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As stated earlier, water thrown over a tent in the hot desert sun will cause cooling. Evaporation of a liquid needs heat energy. The latent heat required for the evaporation to take place is produced by the sensible heat from the tent interior which, in turn causes cooling of the tent.
If you dip your fingers in a volatile liquid like gasoline or alcohol and then allowed the liquid to evaporate, you can feel a cooling effect. The same thing will happen with water but, will not be as noticeable, as the evaporation process is slower. The cooling effect, again, comes from the heat being removed from your fingers to evaporate the liquid.
Large, modern refrigeration units use the evaporation principle to produce the low temperatures necessary to do the job required. The process consists of a cycle of compression, cooling and condensation, then the expansion of the liquid, evaporation and re-compression of the vapour.
THE VAPOUR COMPRESSION REFRIGERATION CYCLE
The principles used in a vapour compression refrigeration system are:
Compression of a gas causes its temperature to increase. When the gas is cooled and sensible and latent heat removed, the temperature decreases and the gas condenses to liquid which is also the boiling point of the liquid. (The compression also increases the temperature at which the liquid boils). The liquid is then further cooled to around atmospheric temperature.
When the liquid is expanded (volume increased) into a lower pressure system, it will boil and cause the liquid temperature to decrease rapidly as it gives up sensible heat to provide the latent heat of partial vaporization of the liquid. The cold liquid and vapour, (the latent heat does not increase the vapour temperature), now pass through the coils inside the ' Cold Box ' (or Evaporator) of the system.
Exchange of heat between the refrigerant and the material or space being cooled, adds more heat to the refrigerant liquid which continues to evaporate. The refrigerant, on leaving the cooling system is now all cool vapour and passes to the suction of the compressor to begin the cycle again.
The sequence of the refrigeration cycle is as follows:
- Compression of the refrigerant gas.
- Cooling and condensation of the refrigerant to liquid.
- Expansion and partial evaporation of the liquid into a lower pressure which causes cooling.
- Continued evaporation of the liquid in the 'cold box' - further heating by removal of sensible heat from the item being cooled, to provide the latent heat of vaporization of the refrigerant.
- Re-compression of the vapour to begin the cycle again.
Figure: 7, shows a simple block diagram of the refrigeration thermo-dynamic cycle.
The type of compressor used in refrigeration systems may be reciprocating or centrifugal in operation. Large industrial units may have multi-stage compression systems with inter-stage cooling, in order to achieve the required refrigerant pressure without excessive temperature increases.
The cooling / condensing unit of a system can consist of cooling by natural convection - as in a household fridge, or by forced air cooling - i.e. a fan or fans to force the air over the cooling coils (similar to a car radiator), or by water cooled heat exchange equipment.
(Shown in Figure: 8).
Control of a refrigeration unit can be by a thermostatic system which will start and stop the compressor - (a bi-metal strip switch or mercury switch), or, in large units, by control of the expansion valve.
When the unit shuts down, the expansion valve will also close by activation of a solenoid valve.
Following are some typical uses of refrigeration in industry and everyday life.
- To Reduce the Rate of Chemical Reactions - and Storage of Food and Other
- To Store Flammable Materials
- To Condense the Vapour of Low Boiling Point Liquids
- For Air Conditioning
- For Freeze Drying of Materials
- For Separation and Recovery of Process Fluids
- To Produce Cryogenic Temperatures