A major problem with heating with steam is the potential for coil freeze. A necessary byproduct of steam is condensate, or water. This is in theory a good thing as it indicates that the process is working. As the heat from the steam is transferred to the medium, in this case airflow, the steam condenses back to water. At this point it should be removed from the system via the steam trap, and in an ideal world this always happens. However, we all know about the ideal world.
The typical freeze scenario comes into play because of system stall. Conventional steam coil design utilizes a modulating valve to control steam flow. Now, while I note this is conventional design, it is not necessarily optimal design. In most cases of using steam in HVAC applications I recommend the Integral Face and Bypass Coil, as noted in a previous entry. However, that design is not in itself always optimal, or always utilized; in which case the designer, plant engineer, or technician must deal with the situation at hand. As it stands, conventional design utilizing a modulating valve will run into trouble at the point the valve begins to close as demand is satisfied. If in this situation the valve closes so that downstream pressure is equal to or greater than upstream pressure the coil will be in a condition of stall. The condensate will accumulate in the coil and is thus subject to freezing.
My choice solution for an existing conventional system is to utilize a freeze valve that will sense the water and remove it. I use the Thermomegatech HAT valve with a setpoint of 100 Deg. F. I install the valve in the condensate line between the coil and the strainer and/or steam trap. I pipe the condensate to drain. I have seen similar devices used by the steam specialty companies that were marginal or ineffective. The primary reason being they had a setpoint in the area of 35 Deg. F. This is too low and too close to freezing temperature. The condensate may not be able to escape the coil before freezing. On the other hand, having a setpoint of 100 F. seems to work well. 100 F. is way too cold for condensate, which should typically be over 210 Deg F., yet it is warm enough the condensate won’t freeze.
I have had positive results with this system. While it is recommended for conventional steam heating, it can be applied in most applications where freezing condensate or system stall may be a problem. Another problem with system stall outside of freezing is the introduction of air and non-condensables into the system through the vacuum breaker. This is often seen in process heat exchangers utilizing modulating valves as discussed above.