In this article, we learn what is refrigeration, the applications of refrigeration, vapour compression refrigeration system, and household refrigerator.
What is Refrigeration?
Refrigeration is defined as the branch of science that deals with the process of reducing and maintaining the temperature of that space or material below the temperature of its surroundings.
The system maintained at lower temperatures is called a refrigerated system and the equipment used to produce this is called a refrigerator.
The cooling effect produced by refrigerators is termed as refrigerating effect and the working substance used to produce this effect is known as refrigerant.
The heat withdrawn from the refrigerated space is rejected to the atmosphere which acts as a natural heat reservoir.
The heat transfer from the system to the surroundings is reduced by providing insulating materials.
Applications of Refrigeration
Refrigeration can be used in the following areas:
Formation of ice.
Preservation of foodstuffs, fruits, vegetables, etc. in the storage area or transportation.
Preservation of medicines and blood.
Cooling of liquids in chemical process plants.
Preservation of photographic films, archeological documents, etc.
Providing a comfortable environment under difficult working conditions.
Liquefaction of gases like oxygen, nitrogen, hydrogen, etc.
It is used for processing textiles, processing work, photographic material, etc.
Processing of farm crops,
For pre-cooling and pasteurization of milk and in preparation of butter.
Methods of Refrigeration
The refrigerating effect can be produced by using the following methods:
1. Ice refrigeration
2. Steam jet refrigeration
3. Refrigeration by air expansion
4. Vapour absorption cycle
5. Vapour compression cycle
Out of all these methods, the vapor compression cycle is commonly used for applications like refrigerators, deep freezers, and air-conditioners.
Unit of refrigeration
The capacity of the refrigeration system is given in Tons of Refrigeration (T.R).
A ton of refrigeration is defined as the quantity of heat required to be removed, to form one ton of ice at 0 °C in 24 hours when the initial condition of the water is 0 °C because the same cooling effect will be obtained by melting the same ice.
S.I. unit of 1 ton of refrigeration is,
1 TR = 12660 kJ/hr = 211 kJ/min or
1 TR = 3.517 kJ/sec = 3.517 kW
1 TR = 3.517 kW
TR actually measures the rate of heat transfer.
Coefficient of performance (COP)
COP of the refrigerator is defined as the amount of heat extracted from the refrigerator to work supplied.
QA = Amount of heat extracted in refrigerator
Wnet = Net work supplied to the system
COP = QA/Wnet
Vapour Compression Refrigeration System
Vapour compression refrigeration system is the most commonly used method of refrigeration for refrigerators, air-conditioners, etc.
In this system, the liquid refrigerant boils in the evaporator which at low pressure, by absorbing latent heat.
The vapours formed are condensed in the condenser which is at high pressure by rejecting latent heat.
Thus, in this cycle heat is transferred in the form of sensible heat as well as latent heat which gives higher COP.
The figure shows the simple vapour compression refrigeration system.
In this system, commonly used refrigerants are NH3, R-11, R-12, and R-22. In modern refrigerators, Freon-22 and 134-a refrigerants are used.
A simple vapour compression refrigeration system consists of four different processes.
4. Evaporation (Vaporization)
During the suction stroke of the compressor, low-pressure vapour in the dry state is drawn from the evaporator. Here, the temperature and pressure of vapour increase until the vapour temperature is greater than the condenser temperature.
During condensation, high-pressure refrigerant vapour enters the condenser where the cooling medium absorbs the heat and converts the vapour into liquid.
After condensation, the liquid refrigerant is stored in the receiver, and from the receiver, it is passed to the evaporator through an expansion or throttle valve. This valve reduces the pressure by keeping the enthalpy constant (Throtting process).
4. Evaporation (Vapourization)
After expansion, the low-pressure liquid refrigerant enters an evaporator where a considerable amount of heat is absorbed by it and converted into vapour. This low-pressure vapour is sucked by the compressor and the cycle repeats.
A household refrigerator is the application of vapour compression refrigeration system
The figure shows the arrangement of household refrigerators.
A household refrigerator consists of the following main elements:
4. Capillary tube
In household refrigerators generally, a hermetically sealed type of reciprocating compressor is used. It is mounted on the rear bottom side of the refrigerator. The compressor is driven by an electric motor.
In household refrigerators, the condenser is placed on the rear side. The heat transfer takes place through the circulation of air naturally. It is made of tubing of copper.
In the receiver, the refrigerant is stored under high pressure.
4. Capillary tube
In household refrigerators, the capillary tube will acts as a throttle valve where the expansion of refrigerant takes place.
Household refrigerators are commonly equipped with two evaporators. Out of them, one is used for frozen food and the other for regular refrigerating temperatures. But nowadays new models of refrigerators provide much more storage capacity using the same outside dimensions as the older type refrigerators, Modern refrigerators also provide more effective and thinner insulation. Evaporator coils are generally made of aluminum or stainless steel.
The working of household refrigerators is similar to vapour compression refrigeration systems as discussed in the previous section.