A refrigeration system is to be designed for a cooling capacity of 7.5 tons of refrigeration at saturation pressure corresponding to –200C. It uses refrigerant R-22. Condenser pressure is to be corresponding to saturation temperature of 400C. The refrigeration system uses a liquid-vapour regenerative heat exchanger. The refrigerant vapours coming out of the evaporator are superheated by 50C and then pass through the regenerative heat exchanger to cool the liquid refrigerant coming from the condenser. The temperature of the refrigerant vapours at the exit of heat exchanger is 200 C. The liquid refrigerant is subcooled to 360C at the exit of the condenser. Make a sketch of the system and represent it on P-h and T-S diagrams.The system uses two single-acting cylinders with bore to stroke ratio of 0.8, speed of compressor is 1420 rpm, clearance factor for compressors is 0.04 and polytropic index of compression is 1.1 Mechanical efficiency of the compressor is 0.8. Determine:
(i) temperature of liquid refrigerant at the exit of regenerative heat exchanger, assuming specific heat of the liquid to be 1.37 kJ/kg K.
(ii) mass flow rate of the refrigerant.
(iii) the temperature of the refrigerant vapours at the exit of compressor, assuming ideal gas behaviour in the superheat region, specific heat of refrigerant vapours in the superheat region may be taken as 0.85 kJ/kg K.
(iv) Power input to the compressor.
(v) COP of the system.
(vi) Volumetric efficiency of the compressor, and
(vii) Bore and stroke of the compressor.
The following properties of R - 22 are given:
|
Sat. temp
ts(°C)
|
Sat pressure
Ps (bar)
|
Sp. Vol. of
Sat. vapour
(m3 / kg)
|
Enthalpy (kJ/kg)
|
|
|
|
|
Sat. Liquid
hf
|
Sat. vapour
hg
|
|
–20
|
2.448
|
15.335
|
177.4
|
397.5
|
|
40
|
15.335
|
0.0151
|
249.08
|
415.95
|
For superheated refrigerant vapour at 2.448 bar and saturation temperature of –200C
|
Temperature
0C
|
Specific volume
(m3 / kg)
|
Enthalpy
kJ / kg
|
|
–15
|
0.0951
|
400.7
|
|
20
|
0.1107
|
423.9
|
(ESE-ME-11) (20 M)