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View Subscription Details: Letter ( 10/24/2008 ) - HTML Format

Flexible defrost

 

In order to maximize the efficiency of a refrigeration plant frequency of hot gas defrosting should be changed from time to time. Why should we do that?

The frequency of defrosting should be adjusted to the rate of frost formation. This rate depends of 3 factors:

-         temperature difference between the evaporator coil temperature and the air temperature in a refrigerated room.

-         humidity of the air in the refrigerated room.

-         evaporator coil operating time.

 

An evaporator coil operates as a magnet; it attracts the moisture from the surrounding air and freezes this moisture. The strength of this magnet depends on the temperature difference between the coil temperature and the air temperature in the refrigerated space. Greater temperature differences directly correlate to higher rates of frost formation.

Humidity of the air in the refrigerated room is probably the most important factor of frost formation. This humidity will increase during the summer because of higher moisture content in the ambient air.

Assume that the optimum defrosting should be done at 90% of coil capacity. At 90% of coil capacity frost thickness will be 1/8 inch. Evaporator coil should run for 12 hours during the winter operation in order to get 1/8 inch of frost. During summer operation the same coil should run 4 hours to get 1/8 inch of frost. Therefore, this evaporator should be defrosted every 4 hours during summer operation and every 12 hours during winter operation. Eight hours defrost will probably be a good choice for spring and fall operation.

My experience has shown that the optimum length of defrosting should be 25-30 minutes for the freezers and 15-20 minutes for the coolers. Large coils in extremely cold freezers (-20°F or colder) should be defrosted for 40-45 min. Hot gas should not be oversupplied because oversupply creates a lot of ammonia condensate. It is difficult to drain this condensate from the overfeed coil because of small orifices at the entrance of each coil circuit. Very often, undrained ammonia condensate is the reason of poor hot gas defrosting, especially for evaporator coils in extremely cold rooms.

 

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