Basic Operation : Pressurized and hot air coming from the airplane Bleed System enters the Primary Heat Exchanger where it’s temperature is reduced, then enters the compressor section of the Air Cycle Machine where it’s temperature and pressure is raised again, then the air enters the Secondary Heat Exchanger reducing again it’s temperature, and finally it enters the Turbine section of the Air Cycle Machine expanding and releasing it’s energy to rotate the Turbine and the Compressor wheels being on the same shaft, with the temperature dropping down to 0 degree Celsius or below. At this temperature the moisture is condensed and is removed by a Water Separator.

The early Jet airplanes Air Conditioning System configuration was close to the basic system in addition of a Flow Control Valve and an Ice control system.

Last generation Airplane Air Conditioning System Cooling Pack consists ofa Flow Control Valve, Primary and Secondary Heat Exchangers, Air Cycle Machine, Primary and Secondary Temperature Control Valve, Re heater, Condenser, Ozone Converter, a Water Separation System, Check Valves, and many sensors.

Operation of the Cooling Pack : When the pack is operating on the ground, ram air is pulled across the heat exchangers by the impeller fan powered by the turbine section of the air cycle machine, or in older configurations by a turbofan where the fan is powered by a turbine rotated by high energy bleed air of the pneumatic system, or by powerful three phase alternating currentelectric blowers, while for the in flight operation the ram air is more than enough to cool the heat exchangers, and the operation of the fans stop in case they are electrically or bleed air powered, or they are bypassed by a check valve into the ram air duct in case they are powered by the turbine section of the air cycle machine.

The bleed air enters the Cooling Pack through an Ozone Converter which transforms ozone contained in bleed air into Oxygen by Catalytic Effect, then passes throughFlow Control Valve which regulates the flow according to the Air Condition System and Cabin Pressurization System demands.

Flow controlled bleed air then goes first to the Primary Heat Exchanger at an average pressure of 30 to 40 p.s.i.and temperature of 150º C, where ram air removes heat from the bleed air. The temperature drop of the air passing the primary heat exchanger is about 100º C.

The Primary Heat Exchanger is made of Aluminum alloy and it is of Plate and Fin type, and has a single-pass cross flow configuration.

The partially cooled bleed air goes to the compressor section of the air cycle machine which is powered by the turbine section of the air cycle machine.

The compressor section compresses the air and increases the pressure and temperature. The temperature rise of the air passing the compressor section is about 30 to 40º C. This high pressure and temperature air goes through the Secondary Heat Exchanger, which is similar to the Primary Heat Exchanger Plate and Fin type made of aluminum, to remove the heat added by the compressor. The temperature drop of the air passing the secondary heat exchanger is about 60 to 70º C.

The water extractor duct removes some moisture condensed after the air passed and cooled from the Secondary Heat Exchanger, using the centrifugal effect swirling the air through vanes to separate water particles from the air and sends the moisture to a water collector.

Air leaving the Secondary Heat Exchanger is cooled prior to entering the condenserpassing the re heater. The re heater is a cross flow single pass air to air heat exchanger.

After re heater air is passing through the condenser. The Condenser is a cross flow single pass air to air heat exchanger. The core consists of two smaller core modules arranged in a paralleled flow configuration with a space between the modules. This allow a portion of the Turbine outlet air to flow through even in excessive icing conditions. The cold air that flows across the condenser coming of the outlet of the turbine section of the air cycle machine, causes the warm air going to the water extractors to cool below the due point and the excess moisture to be condensed into water particles easy to be removed from the next water extractors of the system using again the centrifugal effect to separate water from the air. Water collected from both water extractors is fed to spray nozzles into the the ram air duct to improve cooling efficiency of the heat exchangers.

The dry air goes through the re heater to warm the air that goes to the turbine to increase the turbine efficiency. The inlet temperature of the turbine section of the air cycle machine is at a level of 20 to 10º C

Rapid expansion and energy extraction across the turbine lowers the temperature of the air down to -10º C and rotates the turbine wheel together with the wheels of the compressor and the fan being on the same shaft to a speed of 40.000 to 50.000 rpm.

All these temperature and speed values above apply when the cooling pack is operating at the maximum cooling capability usually in hot and humid whether conditions on the ground. When the cooling demand is lower than the maximum (in flight), or when there is ice built up in the condenser or the exit of the turbine, some air is bypassing the system of the heat exchangers and the air cycle machine through a valve called ”Temperature Control Valve”. The hot bleed air melts the ice and frees the exit of the system, and at the same time the air passing through the heat exchangers and the air cycle machine is less,the speed decreases and the cooling pack delivers hotter air to the Air Conditioning System.

When in cruising altitude the need for cooling is at a minimum because of the very low ambient temperature at that flight level, the most of the bleed air is passing just through the heat exchangers being cooled from the ram air. In this case the speed of the air cycle machine is the minimum or in the last generation systems it is completely stopped.

References :

Commercial Airliner Environmental Control System

Engineering Aspects of Cabin Air Quality

Elwood H. Hunt, Dr. Don H. Reid,

David R. Space, and Dr. Fred E. Tilton

Boeing Aircraft Maintenance Manuals