterrnined only by actual test of the fan. (This does not include the drive losses between motor and fan.)
was defined as Coefficient ofEntry in previous editons.
Capture Velocity: The air velocity at any point in front of the
Humidity, Absolute: The weight of water vapor per unit vol-
hood or at the hood opening necessary to overcome opposing air currents and capture the contarninated air at that point by causing it to flow into the hood.
ume, pounds per cubic foot or grams per cubic centimeter.
Humidity, Relative: The ratio of the actual partial pressure of the water vapor in a space to the saturation pressure of pure water at the same temperature.
Comfort Zone (Average): The range of effective temperatures over which the majority (50% or more) of adults feel comfortable.
Inch of Water: A unit of pressure equal to the pressure exerted by a column of liquid water one inch high at a standard temperature.
Convection: The motion resulting in a fluid from the differences in density and the action of gravity. In heat transrnission this meaning has been extended to include both forced and natural motion or circulation.
Lower Explosive Limit: The lower lirnit of flamrnability or explosibility of a gas or vapor at ordinary ambient temperaX
1i
General Industrial Ventilation
tures expressed in percent of the gas or vapor in air by volmne. This limit is assmned constant for temperatures up to 250 F. Above these temperatures, it should be decreased by a factor of0.7 since explosibility increases with higher temperatures.
Manometer: An instrmnent for measuring pressure; essentially a U-tube partially filled with a liquid, usually water, mercury or a light oil, so constructed that the amount of displacement ofthe liquid indicates the pressure being exerted on the instrmnent.
Micron: A unit of length, the thousandth part of 1 mm or the millionth of a meter (approximately 1/25,000 of an inch).
Minimum Design Duct Velocity: Minimmn air velocity required to move the particulates in the air stream (fpm).
:
Mists: Small droplets ofmaterials that are ordinarily liquid at normal temperature and pressure.
Plenum: Pressure equalizing chamber. Pressure, Static: The potential pressure exerted in all directions by a fluid at rest. For a fluid in motion, it is measured in a direction normal to the direction of flow. Usually expressed in inches water gauge when dealing with air. (The tendency to either burst or collapse the pipe.)
Pressure, Total: The algebraic smn of the velocity pressure and the static pressure (with due regard to sign).
Pressure, Vapor: The pressure exerted by a vapor. If a vapor is kept in confinement over its liquid so that the vapor can accmnulate above the liquid, the temperature being held constant, the vapor pressure approaches a fixed limit called the maximmn or saturated vapor pressure, dependent only on the temperature and the liquid. The term vapor pressure is sometimes used as synonymous with saturated vapor pressure.
Pressure, Velocity: The kinetic pressure in the direction of flow necessary to cause a fluid at rest to flow at a given velocity. Usually expressed in inches water gauge.
Radiation, Thermal (Heat): The transmission of energy by means of electromagnetic waves of very long wavelength. Radiant energy of any wavelength may, when absorbed, become thermal energy and result in an increase in the temperature of the absorbing body.
Replacement Air: A ventilation term used to indicate the volmne of controlled outdoor air supplied to a building to replace air being exhausted.
xi
Slot Velocity: Linear flow rate of contaminated air through a slot, fpm.
Smoke: An air suspension (aerosol) of particles, usually but not necessarily solid, often originating in a solid nucleus, formed from combustion or sublimation.
Specific Gravity: The ratio of the mass of a unit volmne of a substance to the mass of the same volmne of a standard substance ata standard temperature. Water at 39.2 F is the standard substance usually referred to. For gases, dry air, at the same temperature and pressure as the gas, is often taken as the standard substance.
Standard Air: Dry air at 70 F and 29.92 (in Hg) barometer. 3
This is substantially equivalent to 0.075 lb/ft . Specific heat of dry air = 0.24 BTU/lb/F.
Temperature, Effoctive: An arbitrary index that combines into a single value the effect of temperature, humidity, and air movement on the sensation of warmth or cold felt by the hmnan body. The nmnerical value is that of the temperature of still, saturated air that would induce an identical sensation.
Temperature, Wet-Bulb: Thermodynamic wet-bulb temperature is the temperature at which liquid or solid water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature. Wet-bulb temperature (without qualification) is the temperature indicated by a wet-bulb psychrometer constructed and used according to specifications.
Threshold Limit Values (TLVs®): The values for airborne toxic materials that are to be used as guides in the control of health hazards and represent time-weighted concentrations to which nearly all workers may be exposed for 8 hours per day over extended periods of time without adverse effects (see Appendix).
Transport (Conveying) Velocity: See Minimmn Design Duct Velocity.
Tum-Down Ratio: The degree to which the operating performance of a system can be reduced to satisfy part-load conditions. Usually expressed as a ratio; for example, 30: l means the minimmn operation point is 1130th of fullload.
Vapor: The gaseous form of substances that are normally in the solid or liquid state and that can be changed to those states either by increasing the pressure or decreasing the temperature.
ABBREVIATIONS HV .................humid volume (ft3 mix!lbm dry air) HVAC ..........heating, ventilation, and air conditioning in ............................................ inch . 2 . h m ......................................square me "wg .............................. inches water gauge lb ...........................................pound lbm ....................................pound mass LEL ........................... .lower explosive limit ME ............................mechanical efficiency mg ....................................... milligram min ........................................minute mm ......................................millimeter MRT ........................mean radiant temperature MW ............................... molecular weight
A ............................................ area acfm ...................... flow rate at actual condition AH .................................. air horsepower AR ..................................... aspect ratio As ........................................ slot area B ................................barometric pressure bhp ................................brake horsepower bhpa .........................brake horsepower, actual bhps .....................brake horsepower, standard air BTU ............................British Thermal Unit BTUII ................................ BTU per hour Ce .............................. hood flow coefficient CLR ................................ centerline radius D .........................................diameter df .............................. overall density factor dfe ............................ elevation density factor dfp ............................pressure density factor df¡ .......................... temperature density factor dfm ............................ moisture density factor dscf ........................... dry standard cubic feet dscfm ................ dry standard cubic feet per minute ET .............................effective temperature f .................... Moody diagram friction coefficient F ................................. degree, Fahrenheit Fh ..........................hood entry loss coefficient F el .............................elbow loss coefficient F en . . • • • • . • . . . . . . . • . . . . . . . • . . . . . . entry loss coefficient fpm .................................. feet per minute fPs ..................................feet per second Fs ................................slot loss coefficient 2 ft ...................................... square foot 3 ft ••.......•...•••••••••...•......••••••• cubic foot g ......................... gravitational force, ftlsec/sec gpm .............................. gallons per minute gr ........................................... grains hh ...................................hood entry loss he ............................. overall hood entry loss he1 .......................................elbow loss heu .......................................entry loss hf ............................ .loss in straight duct run HEPA ...............high-efficiency particulate air filters Hf ............................... duct loss coefficient hp ......................................horsepower hr ............................................hour hs ........................... slot or opening entry loss
P · · · · · · · · · · .....................density of air in lb/ft
3
ppm ................................parts per million psi ............................pounds per square inch PWR ........................................power Q ...................................flow rate in cfm Qcorr . . . . . . . . . . . . . . . . . . . corrected flow rate at a junction R .................................... degree, Rankin RH ................................relative hurnidity rpm ...........................revolutions per minute scfm ....................standard cubic feet per minute sfpm .......................... surface feet per minute SG .................................. specific gravity SP ................................... static pressure SPgov . . . . . . . . . . higher static pressure at junction of 2 ducts SPh ..............................hood static pressure SPs .................... SP, system handling standard air STP ..................standard temperature and pressure TLV® .......................... Threshold Limit Value TP .................................... total pressure V ..................................... velocity, fpm Vd .................................... duct velocity VP .................................velocity pressure VPd ............................ duct velocity pressure VPr . . . . . . . . . . . . . . . • • • . • . . . . .resultant velocity pressure VP s ............................. slot velocity pressure V s .....................................slot velocity V 1 . . . . . • • . • . . . . . • • . . • • • . • . • . . . . duct transport velocity W ............................................ watt ro ............... moisture content (lbm H 20/lbm dry air) z ...................... elevation in feet above sea level
xü
Chapter 1
EXPOSUREASSESSMENT
1.1 1.2 1.3
INTRODUCTION ooooooooooooooooooooooooooooool-2 HAZARDS OF THE OPERATION ooooooooooooooo01-2 IDENTlFY THE INHERENT HAZARDS oooooooooool-2 1.301 Health Hazards ooooooooooooooooooooooooo01-2 1.302 Flammability Hazards ooooooooooooooooooo01-3 1.3.3 Reactivity Hazards oooooooooooooooooooooo.1-3 1.304 Physical Hazards ooooooooooooooooooooooo01-3 1.305 Regulatory Issues Pertaining to Hazards ooooool-3 1.4 POTENTIAL EXPOSURE DURlNG NORMAL EQUIPMENT OPERATION ooooooooooooooooooooo01-3 1.5 POTENTIAL EXPOSURE OTHER THAN DURlNG NORMAL OPERATION oooooooooooooooooooooooo01-6 1.6 POTENTIAL SOURCE IDENTIFICATION oooooooo.1-7 1.7 ASSESSING THE EXPOSURE oooooooooooooooooo01-7 1.8 HIERARCHY OF EXPOSURE CONTROL OPTIONS oooooooooooooooooooooooooooooooooooo.1-7 1.9 COMMON AIRBORNE HAZARDS oooooooooooooo01-9 1.10 AIRBORNE CONTAMINANTS ooooooooooooooooo01-9 1.1001 Particulates oooooooooooooooooooooooooooo01-9 1.1002 Liquid Aerosols ooooooooooooooooooooooo01-ll 1.1003 Fumes ooooooooooooooooooooooooooooooool-12 1.10.4 Vapors ooooooooooooooooooooooooooooooo.1-12 1.11 INDOORAIRQUALITY ASSESSMENTISSUES o01-12 1.12 EXPOSURE MONITORING ooooooooooooooooooo01-12 1.1201 Personal Monitoring oooooooooooooooooooo.1-13 1.1202 TWA Monitoring oooooooooooooooooooooool-13
STEL Monitoring oooooooooooooooooooooo01-13 Ceiling Exposure Monitoring oooooooooooo.1-13 Engineering Monitoring ooooooooooooooooo.1-13 Video Use oooooooooooooooooooooooooooool-14 Monitoring Equipment Calibration oooooooo01-14 Selecting a Laboratory for Processing Monitoring Results ooooooooooooooooooooo01-14 1.1209 Monitoring for Air Contarninants in Confmed Spaces ooooooooooooooooooooooooooooooool-14 1.13 LEGALAND CODE REQUIREMENTS ooooooooool-14 1.1301 NFPA ooooooooooooooooooooooooooooooo01-14 1.1302 Building Codes oooooooooooooooooooooooo01-15 1.1303 State and Municipal Fire Codeso oooooooooo01-15 1.1304 Other Code Requirements ooooooooooooooo.1-15 1.1305 Emission Requirements ooooooooooooooooo.1-15 1.1306 Air Ernission Surveys oooooooooooooooooo.1-15 1.1307 Perrnits oooooooooooooooooooooooooooooool-15 1.14 SETTING AN EXPOSURE CONTROL STRATEGY 1-15 1.1401 Exposure Control Strategy Documentation oo01-16 lol5 VENTILATION SYSTEM WORKER SAFETY AND HEALTH ISSUES oooooooooooooooooooooooooooo.1-16 1.1501 Toxic Materials oooooooooooooooooooooooo.1-16 1.51.2 Fall Protection oooooooooooooooooooooooool-18 1.1503 Machine Guarding ooooooooooooooooooooo01-18 1.15.4 Lockout oooooooooooooooooooooooooooooo01-18 REFERENCES oooooooooooooooooooooooooooooooooooo01-18
Figure 1-1 Displaced Air Containing Fine Particulates ooo.1-10 Figure 1-2 Dust Expulsion by Mechanical Compression oo01-10
Figure 1-3 Entrained Air with Dust from Falling Product Stream oooooooooooooooooooooooooooooooo01-10
Table 1-1
Table 1-4
Table 1-2 Table 1-3
Visualizing the Potent Compounds Containment Challenge ooooooooooooooooooooooooooooooool-4 Deflagration Conditions oooooooooooooooooooool-5 Example Task Based Exposure Assessment oooool-8
1.1203 1.12.4 1.1205 1.1206 1.120 7 101208
Table 1-5
Particle Size Ranges and Classifications for Aerosols oooooooooooooooooooooooooooo.1-1 O Containment Tools to Reduce Exposures ooooool-17
1-2
1.1
Industrial Ventilation
INTRODUCTION
Adverse health effects can occur when employees are exposed to occupational hazards. Exposure to a hazard depends on the frequency, duration and magnitude of exposure events. Adverse health effects may occur immediately after exposure (such as the effects of carbon monoxide), or after a long latency period (such as the effects of asbestos). Exposure assessment involves the tasks of evaluating the nature and severity of occupational hazards present in the workplace. This assessment should be based on knowledge in the disciplines of industrial hygiene, toxicology, and epidemiology. The purpose of the assessment is to prevent hazardous exposures and any resulting adverse health effects. Industrial hygienists possess skills specific to conducting exposure assessments. The order of practice in industrial hygiene is hazard 1) anticipation, 2) recognition, 3) evaluation, and 4) control. This places exposure assessment (evaluation) as the third step in the industrial hygiene procedure and control as the fmal step. When considering industrial ventilation systems as a solution to occupational exposures, a three part methodology should be considered: 1) Evaluate whether the process generates potential chemical and/or physical hazards (Section 1.3); 2) Determine if employees are potentially exposed to the hazards (Sections 1.2 - 1.5); and 3) Determine if exhaust ventilation is the preferred method ofhazard control (Section 1.8). Due to the initial and long-term capital expenditures required to implement control systems, the installation of an exhaust ventilation system should only occur if other easier and less costly methods of control are not feasible. The method of answering the three basic questions will vary based on whether the process currently exists or is under proposa!. However, both scenarios require a thorough process review conducted with the input of an experienced occupational safety and health professional. Review will typically include the following steps: l) Identify potential hazardous chemicals and physical agents. Review the corresponding physical, chemical and toxicological properties and applicable exposure criteria. 2) Research the documented exposure levels and necessary control approaches for similar operations or processes. These can be either interna! or externa! to a specific facility. 3) Evaluate the process using a process management approach, investigating worst case scenarios and control approaches necessary to reduce the potential for adverse health effect. 4) Evaluate the process from the mindset ofthe tradition-
al industrial hygiene hierarchy-of-controls may be chosen to provide a relative measure of the inherent hazard. The pharmaceutical industry has been using OECBs successfully for years. Only the top OECBs (Band 1) can be effectively controlled without additional measures. OELs and OECBs are numbers that may be difficult to comprehend from a physical standpoint (Table 1-1 ). Air sampling equipment is readi1y available for analyzing a number of airbome health hazards. However, choosing the correct monitoring and analytical procedure, calibrating equipment and conducting monitoring in a manner that provides meaningful, accurate, and significant results can be difficult. If the facility does not employ an industrial hygiene, safety or plant engineering staff capable of perforrning personnel or process monitoring, an industrial hygiene consultant should be contacted. TheAmerican Board oflndustrial Hygiene (ABIH) certifies industrial hygiene professionals in a range of industrial hygiene practices with the designation certified industrial hygienist (CIH). A list of board certified industrial hygienists can be found at www.abih.org. 1.3.2 Flammabi/ity Hazards. Organic molecules and sorne inorganic molecules have the potential to hum very rapidly, generating large amounts of combustion gases in a small timeframe. If this rapid combustion were to occur in process equipment it could burst or rupture from over-pressurization. A deflagration propagates the combustion zone at less than the speed of sound. A detonation propagates faster than the speed of sound and cannot be controlled. Similar to the Fire Triangle (ignition plus oxygen plus fuel), the Explosion Pentagon lists five conditions for a deflagration to occur:
1) ignition source 2) fuel 3) oxygen or other oxidizer 4) mixing, and
5) confinement. The dry environment of sorne operations can also increase the risk of static electric discharge. The inherent flammability and combustibility hazards described must be known to pre-
vent a potential deflagration or explosion in accordance with National Fire Protection Association (NFPA) standards (Table 1-2). 1.3.3 Reactivity Hazards. Runaway reactions can be caused by materials that are readily capable of water reaction, detonation, explosive decomposition, polymerization, or selfreaction at normal temperature and pressure. Oxidizers are another category of physical hazard. Consideration should be given to possible conditions that can impact the design of ventilation systems, especially when venting closed chemical processes. 1.3.4 Physical Hazards. Other hazards can exist when installing a new process or when altering existing plant operations. These can include (but are not limited to) noise, vibrations, heat, skin contact with contaminants, and excessive moisture. In sorne cases, ventilation can be used to deal with these hazards, but often other corrective actions must be taken. 1.3.5 Regulatory lssues Pertaining to Hazards. Many chemicals have specific handling requirements in environmental or occupational health and safety regulations. Govemment agencies issue environmental permits for air emissions, wastewater discharges and hazardous/solid waste disposal. The Drug EnforcementAgency has regulatory authority for certain controlled substances. Check with environmental, health, and safety resources to determine what requirements apply to the project.
1.4
POTENTIAL EXPOSURE DURING NORMAL EQUIPMENT OPERATION
Once the potential hazards have been identified, the next step is to identify exposure criteria related to the individual hazards. Sorne exposure criteria are not guidelines, but are legal standards and regulations that require adherence. The majority of the legal standards and regulations relating to occupational exposures in the United States are established by the Occupational Safety and HealthAdministration (OSHA) in the U.S. Department ofLabor (USDOL) and the Occupational Safety and Health Administration (OSHA). These are the legallimits usually encountered when conducting occupational hygiene assessments in work environments. When the hazard is one of flammable materials or explosive vapors, OSHA has adopted the criteria developed by the National Fire Protection Association (NFPA) by reference. A number of states have established agreements with OSHA to conduct safety and health inspections within their own states. These agreement "state plan states" are required to establish standards that are at least as stringent as the OSHA standards, but may be more so. If the state where the process or operation occurs is one of the state plan states, then the stan-
......
TABLE 1-1. Visualizing the Potent Compounds Containment Challenge
J..
~
ISO 14&141 # particles f
tt•
IJparticiH 1ft"
#particlq ¡ft"
fl pa111cles 1ft"
9:e. ~
=
= ~
Band 1
(Low Toldcily)
1
~ Q
BandA {Not harmful, not lrritatlng, low
pharmaceufic41 actMty)
' Band2 (lntatmlldiale Toxícity) 1
1 Band ts
, ..1 '1
'®~
21!3
283,000
1
2.270,000,000
28.3
1
28,300
1
227,000,000
2.83
1
2,830
1
22,700,000
U3per 1011'
1
283
1
2,270,000
0.1
(ltarmful, may be irritan! andlor moderale phllrmacological elfacl)
1
1
1
--·--· ..... Band3 (Potaril)
actillily)
1
0,011
101
10.,
0.001
1
1.000
1
BandO (Toxi .... 1
,21
~
e:
a
~
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Ql
'5a.
j
1u
e:
S
~
:§.
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:S
a.
Approval Plant Manager
2-13
2-14
Industrial Ventilation
Class/Rotation/Arrangement Traps/Drains Stack Design Noise Víbration
pe pe e s,pe s,pe
D. Air Pollution Control Device Type of Device Auxiliary Connections Handling of Collected Materials Energy Considerations Construction Materials Specifications Special Temperatura Requirements Future Changes to System Safety Factor
~ees
le Costs Air/Cioth Ratio for Filters
e, e,pe,m e,pe,m o,pe,pm pe e,pe o o,pm,e o,pm,e pm,e pm,p,e e
E. Replacement Air (Air Volume exhausted by LEVs in Operation Area) Number and Type of Units
~Type
Vol ume Temperatura Rise Controls lnsurance Requirements Location Duct Design Supports
m,pe,e pe,e,pm e,pm,pe pe,e,pm pe,c,s p,pe,s pe,pm pe pe
Project Management lssues Cost Estimates Management of Scope Changes Drawing Standards Document Control & Distribution Transfer of Ownership Management of Safety Schedule
o,pm,p pm,p pe p,pe (al!) s,pm p,pm,o
FIGURE 2-1 (Cont.). Sample Team Responsibility Matrix
"T1
G5 e ;:o
m N
~
Project#
Project Title: Target Completion Date:
en
Dl
3
"C
1D
Sign & Date (AII signatures requlred to close PCD)
"'lJ
a
(D"
1. Project Manager must complete thls page before
Project Manager
lssuance of Design Basls.
g.
()
~
Recelvers
CiJ
Operations (O)
e:
o o g
Maintenance (M)
3(J)
Environmental (E)
;:¡.
-=u () .9
2. Checklist items may be completed at any point thereafter.
3. Final closure approvals may be obtained once the checklist is complete.
Plant Manager (PM)
Checklist item exceptions indicated by N/A on the checkllst.
Safety (S) Plant Engineer (PE) Quality (Q) Electrical Controls (C) Purchasing (P) ------------
--
----
----------
----------
_______ j
Extension Requests Revision Date
Reason (Attach list of tasks needed to complete)
Plant Manager (Sign and Date)
1. 2. 3. Final Closure Approvals Operations Manager Plant Engineer
Sign & Date
Signatures lndicate agreement that project is completed and can be closed.
~ = e5' ~
~ el ~
~· N
.... 1
(JI
2-16
Industrial Ventilation
Project: Date: A. Project Description
Oesign Basis 1 Plant: 1 Location:
B. Scope of Design Basís C. Attached or Reference Material: D. Mínimum System Requirements: E. Regulatory Requirements:
F.
Equipment Requirements (LEV System Requirements and Structurallssues):
G. Plant Safety Requirements:
H.
Ouct Specification Requirements:
l. Interface with Other Engineering Disciplines:
J. Power and Controls: K. Transfer of Ownership to Plant:
L. Technical Documentation: M. Technology Transfer and Trainíng to Operators:
Title
Signatura
Date
Project Manager Team Member Team Member Team Member Team Member Use this figure as a guide. Companies are encouraged to reprint and rearrange this figure on their own letterhead.
FIGURE 2-3. Sample Design Basis form
Preliminary Design
Project: Sand System Ventilation Plant: Local Foundry Location: Cincinnati, OH
Date: l/26/05
A. Proiect Description: Provide complete engineercd solution to the emissions from a Green Sand processing operation as sh0\\11 on Plant Layout drawing # lOO. System will exhaust dust from all emission points and convey to a new baghouse to be located outdoors behind the plant. System will meet all requirements for emissions as rcquired by local and applicable federallaws and guarantee a mínimum bag life of one year at full volume. B. Scope of Design Basis: System engineering will include al! hoods and enclosures (including supports), selection of airflow requirements, duct design and routing and selection of control device, fan, motor and drive. Electrical and compressed air connections to the equipment to be fumished by others. C. Attached or Referenced Materials: Plant layout drav.ings showing location of equipment to be ventilated, process requirements of sand (flow rate, moisture content and temperature), applicable emission limits, OSHA in-plant dust levels required, Plant Safety Rules. closures to meet the D. Minimum System Reguirements: System to be guaranteed to eollect dust a requirements ofUSEPA Method 204 for capture as \vell as all applicable operation and emíssion. This will be done while maintaining mínimum transport velocities to kee , ;ftom building up in al! , ust operate under all weather ducts. Desígn to consider moisture from severa! sources in the sabd proce conditions in the plant. Expected winter temperature in,~ic cm e,:SOF. E. Reeulatory Reguirements: System will fall underthé~MAC , c:fitrds as pu · eá in December 2002 and meet all ofthese requirements as well as applicab~~mi.SS~qn timits in~ff-at this date. No turther requirements for future changes are antici,p~ted at 1~i,1 tirrii( ,, F. Eguipment Reguirements (LEV Systent:lttgui!Jnitltts tpd Structurallssues}: New baghouse to be a baghouse with cloth bags al an aír/óio~ ratiQtO:'.guiran~ a bag ti fe of one-year while delivering full design volume and capture at,.elfhoods. ~ags~jll hlve access for changing and maintenance and include screw conveyor and rotary;yalve(s). M-ttmrp.p1 = 20. Fan(s) to be backward-inclined wheel design, Class III or heavier based on volÍime;:U:Bí'pel1tlíUre and pressure calculated. Fan will discharge into a free standing stack complete \\ith test por!s and aqcess'tbr emission testíng. Fan to be belt driven for horsepower up to 150. Fan will be direct drive if c~e~ horsepower is over I 50. Fan to operate on a stable point of fan curve with operating pressure less than 90% of maximum pressure at rated speed. Fan to be equipped with access door, removable wheel, and drain. Motors to be high efficiency. Equipment to be painted per plant specifications. All structural design to meet applicable codes and seismic requirements. G. Plant Safety Reguirements: Design to meet all plant safety requirements and attached rules. Designer to furnish company safety manual fbr approval before commencing and evídence of drug testing and related verification. H. Duct Specification Reguirements: New system to meet all requirements for thickness and stiffening per SMACNA round and squarc duct standards. All fittings to have entry angle of a maximum of 45 dcgrees \\ith 30 degrees preferrcd. Cleanout doors to be located on maximum 20 foot centers and at all elhows and hoods. System to be designed with blast gates for aír balancing. All elbows to have R/D of2.0 minimum with 2.5 preferred. l. Interface With Other Engineering Disciplines: Designer responsible for coordination with in-house structural (Contact name: ) and electrical engineering departments (Contact Name: ) for the design of structural supports, concrete foundations and power and control ofsystem (see Section J). J. Power and Controls: ln-house engineering staff will desígn all electrical power and controls for support of the new system. Designer will coordinate \\ith Electrical Engineer and províde sequence of operation. horsepower and other requirements for design ofthe system. K. Transfer of Ownership to Plant: This Design Basis is for design services only. Design will ha ve reviews with plant project team at 50% completion and 100% completion. After approval of design, information package
foP
FIGURE 2-4. Sample Design Basis
2-17
2-18
Industrial Ventilation
Project: Sand System Ventilation
Date: 1/26/03
(drawings, specifications for purchased equipment and the names ors for all equipment purchased) will be in electronic format with paper copies fessional Engineer. L. Technical Documentation: Designer to furnish ca volumes as well as ACGIH:¡; calculation sheets for the selection of du ressure requirements ofthe fan. Al! drawings to be done in CAD per plant g M. Te bn lo Transfer and Trainin toO provide training program for the installatíon of final system including requiremen or installation manuals and personnel training.
Signatures:
FIGURE 2-4 (Cont.). Sample Design Basis
(Project Manager) (Team Member) (Team Member) (Team Member) (Team Member)
Chapter 3
PRINCIPLES OF VENTILATION E:t.cosure
1
~s~menu JR!Si< At'lli)'Sl$
3.1 3.2 3.3 3.4 3.5 3.6 3.7
j
INTRODUCTION ............................. .3-2 CONSERVATION OF MASS ..................... 3-5 CONSERVATION OF ENERGY ................. .3-6 SYSTEM PRESSURES (STATIC, VELOCITY, TOTAL) ..................................... .3-7 SYSTEM LOSS COEFFICIENTS ................ .3-8 THE FAN IN THE SYSTEM ................... .3-11 APPLYING THE FAN TO THE SYSTEM (SYSTEM CURVE) ............................ 3-11
Figure 3-1 Figure 3-1a Figure 3-2 Figure 3-3 Figure 3-4
Conservation ofMass in a Duct Junction ....... 3-5 Conservation ofMass with Moisture Present ... .3-6 Conservation ofMass through a Heater ....... .3-6 SP, VP, and TP at a Point ................... .3-7 Measurement of SP, VP, and TP in a Pressurized Duct ......................... .3-8 Figure 3-5 SP, VP, and TP at Points in a Ventilation System ................................. .3-8
Table 3-1 Table 3-2 Table 3-3
Primary Physical Quantities ................ .3-2 Useful Symbolic Notation .................. .3-3 Dimensionless Quantities .................. .3-3
3.8
TRACKING PRESSURE VARIATIONS THROUGH A SIMPLE SYSTEM ................ 3-12 3.9 ASSUMED CONDITIONS (STANDARDAIR) ..... 3-13 3.10 ASSUMED CONDITIONS (NON-STANDARD AIR) ....................... 3-14 3.11 DENSITY AND DENSITY FACTOR ............ .3-14 REFERENCES .................................... .3-16
Figure 3-6 Exhaust Hood ............................ .3-9 Figure 3-7 Fan Work Example ........................ 3-11 Figure 3-8 Simple Duct System ..................... .3-12 Figure 3-9 System Curve ........................... .3-12 Figure 3-1 O Variation of SP, VP, and TP through a Ventilation System ..................... .3-12 Figure 3-11 Energy Gained by Air through a Heater ...... .3-14
Table 3-4 Table 3-5
Derived Physical Quantities ................ .3-4 Common Physical Constants ................ .3-5
3-2
3.1
Industrial Ventilation
vey a design airflow, etc.), these basic principies apply.
INTRODUCTION
The importance of clean uncontaminated air in the industrial work environment is well known. Modern industry with its complexity of operations and processes uses an increasing number of chernical compounds and substances, many of which are highly toxic. The use of such materials may result in particulates, gases, vapors, and/or mists in the workroom air in concentrations that exceed safe levels. Heat stress can also result in unsafe or uncomfortable work environments. Effectively designed ventilation offers a solution to these problems. Ventilation can also serve to control odor, moisture, and other undesirable environmental conditions. The application of ventilation to solve the problems of worker exposure and general plant hygiene involves a process of technical solutions to problems of air and particulate movement.¡¡, the flow rate of a hood can be quickly estimated and corrective action can be taken ifthe calculated flow rate does not agree with the design flow rate. This can be a useful tool for troubleshooting systems that may have lost airflow. Ce is an elusive value when designing and measuring a system. Values in this text are estimates for standard hood designs. Field conditions may alter designs and the actual value for Ce would be measured at start-up. The start-up value would be used for comparison rather than using the estimate from the design calculations. EXAMPLE PROBLEM 5 (Stralght Duct Losses) The losses along a length of straight duct are somewhat more complicated. Unlike hoods where the only contributing factors are the shape and the energy transfer of air as it moves into the hood, the losses in a straight duct depend on (are a function of): Velocity of the air moving in the duct (V) Density of the air (p) Length of the duct (L)
L ) 0
[3.13] Since duct lengths are in feet and duct diameters are measured in inches, the equation can be refinad further:
lfwe define F'd =
12f T - F~
where F'd is
a loss coefficient per unit length (feet)
Then the duct pressure loss (hd) is:
[3.14] Where F'd is determinad by the empirical relationship:
F' d
=avb (!)
ae
ft
[3.15]
The original values for friction or loss coefficients (sornetimes also called 'factors') (as a function ofRe and roughness) were provided on the Moody Diagram. (pressure drop across the bag media expressed in ''wg) may be extreme1y low and initial flows may be higher than design. This can have a negative effect on the operation of the system because the higher velocities through the media can embed particles in spaces between the media fibers and retard effective cleaning. In addition, the system may be connected to a process where high flows have anegative impact. Sirnilarly, a high initial flow may give false flow readings as the system is started and balanced. To reduce the impact ofhigh fluctuations in M, pre-coating ofbags may be the best solution. Another method would be to add artificial resistance to the fan by employing an outlet damper and feedback circuit to provide a constant inlet static pressure to the dust collector. The use of a Variable Frequency Drive (VFD) is another possible solution but has higher initial
costs. (Note: If a VFD or inlet fan damper is used for volume control, the requirement will still remain for minimum transport velocities in the duct system.) The designer will need to consider energy usage and other issues, but the design must always be able to provide the design flow at the maximum pressure drop encountered (i.e., baghouse at maximum M). 5.12
DISCHARGE STACKS
The final component of the ventilation system is the exhaust stack, an extension of the exhaust duct above the roof or grade. Assurning all exhaust emission levels are met and maintained, there are still two prime design considerations for the placement of an exhaust stack for a local exhaust ventilation system. First, the air exhausted should escape the building envelope so it does not return directly into building air intakes. Second, once it has escaped the building envelope, the stack should provide sufficient dispersion so that the plume does not cause an unacceptable situation when it reaches the ground. The exhaust stack should incorporate a "stack cap" to prevent entry of precipitation and ice. (In addition, the fan should incorporate a drain port so that moisture does not settle in its housing and cause problems at start-up.) lfthe exhaust stack design includes horizontal runs the duct should be slightly inclined toward a drain point. Large heavy vertical exhaust stacks should not be supported directly by the fan. When placing an exhaust stack on the roof of a building, the designer must consider several factors. The most important is the pattem of the air as it passes the building. Even in the case of a simple building design with a perpendicular wind, the airflow patterns over the building can be complex to analyze. Figure 5-10 shows the complex interaction between the building and the wind. A stagnation zone forros on the upwind wall.
Undisturbed flow Zl Roofrecirculation region Z2 High turbulence region Z3 Roof wake boundry
1.5R
FIGURE 5-10. Effects of building on stack discharge
Design lssues - Systems
Air flows away from the stagnation zone resulting in a down draft near the ground. Vortices form by the wind action resulting in a recirculation zone along the front of the roof or roof obstructions, down flow along the downwind side, and forward flow along the upwind side of the building. The USEPA uses computer modeling/simulations that utilize Gaussian distribution (such as PTMax) to predict resulting ground level concentrations of pollutants emitted from stacks. These predictive tools show 1Oto 100 times the normal ground level concentrations when building wake effects are included (dueto stacks being too short). More guidance in using these tools can be found at www.epa.gov/ttn/scram/, the site for SCRAM (Support Center for Regulatory Atrnospheric Modeling). A recirculation zone forms at the leading edge of the building. A recirculation zone is an area where a relatively ftxed amount of air moves in a circular fashion with little air movement through the boundary. A stack discharging into the recirculation zone can contaminate the zone. Consequently, all stacks should penetrate the recirculation zone boundary. The high turbulence region is one through which the air passes, however, the flow can be highly erratic with significant downward flow. A stack that discharges into this region will contaminare anything downwind of the stack. Consequently, all stacks should extend high enough that the resulting plume does not enter the high turbulence region upwind of an air intake. Because of the complex flow patterns around simple buildings, it is almost impossible to locate a stack that is not influenced by vortices formed by the wind. Tall stacks are often used to reduce the influence of the turbulent flow, to release the exhaust air above the influence of the building and to prevent contamination of the air intakes. Selection of the proper location is made more difficult when the facility has severa! supply and exhaust systems, and when adjacent buildings or terrain cause turbulence around the facility itself. When locating the stack and outdoor air inlets for the air handling systems, it is often desirable to locate the intakes upwind of the source. However, often there is no true upwind position. The wind direction in all locations is variable. Even when there is a natural prevailing wind, the direction and speed are constantly changing. If stack design and location rely on the direction of the wind, the system will clearly fail. The effect of wind on stack height varies with speed: 1) At very low wind speeds, the exhaust jet from a vertical stack will rise above the roof level resulting in significant dilution at the air intakes. 2) lncreasing wind speed can decrease plume rise and consequently decrease dilution. 3) Increasing wind speed can increase turbulence and consequently increase dilution.
5-17
The prediction of the location and the form of the recirculation cavity, high turbulence region and roof wake is difficult. However, for wind perpendicular to a rectangular building, the height (H) and the width (W) ofthe upwind building face determine the airflow patterns. The critical dimensions are shown in Figure 5-10. According to Wilson, the critica! dimensions depend on a scaling coefficient (R) and are given by:
[5.1] where Bs is the smaller and BL is the larger of the dimensions 'H' and 'W'. When BL is larger than 8*Bs, use BL = 8 Bs to calculate the scaling coefficient. For a building with a flat roof, Wilson estimated the maximum height (He), center (Xc), and lengths (Le) ofthe recirculation region as follows:
=0.22 R
Xc = 0.5 R
[5.2) [5.3]
Le= 0.9 R
[5.4]
He
In addition, Wilson estimated the length of the building wake recirculation region by: LR = 1.0 R [5.5] The exhaust air from a stack often has not only an upward momentum dueto the exit velocity of the exhaust air but buoyancy dueto its density as well. For the evaluation ofthe stack height, the effective height is used (Figure 5-11 ). The effective height is the sum of:
1) actual stack height (Hs), 2) the rise dueto the vertical momentum ofthe air, and 3) any wake downwash effect that may exist. A wake downwash occurs when air passing a stack forms a downwind vortex. The vortex will draw the plume down, reducing the effective stack height (Figure 5-12). This vortex effect is elirninated when the exit velocity is greater than 1.5 times the wind velocity. If the exit velocity exceeds 3000 fpm, the momentum of the exhaust air reduces the potential downwash effect. The ideal design extends the stack high enough that the expanding plume does not meet the wake region boundary. More realistically, the stack is extended so that the expanding plume does not intersect the high turbulence region or any recirculation cavity. According to Wilson, the high turbulence region boundary (Zz) follows a 1:1O downward slope from the top of the recirculation cavity. To avoid entrainment of exhaust gas into the wake, stacks must terminate above the recirculation cavity. The effective stack height to avoid excessive re-entry can be calculated by assuming that the exhaust plume spreads from the effective stack height with a slope of 1:5 (Figure 5-1 0). The first step is to raise the effective stack height until the lower edge of the 1:5 sloping plume avoids contact with all recirculation zone boundaries. The zones can be generated by roof top obstacles such as air handling units, penthouses or architectural screens. The heights of the cavities are determined by Equations 5.2,
5-18
Industrial Ventilation
Rise dueto momentum and buoyancy Effective stack height h
FIGURE 5-11. Effective stack height
FIGURE 5-12. Wake down wash effects
5.3 and 5.4 using the scaling coefficient for the obstacle. Equation 5.5 can be used to determine the length ofthe wake recirculation zone downwind of the obstacle. If the air intakes, including windows and other openings, are located on the downwind wall, the lower edge of the plurne with a downward slope of 1:5 should not intersect with the recirculation cavity downwind of the building. The length ofthe recirculation cavity (LR) is given by Equation 5.5. Ifthe air intakes are on the roof, the downward plurne should not intersect the high turbulence region above the air intakes.
When the intake is above the high turbulence boundary, extend a line from the top of the intake to the stack with a slope of 1:5. When the intake is below the high turbulence region boundary, extend a vertical line to the boundary, then extend back to the stack with a slope of 1:5. This allows the calculation of the necessary stack height. The minirnurn stack height can be determined for each air intake. The maximurn of these heights would be the required stack height. In addition, the heights may need to be increased to ensure that plurne does not intersect with the wake zone, as discussed above.
Design Issues - Systems
In large buildings with many air intakes, the above procedure will result in the specification ofvery tall stacks. An alternate approach is to estimate the amount of dilution that is afforded by stack height, distance between the stack and the air intake, and interna! dilution that occurs within the system itself. This approach is presented in the "Airflow Around Buildings" chapter in the Fundamentals volume of the ASHRAE Handbook. 200 fpm) and turbulent mixing. Depending on supply fixture design, outlet velocity and orientation, supply air will continue in its initial direction from the supply point until its energy is lost in the general area at 20--30 feet or more. Contaminated air released from a source located in the Supply Zone will be rapidly mixed (diluted) with the supply air. The Supply Zone ends where the air velocity is less than that of competing air currents induced by outside influences (traffic, thermal air currents, motion of material or equipment, etc.). This is called the General Zone. The first two zones are the province of"general ventilation" (see Chapter 4). The Local Exhaust Zone is the subject of this chapter.
mixed with clean air before reaching workers' breathing zones. Thus, a concentrated source very near a worker is likely to produce high exposures to that worker if only dilution ventilation is employed. By contrast, local exhaust hoods typically can adequately control emissions even when the source is within arm's reach ofthe worker as long as the contaminant cloud is relatively small or is projected away from the worker's breathing zone. 6.1.2 Local Exhaust System Effectiveness. The ability of a local exhaust ventilation system to reduce exposure to air contaminants is determined primarily by three factors:
l. The effectiveness of the hoods (if they have been provided sufficient airflow to contain and capture contaminants) 2. The ability of the fan/duct system to deliver sufficient airflow to each hood 3. Whether workers use the hood when needed, which is strongly affected by the convenience of the hood duringwork.
6.1.1 Local Exhaust Hoods Compared to Dilution Ventilation. Local exhaust systems are ventilation systems that employ local exhaust hoods to control emissions from sources of airbome contaminants, not allowing most of the contaminants to mix with room air prior to collection by the hoods. Hoods control contaminant exposures by controlling airbome contaminants at their source and exhausting them from the area. This is very different from dilution ventilation (see Chapter 4), which allows contaminated air to mix with room air and then exhausts the mixture.
This chapter addresses these issues, as well as issues important to the operation ofhoods. Chapter 13 contains recommendations for hoods for specific processes and tasks based on the general principies in this chapter. The designs in Chapter 13 often can be adapted for different processes and tasks, especially ifthe operating conditions are similar and the degree of hazard is about the same. If there are important differences, any design should be adapted and airflows selected in conformance with the recommendations given in this chapter.
Local exhaust ventilation systems generally need only a small fraction of the airflow required for dilution ventilation of the same sources. Furthermore, since dilution systems not only allow but promote mixing of the contaminated air with room air, dilution is generally adequate only when the contaminant is released at relatively low concentrations or can be well-
There are many hoods designed for specific applications and there are many more where the same basic design is used for different pwposes but are given different names, leading to diverse and sometimes contradictory terminologies. For that reason, the terms used here are mostly descriptive and do not follow any specific conventions.
6-4
Industrial Ventilation
6.1.3 Design Goals. Exhaust airflow into a hood should reduce the worker-user's exposure while working at the hood to "acceptable" levels or lower. The owner of the ventilation system may choose to set acceptable levels of exposure to values required for compliance with govemmental regulations (e.g., OSHA, EPA), conformance with recommended practices (e.g., ACGIH® TLVs®), or other levels. The latter most often occurs when there is no government regulation and no widely accepted recommended standard.
released to the air. Facing upstream often will produce lower exposures than facing downstream if the contaminant cloud does not extend above waist height. That is likely only if the contaminant is released at low velocity exclusively below waist height and immediately upstream of the body. Even under those conditions having the worker face upstream is discouraged because seemingly minor changes in work practices may cause at least sorne contaminant to be dispersed well above waist height.
As in all other engineering design, the goal for hoods is an optimal tradeoff of e:ffectiveness and overall costs while meeting the following goals:
Every blunt body in a flow pattem that is mostly in one direction will have a wake zone, including blunt bodies near the face of capturing hoods and most especially bodies just outside the face of enclosing hoods and within enclosing hoods that are designed to have plug flow. Plug flow will be used here to describe a flow without large scale eddies or swirling. Note that an "aerodynamically'' rounded body will produce minimal wake zones, with the rounding on the downstream side being more important than on the upstream side.
l.
The hood should not introduce a substantial new hazard to workers and should reduce safety hazards where possible.
2. The hood should not increase ergonomic stresses and should reduce them where possible. 3. The hood should use the minimum airflow required to meet goals. Operating and installation costs are roughly proportional to system airflow. 4. Hoods should be designed to minimize the time and e:ffort required for maintenance activities and other interferences with the process. 5. The hood and the materials handling system (entering and exiting the area) must be compatible. 6.1.4 Wake Zones. Understanding wake and separation zones is important when designing or operating hoods that rely on plug flow to protect workers and, to a much lesser degree, capturing hoods. Air passing around any blunt obstruction, including the human body, creates a complex downstream counter-flow known as a "wake zone" that includes more or less stable recirculating airflow patterns called ''vortices" as well as flow back towards the obstruction. Wake zones are crucial to exposure to contaminants and understanding them is crucial when designing hoods.
If the contaminant is released within the wake zone downstream of a human body, it can circulate in that zone while gradually dissipating due to dilution and sudden downstream movement of vortices called "shedding." Meanwhile, the backflow can carry contaminants released several feet downstream of the obstruction back towards the body and up to the breathing zone. If the flow is from the side or front of the body, the wake zone is on the other side or the back of the body, respectively. Since the mouth and nose generally face towards the front of the body, they are not in the wake zone unless flow is from the back. Thus, wake zones are generally of concem when the flow is from the back, (sometimes the case when standing in cross-drafts and nearly always the case when standing in front of enclosing hoods and especially when standing inside of a spray booth). Facing 90° to the cross-draft may provide the lowest exposures, depending on how the contaminant is
Although there may be larger blunt bodies with larger wake zones than the human body, the human body is the most important because the backflows in its wake zone can draw contaminants to the person's breathing zone. Separation of flows from surfaces produces conditions with sorne similarities to the wake zones downstream ofblunt bodies. Anytime airflow changes direction when flowing around a surface, its momentum causes sorne degree of separation of the flow from that surface. In the volume between the separation boundary and the surface ("separation zone" or "separation region") there will be flows with sorne similarities to those due to flow around rounded bodies. The greater the change in direction and the more abrupt it is, the greater the size of the separation region. The circulation velocity of the vortices within the wakes will increase with greater flow velocities. Contaminants released into a separation zone will only gradually dissipate with time. For enclosing hoods there are separation zones associated with the momentum-induced separation of flows around the perimeter of the hood (Figures 6-1 and 6-2). Ordinarily, contaminant that reaches those zones probably would not be a problem ifthe worker was centered on the hood or ifthe contaminant never reached the perimeter. However, if a highvelocity cross-draft approaches the hood from 90°, the size of the separation zone on that side may be large enough to intersect the wake zone ofthe worker's body, allowing transfer of contaminants between the two zones. 6.1.5 Hood Types. Hoods may have a wide range ofphysical configurations but can be grouped into two main categories: enclosing and capturing. lf a contaminant is released in front of the opening into which exhaust air flows, the exhaust opening is said to be a "capturing" hood since the movement of air induced to flow into the opening carries or "captures" sorne or all of the contaminant released in front of the hood (Figure 6-3a). Ifthe contaminant is pushed by mov-
Design lssues - Hoods
6-5
FIGURE 6-3a. Flow into a capturing hood
FIGURE 6-1. Flow with no crossdraft
to equipment inside, or they are intended to completely minimize openings, and allow little or no access. With careful design, sorne enclosing hoods can be used with workers inside.
ing air, thennal buoyancy, or the momentum from the contaminant release towards the capturing hood, the capture hood is called a "receiving" hood. A capturing hood can be large or small, depending mostly on the size of the source and its distance from the hood opening. Sorne capturing hoods protect workers working very near them (e.g., welding hoods) and others serve to reduce background concentrations (e.g., high canopies over furnaces ).
There are many specific terms to label different hoods within those two categories based on their specific purpose or aspects of their designs. Both types of hoods can be effective for cases where either the contaminant generation rate or the amount of dispersion or both are relatively low. If the contaminant generation rate is very high and highly dispersed, then only enclosing hoods are likely to be reliably effective, and then only if their openings are minimized and the worker does not enter the hood without appropriate respiratory protection (also see Appendix A6 ).
If the contaminant is released within the confines of a ventilated structure that is exhaust ventilated, the structure can be called an "enclosing" hood (Figure 6-3b). Enclosing hoods can be large or small. Sorne are intended to allow frequent access
6.2
ENCLOSJNG HOODS- JNTRODUCTJON
Enclosing hoods are ventilated boxes completely or partially enclosing one or more contaminant generation points. Enclosing hoods prevent the escape of contaminant by physically lirniting the openings through which contaminated air can escape and by the movement of air through those openings to prevent its escape. Enclosing hoods can be large or small, depending on the needs of the process and materials handling. There are many hoods used for very specific applications to meet special requirements for materials handling or high toxicity.
In general, enclosing hoods are the most effective means of contaminant control, but an irnportant functional consideration that drives selection or design is the degree and quality of the
FIGURE 6-2. Flow with crossdraft
FIGURE 6-3b. Flow into an enclosing hood
6-6
Industrial Ventilation
necessary access by workers (see Section 6.10 on Ergonomics). In general, the smaller the total area ofpermanent openings, the less airflow is required and the better the containment of the contaminants inside the enclosure. The trade off for the containment efficiency of such hoods is generally very high concentrations inside the hood, making them unsafe for worker entry without appropriate protection (see Section 6.3- Total Enclosures). If workers spend substantial durations reaching through permanent openings to manipulate objects inside the enclosure, then those openings must be large enough to access efficiently and conveniently. To be safe for this use, there are important issues in design and operation that must be addressed. Very often such hoods are mounted on stands, cabinets, or tables so that the opening extends from roughly waist height to above the head of the worker. For that reason, such hoods can be referred toas "bench top" hoods. Because ofthe importance of designing for work efficiency, they could also be called "bench top workstation hoods." A "laboratory" hood is a bench top workstation hood. If workers must occupy a hood to work, then one side of the enclosure may be completely open for ease of access, material handling and to allow uniform flow of hood into the hood. Protection ofworkers in such hoods depends very much on the uniformity of flows down the length of the hood since lateral and upstream flows will draw contaminants from downstream sources to the workers' positions. Because workers generally spend a great deal of time inside these hoods, they also should be considered work stations and designed accordingly. The most noteworthy application of this design is for spray painting large objects, so these hoods are sometimes called "spray booths" even when not used for spray painting. Since bench top workstation hoods also can be used for spray-painting, they will be henceforth referred to as "occupied enclosing" hoods. As with bench top enclosing hoods, critica! design and operational issues must be addressed for workers to use these hoods safely (see Section 6.10, Ergonomics). 6.3
TOTALLY ENCLOSING HOODS
Total enclosures actually include a broad range of hoods with varying degrees of enclosure. True ''total" enclosure (i.e., no openings whatsoever) would be a mistake in many cases since there could be no airflow and concentrations would build up during use. Whenever access was finally needed, the user would be exposed to a potentially very high concentration. If the materials were volatile (e.g., solvents), their evaporation could develop significant pressures in a tightly sealed enclosure, leading to an outward rush of air when the enclosure is opened for any reason. Instead, total enclosures have varying degrees of completeness and employ varying levels of stringency in attempting to prevent contaminant escape through whatever openings are in place (Figure 6-4).
Transparent
[5]
window
Hinged
access panel
O
..
-z___, ¡
~Access
Ports
FIGURE 6-4. Near-total enclosure
6.3.1 lssues in Common. A crucial commonality in totally enclosed hoods is the relatively high concentrations of contaminant inside them when compared to hoods that are designed to have uniform velocities (i.e., plug flow) as the air flows through them. The concentrations are generally higher because their high containment efficiency leads to use of relatively low airflows compared to the generation rate of contaminants, and they are not designed for plug flow so there often are back flows and eddies. The stagnation zones due to eddies can produce large differences in concentrations within the structure and therefore "hot spots" of high concentrations. Hence, because of back flows and stagnation zones, concentrations at openings can be relatively high, making these hoods unsuitable for operations where workers must frequently reach into the hood through openings.
In almost all total enclosures, at least sorne material handling typically is done through openings that are kept blocked by panels and doors. When uncovered, these larger openings can dramatically compromise the effectiveness ofthe hood. When larger openings are covered, well-designed total enclosures often are the only hoods capable of adequately controlling sources that are highly hazardous due to toxicity, rate of emissions, and energetic dispersion. In all cases, the amount of air continuously exhausted from the hood must greatly exceed the amount of contaminant produced by evaporation and other mechanisms, including rapid displacement ofthe air in the hood due to rapid inflow of materials and to thermal expansion. The recommended airflow may be specified (see
Design Issues - Hoods
Chapter 13) based on the expected eflluent level for specific applications but typically is stated as a minimum velocity through ports and other openings (see USEPA Method 204 as used for the determination of hoods containing VOC compounds). In either case, either the inlet velocity must be great enough to overcome the momentum of airflows that impact areas near the openings or the ports should be shielded from such impacts. Likewise, inward velocities should be higher for hot processes to overcome hydraulic forces due to buoyancy ofhot air. The inlet air ports and the exhaust port should be located such that stagnant regions do not develop, especially ifvolatile materials are being contained. If the equipment inside the hood allows, it is desirable to create plug flow. For cases where high velocity air movements are created inside the enclosure by the process, it is important to either avoid placing ports where high velocity air can impact them or to shield the ports so that high velocity air cannot impact them directly. Normally sources in the enclosure should be at least 4 equivalent diameters away from any opening. The size of any total enclosure must be great enough to contain the equipment and materials inside it. lt is also recommended that it be large enough to allow highly energetic contaminant releases to dissipate momentum before striking the sides of the enclosure at locations with ports or other pathways to escape. Any hood can lose containment if it is too small considering the energy level and generation rate of the contaminants it seeks to control. At the same time, all enclosures must be evacuated at a flow rate that will provide operation at levels below the LEL. Assuming adequate levels of airflow and avoidance of stagnant regions, the range of containment e:fficiency with total enclosures of different types is strongly affected by the care tak:en in minimizing opportunities for contaminants to escape. For purposes of discussion, they are divided here into the functional groups: Extremely High Control, Very High Control, High Control, and Moderately High Control. The actual degree of control of each is determined not only by their initial design but how well they are installed and operated. The adequacy ofthe degree of control is strongly affected by the thermal and kinetic energy of the contaminant, as well as its generation rate and toxicity. A hood designed for one contaminant and set of conditions may fall short of requirements when another material is to be contained or the generation rate, temperature, or other conditions are changed. 6.3.2 Extreme/y Effeetive Total Ene/asures. Sorne processes are so hazardous that extreme care must be tak:en to minimize escape from the hood. Examples are the handling of radioactive dusts and gases, deadly bacteria and viruses. To achieve extremely high containment effectiveness, hoods must have a high degree of enclosure and extreme care must be tak:en to minimize escape through ports and openings. The highest containment e:fficiencies within this group are obtained
6-7
by ventilated boxes for which no access at all is required when contaminants are inside the enclosure. Manual access may be provided by manipulators inside the enclosure controlled from outside the enclosure. The enclosure is opened only after a substantial purge period and thorough intemal vacuurning of toxic dusts, viruses or bacteria. To assure constant dilution, the inlet ports should be very numerous and small. Ideally, outflow due to the momentum of air movements or pressure waves inside the enclosure are minimized by forcing circuitous paths to the ports and by resistance to flow through the port. The latter can be provided by filter media such as high e:fficiency particle arrestors. Filters at the ports would also provide secondary protection in case of fan failure. Special regulations and standards should be consulted for these design requirements. 6.3.3 High/y Effective Total Enelosures. Somewhat lower protection but still extremely high control is offered by "glove boxes" (see Chapter 13, VS-35-20) that are total enclosures with impregnable gloves securely attached to interna} ports. The operator inserts his or her arms into the gloves and views the inside of the glove box through a plastic glass or larninated safety glass window. In most designs adding or removing materials or equipment to the glove box is done through an "air-lock" of two small doors in series. The user opens the outside door, places the object in the space between the two doors, closes the outside door, then opens the inside door and retrieves the object with the built-in glove. Even this arrangement will allow sorne transfer of airbome contaminants to the room unless grilles are placed in the airlock doors to provide continuous dilution of the chamber between them. Sometimes the chambers have their own duct tak:eoffs if it is desirable to minimize contamination. Settled or condensed material in the airlocks is likely unless extraordinary measures are made to clean the chamber, preferably using an intemally mounted vacuum cleaner hose. E ven without such cleaning, the amount transferred by handling should be very small.
The gloved port with window idea can be applied to almost any enclosure to good effect. For example, one can place a gloved port insert under the sash of a lab hood. Likewise, one could place glove ports on the wall outside a room, allowing manipulation of objects within reach ofthe gloves. Glove boxes are not necessarily highly effective. For example, the level of control oflab-hood glove box inserts would be deterrnined mostly by the quality of the seals for the insert and by the care tak:en to purge the hood before removing the inserts. Manufacturer standards and regulatory standards must be checked before usage and specification. 6.3.4 High Control Total Ene/asures. If a hood has a high degree of enclosure but less care is tak:en to prevent contaminants from escaping through ports and other openings, it can still be capable of providing an effective control of containment. Assurning that the enclosure must be opened substantially at regular intervals, the most critica} deterrninants of effec-
6-8
Industrial Ventilation
the blocked end. Because they are often filled with large pieces of process equipment (e.g., melting furnaces, etc.), it is sometimes necessary to add additional inflow locations to ensure that air flows through otherwise blocked areas. In placing any opening, it is important that the opening not be in-line with a jet of contaminated air issued within the enclosure. A jet of high velocity air will blow through any opening and overpower lower velocity air drawn into that opening.
tiveness are: 1) sufficient exhaust airflow, 2) prevention of outward flow through the inlet ports, 3) the quality ofthe seals of doors, panels, and windows, and 4) allowing sufficient purge time before opening.
An example is what could be called ''rough glove boxes" used to handle hazardous (but not extremely hazardous) processes. For example, sorne sandblasting can be done in small rooms with the operator standing outside the room to manipulate the sand blast hose through gloved ports. An example application of the latter is "sandblast sheds" used in the manufacture of grave markers and other stone monuments. Because the seals in sorne cases may not be tight and because the operation depends on effective work practices (i.e., waiting for the enclosure to purge itself of dust before access), exposure levels can be exceeded. Ventilated storage cabinets can be designed with an exhaust port and multiple grilles to allow entry of supply air into the cabinet. The exhaust port and grilles should be positioned at each end of the cabinet with the grilles placed to avoid stagnant zones within the cabinet. A door to add or remove the stored chemicals or gas cylinders is a potential vulnerability for two reasons: 1) if it is not shut, the control offered by the cabinet will be poor, and 2) if a stored liquid spills or leaks from the storage vessel, the fluid can seep underneath the door unless the vessel stands in a bucket with sufficient volume to hold spilled liquids. Storage cabinets also can fail under other conditions. For example, if a gas cylinder stored in such a cabinet developed a massive leak, the resulting pressure could exceed the negative pressure in the cabinet, allowing toxic gases to flow through the grille. If the pressure were high enough, the escape could be at high speed. 6.3.5 Moderate Control Total Ene/asures. If the total enclosure has a somewhat lower degree of enclosure and still less effective measures to prevent escape through ports and openings, it can be moderately effective compared to the preceding hoods, though potentially much more effective than plug flow hoods and capturing hoods. If the enclosure is relatively large and the velocity through openings is relatively high (e.g., 150-200 fpm), it can provide a sufficiently high degree of reliable control. They also generally provide the most reliable control of very hot and large quantities of contaminated air. Examples of these hoods are shown in Chapter 13, Section 13.73, Hot Processes.
There are three critica} points concerning these hoods: l. Even if large enough for operator entry, they are seldom designed or suitable for human occupancy. For a worker to enter one safely, the process may have to be shut down. In sorne cases, it may be possible to enter safely while wearing appropriate protection. 2.
The location of the entry points and the exhaust point are important. Generally, they should be designed for flow from one end to another. If the inlet end is blocked, airflow can be drawn around the perimeter at
3. To operate with very high effectiveness, all openings with substantial areas must be opened only for short periods of time. Avoid times when the emissions are highly concentrated or energetic as much as possible. Note that the opening for supply air can be quite large yet still be effective for large, energetic sources if plug flow or near plug flow is established and the inlet is far from the workers' breathing zones and is not used for worker access. 6.4
ENCLOSING HOODS THAT RELY ON PLUG FLOW TO PROTECT USERS
In many cases, work tasks require workers either to stand or sit and reach into the enclosure frequently or, for very large enclosures, to work inside the hood. In these applications, the hood can sufficiently protect the worker only if great care is taken in the design and operation ofthe hood. In particular, the contaminant cloud inside the hood must be largely prevented from reaching the breathing zone. This is best accomplished by preventing the contaminant cloud from mixing with the wake zone ofthe worker as muchas possible. 6.4.1 lmportance of Plug Flow. If the worker is at the face of the hood reaching into it to work, then the inflowing air must push the contaminant towards the back of the hood and the contaminant should not recirculate to the face of the hood once it enters into the enclosure. The primary strategy is to provide relatively uniform velocities at the face of the hood and well into the enclosure. A flow that has a uniform velocity will show little swirl (spiraling flow), no large-scale eddy currents (thus no stagnation zones with rotating flow) and no flow back toward the face. The air is said to move as if it were a fixed volume or "plug." Obstructions and competing air movements tend to disrupt the uniformity of the airflow and thus reduce the protection provided by the hood.
The same issues apply to personnel who work inside a large enclosing hood. It is imperative that the movement of air separate their wake zones from the contaminant cloud. The separation is best achieved by distance, uniformity of velocities through the enclosure, and by keeping contaminant clouds downstream of or to the side of workers. Obstructions and competing air motions can disrupt the flow in ways that move the cloud toward the workers inside the enclosure. To better accomplish plug flow, such hoods generally have a completely open face that is the same cross-section as the enclosure. For occupied hoods, the face can be a wall offilters
Design Issues - Hoods
6-9
to remove room air dust, especially for spray-paint booths. While not "open," a cross-section ofthe wall offilters equal to enclosure size can provide relatively uniform flow.
2. Rate of generation ofthe contaminant. A higher generation rate generally requires velocity closer to the top of the recommended range.
If the hood face is partially blocked so that little or no flow passes through substantial portions of the face, the result will be large-scale eddy currents, along with accompanying stagnant zones as well as lateral and vertical movement of contaminated air. Ifworkers are inside the hood, such blockages will increase their exposures. If the worker is at the face of the hood, such partial blockages can draw contaminants toward the face of the hood, increasing exposures to the worker. In the case of a laboratory hood, the barrier is a sash that can be raised and lowered or moved laterally. lt is intended to keep the user's face out ofthe hood. A vertical sash also serves to keep the worker's face well above the bottom ofthe sash. That is critical because the sash (and toa lesser degree, the "dome" inside the hood) produces a large vortex that rolls on a horizontal axis just behind the sash, bringing contaminants from throughout the hood to the bottom ofthe sash. Lab hoods (see Chapter 13, Section 13.35) have non-plug flow inside the enclosure, but the sash protects users by keeping their heads outside of the hood well above the bottom of the sash.
3. Strength of competing air motions inside the enclosure (e.g., pneumatic spraying) and outside the hood (e.g., cross-drafts, personnel cooling fans, passing vehicles). Very strong competing air motions may warrant face velocities above the range typically recommended. It is also quite possible that for very poor conditions exposures sirnply cannot be controlled sufficiently to protect a worker who is very clase to the source. Based on a study of laboratory hoods, one source-----~~---a
FIGURE 6-32. Roll out hood
6-34
Industrial Ventilation
Hinges to i~ve overbead and sídea
~
aocess
Light ñxture
......
~
1'
"
For infrequent access for maintenance of items within arm's reach, provide a sliding or hinged panel. Avoid bolts or complex fasteners for securing serviceable parts. If particulates or liquids are released or sprayed in the hood, provide a means to remove them periodically or continuously. Also, if it will be necessary to work on the top of an enclosure (e.g., to change the lights) or in any case where falls from an elevation are possible, ensure that proper fall protection and clip on points are provided. 6.13
Side View (Enclosure transparent)
FIGURE 6-33. Turntable
6.12
MAINTENANCEAND CLEANING FORALL HOODS
Collection of settled material should be made convenient whenever particulates (e.g., dusts or rnist) may settle inside the enclosure or from the plenum of a slot plenum hood. For liquids, provide inclined pathways toa drain (Figure 6-34). For dusts, relatively steep slopes should be used where possible to encourage settled material to slide to the bottom (Figure 6-35). Access via panels or doors should be convenient. The panel or door should be hinged rather than hung from supports to assure that they do not "walk away" from the area Access for maintenance of the hood and equipment within should be considered in the initial hood design. For example, easy access should be provided for maintenance of the lighting apparatus, preferably by installing on top of the hood and letting the light shine through a sealed plastic glass or larninated safety glass window (Figures 6-31,6-32, and 6-33).
MAN-COOLING FANS
Man-cooling fans move large quantities of air at very high velocities. That air movement will overwhelm enclosing hoods, blowing the contarninant out ofit even ifblown directly at the face. Even ifthe flow is perpendicular to the hood face, it is likely to radically reduce the effectiveness of the hood and prevent the escape of contarninant. Although man-cooling fans are likely to reduce the effectiveness ofboth enclosing and capturing hoods, the hood user will not necessarily be overexposed to airbome contarninants as a result. The fan may simply blow the contarninant away and cause it to rnix with the ambient air ofthe room. Ifthe generation rate of the source is relatively small, the rise in room concentrations may be acceptably low, especially if there is an effective dilution ventilation system in operation. If the emission source is large the man-cooling fan may simply spread the contaminant around the worker, raising the area concentrations considerably. A simple response to the effects of man-cooling fans is to han them in areas ventilated by hoods. However, if the worker would experience heat stress without a man-cooling fan, it may be better to measure worker exposures and ambient concentrations with the fans both on and off. Ifthe results are acceptably low, one could consider allowing use of the man-cooling fans despite their disruption of hood performance. However, it
Heac:J Plenum Slopesto Drain
12" mln.
Side plenum slopes down to head plenum wlth draln FIGURE 6-34. Diptank with draining for water that enters through ventilation slots on sides and front
Design lssues - Hoods
-0 -- -- -- - TlTLE
®
TERMINOLOGYFORFANS;
FIGURE
7-2 ~~~ AND n.:-n...---1-0-7---1
CHECK CODES. REOULATIONS, ANO LAWS (LOCAL, STATE,ANDNATIONAL) TO ENSURE THAT DESION !S COMPLIANT.
Fans
Diverter
Side sheet
1 1 1
.... , /
~ /
Rim
lnlet collar
Reprinted ftom AMCA Publication 201·90, FANS ANO SYSTEMS, by permissíon ofthe Air Movement and Control Association, Inc.(7 3 >
TERMINOLOGY FOR CENTRIFUGAL FANS; COMPONENTS CHECK CODES, REGULATIONS. ANO LAWS (WCAL STATE. ANO NA110NAL) TO ENSURE THAT OESIGN IS COMPLIANT.
1 1
7-3 1-07
7-5
7-6
Industrial Ventilation
units can be obtained with either downward deflecting or upblast discharges.
Fan and Dust Collector Combination: There are several designs in which fans and dust collectors are packaged in a unit. If use of such equipment is contemplated, the manufacturer should be consulted for proper application and performance characteristics. 7.3
FAN SELECTION
Fan selection involves not only fmding a fan to match the required flow and pressure considerations but all aspects of an installation including the air stream characteristics, operating temperature, drive arrangement, and mounting. Section 7.2 discussed the various fan types and why they rnight be
TYPE
o
~
< u
m ~
¡;,.. ¡;,..
w
Operating characteristícs of this fan are similar to the airfoil fan mentioned above. Peak efficieney for thís fan is slightly lower than the airfoil fan. Normally unstable 1cfl of peak prcssure.
Samc heating, ventilating, and uirconditioning applications as the airfoil fan. Also uscd in some indnstrial applications wherc the airfoil blade is not acceptable
bccanse of corrosive andlor erosion environment
6 8 2 4 lO VOLUME r:LOW RATE
8
Hígher pressure characlelistics tlwn the abo ve mentioned fans. Power rises continually to freedclivcry.
Pres.il!re curve is less steep titan that of backward-curved bladed íllns. Therc ís a dip in the pressure curve left of the peak prcssure point and bíghest etrtciency occurs to the rigbt of peak prcssurc, 40 to SO% of Wid~Hlpe~l volumc. Fan sbould be rated to the right of peak prcssure. Power curve rises contínually toward 1M delivcrv and litis mnst be taken ínto account Wben motor is sclected.
U sed primarily for material handling applieations in indnstrial plants. Wbcel can be of rugged construction and is simple to repair in the field. Wbeel is sometimes coated wílh specinl material. Tbis dcsign also uscd for bigh-pressure industrial requirements. Not collUilonty found in HVAC applícations.
Used primarity in low-prcssure beating vcntilating and air-conditioning applications sttcb as domcstic furnaccs, central station units, and packaged uir-conditioning cquipmcnt from room air-conditioning units to roof top units.
FIGURE 7-4a (Cont). Centrifuga! fans: performance curves, characteristics and applications (*These performance curves reflect the general characteristics of various fans as commonly employed. They are not intended to provide complete selection criteria for appliation purpose, since other parameters, such as diameter and speed, are not defined.)
7-8
Industrial Ventllation
IMPELLER DESION
TYPE
~
;:.¡ -t
~
~
~
~
~
>
" , _ Melllod Calculalion Slleel 1
~- Pfalllem 1 ,..... u t• 2"
r ~
T Qact VI
;,.;
t}f DlllliF_F_
~
ft
ktl
VPIII
a-u.~
,._, ........
-Et*JU.~
T-7
No.
·~
- - -
Dlllllflklloni.GMioVP a-1.-roVP DlllliU.IoVP
DlllliU. Olhor'-Wlllgl!lld " - VP
----
--~ ~--
v,_ ~VIIoollr "':.... ~~-
... ...
201Jol = 0.0311(VII.IIH/Q "'")
...
31* 1.81
2.45
2.1
~
t)
~
0.214 11.1t 0.17
81'1111Cb Elllry l.oM Coelllc-
G.24
0.13
eo.HJ•-
u
211
VP/Iillina
Otllllt~
VP,
V...,
2.00 2.110
~ Enlly l.ou Coollclonl
Tc«t~~DI.IQ~Mt
VP = di{V/4005) 2
10 c.., EI!Miwi.OM (5Pieca) BIJ.L._fal
20
lfl
~l.oulnVP
V = 40D6./VPiii
"• o.m + CII(10&1+0.444n
19
lorO
Hoodl!ntryL-~
Q..r(a,..l(1+w))df
18 7
lpm
VPII!OOd
Hoodl!ntryl.ou
dfr = (1130)/(T + 410) g (1+111j/(1+ UI07m)
df,.
111" 17'
VP,
111=1~0/tllly Alr
~
Oor1
'"'
46
o
38343
217111 4000 210 1313 0.71 17600 T.1t
4000
G-Il 176
VPitlol
35 38 37 38"
1 Raised floor o
Raised floor with depressed s1ab. Air is retumed through a utility compartment to the air supply system.
Airflow
.:
•
Low sidewall grille retum through a utility compartment to the air supply system. Room width is limited to 14 feet iflaminar airflow is to be achieved. The distance from the top of the grille to the floor should not exceed 18 inches.
TITLE
CLEANROOM RETURN AIR ARRANGEMENTS
FIGURE
DATE
CHECK CODES, REGULATIONS, ANO LA WS (LOCAL, STA TE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-10-03 1-10
Specific Operations
13.15
FILLING OPERATIONS
Filling operations have special considerations that should be addressed when designing hoods. An enclosed space is not empty but rather is filled with air. When material enters the space, it forces the air out which in turn can carry sorne of the material with it. Also, additional air can be entrained by the material stream entering the enclosed space. This effect is a function of the size of the partides and the distance the material must fall. These two effects must be considered when designing hoods for material handling situations. If there are any openings in the walls of the container which is being filled, sorne "splashing" of the material can occur. This can lead to loss of material through cracks and openings in the receiving vessel. The design of the ventilation system should take this effect into account. The proper choice of exhaust flow rate is critica!. If too little air is exhausted, the air displaced by the falling material may exceed the exhaust rate and the contaminant may not be adequately controlled. If too much air is exhausted, excess material could be entrained into the exhaust air stream. As this material often is the product, excess product loss could occur. VS-15-0 1 illustrates four different ways of controlling barrel or drum filling operations. VS-15-02 illustrates bag filling and weighing. VS-15-03 depicts a bag tube packer. VS-15-1 O and VS-15-11 depict a weighing hood where dry materials are removed from a bulk pack and weighed into smaller bags_ Ir) 1
r:/)
~
+
o
1" slot
2
Q = 100 acfin/ft barre! top (mínimum) Mínimum duct velocity = 3500 fpm he= 1.78 VPs + 0.25 VPd
Q = 150 acfin/ft2 of open face area Minimum duct velocity = 3500 fpm h.= 0.25 VPd (45° taper)
Q
1 Exhaust duct PufferHood
Feed spout 3" min. dia.
Flex duct
Q =50 acfm X drum diam. (ft)
Q = 300-400 acfm
Minimum duct velocity = 3500 fpm h.=0.25 VPd
Mínimum duct velocity = 3500 fpm h_=0.25 VPd
Note 1: A ir displaced by material feed rate may require higher exhaust flow rates. Note 2: Excessive airflow can cause loss ofproduct. Note 3: When transferring flammable or combustible liquids, bonding and grounding requirements ofNFPA Code 77 should be followed. Reference: 13.15.4 TITLE
FIGURE
BARREL FILLING
DATE
CHECK CODES, REGULA TIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-15-01 1-10
Specific Operations
Hood attached to hin
Principal dust source 500 fpm maximum Scale support
Bag
Q = 400-500 acfm - non-toxic dust 1000-1500 acfm - toxic dust Mínimum duct velocity = 3500 fpm he= 0.25VPd Note:
Care must be taken so that too much air is not used, as valuable product will be pulled into the exhaust system.
Reference: 13.15.2 TTTLE
FIGURE
BAGFILLING
DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-15-02 1-10
13-15
13-16
Industrial Ventilation
Feed hopper
-B
----11"~--
¿!!!!!" Spill hopper
. .•'., ...
-e
1
'
'
Q = 500 acfm per Feed hopper (A) = 500 acfm at filling tube (B) =
950 acfm at Spill hopper (C)
Minimum duct velocity = 3500 fPm
he= 0.25 VPd for take-off atA ande 1.0 VPd for take-off at B
Reference: 13.15.5 FIGURE
TITLE
BAGTUBEPACKER
DATE
CHECK CODES, REGULATIONS, ANO LA WS (LOCAL, STA TE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-15-03 1-10
Specific Operations
13-17
o ....... 1
"' 00 ....... 1
>
;
Air Shower
Dry Material Container Hood
Dry Material Container
Piano Hinge for Cleanout Access Magnetic Late hes at Both Ends
NOTE: See VS-15-11 for design details See Chapter 6 for further details on hood plenum cleaning
Reference 13.15.1 TITLE
WEIGH HOOD ASSEMBLY DRYMATERIAL
FIGURE
VS-15-10 DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STA TE, AND NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
1-10
13-18
Industrial Ventilation
,......
-> 1
Ir)
BOOTH
1
rJJ.
~
2
QB = 50 acfm/ft of face open area. L and W to fit operation Mínimum duct velocity = 3500 fpm he= l. 78 VPs +0.25 VPd
Configure to fit equipment
AIRSHOWER
Q 8 = 100 Lsacfm
ls = 3 feet. (Can be longer ifrequired to fit workstation but do not exceed 1/2 booth length; L)
•1:"" t:
.,''"
0.25: 1 pegboard or equivalent, 20 percent maxtmum open area.
DRY MATERIAL CONTAINER HOOD Hood is extension ofbooth slot. An additional takeoff(s) may be used if required for hood airflow distribution. Airflow and hood slot design per VS-15-10 12" to 24" Diameter 24" Maximum Reference 13 .15 .1 FIGURE
TTTLE
WEIGH HOOD DETAILS DRY MATERIAL
DATE
CHECK CODES. REGULATIONS, ANO LAWS (LOCAL. STA TE, ANO NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-15-11 1-10
Specific Operations
13-19
o.
Light inside hood
Compactor to take fiber bags ----~ Open grille work shelf -+----.._ ( under hood)
Hopper connected to screw feed, chute, etc.
2
Q. = minimum 250 acfm/ft ofhood face open area Mínimum duct velocity = 3500 fPm he=0.25 VPd
:a
Qb = minimum 250 acftn/ft of compactor open area
Minimum duct velocity = 3500 fPm he= 0.25 VPd (tapered takeoff, a= 45 °)
Reference: 13.15.2 FIGURE
TITLE
TOXIC MATERIAL BAGOPENING
DATE
CHECK CODES. REGULA TIONS, ANO LAWS (LOCAL, STA TE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-15-20 1-10
13-20
Industrial Ventilation
lmpeller shaft
Process fluid
Optional drain plug Optional slinger U sed to prevent the process fluid from creeping along the shaft.
t:
.,.
, 2
Q = 500 acfm/ft of open area (typically 10-40 acfm) Note: Sufficient air must be provided to dilute flammable gases and/or vapors to below 25% ofLEL. See Chapter 4. Duct velocity = 2000 fpm he= 1.78VP8 + 0.25VPd Note: Similar hood is appropriate for union fittings.
Reference: 13.15.3 TITLE
SHAFTSEALENCLOSURE
FIGURE DATE
CHECK CODES, REGULATIONS. AND LAWS (LOCAL, STATE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-15-21 1-10
Specific Operations
13-21
o
M
1
1/")
Process line or vessel
1
Cl.l
~> Ram type sampling valve
Slots or perforated plate
Door Swing out or vertical sliding interlock desirab1e to prevent sample extraction unless door is closed.
Q = 125 acfm/ft 2 of open area (door area) minimum Duct ve1ocity = 2000 tpm he= 1.78 VP8+ 0.50 VPd
NOTE:
Sufficient a ir must be provided when door closed to dilute flammable gases aml/or vapors to 25% of LEL and meet all NFPA requirements. See Chapter 2.
Reference: 13.15.3 FIGURE
TITLE
SAMPLING BOX
VS-15-30 DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
13-22
Industrial Ventilation
13.20
FOUNDRY OPERATIONS
Foundry operations include many operations common to other industries. Sorne of these operations are covered in the following subsections of this chapter: 13.45 13.50 13.55 13.60 13.80 13.90
Machining Material Transport Metal Melting Mixing Surface Cleaning Welding and Cutting
This subsection addresses operations that are more unique to the foundry industry: casting shakeout and core making. 13.20.1 Casting Shakeout Foundry shakeout ventilation rates depend on the type of enclosure and the temperature of the sand and castings. The enclosing shakeout hood (VS-200 1) requires the smallest airflow rate. The side draft shakeout hood (VS-20-02) requires additional airflow rate but provides improved access for casting and sand delivery and for casting removal. The downdraft shakeout (VS-20-03) is the least effective in controlling contaminant and requires the highest ventilation rate. It is not recommended for hot castings. The shakeout hopper below the shakeout table requires additional exhaust ventilation equivalent to 1O percent of the shakeout hood exhaust rate .
.,., •1
t,:
Moisture re1eased during shakeout operations can condense on duct and hood surfaces. Systems may require the addition of heat to keep moisture in vapor forrn, especially if duct is connected to dry filter (baghouse). Particular attention should be paid to the conveyor removing sand from the shakeout. This conveyor requires hoods and ventilation as described in Section 13.55. Rotary tumble milis used for shakeout should be treated as an enclosing hood with a mínimum inward velocity of 150 fpm through any opening. 13.20.2 Core Making. Core making machines require ventilation to control reactive vapors and gases such as amines and isocyanates that are used in the core making process. A mínimum capture velocity of 75 fpm is required. However, a ventilation rate as high as 250 acfm/ft2 of opening may be necessary for adequate control of contaminant emissions. When cores are cured in ovens, adequate ventilation control of the oven is required.
REFERENCE
13.20.1
American Foundrymen's Society, lnc.: Managing the Foundry lnplant Air Environment. Des Plaines, IL (2004).
Specific Operations
13-23
-
o1 o
ENCLOSING HOOD
N1
Provides best control with least flow rate Minimum duct velocity = 4000 fpm he=0.25 VPd
(/.]
~
Working openings, keep as small as possible
Castings outhere
Mold Shakeout Shakeout exhaust, minimum * Type ofhood
Hot castings
Cool castings
200 acfm/ft2opening
200 acfrnlft2 opening
At least 200 acfm!ft grate area
2
At least 150 acfm!ft2 grate area
Two sides and 113 top area enclosed ** VS-20-02
300 acfrnlft2 grate area
275 acfrnlft2 grate area
Side hood (as shown or equivalent) ** VS-20-02
400-500 acfrn!ft2 grate area
350-400 acfm/ft grate are a
400 acfrnlft2 grate area
300 acfrnlft2 grate area
Enclosing
** VS-20-01
Double side hood
** VS-20-02
2
* Choose higher values when (1) Castings are hot (2) Sand to metal ratio is low (3) Cross-drafts are high
**
Shakeout hoppers require an additionallO% exhaust. TITLE
FIGURE
FOUNDRYSHAKEOUT ENCLOSURE
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, ANDNATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-20-01 1-10
>
13-24
Industrial Ventilation
8 1
o
Moveable panels to secure desired distribution
N
1
U')
1.5 L - - - - - - 1
~ H=L
J
Baffie to edge --'---, ofgrate
Shakeout 1---- L ----1
Mínimum practica! clearance Velocity through openings 2000 tpm SIDE-DRAFT HOOD Mínimum duct velocity = 4000 tpm he= 1.78 VP8 + 0.25 VPd Blank wall in this position is almost as good as double hood
,,:11' ,.,,
' 1
'••
Minimize
d""~" ~
DOUBLE SIDE-DRAFT Proportions same as single side-draft hood except for overhang Mínimum duct velocity = 4000 tpm Slots sized for 2000 tpm he= l. 78 VP8 + 0.25 VPd See VS-20-01 for exhaust rates TITLE
FIGURE
FOUNDRYSHAKEOUT SIDEDRAFT
DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-20-02 1-10
>
º~
If belt enclosure is over lO feet long, provide exhaust at cover. Qa = 350 acfm/ft belt width
n o
Ref. VS-50-20
~ ...¡Cil
:j ....,
o~
¡;;
~o
~~
trl:j -lO
~
[
~o ~e:
~~
oz
mo
t:Tj~ ~"""
z~
r;;~
Grate
-------
1
Shield~
\
~~ ~o ~~
5!;:
___ rGrate
~--------------------------[7\
tT:1cn
'~-------------------------- ~
8r- > ~~ .~~~ C/Jo
Mínimum face area = 4 x duct area ENDVTEW
. ...., e: ~
'-----Enclose pan feeder or belt completely. - - - - J Exhaust at transfer to elevator.
....,
~ z
See VS-50-20
~1
....,
~,
= 1"1
. . . 1~
o
...g
g;
1
1
1
Top view oftake-off connection
Ro ll-over handle Hood: Closed on ends, top and sides
l
t
Rotating connection
1111
Seal around shaft
SIDE VIEW
2
Q = 200 acfm/ft of open face area Minimum duct velocity = 3500 ipm he= 0.25 VP d Note:
Elbow and rotating connection losses not included
1 TITLE
CORE MAKING MACHINE SMALL RO LL-OVER TYPE
FIGURE
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-20-11 1-10
13-28
13.25
.•.'
l;: ~!
'IL. ~:
l!'
Industrial Ventilation
GAS TREATMENT
prevent EtO buildup.
The handling of gas cylinders for industrial operations requires special attention. In addition to the potential safety problems associated with transportation and use of compressed gas cylinders, the gas inside the cylinders can escape through leaking valves and fittings. During connection and disconnection of the gas lines, due to the operating pressures, gas can be released.
Use redundant control valves (double block with leak check) on the EtO line. Install valve position sensors on critica! valves. Install flow-lirniting device in the vacuurn purnp inlet to prevent overfeeding the OECD. Vent confined spaces to the outside after a power loss.
This section ofVS-prints illustrates uses oftoxic gases during fumigation (VS-25-01 and -02) and during ethylene oxide sterilization (VS-25-10, -11,-12, and -13).
Provide lirnited access to the override controls.
13.25.01 Ethylene Oxide Sterilizer-ln hospitals the sterilizer rooms should be positively pressurized with respect to adjoining rooms to prevent contaminants from entering the sterile environment and the direction of contaminated air should go from clean to contaminates equipment rooms5 13·25· 1• 1325 ·2) A typical hospital system is shown in Figure VS-25-10.
Elirninate back vents, if possible, through equipment and cycle design. Most explosions have occurred when an automatic system opened a sterilizer door that triggered back vent operations,< 13 ·25.1) a manual switch triggered back vent operations50 fpm or > 0.25 mis) between the emission point and the canopy. Careful consideration should be given to such process information as the required worker access and the hazard potential of the process contaminants when determining the proper hood (see Chapter 6). When feasible, ventilated enclosures are recommended for hot processes. This has the twofold advantage of eliminating contaminants from the workplace, and providing a physical barrier between the hot process and the worker (i.e., additional safety). Less desirable controls include capture hoods (with or without slots); however, these are advantageous in sorne cases because they can be incorporated at or near the surface of the process, while not interfering with the process or worker. These hoods provide good control at or near the slot face. However, capture velocity diminishes inversely with the square of the distance from the slot. This practically limits slot hood effectiveness to less than 3 to 4 feet (~ 1 m) from the hood face. Furthermore, slot hoods are diminished if the velocity of the heated plume of air rising from the process is moving rapidly (i.e., smelting processes) since the plume could be moving upward at > 20% of the slot face velocity. Although control may sometimes be achieved by implementing a side-draft multiple slot hood, it is not recommended unless the application is similar to specific heated operations documented in this chapter. 13.27.3 Canopy Hoods. Canopy hoods are the remaining choice if the process to be controlled does not fit applications for enclosures or capture hoods. There are four combinations of canopy hoods that will be discussed; low and high canopy hoods, with and without side walls. Low or high canopy hoods with side walls on all sides are preferred, in that they provide similar benefits as ventilated enclosures. The side walls should extend slightly below the top surface of the hot process to ensure capturing contaminants. Iftemporary access to the process is required, side walls can include hinged or sliding doors for access. Another variant is to partially enclose the process by installing side walls on one, two or three sides of the process. Although not ideal, partial enclosures are an improvement over canopy designs with no side walls. 13.27.4 Canopy Hoods Without Sidewal/s. Low and high canopy hoods without side walls are the least effective and least efficient method of controlling hot processes. Although canopies are included in Chapter 6 as a recommended design, there are restrictions to their application. For low canopy hoods, the distance from the hood face to the surface of the hot process should be no greater than the diameter of the source or 3 feet, whichever is smaller.< 13·27.l, 1327·2l lt is also recommended that the size of low hoods and hoods with side walls should be a foot larger on all sides and have the diameter of the hot process at its widest cross-section (Figure 13-27-1 ).
13-80
Industrial Ventilation
o oV) 1
1
r.l.)
he= 0.25 VPd
Closed top
Belt
Bin
...~ Belt
- - or Locate remote from loading point
MECHANICAL LOADING Flow rate (Q)
Belt speed Less than 200 fpm
350 acfm/ft ofbelt width. 2 Not less than 150 acfm/ft of opening.
Over 200 fpm
500 acfm!ft ofbelt width. 2 Not less than 200 acfm/ft of opening.
Booth to '"ommodolo bmol, bag, ""·
\
Booth to cover as much of hopper as possible
\
Hopper
¡ Mínimum duct velocity = 3500 fpm .Q = 150 acfm/ft2 face he= 0.25 VPd MANUAL LOADING FIGURE
TTTLE
BIN &HOPPER VENTILATION
VS-50-10 DATE
CHECK CODES, REGULATIONS, ANO LA WS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
>
Specific Operations
13-81
-,-
24"min
1
¡-24" min ¡
l) Close face to bottom ofbelt - - -
As close as
l. Conveyor transfer less than 3' fall. For greater fall, provide additional exhaust at lower belt. See 3 below. lle= 0.25 VPct
s~T=o=te=b=o=x=?
practica! 2. Conveyor to elevator with magnetic separator. he= 0.25 VPct
2 x belt width
DESIGNDATA Transfer points:
113 belt width
Endose to provide 150- 200 tpm indraft at all openings. (Underground mining tunnel ventilation will interfere with conveyor exhaust systems.) Q = 350 acfin/ft belt width for belt speeds under 200 tpm. (minimum) = 500 acfm/ft belt width for belt speeds over 200 tpm and for magnetic separators. (minimum) Minimum duct velocity = 3500 tpm he= 0.25 VPd
Rubber skirt
3. Chute to belt transfer and conveyor transfer, greater than 3' fall. Use additional exhaust at @ for dusty material as follows: Belt width 12"-36", Q=700 acfm Belt width above 36", Q=lOOO acfm he= 0.25 VPct
Conveyor belts: Cover belt between transfer points Exhaust at transfer points Exhaust additional 350 acfm/ft ofbelt width at 30' intervals. Use 45° minimum tapered connections.
2" clearance for load on belt
Note:
Dry, very dusty materials may require exhaust flow rates 1.5 to 2.0 times stated values.
DETAIL OF BELT OPENING L
,_
. ~
TITLE
'
FIGURE
CONVEYOR BELT VENTILATION
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, ANDNATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-50-20 1-10
13-82
Industrial Ventilation
.N
1
oIr¡ 1
U'J
~ Totally enclosed conveyor Settling box
Interna! skirt board
Troughing belt Cleanout and inspection doors Scraper conveyor
Return belt scraper orbrush
Q = 250 acfm/ft 2 of open area Minimum duct velocity = 3500 tpm
he= 0.4 VPd
Reference: 13.50.3 TITLE
TOXIC MATERIAL BELT CONVEYING HEADPULLEY
FIGURE DATE
CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-50-21 1-10
>
Specific Operations
13-83
N N
1
o
tr\ 1
tZl
>
Square settling box (Designed for 150-200 fpm velocity)
Exhaust duct
Interna! dust shield
Inspection door
Rockbox
Completely enclosed conveyor
Flexible skirt board
Troughing belt Clean-out and inspection
2
Q = 250 acfmlft of open area Mínimum duct velocity = 3500 fpm he= 0.4 YPct
Reference: 13.50.3 TITLE
TOXIC MATERIAL CONVEYOR BELT LOADING
FIGURE
VS-50-22 DATE
CHECK CODES, REGULA TTONS, AND LA WS (LOCAL, STATE, AND NA TTONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
13-84
Industrial Ventilation
Power hoist
---~
8400 scfm air suction
4 ft. vertical travel (min.)
Reference 13.50.4
FIGURE
TITLE
®
RAIL LOADING
DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-50-30 1-10
Specific Operations
Q
Powerhoist
Air conduit must be flex hose or swivel joint grain spout
t~
A ir velocity 1500 fpm
Outer sleeve must be telescoping or collapsible
Plug relief
Must maintain 12" clearance to be most effective
Grain pile
Reference 13.50.4 TITLE
TRUCK LOADING CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STA TE, ANO NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-50-31 1-10
13-85
13-86
13.55
Industrial Ventilation
METAL MELTING FURNACES
This set ofVS-prints describes hood designs for a variety of metal melting furnaces, including electric induction, carbon are, convection, and crucible, which use natural gas or electric resistance elements as the heat source. Exhaust ventilation is usually required to control fumes generated from the metal being oxidized during the melting as well as the combustion of contaminants carried with the scrap charge. In sorne cases, a single hood will suffice for charging, melting, and pouring (tapping). In other cases, a separate hood, remote from the primary melter, may be required for charging because of the nature of the charge or the large open area necessary at this phase of the operation. This is particularly true of electric are furnaces which, in most cases, are completely open for charging. After charging, the direct exhaust can be used to achieve control during the remainder ofthe melting cycle. During tapping ofthe hot metal, either the remote (high canopy) hood or a side draft hood at the ladle lip can achieve control. All metal will produce a slag that must be removed prior to tapping. This activity can produce a significant release of oxides and will require a separate exhaust system for the oxide/dross control. The remote or high canopy hood can often provide the control. Where metal purification is performed directly within the furnace or melting vessel, such as the addition of oxygen or chlorine, additional exhaust may be required to contain the rapidly generated plume. All systems must be designed to include the increase in air temperature under operating conditions to insure an adequate airtlow into the hood. The air temperature rise is usually relatively low except where metal inoculants or oxidizers are added to the molten charge. During this phase of metal melting, a significant temperature rise will occur and it is customary to provide a large hood in which gas expansion can take place.
13.55.1 Electric Are Fumace.l13·55·1l Small, low power (lO MVA or lower) Electric Are Furnaces (EAFs) normally use custom designed side draft hoods. From the graph, either the melt rate or the oxygen lance rate can be used to determine the exhaust volume (Q) and the temperature of the exhaust gas. The duct velocity at the flange is a mínimum of 3800 fpm. A damper is placed in the duct leading from the slag door hood to the main duct. Eighty percent of Q is exhausted from around the electrodes and 20% from the slag door. A high canopy hood is required to capture charging and tapping fumes (VS-55-03B). Most EAFs today are large Ultra High Powered (UHP) furnaces with melt rates of 100 TPH or more and oxygen lance rates well in excess of 1600 acfm. The volume (Q) for the melt shop can be approximated from the geometry of the melt shop. VS-55-03B provides the information to approximate the overall 'Q' evacuated from the high canopy hood during charging and tapping. However, the heat load coming out of the EAF through the direct evacuation system (DES) must be considered. (The calculation ofthe DES heat load is beyond the scope of this Manual.) Approximately 20% of the energy entering the EAF leaves in the DES. The DES route generally removes 25% of the total Q during melting and contains approximately 80% of the dust and metallurgical fume. During melting and refining, the high canopy hood exhausts 75% ofthe total volume and contains only 20% of the dust and fume. REFERENCES
13.55.1
Walli, R: Private Communication to Gerry Lanham (2009).
13.55.2
American Foundrymen's Society, Inc.: Managing the Foundry Inplant Environment (2004).
Specific Operations
13-87
o1
on on1
V'J
>
¡ 45° min.
1 45° min.
Close end wit~ panel
Sliding panels on rollers -----....._ Track for panels
2
Q = 200 acfm/ft of opening including doors, plus products of combustion corrected for temperature. Mínimum duct velocity = 3500 fpm
Row of crucibles
he=0.5 VPd
Note:
Same principie ofsliding or swinging doors is applied to individual fumace enclosures.
Exhaust stack
t
Canopy to clear crane; or provide slot for bridge crane; or separate cranes inside and outside; or use manual crucible removal.
7'- 8'
Q = 200 acfmlfloftotal opening, mínimum, plus products of combustion corrected for temperature. TITLE
MELTING FURNACE CRUCIBLE NON-TILT
FIGURE DATE
CHECK CODES, REGULA TIONS, AND LA WS (LOCAL, STATE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-55-01 1-10
13-88
Industrial Ventilation
Door guides
Counterweighted or spring-loaded sliding doors front and back if necessary
1+----L----+l
,-----
Solid side panels
1
~~---w--~ 1 1
~----, Door to extend below top offumace if possible.
~~
t~0
1 1 1 1 1
/"Á\ 1 1--\\
1
1 1
1 1 11 1 1
1Y; -=-'~1 1
Q = 200 LW; but not less than 2
200 acfrn!ft of all openings with doors open. Correct for products of combustion and temperature. Mínimum duct velocity = 3500 fpm he= 0.25 VPct
FIGURE
TITLE
MELTING FURNACE TILTING
VS-55-02 DATE
CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
Specific Operations
-
Q (See 13.55.1)
Close capture at electrodes Furnace body
Verify Q, SP and operating temperature with manufacturer. Altemate designs: l. Sorne exhaust designs utilize direct furnace rooftap. For details, consult manufacturer.
SECTION FURNACE HOOO ANO EI..ECTROOES
2. Canopy hoods can be used as secondary hoods to capture fugitive emissions and charging. (See VS-55-03B)
H = Q 1 [3500
1600
30
1400
28
::;;: 1200 ~
u
-
§
Work gives offvapors after removal from tank
To suit work
_L L-------1
Section
D. PICKLING TANK
Extend over tank as far as possible ~ Slot
----=:::::-
Tank L----1
E. LATERAL
w lnside radius desirable if space permits Max. plenum velocity = 112 slot velocity _[_ 12" mínimum
Slot
-j
1-
,__ _____;. . . ._., _t
--+-- ~--~1
2 S mínimum
Ls
1
Slot velocity 2000 fpm Sloped plenum desirable
F. END TAKE-OFF See Section 13.70. TTTLE
FIGURE
OPEN SURFACE TANKS
VS-70-02 DATE
CHECK CODES, REGULA TTONS, AND LA WS (LOCAL, STATE, AND NATTONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
13-122
Industrial Ventilation
o 1 o
('1
r-1 00
>
~
""
No slot near take-off
lnside radius desirable
Maximum plenum velocity = 1000 fpm
~en not in use
L-------1
A_j
Section A-A
Q=SOLWacfm Slot velocity = 2000 fpm he= 1.78 V~+ 0.25 VP d Duct velocity = 2000 fpm mínimum
Also provide:
l. Separate flue for combustion products. 2. For cleaning operation, appropriate respiratory protection is necessary. 3. For pit units, the pit should be mechanically ventilated. 4. For further safeguards, see VS-70-21. NOTE: Provide downdraft grille for parts that cannot be 2
removed dry; Q =50 acfm/ft grille area.
1 !
See VS-70-21 TITLE
SOLVENT DEGREASING TANKS
FIGURE DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-70-20 1-10
Specific Operations
Solvent vapor degreasing refers to boiling liquid cleanin\systems utilizing trichloroethylene, perchloroethylene, methylene chloride, freons or other halogenated hydrocarbons. Cleaning action is accomplished by the condensation ofthe solvent vapors in contact with the work surface producing a continuous liquid rinsing action. Cleaning ceases when the temperature ofthe work reaches the temperature ofthe surrounding solvent vapors. Since halogenated hydrocarbons are somewhat similar in their physical, chemical and toxic characteristics, the following safeguards should be provided to prevent the creation of a health or life hazard: l. Vapor degreasing tanks should be equipped with a condenser or vapor level thermostat to keep the vapor leve] below the top edge of the tank by a distance equal to one-halfthe tank width or 36 inches, whichever is shorter. 2. Where water type condensers are used, inlet water temperature should not exceed 80 F (27 C) and the outlet temperature should not exceed 110 F (43 C). For sorne solvent, lower water temperatures may be required. 3. Degreasers should be equipped with a boiling liquid thermostat to regulate the rate ofvapor generation, and with a safety control atan appropriate height above the vapor line to prevent the escape of solvent in case of malfunction. 2
4. Tanks or machines ofmore than 4 ft ofvapor area should be equipped with suitable gasketed cleanout or sludge doors, located near the bottom, to facilitate cleaning. 5. Work should be placed in and removed slowly from the degreaser, ata rate ofno greater than 11 fpm to prevent sudden disturbances of the vapor leve!. 6. CARE MUST BE TAKEN TO PREVENT DIRECT SOLVENT CARRYOUT DUE TO THE SHAPE OF THE PART. 7. Maximum rated workloads as determined by the rate ofheat transfer (surface area and specific heat) should not be exceeded. 8. Special precautions should be taken where natural gas or other open flames are used to heat the solvent to prevent vapors* from entering the combustion air supply. 9. Heating elements should be designed and maintained so that their surface temperature will not cause the solvent or mixture to breakdown* or produce excessive vapors. 1O. Degreasers should be located in such a manner that vapors* will not reach orbe drawn into atmospheres used for gas or electrical are welding, high temperature heat treating, combustion air or open electric motors. 11. Whenever spray or other mechanical means are used to disperse solvent liquids, sufficient enclosure or baflling should be provided to prevent direct release of airbome vapor to the top of the tank. 12. An emergency quick-drenching facility should be located in near proximity to the degreaser for use in the event of accidental eye contact with the degreasing liquid. *Electric ares, open flames and hot surfaces will thermally decompose halogenated hydrocarbons to toxic and corrosive substances (such as hydrochloric and/or hydrofluoric acid). Under sorne circumstances, phosgene may be formed. TITLE
SOLVENT VAPOR DEGREASING
FIGURE DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-70-21 1-10
13-123
13-124
13.72
Industrial Ventilation
PUSH-PULL VENTILATION 30
Local exhaust ventilation consists of an exhaust hood which in.effect creates a flow of air over a contaminant source capturing the contaminant and removing it from the work~ place. Local exhaust can be very effective where the contaminant is ernitted over a narrow area and where the exhaust hood can be placed in reasonably close proximity to the ernission source. The capture velocity of the hood decreases inversely with the square of the distance from the hood limiting the effectiveness of the hood to 3 to 4 ft. Conversely, a jet of air can be blown up to 30 or more feet in a coherent fashion. As such, local exhaust ventilation options may be limited for large open surface vessels, areas where contaminant generation occurs across a large area or where the process configuration makes local exhaust ventilation difficult. In such cases, pushpull ventilation may be appropriate. The jet coupled with an exhaust hood forms a push-pull system. The push-pull system consists of a manifold, usually a pipe with drilled boles, which directs the jet across the contaminant ernission area. The jet captures the contaminant and carries it into the exhaust hood. The primary purpose of the exhaust hood is to capture and remove the jet, not to provide capture velocity. A jet is essentially a constant momentum process. The jet leaves the nozzle ata flow rate (Qo) and high velocity (Vo). Downstream the velocity decreases and the flow rate increases due to the entrainment of the surrounding air. The product of the nozzle velocity and the nozzle flow per foot is referred toas the initial kinetic momentum (KMo). The selection ofthe initial jet flow and velocity values to achieve a momentum necessary for control is not arbitrary but is a function of the push distance. 13.72.1 Open Surface Tanks. Careful consideration must be given to the application ofpush-pull systems to open surface tank operations. When applied to plating or cleaning tanks, a properly applied jet will "attach" to the fluid surface and take on the characteristics of a wall jet. (l3.?2.1J This tends to hold the jet within the tank and near the tank liquid surface. Ideally, the push manifold should be mounted on the tank such that there is no gap between the nozzle and the tank edge. This will optirnize attachment of the jet. If the push manifold cannot be mounted without a gap, it should be located as close to the tank edge as possible and be baflled, if possible, to close the gap. Provisions must be incorporated to angle the jet downward into the tank and under tank obstructions. See VS-72-0 l.
The jet will usually flow around small obstructions such as parts hangers, parts, and parts baskets during placing and removal. However, large objects that may not cause a problem during the plating or cleaning operation may result in jet deflection and spillage when removing or placing the part in the tank. The flow of entrained air into the jet will offer sorne capture and removal of these spillage and dragout ernissions. Possible solutions may be to shut the jet off during this part of the oper-
25
20
E" .E ! ¡¡:
15
d' 10 _.., 1/4 in hole dia
-7/32inholedia
5
....... 3116 in hole dia
o 4
12
8
16
Push Distance (ft)
20
24
28
FIGURE 13-72-1. Push flow per foot nozzle length for holes on 2-inch centers
ation, or if the system configuration permits, configuring the jet to be parallel to the part or tank obstructions rather than perpendicular. In sorne instances, a second jet located above the tank may be beneficia! in capturing dragout and jet spillage. There are many situations such as process automation, high speed production, cables, bus bars, heaters, monitors, hoists, barreis, drives, racks, etc., which create obstructions that may hinder the performance of a push-pull system. Elevated process temperatures and automated finishing processes such as those discussed above may negate a reduction in exhaust volumes and, in severe cases, may preclude the use ofpushpull. 13.72.2 Push Jet Characteristics. The push jet strength for open surface vessels is referred to as kinetic momentum and is a function ofthe jet flow rate and velocity at the jet nozzle.
KMo = 12291x
(13.72.1]
KMo= QoVo
[13.72.2]
KMo"" Vo2Ao
[13.72.3]
where: KMo "" jet kinetic momentum x
=
tank width, ft
Qo "" jet nozzle flow per ft, acfin!ft nozzle length Vo
=
jet nozzle velocity, fpm
Ao
=
jet nozzle area, ft2/ft nozzle length
KMo values have been determined experimentallym
For construction and safety, consult NFPA Code
§
o = = = =
1" clearance
o
o
o
¡
!
= =
o
1
00
1"
c::=::===::J
1
o=
1" space
4
1 1 1 1
1 1 1 1
1 ,¡
I/ 1 .lj 1
1
1 ' 1l ¡
i
Ring attached to hood at convenient locations
1~-b+-::~
Adjustable to clear grinder
Angle of slots to be in relation to rotation EXHAUST FLOW RA TE, acfm Disc diameter inches Up to 20 20 to 30 30 to 53 53 to 72
No.*
1/2 or more of di se covered Exhaust flow rate, acfm
1 2 2 2
No.*
500 780 1800 3100
Disc not covered Exhaust flow rate, acfm 780 1500
2 2 4 5
3500 6000
* Number of exhaust outlets around periphery ofhood or equal distribution provided by other means. Mínimum slot velocity = 2000 fpm Minimum duct velocity = 4000 fpm he= 1.0 VP8 + 0.5 VPct FIGURE
TITLE
VERTICAL SPINDLE DISC GRINDER
DATE
CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-80-14 1-10
13-150
Industrial Ventilation
lrl
....... 1
o
00 1
r::l.l
Close clearance
~
Work
Endless belt conveyor or any other method. Disc diameter (D) inches
Exhaust flow rate aefin
Up to 19
610 880 1200 2000
19 to 25 25 to 30 30 to 53 53 to 72
SectionA-A
Note:
6300
Ifthe disc is tightly enclosed by machine housing, then exhaust from the housing is acceptable.
Minimum duct velocity = 4000 fpm he= 0.65 VPd straight take-off he= 0.45 VPd tapered take-off TITLE
FIGURE
HORIZONTAL DOUBLESPINDLE DISC GRINDER
DATE
CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STATE, ANDNATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-80-15 1-10
>
Specific Operations
13-151
'-0 1
o
Branch takeoff at top or back. Central location or multiple branches if severa! booths are used ~
00 1
r:/)
>
§
Additional adjoining
""'bootb. if n') / / 1
1
1, 1\
1
1)---"" 11 1 1
1 1
1
1
)
1
Adjustable hopper /
'
~
r
U se one branch duct for each wheel Hinged access doors for maintenance, normally closed
D Slow speed belt conveyor
Q = 500 acfm/wheel, minimum Not less than 250 acfrn!flhotal open area Minimum duct velocity = 3500 fpm, 4500 fpm if material is wet or sticky he= 1.78 VP5 + 0.25 VPd Note: l. Consult applicable NFPA standards.CBB0.2) 2. Caution: Do not mix ferrous and non-ferrous metals in same exhaust system. 3. Wheel adjustments on outside ofenclosure. 4. For highly toxic material endose the return strand of the belt conveyor.
FIGURE
TTTLE
STRAIGHT LINE AUTOMATIC BUFFING
DATE
CHECK CODES, REGULATIONS, ANO LAWS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-80-33 1-10
Specific Operations
13-159
"'1"
M
1
o00 1
tZl
~
D Access door
Q = 500 acfm/wheel, mínimum 2
Not less than 250 acfrnlft total open area Mínimum duct velocity = 3500 fpm, 4500 fpm if material is wet or sticky he= 1.78 VPs+ 0.25 VPct On small (2 or 3 spindle) machines one tak:e-offmay be used. Multiple take-offs desirable. Note: l. Consult applicable NFPA standardsSl3802) 2. Caution: Do not mix ferrous and non-ferrous metals in same exhaust system.
TTTLE ®
FIGURE
CIRCULAR AUTOMATIC BUFFING
VS-80-34 DATE
CHECK CODES, REGULA TIONS, ANO LA WS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
>
13-160
Industrial Ventilation
...
..
..
.
....
•
•
Specific Operations
13.90 WELDING ANO CUTTING
The purpose of welding and cutting ventilation is to control gases, fumes, and particulate generated during the welding and cutting operations. 13.90.1 Hazards. The generation rate of fumes and gases varies with the composition of the base metal, fluxes, and fillers, and with the rate and depth of welding. Exposure to the welder varies with the generation rate, duration and frequency of operations, work practices (particularly distance of the plume from the breathing zone), and the effectiveness of ventilation.
Contaminants from welding may include: l.
Fume from the base metals and filler or electrode metals
2. Fume from coatings (e.g., zinc oxide from galvanized surfaces, thoria from T.I.G welding, and fluorides and N02 from electrode coatings) 3.
Ozone due to ionization of oxygen by the ultraviolet light from are welding
4.
Carbon monoxide from ultraviolet effects on carbon dioxide in shield gas
5.
Shield gases such as carbon dioxide, helium, and argon
6.
Fluoride gases and other thermal decomposition products of flux es and electrode coatings and
7. Flammable gases such as acetylene. There are welding tasks that present enhanced hazards such as welding on materials containing or contaminated with heavy metals or welding in the presence of flammable vapors or halogenated hydrocarbons. If such welding is required, extraordinary precautions must be taken on a case-by-case basis. Even in the absence of such hazard materials, any welding operation in a confined space is potentially lethal and requires continuous and copious dilution ventilation. 13.90.2 General Recommendations.
l.
Choose hood designs in the following descending order of effectiveness: enclosing hoods, vacuum nozzles, fixed slot/plenum hood on a worktable or rectangular hood fixed above a worktable, moveable hood above a worktable, moveable hood hanging freely or overhead canopy, dilution ventilation.
2. Integrate planning for ventilation systerns with planning for materials handling. 3. Place welding curtains or other barriers to block crossdrafts. 4.
Install tumtables, work rests, and other aids to improve utilization of the hoods.
5. Avoid recirculating filtered air from welding hoods back into occupied spaces unless the welding is low-hazard and produces low quantities of gaseous contaminants. 6.
Face velocity for enclosing hoods should be 100--130
13-165
fpm with the higher values used for poor conditions such as high cross-draft velocities. 7.
Capture velocity for non-enclosing hoods should be 100--170 fpm with the higher values used for poor conditions such as high cross-draft velocities and with higher hazard levels.
Enclosing hoods are by far the most effective in controlling welding contaminants; however, they restrict access and force reconsideration of material and product handling. Capturing hoods are less effective than enclosures but for low hazard conditions can be adequate if properly used. 13.90.3 Dilution (general) Ventilation. Dilution ventilation should be used to complement local exhaust hoods. However, dilution ventilation may be adequate without local exhaust ventilation under the following conditions:
1) It is not a confined space, 2) The contaminants are relatively low tox1c1ty (e.g., welding on mild steel or iron without coatings on the steel or unusual rod coatings), 3) Any impediments to air movement are at least 7 feet away (e.g., welding curtains), 4) The welding is interrupted by long periods of other activities (e.g., setup, material movement, etc.) so that the total generation rate is much less than would occur in "production" welding, 5) For very short periods of welding with moderately toxic contaminants if an appropriate NIOSH-approved respirator is properly wom and maintained. General ventilation may not be sufficient for toxic materials. Local ventilation and respiratory protection equipment will probably be necessary. Even if low toxicity welding, it is best to use local exhaust ventilation if at all possible rather than rely on general ventilation. Airflow requirements for low-toxicity, non-production, non-confined space dilution ventilation depend on 1) the rate of generation, and 2) the effectiveness of cross-drafts in dispersing the welding effiuent. For such welding in open areas where welding fume can easily rise away from the breathing zone: Ot = 800 acfm x lb/hour of the rod used For such welding in open areas where welding fume is moderately blocked by welding curtains and other obstructions to cross-drafts that are not closer than about 7 feet the airflow requirements should be roughly twice as much: Ot = 1600 acfm x lb/hour of the rod used 13.90.4 Movable Capturing Hoods. Use mobile hood system (see VS-90-02) ifit is necessary to move the hood to keep it elose enough to the point of weld as the welder moves about his or her work station.
13-166
Industrial Ventilation
Welders may forget or choose not to move the hood to keep it elose enough to be effective. This is especially likely if the capture distance (X) is relatively short (e.g., less than 12"). For low values of X the velocity gradient is extreme. Therefore, moving the hood an inch or two closer has substantial effects on shield gas. Moving it an inch or two further out substantially reduces its effectiveness. The need for hood movement can be reduced by using relatively large values of X in selecting airflows and by making the hood wide. The former reduces required up and back movements and the latter reduces the need for lateral movements. Increasing X greatly increases airflow requirements. Increasing the width moderately increases airflow requirements. If the hood can be ftxed in place, consider doing it since it would reduce the likelihood that the hood is left at an excessive distance from the weld. The hood in VS-90-02 can be positioned within the range of the flexible ducts connecting it to a fan. There are commercially available portable hoods that have the hood and duct system both on a wheeled platform, typically with an aircleaning device as well. The airflow requirements in VS-9002 should apply to these hoods. The advantage ofthese hoods is the ability to provide temporary ventilation almost anywhere an electrical cord can reach. Sorne concerns in using a portable hood include: l. The portable hood "scoop" is typically small and the airflows are typically relatively low. For that reason, the hood must be kept very close to the weld to be effective, producing the problems described above 2. The air cleaner may be ineffective in sorne models even when new and the air cleaner in all of them must be properly maintained to remain effective. 13.90.5 Slot Hoods. The side panels on the slot "welding" (it could be used for many other tasks) hood (see VS-90-1) are intended to block cross-drafts. If recommended airflows are provided for a given slot length and depth of the work bench, then the design capture velocity will be provided or exceeded at any location on the bench. Thus, a welder can weld at any location on the bench, a great advantage over small capturing hoods.
Sorne concerns in using a slot hood include:
l. If welding the front of a bulky object, the object itself may block the flow such that the control velocity may be low. In addition, the plume could rise directly into the welder's face before flowing back to the slots. 2. The top slot must be located well above the height of the tallest object that will be welded to prevent the rising plume from escaping. It is desirable to have at least two slots, one low (e.g., 12" high) and one higher (e.g., 24" high). 3. The inward velocity close to the slots may be high enough to disrupt shield gases. The velocity gradient is
1 elose to linear (as opposed to quadratic for the non-slot capturing hoods), so welding can be done perhaps halfway between the front edge of the table and the slots. 13.90.6 Bench Top Enclosing Hoods. It is generally assumed that enclosing hoods are more effective than capturing hoods. The disadvantage of enclosing hoods is reduced access, making it more difficult to move materials to be welded and more difficult to access more than one side of an object to be welded Sorne objects may be too large to fit into the hoods. Ifthe user finds that his or her job is made more difficult by the hood, he or she may sabotage, jerry-rig "improvements," or fail to use it when welding.
For those reasons, the design and use of an enclosing hood requires careful thought and consideration. The hood should be designed for a large fraction ofjobs, but it is often a mistake to try to make one hood fit every possible welding task. Very large objects should be welded elsewhere and ventilated using a different strategy. However, most objects that are smaller than the dimensions of the hood can be welded in an enclosing hood. Likewise, problems of inconvenience often can be overcome by modifying the materials handling systems and by designing the hood to enhance usability. For example, convenient access to all sides of many objects to be welded can be obtained by installing a rotating work platform on the floor of the hood. Hoisted items can be set on to a heavy-duty roll-out platform. Sorne long, narrow objects can be passed through small holes in each side of the hood to allow welding at different points along their width. The hoods also can be made more convenient for the welder by installing holders for welding rods and rests for the electrical cables. In sorne cases, the electrical cables can be passed through an eye-bolt that is screwed into the hood ceiling, reducing the dead weight of the cables. Although it is generally true that enclosing hoods are more reliably successful than any other type of hood, it is possible that capturing hoods can be more effective than the bench top hood under sorne conditions. That is most likely when the welding operation requires the welder to put his head even partially into a concave object. On the other hand, if the welder is likely to fail to keep the capturing hood close enough to the source, then an enclosing hood is likely to work better in practice. Face velocities for enclosing hoods should be 100 to 130 fpm through the enclosure face area. 13.90.7 Specialty Welding Hoods. VS-90-03 to VS-90-30 show designs for hoods for specific types of welding and welding-related activities.
REFERENCE 13.90.1
American Welding Society (AWS D. 1-72), P.O. Box 351040, Miami, Florida 33135.
Specific Operations
13-167
o1 o
0'1 1 crJ
Slots-size for 2000 fpm
;:>
~
Maximum plenum velocity 1/2 slot velocity
Q = 350 acfm/ft ofhood length Hood length = required working space W = 24" maximum, ifW greater than 24" see Chapter 3 Minimum duct velocity = 2000 fpm he = 1.78 VP8 + 0.25 VP d General ventilation, where local exhaust cannot be used: Rod, diam.
acfm/welder
5/32
1000
3/16
1500
1/4
3500
3/8
4500
A. For open areas, where we1ding fume can rise away from the breathing zone: acfm required = 800 x lblhour rod used or
B. For enclosed areas or positions where fume does not readily escape breathing zone: acfm required = 1600 x lblhour rod used
For toxic materials higher airflows are necessary and operator may require respiratory protection equipment. Other types ofhoods Local exhaust: See VS-90-02 Booth: For desifl see VS-90-30 Q = l 00 acfm/ft of face opening MIG welding may require precise airflow control TTTLE
WELDING VENTILATION BENCHHOOD
FIGURE
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STA TE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-90-01 1-10
13-168
l
Industrial Ventilation
M
o
1
o
Overhead suppmt
O\ 1
Use middle hood system if necessary to move hood to keep it close enough to the poinr ofweld. Otherwise, use flxed duct.
¡z¡
Conical or 45-degree tapered, rectangular hood.
Swivel
Flanged duct opening as a hood
Plain duct opening as a hood
APPLICATION OF MOVEABLE CAPTURING HOODS l. Not suitable for confined spaces unless coupled with appropriate use of respiratory protection and copious dilution ventilation with good mixing. 2. Not suitable for highly hazardous contaminants (use enclosing hood instead). 3. The distance ofthe center ofthe hood faceto the weld (X) is critical. The welder must be diligent in keeping the hood close to the weld for these hoods to be effective.
RECOMMENDED EXHAUST RATES (Qt) AND DETERMINA TION OF HOOD DIMENSIONS Plainduct Flange or X inches acfm cone acfm 280 380 6" Cr = 0.75 for flanged or tapered 580 9" 770 Cr = 1.0 for plain duct 1000 12" 1350 .. l. Q1 values above assume that the hood is elevated above the welding source to account tor the nsmg plume. Ifthe hood 1s level with the source, Q1 mus! be much higher.
Q1 =125 Cr (10x2 + Arearacel acfm
2. Q, values above are based on low toxicity welding. For moderate toxicity welding, increasing airflows by 20-50% (see also VS-90-01) may be sufficient. For still higher toxicity welding, an enclosing hood (VS-90-30) and respiratory protection may be necessary. 3. Velocities above 150 fpm at the point ofthe weld may disturb shield gas. 4. The greater the width ofthe hood the less the hood must be moved left to right, which is often awkward. The hood width should be greater than the distance, X, from the face of the hood to the source. 5. The velocity gradient becomes extreme for X-distances less than 10". It is best to design for X greater than 16".
DUCT SIZE SELECTION AND COMPUTATION OF SPh l. Mínimum design velocity for ducts should be about 2,500- 3,000 fpm. 2. The hood static pressure is computed from the entry loss coefficient (Fh) and the duct velocity pressure (VP):
TITLE
CAPTURING HOOD FOR LOW TOXICITY WELDING
Hood Plain duct Flanged duct Conical 45-rectangular FIGURE DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, AND NATJONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
Fh 0.93 0.50 0.15 0.25
VS-90-02 1-10
~
"'-
>
Specific Operations
13-169
M
o1 o
0'1 1
(/)
....
•
~ Replaceable filter media Window Grilles 12" to 30" high Design for 300-500 ipm
Exhaust stack
Car silhouette
/
F1ow line
Q = 1000 to 1200 acfm/linear ft ofbooth he= filter loss + 0.5 VPct Minimum duct velocity = 3500 ipm
TTTLE
FIGURE
PRODUCTION LINE WELDING BOOTH
VS-90-03 DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STA TE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
>
¡
13-170
i
Industrial Ventilation
o 1 o
0'1 1
r:n
>
1 6 foot center to center maximum
Slots sized for 2000 fpm
/
/
45° taper angle
Cleanout doors
~--'-
Enclose base ofbench
2
Q = 150 acfm/ft of gross bench area Minirnum duct velocity = 4000 fpm he= 1.78 VP8 + 0.25 VPct
TITLE
FIGURE
TORCH CUTTING VENTILATION
DATE
CHECK CODES, REGULATIONS, ANO LA WS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-90-10 1-10
Specific Operations
_l_ 1-112 ..
10"
13-171
--1
T Slot velocity = 2000 fpm Hood located within 4" from source to maintain 200 fpm capture velocity
/ To exhaust blower
Robot
Swivel
Slot nozzle see detail above
'-
Rotation
1· !¡
Floor line
~1
he= 1.78 VP8 + 0.25 VPd Minimum duct velocity = 3500 fpm TITLE
1 FIGURE
ROBOTIC APPLICATION
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STATE, ANDNATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-90-20 1-10
1 ~ ¡
f
13-172
IndusnialVentilation
45° taper angle Slot velocity = 2000 fpm
~---------------
•
-~
- - - - - - - - - - - - - - --1 L___________ _____ J ~---------------~ -------________ j
---------1
~~~~~~~1 ("~~~~~~~~~~~~~~~ ~::::6::::4::::44""
Face open
1 1
1 1
1
1
1 1
Grille top work bench
Clean-out doors METALIZING BOOTH Non-toxic:
2
Q = 150 acfin!ft face area
Provide appropriate NIOSH certified respirator 2 Q = 200 acfin!ft face area
Toxic:
Minimum duct velocity = 3500 tpm he= 1.78 VPs + 0.25 VPd Smalllathe, etc., may be mounted in booth
1---
12" ruin.
____,
Flex duct to allow movement full length ofwork
Gun (on tool post)
~ Hood extends as low as possible to
LOCAL HOOD
clear lathe raíl. Hood may be connected to move with too! rest.
Note: Local hood may not be satisfactory for spraying toxic metals. 2
Q = 200 acfinlft face openings Mínimum duct velocity = 3500 tpm he=0.25VPd TITLE
FIGURE
METAL SPRAYING
DATE
CHECK CODES, REGULATIONS, AND LAWS (LOCAL, STA TE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-90-30 1-10
Specific Operations
13.95
WOODWORKING
Woodworking equipment generates large amounts of wood dust by abrasive or cutting action. lt is important to provide good ventilation for all equipment as the broad particle size distribution of wood dust creates the potential for health, housekeeping problems, and tire hazards. Excessive amounts of dust, if allowed to accumulate inside equipment and in shop areas, can create frre or explosion hazards. An additional consideration should be the toxicity of the wood species used.
13-173
the specific equipment intended for use in their shop. In many cases, the recommended exhaust flow rates or hood static pressure must be increased to accommodate single exhaust ports, smaller duct connections, non-tapered entries, openings in equipment bases, etc. Additional information for hand held sanders using Low Volume-High Velocity (LV-HV) can be found in sub-section 13.40. Where information for a specific operation is not provided, data for similar listed operations can be used.
In many instances, woodworking equipment, such as saws and sanders, generates airflow patterns that make dust control difficult. Exhaust hoods should endose the operation as much as possible. Where the equipment tends to eject wood dust (e.g., at sanding belt pulleys), the exhaust hood should be placed in the ejection path.
REFERENCES
13.95.1
Hampl, V.; Johnston, 0.: Control ofWood Dust from Horizontal Belt Sanding. American Industrial Hygiene Association Joumal, 46, 10, pp. 567-577 (1985).
Enclosures must incorporate cleanout doors to prevent dust build-up. Duct velocities should be maintained at a mínimum of 3500 fpm to prevent settling and subsequent clogging of the duct.
13.95.2
Hampl, V.; Johnston, 0.: Control ofWood Dust from Disc Sanders, Applied Occupational Hygiene, 6(11 ): 938-944 (November 1991).
13.95.3
Topmiller, J.L.; Watkins, D.A.; Schulman, S.A.; Murdock, D.J.: Controlling Wood Dust from Orbital Hand Sanders. Applied Occupational Hygiene, 11 (9): 1131-1138 (September 1996).
13.95.4
Hampl, V.; Toppmiller, J.L.; Watkins, D.S.; Murdock, D.J.: Control of Wood Dust from Rotational Hand Sanders. Applied Occupational and Environmental Hygiene, 7(4): 263-270 (April1992).
Exhaust flow rates will vary with equipment type and size. Design data are provided for a number of operations in VS-95-01 through VS-95-20 and in Table 13-95-1. The exhaust flow rates for many hoods shown in this section were developed based on the specific configuration shown in the drawings. The drawings show well-designed hoods that may not be found in woodworking equipment purchased "off-theshelf." Readers are cautioned to evaluate the configuration of
1
1
13-174
Industrial Ventilation
TABLE 13-95-1. Miscellaneous Woodworking Machinery Not Given in VS Prints The following list of recommended exhaust volumes is for average-sized woodworking machines and is based on many years of experience. lt must be noted that sorne modem, high-speed, or extra-large machines will produce such a large volume of waste that greater exhaust volumes must be used. Similarly, sorne small machines of the home workshop or bench type may use less exhaust air than listed. SELF-FEED TABLE RIP SAW
DOUBLE PLANERS OR SURFACERS Exhaust Flow Rate, acfm
Saw Diameter, inches
Exhaust Flow Rate, acfm
Bottom
Top
Total
Bottom
Top
Total
Up to 16 inclusive
440
350
790
Up to 20" knives
550
785
1335
Over 16
550
350
900
Over 20" to 26" knives
785
1100
1885
Self-feed, not on table
800
550
1350
GANG RIP SAWS Exhaust Flow Rate, acfm Saw Diameter, inches
Saw Diameter, inches
Over 26" to 32" knives
1100
1400
2500
Over 32" to 38" knives
1400
1800
3200
Over 38" knives
1400
2200
3600
MOLDERS, MATCHERS, & SIZERS
Bottom
Top
Total
Up to 24, inclusive
550
350
900
Size, inches
Bottom
Top
Right
Left
Over 24 to 36, incl.
800
440
1240
Up to 7 incl.
440
550
350
350
Over 36 to 48, incl.
1100
550
1650
Over 7 to 12, incl.
550
800
440
440
Over48
1400
550
2060
Over 12 to 18, incl.
800
1100
550
550
VERTICAL BELT SANDERS (rear belt and both pulleys enclosed) and TOP RUN HORIZONTAL SANDERS Belt Width, inches
Over 18 to 24, incl.
1100
1400
800
800
Over24
1400
1770
1100
1100
Exhaust Flow Rate, acfm
Exhaust Flow Rate, acfm
Up to 6, inclusive
440
Over 6 to 9, inclusive
550
Over 9 to 14, inclusive
800
Over 14
1100
Swing Ann Sander
2200
SINGLE PLANERS OR SURFACERS Exhaust Flow Rate, acfm Up to 20" knives
Exhaust Flow Rate, acfm
785
Over 20" to 26" knives
1100
Over 26" to 32" knives
1400
Over 32" to 38" knives
1765
Over 38" knives
2200
Sash stickers Woodshapers Tenoner Automatic lathe Fonning lathe Chain mortise Dowel machine Panel raiser Dovetail and lock corner Pulley pockets Pulley stile Glue jointer Gainer Router Hogs Up to 12" wide Over 12" wide Floorsweep 6" to 8" diameter
500 440 to 1400 Same as moulder 800 to 5000 350 to 1400 350 350 to 800 550 550 to 800 550 550 800 350 to 1400 350 to 800 1400 3100 800 to 1400
Specific Operations
13-175
Blade
Hinged door for cleanout
A Table Slotted wood block Tophood
TOP HOOD DETAlL
Entire base enclosed on all sides
b
l 1 t
Exhaust flow rate, acfm
Blade width, inches
Bottom
Top
Total
Up to2 2 to 3 3 to4 4 to 6 6 to 8
350 350 550 550 550
350 550 800 1100 1400
700 900 1350 1650 1950
Mínimum duct veloci!}' = 3500 ipm he= 1.75 VPd(Point@ in duct riser) TTTLE
FIGURE
BANDSAW
DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STATE, AND NATIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-95-01 1-10
/ 13-176
IndusniaiVentilation
Connectto Overhead Support \ Saw Guard required F or Saw Guard ventilation see VS-95-05
A
L.r--¡ ¡---;_J 11 11
1 -- 1 11 1' 1 1 1,.---1
11
11 r------~
--
-----
~
A,.--~ IV '\1 \ 1 ( 11 1 1 1 1
1 1 1 1 1 _ _ _ _-:_-:_-;:,
----
rr---L
--- --- ---
\ '.,
1 1
--"--...:::::~--
----
-x
---
-- --
1'
1'
---~---
.-- - - -
Floor A
lnclined plate inside housing
Saw blade diameter, inches
Exhaust flow rate, acfm Base Guard 100 545 785 100 785 100
upto 16"0 over 16"0 Saw with dado blade
Minimum duct velocity = 4000 fpm he= F8 VPs + 0.25 VPd
TITLE
F
Shape
S
--
1.78
!
1.00
1 1
--__L
0.49
r
FIGURE
FLOOR TABLE SAW
VS-95-02 DATE
CHECK CODES, REGULA TIONS, ANO LA WS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
1-10
Specific Operations
13-177
M
o
1
V)
0\ 1 í/l
~ 500 acfm
t A
430 acfm
30"
Blast gate
Table Hood 4 1/2" wide
Minimum duct velocity = 4000 fpm he= 3.5 VPd (point A in duct riser) For booth enclosure, see VS-80-17 Note: Design cannot be used for diagonal or longitudinal cuts
TTTLE
FIGURE
RADIAL ARM SAW
VS-95-03 DATE
CHECK CODES, REGULATIONS, AND LA WS (LOCAL, STA TE, AND NA TIONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
1-10
>
13-178
Industrial Ventilation
Type ofhood where table is cut through
Front view ofhood
Front view ofhood
Type ofhood where table is not cut through
Saw diameter, inches
Exh. flow rate acfin
Upto 20 incl.
350
over 20
440
Mínimum duct velocity = 4000 fpm he= l. 78 VP8 + 0.25 VPd TTTLE
FIGURE
SWINGSAW
DATE
CHECK CODES, REGULATIONS, ANO LA WS (LOCAL, STATE, ANO NATIONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
VS-95-04 1-10
Specific Operations
Qo= lOOACFM
Saw Table
1 1
: 1 1 1 1 1
Table
Saw Blade
End View
Side View
FIGURE
TITLE
TABLE SAW GUARD EXHAUST
VS-95-05 DATE
CHECK CODES, REGULA TTONS, AND LA WS (LOCAL, STATE, AND NATTONAL) TO ENSURE THAT DESIGN !S COMPLIANT.
1-10
13-179
13-180
Industrial Ventilation
Smooth Wall Flex Duct
Compressed Air Jet
Bits< 3/4" and
§ Cleanout
Drum covers necessary. Hinge or otherwise provide for maintenance.
Exhaust flow rates Drum length
Total e:xhaust for machine acfrnldrum *
Upto31"
550
31" to 49"
790
49" to 67"
1100
over67"
1400
Brush rolls
350 acfm at brush
*Note: Provide one more take offthan the number of drums. Mínimum duct velocity = 3500 fpm he= 0.25 VPd
TITLE
FIGURE
MULTIPLEDRUM SANDER
DATE
CHECK CODES, REGULA TTONS, AND LAWS (LOCAL, STATE, AND NATTONAL) TO ENSURE THAT DESIGN IS COMPLIANT.
VS-95-11 1-10
Specific Operations
r-r:::::. :
'
1
~-+-i
\ i(-', 1
'
1
1 '\
1
•
\
'
P;\\\ 1 1 1
1 1 1
::
1
r
Duct-C
\ '\ \ \ \ '\\
\
1
1
'\
\\ \' \\ \ \ ':...._
/.:¡c--~~~~-
¡ 1 1
11 11
~--r--
~----
45 o V
:
Duct-A
1 1
o..,t-B
~
1 1 1 1 1 1 1
1
:
:4 51Jm; aspect ratio ?. 3:1, as determinad by the membrana filler method al 400-450X magnification (4-mm objective), using phaSEH:Ontrast illumination. (G) As measured by the vertical elutriator, cotton-dust sampler; see the TLve Documentation. (H) Aerosol only. (I) lnhalable fraction; see Appendix C, paragraph A. (IFV) lnhalable fraction and vapor; see Notations/Endnotes section. (J) Does not indude stearates of toxic metals. (K) Should not exceed 2 mg/m3 respirable particulate mass. (L) Exposure by all routes should be carefully controlled to levels as low as possible. (M) Classification refers to sulfuric acid contained in strong inorganic acid mists. (O) Sampled by method that does not collect vapor. (P) Application restricted lo conditions in IM!ich there are negligible aerosol exposures. (R) Respirable fraction; see Appendix C, paragraph C. (T) Thoracic fraction; see Appendix C, paragraph B. (V) Vapor and aerosol. B = Background; see BEIIntro. BEI = Substances for IM!ich there is a Biological Exposure lndex or Indicas (see BEF section). BEI,: see BEI~ for Acetylcholinesterase lnhibiting Pesticidas BE l.: see BE¡e for Methemoglobin lnducers BEI.: see BEI® for Polycydic Aromatic Hydrocarbons (PAHs) rvwv =Molecular weight. NOS =Not otherwise specified. Nq =Nonquantitative; see BEIIntro. Ns = Nonspecific; see BEIIntro. SEN =Sensitization; see definition in the "Definitions and Notations' section. Skin = Danger of cutaneous absorption; see discussion under Skin in the "Definitions and Notations" section. Sq = Semi-quantitative; see BEI lntro. STEL = Short-term exposure limit; see definition in the "lntroduction lo the Chemical Substances." TWA =8-hour, time-weighted average; see definition in the "lntroduction lo the Chemical Substances.' ppm = Parts of vapor or gas per million parts of contaminated air by volume al NTP conditions (25°C; 760 torr). mg/m3 = Milligrams of substance per cubic meter of air.
Substance [CAS No.]
ADOPTED VALUES TWA (pplllltng/m 3)
STEUC (pplllltng/m3)
Acetaldehyde [75-07-0] (1992) C25ppm Acetic acid [64-19-7] (2003) 10ppm 15ppm Acetic anhydride [108-24-7] (1990) 5ppm Acetona [67-M-1] (1996) 500 ppm 750 ppm Acetone cyanohydrin [75.a6-5], as CN (1991) C5 mg/m3 Acetonitrile [75-05-a] (1996) 20ppm Acetophenone [98.a6-2] (1990) 10ppm Simple asphyxiant