What is 'sub cooling' in refrigeration systems?

• A. Heating a vapour above its saturation temperature
• B. Cooling a liquid below its saturation temperature
• C. Increasing the saturation temperature
• D. Vaporizing the liquid refrigerant

Answer: (B)

Sub cooling in refrigeration systems refers to the process of cooling a liquid refrigerant below its saturation temperature corresponding to the pressure at which it resides. This is important because it ensures that the refrigerant is entirely in liquid form before it enters the expansion valve. By being below the saturation temperature, it eliminates the risk of the refrigerant vaporizing prematurely, which can lead to inefficiencies and damage to the system.

When the refrigerant is sub-cooled, it has a greater thermal capacity, which can enhance the overall efficiency of the refrigeration cycle. This leads to improved performance, as the sub-cooled liquid absorbs more heat when it evaporates in the evaporator.

In the context of the multiple-choice options, the other choices discuss processes that either involve heating or changing the state of the refrigerant in ways that do not accurately define sub cooling. Heating a vapor above its saturation temperature relates to superheating, increasing the saturation temperature pertains to changes in pressure, and vaporizing the liquid refrigerant refers to the process of converting liquid to gas, which is not what occurs during sub cooling.