1st step :
The hot air coming from the engine bleed air system first goes through a flow control valve FCV whose main function is to adjust the air flow to the required level necessary to cool the cabin.
2nd step :
This air then flows into the primary heat exchanger PHX cooled down during flight by ram air (see hereafter). Air temperature and pressure are consequently decreased.
This ram air is generated by the aircraft flight speed and is driven into the primary heat exchanger by a special scoop. This scoop has often a variable geometry electrically driven depending on of the flight phase (take off, cruise,.... ).
This is mostly integrated in the lower part of the fuselage and explains the various shaped ports that we can see beneath an aircraft :
3 rd step :
The air now enters the ACM air cycle machine compressor stage where the pressure is raised. As accordingly the temperature increases, the air enters into the main heat exchanger MHX supplied as the primary heat exchanger PHX by ram air .
The air temperature is decreased.
The purpose compressor's phase is to increase again the pressure so that the expansion inside the turbine be higher. In fact during the cooling inside the two heat exchangers the temperature goes well down but the air pressure also. As we can see on the following diagram, without this compression phase , the turbine supply pressure is lower and the turbine efficency also. ( Turbine efficency is bound to the ratio of the inlet pressure to the outlet pressure )
4 th step :
All the next operations will be dedicated to water extraction to avoid icing before the air enters the turbine stage where the temperature is going well below zero degree celsius at the outlet. ( water vapor + very low temperature = ice )
Now the air goes across a reheater RHX , then in a condenser CD where water is converted from vapor to thin water drops. These drops are finally removed from the air flow in the water extractor WE .
5 th step :
The " dried air" can now enter the turbine stage, the final step of the travel !
In a turbine the air expands and its temperature and pressure are decreased. The temperature is very close from zero degree celsius or even negative.
For this explanation all steps have been presented separately.
In a cooling pack all these steps are interconnected and particularly :
For more informations go to the following pages :
air cycle machine
animated air cooling pack
MAIN SITE NAVIGATION :
bleed air system / air cooling system / zone temperature control system / cabin pressure control system