Phase change heat pump
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A phase change heat pump employs a liquid with a low boiling point to transfer heat from cooler space to a warmer space; generally in a refrigeration application. It is the most common heat pump used in domestic refrigerators, and heat pumps are also used in air conditioning systems to transfer heat from the outside of a building to the inside or vice versa depending upon a valve position.
It employs a liquid, known as a refrigerant which has a low boiling point. The liquid requires energy (called latent heat) to evaporate, and it drains that energy from its surroundings in the form of heat (in the same way that sweating cools the body). When the vapour condenses again, it releases the energy, again in the form of heat.
The pump operates a cycle where the refrigerant repeatedly changes state from liquid to vapor and back to liquid, the process being known as a refrigeration cycle. The refrigerant is condensed to release heat in one part of the cycle and is boiled (or evaporated) to absorb heat in another part of the cycle.
Since the 1930s, the refrigerant has typically been the chlorofluorocarbon (CFC) known as R-12 (dichlorodifluoromethane), one in a class of several refrigerants using the brand name Freon, a trademark of DuPont. Its manufacture was discontinued in 1995 due to the damage that it does to the ozone layer if released into the atmosphere. The widely-adopted replacement refrigerant is the hydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Other substances such as liquid ammonia, or occasionally the less corrosive but flammable propane or butane, can also be used.
The most common form of phase change heat pump uses an electric motor to drive a mechanical compressor. The compressor does not create a cooling effect directly. The cooling effect is created when the refrigerant boils and absorbs heat from the cooled space through a heat exchanger. The cycle can be divided into two parts — the liquifaction stage and the evaporation stage:
Liquifaction stage
The first part of the cycle causes refrigerant vapor to be recycled into its liquid form by extracting heat from a comparatively high temperature vapor. The compressor compresses a relatively low-pressure and low-temperature refrigerant vapor drawn from the evaporator coil. During compression, the refrigerant vapor is heated by compression itself (PV = nRT) and the work of compression to create a high-temperature and high-pressure vapor. Then the vapor is pushed into a heat exchanger known as a condenser located in a higher temperature heat sink that is located outside of the space being cooled. In the condenser, heat is removed from the refrigerant so that it condenses to a liquid state.
Evaporation stage
The second part of the cycle begins after the liquid refrigerant leaves the condenser as a high temperature, high-pressure liquid and passes through a refrigerant metering device (different types are known as a capillary tube, fixed orifice, thermostatic expansion valve, and an expansion valve). The valve allows a specific quantity of liquid coolant to pass into the evaporation chamber. Evaporation chambers are relatively low pressure and this encourages coolant evaporation. Newly evaporated coolant is drawn though the cooling coils (typically a fan is used to blow air over the coils). Thus, the evaporative process produces the cooling effect refrigerators are designed for. The refrigerant then is pulled to the compressor in the suction line where it will be compressed into a high temperature, high pressure gas and sent to the external heat sinking coils.
Performance
How much heat can be pumped depends on the temperature difference between the cold area and the hot area. This is important in air conditioning applications since in very cold weather the machine stops working when the outdoor part, the condenser, freezes. In these conditions a simple electric heater works better unless the phase change heat pump is replaced by a more suitable type such as a gas compression device.
The four essential components of the mechanical refrigeration cycle for a phase change heat pump are the compressor, the condenser, the refrigerant metering device and the evaporator. These four components must be selected or matched for the application and to each other in order for the system to work well and efficiently. None of these parts produce a refrigeration effect. Boiling (or rapidly evaporating) refrigerant absorbs heat and creates the benefit of refrigeration. The refrigeration cycle allows a small amount of refrigerant to be cycled and recycled for decades of use.
See also
External links
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