The fundamental need to provide life-supporting air to pressurised airliner cabins has challenged engineers since the late 1930s. Even with advancing technology, there seems to be no let-up in the challenge, as operators look for smaller, lighter and more efficient systems, and passengers demand ever better aircraft air quality.

Humans are optimised to breathe at sea level, their efficiency diminishing with increasing altitude and reduced atmospheric pressure. Oxygen becomes scarce as the air ‘thins’ at height, causing hypoxia and, ultimately, death.

An optimised airliner, meanwhile, flies best at altitudes above the weather, where the thinner atmosphere causes less resistance to forward momentum, minimising fuel consumption and helping maximise performance.

Delivering clean, safe, breathable aircraft air quality under pressure to the cabin has therefore been a fundamental life-support requirement for every airliner since the Boeing Stratoliner, the first pressurised public transport aircraft, entered service in 1940, offering the potential for regular passenger service above 20,000ft.

A system is driven by engine supercharging generated the pressurised air for the Stratoliner’s cabin, but engine bleed became the standard source of pressurised cabin air in the turbine era.

The bleed air used to pressurise the majority of modern airliner cabins (with the notable exception of the Boeing 787) is drawn from the engine’s compressor section. It has not been through the motor’s hot section, where combustion with fuel occurs, so it is essentially compressed, heated fresh air, but still in need of further treatment before it reaches the cabin.

Ironically, the advanced piston-powered airliners that followed the Stratoliner into service, including Boeing’s Stratocruiser, employed electrical compressors to generate cabin pressure and similar systems, albeit benefitting from 70 years of technological development, are employed on the 787.

Tom Hart, vice president and general manager for Air and Thermal Systems, Honeywell, leads a section of the business producing air management systems including environmental control system (ECS), air conditioning, bleed air management and cabin pressure.

Tom explains: “Air is bled from the engines into the air management system, where it’s cooled and compressed, then expanded and delivered to the cabin. Cabin pressurisation is derived from the bleed air pressure, with outflow valves in the fuselage allowing pressure to ‘escape’, enabling pressure to increase as an aircraft flies higher and decreases as it flies lower. It’s all maintained as delicately as possible to maximise passenger comfort.”

Considering the complex engineering and regulatory requirements of an air management system, Paul D’Orlando, business development director, Air Management Systems, Collins Aerospace, explains: “Key safety regulations have been implemented regarding minimum fresh air flow rates based on number of occupants, maximum bleed air temperature as it travels through different zones in the aircraft, component insulation requirements due to composite structures, and mechanical and control redundancies in the event of a component failure during flight.”

He says the ECS is, therefore, an integral part of aircraft design. “Changes in maximum cruise altitude, airspeed, maximum passenger count, and engine power (bleed and electric) availability affect the definition of every major part of the pneumatic system.

“The airflow rate directly impacts the size of major components, including air cycle machines, heat exchangers, pneumatic control valves and ducts. Safety regulations that drive redundancy can affect the design through duplicate parts, but also size parts larger than needed. For example, the size of a typical heat exchanger is determined on the assumption that other components have failed. In effect, the heat exchanger is oversized for normal operation.”

Collins has ECS equipment on Boeing, Airbus, Embraer and Bombardier platforms, and D’Orlando describes the scope of a typical system: “Our ECSs include the pneumatic bleed air control, cabin pressurisation and air conditioning functions required for passenger comfort and safety. Key components among them are pneumatic control valves; heat exchangers; temperature, pressure and flow sensors; electronic controllers; and rotating machinery, including air cycle machines, cooling turbines and compressors.”

Quality air

Air bled directly from an engine may contain elements undesirable in an aircraft cabin. Among them, volatile organic compounds (VOCs), typically derived from lubricants and fuel, have the potential to be harmful, while ozone is inevitably present, its concentration increasing with altitude, particularly at the poles.

Victor Leung, global marketing manager, Clean Air at BASF explains: “The air at high altitude contains significant levels of ozone. If left unchecked, it enters aircraft through air conditioning ducts via the bleed air system. Ozone exposure is known to cause adverse health effects, including headaches, fatigue, shortness of breath, chest pains, coughing and irritation of the eyes, nose or throat.”

Among the products available for converting ozone into oxygen, BASF offers its Deoxo range of ozone/VOC converters. “VOCs, which can cause unpleasant odours, can be an issue. Sources include lubricant leaks, on-ground maintenance activities and the ingestion of ground exhaust and de-icing fluids. Our Deoxo technology reduces both harmful ozone and VOCs.

“The ozone converter was introduced more than 35 years ago, in response to Federal Aviation Regulations 25.832, which set the maximum allowable ozone exposure. BASF introduced its dual function ozone/VOC converter 15 years ago and since then the major change we’ve noted is that the flying public and aircrews have become more aware of cabin air quality.”

Honeywell offers what Hart refers to as an ‘industry-leading’ range of ozone conversion products. “We also produce catalytic ozone converters. It’s an area where we’re focussing R&D effort and a direction in which I believe the industry is moving. Our experience began in the automotive sector, where we had technology used in the original catalytic converters for cars; we’ve been modifying that for aircraft. Today we provide catalytic ozone converters that also consume VOCs, should they find their way into the air.

“We’re working on technologies that will allow these converters to work at lower temperatures, making them more efficient and able to convert more VOCs, helping further reduce smells and organic compounds entering the cabin and affecting the aircraft air quality.”

While BASF and Honeywell are in the business of removing undesirables from cabin air, Sweden’s CTT Systems makes up for a missing component. High-altitude air contains less water than the air we breathe at sea level and Peter Landquist, CTT’s VP sales & marketing, says its humidifier restores cabin ambient conditions from an ‘uncomfortable’ 5–10 per cent relative humidity (RH) in first and business class, to a ‘comfortable’ 22 per cent.

For now, the use of cabin humidifiers is restricted, however. “We started in the VIP business in early 2000 and now almost every large VIP aircraft is equipped with a cabin humidification system,” Landquist notes. Humidification’s limited employment no doubt reflects its perceived benefits, which Landquist describes as ‘affecting the sensory experience in a positive way’.

There is no regulatory imperative for installing systems, but, he reckons: “The high-quality products and services offered in first and business class cabins can only truly be experienced with a cabin humidifier installed. The increased cabin humidity makes passengers feel and sleep better, improves their ability to taste and enjoy food, and helps them more quickly recover from their flight. The reality is that even a first-class suite or business class seat is not comfortable if the cabin is too cold, too warm, too draughty, too noisy and too dry.

“The airlines have recognised the benefits of humidification for pilots and cabin crew and we have around 2,500 humidifiers in service for flight decks and cabin crew rest areas. During 2018, four major airlines selected our humidifier for their A350 and 777X first and business class cabins and our systems are basic or selectable line-fit options on the 787, 777X, A380, A350 and MC-21.”

Landquist confirms that CTT’s humidifiers and zonal dryers are available as retrofit equipment for the majority of Airbus and Boeing types, also explaining the basics of how they operate.

“Their function is based on the cold-evaporation principle. A humidifier is installed in the air supply duct for the cabin zone to be humidified. Water from the potable water system is sprayed in sequence – for approximately two seconds every minute – over an evaporation pad that rapidly absorbs it. At the same time, the water evaporates into the ECS air as it passes through the evaporation pad. The humidifier is activated automatically when the aircraft reaches cruise altitude and deactivated at the beginning of the descent.”

Of course, water is less than ideal in a corrosion-sensitive structure like an aircraft fuselage, while pooled condensation might also provide a breeding ground for bacteria and mould. For that reason, Landquist says: “When installing for the passenger cabin, humidifiers are always combined with zonal dryers to control the humidity balance in the pressurised area of the aircraft.”

As well as lack of humidity, Landquist hits on another perennial cabin air issue – noise. Generating cabin air is an inherently noisy process and much of the background ‘hiss’ passengers hear at altitude, in the cruise, is from conditioned air emerging from the ‘gaspers’ in the overhead PSUs. Noise travels along ECS ducts, bouncing from wall to wall as it moves, losing some of its energy on the way but still emerging with an obvious signature.

Careful gasper design helps minimise the issue, but alternative ducting materials could mean noise has all but dissipated even before it reaches the PSU. The UK’s Zotefoams manufactures a wide range of aviation-grade products in its unique ZOTEK F material, an incredibly versatile foam that absorbs sound, leaving very little to reach the gasper.

Flexible, boasting excellent fire and toxicity qualities, water-resistant, and remarkably tough, ZOTEK F is also extremely light. Zotefoams manufactures extraordinarily light ECS duct sections, each comprising a ZOTEK F sheet welded to create a tube, with a connecting part and sealing ring at either end, enabling simple compatibility with existing systems and seemingly impossible flexibility for installation in difficult spaces.

New age air

Collins Aerospace produces the ‘electric’ ECS for the 787 which, Paul D’Orlando says, “Enables a significant aircraft fuel burn improvement.” As such, he believes, “Electric power generation and conversion technology is now a key enabler for future improvements in pneumatic systems. The traditional pneumatic bleed air control system is replaced by electric motor-driven compressors that feed the rest of the ECS.

“With the arrival of ‘more electric’ aircraft, there is now a consideration between traditional pneumatic or a more electric function. The aircraft designer’s preferences are influenced by balancing overall energy optimisation at the aircraft level.

“Bleed air use is considered a fuel penalty since it takes away energy generated by the engine for a purpose other than propulsion. Depending on the aircraft and ECS configuration, for example, it could be more efficient to generate pressurised air using an electric compressor.”

Honeywell’s Tom Hart notes the lowering of cabin pressurisation altitudes. “Airliners today typically have cabin altitudes around 8,000ft, but there’s a demand for pressurising to a lower altitude, maybe 6,000ft. That creates higher pressure in the cabin, which wasn’t always possible with legacy aluminium fuselage technology.

“Now it’s feasible, with advanced composite materials, and passengers are feeling the benefits of lower cabin altitude, but it requires considerable cabin pressure management. We’re therefore working with our OEM air management system customers to provide that experience.

“Honeywell is also working to reduce air management system size and weight, while increasing efficiency. These systems are naturally large, so it’s not an easy task. Advanced alloys can save weight, but at an inevitable increase in cost; there’s always a trade between weight and cost.

“We’ve already seen efficiency improvements in recent years. Two decades ago, the bearings on air cycle machines were legacy oil, ball or early-generation air bearings. Now, Honeywell uses what we consider fifth-generation air bearing technology that allows the air cycle machine to spin at 50,000 to 70,000rpm. That means a more compact, more efficient machine that’s oilless, frictionless and longer-lasting.

“We’re really excited about the advances we’re making on catalytic ozone conversion and VOC reduction. We see them among the key future attributes to how aircraft air quality will become even purer than it is today.”

Peter Landquist also predicts a growing demand for improved aircraft air quality, as passengers become more aware of and concerned by the travelling environment. “I believe that within five to ten years cabin humidification will become mandatory for the airlines.

“We’re seeing growing interest in improving the dry cabin environment, especially as flights become longer and longer. Today’s modern premium passengers are also more demanding. They want to rest well, sleep and be in good shape when they arrive, ready to work or enjoy getting back to their family. It’s time to consider how we perceive the travel experience.”