We have read about heat engines that convert heat energy into work and its application in various fields of thermodynamics. Can we think of a device that is the opposite of a heat engine, i.e. a device that helps in the conversion of work into heat? In this section, we will learn about refrigerator and heat pump that works on the opposite principle of a heat engine.
In the refrigeration cycle, there are five basic components: a fluid refrigerant; a compressor, a condenser coil, an evaporator coil and an expansion device. The compressor constricts the refrigerant vapour, raising its pressure, and pushes it into the coils on the outside of the refrigerator. When the hot gas in the coils meets the cooler air temperature of the kitchen, it becomes a liquid. Now, in the liquid form at high pressure, the refrigerant cools down as it flows into the coils inside the freezer and the fridge. The refrigerant absorbs the heat inside the fridge, cooling down the air. Lastly, the refrigerant evaporates and then flows back to the compressor, where the cycle repeats itself.
- Separation of gases: Separation of air into its constituents by fractional distillation as different components of air liquefies at different temperatures.
- Condensation of gases: In industries, gases such as ammonia are condensed before storage and shipment.
- Dehumidification of air: Air is dehumidified by liquefying and separating the moisture present in it.
- Cooling for preservation: Vegetables, organic-chemical and explosives are kept in cold storages for preservation.
A heat pump is a mechanical compression cycle that can be reversed to either heat or cool a controlled space.
A typical heat pump consists of two parts: an indoor unit called an air handler and an outdoor unit similar to an air output unit. A compressor circulates a refrigerant that absorbs and releases heat as it travels between these two units. Here, the working fluid or the refrigerant (in its gaseous state) is pressurized by a compressor and circulated through the system. The process of compression makes the fluid hotter. The hot and pressurized vapour, on the discharge side of the compressor, is cooled in a heat exchanger called a condenser, until it condenses into a high pressure, moderate temperature liquid. The pressure of the condensed fluid is reduced using a pressure lowering devices such as a capillary tube or an expansion valve. The temperature of the low-pressure liquid is increased in a heat exchanger after which the refrigerant is made to return to the compressor and the cycle is repeated.
- Space heating: Heat pump is used to heat an enclosed area such as a workspace, greenhouses and houses.
- Water heating: Water in industries and household is heated using heat recovered from other reactions using the heat pump.
- Process heating: In industries, the heat pump is used to heat the process fluid before the reactions.
- Heat recovery: Heat pump is used to recover process heat from other reactions.
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