Air Conditioning on Airplanes

modern aircraft require air conditioning for occupants

Maintaining a correct temperature on a commercial aircraft with hundreds of passengers on board traveling at 40,000 feet (12,000 meters) represents a huge challenge for air conditioning specialists.

Achieving a comfortable cabin environment for the passengers and crew of an aircraft requires the control of different variables such as: Air Temperature, Humidity, Pressure, Air Quality Control.

These are the factors involved in the operation of the air conditioning system. The cabin of a commercial aircraft represents a huge volume for the only source of air available for the conditioning of the same, we refer to the hot pressurized air that is taken out of the engines. So it is normal that there are limitations on the amount of air that can be extracted in the different phases of flight, because among the main objectives of the air that passes through the engine is not the conditioning of the air for the passengers but the propulsion for the plane to fly, and only a small part of it must be diverted to this function in addition to attending to pressurization and other services required in the aircraft cabin. To better understand the complication of air conditioning in aircraft, other issues must also be considered, such as the limitations of air on board and the conditions in which the system must operate, facing external temperatures of up to -60º C, as well as the lack of humidity.

The cooling systems used in aviation are the air cycle and the steam cycle. The first is based on the principle of heat removal by transformation of heat energy into mechanical work, this is used in commercial aircraft, military transport and combat aircraft, works with the air that is extracted from the turbojet compressor, said hot and pressurized air, is used for heating, cooling and even for the pressurization of the cabin. The second system by steam cycle is more limited since it provides only the cooling of the air; it works by evaporating a coolant into a unit very similar to the one that is widely used in the automotive industry; such a system is generally used in flights made at low altitude and short distance. It should be noted that in turboprop aircraft both types can be used.

Now with respect to the air conditioning unit, the aircraft, depending on its size, has one or more independent units. Figure 1 shows the scheme of this unit, it consists of: a) an impact air duct, b) a heat exchanger, c) the cooling turbine assembly.

a) The impact air duct.- is a pipe that is connected to a gate located on the outer surface of the aircraft, so that in flight the cold air and with sufficient dynamic pressure enters through the gate and follows the course of the duct. For this reason this current is called "impact air". The duct (in blue in the figure) is open at both ends, so that it continuously enters and exits the air captured from the outside of the aircraft.

b) Heat exchanger.- Inside this duct is the heat exchanger; through it circulates the hot and pressurized air extracted from the turbojet compressor. The heat exchanger is simply a radiator that is bathed in the current of cold impact air.

c) Cooling turbine assembly.- It consists of an impact air suction fan and the turbine wheel. The fan and the turbine wheel are joined by an axis, so that they rotate at the same speed. Actually the turbine rotor moves the fan except when the aircraft is on the ground, where there is no impact air current. In this case the fan is moved by an electric motor that maintains air circulation.

Its operation is based on extracting air from the engine through one or more valves that are in the turbojet compressor and is driven into the heat exchanger. This flow of air extracted from the engine is destined to finally enter the cabin of the aircraft which is not the impact air as one might think. For its part, the fan of the unit sucks outside air. The impact air is expelled into the atmosphere after passing through the heat exchanger and cooling the hot air expelled by the engine.

The hot air coming from the compressor runs its course and passes into the turbine. The turbine wheel is of a small diameter and rotates at 90,000 r.p.m; which is placed in the circuit so that the air expelled from the engine experiences a very strong expansion when passing through it. With the expansion of air two things happen: its temperature decreases (law of perfect gases) to the point where ice is normally formed at the outlet of the duct if there is enough moisture in the air and secondly, the expansion produces the rotational movement of the wheel and it drives the fan located on the same axis. In flight we have a machine with autonomous movement. This provides a very cold air source for the cabin. It is well understood that if this airflow is introduced directly into the cabin the cold would be extreme freezing the passengers, so it is clear that its temperature must be modulated as follows.

The modulating function is done with the bypass valve of the VDT cooling turbine. This valve is located in a duct parallel to the heat exchanger.

If the VDT valve is very open the flow of air expelled by the engine mainly follows this route, since it is easier for the air, than to enter the internal path of the heat exchanger which is made of narrow tubes. The path through the VDT bypasses the passage of air through the heat exchanger and the expansion turbine, so that it is a flow of hot air, more or less at the air temperature in the engine compressor. On the contrary, if the VDT valve is very closed, almost all the air is forced to pass through the path of the heat exchanger, that is, along the path where the air is subjected to a strong cooling.

The cabin air temperature is regulated by the position of the VDT valve. If the valve is wide open, heat is sent to the cab; if it is very closed the air that comes out of the unit is cold because it has mixed little hot air from the VDT duct. This would be the typical case of operation of the unit on a high-altitude cruise flight, where the aircraft experiences heavy heat losses and the cabin needs heating. In low-altitude flight, it is usually necessary to introduce a large amount of cold air into the cabin, so that the VDT tends to be placed in more closed positions. The pilot has a control to select the position of the valve, but it is normal for the system to operate in automatic mode by regulating the cabin temperature to a previously established value.

Current commercial aircraft require special air conditioning units. Much of the equipment is made of aluminum to save weight. Refrigeration and air conditioning units for large aircraft are usually set on the wings. In a smaller aircraft, these units may be in the body of the aircraft. The air from the unit is distributed to the cab through the diffusers and grilles located on the roof.

The cabin air conditioning system on current aircraft is designed to provide a safe and comfortable cabin environment at flight altitudes that can reach above 40,000 feet.

The system is designed to provide a safe and comfortable cabin environment

By a government and FAA regulation, cabin pressure cannot be less, at the maximum altitude of the crossing, than the equivalent of outside air pressure at 8,000 feet high. The air conditioning system in addition to controlling the temperature and providing comfort to passengers controls the pressurization of the cabin, the flow of air and the filtration of the same in the following way. The cooled air coming from the air conditioning unit flows into a compartment where it mixes with an approximately equal amount of highly filtered air from the passenger cabin.

Combined the outside air and the filtered air is channeled to the cabin and distributed through the diffusers located on the roof of the cabin. Inside the cabin, the air flows follow a circular pattern and their exit is through grills located on the floor or on any side of it, in some aircraft this exit is made through ducts located above the hand luggage compartments. The air coming out is sent under the cockpit floor in the lower lobe of the fuselage. Air circulation is continuous and quickly eliminates odors while also maintaining a comfortable temperature in the cabin.

About half of the air leaving the cabin is immediately removed from the aircraft through an outlet valve in the lower lobe, which also controls the pressure of the cabin. The other half is directed by the fans through the special filters under the floor, and then mixed with the outside air coming from the engine compressors. These high efficiency filters are similar to those used to keep the air clean in hospitals, such filters are very effective on microscopic particles as they are able to filter bacteria and viruses. It is estimated that between 94 and 99.9 percent of the airborne microbes that reach these filters are captured.

There are several features of the cabin air conditioning system that deserve special emphasis:

Recent studies have confirmed the safety and full effectiveness of aircraft cabin air conditioning systems. One of the studies, conducted for the U.S. government, was the most comprehensive of all. It involved an independent test that took air samples on 92 randomly selected flights from a commercial airline. Levels of contaminants such as fungi and bacteria were evaluated and turned out to be similar to or lower than those found in normal indoor environments. Also, carbon dioxide levels were evaluated, resulting in an average being less than half the limit recommended by the American Conference on Industrial Hygiene.

Therefore, we can see that it is implausible that the cabin air of a commercial aircraft contains enough pollutants to cause the occasionally reported conditions such as headache, fatigue, nausea or respiratory problems.

These conditions are more likely to be caused by the complex interplay of factors such as: the individual's health, flight delay, medication use, alcohol consumption, and motion sickness (vertigo) along with effects of cabin altitude and low cabin humidity.

Recent studies have confirmed the safety and full effectiveness of aircraft cabin air conditioning systems.

It is worth mentioning that the smoke outlet sometimes through the air conditioning ducts of an airplane is a simple sign that the equipment works perfectly because if the temperature at the airport and the humidity of the air is high, the exit of cold air through the ducts, produces the rapid condensation of that water, with the consequent appearance of smoke that is nothing more than water vapor similar to that which forms the clouds.

When the cold air mixes with the hot air and, therefore, the temperature of this increases (while lowering that of the hot one), the water returns to its gaseous and colorless state so that that vapor disappears after a few seconds. Only in extreme cases does the presence of steam last beyond a few seconds without representing any problem.

The proper functioning of the system is of vital importance for the operation of the entire aircraft

The air conditioning system of an aircraft is of such importance that last August a Boeing 737-300 type aircraft of the Cypriot company Helios Airways which was on an international flight from Larnaca, Cyprus to Athens, Greece, lost communication with air traffic control shortly after the crew reported a problem in the air conditioning system of the plane so a Greek F-16 aircraft he was sent to intercept the passenger plane and reportedly observed at least one person who was not a crew member inside the cockpit. The co-pilot seemed to be unconscious and did not see the captain in the cockpit. The plane apparently flew to run out of fuel crashing into a mountainous area approximately 40 kilometers from Athens, killing all 6 crew members and their 115 passengers. Shortly after the rescue services reported that most of the victims had died of frostbite before the plane crashed so it is presumed that there was a failure in the unit of the air conditioning system at the altitude of cruise flight that the plane made causing the interior temperature to drop dramatically to approximately 60 ° C below zero, causing tragedy.

As we can see, the perfect functioning of the air conditioning system in airplanes is essential to make a safe flight at the altitudes that current aircraft fly.

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