What does sub-cooling refer to in refrigeration?

Sub-cooling refers specifically to the process of cooling a liquid refrigerant to a temperature that is lower than its condensation point at a given pressure. This occurs after the refrigerant has condensed from a vapor into a liquid but before it enters the expansion valve or evaporator. Sub-cooling is crucial because it ensures that only liquid refrigerant enters the evaporator, maximizing efficiency and preventing compressor damage caused by liquid refrigerant.

When the refrigerant is sub-cooled, it is often at a lower temperature than the surrounding environment and is in a denser, more energy-efficient state that aids in the absorption of heat while evaporating in the evaporator coil. Proper sub-cooling improves the overall efficiency of the refrigeration cycle, reducing energy consumption and enhancing cooling performance.

The other options describe processes that do not accurately define sub-cooling: cooling of a gas at low pressure refers to a different stage of refrigeration, heating of a liquid before evaporation implies an increase in temperature rather than a decrease, and cooling of a vapor above its boiling point suggests a situation that is not relevant to the sub-cooling process.