The Refrigeration Cycle

How a vapor compression system moves heat from where it is not wanted to where it can be rejected.

Concept

Beginner

#refrigeration#cycle#fundamentals#pressure

Safety

Work on refrigeration systems only with proper certification, recovery equipment and personal protective equipment. Never vent refrigerant to atmosphere.

What it is

The refrigeration cycle is the process every air conditioner, heat pump, refrigerator and chiller uses to move heat. Nothing creates cold. A refrigerant is pushed through a repeating loop of pressure and temperature changes so that it absorbs heat in one place and releases it in another.

Vapor compression

A closed loop in which refrigerant boils at low pressure to absorb heat, is compressed to a higher pressure and temperature, and then condenses to release that heat.

Why it matters

Almost every cooling complaint, charging decision and compressor failure traces back to something happening inside this loop. Once you can picture where the refrigerant is boiling, where it is condensing, and what pressure it is under, readings stop being numbers and start being a description of the system.

How it works

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 four core components of the loop

1The compressor raises the pressure and temperature of refrigerant vapor leaving the evaporator.
2In the condenser, that hot high pressure vapor gives up heat to outdoor air or water and condenses into a liquid.
3The metering device drops the pressure sharply, which also drops the boiling point of the liquid refrigerant.
4In the evaporator, the low pressure refrigerant boils as it absorbs heat from indoor air, and the vapor returns to the compressor.

Technician tip

High side and low side are separated by exactly two points: the compressor and the metering device. Knowing that alone tells you which side a reading belongs to.

What can go wrong

Low charge or a restriction starves the evaporator, so superheat climbs and capacity falls.
Overcharge or a dirty condenser raises head pressure and reduces subcooling control.
Poor airflow across the evaporator lowers suction pressure and can flood liquid back to the compressor.
A failing metering device cannot hold the correct pressure drop, so feed becomes unstable.

How to diagnose it

Measure suction and liquid pressures and convert to saturation temperature

Establishes where the refrigerant is actually boiling and condensing, not just what the gauge face says.

Calculate superheat at the compressor inlet

Tells you how completely the evaporator is being fed.

Calculate subcooling at the liquid line

Tells you whether the condenser holds enough liquid before the metering device.

Check indoor and outdoor airflow

Airflow problems imitate charge problems and must be ruled out first.

Safety

Refrigerant handling

Refrigerant is handled under pressure and can cause frostbite and asphyxiation hazards. Recovery, charging and disposal are regulated activities that require proper certification, recovery equipment and eye and hand protection.