EXXON MOBIL - Heat Transfer & Heat Exchangers [PDF]

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HEAT TRANSFER AND HEAT EXCHANGERS

Slide 1

What We Will Cover

• Heat transfer theory-review • Relation of heat transfer theory to shell and tube heat exchangers • Design of a S&T exchanger--procedure outline • Design features and parameters of shell and tube exchangers

Slide 2

BASIC HEAT TRANSFER CONCEPTS

• Flow of heat behaves like flow of fluids and flow of electrons

Rate  K x

Driving Force Resistance

QK x

Pressure Drop Resistance

I = 1.0 x

Voltage Resistance

Temperature Difference QK x Resistance Slide 3

(General)

(Fluids)

(Electricity)

(Heat)

COMPARISON WITH FLUIDS Fluids:

Heat:

Q A

2

= K x (P2 - P1)

(Remember Section 3?)

fL D Q = 1 x (T2 - T1) A

RT FLUIDS

Q = Volume / Second P2, P1 = Higher, lower pressures A = Area available for flow

fL 4 * D = Number of fluid flow resistance units Slide 4

HEAT

Q = Btu / Hour T2, T1 = Higher, lower temperatures RT = Total specific resistance A = Area available for flow of heat

BASIC HEAT TRANSFER EQUATION

Q = 1 x (T2 - T1) = 1 x T A RT RT RT = Total Resistance, Hr x FT2 x °F / Btu I = Total Conductivity = U° Btu / Hr x Ft2 x °F RT Q = 1 x U° T A Q = U° x A x T Btu / Hr U° is Referred to as the “Overall Heat Transfer Coefficient” Slide 5

TOTAL RESISTANCE TO HEAT FLOW - HEAT EXCHANGERS

• There are two areas through which heat must flow: The inside tube area and the outside tube area. Resistance occurs at both areas. • The Industry Standard Reference Area is the Outside Tube Area.

Slide 6

INDIVIDUAL COMPONENTS OF THE TOTAL RESISTANCE Inside Film Resistance = R io = R i Inside Fouling Resistance = r io = r i

Ao Ai Ao Ai

Tube Wall Resistance = r w = w / k w Outside Fouling Resistance = ro Outside Film Resistance = Ro

Rio + rio + rw + ro + Ro = RT = w = Wall Thickness, Feet

I U°

Kw = Thermal Conductivity, Btu / Hr x Ft 2 x °F Ft r = Resistances, Hr x Ft2 x °F/Btu

Slide 7

INDIVIDUAL COMPONENTS OF THE TOTAL RESISTANCE

Slide 8

TYPICAL RESISTANCE VALUES

Very Low

Typical

Very High

0.00050 (2000)

0.004 (250)

0.04 (25)

0.001 (1000)

0.002 (500)

0.01 (100)

Wall Resistance Inverse

0.000030 (32,000)

0.00027 (3760)

0.00049 (2030)

Total Resistance Inverse

0.00303 (330)

0.01227 (81)

0.10050 (10)

Film Resistances (Each) (Inverse = h) Fouling Resistance (Each) Inverse

Slide 9

THE CONTROLLING COEFFICIENT •

Frequently One of the two film coefficients determines the value of the overall coefficient:

Out side Coefficient, Inside Coefficient,

h° hio Ro Rio rw + rio + ro RT U° Improvement

•

Slide 10

= = = = = = = =

75 1000 0.01333 0.00100 0.00070 0.01503 66.5 Base

75 3000 0.01333 0.00033 0.00070 0.01436 69.6 +4.6%

150 1000 0.00667 0.00100 0.00070 0.00837 119.5 +80%

Hence h° is the “Controlling Coefficient”, and efforts to improve exchanger performance should concentrate on this side of the exchanger.

TEMPERATURE DROPS ACROSS THE RESISTANCES

• Temperature drop across each of the resistances is directly proportional to each resistance. • For example, If T2 = 200 and T1 = 80, then total temperature drop = 120•F, and: Temperature Drop

Ro Rio rw rio+ ro RT

Slide 11

= = = = =

0.01333 0.00500 0.00030 0.00200 0.02063

77.6 29.1 1.7 1.6 120°F

=

0.01333 0.02063

x 120

TEMPERATURE DROPS ACROSS THE RESISTANCES

Q A

A Useful Concept is Heat Flux =

=

Btu Hr x Ft2

Q = U x A x (T2 - T1) = U x A x T Then T =

Q UxA

Then Q = T = A RT

=

Q A

* x

R

=

Flux x Resistance

120 0.02063

= 5817 Btu , and T across Ro = 5817 x 0.01333 = 77.6 °F Hr x Ft² as shown on that slide. Slide 12

BACK TO BASICS •

We’ve looked at basic theory, and discussed Q = U° x A x T. In refinery work we usually know either Q or A, and need to calculate the other value. How do we do it?

•

Either question requires calculating U° or T.

•

We’ll talk about U° later, first let’s discuss T, the temperature driving force.

•

Note that capital letter T denotes the hot stream, while lower case t denotes the cold stream: T1 = Hot In t1 = Cold In

Slide 13

T2 = Hot Out t 2 = Cold Out

FLOW PATTERNS AND TEMPERATURE DRIVING FORCE

Slide 14

FLOW PATTERNS AND TEMPERATURE DRIVING FORCE

Slide 15

FLOW PATTERNS AND TEMPERATURE DRIVING FORCE

Slide 16

FLOW PATTERNS AND TEMPERATURE DRIVING FORCE

Slide 17

TEMPERATURE DRIVING FORCE •

• •

From the preceding slides, it is clear that some sort of average driving force must be used in design calculations. What is this average? The average is called “The Effective Mean Temperature Difference”, or MTDe. For true countercurrent and true cocurrent flow, the effective driving force equals the log mean average of the two extreme (largest and smallest) deltas.

Te = LMTD =

(T1 - t2) - (T2 - t1) (T1 - t2)

LN

(T2 - t1)

This is precisely true only when the heat release curves are straight lines. Otherwise it is an approximation.

Slide 18

TEMPERATURE DRIVING FORCE • What about mixed flow: Shell and Tube Exchangers? • The complex flow in these units was analyzed mathematically many years ago, resulting in rigorous equations for a Correction Factor, Fn. This is multiplied by the LMTD to give the correct MTDe. MTDe = Fn x LMTD • Equations are valid only when heat release curves are linear. • Similar relations are available for transverse flow (air fin coolers, for example).

Slide 19

CALCULATION OF Fn • Depends on the number of shells in series (“Shell Passes”)

• The more shells one has in series, the closer Fn approaches 1.0 • Typically the minimum acceptable value of Fn is 0.8

• What exactly do we mean by “shells in series” or “shell passes”?

Slide 20

CALCULATION OF Fn - SHELL PASSES

Slide 21

CALCULATION OF Fn - SHELL PASSES

Slide 22

CALCULATION OF Fn • • •

Slide 23

Complex equations simplified to charts See TEMA Section 7, or Exxon DP IX-D Applicable only to linear heat curves

CALCULATION OF Fn

Example

T1 = 300 T2 = 105

t1 = 85 t2 = 115

P = j = 115 - 85 = 0.14 300 - 85 R = 300 - 105 = 6.5 115 - 85

R n (1 Shell) =