How often and for how long should an air unit evaporator in a cooler or freezer be defrosted?

Hot gas and electric defrosting of air units that are installed in walk-in coolers, freezers, and warehouse complexes involves a lot of expense.  If the unit is not defrosted often enough, there is the wasted energy of operating an evaporator that is working inefficiently.  If the unit is defrosting too often, there is the unnecessary expense of the defrost cycle and removal of the heat that was added during the cycle.  Therefore, it is important that the defrosting of these evaporators be properly sequenced.

The source of the frost on the evaporator fins and tubes is the water vapor in the air.  The amount of frost formed will be different on hot and humid summer days than it will be during the cold and dry winter season.  Therefore, the frequency and duration of the defrost cycle will likely need to be adjusted to accommodate the different seasons.

The following steps will help determine the frequency and duration of the defrost cycle:

  1. Start out with a “clean” coil.  Initiate a defrost cycle through the use of the defrost clock (or other controller) and ensure that the evaporator coil and pan are free and clear of all frost and ice. 
  2. Note the time of day and place the unit back in normal refrigeration.
  3. Check on the evaporator every hour or so throughout the day and observe how rapidly frost is reforming on the tubes and fins of the coil.  Common sense will tell you when the frost is sufficient to begin restricting the air flow through the unit.
  4. Note the time of day when enough frost has formed that you feel the air is restricted and it is time to defrost again.  If the time from start to the time for defrost took 8 hours, then you need to set the time clock for three defrosts per day. If 12 hours, then two defrosts per day.
  5. Now adjust the timer to start a defrost cycle and observe the process.  How long is it taking to clear the frost & ice and return the unit to the “clean” condition that we had in Step 1?  This time tells you how long the electric defrost heaters or hot gas valves need to be ON.  Some evaporators and their controls have a “temperature termination” feature.  This feature uses a thermostat that is monitoring the temperature of the tubing in the evaporator and terminates the defrost cycle when the tube reaches approximately 50°F.  Observation of the evaporator during the defrost period will tell if this feature is working properly.

In summary, determining the frequency and duration of a defrost cycle is quite simple, but it is often overlooked and contributes to unnecessary expense.