Rhoss Air cooled screw chillers on a white background showing the outer and internal parts.

Refrigerant is an essential part of most HVAC systems in both commercial and domestic settings. 

Refrigerant is responsible for the movement of heat from one set of coils to another; this is the process called heat exchange. In a heat pump this transfer of heat is used warm a building – the refrigerant absorbs heat from the outdoor coils and releases it to the indoor coils. In cooling systems, such as air conditioning, the refrigerant transfers heat out of a building.

What is the refrigerant cycle?

The refrigerant cycle is the process of the refrigerant moving round the heat pump or air conditioning system. It consists of four main components as shown in the diagram below:

  • Compressor
  • Condenser
  • Metering device
  • Condenser 

The diagram shows how the refrigerant moves around – from the top, travelling in clockwise direction:



Compressor -> Discharge line -> Condenser -> Liquid Line -> Metering Device -> Expansion Line -> Evaporator -> Suction line -> and then back to the Compressor.




The refrigeration cycle described works for all air conditioning units and chillers. For heat pumps, when in heating mode, the cycle is reversed and the refrigerant circles the other way around in the system – the condenser becomes the evaporator and the evaporator becomes the condenser.

The different parts of the refrigerant cycle, explained:

What is a compressor?

The compressor is the only mechanical component in a basic refrigeration system. Without the compressor to move the refrigerant around, the system wouldn’t work.

How does the compressor work?

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser, and liquid line) and low pressure on the low side (suction line, evaporator, and expansion line).

There are many different types of compressors – scroll and reciprocating-type compressors are the most commonly used.

  • Reciprocating compressor: A reciprocating compressor uses pistons, similar to a car engine.
  • Scroll compressor: Scroll compressors use an oscillating movement to compress and pressurise the low-pressure vapour into high-pressure vapour. This also causes the temperature of the gas to increase and the volume to decrease.

The compressor relies on good lubrication and the cool suction gas from the evaporator to reduce the temperature properly; a precise balance is required to keep the compressor from being flooded while ensuring it doesn’t overheat.

What is a condenser?

Condensers, or the outdoor unit, reject heat from the refrigerant changing its state from liquid into vapour.

The condenser has three critical functions:

  1. To de-superheat the refrigerant (reduce the temperature to the condensing temperature)
  2. To condense (saturate) the refrigerant (reject heat until all the refrigerant turns to liquid)
  3. To sub-cool the refrigerant (reduce the temperature of the refrigerant below the condensing/saturation temperature).

The condenser’s important job is to reduce the temperature of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid so it can boil in the evaporator.

How does the condenser work?

The hot vapour from the compressor enters the condenser, and the superheat (temperature above condensing temperature) is then removed at which point the refrigerant begins to change state from vapour to liquid (condensing – the refrigerant maintains a constant temperature until every molecule of vapour is condensed).

The temperature of the liquid then falls again; this is known as sub-cooling (when measuring sub-cooling, we measure the temperature rejected once the refrigerant has turned completely to liquid).

So, when something is fully vapourised (like the air around us), it will be superheated, and when it is fully liquid (like the water in a glass), it is sub-cooled.

What is the metering device?

The metering device creates a pressure drop to facilitate refrigerant boiling in the evaporator coil. There are different types of metering devices such as thermostatic expansion valves (TEVs) or electronic expansion valves (EEVs).

The metering device is located between the liquid-line and the evaporator, where the liquid-line is full of high-pressure liquid refrigerant.

How does the metering device work?

When the high-pressure liquid reaches the small piston in the metering device, the pressure reduces to such a degree that the saturation temperature is lower than the temperature of the air surrounding the refrigerant tubing. At this point, the refrigerant starts to change from liquid to vapour – so called ‘boiling’ or ‘flashing’.

The ‘boiling / flashing’ process brings the refrigerant down from the liquid-line temperature to the boiling (saturation) temperature in the evaporator. In this process, a percentage of the refrigerant is immediately changed from a liquid state into vapour.

What is the evaporator?

The evaporator is also known as the cooling coil. The purpose of the evaporator is to absorb heat from a room by changing the refrigerant from liquid to vapour (boiling).

How does the evaporator work?

The boiling process starts as soon as the refrigerant leaves the metering device and continues until the refrigerant has absorbed enough heat to complete the transition.

As long as the refrigerant is boiling, it will remain at a constant temperature – referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is finished boiling, the temperature starts to rise – superheat.

When the indoor air temperature or the airflow going over the coil is higher, the evaporator pressure and temperature will also be higher due to the increased heat being absorbed into the coil. Conversely, when the air temperature or airflow over the coil is lower, it will have lower pressure and temperature. The refrigerant leaves the evaporator, travels down the suction line, and heads back to the compressor, where the cycle starts all over again.


We hope you find the above information helpful. If you want to find out more, or discuss your heating or cooling needs, please contact us.

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