HVAC Fundamentals: Heat Pumps
A heat pump is a refrigeration device used to move heat from one room or space to another. The heat pump is designed to take heat from a medium-temperature source, such as outdoor air, and transform it to higher-temperature heat for dispensing within a structure. By method of a specifically designed reversing valve, the pump can also extract heat from the indoor air and expel it outdoors.
Because a heat pump system uses the reverse-cycle rule of operation, its operating rule is sometimes referred to as reverse cycle conditioning or reverse-cycle refrigeration. The latter term is not correct because there are basic differences between the operating principles of a heat pump and a true refrigeration unit. The confusion probably stems from the fact that during the cooling cycle, the operation of a pump is identical to that of the mechanical refrigeration cycle in a packaged air conditioning unit. The indoor wire function as an evaporator, cooling the indoor air. The outdoor wire is condenser, in which the hot refrigerant gas releases heat to the outside air.
Heat Pump Operating Principles
The two principal phases of heat pump operation are the heating and cooling cycles. A third phase, the defrost cycle is used to protect the coils from excessive frost buildup.
The heating cycle of a heat pump begins with the circulation of a refrigerant by the outdoor coils. Initially, the refrigerant is in a low-pressure, low-temperature liquid state but it soon absorbs enough heat from the outdoor air to raise its temperature to the boiling point. Upon reaching the boiling point, the refrigerant changes into a hot vapor or gas. This gas is then compressed by the compressor and circulated under high pressure and temperature by the indoor coils, where it comes into contact with the cooler room air that circulates around the coils. The cooler air causes the gas to cool, condense and return to the liquid state. The condensation of the refrigerant vapor releases heat to the interior of the structure. After the refrigerant has returned to a liquid state, it passes by a special pressure-reducing device and back by the coils where the heating cycle starts all over again.
In the cooling cycle, the reverse valve causes he flow of the refrigerant to be reversed. As a consequence, the compressor pumps the refrigerant in the opposite direction so that the coils that heat the building or space in cold weather cool it in warm weather. In other words, the heat is extracted from the interior, cycled by the heat pump and then expelled outside the building or space during the condensation of refrigerant.
Because the outdoor air is comparatively cool when the heat pump is on the heating cycle, and the outdoor wire is acting as an evaporator, frost forms on the surface of the wire under certain conditioners of temperature and relative humidity. Because this inner of frost on the coils interferes with the efficient operation of the heat pump, it must be removed. This is achieved by putting the pump by a defrost cycle.