Steam Coil Condensate Load

A steam coil does its work by condensing, so the steam it eats is simply the duty divided by the latent heat at the pressure the coil is running at. A 150 kW preheat coil fed 15 psig steam, where $h_{fg}$ is about 2,199 kJ/kg, condenses 150/2,199 = 0.0682 kg/s, which is 246 kg/h or roughly 540 lb/h. Notice that the answer is not really a flow of steam so much as a flow of water: everything that condenses has to leave through the trap, which is why the same number is called the condensate load and why it is the figure the trap, the return line and the receiver all get sized from.

The pressure matters twice over and in opposite directions. Raising it gives a hotter coil and a bigger temperature difference, so the coil transfers more heat per square foot, but it also LOWERS the latent heat each kilogram carries, so the mass flow climbs faster than the duty does. That is the trade-off behind every steam coil selection.

Then there is the number this page does not give you, and it is the one that matters on a cold morning. The running load calculated here is a steady-state figure. Start-up is worse, sometimes far worse: the coil metal is cold, the air on it is cold, and condensate forms faster than the coil will ever produce it once warm. Traps are selected on two to three times the running load for that reason. Undersize the trap and condensate backs up into the tubes, where it subcools, and in a coil facing outdoor air in a Canadian winter it freezes and splits them — which is exactly why steam-distributing "non-freeze" coils exist, with an inner perforated tube that keeps live steam against the full length of the outer tube instead of letting a stalled slug sit in the bottom.