Shell-side seal strips

Shell-side seal strips. See if you cannot find an old tube bundle lying around your plant. Many such bundles have pairs of metal strips set around the edge of the tube bundle. These metal strips are typically V4 in thick and 4 in wide. They extend down the length of the tubes. As seen in Fig. 19.4, they are inserted in grooves cut in the tube support baffles. These seal strips often increase heat-transfer efficiency by 5 to 10 percent. 

Tube-side improvement use tube inserts, twisted tubes and tubes with internal fins Shell-side improvement use helical baffles, EM baffles, externally finned tubes, twisted tubes and shell-side seal strips (Fieberman, 2010)... 

Shell-and-Tube Heat Exchangers 333 26.1.1.4 Shell-Side Seal Strips... 

When tubes are omitted from the tube layout to provide entrance area about an impingement plate, the need for sealing strips or other devices to cause proper bundle penetration by the shell-side fluid is increased. 

Figure 10-147. Shell-side jn factors for bundles. One sealing strip per 10 rows of tubes and TEMA clearances. (Source Engineering Data Book, 2" Ed., 1960. Wolverine Tube, Inc. Used by permission Kern, D. Q., and Kraus, A. D. Extended Surface Heat Transfer, p. 506, 1972. McGraw-Hill, Inc. All rights reserved.)...

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. 

The use of an impingement plate requires the removal of several rows of tubes. This creates a large gap between the edge of the tube bundle and the shell. It is the primary purpose of the seal strips to reduce this gap (discussed in the next section). For the seal strips to fulfill this function, one of the seal strips in each pair must extend the full length of the tube bundle. This is rather complicated to explain. However, if you will look at a tube bundle that has been extracted from its shell, you will understand why this is needed, and how impingement plates render seal strips essential for good sensible heat transfer from a shell-side fluid. 

For exchangers with vertically cut baffles, the shell-side impingement plate will be opposite the shell-side inlet nozzle. Contrary to the location of the seal strips shown in Fig. 26.4, the critical seal strips will be set on either side of the impingement plate (see Fig. 28.1 in Chap. 28). Additional pairs of seal strips can also be installed as shown in Fig. 26.4. (See Chap. 28, "Shell-and-Tube Heat Exchangers ... 

Figure 28.1 The seal strips avoid shell-side bypassing.

However, when I wrote that the seal strips needed to be as long as the tubes, that is not quite right. One of the pairs of seal strips must have one strip that stops at the first tube support baffle. If not, the shell-side feed would be trapped above the impingement plate. 

If we have vertically cut the tube support baffles (which is normal, at least in the United States), then the seal strip that is aligned with the edge of the first shell-side tube support baffle should extend to the tubesheet. 

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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