When is a refrigerator a heater? Making machines more efficient.

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When is a refrigerator a heater? Making machines more efficient.

Praveen Ravi

G 11

Physics essay

A refrigerator is a cooling device meant to keep substances at temperatures lower than the outside temperature, as low as 2 to 3 °C. A refrigerator works on the basic principle of heat transfer like a heat engine. While a heat engine takes in heat from a higher temperature, expends some of it for doing work and rejects the remaining as exhaust at a much lower temperature, a refrigerator does quite the converse. It takes in heat from a body making it cold, has work done upon it and rejects the heat at a much higher temperature (Fig.1; Appendix). A heat pump as the name suggests, is a device which can pump heat in or out from an enclosure like a room. It is very similar to that of a refrigerator, the only difference being that it takes in heat from a cold reservoir like the outside air and passes it to a room at a higher, suitable temperature. It can also operate vice versa by introducing a valve where the process of cooling or heating can be adjusted. Theoretically, a refrigerator should be able to work like a heat pump. However, it has its own possibilities and limitations which shall be discussed in this essay.

A schematic diagram of a refrigerator is shown in the Appendix (Fig.2). The working of a refrigerator comprises of four major steps namely - evaporation, compression, condensation and expansion. A chemical called a refrigerant is used in this process to remove the heat from the refrigerator. Refrigerants must have certain specific properties - they must be easily liquefiable, they must have low specific heat capacities, they must be non-toxic, non-flammable and non reactive with foodstuff etc. The first refrigerants used were ammonia, ether and chemogene (a mixture of petrol ether and naphtha). However, nearly all were toxic, flammable and reactive. Thomas Midgley made a new species of compounds from fluorine called Chloro-Fluoro Carbons (CFC)1. They were non-toxic, non flammable and removed large quantities of heat from the refrigerator and were ideal refrigerants.

The refrigerant is contained in the refrigerant tank in a cold liquid state. First, it goes to the evaporator. The evaporator is a snake like pipe which is in close contact with the refrigerator cabin. The refrigerant absorbs this heat cooling the cabin. It uses it as latent heat and changes from cold liquid to cold vapour without any change in temperature. Thus, the evaporation is isothermal.
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The cold vapour then goes to the compressor. The compressor is of a big volume with a piston attached to it. Electrical energy is used to move the piston. The cold vapour is compressed suddenly increasing the pressure and decreasing the volume rapidly. Because of the sudden compression, there is no time for the heat generated in the vapour to flow out and temperature increases. Thus, this process is adiabatic. Since work is done on the gas, W < 0. The cold vapour now changes into very hot vapour.

The hot vapour then flows into the condenser. ...

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