1. Applied Thermodynamics and Heat Engines
S.Y. B.Tech. (Production)
Assignment - IV
1. (i) State the elements of a refrigeration system.
(ii) What is a standard rating of a refrigerating machine ?
2. Derive the expression for COP of an air refrigeration system based on Bell – Coleman
cycle.
3. In an aircraft cooling system, air enters the compressor at 0.1 MPa, 4 °C and is
compressed to 0.3 MPa with an isentropic efficiency of 72 %. After being cooled to 55 °C
at constant pressure in a heat exchanger, it then expands in a turbine to 0.1 MPa with an
isentropic efficiency of 78 %. The low temperature air absorbs the cooling load of 3
tonnes of refrigeration at constant pressure before re-entering the compressor, which is
driven by the turbine. Assuming air to be an ideal gas, determine the COP of the
refrigerator, the driving power required and the air mass flow rate.
(ANS : COP = 0.2, Power = 48.6 kW, mair = 0.66 kg/sec)
4. Compare the ‘Vapour Compression System’ and ‘Air Refrigeration System’.
5. A refrigerating plant works between temperature limits of -5 °C and 25 °C. The working
fluid ammonia has a dryness fraction of 0.62 at the entry to the compressor. If the
machine has a relative efficiency of 55 %, calculate the amount of ice formed during the
period of 24 hours. The ice is to be formed at 0 °C from water at 15 °C. and 6.4 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 NH3 :
Temp Liquid Heat Latent Heat Liquid Entropy
(°C) (kJ/kg) (kJ/kg) (kJ/kg)
25 298.9 1167.1 1.124
-5 158.2 1280.8 0.630
(ANS : mice = 1896.1 kg / 24 hr)
6. Name the desirable properties of an ideal refrigerant.