Defrost in Commercial Refrigeration w/ Dick Wirz

Dick Wirz, author of Commercial Refrigeration for Air Conditioning Technicians, talks to us about refrigerator and freezer defrost strategies. Check out Dick's book HERE.

In commercial refrigeration, we deal with much lower evaporator temperatures than residential HVAC. Although an evaporator temperature of 40°F may be commonplace in residential HVAC, you can expect evaporator temperatures from 25-30°F in refrigeration. Even though having ice on the coil is a negative thing in residential HVAC, it is perfectly normal in refrigeration.

The purpose of defrosting is to bring the evaporator temperature above freezing to melt off the frost. We can defrost a coil in a few different ways, including a mere off-cycle defrost in medium-temperature refrigeration. When the system shuts off, the evaporator coil can start defrosting. However, if too much heat is introduced to the system, more frost can accumulate on the evaporator coil. As such, a planned defrost may be in order. These defrosts occur on a timer and turn the system off overnight. Alternatively, these defrosts may use electricity or hot gas to remove ice from the coil more rapidly, especially in low-temperature applications.

Electric and hot gas defrost are common defrost types. The hot gas method generally reverses refrigerant as a heat pump does; hot discharge gas runs through the evaporator coil and melts the ice off the coil. However, hot gas is an expensive method and can negatively impact system longevity if used improperly. The electric method is cheaper than the hot gas method; this method relies on electric heat outside the coil to melt the frost from the outside.

Dick also talks about:

• Warm air infiltration
• Coil-sensing thermostat controls
• Defrost failsafe
• Defrost termination
• "Snowing" in the box and fan delays
• Drain pan heaters and drain complications
• Paragon timers
• Demand defrost setups/clocks

 

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