A TECHNICAL EXPLANATION...A heat pump uses a compression refrigeration cycle to transfer heat. The temperature at which the refrigerant vaporizes or condenses is controlled by regulating the pressure in the different parts of the system. A low pressure is maintained in the evaporator, so the refrigerant can vaporize at low temperatures.
Conversely, the condenser is maintained at a high pressure so that the vapor is forced to condense at relatively high temperatures. The compressor compresses the refrigerant vapor and adds heat of compression.
The hot vapor (under high pressure) flows from compressor to condenser, where the air or water to be heated passes over and absorbs heat, thereby cooling the refrigerant vapor. As this happens, the vapor condenses to a liquid and gives up its latent heat of condensation. The warm liquid refrigerant (still under high pressure) flows from condenser to a metering valve which controls the flow of liquid refrigerant. The downstream side of this device is under low pressure, being connected through the evaporator to the suction side of the compressor.
The liquid passing through the metering device begins to evaporate under low pressure as it enters the evaporator heat exchanger. The temperature of the liquid drops as it releases the latent heat of vaporization to the refrigerant vapor. The cold fluid in the evaporator absorbs heat from the source, and the liquid evaporates. The cool vapor from the evaporator is drawn to the suction side of the compressor where it is compressed and the cycle is repeated.
Most heat pumps have a reversing valve which allows them to cool as well as heat the building. This valve changes the flow of the fluid such that the coil in the building becomes the evaporator and the outdoor coil becomes the condenser.
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