Conventional Jackets [PDF]

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Conventional Jackets

Figure 1: Conventional Jacket "Conventional jackets" can be divided into two (2) main categories: baffled and non-baffled. Baffled jackets often utilize what is known as a spirally wound baffle. The baffle consist of a metal strip wound around the inner vessel wall from the jacket utility inlet to the utility outlet. The baffle directs the flow in a spiral path with a fluid velocity of 1-4 ft/s. The fabrication methods does allow for small internal leakage or bypass around the baffle. Generally, bypass flows can exceed 1/3 to 1/2 of the total circulating flow. Conventional baffled jackets are usually applied with small vessels using high temperatures where the internal pressure in more than twice the jacket pressure. Spirally baffled jackets are limited to a pressure of 100 psig because vessel wall thickness becomes large and the heat transfer is greatly reduced. In the case of an alloy reactor, a very costly vessel can result. For high temperature applications, the thermal expansion differential must be considered when choosing materials for the vessel and jacket. Design and construction details are given in Division 1 of the ASME Code, Section VIII, Appendix IX, "Jacketed Vessel". Heat Transfer Coefficients: Conventional Jackets without Baffles (hj De / k) = 1.02 (NRe) 0.45 (NPr) 0.33 (De/ L) 0.4 (Djo/ Dji) 0.8 (NGr) 0.05

Figure 2: Schematic of Conventional Jacket Where: hj = Local heat transfer coefficient on the jacket side De = Equivalent hydraulic diameter

Eq. (1)

NRe = Reynolds Number NPr = Prandtl Number L = Length of jacket passage Djo = Outer diameter of jacket Dji = Inner diameter of jacket NGr = Graetz number The Reynolds Number is defined as: NRe = DVρ/μ Where D is the equivalent diameter, V is the fluid velocity, ρ is the fluid density, μ and is the fluid viscosity. The Prandtl Number is defined as: NPr = Cp μ / k Where Cp is the specific heat, μ is the viscosity, and k is the thermal conducitivity of the fluid. The Graetz Number is defined as: NGr = (m Cp) / (k L) Where m is the mass flow rate, Cp is the specific heat, k is the thermal conducitivity, and L is the jacket passage length. The equivalent diameter is defined as follows: De = Djo-Dji for laminar flow De = ((Djo)2 - (Dji)2)/Dji for turbulent flow

Conventional Jackets with Baffles For conventional jackets with baffles, the following can be used to calculate the heat transfer coefficient: hj De/k= 0.027(NRe)0.8 (NPr)0.33 (µ/µw)0.14 (1+3.5 (De/Dc) ) ( For NRe > 10,000) hj De/k = 1.86 [ (NRe) (NPr) (Dc/De) ] 0.33 (µ/µw)0.14 ( For NRe < 2100 )

Eq. (2) Eq. (3)

Figure 3: Schematic of Conventional Jacket with Baffle Two new variables are introduced. Dc is defined as the centerline diameter of the jacket passage. It is calculated as Dji + ((Djo-Dji)/2). The viscosity at the jacket wall is now defined as µw. When calculating the heat transfer cofficients, an effective mass flow rate should be taken as 0.60 x feed mass flow rate to account for the substantial bypassing that will be expected. De is defined at 4 x jacket spacing. The flow cross sectional area is defined as the baffle pitch x jacket spacing.