In vapour absorption refrigeration system the the compressor is replaced by-
c) Generator/solar heat/ waste heat
AQUA AMMONIA VARS
These components perform the same function as that of a compressor in VCRS. In the vapour absorption system, the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by weak solution of the refrigerant forming a strong solution. This strong solution is pumped to the generator where it is heated by some external source. During the heating process ,the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquified. The liquid refrigerant then flows into the evaporator and thus the cycle is completed.
The C.O.P. of a vapour absorption system is given by
C.O.P. = Te/Tg(Tg-Tc/Tc-Te)
Where, Tg= Temperature at which heat is given to the generator
Tc= Temperature at which heat is discharged to the cooling water from the condenser and absorber
Te= Temperature at which heat is absorbed in the evaporator
Domestic Electrolux(Ammonia Hydrogen) Refrigerator
The main of domestic electrolux refrigerator is to eliminate the pump so that in the absence of moving parts the machine becomes noise less. This type of refrigerator is also called three fluids absorption system. The three fluids used in this system are ammonia, hydrogen and water. The ammonia is used as a refrigerant because it possesses most of the desirable properties. The hydrogen being the lightest gas,is used to increase the rate of evaporation of the liquid ammonia passing through the evaporator. The hydrogen is also non corrosive and insoluble in water. The water is used as a solvent because it has the ability to absorb ammonia readily.
The C.O.P. of this type of refrigerator is given by the ratio of heat absorbed in the evaporator to the heat supplied in the generator.
Lithium Bromide Absorption Refrigeration system
The lithium Bromide absorption refrigeration system uses a solution of lithium bromide in water. In this system the water is being used as a refrigerant whereas lithium bromide, which is a highly hygroscopic salt,used as an absorbent. The lithium bromide solution has a strong affinity for water vapour because of its very low vapour pressure. Since lithium bromide solution is corrosive, therefore inhibitors should be added in order to protect the metal parts of the system against corrosion. This system is very popular for air conditioning in which low refrigeration temperature (not below 0°C) are required.
NOTE- Since water is used as a refrigerant in this system,hence the refrigeration temperature must be kept above the freezing point of water.
The refrigerant is a heat carrying medium which absorb heat from a low temperature system and discard the heat so absorbed to a higher temperature system during their cycle in the refrigeration system.
Properties of a refrigerant :
A good refrigerant should have following properties.
1.) Low boiling point
2.) High critical temperature
3.) High latent heat of vaporisation
4.) Low specific heat of liquid
5.) Low specific volume of vapour
6.) Non corrosive to metal
7.) Non flammable and non toxic and non explosive
8.) Easy to liquify at moderate pressure and temperature
9.) Easy of locating leaks by odour or suitable indicator
10.) Mixes well with oil
Mostly used refrigerants :
1.) R- 11(CCl3F) ; It is stable, non flammable and non toxic. It is considered to be a low pressure refrigerant. It has a low side pressure of 0.202 bar at -15°C and high side pressure of 1.2606 bar at 30°C. The latent heat at -15°C is 195 kj/kg. The boiling point at atmospheric pressure is 23.77°C. Due to its low operating pressures, this refrigerant is exclusively used in large centrifugal compressor systems of 200TR and above. The leaks may be detected by using a soap solution,a halide torch or by using a electronic detector. The cylinder colour for this refrigerant is orange.
2.) R-12(CCl2F) ; It is a very popular refrigerant. It is a colourless and almost odourless liquid with boiling point of -29°C at atmospheric pressure. It is non toxic, non corrosive, non irritating and non flammable. It has relatively low latent heat value which is an advantage in small refrigerating machines. The refrigerant is used in many different types of industrial and commercial applications such as refrigerators, freezers, water coolers, room and window air conditioning units.
3.) R-22(CHClF2); It has also been successfully used in air conditioning units and in household refrigerators. It is used with reciprocating and centrifugal compressors. It is not necessary to use R-22 at below atmospheric pressure in order to obtain the low temperatures.
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