Types of GHP
The familiar kitchen refrigerator is perhaps the simplest example of a heat pump and uses all the basic principles described above to move heat out of its cold box. It has four main components
Compressor, which increases the pressure of the refrigerant vapor, pushing it through the system, and increasing the vapor's temperature above that of the surrounding kitchen.
Condenser, usually behind the refrigerator, where the refrigerant vapor condenses to a liquid.
Expansion valve, which causes a sudden drop in refrigerant pressure, causing it to boil; also called a "metering" valve, since it passes only as much liquid as can be completely vaporized in the evaporator.
Typical pressure and temperature conditions for such a refrigerator are illustrated in Figure
For kitchen refrigerators, the compressor typically operates at a compression ratio in the range of 4 to 5, depending on what refrigerant is used. For example, if R134a (tetra-fluoroethane) is used, an evaporator pressure of 1.6 atm is required to achieve a vaporizing temperature of 5°F, while a condenser pressure of 7.6 atm is required to achieve a condensing temperature of 85°F. On the other hand, if R717 (ammonia) is used, the required evaporator and condenser pressures for the same phase change temperatures are 2.3 atm and 11.5 atm, respectively.
The refrigerant's vaporizing temperature (and associated pressure in the evaporator) is chosen such that it is significantly colder than the target temperature of the compartment to be refrigerated. This creates a temperature gradient for spontaneous heat flow to the evaporator, vaporizing the refrigerant, which thereby gains latent heat from the cold compartment. Likewise, the condensing temperature (and asociated pressure in the condenser) is chosen such that it is warmer than room temperature. This creates a temperature gradient for spontaneous heat flow away from the condenser, causing the refrigerant to liquify and thereby lose the latent heat that it had gained from the cold compartment. In this way, heat is pumped out of the refrigerator.