776 ENGINEERING THERMODYNAMICS
dharm
\M-therm\Th14-4.pm5
The ice is to be formed at 0°C from water at 20°C and 6 kg of ammonia is circulated per minute. Specific
heat of water is 4.187 kJ/kg and latent heat of ice is 335 kJ/kg.
Properties of a ammonia (datum – 40°C) :
Temp. Liquid heat Latent heat Entropy of liquid
K kJ/kg kJ/kg kJ/kg°C
298 298.9 1167.1 1.124
268 158.2 1280.8 0.630
[Ans. 1640.5 kg]- A food storage locker requires a refrigeration system of 2500 kJ/min capacity at an evaporator temperature
of – 10°C and a condenser temperature of 30°C. The refrigerant used is Freon-12 and sub-cooled by 5°C
before entering the expansion valve and vapour is superheated by 6°C before leaving the evaporator coil.
The compression of refrigerant is reversible adiabatic. The refrigeration compressor is two-cylinder single-
acting with stroke equal to 1.3 times the bore and operates at 975 r.p.m. Determine (using thermodynamic
tables of properties for Freon-12) :
(i) Refrigerating effect per kg.
(ii) Mass of refrigerant to be circulated per minute.
(iii) Theoretical piston displacement per minute.
(iv) Theoretical power required to run the compressor, in kW.
(v) Heat removed through the condenser per minute.
(vi) Theoretical bore and stroke of compressor.
Properties of Freon-12
Saturation Absolute Specific Enthalpy Entropy
temp. pressure volume of
°C vapour Liquid Vapour Liquid Vapour
m^3 /kg kJ/kg kJ/kg kJ/kg K kJ/kg K
- 10°C 2.19 0.0767 26.9 183.2 0.1080 0.7020
30°C 7.45 0.0235 64.6 199.6 0.2399 0.6854
Take : Liquid specific heat = 1.235 kJ/kg K
Vapour specific heat = 0.735 kJ/kg K.
[Ans. (i) 129.17 kJ/kg (ii) 19.355 kg/min (iii) 1.518 m^3 /min (iv) 7.2 kW
(v) 2931 kJ/min (vi) 91 mm, 118 mm]
- A vapour compression refrigerator uses methyl chloride and works in the pressure range of 11.9 bar and
5.67 bar. At the beginning of the compression, the refrigerant is 0.96 dry and at the end of isentropic
compression, it has a temperature of 55°C. The refrigerant liquid leaving the condenser is saturated. If the
mass flow of refrigerant is 1.8 kg/min. Determine :
(i) Co-efficient of performance.
(ii) The rise in temperature of condenser cooling water if the water flow rate is 16 kg/min.
(iii) The ice produced in the evaporator in kg/hour from water at 15°C and ice at 0°C.
Properties of methyl chloride :
Saturation Pressure Enthalpy (kJ/kg) Entropy (kJ/kg K)
temp. (bar)
(°C) hf hg sf sg
- 20 11.9 30.1 455.2 0.124 1.803
25 5.67 100.5 476.8 0.379 1.642