With expansion valve it forms a tight unit. The condition of the refrigerant supplied to the evaporator by the expansion valve starts, vapor from the liquid. This is needed because the refrigerant injected needs a lot of heat to evaporate, which is achieved by passing the warm air of the passenger compartment through a fan on the evaporator heat exchanger plates. Meanwhile, the air transfers the heat to the evaporator and the heat is taken up by the refrigerant to evaporate, while the air temperature drops considerably, bringing the cooled air back into the passenger compartment. Steam coolant from the evaporator is sucked in, compressed by the compressor, raising its pressure and temperature. This process is repeated until the passenger compartment temperature has been set to the desired value. To ensure that this process is always carried out under optimum controlled conditions, system monitoring temperature and pressure sensors and control elements are required. These elements are the thermostat, the pressure switches and the quantity and temperature control elements of the expansion valve.