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When does it become economical to enhance the surface or add fins to a shell-and-tube heat exchanger? How do you know whether the investment is worth it? I have a gas-to-gas, single-pass, counter-flow exchanger currently.

The area added by fins is not as efficient as bare tube surfaces due to the resistance to conduction through the fin. Extended surfaces, or fins, are used most commonly where the film coefficient on the outside of a tube is lower than that on the inside. In this case, fins greatly extend the rate of heat transfer. A good example is steam heating of air. The film coefficient for air is much less than that of steam, so the fins greatly increase the heat transfer rate. When fouling is present, fins are not a good solution.

For gas-to-gas heat transfer, in general, the film efficiencies are probably similar enough, adding fins is not a good investment. However, if one of the gases is condensing, adding fins is a good investment. -- Alan Levine, Engineering Solutions Inc., alevine@eng-sol.com