With an above-32º box temperature the air in the box is moved across the evaporator coil even when the compressor is off on the temperature or pressure control.
To get box temperatures below about 45º the evaporator will trend below 32ºF. and so develop frost. In a properly design system this frost melts off during the compressor's "off cycle". That's why it's called off-cycle-defrost.
This is why there is a Refrigeration-Day consideration. Of course there are 24 hours in a Standard Day. But all defrost time must be subtracted from our 24 hour Standard-Day - because all the BTU's have to be removed during the non-defrosting periods - when the compressor is operating.
For example; if six one-hour defrost periods are required to keep the evaporator ice free that only leaves 18 hours in which to get all the BTU's out of the box.
If the total heat load is, say; 10,000 BTU's per hour - because our Refrigeration Day is only 18 hours long; the refrigeration system's capacity will have to be 25% greater, or: 12,500 BTU's per hour.
Because 24 hours is 25% more than 18 hours - and we have to get all the refrigerating work done in 18 hours to allow the other 6 hours to be devoted to defrosting time.
Is that what you wanted to know?
PHM
---------------
Originally Posted by
hvactrekkie
Off cycle defrost? Could you explain? I just started a job where they expect me to fix ice machines, which is ok. But refrigerators I do not know. Thanks.
PHM
--------
When faced with the choice between changing one's mind, and proving that there is no need to do so, most tend to get busy on the proof.