Why can oil gather in the evaporator at the lowest point in an ammonia system?

In an ammonia system, oil tends to accumulate in the evaporator at its lowest point primarily because oil has a higher boiling point than ammonia. This characteristic means that while ammonia, a refrigerant, evaporates easily at lower temperatures, oil does not vaporize as readily due to its higher boiling point. As the system operates, ammonia vaporizes and circulates through the system, while the oil, remaining in liquid form, settles to the lowest point, where it collects in the evaporator. This behavior is significant because it affects the efficiency of the system and the potential for oil circulation back to the compressor for lubrication purposes. Understanding the boiling points and physical properties of these substances helps in managing and troubleshooting refrigeration systems effectively.