Fig 4.1 Contents of this post
Aircraft Cooling System:
In an aircraft cooling system required to maintain cabin temperature at comfortable level for occupants in aircraft.Despite outside temperature is low refrigeration is required in inside the occupant cabin because :
1. large internal heat is generated due to occupant and equipment inside the cabin.
2.Heat generation due to skin friction caused by the fast moving aircraft.
3. At high altitude, the outside pressure will be sub-automatic.
4.Solar radiation.
For low speed aircraft flying at low altitudes, cooling system may not be required, however, for high speed aircraft flying at high altitude, a cooling system is must. Even the COP of air cycle refrigeration is very low compared to vapor compression refrigeration system, it is still found to be most suitable for aircraft refrigeration system as:
1. Air is cheap, safe, nontoxic and non-flammable. Leakage of air not a problem.
2.  Cold air can be directly be used for cooling thus eliminating the low temperature heat exchanger leading to lower weight.
3. The aircraft engine already consist of a high-speed turbo-compressor, hence separate compressor for cooling system is not required. This reduces the weight per kw cooling considerably. Typically, less than 50% of an equivalent vapor compression system.
4. Design of complete system is much simpler due to low pressure. Maintenance is also less.

1. Simple Aircraft Refrigeration system 
Fig 4.2 Simple aircraft Refrigeration system
 Fig 4.2 shows the schematic of a simple aircraft refrigeration system and the operating cycle on T-S diagram shown in figure 4.3.
Fig 4.3 T-S Diagram of Simple Refrigeration system
This is open system as shown on diagram outside air temperature at low temperature and low pressure (state 1) is compressed to ram pressure (state 2). During this process temperature is increased from 1 to 2. This air is compressed to 2 to 3 in main compressor and is cooled to to 4 at air heat exchanger.
Then expansion process take place at 4-5 and cold air at 5 is supplied to cabin.
Fan and turbine mounted on same shaft.Fan keeps up the obliged wind stream over the air cooler. This basic framework is useful for ground cooling(when airplane is not moving)as fan can maintaining wind stream over the air cooler.

 2. Bootstrap Refrigeration System:
Fig 4.4 Bootstrap Refrigeration System
Fig 4.4 shows schematic diagram of bootstrap refrigeration system consist of two heat exchanger (air cooler and aftercooler) named as heat exchanger 1 and heat exchanger 2 as above. 
Compressor and turbine mounted on same shaft and compressor run on turbine and in between them heat exchanger is placed. First heat exchanger is placed between main compressor and auxiliary compressor. Output of turbine is cold air given to cabin.

Fig 4.5 T-S Diagram of Bootstrap Refrigeration System
Fig 4.4 shows schematic diagram of bootstrap refrigeration system consist of two heat exchanger (air cooler and aftercooler) named as heat exchanger 1 and heat exchanger 2 as above.
Compressor and turbine mounted on same shaft and compressor run on turbine and in between them heat exchanger is placed. First heat exchanger is placed between main compressor and auxiliary compressor. Output of turbine is cold air given to cabin.
Fig 4.5 shows T-S diagram of total system first of all surrounding air which is at temperature at 1 which is low pressure and low temperature in compressed to pressure 2 by ram effect. Ideal compression shown by 1 to 2’.
This air is further compressed to state 3 in main compressor. Then this air cooled to air cooler up to state 4. Heat rejected by air absorbed by ram air in heat exchanger 1. The air at state 4 is then compressed to 5 in secondary compressor. Then it is cooled to state 6 in heat exchanger 2.
After air expanded in turbine and is supplied to cabin at temperature T7. Since the system does not consist of separate fan for driving the air through the heat exchanger, it is not suitable for ground cooling.
However, in general ground cooling is done by external air conditioning system as it is not efficient to run the aircraft to provide the cooling when it is grounded.
Another advantage is that ram air is used to cool the heat exchanger 1 and 2. Cool air obtained at state 7 for occupant in cabin.