Analysis

Cabin Air Systems

Cabin air systems enable passengers and crew to travel in relative comfort at typical aircraft cruising altitudes, where the outside air is too thin to breathe and too cold to survive, says Bernard Fitzsimons.

The upper atmosphere where airliners typically cruise is a hostile environment for human beings, with temperatures which can fall below -50°C, ambient pressure as low as one fifth that of the Earth’s surface and frequent high concentrations of ozone. So the environmental control system effectively constitutes a communal life support system for passengers and crew.

The air outside may be cold, but by the time it is extracted from the engine compressor stage to feed the air conditioning and other pneumatic systems it can be as hot as 400°C. So it is first pre-cooled by heat exchangers using ram air from outside the aircraft before reaching the air conditioning pack at a temperature of around 200°C. The pack uses air cycle machines combining turbines and compressors to produce air at the required low temperature and ambient pressure. Catalytic ozone converters installed in the bleed air ducting upstream from the air conditioning packs reduce ozone concentration to an acceptable level.

A mixer unit under the cabin floor then mixes the cooled outside air with air drawn by fans though recirculation filters from the cabin floor area. The recirculated cabin air contains moisture from the passengers which mitigates the extreme dryness of the outside air, but high efficiency particle air (HEPA) filters remove at least 99.95 per cent of 0.3 micron particles, and 99.999 per cent of bacteria and viruses.

From the mixing unit, the air is distributed to overhead outlets in the various cabin zones, where trim air valves use small quantities of hot bleed air from the pre-cooler outlet to regulate the cabin outlet temperature. The temperature of the air supplied to each zone — there are two in an A320, for example, and up to six in an A330 or A340 — can be adjusted independently to take account of passenger density. Cabin pressure is regulated by an outflow valve that monitors the pressure and releases cabin air to the outside in accordance with the aircraft altitude and the external static pressure.

Integrated air management

Traditionally, the various components of the air system — engine bleed air, air conditioning, cabin pressure control, airframe and engine anti-icing, fuel tank inerting, and additional cooling for avionics, galleys, hydraulics and electronics — were procured individually. Bombardier tried a new approach with the integrated air management system it specified for the Global Express: Liebherr-Aerospace, which supplied that system, has gone on to build up a 40 per cent market share in civil air systems. The company is currently developing or supplying the integrated air systems for Comac's ARJ-21 and C919, the Sukhoi Superjet  and Bombardier's CRJ1000 and Global 7000/8000, plus the bleed air and air conditioning systems for the 747-8 and A320neo, and supplementary cooling for the A380.

Both the A320neo and CSeries, Liebherr says, feature the latest generation of electro-pneumatic engine bleed air systems using switchless compact valves. They offer lower weight and higher reliability than previous-generation systems, despite being installed in new-generation engines — the Pratt & Whitney (P&W) PurePower geared turbofan and CFM LEAP X — which feature tougher design and operating conditions. “There is less space available in the nacelle and pylon for integrating the equipment, while ambient nacelle temperature, bleed pressure and temperature and vibration levels are all higher,” says Liebherr.

For the future, the company says that as well as continuing to improve conventional pneumatic systems with incremental innovations, it is preparing for the next generation of more electric aircraft. “We have developed unique electrical air system technologies that will enable future more-electric commercial aircraft to substantially increase their performance while reducing carbon dioxide emissions.” The electrical air management technologies have completed laboratory testing successfully and are about to undertake in-flight testing. Meanwhile, the company says, both airframers and aircraft operators have shown their satisfaction with “the tremendous growth in system reliability that they experienced through the introduction of Liebherr’s new-generation systems”.

For the derivative 747-8, Boeing retained the use of bleed air but introduced a digitally controlled system designed to be lighter, more reliable and easier to maintain. It has fewer line replaceable units (LRUs) to simplify troubleshooting, while changes to the air supply control system (ASCS) and temperature control system (CACTCS) combine to provide increased cooling capacity.

The system takes bleed air from ports at the fourth and tenth compressor stages of each GEnx-2B engine, regulating its temperature and pressure before delivering it to the CACTCS and other aircraft systems. The air conditioning pack is described as a true subfreezing pack, one which will operate to temperatures below the freezing point of water at all altitudes. Enabling technologies include high-pressure water separation to mitigate the build-up of ice within the air cycle machine and integrated pack control features that lessen ice formation within the air conditioning pack. Recirculated air and pack outlet air are combined in a compact mixing section at the turbine outlet.

Bleedless Boeing

For the clean-sheet 787, on the other hand, Boeing opted to do without bleed air for everything except nacelle anti-icing. For environmental control the two electrically driven CACTCS packs use air taken directly from the outside.

Boeing says that as well as improving fuel efficiency, the no-bleed air system eliminates the risk of engine oil decomposition products being introduced into the cabin supply air in the rare event of a failed engine compressor seal. The 787’s ventilation system controls allow the outside air ventilation rate to be adjusted per flight based on passenger loading, a capability the airframer says reduces fuel burn and maintains higher relative humidity in the cabin.

UTC Aerospace (including the former Hamilton Sundstrand), which developed the 787 system, says each of the two packs generates enough air conditioning power to cool 25 typical homes. As well as HEPA filters the system incorporates gaseous air purification for the recirculated cabin air in the form of the Donaldson Air Purification System. While the particulate filtration element of the APS removes dust, allergens, bacteria, viruses and other irritating particles, a chemical process called gas phase adsorption removes bio-effluents, fuel by-products, cleaning agents, emissions from internal materials, cosmetics and other personal care products. Donaldson says the same technology can be repackaged for installation on 747s and 767s.

The 787’s ECS also incorporates 48 Maxon motors, including drives for the cabin ventilation, for cooling the electronics and to open and close the air inlet on the outside of the aircraft. Maxon’s EC 45 flat motors, light brushless motors designed to fit small spaces and operating at 4,000 rpm, drive the 36 shut-off valves in the cabin ventilation system, and EC32 motors modified with low temperature Hall sensors power the linear drives for the air inlet doors.

Honeywell, the developer of the air management system for the A350 as part of its $16bn contract for the new type’s Extended Mechanical System Perimeter, is using an air system integration bench (AirSIB) at its facility in Mexicali, Mexico, to confirm interactions between control systems and the physical characteristics of mechanical hardware. The EMSP includes the HGT1700 APU along with the environmental and cabin pressure control systems, supplemental cooling, ventilation and fans, bleed air systems and wing anti-ice systems.

Humidity control

The low external temperatures lead to condensation in the form of frost and ice on the inside of the aircraft’s outer skin. When it melts at lower altitudes it leads to an accumulation of moisture in the insulation blankets, increasing aircraft weight, potentially interfering with electrical systems, encouraging corrosion and damaging the insulation.

CTT Systems developed its 'Zonal Drying' system to remove this moisture. The 'Zonal Dryer' uses a fan to draw air through a glass fibre rotor impregnated with silica gel and feed the dry air to the area to be dried. Some of the air — about 20 per cent — is heated before passing through to dry the rotor and is then either added to the air conditioning supply or vented overboard. The 'Zonal Dryer' can also be paired with one or more humidifiers to form the 'Cair' system. The humidifier contains a pad of glass fibre with specially designed water channels moistened via a spreader system to cool and humidify air passing over it.

'Zonal Drying' is basic equipment on the 787, and CTT humidifiers are an option for crew rest compartments and the flight deck. The humidifiers are also an option in crew rest compartments for the A380, and 'Cair' is operational aboard Lufthansa A380s in first-class cabins. Both 'Zonal Drying' and 'Cair' are options for the A350.

There have also been many retrofit installations, says Peter Landquist, VP sales, marketing and customer support at CTT. 'Zonal Drying' is operational aboard 757, 737 classic/NG, A320/321 and A330 aircraft operated by Air New Zealand, Transavia, Titan Airways, flydubai, Jet2.com, Thomas Cook Scandinavia, Monarch, Luxair, Lufthansa City Line, Adria Airways, Air Berlin, United and Nova Airlines.

In February, easyJet announced that it would install CTT 'Zonal Drying' units on four of its A320s for a one-year trial. The sub-30kg units are expected to reduce aircraft weight by up to 250kg, or the equivalent of 12 hold bags. Flight operations manager Chris Foster said the weight reduction could save as much as 4.5 million kilos of fuel annually across the airline’s fleet of 200-plus A319s and A320s.

The majority of 787 customers — 85-90 per cent of them — have selected the flight deck humidifiers. For the 787 cabin crews, the humidifiers form part of the crew bunk selection, “so our humidifiers are on every cabin crew bunk that the airlines select”, Landquist says. So far that is about 50-60 per cent of them.

For both narrow- and widebody VIP aircraft CTT is the market leader, Landquist adds: “The 'Cair' system has so far been selected for seven of the eight 747-8 VIP aircraft sold.” The 'Cair' system in Lufthansa’s first-class A380 cabins has been in operation for four years now, and like the cabin crew humidifier that has been operational aboard Singapore Airlines A380s since the aircraft’s introduction, has demonstrated what he describes as “excellent performance and reliability”.

Liebherr has also developed cabin air humidification systems to counteract the discomfort of dry noses and throats and sore eyes resulting from the low humidity level at high altitude. The Liebherr humidification system uses boiled sterile water and mineral-free steam to provide a relative humidity level of 20 per cent, which significantly improves the comfort of crew and passengers. The level is constantly monitored and controlled by a closed loop multi-zone humidity regulator.

Two different technologies are offered, based on either electrical or bleed systems. The electrical humidifier is well adapted to existing aircraft as it is easier to integrate the unit in limited space without excessive aircraft modification. The system using bleed air is perfect for new aircraft as it is highly integrated with the air management system and does not need electrical power.

Liebherr says its highly reliable electrical heaters with plug-in technology enhance the temperature control, including over-heat protection, and are ideal for VIP aircraft cabin comfort and crew rest areas. The heaters increase the temperature of fresh air to meet the requirements of lower deck facilities. They provide a stand-alone operation with the option to integrate a low-flow device. The system can be switched on and off by a separate discrete switch and does not require scheduled maintenance. Heaters are available from 1.2 kW up to 4.0 kW.

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