https://www.mesubjects.net/wpadmin/post.php?post=358&action=edit Gas Refrigeration1
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https://www.mesubjects.net/wpadmin/post.php?post=7902&action=edit Summary of Air Conditioning
AIRCRAFT
COOLING SYSTEMS
Air craft cooling is a must. It may on
the ground or in the air. Different
cooling methods are used. These are Boot
strap, generative and reduced ambient
cooling systems.

Boot Strap Aircraft cooling
This Bootstrap system has two compressors, two heat exchangers and one turbine. Expansion turbine also runs the second compressor. Boot strap system is of two types:
1. System with evaporative cooling
There is a third heat exchanger. This is water cooled. It is placed after the second heat exchanger. This further cools air entering the turbine. This increases the cooling effect.
It has two compressors and two heat exchangers.
Analysis is common to simple cooling system up to point 3.
Determination of temperature T_{4} after the HE I.
Є_{HE I} = (T_{3} – T_{4})/(T_{3} –T_{2}),
Assume Є_{HE I} (effectiveness) as 80 % if not given.
Then it goes to the second compressor.
Determination of temperature T_{5’} after the second compressor
T_{5’}/T_{4} =( p_{5’} /p_{4})^{(γ—1)/γ}
Determination of temperature T_{5}
η_{comp II} = (T_{5} –T_{4})/ (T_{5’} –T_{4})
Determination of temperature T_{6 }after the second heat exchanger
Є_{HE II} = (T_{5} – T_{6})/(T_{5} –T_{2})
Assume Є_{HE II} (effectiveness) 80 % if not given.
Then it goes to the Turbine.
Determination of theoretical temperature T_{7’ }after the turbine
T_{7’}/T_{6} =( p_{7’} /p_{6})^{(γ—1)/γ}
Determination of actual temperature T_{7 }after the turbine
_{η (turbine)} = (T_{7} –T_{6})/ (T_{7’} –T_{6})
Assume efficiency of the turbine as 80 % if not given.
CALCULATE THE FOLLOWINGS:
Cooling effect per kg, N = 1 x c _{p }(T _{cabin} –T _{7})
Work input per kg in First compressor W _{c 1} = 1 x c _{p}(T _{3} –T_{2})
Specific Work used in second compressor W _{c 2} = 1 x c _{p}(T _{5} –T _{4})
Specific Work obtained from the turbine W_{T} = 1 x c _{p}(T _{6} –T _{7})
Find W _{net} = W _{c 1} + W _{c 2} –W_{T}
COP = N/ W _{net}
Determine mass flow rate of air
m^{. }N = TR x 211
where TR is the cooling capacity in tons and N is the cooling effect per kg
Determine volumetric capacity ( V_{2}^{.}) of compressor 1 at the INLET OF COMPRESSOR
P_{ 2} _{ } x 100 V_{2}^{. } = m^{.} R T_{2}
Determine volumetric capacity ( V_{4}^{.}) of compressor 2 at the INLET OF COMPRESSOR
P_{ 4} _{ } x 100 V_{4}^{. } = m^{.} R T_{4}
Determine volumetric capacity ( V_{7}^{.}) of turbine at its outlet
P_{ 7} _{ } x 100 V_{7}^{. } = m^{.} R T_{7 2. Bootstrap without evaporative cooling}
_{As compared to case 1, last heat exchanger is absent. }REGENERATIVE AIRCRAFT COOLING SYSTEM
Some air coming out of the cooling turbine does the cooling in the heat exchanger just before the turbine. In this, there is no cooling by rammed air in the first heat exchanger.
Calculations
Same calculations up to point 3 as in the case of simple system of aircraft cooling.
Now cooled air at point 5 (outlet of turbine) enters the heat exchanger for cooling.
Remember T _{5} is still unknown.
ASSUME T_{5} say= 100 ^{0 }C.
Then calculate T4 from HEX effectiveness = 0.80 = (T3 –T4)/(T3–T5)
Use T4 to calculate T5 from efficiency of turbine.
This T5 may not match with the assumed T5.
An ITERATIVE PROCEDURE minimizes the difference between assumed T5 and calculated T5.
Reduced Ambient Cooling System
It uses two turbines and one compressor. Turbine 1 uses rammed air. The air from turbine 1 enters the heat exchanger. Turbine 2 uses air after the heat exchanger for further cooling.
Turbine 1 has input and output points as 2 and 4.
Cooled air from turbine 1 enters the first Heat exchanger at point 4.
Heat exchanger has input at points 3 and 5.
CAUTION: RAMMED AIR IS FED TO TURBINE 1.
Turbine 2 has input and output points as 5 and 6.
Procedure of calculation is similar to the BOOT STRAP SYSTEM.
Fig.
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