How do Steam Coils Work?

Steam coils commonly are used to heat large commercial and industrial spaces with exceptionally large heat loads, but how do they work, and why should you use them?

Steam coils heat air, or another type of gas, by moving the air across a bank of finned tubes.  The steam is contained in the tubes, while the air passes through the finned section.  The steam enters from a supply manifold, flows through the finned tubes where it condenses back into water, and drains out the return pipe manifold.

Steam is the very best heat transfer fluid available.   Steam has a high heat of condensation, so it can carry a lot of heat to the heat exchanger with ease.  It has a very high heat transfer coefficient when it is condensing, which allows the heat to flow from the steam into the metal of the heat exchanger, very easily.  When condensing, steam temperature and pressure are linked, so the temperature can easily be controlled by adjusted pressure valves.  Steam comes from water and water is very easy to get compared to other heat transfer fluids, such as refrigerants or oils.  Lastly, steam condenses into water in the heat exchanger, and can easily be pumped back to the boiler to be turned into steam again.

Air, on the other hand, does not heat as easily as steam.  The average heat transfer coefficient is about 100 times less than steam. The steam’s heat flows into the metal heat exchanger 100 times better than from the metal heat exchanger to the air.  In order to make up for the difference, fins are added to the outside of the tubes, so that the air has a lot of heat transfer surface area touching it.