What phenomenon causes refrigerant to move through the system to the compressor crankcase during the off-cycle?

Refrigerant migration is the phenomenon that causes the refrigerant to move through the system and collect in the compressor crankcase during the off-cycle. This movement can occur due to differences in pressure and temperature within the system. When the air conditioning or refrigeration unit is not running, the refrigerant can condense and a significant portion can return to the compressor.

This is especially prevalent in systems where the refrigerant is a vapor at normal operating conditions, but may condense and flow back to the crankcase when the system is off. This migration process is particularly important to understand because if too much refrigerant gathers in the compressor during the off-cycle, it can adversely affect the compressor's ability to function properly when the system is restarted. Understanding refrigerant migration helps in designing systems that appropriately manage refrigerant during both operation and non-operation periods.

The other options do not specifically refer to the same process: backflow implies a reversal of direction in fluid flow, pooling suggests stagnation in one location, and transfer generally refers to the movement of refrigerant but does not specifically address the time frame of the off-cycle or the pathway to the compressor crankcase.