Methods of Refrigeration: Vapor Compression Cycle

In the cyclic process of refrigeration the heat is removed from the low temperature reservoir and is thrown to high temperature reservoir. As per the second law of thermodynamics the natural flow of heat is from the high temperature reservoir to low temperature reservoir. In the cyclic refrigeration process since the flow of heat is reversed, the external work has to be done on the system. The cyclic process of refrigeration is also reverse of the thermodynamic power cycle or Carnot cycle in which the heat flows from high temperature reservoir to low temperature reservoir; hence the cycle of refrigeration is also called as Reverse Carnot Cycle.

There are two types of cyclic process of refrigeration: vapor cycle and gas cycle. The vapor cycle is classified into vapor compression cycle and vapor absorption cycle. Let us see all these processes one-by-one.

The vapor compression cycle is the mostly widely used method of refrigeration in the modern applications. Your household refrigerator, water cooler, deep freezer, air-conditioner etc, all run on vapor compression cycle. The cycle is called as vapor compression cycle, because the vapors of refrigerant are compressed in the compressor of the refrigerator system to develop the cooling effect.

Here are the various processes of vapor compression cycle (refer the figure give below):

1) Compression: The vapors of refrigerant enter the compressor and get compressed to high pressure and high temperature. During this process the entropy of the refrigerant ideally remains constant and it leaves in superheated state.

2) Condensation: The superheated refrigerant then enters the condenser where it is cooled either by air or water due to which its temperature reduces, but pressure remains constant and it gets converted into liquid state.

3) Expansion: The liquid refrigerant then enters the expansion valve or throttling valve where sudden expansion of the refrigerant occurs, due to which its temperature and pressure falls down. The refrigerant leaves expansion valve in partially liquid state and partially in gaseous state.

4) Evaporation or cooling: The partially liquid and partially gaseous refrigerant at very low temperature enters the evaporator where the substance to be cooled is kept. It is here where the refrigeration effect is produced. The refrigerant absorbs the heat from the substance to be cooled and gets converted into vapor state.

This low pressure refrigerant is then absorbed by the compressor where it is compressed again and the whole cycle of refrigeration repeats again. The vapor compression cyclic process is used for refrigeration in domestic as well as commercial applications.

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