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, counterflow exchanger currently. -- D.L.

The shell-side heat transfer coefficient must be considerably less than the tube side for this to be beneficial. Then, the increase in the shell-side pressure drop must be considered in the analysis. -- Brian Emery, Industrial Heat Recovery Equipment, (800) 422-1333.