Use a pressure-temperature, or P-T, chart when analyzing a refrigerant system to set a coil pressure to produce a certain temperature, to determine the amount of superheat over the saturated vapor temperature at the evaporator outlet and to determine the amount of subcooling under the saturated liquid condition at the condenser outlet. A P-T chart typically has two columns listing the saturated refrigerant pressures corresponding to certain temperatures. Some charts present a series of columns with figures for various refrigerants.
The pressure and temperature figures presented on a P-T chart correspond to saturated states where there is a mixture of liquid refrigerant and vapor. The three places in an operating refrigeration system where this state exists are at the evaporator, the condenser and the receiver. Determine the saturation temperature by taking a pressure reading at one of these three points and find the corresponding temperature on the P-T chart, or take a temperature reading to determine the saturation pressure.
When the temperature measured at the evaporator outlet, for example, is higher than the saturation temperature for a given pressure, the difference between the two temperature readings is the degree of superheat at that point. Similarly, the refrigerant leaving the condenser may have a lower measured temperature than the saturation temperature at a given pressure. The difference between these two temperature readings is the degree of subcooling.