The Vapour Compression Cycle

This consists of 4 primary components:


This is a heat exchanger which takes low temperature heat out of the source (ground or air) and transfers it to the circulating refrigerant. The refrigerant is forced by an expansion valve and the suction effect of the compressor to change from a liquid to a gas within the evaporator. This normally requires much heat* (e.g. Boiling off water to its vapour form) and therefore the evaporator becomes very cold drawing in the required heat from its surroundings (usually air or water).

*Latent Heat of Evaporation is much greater than sensible heat capacity.


is designed so that high pressure hot gas gives up its heat and turns to liquid — it condenses. This is the reverse of the process of evaporation so the latent heat of evaporation must be released (or rejected) and the condenser gets very warm. This heat is transferred into the water or air circulating around the heating system.


This compresses the low temperature, low pressure gas into a high pressure gas, which causes it to increase in temperature to a high temperature gas (like the bicycle pump example). It creates the low (suction) and high (delivery) pressures which enables phase change (liquid-gas-liquid) to occur at different, more favourable temperatures.

Expansion Valve:

The high pressure refrigerant liquid from the condenser travels through the expansion valve, which acts as a pressure differential valve (high pressure one side low pressure the other), turning it into a low pressure, and hence low temperature liquid very near its boiling point (by reducing pressure — similar to lower boiling point of water up Mount Everest).

The process is then repeated as a continual cycle.

Because change of state is happening i.e. Latent heat of evaporation large quantities of heat can be transferred, larger than just by temperature change alone and this is enabled by changing the pressure of the refrigerant.

The following diagram shows a bit more detail and uses the example of an Air-to-Air heat pump (Source-Delivery medium). Vapour compression heat pumps use a ‘refrigerant’ to transfer heat and his hence similar to an air conditioning unit.

This diagram also illustrates how easy it is to utilise the key components but to reverse the process i.e. to pump heat out rather than in, thus becoming an ‘air conditioning’ unit (more correctly called a cooling unit).

You can clearly see that all the main components are the same with just the flow reversed (via the 4 way reversing valves) and some additional minor components (e.g. check valves and specific expansion valve).

This leads some to question “Is a heat pump unit just an air conditioning unit in reverse?”
(see FAQs)