How To Defrost Coils : Information from Gartner Refrigeration for mechanical contractors

TIPS & TOOLS:
How To Defrost Coils

Defrosting of evaporators can sometime be a finicky process. Coil manufacturers do not have set recommendations, and generally, it takes some experimentation by the operator to nail down the process. We want to clear the coil of frost, get rid of the frost melt down the drain, and remove moisture from the room and put the coil back into operation as quickly as possible. One note is that the drain lines should always have a trap.

There are a few ways to initiate a defrost cycle such as; time of day, by run time in refrigeration and frost sensors on the evaporators. Once a defrost cycle is initialized the liquid feed ceases and the remaining refrigerant is vaporized and exits the coil. We suggest starting with a 15 minute pump out period. After the pump out, the hot gas is activated and flows into the coil, and condenses as the heat is transferred from the refrigerant to the frost. Twenty minutes is a good starting point for the hot gas step. After the hot gas shuts off, there is a bleed period where the pressure of the coil is slowly relieved back to suction pressure. In most applications, a small ½” or ¾” solenoid is installed around the suction stop valve, to SLOWLY relive pressure. There are newer models of valves on the market which do not require a separate bleed solenoid, but have an internal bleed feature. Three minutes is our suggestion for the bleed cycle. After the bleed cycle the liquid feed is restarted, however the fans remain off, for a period of 3-5 minutes. If the fans come on too early any remaining water droplets will be blown into the air and potentially product and the floor. Hot gas pressure of around 120 PSIG with the defrost pressure regulator set at 70# promotes good even defrost.

There are a number of factors that can influence how often defrosting is necessary. One of the biggest culprits in frost accumulating fast is infiltration. This unwanted air and moisture combo comes from air leaks in the walls, roof, and pipe penetrations. Good freezer doors between a dock or other area and a freezer help to greatly reduce the infiltration load. The season can also have an impact. In the summer hot gas pressure might be too high, and warm humid ambient conditions pronounce infiltration. During the winter low hot gas pressure may contribute to a poor defrost, but in contrast the quantity of defrosts required are lowered.

In conclusion, it is best for the operator to experiment with defrost sequences to determine the best way to defrost. There are a number of factors that can influence the defrosting of an evaporator, but these guidelines are a good place to start particularly if you are having issues with improper defrosts.