Question
What is refrigerator? Draw its block diagram. Discuss its theory.
Solution
Refrigerator is a device used to cool the bodies to a temperature lower than the temperature of their surrounding.
According to second law of thermodynamics, it is impossible to have a purely automated machine that can transfer the heat energy from a body at low temperature to a body at high temperature.
Therefore, in order to cool a cold body, it is necessary to expend some energy.
The refrigeration involves the use of a working substance (called refrigerant) going about a cycle. In te process, there is the removal of heat from a cold body.
Work is done on the body and a large amount of heat is rejected to the hot body.
The above diagram shows the block diagram of a refrigerator.
Let the refrigerant extract Q2 heat from cold body.
Let W be the work required to be done on the refrigerant to reject this heat to hot body and Q1 be the net heat rejected to hot body.
As the refrigerant undergoes a cycle, there is no change in internal energy.
Using first law of thermodynamics,
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Thus the heat rejected to hot body is greater than the heat absorbed from cold body.
The best quality of refrigerator is one which extracts maximum heat and expends minimum work to reject it.
The quality of refrigerator is measured in terms of coefficient of performance and is defined as the ratio of the heat removed from the cold body to the work spent in rejecting the heat to hot body.
Mathematically, coefficient of performance is given by,
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∴
According to second law of thermodynamics, it is impossible to have a purely automated machine that can transfer the heat energy from a body at low temperature to a body at high temperature.
Therefore, in order to cool a cold body, it is necessary to expend some energy.
The refrigeration involves the use of a working substance (called refrigerant) going about a cycle. In te process, there is the removal of heat from a cold body.
Work is done on the body and a large amount of heat is rejected to the hot body.
The above diagram shows the block diagram of a refrigerator.
Let the refrigerant extract Q2 heat from cold body.
Let W be the work required to be done on the refrigerant to reject this heat to hot body and Q1 be the net heat rejected to hot body.
As the refrigerant undergoes a cycle, there is no change in internal energy.
Using first law of thermodynamics,
Thus the heat rejected to hot body is greater than the heat absorbed from cold body.
The best quality of refrigerator is one which extracts maximum heat and expends minimum work to reject it.
The quality of refrigerator is measured in terms of coefficient of performance and is defined as the ratio of the heat removed from the cold body to the work spent in rejecting the heat to hot body.
Mathematically, coefficient of performance is given by,
For ideal case,
∴
