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TECHNOLOGY FOR THE GOOD CLIMATE ON BOARD

Today, commercial airliners are flying at altitudes up to 12000 meters. No human being would survice the hostile environment at that level without protective measures: The air only has one quarter of the pressure that it has on the surface of the Earth, the temperature is around minus 60 degrees centigrade and the relative humidity is as low as in the driest regions of the world.

Since high-flying long-range commercial aircraft were introduced in the fifties, engineers had to come up with solutions to create atmospheric conditions on board which make air travel possible. The main focus of the technological challenge is on the cabin pressure altitude.

With an increase in flight altitude not only the air pressure is decreasing (by 50 percent at 18000 ft) but also the partial pressure of the air's oxygen content which in turn is necessary to keep up the oxygen supply to the human body. With this in mind, it became necessary to maintain a high pressure inside the cabin.

However, to maintain the same atmospheric conditions which we have at sea level would require a such a strong airframe structure that such a design would be very difficult to operate economically. Early medical studies revealed that the human being can stay for a longer period of time at altitudes around 8000 ft without a health concern and without loosing to much comfort. Thats why the cabin pressure of airliners is lowered during flight down to a certain limit. Worldwide, there is an agreement to limit the cabin altitude in commercial air traffic to 8000 ft.

A sophisticated cabin pressurization system is actually pumps up the cabin at altitude to counteract the low outside air pressure. The pressure on the inside of the aircraft fuselage can be up to one ton per square meter at high altitudes. The change in pressure inside the cabin during climbs or descents is systemwise limited to a passenger friendly 150 m per minute during climb and 100 m per minute in descents, while the aircraft itself may be actually changing altitude much more quickly.

The actual air demand per passenger is at around 6,8 liters per minute. When calculating the needed amount of air, Airbus, for example, uses a value of 9,5 liters per second, taking into consideration the maximum number of passengers for the specific aircraft type. For the long-range jet A340 this computes to an amount of 3300 liters of air per second which has to be moved by the system. This also means that each of the passengers actually gets 80 times the actually needed amount of oxygen. The idea behind this large amount of air is a quick removal of used air (carbon dioxide) as well as an equal temperature distribution.

However, having just a quick exchange of air is not sufficient for creating a comfortable climate on board of an aircraft. In airliners, the so called environmental control system (ECS) is responsible for an adequate and comfort oriented processing of the cabin air. Two of these ECS systems need to be on board for safety (redundancy) reasons, very large airliners, such as the Boeing 747-400, even need three.

At outside air temperatures of minus 60 degrees centigrade one would assume that the ECS main work is to heat the air to a comfortable temperature. However, since the fresh air supply for the cabin is mainly bleed air from the engine compressor stage and has a temperature of 300 degrees, the air conditioning is mainly needed for cooling purposes.

Taken directly from the engine compressor, the bleed air is first routed through a turbine which cooles it down to approximately 200 degrees. Furtheron, the bleed air is routed through ductings in the wings and into the fuselage. Part of the still very hot air is then ducted into the cooling packs which can cool down the air to below zero degrees. Depending on the actual temperature need, hot air is mixed into the cool air at a later stage. Two independent computers are responsible for the regulation of the cabin temperature and the pack outlet temperature respectively.

The second reason why the air conditioning system is rather a cooler than a heater is the heat which is generated by the passengers themselves. Each person generate about 80 to 1000 Watt. For an A340 seated with 300 passengers this means a continous heat emission of approximately 25 Kilowatts. This is added by other heat sources such as the galleys and the lighting. Because of these secondary heat sources the air entering the cabin from the ECS only needs to have a temperature of 18 degrees to keep the actual cabin temperature at 24 degrees.

Still, there is a reason why the air quality on board is often critisized by the occupants. That is because of the extremely low relative humidity. On Earth, we are used to a humidity of approximately 40 to 70 percent. However, at typical cruise flight levels, the air only has a relative humidity of roundabout three percent. While early airliner generations used solely fresh air (and such only the extremely dry air) for the cabin, in modern aircraft approximately 40 percent of the cabin air is recirculated and reused (in some aircraft, such as the 747-400 this percentage can even be up to 54 percent).

The recirculated air is ducted through a special filter system in order to filter out dust, tobacco particles, bacteria and viruses. However, the filtered air keeps its humidity such slowly increasing the relative humidity of the cabin air. Depending on the cabin zone, the humidity can go up to five percent in the First Class (with only a few occupants) and to 15 percent in a fully-seated Economy Class.

Using recirulated air on board of airliners has already been the topic of many controversies in the past. The main accusation and perception among many passengers and flight crew is that flying on commercial aircraft is unhealthy because recirculated air spreads bacteria and viruses among passengers and crew. The critics continue, saying the only reason why airlines are using recirculated air is that this procedure reduces the amount of air that has to be taken from the engines. The more bleed air is taken from the engines the higher is the fuel consumption.

Airlines and aircraft manufacturers are increasingly installing highly efficient filters in their aircraft. Since the end of the eighties all Airbus aircraft are fitted with so called HEPA filters (high efficiency particulate arrestor) which filter out up to 99,97 percent of all particles in the air. According to Airbus, the bacteria concentration in the air supply is at least equivalent to intensive care room recommendations.

Additionally to that, says Dr. med. Lutz Bergau, the medical director from Lufthansa, in a medical report, the extreme low relative humidity significantly reduces the survivability of viruses and bacteria on board. Bergau sees a much higher chance of getting infected by taking public transportation such as busses or subways. According to Airbus, the biggest health risk may arise from person to person contact (droplet infection).

Even today, aviation doctors, airlines and aircraft manufacturers are discussing the optimum mixture of fresh and recirculated air. From the medical point of view, doctors are already warning of potential plans by the airliner manufacturers to completely rely on recirculated air in the future by using a new level of filter quality.

Cabin comfort is one of the most important marketing arguments for the airlines. One improvement is already being introduced more widely in the airline industry. By putting ozone converters in the fresh air supply, the level of harmful ozone in the cabin can be reduced to harmless values (the air's ozone concentration increases with altitude and can lead to health problems). Another (future) option could be to increase the humidity of the cabin air.

This has been rejected so far because of technical reasons (corrosion and weight penalty for the extra water). While none of the manufacturers has concrete plans to go in that direction, the creating of some kind of "green house atmosphere" could offer a new quality of air travel for First Class, VIP or business travellers.

From page 96 of FLUG REVUE 10/99