Q: Explain the working principle of the Bell Coleman air refrigeration system. Also, find the coefficient of performance.
Working principle of the Bell Coleman air refrigeration system :
The Bell Coleman air refrigeration system is based on a reverse Joule cycle.
- The components of the Bell Coleman air refrigeration system are shown in figure (a). In this system, the air is taken into the compressor from the atmosphere and compressed.
- The hot compressed air is cooled in a heat exchanger up to the atmospheric temperature (in ideal conditions). The cooled air is then expanded in an expander.
- The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion.
- The low-temperature air coming out from the expander enters into the evaporator and absorbs the heat. The cycle is repeated again.
- The working of the air refrigeration cycle is represented in figure (b).
Process (1‐2): It represents the suction of air into the compressor.
Process (2‐3): It represents the isentropic compression of air by the compressor.
Process (3‐5): It represents the discharge of high-pressure air from the compressor into the heat exchanger. The reduction in the volume of air from V3 to V5 is due to the cooling of air in the heat exchanger.
Process (5‐6): It represents the isentropic expansion of air in the expander.
Process (6‐2): It represents the absorption of heat from the evaporator at constant pressure.
- The compression and expansion processes are reversible adiabatic processes.
- There is a perfect inter‐cooling in the heat exchanger.
- There are no pressure losses in the system.
COP = Net refrigeration effect/ Net work supplied
Work done per kg of air for the isentropic compression process 2‐3 is given by,
Wc = C p (T3 – T2 )
Work developed per kg of air for the isentropic expansion process 5‐6 is given by,
We = C p (T5 –T6 )
Net work required :
Wnet = (Wc – We ) = C p (T3 – T2 ) – C p (T5 – T6 )
The net refrigerating effect per kg of air is given by,
Qadd = C p (T2 – T6 )