We learned from the previous page that compressed air at high temperature is available from the engine bleed air system. The next step will be to convert this air to a breathable air to be injected into the cabin.
This is made with air cooling packs using mainly an air cycle machine to produce very cold air . Later we will see on the distribution and zone temperature control system page, how this very cold air is mixed to air withdrawn from the cabin to provide the final air needed to the passenger's comfort.
BUT BEFORE, SOME WORDS OF PHYSIC !...( in very few words )
If we take a volume of air and if we decrease this volume
( compression ): pressure and temperature increase.
If we take this same volume and we increase it (expansion ) : pressure and temperature decrease.
Accordingly if you wish to get cold air, you only need to increase its volume.
And now let's start,
WHAT IS A COOLING PACK ?
The purpose of the air conditioning pack is to decrease the temperature and remove the water contained in the hot air bled from the engines.
Schematically as shown on the following figure, the pack transforms air with characteristics from the red area to air of the blue area.
The main target of the operation carried out by the air cooling pack is to put the air into a device where its volume will be increased and accordingly its temperature will decrease (see above some words of physic ) .
The following explanations describe how this is achieved.
Two heat exchangers decrease, in two steps, the temperature of this air. Between theses two steps , an air cycle machine, in its compressor stage compresses the air for thermodynamic reasons.
Then in the turbine stage of this same air cycle machine, the air expands thus lowering the air temperature to a very low level.
To avoid ice built up, some operations are added to the previous steps described hereabove to remove water. This is done in a condenser which condenses the water vapor included in the air and a water extractor which removes this water outside.
For more informations go to the
AIR COOLING PACK DETAILED EXPLANATIONS PAGE:
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bleed air system / air cooling system / zone temperature control system / cabin pressure control system