Gas Cooling
U-Tube Heat Exchangers

Gas Cooling U-Tube Heat Exchangers

Low Coefficients Against a Fixed Pressure Drop Budget

Gas-side film coefficients sit roughly an order of magnitude below liquid, so the gas side controls the overall coefficient almost regardless of what runs on the other side. The consequence is a lot of surface area for a modest duty, and a design where every decision is made on the gas side first.

The obvious fix and the obvious problem. Raising gas velocity improves the coefficient, and pressure drop rises with roughly the square of velocity. On compressed gas that drop is compression work you have already paid for, so the pressure drop budget usually sets the design before surface area does. On low-pressure gas the budget is small in absolute terms and just as binding.

Which side the gas runs on. High-pressure gas is cheaper to contain in tubes than in a shell, and that frequently decides it. Where gas is at low pressure and drop is critical, the shell side offers a much larger flow area and opens up the option of finned tube, which puts extra surface exactly where the resistance is.

Design for the condensate, not the gas. As gas cools past its dew point, the first liquid to form concentrates whatever acid or chloride the gas carries into a very small volume against the tube wall. That film is what corrodes, and it can be far more aggressive than the dry gas analysis suggests. Selecting material against the gas composition and then finding the cold end corroded through is a common and expensive sequence.

Partial condensation changes the duty profile. Latent heat is released as condensation begins, so the heat load is not distributed the way a sensible-cooling calculation predicts. The rating needs a condensing curve rather than a dew point, and the design needs somewhere for the liquid to go — usually a knockout separator sized for the expected condensate rate.

Fouling on dirty gas. Gas carrying compressor oil, dust or polymer-forming components will foul, and it will foul preferentially at the cold end where the deposit does the most damage to the approach. A removable bundle earns its keep here, and generous tube pitch makes the shell side genuinely lanceable.

Related pages: process cooling, vapor condensing, duplex and high alloys and shell selection.