https://www.mesubjects.net/wp-admin/post.php?post=358&action=edit Gas Refrigeration-1
https://www.mesubjects.net/wp-admin/post.php?post=483&action=edit Q. Ans Air Refrigeration-2
https://www.mesubjects.net/wp-admin/post.php?post=415&action=edit Q. Ans Refrigeration
https://www.mesubjects.net/wp-admin/post.php?post=7902&action=edit Summary of Air Conditioning
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
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 T4 after the HE I.
ЄHE I = (T3 – T4)/(T3 –T2),
Assume ЄHE I (effectiveness) as 80 % if not given.
Then it goes to the second compressor.
Determination of temperature T5’ after the second compressor
T5’/T4 =( p5’ /p4)(γ—1)/γ
Determination of temperature T5
ηcomp II = (T5 –T4)/ (T5’ –T4)
Determination of temperature T6 after the second heat exchanger
ЄHE II = (T5 – T6)/(T5 –T2)
Assume ЄHE II (effectiveness) 80 % if not given.
Then it goes to the Turbine.
Determination of theoretical temperature T7’ after the turbine
T7’/T6 =( p7’ /p6)(γ—1)/γ
Determination of actual temperature T7 after the turbine
η (turbine) = (T7 –T6)/ (T7’ –T6)
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 –T2)
Specific Work used in second compressor W c 2 = 1 x c p(T 5 –T 4)
Specific Work obtained from the turbine WT = 1 x c p(T 6 –T 7)
Find W net = W c 1 + W c 2 –WT
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 ( V2.) of compressor 1 at the INLET OF COMPRESSOR
P 2 x 100 V2. = m. R T2
Determine volumetric capacity ( V4.) of compressor 2 at the INLET OF COMPRESSOR
P 4 x 100 V4. = m. R T4
Determine volumetric capacity ( V7.) of turbine at its outlet
P 7 x 100 V7. = m. R T7 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.
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 T5 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 ex-changer 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.