The document discusses air refrigeration systems using the Bell-Coleman cycle, which utilizes air as a refrigerant, negating atmospheric damage associated with traditional refrigerants. It outlines the cycle's processes, including isentropic compression, constant pressure cooling, isentropic expansion, and constant pressure expansion. Additionally, it details the coefficient of performance calculations and an example applied to an aircraft cooling system.

1. AIR REFRIGERATION BY
USING BELL-COLEMAN
CYCLE /REVERSED
BRAYTON CYCLE.

2. Introduction
•
Air refrigeration system generally uses air as medium, whereas
other refrigeration systems use refrigerants (Freon’s, ammonia
etc.,) as medium. Since air is used as refrigerant no damage to
atmosphere is done. By using other refrigerants damage to
atmosphere such as ozone layer depletion takes place.
• Other refrigeration systems have high COP but have severe
impact on atmosphere. Moreover these refrigerants are too
expensive and handling of these refrigerants is difficult.

3. Bell-Coleman cycle refrigeration system
Components of Bell-Coleman cycle refrigeration system:
• Air Compressor
• Heat Exchanger
• Turbine
• Evaporator (Cabin)

4. Working of Bell-Coleman cycle
1. Isentropic compression process
2. Constant pressure cooling process
3. Isentropic expansion process
4. Constant pressure expansion process

5. Working of Bell-Coleman cycle
1. Isentropic compression process: The cold air from the
refrigerator or atmosphere is drawn into the compressor cylinder
where it is compressed isentropically in the compressor as shown
by the curve 1-2 on p-v and T-s diagrams. During the compression
stroke, both the pressure and temperature increases and the
specific volume of air at delivery from compressor reduce from v1
to v2. We know that during isentropic compression process, no
heat is absorbed or rejected by the air.
2. Constant pressure cooling process: The warm air from the
compressor is now passed into the cooler where it is cooled at
constant pressure P3 (equal to P2), reducing the temperature from
T2 to T3 (the temperature of cooling water) as shown by the curve
2-3 on p-v and T-s diagrams. The specific volume also reduces
from v2 to v3.

6. 3. Isentropic expansion process: The air from the cooler is now drawn
into the expander cylinder where it is expanded isentropically from
pressure P3 to the refrigerator pressure P4 which is equal to the
atmospheric pressure. The temperature of the air during expansion
falls from T3 to T4 shown by the curve 3-4 on p-v and T-s diagrams.
The specific volume of air at entry to the refrigerator increases from
v3 to v4. We know that during isentropic expansion of air, no head is
absorbed or rejected by the air.
4. Constant pressure expansion process: The cold air from the
expander is now passed to the refrigerator where it is expanded at
constant pressure P4 (equal to P1). The temperature of air increases
from T4to T1. This process is shown by the curve 4-1on p-v and T-s
diagrams. Due to heat from the refrigerator, the specific volume of
the air changes from v4 to v1.

7. Equation of Coefficient of performance (COP) of Bell Coleman cycle
Heat absorbed during cycle per kg of air Q(4-1) = Cp(T1 -T4)
Heat rejected during cycle per kg of air Q(2->3) = Cp(T2 -T3)
Work done per kg of air during the cycle is = Heat rejected –
Heat absorbed = Cp(T2 -T3) - Cp(T1 -T4)

8. Coefficient of
performance:
For isentropic compression process 1-2:
where γ =
Cp/Cv

9. For isentropic expansion process
3-4:
Since, P2 = P3 and P1 = P4, therefore from the above equations:
Substituting this
in the COP
equation:

10. In an aircraft cooling system, air enters the compressor at 0.1
MPa, 4 C, and is compressed to 0.3 Mpa with an insentropic
efficiency of 72%. After being cooled to 55 C at constant pressure
in a heat exchanger the air then expands in a turbine to 0.1 MPa
with an isentopic efficiency of 78%. The low temperature air
absorbs a 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 C P of the
refrigerator, thedriving power re uired, and the air mass flow rate.