When might a compressor lose oil?

A compressor can lose oil when the velocity of refrigerant in the risers is too low because the refrigerant needs a certain speed to properly circulate oil back to the compressor. If the refrigerant flows too slowly, it can impede the oil return, leading to insufficient lubrication for the compressor. This can result in oil pooling in the evaporator or other parts of the system instead of returning to the compressor, and over time, this can cause the compressor to run with insufficient lubrication, potentially leading to damage.

Other scenarios, such as having air in the system or experiencing medium pressure drops in the evaporator, can affect system performance, but they do not directly correlate with the loss of oil in the same way that poor refrigerant velocity does. Small load variations may also not significantly impact oil retention, as they might be within acceptable operational parameters. Thus, the correct choice emphasizes the importance of refrigerant motion in maintaining oil levels for effective compressor function.