Exchanger , longitudinal baffle

Longitudinal baffles must also be compatible with the shell-side fluid, so they normally will be of the same material as tubes or baffles. This baffle never extends the full inside length of the shell, because fluid must flow by its fer end for the return pass in reaching the exchanger oudet. 

The critical feature of this exchanger is the longitudinal baffle, which extends down the length of the shell. The baffle is fitted directly to, and thus becomes physically part of, the tube bundle. The function of the baffle is to force the shell-side fluid to flow down the entire length of the... 

The reason illustrates the true nature of the shell-and-tube heat exchanger. It is a compromise between an ideal heat-transfer configuration and practical mechanical limitations. In this case, the difficulty is preventing leakage around the longitudinal baffle. Such leaks permit the shell-side fluid to short-circuit the tube bundle that is, a percentage of the inlet flow, may flow directly to the outlet nozzle. In extremely serious cases, I have seen the bell head (shown in Fig. 19.8) 100°F colder than the shell-side outlet temperature. 

Figure 19.9 Detail illustrating how a modem longitudinal baffle seals up against the inside of the exchanger shell.

I have seen new exchangers of "two-pass shell" configurations work well when new. However, with time and typically after a turnaround, performance has been degraded, due to leakage around the cross-flow (longitudinal) baffle. 

Multipass Flow. True countercurrent flow is not attained in most multipass exchangers. In such exchangers, the fluid is guided back and forth in the shell by means of longitudinal baffles, and in the tubes by means of partitions in the heads. Efficiencies (or multipliers for the log-arithmetic mean difference) are shown in Figs. 17-4 and 17-5 in terms... 

Twisted Tube construction (Figure 6.32) eliminates the need for baffles altogether, since the tubes can be oriented ( tuned ) to contact surrounding tubes periodically along the exchanger. The shell-side flow is longitudinal, and special correlations have been developed for both heat transfer and pressure drop on both sides of the surface. Laminar flow inside the tube is enhanced by the secondary flow induced by the twisted flow path. 

For this formulation to apply to shell-and-tube exchangers, it is assumed that each tube pass has the same number of tubes and that the tube-side coefficient in each pass is the same. The analysis also requires that there be sufQcient baffles that the shell-side flow can be treated as longitudinal to the tubes, rather than as a series of cross-flow sections. Three or four baffles may be sufficient to meet this criterion if the shell-side temperature change is less than the minimum temperature difference between shell-side and mbe-side fluids, and eight or more baffles are almost always sufficient for the assumption to be satisfied. 

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