Two-pass exchangers

Figure 1.7 Simple detail of shell-and-tube heat exchanger. The water box may be designed for as many as eight passes, and a variety of configurations of shell-side baffles may be used to improve heat transfer, (a) Several water box arrangements for tube-side cooling, (b) Assembly of simple two-pass exchanger with U-tubes. [Fig. 38.2, The Nalco Water Handbook, 1st ed. (1979), reprinted with permission from McGraw-Hill, Inc.)...

The pass partition baffle shown in Fig. 14.5 makes this cooler a two-pass exchanger. These baffles are subject to failure, due to corrosion. More often, they break because of excessive tube-side pressure drop. The differential pressure across a two-pass pass partition baffle equals the tube-side AP. 

The resulting four-pass tube bundle will have a tube-side velocity twice as high as it did when it was a two-pass exchanger 3 ft/s. Experience has shown that in many services, doubling this velocity may reduce fouling rates by an order of magnitude. That is fine. But what about pressure drop ... 

Mean-temperature-difference (MTD) correction factor. When the outlet temperatures of both fluids are identical, the MTD correction factor for a 1 2 shell-and-tube exchanger (one pass shell side, two or more passes tube side) is approximate 0.8. For a single-pass aircooled heat exchanger the factor is 0.91. A two-pass exchanger has a factor of 0.96, while a three-pass exchanger has a factor of 0.99 when passes are arranged for counterflow. 

Also, AP, as discussed above for the shell-side flow (see preceding section on cross-flow velocity), varies with the number of passes, cubed (not squared). That is, flow path length doubles when going from two to four tube-side passes. Meaning, if I convert a two-pass exchanger to four-pass, the fluid has to flow twice as fast and twice as far. 

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