Vapour Absorption Refrigerator
In vapour absorption system (Figure ) the heat energy is added externally. This addition may be from an external source such as boiler or heating coil or gas flame. It is this energy, which makes the low temperature, low-pressure vapour into high temperature, high-pressure vapour.
The advantage of vapour absorption system over the vapour compression system is reduced noise due to the absence of compressor.
The widely used vapour absorption refrigeration system is the “Aqua-ammonia refrigeration system.” Ammonia is the refrigerant in the system, The following are the basic parts of a simple vapour absorption refrigerator.
- Expansion valve
The strong solution of aqua-ammonia received from the absorber through heat exchanger is heated by some external source such as gas flame or steam or electric heating coil. During the heating process, ammonia in the aqua-ammonia solution vapourises at high pressure leaving behind the hot weak ammonia solution (mostly water and traces of ammonia) in the generator.
The weak ammonia solution flow to the absorber through pressure reducing valve and heat exchanger.
The high-pressure ammonia vapour from the generator flows to the condenser. Cold water circulates outside the condenser. The ammonia vapour gets cooled by the circulating cool water and condenses to high-pressure ammonia liquid. It then flows to the expansion valve.
3) Expansion Valve
The expansion that takes place in the Expansion valve is throttling. The liquid refrigerant passes through the expansion valve (which is also known as throttle valve) and is throttled to a lower pressure and temperature. After throttling, the medium has little vapour and maximum liquid.
Evaporator is the actual freezing unit (usually the cooling section in the refrigerator). The ammonia refrigerant passing through the evaporator absorbs the heat (i… heat from the food stuff or any material kept for cooling) and evaporates. Then the low pressure vapour ammonia passes to the absorber.
The absorber receives low-pressure ammonia vapour from the evaporator and weak ammonia solution (mostly water) from the generator. The purpose of the absorber is to make this mixture into a strong solution, which is pumped back to the generator.
Water has the ability to absorb very large quantities of ammonia vapour and the solution thus formed is known as “aqua-ammonia” (strong solution). The absorption of ammonia vapour in water lowers the pressure in the absorber, which in-turn draws more ammonia from the evaporator and thus raises the temperature of solution.
The outer surface of the absorber is constantly cooled by cooling arrangement (usually air or water cooling), this arrangement is used in the absorber to remove the heat of solution evolved there. This Cooling is necessary in order to increase the absorption capacity of water, because at higher temperature water absorbs less ammonia vapour.
The strong solution thus formed in the absorber is pumped to the generator by the liquid pump. The pump increases the pressure of the solution (normally 10 to 20 bar). This completes the simple vapour absorption cycle.
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See More: Vapour Compression Refrigeration System