Steam coils are a common heat source in an air handler and provide heating for commercial and industrial buildings. Similar to a hot water coil, the supply and return connections are the same on these coils. Despite this similarity, steam is very different than hot water, and steam coils are specially built for and circuited for steam.
There are two types of steam coils used in commercial and industrial applications: standard steam coils and non-freeze (steam distributing) coils. Standard coils have a single-tube contraction, while non-freeze coils have a dual-tube construction.
Standard coils are made with 5/8” tubes, and the tube thickness differs based on the application and steam pressure. They are used when entering air temperatures are above 40ºF. It’s recommended to pitch the casing on almost all steam coils to remove the condensate.
In a non-freeze steam coil, a tube is set inside another tube. The steam on the inner tube prevents the condensate in the outer tube from freezing. Coils were initially designed to distribute steam evenly along their length and to eliminate any dead spots on the coil. Because of this, the coils were not as likely to freeze, so they became known as “non-freeze”, which is mostly accurate. In reality, any coil can freeze under certain conditions, but this design is what needs to be used when the entering air is under 40ºF.