2 Refrigerant Pressures and Pressure-Temperature Charts

Pressure and saturation temperature

For a given refrigerant, pressure and saturation temperature are linked: higher pressure corresponds to higher saturation temperature, and lower pressure corresponds to lower saturation temperature. At saturation, liquid and vapor can coexist. While refrigerant is boiling or condensing, the saturation temperature is the temperature at which that phase change occurs at the measured pressure.

Using a P–T chart

A P–T chart relates saturation pressure and saturation temperature for a particular refrigerant. Identify the refrigerant first; never use another refrigerant’s chart column. Then find the measured pressure and read across to its corresponding saturation temperature, or begin with a temperature and read the corresponding pressure.

The relationship is refrigerant-specific. At a saturation temperature of $70\, ^\circ\text{F}$, one Sporlan chart lists about $121.4\,\text{psig}$ for R-22, $201.8\,\text{psig}$ for R-410A, and $71.1\,\text{psig}$ for R-134a. These are saturation pressures at that temperature, not universal normal operating pressures for a system.

Pressure units and reference

Check the chart’s pressure units and reference before interpreting a reading. Gauge pressure (psig) is measured relative to atmospheric pressure, whereas absolute pressure (psia) includes atmospheric pressure. Near sea level, the approximate relationship is:

The cited Sporlan chart reports psig at sea level. Do not compare values expressed in different pressure units without converting them.

Temperature glide in blends

Some refrigerant blends change temperature as they evaporate or condense at nearly constant pressure. This temperature change is called temperature glide. Such blends can have distinct saturation values at the dew-point and bubble-point boundaries.

• Dew point: the saturated-vapor boundary; use its saturation value when calculating superheat.

• Bubble point: the saturated-liquid boundary; use its saturation value when calculating subcooling.

Use a chart or approved tool that identifies the correct dew- and bubble-point values for the refrigerant.

What operating pressures reflect

In a typical vapor-compression system, suction pressure reflects conditions around the evaporator, while discharge or condensing pressure reflects conditions around the condenser. The evaporator refrigerant must be cool enough to absorb heat, and the condenser refrigerant must be warm enough to reject heat.

Operating readings vary with refrigerant and system conditions, including indoor and outdoor temperatures, heating or cooling load, airflow, coil cleanliness, compressor operation, metering-device control, refrigerant charge, and pressure drop through the system. A change in any of these can shift operating pressures. Pressure at the compressor may also differ from pressure at a coil because refrigerant lines have pressure drop.

Interpreting readings

There is no single correct suction or discharge pressure for all systems, or for one system under every condition. Compare readings with the equipment manufacturer’s specifications and the actual operating conditions.

A P–T chart describes saturation; by itself, it does not establish whether a system is correctly charged or operating properly. Compare a measured line temperature with its saturation temperature to determine superheat or subcooling. Use the vapor saturation temperature for superheat and the liquid saturation temperature for subcooling. Interpret pressure alongside line temperatures and relevant measurements such as superheat and subcooling; pressure alone is not a reliable diagnosis or charging method.