Cooling turbine
A cooling turbine ( air cycle machine ) is the cooling unit in an air conditioning system that is mainly used in aircraft ( packs ).
Packs
The air conditioning packs (short packs ; the cooler of the aircraft air conditioning system) composed mainly of a cooling turbine, a water condenser, two heat exchangers and control valves.
Most planes have two packs, each with a cooling turbine. Very large aircraft (e.g. the B747-400) have up to three packs that are supplied with bleed air via a central line . In the case of aircraft with two packs, the failure of a cooling turbine can already be a reason for a safety landing at an alternate airport.
functionality
After the ambient air has been sucked in by the engine, it is compressed in the compressor stage of the engine. Due to the strong compression, the air is strongly heated. This air is passed on to the packs as bleed air. Before entering the packs, the air is cleaned of ozone in an ozone converter , which has a higher concentration at high altitudes than near the ground. After the bleed air at 220 ° C has been slightly cooled in a (usually present) first heat exchanger ( precooler ), the air enters the cooling turbine. Fan air is used as cooling air for the precooler . By cooling the air beforehand, the compressor of the cooling turbine can be made of light, but not very temperature-resistant aluminum. The air is compressed further in the compressor stage (turbo compressor; turbofan with radial turbine and axial fan; turbo compressor with radial turbine and radial compressor) of the cooling turbine. It heats up strongly (compression heat; 50 ° C temperature increase). In the downstream second heat exchanger (more precisely: the only heat exchanger within the cooling turbine), the air is cooled down (recooling). The cooling air required for this is taken from the (uncompressed and therefore not heated) ambient air. In the turbine stage of the cooling turbine, which is connected to the compressor stage via a shaft, the air expands again - the pressure drops sharply. This also causes the temperature to drop sharply. This cooling air now has a temperature below the ambient temperature. Outside the cooling turbine, the cooling air is usually mixed with a small proportion of hot bleed air (which is called “trim air” in the case of the air conditioning system), and also that which is diverted from the aircraft cabin (the air is “refreshed”, admixing the recirculation air; eng - partial recirculation mode - means of circulating blower. recirculation fan ), because the cooled air from the cooling turbine almost back to cold (0 ° to 15 ° C) for the passengers.
physics
The basic principle of the cooling turbine is based on the counterclockwise Joule process ( thermodynamic cycle ). The air that is compressed and heated in the compressor stage of the cooling turbine is cooled in the heat exchanger, then expanded in the turbine stage and cooled to a lower temperature than the air supplied.
The air conditioning in the aircraft is one of the units with the greatest secondary power requirement. In the case of large aircraft with 350 passengers, 52 kW are required for this (150 to 200 W per passenger). On hot days, around 25% of the energy required for air dehumidification is required on the ground.
Throughput
The circulation volume is measured in such a way that the air is circulated every 2 minutes (Airbus) to 6 minutes (in comparison, air changes in public buildings only take place 5 to 8 times an hour). The approval regulations (FAA, JAA) require an air flow rate of 4.7 l / s fresh air per passenger, plus the proportion of recirculated air (approx. 40–50%). Even in the event of an error, the value must not fall below 73%.
Especially when descending, when the engines are idling and only generate low pressure, the bleed air hardly has enough pressure to meet these values. The air conditioning systems must accordingly be designed for these "critical" flight conditions. For this reason, the bleed air is taken from the high-pressure bleed air extraction of the engine when descending, while the bleed air is derived from the low-pressure extraction during cruising flight. Both approvals are on the high pressure compressor, which has a speed of around 15,000 revolutions per minute.
The cooling turbine does not have an additional drive motor - the turbine stage is driven by the air flow and in turn drives the compressor stage via a shaft. The energy for the cooling process is supplied by the bleed air supplied under pressure.
Recirculation
The recirculation air is cleaned by HEPA filters, which are replaced every 6 to 18 months, depending on the aircraft type. For example, the Airbus A340 has eight of these recirculation filters.
ICE
Cooling turbines are also used to air-condition the ICE coaches . Here they are driven by electric motors. The cooling capacity is controlled via the speed control of the electric motors. Since the shaft of the cooling turbine is magnetically supported and floats without contact, it runs without lubricant or coolant.
One distinguishes
- air-assisted air conditioning systems, to which the cooling turbine belongs ( air cycle machine ) and
- Cold vapor air conditioning systems ( vapor cycle machine ; cold vapor process ).
Air-supported air conditioning systems are operated as an open circuit (open cold air air conditioning system), while cold steam air conditioning systems are operated as a closed circuit. In cold steam air conditioning systems, the fresh air is supplied independently of the cooling process.
The turbine stage of the cooling turbines works as an expansion cooling system ( expansion cooling turbine ). Motorized cooling turbines are sometimes also used in shipbuilding and military technology. They currently play practically no role in building air conditioning.
Pc
High-performance fans on the PC are also called cooling turbines.