Pressure and Temperature Relationship

Why a pressure reading tells you a saturation temperature, and how technicians use that link every day.

Concept

Apprentice

#fundamentals#refrigeration#pressure#saturation

Safety

Refrigerant systems hold pressure even when off. Wear eye protection and gloves, and never vent refrigerant to atmosphere.

What it is

For any refrigerant that has both liquid and vapor present, pressure and temperature are locked together. Know one and the pressure temperature chart gives you the other. That saturation temperature is the temperature at which the refrigerant is boiling or condensing at that pressure.

Saturation

The condition where liquid and vapor exist together and the refrigerant is changing state. Adding heat boils more liquid instead of raising the temperature.

Why it matters

This relationship is what makes superheat and subcooling possible. You cannot put a thermometer inside the coil, but you can read the pressure and convert it to the boiling temperature happening inside. Comparing that to the actual line temperature tells you what the refrigerant is doing.

Suction pressure converted to saturation gives you the evaporator boiling temperature.
Discharge or liquid pressure converted to saturation gives you the condensing temperature.
The gap between saturation and measured line temperature is superheat or subcooling.

How it works

Raising pressure on a refrigerant raises the temperature at which it will boil, and lowering pressure lowers it. The compressor and the metering device exist to create and hold that pressure difference so the same refrigerant can absorb heat at a low temperature indoors and reject it at a high temperature outdoors.

Step 1 of 4

Compressor: Low pressure vapor is compressed into hot, high pressure discharge gas.

Vapor compression refrigeration cycle — heat is absorbed in the evaporator and rejected at the condenser

The metering device and compressor create the two pressure zones that make the temperature difference possible.

Blends behave differently

Zeotropic blends have a temperature glide, meaning the boiling and condensing temperatures are not the same number. Use the bubble point column for subcooling and the dew point column for superheat.

What can go wrong

Using the wrong refrigerant column on the chart or the wrong setting on a digital manifold.
Reading pressure at a service port far from the point of interest, so line losses skew the saturation value.
Assuming pressure alone diagnoses charge. Pressure is a symptom, superheat and subcooling are the measurement.

Refrigerant under pressure

Wear safety glasses and gloves when connecting hoses. Liquid refrigerant contacting skin can cause frostbite, and vapor displaces oxygen in confined spaces.

Where it is used

Charging, leak checking with a standing pressure test, evacuating to a target vacuum, and diagnosing restrictions all rely on reading pressure as a temperature.