Shell-side pressure-drop effect

Effect of shell-side pressure drop. Reducing the baffle spacing increases cross-flow velocity and improves heat transfer. But it also increases the shell-side pressure drop. Reducing the baffle cut also improves heat transfer, but increases AP. 

The Kern method is inaccurate for calculating the shell-side heat-transfer coefficient and shell-side pressure drop (Ref. E2, p. 545). Kern makes no account of the effect of bypss and leakage in the shell-fluid passage. For these reasons the Bell Method (Ref. E9) is employed to serve as a check on the preliminary design. 

The shape of the coohng and warming curves in coiled-tube heat exchangers is affected by the pressure drop in both the tube and shell-sides of the heat exchanger. This is particularly important for two-phase flows of multicomponent systems. For example, an increase in pressure drop on the shellside causes boiling to occur at a higher temperature, while an increase in pressure drop on the tubeside will cause condensation to occur at a lower temperature. The net result is both a decrease in the effective temperature difference between the two streams and a requirement for additional heat transfer area to compensate for these losses. 

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

Figure 10-57. Effect of velocity on heat transfer rates and pressure drop shell-side and tube-side. (Used by permission Shroff, P. D. Chemical Processing, No.4, 1960. Putnam Publishing Co., Itasca, III. All rights reserved.)...

The effect of the fouling on the shell-side flow is to increase the cross-flow and increase the overall heat transfer coefficient for a fixed pressure drop (assuming the same fouling coefficients in both cases). 

The shell-side exchanger pressure drop, which includes the effect of the baffle height. 

The pressure drop due to friction when a fluid is flowing parallel to and outside of tubes can be calculated in the normal manner described in Chap. 14 by using a mean diameter equal to four times the hydraulic radius of the system and by including all frictional effects due to contraction and expansion. In heat exchangers, however, the fluid flow on the shell side is usually across the tubes, and many types and arrangements of baffles may be used. As a result, no single... 

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