If a PT chart for a refrigerant has a Dew point and Bubble point associated with the refrigerant, which one should be used to charge for subcooling?

To determine the appropriate point to use for charging the system when dealing with refrigerants, it's essential to understand the physical meanings of the dew point and the bubble point on a pressure-temperature (PT) chart.

The bubble point is defined as the temperature at which a refrigerant begins to boil or vaporize when at a certain pressure. In contrast, the dew point is the temperature at which a refrigerant starts to condense from a vapor into a liquid at that pressure. When it comes to charging a refrigeration system for subcooling, the bubble point is the relevant reference point.

Charging for subcooling means ensuring that the refrigerant is in a fully liquid state before it enters the expansion device, preventing any vapor from entering the system, which can lead to inefficiencies or damage. The bubble point indicates where the refrigerant is about to become a vapor, so you want to charge to a point below this temperature to ensure that the refrigerant is indeed fully subcooled and remains in a liquid state in the condenser and before it reaches the expansion valve.

In practical terms, using the bubble point during charging ensures that the system operates efficiently, achieving optimal performance and preventing potential issues associated with vapor entering a liquid line. This understanding is crucial