HOW DOES A “NON-FREEZE” STEAM COIL OPERATE?
There is no such thing as a “non-freeze” steam coil. Under the right circumstances any steam coil can freeze, including those labeled “non-freeze”! The proper term to describe a steam coil is ” Steam Distributing”. The whole idea behind this type of coil is to insert an inner tube down the entire length of the outer tube to distribute steam evenly down the inner tube.
To accomplish this, holes are spaced approximately 9 to 12 inches apart in the inner tube. As the steam turns to condensate — or travel down the inner tube — the condensate is released through these holes to the outer tube. Whatever steam doesn’t turn to condensate is eventually dumped out the end of the open inner tube to the outer tube. The outer tube is “capped” with no return bends so that the condensate under pressure has no place to go but back down the outer tube toward the manifold where the steam originally came from. The idea behind the original design of this coil was to evenly distribute the steam and the condensate throughout the coil so there are no “dead spots” or “cold spots” in the coil.
After much testing, it was found that a residual benefit with this type of coil was a higher resistance to freezing over the standard conventional steam coil, hence the name “non-freeze” surfaced. The steam traveling down the inner tube kept the condensate traveling the opposite direction in the outer tube from freezing. Today it is an accepted industry practice to install these coils in applications where entering air temperatures are 40°F or below. Keep in mind however, that non-freeze coils don’t freeze easily, but it is possible to freeze them under the right conditions, and it happens all the time.
5/8″ Outer Tube VS 1″ Outer Tube
Many companies sell and build 5/8″ or 1″ non-freeze steam coils as though they are interchangeable. Whatever is quicker or cheaper is what they use depending on the job. At Hydronic Systems, we want our customers to understand that there is a major difference in the construction of these coils and proper usage depends on the job at hand.
5/8″ Outer – 3/8″ Inner Steam Distributing Coil
Most steam distributing coils have a 5/8″ outer tube with a 3/8″ inner tube. This leaves a space between the tubes of 1/4″, or an 1/8″ on each side of the tube. The thickness of the outer tube must be subtracted from this ⅛. This leaves very little space between the outer and inner tube to pass condensate back through the outer tube.
1″ Outer – 5/8″ Inner Steam Distributing Coil
The real O.D. of 1″ Steam Distributing coils is 1-1/8″ with a 5/8″ inner tube. This leaves more than twice the room to pass the condensate through the outer tube. The wall thickness als is .035″ in lieu of .025″. When using a lot of outside air across a non-freeze coil, the temperature rise and the amount of BTUs becomes larger. As a result, more lbs. per hour of steam are needed and more pounds per hour of condensate passess through the coil.
For a high air temperature rise or a low entering air, a 1″ non-freeze steam coil should be used. If there is mostly recirculated air or need only 30°F or 40°F air temperature rise is needed, then a 5/8″ non-freeze coil will suffice.
The major concern here is cost. As expected, 1″ steam coils are more expensive than 5/8″, and some companies do not even build 1” steam coil resulting in 5/8″ steam coils being installed despite the conditions. This solution often does not work.
On long coils over 72″, the problem can be diagnosed by feeling the far end of the coil away from the manifold. If the coil feels cold, the condensate is trapped and blocking the steam from getting down the coil which indicated the coil is most likely the incorrect design for that application and should be replaced.
USA Coil and Air, Inc.is a good choice for steam distributing coils because the manufacturer builds all types of steam coils so the proper size coil can always be used for maximum operating efficiency.
To view USA Coil & Air, Inc’s PDF of this article in its entirety, go to USA Coil & Air, Inc. Non-Freeze Steam Coil.
Source: USA Coil & Air Website