Pressure-Temperature Chart Basics for Refrigerants

When a refrigerant's liquid and vapor are together in a closed space and settled, the refrigerant is saturated. At saturation, pressure and temperature are locked together: for a given refrigerant, each temperature has exactly one pressure. Warm the cylinder and the pressure rises. Cool it and the pressure falls. A P-T chart lists those matching pairs.

Each refrigerant has its own curve, which is why charts have a column per refrigerant. It is also how the regulation sorts refrigerants into pressure classes, using the saturation pressure at 104 °F: low-pressure below 45 psia, medium 45 to 170 psia, high 170 to 355 psia, and very high above 355 psia or with a critical temperature below 104 °F (40 CFR 82.152).

The chart only applies where liquid and vapor coexist. In a running system that means inside the evaporator and the condenser, and in a cylinder or a system that has been off long enough to settle. Pure vapor, like the superheated gas in the suction line, or pure liquid, like the subcooled liquid line, will not match the chart. That difference is what superheat and subcooling measure.

Service gauges read psig, pressure above the surrounding atmosphere. Absolute pressure, psia, counts from a perfect vacuum. At sea level the atmosphere is about 14.7 psi, so psia = psig + 14.7. EPA's Type II topics specifically mention adding 14.7 to convert a gauge reading to absolute.

Below 0 psig, gauges read in inches of mercury (in. Hg) vacuum. Standard atmospheric pressure is 29.9 inches of Hg, the reference used in the evacuation table in 40 CFR 82.156. Deep vacuum is measured in absolute units, such as the 25 mm Hg absolute required for low-pressure appliances, or microns.

Figures below are rounded saturation pressures at sea level, given only to show how differently common refrigerants behave at the same temperature. Use the manufacturer's chart or a calibrated digital gauge for real work.

EPA's Type I and Type II topics include using pressure and temperature to identify a refrigerant and detect noncondensables. The method uses a system or cylinder that has been off and has settled to the temperature around it.

Measure the pressure and the actual temperature of the refrigerant, for example the ambient temperature of a unit that has sat unused, or of a cylinder stored at room temperature. Look up that temperature on the chart. If the pressure matches a refrigerant's column, that is very likely what is inside, and it should match the nameplate.

If the pressure is noticeably higher than the chart value for the refrigerant that should be there, suspect noncondensables, usually air, or a mixed or wrong refrigerant. Noncondensables collect in the condenser or the top of a cylinder and add their own pressure on top of the refrigerant's. A low reading on a settled system usually means there is not enough refrigerant for liquid to be present, so the chart no longer applies.

Superheat is how far a vapor's temperature is above its saturation temperature at that pressure. Measure suction pressure, read the saturation temperature from the chart, and subtract it from the actual suction line temperature. The difference is superheat. EPA's Type II topics list excessive superheat as a sign of leakage in high-pressure systems: with too little refrigerant, the evaporator boils off its charge early and the vapor keeps picking up heat.

Subcooling is how far a liquid's temperature is below its saturation temperature at that pressure. Read the saturation temperature for the liquid line pressure and subtract the actual liquid line temperature. Manufacturers give target superheat or subcooling values for charging. Follow the equipment's own charging method.

Remember where each one is measured. Superheat is a vapor-side reading, usually on the suction line. Subcooling is a liquid-side reading, on the liquid line leaving the condenser.

Low-pressure refrigerants such as R-11 and R-123 boil at around room temperature at atmospheric pressure; R-123 boils at roughly 82 °F. To boil at the low temperatures needed to chill water, they have to be below atmospheric pressure. So the evaporator of a low-pressure centrifugal chiller normally runs in a vacuum.

That turns leaks around. Instead of refrigerant leaking out, air and moisture leak in. A purge unit removes the noncondensables that build up, and EPA's Type III topics list excessive purging as a sign of leakage. Leak-testing a low-pressure system means raising its pressure, preferably with controlled hot water or a built-in heating or pressurization device, or with nitrogen (EPA: Test Topics).

The P-T relationship also explains the Type III charging rule. Liquid refrigerant entering a deep vacuum flashes and gets very cold, which can freeze the water in the tubes. That is why you introduce vapor first, until the pressure is high enough that the saturation temperature is above freezing, then switch to liquid.

A single refrigerant, or an azeotropic blend in the 500 series, boils at one temperature for a given pressure. A zeotropic blend in the 400 series does not. At a given pressure it starts boiling at the bubble point and finishes at the higher dew point. The spread between the two is temperature glide.

P-T charts for blends with real glide show two columns, bubble and dew. Use the dew point for superheat, since you are measuring vapor, and the bubble point for subcooling, since you are measuring liquid. Using the wrong column can throw a reading off by up to the amount of the glide.

Blends with glide can also fractionate. Their components can leak at different rates, changing the mix left in the system (EPA: Test Topics). This is one reason they are charged as liquid.

Several recovery practices on the test come straight from the P-T relationship. Heating the appliance or the vessel you are recovering from raises its pressure. Chilling the recovery cylinder lowers its pressure. Both speed up transfer. Recovering liquid first is faster because you move refrigerant without boiling it.

After reaching the required vacuum, wait a few minutes and watch the gauge. If the pressure rises, liquid refrigerant is still boiling off somewhere in the system or out of the oil, and you need to keep recovering (EPA: Test Topics). Heating oil in a low-pressure system to 130 °F before removing it drives refrigerant out of the oil for the same reason.