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.