Aircycle cooling system in aircraft

Why we need aircraft's cabin pressure and temperature to be controlled?

At atmospheric condition we have sufficient pressure to breath freely. As the altitude increases pressure drops. Till 8,000 ft altitude the pressure variation won’t affect us but beyond that we will get affect (hypoxia), so in order to have same pressure to breath freely in the cabin of aircraft, we need to go for maintaining the pressure artificially.

As like pressure temperature also drops as the altitude increases, but up to certain layer of atmosphere which is stratosphere. Most of the aircraft are operating within Stratosphere. So we need to have a constant temperature in our cabin, so that passengers feel the comfort of their travel. 

What are the methods in practice for maintaining cabin pressure and temperature?

Air cycle system

Vapor cycle system

Explain Air cycle cooling system of cabin atmosphere control with neat figure?

.     Left engine

2.     Right engine

3.     Flow limiter

4.     Primary heat exchanger

5.     Primary heat exchanger bypass valve

6.     Shut off valve

7.     Compressor

8.     Secondary heat exchanger

9.     Water separator

10.    Secondary heat exchanger bypass valve

11.     Refrigeration unit

1. Air from the compressor sections of the two engines is taken for air conditioning and pressurization. We are making a cross connection so that it can supply uniform flow of air.

2. Air is going to the flow limiter. Flow limiter limits the amount of compressed air to entry into the system. Suppose if there is any pipeline ruptured in air cycle system, then flow limiter won’t allow the compressed air to enter into the air cycle system.

3. Partial amount of air from the flow limiter goes to the primary heat exchanger. Primary heat exchanger utilizes ram air for cooling purpose. The compressed air taken from the engine compressor may be at temp range of 200 deg to 400 deg Celsius. We are cooling the air without reducing much pressure by using heat exchanger, and heat exchanger works on Convective type.

4. Another amount of compressed air from flow limiter goes to mingle with the heat exchanger outlet air to make a constant temperature air of 300 °F. This constant temperature can be attained only by proper operation of Primary heat exchanger (PHE) bypass valve. For example if heat exchanger outlet air is at 200°F, but I need an output of 300°F at outlet portion of Primary heat exchanger bypass valve, so we are opening the bypass valve for some designated time and mixing the hot(directed from flow limiter) and cold (PHE outlet ) flows.

5 .This 300°F temperature air is going to split for three Purpose

a)     For Anti-icing

b)    To supply hot air to cabin(if required)

c)     To refrigeration unit

6. This 300°F air can be directly used for anti-icing and de-icing purpose. This air will be taken by the tubes and will be sprayed on the leading edge through suitable arrangements. And one part of the 300°F air flow is directed to the cabin for hot air supply.

7.  Remaining part of the air is directed towards refrigeration unit for further cooling. On the way there is main control that is “Main shut off Valve”. This can be directly controlled by the pilot.

8. After main shut off valve there is Refrigeration unit, this contains

a)     Compressor

b)    Turbine

c)     Water Separator

Both turbine and compressor are connected by the same shaft.

9. The air flows to the compressor region; there it strikes the compressor blade and makes rotating at initial. After compressor starts it compresses the air and Pressurizes the air so some amount of heat may be added to the air. Then Air is going to the Secondary heat exchanger (SHE), where the heat from the air is taken by the ram air by convection. The air has been cooled now.

10. The output of SHE goes to the Turbine. Since because of the turbine, here the cold gas is allowed to expand, so that pressure drops, temperature again drops, it may be on some minus °F sometimes.

11. After expansion in the turbine, air which is in a circular motion is allowed to go for the water separator region for separation of water particles in the cold air. Since water particles are denser than air they get attached to the walls of the water separator due to centrifugal force. In some specified place there are some holes made on the water separator to drain the water particles attached.

12. In the cabin we need only a temperature range of 60°F-125°F (15°C-51°C) and a pressure of 3p.s.i. which is suitable for human.

13. By opening and closing the refrigerant bypass valve, we can mingle the pure cold air and 300°F air, to make a possible living temperature for human beings.

14. After maintaining to proper temperature and pressure the air is allowed to go to the cabin by suitable pipelines.

15. If there is any problem on the total system means, we can directly mix the hot air supply with ram air (which is taken near SHE) and maintain the proper temperature by proper mixing. But this method is only for emergency purpose. Please note when using this ram air method of cooling pressurization should be done separately by cabin superchargers or whatever the device builds up pressure.