774 ENGINEERING THERMODYNAMICS
dharm
\M-therm\Th14-4.pm5
- State merits and demerits of ‘Vapour compression system’ over ‘Air refrigeration system’.
- State the functions of the following parts of a simple vapour compression system :
(i) Compressor, (ii) Condenser,
(iii) Expansion valve, and (iv) Evaporator. - Show the vapour compression cycle on ‘Temperature-Entropy’ (T-s) diagram for the following cases :
(i) When the vapour is dry and saturated at the end of compression.
(ii) When the vapour is superheated after compression.
(iii) When the vapour is wet after compression. - What is the difference between ‘Wet compression’ and ‘Dry compression’?
- Write a short note on ‘Pressure Enthalpy (p-h) chart’.
- Show the simple vapour compression cycle on a p-h chart.
- Discuss the effect of the following on the performance of a vapour compression system :
(i) Effect of suction pressure (ii) Effect of delivery pressure
(iii) Effect of superheating (iv) Effect of sub-cooling of liquid
(v) Effect of suction temperature and condenser temperature. - Show with the help of diagrams, the difference between theoretical and actual vapour compression cycles.
- Define the terms ‘Volumetric efficiency’ and ‘Clearance volumetric efficiency’.
- Derive an expression for ‘Clearance volumetric efficiency’.
- Explain briefly the term ‘Total volumetric efficiency’.
- Explain briefly simple vapour absorption system.
- Give the comparison between a vapour compression system and a vapour absorption system.
Unsolved Examples
- The co-efficient of performance of a Carnot refrigerator, when it extracts 8350 kJ/min from a heat source,
is 5. Find power required to run the compressor. [Ans. 27.83 kW] - A reversed cycle has refrigerating C.O.P. of 4,
(i) Determine the ratio T 1 /T 2 ; and
(ii) If this cycle is used as heat pump, determine the C.O.P. and heat delivered.
[Ans. (i) 1.25 (ii) 50 kW, 5] - An ice plant produces 10 tonnes of ice per day at 0°C, using water at room temperature of 20°C. Estimate
the power rating of the compressor motor if the C.O.P. of the plant is 2.5 and overall electromechanical
efficiency is 0.9.
Take latent heat of freezing for water = 335 kJ/kg
Specific heat of water = 4.18 kJ/kg. [Ans. 21.44 kW] - An air refrigeration system operating on Bell Coleman cycle, takes in air from cold room at 268 K and
compresses it from 1.0 bar to 5.5 bar. The index of compression being 1.25. The compressed air is cooled to
300 K. The ambient temperature is 20°C. Air expands in an expander where the index of expansion is 1.35.
Calculate : (i) C.O.P. of the system (ii) Quantity of air circulated per minute for production of 1500 kg of ice
per day at 0°C from water at 20°C. (iii) Capacity of the plant in terms of kJ/s.
Take cp = 4.18 kJ/kg K for water, cp = 1.005 kJ/kg K for air
Latent heat of ice = 335 kJ/kg. [Ans. 1.974 ; 5.814 kg/min ; 7.27 kJ/s] - The temperature in a refrigerator coil is 267 K and that in the condenser coil is 295 K. Assuming that the
machine operates on the reversed Carnot cycle, calculate :
(i) C.O.P.(ref.)
(ii) The refrigerating effect per kW of input work.
(iii) The heat rejected to the condenser. [Ans. (i) 9.54 (ii) 9.54 kW (iii) 10.54 kW]