Air cycle machine

A cooling turbine (german air cycle machine) is the cooling unit in an air conditioner, which is mainly used in aircraft ( packs).

Packs

Main article: Air conditioning (aircraft)

The air conditioning packs ( packs short, the cooler the aircraft air conditioning system ) consist mainly of a cooling turbine, a water separator, two heat exchangers and control valves.

Most aircraft have two packs, each with a cooling turbine. Very large aircraft (eg B747 -400) have up to three packs that are supplied via a central line with bleed air. For airplanes with two packs of the failure of a cooling turbine may already be a reason for a precautionary landing on a alternate aerodrome.

Operation

After aspiration of the ambient air by the engine this is compressed in the compressor stage of the engine. Due to the strong compression, the air is strongly heated. This air is then passed as a bleed air to the pack. Before entering into the pack, the air is cleaned in an ozone converter of ozone at high altitudes has a higher concentration than at ground level. After a slight cooling of 220 ° C in a hot bleed air (usually existing ) first heat exchanger ( Precooler ), the air in the cooling turbine. As cooling air for the fan air is used Precooler. Due to the previous cooling of the air compressor of the cooling turbine can be made ​​of light, but not very temperature resistant aluminum. In the compressor stage (turbo compressors, turbo fan with radial turbine and axial fans, centrifugal compressor with radial turbine and radial compressor) cooling the turbine, the air is further compressed. They strongly heated ( heat of compression, 50 ° C temperature increase). In the subsequent second heat exchanger ( strictly speaking, the only heat exchanger inside the cooling turbine), the air is cooled ( recooling). The required cooling air is taken from the ( uncompressed and therefore not heated ) ambient air. In the turbine stage of the air cycle machine, which is connected via a shaft to the compressor stage, the air expands again - the pressure thus greatly falls. Thereby the temperature drops sharply. This cooling air is now at a temperature below the ambient temperature. Outside of the cooling turbine, the cooling air is usually a small proportion of hot bleed air ( that is mentioned in the case of the air conditioning system " trim air" ) is added, and in addition those derived from the aircraft cabin ( the air is "refreshed " admixture of recirculation; partial recirculation mode - through circulating fan - engl recirculation fan ), because the cooled air from the cooling turbine to cold ( 0-15 ° C) for passengers is almost back. .

Physics

The basic principle of the cooling turbine is based on the left-handed Joule process ( thermodynamic cycle ). The compressed and heated in compressor stage of said air cycle air is cooled in the heat exchanger, then expanded in the turbine stage, and cooled to lower temperatures than the air supplied.

The air conditioning in the plane is one of the units with the largest secondary power requirements. For long-range aircraft with 350 passengers are required to 52 kW (150 to 200 W per passenger). On hot days, about 25 % of the required energy for air dehumidification is needed on the ground.

Throughput

The circulating volume is such that the air is circulated every 2 min ( Airbus ) to 6 min (compared to find a change of air in public buildings is a 5 - to 8- times an hour instead ). The licensing requirements (FAA, JAA) require an air flow rate of 4.7 l / s (0.55 lb / min) of fresh air per passenger and additionally there is the proportion of recirculated air (about 40-50 %). In the event of an error 73 % of this value does not fall below. Particularly in descent, when the engines operate at idle and produce only low pressure, the bleed air is barely sufficient pressure to meet these values. The air conditioning systems have to be designed accordingly to these "critical" flight modes. Therefore, during descent, the bleed air is withdrawn from the high pressure bleed air of the engine, while the bleed air is discharged from the low-pressure discharge in the cruise. Decreases both are at the high- pressure compressor having a rotational speed of about 50,000 revolutions per minute.

The cooling turbine has no additional drive motor - the Tubinenstufe is driven by the air stream and drives via a shaft turn the compressor stage to. The energy supply for the cooling process is carried out by the supplied pressure on the bleed air.

Recirculation

The recirculated air is cleaned by HEPA filters that are exchanged according to aircraft every 6 to 18 months. The Airbus A340 has, for example, eight of the recirculation filter.

ICE

Also for the air conditioning of the passenger coaches of the ICE cooling turbines are used. Here they are driven by electric motors. The control of the cooling power over the speed control of electric motors. Since the shaft of the cooling turbine is mounted magnetic and contactless floats, it will run without lubricant or coolant.

A distinction

  • Air-supported air systems to which the cooling turbine part (german air cycle machine) and
  • Cold vapor air systems ( engl. vapor cycle machine; cold vapor process).

Airborne Air systems are operated as an open circuit ( open cold air air conditioning system ), while cold air steam systems are operated as a closed circuit. In cold climate vapor systems, the supply of fresh air is independent of the cooling process.

The turbine stage of the cooling turbine operates as an expansion cooling system ( flash cooling, expansion cooling; engl expansion cooling turbine. ). Motorized cooling turbines are sometimes even used in shipbuilding and in military technology. In the air conditioning of buildings they currently play virtually no role.

PC

High performance fans on the PC are also referred to as a cooling turbine.

  • Turbine
  • Aeronautical engineering
  • HVAC
  • Refrigeration technology
  • Cooling unit
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