There are four main processes in a vapor

compression refrigeration cycle.

(i) Compression

(ii) Condensation

(iii) Expansion

(iv) Evaporation

It is most commonly used & the oldest refrigeration cycle. It is a single stage refrigeration cycle.


Clockwise sequence of components is compressor,  condenser,  expansion valve, evaporator and compressor . It can be condenser, expansion valve, evaporator, compressor and condenser

Fig. Vapor Compression Refrigeration Cycle


Fig. Simple vapor comp refrigeration cycle on p-h and T-s charts

Starting from the compression process

Compressor compresses the low temperature & low pressure vapor refrigerant coming from the evaporator. This compression makes the refrigerant as high temperature high pressure superheated vapor.


The high pressure and high temperature vapor refrigerant passes through the condenser. Outside cooling media air or water condense it. Condensation results high pressure high temperature liquid refrigerant. Refrigerant loses latent heat to the cooling media.


The high pressure high temperature liquid refrigerant enters the expansion valve and reduces its temperature and pressure. Some of the liquid evaporates passing through the expansion valve. Greater amount of liquid refrigerant at low temperature low pressure remains after the expansion valve.


Low pressure low temperature liquid refrigerant evaporates in the evaporator. It takes latent heat from the cooled space. Evaporator converts the refrigerant into low pressure low temperature vapor. Compressor sucks it and compresses the vapors. The cycle repeats giving continues cooling effect.

Refrigerant changes phase twice in the cycle.  Refrigerant condenses  from vapor to liquid in the condenser. It rejects latent heat to the surrounding cooling media. It absorbs its latent heat from the space cooled around the evaporator. The refrigerant changes from liquid to vapor phase.


  1. Theoretical vapor compression cycle

Line diagram            Cycle on p-h    Cycle on T-s

Dry saturated vapor enters the compressor( point 1)

isentropic compression (point 1- 2)

saturated liquid comes out of condenser(point 3)

throttling expansion (point 3- 4) in the expansion valve

Details as per Fig.

  1. 1-2 process is isentropic compression
  2. 2-2′ process  is de-super-heating at constant pressure in the condenser
  3. 2′-3 process is condensation
  4. Process 3-4 is throttling expansion at constant enthalpy
  5. 4-1  evaporation in the evaporator


2. Standard refrigeration cycle

Line diagram     Cycle on p-h    Cycle on T-s

5C super-heated vapor enters the compressor

isentropic compression

5C sub-cooled liquid comes out the condenser

throttling in the expansion valve

Details as per Fig.

1′–1 super-heat of 5C in the evaporator at constant pressure,

1–2 isentropic compression

2-2′ is de-super-heating at constant pressure in the condenser

2′-3 is condensation at constant pressure and constant temperature

3–3′  sub-cooling of 5C in the condenser at constant pressure

3–4 isenthalpic expansion( Throttling) in the expansion valve

4-1 is the evaporation at constant pressure and constant temperature

3. Actual refrigeration cycle

Variable degree of super-heat

variable degree of sub-cooling

Poly-tropic compression

Pressure drops in the


suction pipe,

suction valve

discharge pipe 


Liquid line

Liquid vapor line

leakages across the suction valve, discharge valve and the piston

Details as per Fig.

Line diagram    Cycle on p-h  Cycle on T-s

Variations of actual refrigeration and theoretical cycle

(i) Variable degree of super-heat in the evaporator  1′- A at constant pressure

(ii) Pressure drops in the evaporator AB

(iii) BC superheat in suction pipe

(iv) CD=Pressure drop in suction pipe 2E

( v  ) DE  Pressure drop across suction valve

( vi ) E- 1 Superheat at entry to compressor cylinder

(vii) 1-2 Polytropic compression

(viii) Pressure drop across discharge valve 2–F

(ix) Surrounding air causes de-super heating, FG, in the discharge pipe

(x) GH Pressure drop in the discharge pipe

(xi)H- 2′ De-super-heating in the condenser

(xii) Change of vapor to liquid  2′ –3′

(xiii) Sub-cooling in the condenser  3′ 3″

(xiv) 3″- 3Pressure drop in the condenser

( xv  ) 34 Pressure drop across the expansion valve

( xvi )4- 1 evaporation in the evaporator

Types of Vapor Compression Refrigeration 

 (a) High temperature refrigeration produces temperatures between 2 to 18C

         Used in comfort air conditioning, cooling of cold drinks and cooling of drinking water

(b) Medium temperature refrigeration produces temperatures between -2 to 5C

        Used in space of  fridges, cold storage’s

(c) Low temperature refrigeration produces temperatures between -5 to –30C

          Used in fridges in freezer section,  ice cream manufacture and ice manufacture               Equipments VCR SYSTEM        MCQ Vapor Comp Ref