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The diagram shows refrigerant fluid flowing through a set of components in a circle starting at a point on the left side labelled ‘Expansion valve’. The refrigerant flows in an anticlockwise direction. The refrigerant as a liquid passes from the valve downwards into the area labelled ‘1’, where the liquid, coloured blue, is labelled ‘Cool liquid’ and then flows in a pipe in the direction of small arrows to the right into a unit labelled: ‘Evaporator’ and ‘Low pressure’. A large upward-pointing blue arrow at the bottom of the diagram labelled ‘Heat source’ points towards the evaporator. From the evaporator the fluid now labelled ‘Cold vapour’ then passes to the right into an area labelled ‘2’ and upwards into a unit labelled ‘Compressor’. A small leftward-pointing arrow at the right of the diagram labelled ‘Electricity’ points towards the compressor. From this the heated fluid is pumped upwards into the area labelled ‘3’ following the direction of small arrows. The fluid, now labelled ‘Hot vapour’ and coloured red passes to the left into a unit labelled: ‘Condenser’ and ‘High pressure’. Here, a large red upward-pointing arrow labelled ‘Heat Sink’ shows heat flowing away from the condenser to cool the fluid. At the left of the condenser the fluid, now labelled ‘Warm liquid’, passes into an area labelled ‘4’ and then on back to the expansion valve to repeat the cycle.

 3.3.1 Heat pumps