Water and steam have been used to cool and heat air in finned-tube heat exchange coils almost since the inception of heating and air conditioning. Freezing of the fluid and the resultant coil damage have also been around for the same length of time. It is a systematic problem that many times is preventable.
But we all know that this is not a perfect world. Nor are the HVAC systems that have been in service for decades.
It is fairly simple to understand the basics of a liquid phase freeze. The ambient conditions must be at or below 32°F. That can, in turn, cause the water inside the coil to fall below 32° as well. If kept below the freezing temperature of fluid long enough, the coil may be damaged by this condition.
WHY COILS FAILThere are many different reasons for coil failures due to a freeze condition. Some of those include:
• Controller (actuator) malfunctions and the outside air damper stays open. At the same time, fluid is not being pumped through the coil.
• Freezestat wrapped on the leaving side of the coil either is defective or does not cover the entire coil area. Thus, it does not shut down the system when a freeze condition is present.
• Coil is not drained properly for winterizing. Simply, water is lying in some or all parts of the coil tubes, or return bends, and/or headers during peak winter months.
• Coil does not have adequate antifreeze solution added for winterizing.
• Valves, traps, or other water or steam accessories malfunction, trapping water or steam condensate inside the coil with low ambient conditions around the coil.
• Improper water coil design may trap liquid (not completely drainable).
• Steam distributing coils with long tube lengths or small-diameter inner and outer tubes may not be able to remove condensate quickly enough, trapping liquid in coil tubes and headers.
• Coil is so long that it should have a dual-feed design (supply connections at both ends) to provide an even flow of steam and condensate so that traps can remove the condensate. Most times this option is not selected because of the physical restraints near the unit such as wells or other equipment that do not allow steam piping on the other side of the unit.
PRESSURE: THE CULPRITThe key to understanding coil damage due to a liquid line freeze relates to the extreme pressure produced during the formation of ice. The area that contains this ice can only handle this added pressure until it reaches a limit that causes heat exchanger damage and subsequent failure.
The pressure limit is a variable limit based on many different factors, including coil construction, especially the tubes and return bends and also systematic life deterioration. The original coil construction deteriorates the longer it is in service. Walls of the tubing, and especially return bends, thin out because of water or steam velocity.
There also may be corrosive agents involved that can cause stress corrosion cracking, crevice corrosion, or general corrosion fatigue, thus reducing the maximum freeze pressure of the coil.
Most new initial coils are constructed to withstand well over 1,000-psi easily. Bursting pressures of bends and tubes are such that they can individually handle well over 1,800-psi. It then must be very obvious that the pressure inside a heat exchanger coil during a freeze cycle would have to be very high.
Where does a coil fail? The answer is fairly simple and consists of two main factors: The circuitry of the coil where the pressure builds, and the weakest point in that circuit. Extensive testing has shown that the failure will appear as a bloated area in the tube header or bend that has expanded. This, in most cases, is the area that will rupture.
This area at the weak point almost always looks like “12 pounds in a 10-lb. bag.” It is clear that the point has experienced great stress and has tried to contain pressure by deforming (expanding) and then finally rupturing. Natural pressure relief is simply destined to be at the weak point in the circuit.
PRESSURE RELIEF: THE ANSWERTake note that coils constructed with copper tubing and headers (the most widely used construction for HVAC coils) have a steam pressure rating of 250 psig from -30 to 250° and a test pressure of 400 psig for water coils. Steam coils are 100 and 400 psig, respectively. The object is to provide a pressure-relief device that will automatically fail above the test pressure (since it is the highest coil pressure for any given construction) and yet still below the pressure of a liquid phase freeze.
To summarize: Adverse pressure is the cause of freeze damage to a coil, and the relief of that pressure is the solution. However, modern technology (such as Sentry Guard™ heating and cooling coils) has addressed this issue. Such coils feature a pressure relief design, are economically feasible and do not affect the other performance characteristics of any coil. Along with the capability of replacing or fabricating almost any coil, regardless of its age, make, or construction, this new coil can relieve the owner of a major problem that has existed since the inception of heat transfer.