07-11-2012, 04:41 PM
REFRIGERATION USING WASTE HEAT IN CARS
REFRIGERATION.doc (Size: 38 KB / Downloads: 94)
ABSTRACT:
Quite often, people’s own desire for comfort increases the burden on environment, the use of private cars and air conditioning units in them are an example of this. These add comfort to the people, increasing the burden on environment. In this project we have tried to give positive life cycle environmental impact by reducing the emission of exhaust gas. This reduction is achieved using vapor absorption cycle instead of vapor compression cycle and by using refrigeration motor run by waste heat that is normally used.
At present vapor compression system is widely being used in automobiles for refrigeration purpose. The major disadvantages of this system are
• Compressor runs by the power from engine crankshaft, thus reducing the efficiency of the engine
• Use of refrigerant increases the cost and the environmental impact
Being a salt, lithium bromide does not exert any pressure and does not boil off when heated. So the refrigerant water vapor boiled off from the absorbent solution in the generator leaves as pure refrigerant vapor. Thus there is no need of rectifier to separate out the absorbent vapor from the refrigerant vapor, as required in water- ammonia absorption cycle.
Lithium bromide salt when mixed with water forms the solution. As the amount of the salt mixed in the solution increases, the solution is said to be stronger and with a reduction of salt content the solution becomes weaker. The concentration of the solution is denoted by the amount of salt by weight expressed as percent of the solution weight. The concentration of the solution does not change with temperature, but its specific gravity does.
The concentration of a solution does not change with temperature, its concentration comes down. This is because the weight of the total solution has increased on absorbing water, but salt content has not changed. A stronger solution can absorb more water vapor than a solution of lower concentration. The rate of absorption of water vapor comes down as temperature of the solution increases.
The refrigerant used in the absorption system is highly soluble in absorbent. The evaporator and absorber are inter-connected.so the refrigerant vapor formed in the evaporator (on the evaporation of the liquid refrigerator) is absorbed by the absorbent in the absorber due to the strong affinity of the absorbent for the refrigerant, thereby maintaining the refrigerant vapor pressure in the evaporator at the low level required for continuous vaporization of liquid refrigerator to obtain refrigeration. To recover the refrigerant from the absorbent it is pumped from the absorber to the generator, where it is heated using steam or hot water.
On getting heated the absorbent releases the refrigerant vapor as a high temperature/pressure vapor. It passes to the comparatively cooler condenser where it is condensed. The liquid refrigerant then passes on to the evaporator thus completing the refrigerant cycle. The absorbent flows back from the generator to the absorber. Heat is generated when the refrigerant is absorbed by the absorbent-known as the heat of absorption or heat of dilution. Further the refrigerant vapor condenses in the absorbent solution and for this the latent heat of vaporization of the refrigerant vapor has to be removed. Therefore the absorber also needs cooling and for this cooling medium (air or water) used for the condenser is first passed through absorber and then to the condenser. Thus the heat rejection of an absorption system is higher and so requires higher flow rate of the coolant (air or water) than the mechanical system. The cooling tower capacity for an absorption machine therefore should be much more than that of a mechanical vapor-compression system of the system capacity.