What is the principle of vapour compression system?


In this type, the vapors condense or evaporate within the working ranga of temperature of the refrigeration systems. Here the working agents undergo changes of state and heat changes are in the form of latent heat.


A schematic diagram of vapor compression system :

The basic components required to make up a refrigeration system are :

  1. A refrigerant
  2. An evaporant
  3. A compressor
  4. A condenser
  5. A liquid metering device.

Another component often used is a separate receiver used to store refrigerant.

A drier is also installed in the liquid line to absorb moisture present in the liquid refrigerant. So that moisture does not enter the narrow cross-section of the expansion valve, which will prevent the choking of the expansion valve.

When these components are functioning properly, the refrigeration system operates at two definite pressure levels such as.

Highpressure side

It includes the following :

(a) The discharge side of the compressor

(b) Delivery line

(c) Condenser

(d) Receiver (optional)

(e) Liquid line up to the metering device.

The high-pressure side is also called head pressure or discharge pressure.

Lowpressure side

It includes the following :

(a) Expansion valve

(b) Evaporator

(c) Suction line up to the compressor.

The low-pressure side is also called back pressure or suction pressure.

Principle of working

The compressor compresses the refrigerant vapors coming out from the evaporator to high pressure and high temperature. These vapors pass through the condenser where vapors are cooled. The commonly used cooling mediums in the condenser are air or water. The refrigerant rejects its heat to the cooling medium and is condensed.

The high-pressure liquid refrigerant next passes through the liquid metering device (expansion valve). Due to expansion, the high-pressure liquid refrigerant turns into low-pressure, low-temperature liquid.

The liquid refrigerant then flows through the evaporator where it absorbs heat from the refrigerant space. The refrigerant evaporates and changes its state from liquid to vapor. Thus the refrigerant space is cooled as it has rejected heat to the refrigerant.

The compressor sucks the vapor refrigerant, which is once again compressed, and the cycle is repeated.

Dry compression vs wet compression

The compression process starting with saturated vapor is called dry compression, if the compression starting with wet vapor state is called wet compression. Dry compression is always preferred to wet compression because the wet compression occurs in the mixture region with droplets of the liquid present which has certain disadvantages in it.

The disadvantages of wet compression

  1. Liquid refrigerant may be tapped in the head of the cylinder and possibly may damage the valves or cylinder head and may lead to bursting of compressor.
  2. The droplets of the liquid may wash out the lubricating oil from the walls of the cylinder thus accelerating wear.


The vapor used as the working agent in the reversed Carnot cycle with slight modifications is called the vapor compression cycle. The basic operations involved in a vapor compression refrigeration cycle are illustrated in the flow.

The corresponding property diagrams such as :

  1. P-V diagram
  2. T-s diagram
  3. P-h diagram are represented.

Process 1-2 – A reversible adiabatic (isentropic) process (dry compression) where entropy remains unchanged.

Process 2-3 – A reversible constant pressure process (condensation) where the refrigerant rejects its heat to the cooling medium

Process 3-4 – An adiabatic throttling process (expansion) where enthalpy remains unchanged.

Process 4-1 – A reversible constant pressure process (evaporation) where the refrigerant absorbs heat from the space.

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