Must Steam Coils Freeze?

A coil that is used to heat air with steam must be able to continuously change the state of the water vapor (steam) to a liquid condensate by removing from the steam nominally 1,000 BTU per pound.

When the entering air temperature is below 32 degrees F, the condensate (liquid water) cools towards its freezing point. If allowed to freeze – DISASTER. As the forming ice expands inside the coil tube, so does the coil tube. There is no reason to criticize a coil that is leaking through a crack that has expanded.

In order to use only the quantity of steam required to satisfy a particular heating requirement, steam “traps” are used that will allow only the condensate to return to the mains. Raw steam must not be wasted.

If the condensate is “trapped” in the coil tubes for any reason, still – DISASTER!!!!!

The real questions is: how can condensate be trapped in the coil tubes and how can we prevent it?

In a steam control system that matches capacity to load, a modulating valve causes a pressure drop across itself that reduces the steam pressure and steam quantity to the amount required. The void created by the change of state from a high steam vapor volume to a much lower liquid water volume must be satisfied. If that void is not filled with air from the surrounding atmosphere, a vacuum is created.

This phenomenon allows the pressure difference across the steam trap to be in a reverse direction and prevents the condensate water from leaving the coil.

The answer to this problem lies in the use of a “vacuum breaker” that allows air to enter and mix with the steam and satisfy the vacuum. The “vacuum breaker” location at either the coil inlet or outlet will satisfy the trap performance. However, when located at the INLET, it will allow steam and air to mix BEFORE they enter the coil and minimize the tendency to stratify.

It becomes apparent that a reduced steam pressure within the coil reduces the driving force pushing the condensate towards the trap inlet. When that pressure difference becomes zero, only the force of gravity remains and it still must overcome the capillary attraction holding the water to the tubes. If any coil tube that contains water is not pitched towards the coil outlet, it carries the potential for internal

It is not enough to rely upon so called “horizontal” installation. Nothing is truly horizontal until it is parallel to the horizon of the earth. Even then, it will not drain until the forces of capillary attraction are overcome.

A third concern lies in the fact that at the low coil operating steam pressure, condensate will only flow to the return main that is at atmospheric pressure by gravity through the orifice of the steam trap. In order for this condensate to flow, a vertical head of water must be formed high enough to overcome the trap and piping pressure drop. The coil tubes and headers MUST NOT be used to hold that required liquid head. Not only might it freeze but it will almost certainly generate that destructive force called “Water Hammer”.

The answer is to include just ahead of the trap a 12″ to 18″ vertical “Liquid Leg” in which the required “Head” can form.

In a related piping concern, it should be realized that, in order for the coil to return to design capacity, the air that was deliberately admitted through the vacuum breaker must now be exhausted. This is best accomplished by the installation of a steam type air vent at the coil outlet, ahead of the liquid leg. Potential dripping from this air vent can be accommodated by piping the vent outlet to the condensate
return main AFTER the trap outlet.

Please realize that we do not expect our readers to become steam coil piping experts just by reading this article. Let your fingers do the walking and leave the remembering to us.

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