In a dry type evaporator, why must liquid refrigerant boil away close to the end of the coil?

In a dry type evaporator, it is crucial for the liquid refrigerant to boil away close to the end of the coil to operate at high efficiency. The primary function of an evaporator is to absorb heat from the surrounding environment, which is achieved through the phase change of the refrigerant from a liquid to a vapor. When the refrigerant boils towards the end of the coil, it ensures that all the liquid refrigerant has transitioned to vapor before exiting the evaporator. This maximizes the heat absorption process, as vaporized refrigerant can absorb more heat than liquid refrigerant.

Efficiency is critical in refrigeration systems because it directly impacts the overall performance, energy consumption, and operational costs. Ensuring efficient boiling near the end of the coil helps to maintain optimal conditions within the evaporator, reducing the risk of liquid carryover to the compressor, which can lead to potential damage and decreased performance.

While the other options mention important aspects such as preventing frost accumulation, sub-cooling the compressor, or ensuring proper oil return, they do not primarily address the reason for the efficiency of the evaporation process, which is the main point of focus for the boiling away of refrigerant within the coil.