Most users normally associate refrigeration with cold and cooling, yet the practice of refrigeration engineering deals almost entirely with the transfer of heat. This seeming paradox is one of the most fundamental concepts that must be grasped to understand the workings of a refrigeration system.
Cold is really only the absence of heat, just as darkness is the absence of light, and dryness is the absence of moisture.
The condition of temperature and pressure at which both liquid and vapor can exist simultaneously is termed saturation. A saturated liquid or vapor is one at its boiling point, and for water at sea level, the saturation temperature is 212° F. At higher pressures, the saturation temperature increases, and with a decrease in pressure, the saturation temperature decreases.