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

Figure 2: Schematic of Conventional Jacket

Where:
h_j = Local heat transfer coefficient on the jacket side
D_e = Equivalent hydraulic diameter
N_Re = Reynolds Number
N_Pr = Prandtl Number
L = Length of jacket passage
D_jo = Outer diameter of jacket
D_ji = Inner diameter of jacket
N_Gr = Graetz number

N_Re = 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:

N_Pr = C_p 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:

N_Gr = (m C_p) / (k L)
Where m is the mass flow rate, C_p is the specific heat, k is the thermal conducitivity, and L is the jacket passage length.

The equivalent diameter is defined as follows:

D_e = D_jo-D_ji for laminar flow
D_e = ((D_jo)2 - (D_ji)2)/D_ji for turbulent flow

Conventional Jackets with Baffles

For conventional jackets with baffles, the following can be used to calculate the heat transfer coefficient:

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 D_ji + ((D_jo-D_ji)/2). The viscosity at the jacket wall is now defined as µ_w. 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. D_e is defined at 4 x jacket spacing. The flow cross sectional area is defined as the baffle pitch x jacket spacing.

Eq. (4) Eq. (5) Eq. (6)