Shell-side bypassing , seal

Figure 28.1 The seal strips avoid shell-side bypassing.

The function of seal strips is to interfere with, and hence reduce, the fluid flow through the bypass area. Often, one pair of seal strips is used for every 18 in of shell ID (inner diameter). These seal strips encourage good shell-side cross-flow velocity and also help reduce localized fouling, caused by low velocity. 

Normal design practice is to end both seal strips at the first tube support baffle. This is wrong design practice. It allows the shell-side flow to bypass the exchanger area between the tubesheet and the first tube support baffle. To avoid this problem, the seal strip at the edge of the first tube support baffle must be extended to the tubesheet (see Fig. 28.1). 

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