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View Subscription Details: Letter ( 04/25/2011 ) - HTML Format

Frost sublimation


Some companies claim that during hot gas defrosting one third or 33% of the frost will sublimate and reenter into refrigerated space as water vapor. I have done a test to check this statement and the result was different.


A cold room (freezer) has penthouse with 4 evaporators. Temperature in this freezer is 0°F. Capacity of each evaporator is 25 TR. Before defrosting, all evaporators were operated for 24 hours. Evaporator 1 was defrosted and switched to cooling mode. Then 3 remaining evaporators were defrosted and amount of water drained was measured. In average 24 gal. of water were produced by each evaporator. It means that rate of frost formation was 1 gal. per hour. Evaporator 1 was operated in cooling mode for 30 minutes when the other 3 evaporators were defrosted and 30 minutes after defrosting. Then evaporator 1 was defrosted and 2.5 gal. of water were drained during this defrosting. At regular rate of frost formation we would have 1 gal. of water. It means that additional frost of 2.5 - 1 = 1.5 gal.  was formed on this evaporator during defrosting of 3 other evaporators. 1.5/3 = 0.5 gal. of the water vapor have reentered refrigerated room. This test has shown that only around 2% of the frost will reenter refrigerated room. This hot gas defrosting was done at 100 psig of the condensing pressure.

Major factor of mentioned sublimation is temperature of hot gas that enters defrosted evaporator. Today the majority of refrigeration plants have screw compressors that have discharge temperature of 170 - 190°F. Temperature of hot gas that enters an evaporator is significantly lower, because this gas will cool down in hot gas main and in the evaporator pan. I think that in worse case scenario sublimation of the frost will be less than 5% and this is significantly lower than 33% mentioned by some companies.



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