Phase change heat pump

From Academic Kids

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 liquid has typically been freon (CFC), but its use has been discontinued because of damage that it does to the ozone layer if released into the atmosphere. The liquid now used is usually R-134a (1,1,1,2-tetrafluoroethane), but 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:

Liquefaction 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=RT) 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 relatively warm, high-pressure liquid and passes through a refrigerant metering device (also referred to as an expansion valve) into the cooling coil or evaporator on the low-pressure side of the system. The compressor pumps the refrigerant out of the evaporator at a rate sufficient to cause the pressure and temperature of the refrigerant to drop well below its boiling point as it moves through an interior heat exchanger coil known as the evaporator. This boiling liquid refrigerant absorbs heat energy from the interior space through the walls of this evaporator. The system is designed to completely evaporate liquid refrigerant into a low-pressure vapor within the interior coil before it returns to the compressor to repeat the cycle.

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 refigeration cycle allows a small amount of refrigerant to be cycled and recycled for decades of use.

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.

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