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Simple Vapour Compression System

Fig. 12.1 shows a simple vapour compression refrigeration system which consists of

• compression • condensation • Expansion • Vaporisation.


(a)  Compression : The vapour at low pressure and low temperature (point 2) enters to compressor where it is compressed reversible adiabatically (isentropically). The pressure and temperature increase and pass through discharge line (point 3).


(b)  Condensation : The vapour at high pressure and temperature enters to condenser where it is condensed to high pressure liquid (point 4) and is collected by receiver tank.


(c)   Expansion : The liquid from receiver tank passes through the expansion valve where it is throttled (isoenthalpy) and passed at a controlled rate after reducing its pressure and temperature.


(d)  Vaporisation: In evaporator the liquid at low pressure and temperature evaporates and is changed into vapour. During vaporisation the liquid absorbs latent heat of vaporisation from the medium (air, water or brine) which is to be cooled.

Vapour compression cycle on T-S and P-h is shown in Fig. 12.2 and Fig. 12.3 respectively.





12.2.1 Coefficient -of Performance of Refrigerator for Vapour Compression Cycle


Coefficient of Performance of refrigerator for vapour compression cycle is given by


COP = h2 – h1 / h3 – h2 ( COP = heat rejected evaporator / work input to compressor)


12.2.2 The Quantity of Refrigerant (m r) Circulated in the Refrigeration Plant


The Quantity of refrigerant is given by

m r = 210 T / h2 – h1 kg / min

where T = Load in tons of refrigeration on the refrigerator


12.2.3 Power Required to Drive the Compressor

Power = m r (h3 – h2) / 60 KW

Where h is in KJ / kg


12.2.4 Quantity of Cooling Water (me) Circulated in the Condenser


Quantity of cooling water circulated the condenser per minute can be found out from the

equation m, Cc (t0 – t;) = m, (h3 – h4).

where              mc = mass of cooling water circulated per minute

Cc = specific heat of cooling water

t0 =outlet temperature of cooling water

t1 = inlet temperature of cooling water.