Freezing Temp.’s & the Effect of Water in Coils
Water and steam have been used to cool and heat air in finned-tube heat exchange coils almost since the beginning of heating and air conditioning. At the same time, the freezing of the internal fluid and the resulting coil and property damage, have also been around for just as long. It is an inherent problem that many times can be prevented. Since the beginning of coil manufacturing, cold temperatures have been a nightmare for coils and the units which they are installed in. It is general knowledge that at 32°F, water will freeze (based on certain conditions of where that 32°F or lower air is coming in contact with the water). When temperatures drop to 32°F and below, the steam and water within these coils will also freeze, causing internal pressure to build. The pressure can then become high enough that the tubing will expand and burst. The key to understanding the damage caused to the tubing directly 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 the heat exchanger damage, and finally, 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 may also 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. Today, most new coils are constructed to withstand well over 1,000-psi with ease. Bursting pressures of bends and tubes are such that they can individually handle well over 1,800-psi. Therefore, it is very obvious that the pressure inside a heat exchanger coil during a freeze cycle could be well above these pressures.
So … 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. A common misconception is that steam coils can’t freeze; however, a cold spot on the coil or a failed steam trap can cause condensate to back up and freeze within a coil. Preventive maintenance should be performed during the summer months to ensure proper operation before the first freeze of the winter season. A frozen coil can cause an avalanche of problems in the building, from water damage, to downtime, and cost associated with replacement of the coil.
What steps can be taken to avoid this issue? Here are some preventive steps that can be taken to assist in the non-freezing of coils.
Properly drain the water out of the coil. This should be done when temperatures are beginning to get colder and closer to freezing.
Use of glycol mixture – Changing the 100% water to a mixture of glycol and water in the hot or chilled water system(s) can lower the freezing point of the solution.
Replace standard steam with steam distribution coils – These coils distribute steam from an inner tube to an outer tube containing the condensate. The inner distribution tube reduces the likelihood of the outer condensate tube freezing.
Coil placement – Coils without freeze protection should be located downstream of heating coils.
Bypass pumps – The pump turns on when the outdoor air is near freezing, so the constant moving of water flows through a coil to reduce the likelihood of freezing.
Preventative maintenance – Preventative maintenance begins during the summer and early Fall months.
Properly designed and installed coils provide fresh air during all conditions without freezing, ensuring the HVAC system can provide ventilation and proper exhaust for occupants. As stated earlier, the best time to check your coils for freeze protection is during the warmer months so any modifications can be made before the next winter.