In HVAC systems, liquid and vapor will exist at the same time and place. We call that condition saturation, or we say that the refrigerant is “at saturation.” Phase changes occur in the evaporator and condenser, so these are spots where liquid and vapor coexist while the system is running.

Saturated conditions occur whenever liquid and vapor occupy the same closed space. Liquid and vapor obey pressure rules when they inhabit the same area in a closed system. These closed systems can be inside HVAC units or tanks and are static (still) when in tanks or when the system is off, and they are dynamic (moving) when the system is running.

When the liquid and vapor exist at the same place at a given temperature in a closed system, they have a known pressure. We call this the pressure-temperature (P-T) relationship. This relationship will exist as long as you have at least a droplet of liquid in a closed system.

However, the refrigerant must be at its saturation point. Saturation can be confusing, so this article will explain saturation and how a P-T chart fits into the concept. It’ll also teach you how to use your P-T chart to determine superheat and subcooling.

Saturation

When something is saturated, it’s full of something else. For example, clothes become saturated with water in a washing machine.

In physics, liquids at saturation are “full” of kinetic energy. When this happens, they’ve reached their boiling point. The “boiling point” can be a somewhat misleading term, though.

Liquids at saturation have reached their boiling point, but they don’t have to boil to evaporate. Temperature is only a measure of average molecular activity. Some individual molecules have a lot more kinetic energy than others. These molecules will escape into the air without boiling. That’s why puddles don’t have to boil for water to evaporate from them.

Boiling occurs only when the vapor pressure and atmospheric pressure are the same. Most refrigerants have a high vapor pressure and will boil easily. Whenever boiling occurs within a closed system, the gas molecules increase the pressure inside the vessel. Gas molecules are far apart from each other and move quickly, and boiling increases the amount of them. The pressure increases when more of those molecules zoom around the closed space.

At some point, the vessel’s pressure will exceed the liquid’s vapor pressure. Boiling will stop when this happens. When boiling stops within a closed system, the temperature and pressure stop increasing.

The refrigerant will reach equilibrium. Molecules evaporate and condense at an even rate at a constant temperature and pressure. When that happens, the refrigerant remains at its saturation point. At saturation, you can use the P-T relationship to predict temperature or pressure.

P-T charts

The P-T chart is a vital yet often overlooked tool. P-T charts use the pressure-temperature relationship to help you determine the refrigerant’s pressure at a given saturation temperature.

The table’s top usually lists common refrigerants, and the left side lists saturation temperatures. The rest of the table has the saturation pressures for each refrigerant at the given saturation temperatures.

You can use this chart to determine the pressure when you read a temperature or vice versa. Refrigerants exist in vapor and liquid states simultaneously in the evaporator and condenser. The coils add or remove heat, which allows phase changes to occur. Before a phase change can take place, the refrigerant must reach saturation.

Remember, this chart is only accurate when liquid and vapor are present at the same time and place. The refrigerant has to be at a certain temperature and pressure because it exists in both gas and liquid phases within a closed system.

Keep in mind that many of us won't use the chart itself very often. We will use apps like Danfoss RefTools or MeasureQuick to give us P-T data, or we will simply look at our gauge, which will have a P-T chart for various common refrigerants printed right onto the gauge face. If the gauge above were connected to an R410a system, we would see the pressure is about 134 PSIG, which points at about 46°F on the R410a (pink) temperature scale printed on the face. If it were R22, the green scale would show us 75°F for the very same pressure.

Superheat and subcooling

The temperature deviates from the P-T relationship outside the evaporator and condenser. In these cases, superheating or subcooling has occurred.

Superheated vapor is hotter than the saturation temperature. The vapor/suction line should contain superheated vapor. Otherwise, vapor-liquid mixtures in that line may indicate flood back. Subcooled liquid is cooler than the saturation temperature, and it should be limited to the end of the condenser and the liquid line.

You can determine the superheat or subcooling by finding the difference between the sensible and saturated temperatures at a given pressure. That’s where your P-T card or P-T app comes in handy.

You’ll find saturation temperatures inside the evaporator and condenser coils. You can take sensible temperatures anywhere in the liquid or vapor lines.

To determine the superheat in the vapor/suction line, locate a specific point on the line. This point can be the coil outlet or anywhere else between the evaporator and the compressor depending on the purpose of the measurement. Take a sensible temperature measurement of the line and pressure reading. Find the pressure on your P-T card, gauge, or app and look for the corresponding saturated temperature. Find the difference between the measured sensible and saturated temperatures. The temperature increase from saturated to sensible is the superheat.

The same principle applies to subcooled liquid in the liquid line. Take a sensible temperature and pressure reading on the liquid line. Find the pressure on your P-T card, gauge, or app and locate the corresponding saturated temperature. The difference is the subcooling amount, and it will always be a lower measured line temperature than saturation when the refrigerant is fully liquid.

The P-T relationship makes your job a lot easier. Still, it only exists under specific conditions. It’s a good idea to understand those conditions fully. That way, you can use the P-T chart to help you determine superheat and subcooling conditions as well as evaporator and condensing temperatures. It can also help us identify what type of refrigerant we have in a tank or if that refrigerant may be cross-contaminated.

This knowledge is one of the basic building blocks of the refrigerant circuit. It always helps to start with a solid understanding of the P-T relationship and saturation.

P.S. – Here's another article on saturation in the refrigeration circuit.