When inputting power, the compressor starts running, the whole system operating in circle. Actually, it is the refrigerant circulated through the heat pump using a compressor to drive the process.
1. The refrigerant enters the compressor, in a gaseous (saturated vapor) state at lower pressure and lower temperature and exits at higher pressure and higher temperature, in a superheated gaseous state.
2. The refrigerant then passes through the hot side heat exchanger, and in so doing changes state into a liquid.
3. The refrigerant then passes through an expansion valve, which forces the liquid refrigerant to flash into a gas and liquid mixture, at a pressure and temperature both lower than before entering the valve. This mixture then passes through the cold side heat exchanger, during which the refrigerant completely converts into a gas.
4. The associated latent heat of vaporization (due to phase change of the liquid portion of the mixture into a gas) is absorbed by the heat exchanger from whatever medium the heat exchanger is in contact with. From this stage the refrigerant then enters the compressor as a saturated vapor, and the cycle repeats.
Four Main Parts of A Heat Pump System