AIRCRAFT REFRIGERATION

 Need of Aircraft Refrigeration

In general aeroplane cruises at an altitude of 10000m above sea level where pressure is 0.15 bar and temperature is -50˚C.

Travelling in such conditions is not possible for humans, A jet fighter travelling at 950 km/hr needs a cooling system capable of 10-20 TR capacity. Also, there is a need to dissipate the heat load from electronic devices. In addition to this, there are solar radiations coming to the plane surface directly of about 1000W/m². Also, there are occupants from where heat needs to be extracted.

So definitely there is a need for an aircraft refrigeration system.

Why air cycle?

We can have many other cycles also but why do we want to use air as a refrigerant for cooling in an aeroplane.

First of all, the air is cheap then it is environmentally friendly, safe and non-toxic, in addition to this it is also inflammable.

Air cycle equipment is extremely reliable because the pressure ratio in the air cycle is less in comparison to the pressure ratio in the vapour compression cycle. So, the maintenance cost is less and systems are reliable,

Also, the performance of the air cycle we need does not

deteriorate as much as that of vapour compression unit when operating away from its design point.

 

Simple air-cooling system

Fig-1

A simple air-cooling system for aircraft is shown in Fig (1) The main components of this system are the main compressor driven by a gas turbine, a heat exchanger, a cooling turbine and a cooling air fan.

The air required for the refrigeration system is bled off from the main compressor. high pressure and high-temperature air is cooled initially in the heat exchanger where ram air is for cooling.

It is further cooled in the cooling turbine by the process of expansion. The work of the turbine is used to drive the cooling fan which draws cooling air through the heat exchanger.

The system is good for ground surface cooling and for low flight speeds.

Fig-2

1.  Ramming process.

a. the pressure and temperature of ambient air are p1 and T1 respectively. The ambient air is rammed isentropically from pressure p1 and temperature T1 to pressure p2 and temperature T2. This ideal ramming action is shown by the vertical line 1-2

b.  In actual practice, because of internal friction due to irreversibilities the actual ramming process is shown by the curve 1-2’ which is adiabatic but not isentropic due to friction

2.  Compression process.

The isentropic compression of air in the main compressor is represented by the lines 2’- 3. In actual practice, because of internal friction, due to irreversibilities, the actual compression is represented by the curve 2’-3’

3.  Cooling process.

The compressed air is cooled by the ram air in the heat exchanger. This is shown by the curve 3’-4. In actual practice, there is a pressure drop in the heat exchanger which is not shown in the figure. The temperature of the air decreases from T3’ to T4.

4.  Expansion process.

The cooled air is now expanded isentropically in the cooling turbine shown by curves 4-5. In actual practice, because of the same reason that is irreversibilities, actual expansion in the cooling turbine is shown by the curve 4 - 5’. The work of this turbine is used to drive the cooling air fan which draws cooling air from the heat exchanger.

3. Refrigeration process. The air from the cooling turbine (i.e., after expansion) is sent to the cabin and cock pit where it gets heated by the heat of equipment and occupancy. This process is shown by the curve 5’-6.

 

Simple air evaporative cooling system

A simple air evaporative cooling system is shown in Fig. (3) It is similar to the simple cooling system except that the addition of an evaporator between the heat exchanger and cooling turbine. The evaporator provides an additional cooling effect through the evaporation of a refrigerant such as water.

Fig-3

At high altitudes, evaporative cooling may be obtained by using alcohol or ammonia. The water, alcohol and ammonia have different refrigerating effects at different altitudes. At 20000 metres height, water boils at 40°C, alcohol at 9°C and ammonia at ~ 70°C.

 

Fig-4

In the T-s diagram as shown in Fig. 4, the thick lines show the ideal condition of the process, while the dotted lines show actual conditions of the process.

If cooling of 45 minutes duration or less is required, it may be advantageous to use evaporative cooling alone.

 

Boot-strap air cooling system

A boot-strap air cooling system is shown in Fig. 5.

Fig-5

This cooling system has two heat exchangers instead of one and a cooling turbine drives a secondary compressor instead of the cooling fan. The air bled from the main compressor is first cooled by the ram air in the first heat exchanger. This cooled air, after compression in the secondary compressor, is led to the second heat exchanger where it is again cooled by the ram air before passing to the cooling turbine. This type of cooling system is mostly used in transport type aircraft.

Fig-6

The T-s diagram for a boot-strap air cycle cooling system is shown in Fig. 6. The various processes are as follows:

1. process 1-2 represents the isentropic ramming of ambient air from pressure p, and temperature T, to pressure p2 and temperature T2. Process 1-2' represents the actual ramming process because of internal friction due to irreversibilities.

2. process 2- 3 represents the isentropic compression of air in the main compressor and process 2'-3' represents the actual compression of air because of internal friction due to irreversibilities.

3. process 3-4 represents the cooling by ram air in the first heat exchanger. The pressure drop in the heat exchanger is neglected.

4. process 4-5 represents the isentropic compression of cooled air, from the first heat exchanger, in the secondary compressor. The process 4 - 5' represents the actual compression process because of internal friction due to irreversibilities.

5. process 5-6 represents the cooling by ram air in the second heat exchanger. The pressure drops in the heat exchanger in neglected.

6. process 6-7 represents the isentropic expansion of cooled air in the cooling turbine up to the cabin pressure. The process 6-7' represents an actual expansion of the cooled air in the cooling turbine.

7. process 7-8 represents the heating of air up to the cabin temperature T8.

 

Bootstrap air evaporative cooling system

A boot-strap air cycle evaporative cooling system is shown in Fig.7.

Fig-7

 It is similar to the boot-strap air cycle cooling system except that the addition of an evaporator between the second beat exchanger and the cooling turbine.

Fig-8

The T-s diagram for a boot-strap air evaporative cooling system is shown in Fig 8. The various processes of this cycle are the same as a simple boot-strap system except for the process 5-6 Which represents cooling in the evaporator using any suitable evaporant.

 

Reduced ambient air-cooling system

The reduced ambient air-cooling system is shown in Fig. 9. This paling system includes two cooling turbines and one heat exchanger.

Fig-9

The air reduced for the refrigeration system is bled off from the main compressor. This high pressure and high-temperature air is cooled initially in the heat exchanger. The air for cooling is taken from the cooling turbine which lowers the high temperature of rammed air.

 

Fig-10

The cooled air from the heat exchanger is passed through the second cooling turbine from where the air is supplied to the cabin. The work of the cooling turbine is used to drive the cooling fan (through reduction gears) which draws cooling air from the heat exchanger. The reduced ambient air-cooling system is used for very high speed (supersonic) aircrafts, when the ram temperature is too high.

 

Regenerative Air-Cooling System

Air Refrigeration Cycle with regeneration initially the processes are same due to ramming action of the pressure and the temp of the air is increased starting from state 1 to state 2, after the state 2, the state 3 is attained in a compressor and after 3, the air coming out of the compressor is cooled at constant pressure, so at constant pressure the cooling of air takes place and state 4 is attained.

Fig-11

After state three, there is a heat exchanger, this heat exchanger as shown in the figure takes air after the expansion of the turbine so part of the heat exchanger that the temp. Of air is further cool to state 5. So due to this heat exchanger instead of attaining the temperature the gas is further cooled up to temp. 5 and the expansion takes place.

In this case, also we get cooler air at the exit of the turbine.

Fig-12

The T-s diagram for the reduced ambient air cycle cooling system is shown in Fig. 12 The various processes are as follows:

 1. process 1-2 represents isentropic ramming of air and process 1-2 represents Actual ramming of air because of internal friction due to irreversibilities

2. process 2-3 represents isentropic compression in the main compressor and process 2-3 represents actual compression of air, because of internal friction due to Irreversibilities.

 

3. process 3-4 represents cooling of compressed air by ram air which after passing through the first cooling turbine is led to the heat exchanger. The pressure drop in the heat exchanger is neglected

 4. process 4-5 represents isentropic expansion of air in the second cooling turbine Upto the cabin pressure. The actual expansion of air in the second cooling turbine is represented by the curve 4-5

5. The process 5-6 represents the heating of air up to the cabin temperature 7.

 

DART and comparison of various air-cooling systems

Fig-13

The performance curves for the various air-cooling systems used for aircraft are shown in Fig.13. These curves show the dry air rated turbine discharge temperature (DART) against the Mach number. From Fig. 13, we see that the simple air-cooling system gives a maximum cooling effect on the ground surface and decreases as the speed of aircraft increases. The bootstrap system, on the other hand, requires the aeroplane to be in flight so that the ram air can be used for cooling

in the heat exchangers. One method of overcoming this drawback of bootstrap system is to use part of the work derived from the turbine to drive a fan that pulls air over the secondary heat exchanger, thus combining the features of a simple and bootstrap system. As the speed of aircraft increases, the temperature of ram cooling air rises and the ram air becomes less effective as a coolant in the heat exchanger. In such cases, a suitable evaporant is used with the ram air so that the cabin temperature does not rise. For high-speed aircraft, the bootstrap evaporative or regenerative systems are used because they give lower turbine discharge temperature than the simple cooling system. In some cases, aeroplanes carry an auxiliary gas turbine for cabin pressurisation and air

 

From Fig. 13, we see that the turbine discharge temperature of the air is variable. Therefore, in order to maintain the constant temperature of supply air to the cabin, it requires some control system.

 

References

· Air conditioning systems for aeronautical applications: a review- Cambridge University Press:  27 December 2019

·     https://nptel.ac.in/courses/112/107/112107208/

·     Refrigeration and air conditioning-C.P. Arora