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View Subscription Details: Letter ( 12/12/2005 ) - HTML Format

Frost on the evaporator coil

When the surface of evaporator coil operates at temperatures below 32°F and also below the dew point temperature of the air, frost will form on the coil.


Sometimes during the winter operation, dew point temperature of ambient air (major source of the moisture) can be lower than evaporator coil temperature of the storage cooler. In this situation the frost will not form on this coil.


However, usually we have the frost on the evaporator coil if temperature of this coil is lower than 32°F.


During the first several hours of evaporator coil operation, capacity of this coil will slightly increase.

Ice crystals (frost) increase heat transfer surface of the coil, but heat transfer resistance of these crystals is not significant. However, gradually the density of the frost will increase as well as heat transfer resistance and coil capacity will decrease.


The two negative effects of frost are:

  • resistance to heat transfer
  • resistance to air flow

Of these two penalties, restriction of air flow is the most serious.


The rate of frost formation depends on the following 3 factors:

  • temperature difference between the coil temperature and air temperature
  • relative humidity of the air in refrigerated space
  • evaporator coil operating time

Evaporator coil operates as a magnet. It attracts the moisture from surrounding air and freezes this moisture. Strength of that magnet depends on the temperature difference between coil temperature and air temperature. At higher temperature difference we have a higher rate of frost formation.


Another important factor of frost formation is relative humidity of air in the refrigerated space. This humidity will increase during the summer and will decrease during the winter.  If we have more moisture (higher relative humidity) in the air, the rate of frost formation will increase.


Several PLCs have a function of evaporator coil cumulative time (total operating time) to trigger defrosting of this coil.

This function is working better than a regular defrost timer, but the operating time is only one of 3 frost formation factors shown above.


If we want to start defrosting at certain capacity of the coil (for example 95%), setting of the cumulative time should be changed several times per year to compensate for the changes in relative humidity(suction  temperature is constant at optimum level).

Summer operating time will be shorter than winter operating time due to difference in relative humidity of air in the refrigerated space.

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