AIRCRAFT COOLING SYSTEMS        Gas Refrigeration-1       Q. Ans Air Refrigeration-2         Q. Ans Refrigeration          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

cooling systems.

  1. 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.
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 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.
Turbine 2 has input and output points as 5 and 6.
Procedure of calculation is similar to the BOOT STRAP SYSTEM.