Final answer:
Begin charging liquid refrigerant into a system when the refrigerant saturation temperature sufficiently increases, indicating a balance in vaporization and condensation rates and ensuring system stability.
Step-by-step explanation:
In the context of charging a refrigeration system with refrigerant, the point at which you may begin charging liquid refrigerant is typically when the system indicates the refrigerant saturation temperature increases sufficiently. This increase signifies that enough vapor refrigerant is present in the system to start adding liquid without risking damage from rapid expansion.
Understanding vapor pressure is crucial in this process. As the temperature of a liquid increases, so does the kinetic energy of its molecules. When vapor pressure equals atmospheric pressure, the liquid begins to boil. In a refrigeration system, you want to start charging liquid refrigerant after the system reaches a steady state, where the vaporization and condensation rates are equal, creating a balance and constant saturation temperature. At this point, additional liquid refrigerant can be added without causing instability in pressure or temperature.