Design for the Condensate, Not the Dry Gas

Gas-side film coefficients sit far below liquid, so on almost any gas duty the gas side controls the design regardless of what is on the other side. That is why the range runs to 168 inch shell lengths rather than larger diameters: length buys surface, and surface is what a low coefficient demands. Sizing a gas cooler on the same basis as a water exchanger of similar duty will produce a unit less than half the size it needs to be.

Pressure drop is usually the binding constraint rather than the thermal duty. Raising velocity improves the coefficient, and drop rises with roughly the square of velocity. On a compressed stream that drop is compression work already paid for; on a ducted stream it is draft the system may not have. Send the allowable drop with the enquiry, because it frequently decides the selection before the heat balance does.

The detail most often missed is what happens as the gas cools past its dew point. Latent heat is released, which changes the duty profile, and liquid appears that has to be drained. More importantly the first condensate to form concentrates whatever acid the gas carries into a very small volume against the tube wall, and that film can be far more aggressive than the dry gas analysis suggests. Material selection should follow the expected condensate chemistry rather than the gas.