Sophisticated climate control technology is what makes it possible for aircraft passengers not only to survive a hostile outside environment, but also to travel in comfort. Cabin air researchers at EADS Innovation Works are adding further refinements to this technology by working on more effective, regenerative air processing techniques and measures to counteract unpleasant odours.
At an altitude of 12,000 metres, the temperature outside an aircraft is around minus 50° Celsius. The air here is so thin that if breathed in, little oxygen would reach the lungs. The outside air can also contain a significant amount of toxic ozone, which is usually formed in the atmosphere during the day through intense solar radiation. Within the cabin, waste matter produced by passengers also comes into play: it has been estimated that on an 18-hour non-stop flight from Singapore to New York, 120 passengers discharge approximately 150 litres of perspiration, belch 144 litres of stomach acid gas and produce around 60 litres of “wind” through the digestive process.
The fact that passengers hardly notice these external conditions and interior cabin air impurities is owing to an advanced air conditioning system that keeps air pressure at a comfortable level, regulates the temperature and pumps fresh air into the aircraft, as well as using a high-efficiency filter to make the on-board air as clean as the air in an operating room.
The system works by bleeding off external air that has been compressed and thereby heated to over 200°C at the engine compressor. The air passes through special converters that turn the ozone into harmless atmospheric oxygen on its way to the cabin. It then passes through the air conditioning “packs”, in which a sophisticated temperature regulator heats or cools the air to the temperature required. This air is then mixed with processed recirculated air and fed into the cabin through ventilation slots and air nozzles installed above the passengers. The air pressure at this stage is equivalent to that of outside air at an altitude of 2400 metres. To ensure a constant air supply, the air introduced into the cabin is also continually removed again by suction. About half of this suctioned air is directly blown back outside.
The other half is cleaned of dust, bacteria and viruses through so-called HEPA (High Efficiency Particulate Arrestor) filters. Activated charcoal layers can also be installed in the HEPA filters to remove unpleasant odours. The recirculated air processed in this manner is then mixed with air from the packs and fed into the cabin. The air conditioning system achieves a very high rate of air exchange, which ensures that passengers enjoy a continuous, ample supply of high-quality air.
Robert Schreiber and Helmut Oberpriller work in the Surface and Chemical Engineering department at EADS Innovation Works in Ottobrunn as part of Christian Wolff’s Cabin Air Quality team, where researchers look for further system improvements, in particular with regard to minimising odours. “The problem with the odour filters used up until now is that they typically have to be replaced every 15 to 18 months (C-check),” says Schreiber, “as their capacity to absorb odours is understandably limited. This is bad for the environment and also costs a lot of money each time the filter is exchanged.” That is why Schreiber is conducting research into regenerative air processing techniques for odour reduction. He is currently focusing on testing two different solutions in the lab: an oxidative and an adsorptive technique. The oxidative approach (photocatalysis) works by trapping organic compounds on a photocatalytic titanium dioxide surface and oxidising them using UV radiation.
The resulting end products are harmless, odour-neutral carbon dioxide and water. The adsorptive process (rotation adsorber), on the other hand, consists of a rotating drum with numerous individual flow-through channels. An adsorptive coating on the internal walls of these channels traps the organic compounds flowing through with the impure air. However, the compounds are not destroyed, but rather separated from the purified air: the rotation allows unwanted compounds to be desorbed into a separate hot air stream. In this way, odours are targeted and fed to the outside in concentrated form.
Schreiber and his team are currently exploring both techniques with the help of the cabin air test rig, in which unwanted odours are simulated under laboratory conditions in order to demonstrate the efficacy of cabin air cleaning technologies. Schreiber can simulate odours like foot or underarm perspiration with products obtained from chemical wholesalers. “To simulate flatulence for example, we use dimethyl sulphide.” The advanced state of air conditioning technology on board passenger aircraft is confirmed by doctors' accounts that the danger of infection during a flight is significantly lower than with other modes of public transport, such as buses or trains.
On an 18-hour non-stop flight from Singapore to New York, 120 passengers discharge approximately 150 litres of perspiration, belch 144 litres of stomach acid gas and produce 60 litres of “wind”.
