In refrigeration practice, what is the reason for sub-cooling the refrigerant?

Sub-cooling the refrigerant is an essential practice in refrigeration systems, primarily to enhance the refrigeration effect. Sub-cooling involves lowering the temperature of the refrigerant below its saturation temperature after it has been condensed and prior to it entering the expansion device. This process ensures that the refrigerant remains in a liquid state, preventing any vapor from entering the expansion device, which could negatively impact system performance.

When the refrigerant is sub-cooled, it can absorb more heat from the environment being cooled, thereby increasing the system's overall efficiency and the amount of heat removed from the refrigeration space. With more liquid refrigerant entering the evaporator at a lower temperature, the refrigeration cycle can draw in a larger amount of heat, ultimately enhancing the cooling effect within the system. This leads to improved energy efficiency and coherence in temperature control.

In contrast, other options focus on different aspects of refrigeration. While lowering evaporation pressure and preventing freezing may seem relevant in various scenarios, they do not directly relate to the primary purpose of sub-cooling. Enhancing compressor efficiency, although a desirable outcome, is more a result of an overall optimized system rather than a direct consequence of sub-cooling itself.