Aircraft Air Conditioning Systems: Types, Parts, Operation
Commercial aircraft and military aircraft are fitted with air conditioning systems to make the planes conducive environments for passengers and flight crew alike. Light General Aviation (GA) aircraft typically do not have air conditioning systems as they are quite expensive. Moreover, the temperatures at the altitude where they fly are quite low, so it’s cool enough for comfort.
Aircraft Air Conditioning Systems
In this article, I will talk about:
- Types of aircraft air conditioning systems
- Common components of aircraft conditioning systems and their operation.
Types of Aircraft Conditioning Systems
The two main types of air conditioning systems used on aircraft are:
- Air cycle air conditioning system: this is typically used in aircraft powered by gas turbine engines and makes use of engine bleed air or Auxiliary Power Unit (APU) pneumatic air. In some aircraft, the cycle is controlled automatically.
The bleed air supplied from the engines via the pneumatic system is too hot to be directly used in the aircraft, therefore, an air conditioning system is needed. Before the air enters the system, it is first cooled by ram air in a heat exchanger.
This cooled air is then passed into the system where it is compressed and then cooled by a secondary heat exchanger. The cooler bleed air then goes on to drive the expansion turbine and cool even more. The final temperature adjustment is done after moisture is removed from the bleed air and the bleed air is mixed with bypassed bleed air.
This air is then passed to the air distribution system. The air conditioning system performs this process separately for the cockpit, and forward and aft cabins.
- Vapour cycle air conditioning system: this is typically used in aircraft powered by reciprocating (piston) engines, but they may also be found in some turbine-powered aircraft. It is the only practical solution for air conditioning in piston engine-powered aircraft since such engines do not produce bleeding air. The system only acts to cool the cabin, by transferring heat from inside the cabin to outside the cabin.
The heat is transferred to a refrigerant liquid which then turns to vapour as it gains extra (heat) energy. The vapour then undergoes compression which makes it very hot. It is then removed from inside the cabin. It releases the gained heat energy to the outside and then returns back to the cabin as a liquid. The cycle continues. It is essentially the same way a fridge works.
Components of Air Conditioning Systems and TheirWorkings.
Air conditioning packs: there are two packs which are normally automatically operated. Their operation is also independent of each other. Within each pack, there are primary and secondary (main) heat exchangers, pack valves, an air cycle machine (ACM), AKA a refrigeration turbine unit, with compressor and turbine sections, and a water separator system, and a cooling air fan.
The bleed air which enters the pack is first passed through the primary heat exchanger. A controlled flow of ram air is also ducted into the exchanger and heat is exchanged between the warm bleed air and the cool ram air.
When the aircraft is on the ground, a fan draws air through the ram air duct for the heat exchange. In flight, the ram air doors move to increase or decrease ram airflow according to the deflection of the wing flaps.
At slow speeds, the flaps are extended, and the doors are open. At high speeds, the flaps are retracted, and the doors move to close so that the ram air intake is reduced.
After passing through the primary heat exchanger, the air flows through the compressor. The compression performed causes the air to heat up and be pressurised, which will be useful in the main heat exchanger as it fosters the heat energy exchange to the ram air. The air then passes through the main heat exchanger in the ram air duct. The water separator system then removes the water from the air which is saturated.
The forced movement of the air through the water separator’s fibreglass sock causes the water to condense then the condensed droplets join. The water and air are then swirled so that water collects on the sides of the separator and is drained. The dry air is then passed to the turbine to drive it and in the process, it expands.
This expansion powers the driving of the compressor and cooling the air fan. The energy loss from the air results in much more cooling so that the air leaves the turbine at a very low temperature.
- Pack control valve: the valve is controlled electrically by the pack controllers but operated via pneumatics. The command on the switch on the air conditioning panel in the cockpit is what controls it. The valve opens, closes and modulates as directed in order to allow the air conditioning system to be supplied with heated, compressed air. If there is no air pressure, a spring which is connected to the valve causes it to close. In the event of operations such as pack overheating, engine start, fire and ditching pushbuttons in ON mode, an electric signal given causes the valve to close automatically.
- Mixer unit: this functions as a bleed air bypass mechanism. It mixes the warm air bypassed by the recirculation fans with the very cool air from the air conditioning packs. This is done so that the air which is distributed to the cabins and cockpit is at a suitable temperature. The flow of both the bypassed air and the air which is to be cooled are controlled by the mixer valve to suit the demands made by the manual or automatic temperature regulators.
- Temperature sensors: these are necessary for the detection of the temperature of the air so that the system can act accordingly to produce air of a suitable temperature. The sensors can be found in the cockpit zone, and the lavatory extraction circuit and galley ventilation system in the cabin zones.
- Ram air: apart from the normal ram air used in the packs, there is an emergency ram air inlet. It vents smoke off the cabin and cockpit and provides ventilation if there is dual pack failure. It is controlled by the RAM AIR pushbutton on the air conditioning panel. The pushbutton, when switched on, causes the ram air valve to open as long as ditching is not selected.
In the A320 aircraft, when the pushbutton is on, the outflow valve opens halfway if it is in AUTO mode and the differential pressure is below 1 psi. In MANUAL mode, the outflow valve will not open automatically, irrespective of other conditions. If the differential pressure is above 1 psi, the check valve situated downstream of the ram air doors will not open even when commanded to do so, such that there is no air supply.
- Hot air pressure regulating valves: they regulate the pressure of the hot air which is upstream of the packs. They are also operated via pneumatics and are electrically controlled using a button in the cockpit panel. If there is no air, the valves are kept closed by springs. They close automatically when there is duct overheating or both cabin zone or cockpit trim valve failure. If either one of the cabin zone trim air valves is functional, the hot air pressure valves remain open.
- Trim air valves: these are also bleed air bypass valves. They are controlled electrically by the zone controllers. When commanded to, they optimise the temperature by adding hot bypassed air to the bleed air. There is one for trim air valve for each zone of the system.
- Zone controller: they control the temperature and pressure of the air for their corresponding zones. Each zone controller has a primary channel and a backup secondary channel. In the A320, the pilot can use the PACK FLOW selector on the AIR COND panel to make adjustments to the backflow based on the number of passengers and the external conditions. Regardless of the selections made, the system delivers HIGH flow when only one pack is operative, and when the APU is the source of bleed air (instead of the engines). The system delivers normal flow if LO flow is selected and the desired temperature cannot be met. When the system is incapable of meeting a cooling demand because the bleed pressure is insufficient, the zone controller sends a pressure-demand signal to the EIU to increase the minimum idle rpm in order to raise the bleed pressure. When the APU bleed valve is open and the desired temperature cannot be met in a zone, the zone controller sends signals to the APU’s Electrical Control Box (ECB) to increase the flow output.
As for temperature regulation, the temperature sensors monitor the temperature of the air in the ducts of the different zones and input them into the zone controllers. The pilot uses the temperature selectors on the AIR COND panel to select the desired temperature. The zone controller then compares the input of the temperature sensors with the desired temperature via logical processing.
An output signal is then sent to the pack controller which works to make the air conditioning packs produce an air of the desired temperature via the operation of the mixer unit. The air temperature is finally optimised by the zone controller using the trim air valves. The range for the temperature selector is between 18 ̊C- 30 ̊C.
- Pack controller: there is one for each zone and each one has primary and secondary channels as well. Pack controllers regulate the temperature of their corresponding packs, in response to the signals sent by the zone controllers. They move the bypass valve in the mixer unit and the ram air doors. The ram air doors close during take-off and landing so that there is no foreign object debris (FOD). During take-off, the doors close automatically once the Take-off power is set and the main landing gear struts are compressed. During landing, the doors close once the main landing gear struts are compressed and the speed is at least 70 knots. They wait 20 seconds to open after the speed drops below 70 knots. The pack controllers also regulate the airflow via the pack control valves.
- Receiver dryer: this is a reservoir that stores the refrigerant in liquid form in a vapour cycle air conditioning system. It also serves as a filter. The refrigerant is typically a halogen compound with a boiling point of about -15 °F (-26 °C). When bubbles form in the glass part of the receiver dryer, it indicates that the dryer needs to be filled with refrigerant.
- Expansion valve: the refrigerant flows from the receiver dryer into an expansion valve. The expansion valve contains a small orifice which blocks most of the refrigerant from flowing out. The pressure in the expansion valve causes the refrigerant to be forced out of the orifice.
- Evaporator: The evaporator assembly is similar to that of a car radiator. It contains coiled tubing into which the refrigerant flows from the expansion valve orifice as tiny droplets. Here, the refrigerant absorbs enough heat energy from the cabin air so it turns to vapour before it is passed out.
- Fan: it blows the cabin air over the evaporator.
- Compressor: the gaseous refrigerant from the evaporator comes here next so it can undergo compression and a resultant temperature increase.
- Condenser: the pressurised high-temperature gaseous refrigerant enters the condenser from the compressor. It has a long assembly of tubing with fins to facilitate heat transfer. External air is passed over the condenser, and consequently, the gaseous refrigerant releases heat into the external air.
This principle of heat exchange is easy to understand when we establish that heat is always transferred from hotter regions to colder regions.
Conclusion
The pressurisation system of turbine-powered aircraft works hand in hand with the air conditioning system. The former ensures that the pressurised air generated by the air conditioning system is regulated. If the pressure is too much, this can cause serious damage to the aircraft structure and the aircraft’s occupants. If the pressure is too low, the aircraft’s occupants will not be able to breathe.
The pressurisation system works well with the air conditioning system to prevent such from happening. Components within the air conditioning system ensure that the supplied air temperature is not too high or too low such that it causes damage to the aircraft components or the aircraft’s occupants. The redundancy provided by backup components within the two systems increases the safety of their operation.
References
- Youtube (2016). A320 CBT #16 Air Conditioning System Abnormal Operation. Available at: https://www.youtube.com/watch?v=3my5xqHvtfA&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=36
- Federal Aviation Administration (FAA) (2018). Aviation Maintenance Technician Handbook-Airframe, Volume 2. Federal aviation administration.
- Air Conditioning and Pressurisation. (n.d.). [ebook] Airbus training. Available at: http://www.smartcockpit.com/docs/A320-Air_Conditioning_and_Pressurisation.pdf
- Aircraft Systems. 2017. Aircraft Air Conditioning Systems. [online] Available at: <https://www.aircraftsystemstech.com/2017/05/aircraft-air-conditioning-systems.html#:~:text=There%20are%20two%20types%20of,often%20used%20on%20reciprocating%20aircraft.> [Accessed 20 July 2022].
