In which conditions is refrigerant considered sub-cooled?

• A. When it's at the boiling point
• B. When it's cooled below its condensing temperature
• C. When it has reached its freezing point
• D. When it is a saturated vapor

Answer: (B)

A refrigerant is considered sub-cooled when it is cooled below its condensing temperature. This means that the refrigerant has been cooled to a temperature that is lower than the temperature at which it would begin to condense from a vapor into a liquid under the given pressure.

Being sub-cooled is important in refrigeration systems, as it allows for improved efficiency and prevents the refrigerant from vaporizing too early in the system. Sub-cooling ensures that all of the liquid refrigerant entering the expansion device is indeed in liquid form, which is crucial for the proper functioning of the refrigeration cycle.

The other conditions mentioned pertain to different states of refrigerant. When at the boiling point, the refrigerant is at equilibrium between its liquid and vapor phases, which is not considered sub-cooled. Reaching its freezing point indicates that the refrigerant is becoming solid, while a saturated vapor refers to refrigerant that exists at a condition where it is poised between being a liquid and a vapor but is not cooled below its condensing temperature. Therefore, the defining characteristic of sub-cooling is being below the condensing temperature, making that the correct choice.