Why might oil accumulate at the lowest point of an evaporator in an ammonia system?

In ammonia refrigeration systems, oil accumulation at the lowest point of an evaporator occurs primarily because oil has a higher boiling point than ammonia. When the system operates, ammonia evaporates and absorbs heat, turning from a liquid to a vapor at low temperatures. Since the boiling point of oil is significantly higher than that of ammonia, during the refrigeration process, the oil remains in a liquid state at those lower temperatures while ammonia vaporizes. This characteristic leads to oil settling at the bottom of the evaporator, where it is less likely to be carried away with the ammonia vapor.

Understanding this characteristic is crucial for maintaining optimal system performance, as excessive oil accumulation can hinder heat transfer efficiency and cause operational issues. Proper oil management strategies are necessary to ensure that oil can be periodically returned to the compressor or removed from the system as needed.