During off-cycle defrost on a medium-temperature walk-in, what are the compressor and the evaporator fans doing?
- Compressor running, evaporator fans off
- Compressor off, evaporator fans off
- Compressor off, evaporator fans running
- Compressor running, evaporator fans running
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C. Compressor off, evaporator fans running
The fans keep moving above-freezing box air through the coil to melt the frost while no refrigerant is being evaporated. Stopping the fans is what is done on electric or hot gas defrost in a freezer, to keep heat out of the box.
Reference: Defrost methods; off-cycle defrost
What is the temperature difference (TD) of a refrigeration evaporator?
- Box air temperature minus the saturated evaporating temperature
- Suction line temperature minus the saturated evaporating temperature
- Air entering the coil minus the air leaving the coil
- Box air temperature minus the suction line temperature
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A. Box air temperature minus the saturated evaporating temperature
Evaporator TD is the gap between the air in the box (entering the coil) and the boiling temperature of the refrigerant found from suction pressure. Suction line temperature minus saturation temperature is superheat.
Reference: Evaporator temperature difference
What does a heavy coating of frost do to an evaporator coil?
- It adds cold surface to the fins, so capacity rises
- It raises the suction pressure and overloads the compressor
- It raises the superheat and overheats the suction line
- It insulates the fins and blocks airflow, so capacity falls
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D. It insulates the fins and blocks airflow, so capacity falls
Frost is a poor conductor and it closes the spaces between fins, so less heat reaches the refrigerant. Suction pressure drops, the coil runs colder, and the box warms up even though the system runs constantly.
Reference: Evaporator frost; need for defrost