Air Cycle Air Conditioning System

Air Cycle Air Conditioning Overview

• Used in most turbine-powered aircraft, it utilizes engine bleed air or APU pneumatic air for conditioning.
• Engine bleed air is prepared to pressurize the aircraft cabin by controlling the temperature and quantity to ensure a comfortable environment on ground and at all altitudes.
• This system is often called the air conditioning package or pack, and it's located in either the lower fuselage or tail section of turbine-powered aircraft.
• The Boeing 737 air cycle system can be found looking up into the air conditioning bay in the lower fuselage.

Pneumatic System Details

• The system is supplied with air by the aircraft pneumatic system, which uses bleed air tap-offs on each engine compressor or the APU pneumatic supply.
• It provides high-pressure air for air conditioning, wing anti-icing, water pressurization, and engine starting.
• Air sources include engines, APU, and high-pressure ground air equipment.
• Engine bleed air is tapped at the Intermediate Pressure (IP) and High Pressure (HP) compressor ports, with the HP port providing air when the IP pressure is insufficient.
• Pre-cooler cools the bleed air with cold air from the motor.
• An external pneumatic air supply can be connected when the aircraft is stationary.
• The high-pressure ground cart is essentially an APU on wheels with a filter protecting it from debris; a pneumatic line connects it to the aircraft's pneumatic manifold.

System Operation Explained

• At high altitudes, bleed air is too hot for cabin use so it is cooled by a heat exchanger using ram air.
• This cooled bleed air is directed into an air cycle machine, then compressed before flowing through a secondary heat exchanger for further cooling.
• The bleed air then flows back into the air cycle machine where it drives an expansion turbine for even more cooling.
• Water is removed at this stage, and the air is mixed with bypassed bleed air for final temperature adjustment before being sent to the cabin's air distribution system.

Pack Valve Component

• A pack valve regulates conditioned air from the pneumatic manifold into the air cycle system.
• It is electrically controlled with a switch from the air conditioning panel in the cockpit.
• Pack valves open, close, and modulate to allow the air cycle air conditioning system to be supplied with a designed volume of hot, pressurized air.

Bleed Air Bypass

• A bypass is in place for some pneumatic air supplied around the air cycle air conditioning system.
• Warm bypassed air mixes with cold air produced by the air cycle system to deliver air to the cabin in a comfortable temperature.
• This is achieved by a mixing vale which also simultaneously controls flow of bypassed air and air to be cooled.
• A means to manually control the valve is through the cabin temperature selector in manual mode.

Primary Heat Exchanger

• Warm air dedicated to pass through the air cycle system first passes through a primary heat exchanger which functions like a radiator in an automobile.
• To reduce the temperature of the air inside the system, a controlled flow of ram air is ducted over and through the exchanger. A fan draws air or ram air when the aircraft is stationary on the ground to facilitate heat exchange.
• Ram air doors modulate to increase or decrease ram air flow to the exchanger while in flight.

Secondary Heat Exchanger Facts

• Warm air passes through from the compressor of the air cycle machine.
• A controlled flow of ram air is ducted over and through the exchanger in order to reduce the temperature of the air inside the system.
• Heat exchange is made possible when the aircraft is on the ground via a fan, which draws air through the ram air duct.
• Ram air doors modulate in flight, and their pupose is to increase or decrease ram air flow to the exchanger

Refrigeration Turbine Unit or Air Cycle Machine and Secondary Heat Exchange

• The refrigeration turbine unit, also known as the air cycle machine (ACM), is the heart of the air cycle air conditioning.
• The system consist of a compressor driven by a turbine on a common shaft.
• Air flows from the primary heat exchanger into the compressor side of the ACM and as this air compresses, its temperature rises.
• System air is then sent to a secondary heat exchanger (similar to the primary heat exchanger) located in the ram air duct.
• Easy exchange of heat energy to the ram air is facilitated by elevated temperature of the ACM compressed air.
• The cooled system air, which is still under pressure, flows through the system and exits the secondary heat exchanger.
• Finally, the cooled air makes its way to the turbine side of the ACM.
• As it passes through and drives the turbine, the ACM turbine's pitch angle extracts more energy from the stream and cools the air even further as it expands at the ACM outlet.
• The system's air temperature lowers to near freezing because of the continuous expulsion of energy stemming from driving the turbine and expanding at the turbine outlet.

Water Separator Facts

• The air becomes so cool from the air cycle machine it can no longer hold the quantity of water it could when it was warm.
• The water must be removed before being sent into the aircraft cabin through the use of the water separator.
• The separator operates with no moving parts.
• Foggy air from the ACM enters and is forced through a fiberglass sock that condenses and coalesces the mist into larger water drops.
• A convoluted interior structure of the separator swirls the air and water while the water collects on the sides and drains, whereas the dry air continues through.
• A blocked bypass valve handles that case.

Refrigeration Bypass Valve

• Air exiting the ACM turbine expands and cools, and in this state it is prone to freezing the water in the water separator, thus inhibiting or blocking the airflow.
• A temperature sensor in the separator controls the refrigeration bypass valve that maintains the air flowing through the water separator above freezing.
• It may be referred as temperature control valve valve, 35°F valve, or anti-ice valve.
• The opening of warm air bypasses the ACM.
• Up stream of the water separator is where heated air prevents freezing and thus, regulating the ACM so it can pass through without freezing.

Cabin Temperature Control Systems

• Cabin, cockpit, conditioned air ducts, and distribution air ducts measure cabin temperature and are sent into a temperature controller or regulator.
• This is typically located in the avionics bay.
• A temperature selector in the cocpit can also be used to input the desired temperature, allowing the temp controller to compare actual temperature signals received from various sensors and the desired input temperature.

Typical System Operations

• Controllers in zones of passenger areas or flight decks give temperature information to controllers.
• The lowest temperature is set by the controllers to control air pack temperature of a zone.
• Those controllers use sensors in the mix manifold to adjust pack temperature, which does so because of the air temperature recirculation.
• Air is warmed by the trim to the controllers who control the trim air system.
• Duct sensors measure zones where their settings are to a higher temperature than the lowest setting.
• Controllers employ duct and temperature sensors to control packs and trim systems alike.
• Supply of trim air is between 2 and 8.5 psi above average, a condition assured by shutoff valves and left and right regulating side trim.
• A usual pressure of 5 psi above cabin pressure results, which is a minimum noise output and efficient pneumatic system. The trim air pressure sensors give information to the controllers for control and indication.

Boeing 777 Environmental Control System Facts

• Two shutoff & flow control valves control air flow coming from the pneumatic systems through each packs.
• Only air is open through one valve at a time.
• Altitudes of 26,000ft (7923m) make it that the superior valve is open.
• In excess of 26,000 ft, the valve below will release the air into an ozone convertor.
• When an airplane lowers to below 24,000 ft (7315mm) then the opposite pattern happens again. The top goes fully opens whereas the bottom remains closes.

Environmental Control Systems Specific to 777

• Ambient ram air is used through heat exchangers in order to cut heat emanating pneumatics.
• More cooling air is directed to the condenser air from the compressor and the system is then connected into the air conditioning machine.
• Remaining water condensate is directed to the ram air through an evaporator.
• Collected air in the reheated is employed in eliminating any ice particle remnants. The same then warms in the condenser, which then moves to the reheating process.
• Turbines of the ACM make expanded air to be employed with the provision of generating cooling.
• The control of the turbine for bypass valve is provided through cabin temperature controller (CTC).
• A similar amount with respect to lower limit with those associated doors relating to the rams can be related accordingly. These controls give adjustment in pack controllers, in other cases controllers perform their use of data which can emanate data by sensors with indications made during testing or testing situations.

Additional Boeing 777 Environmental Control System Details

• When cooling isn't as much as needed, pack goes to energy cooling which releases air by reducing compressor and turbine pressure. Pack airflow is possible through reduced pneumatic pressure.
• Outlets with temperature controllers make it possible for turbine valves to make modulation while doors ram air is possible.
• It's possible for temperature to lower as well with operation. In a scenario where ACM fails there will be failed inlets and valves to cause the failure. So, operation might need checking or air passing, which could create ACM from a possibility of cooling.
• Rams with cooling air doors modulate with heat exchange and can get this at temperature pack outputs.

Cabin Conditioned Air Supply and HEPA

• An airliner duct hose gives hot or cold outputs from a grounded space across its own air distribution system duct.
• Fixed low airports install those systems at their own gate through a replaced pressure air in the grounded carts. Conditioners get used through air with a structure and permanence throughout gates as well.
• In circumstances when quick turn happens for aircraft and power units (auxiliary), conditioning with air to cabins make air systems be supplied to the cycle machine itself.
• As sustainable, the airport is tasked in offering air by handling the unit instead. Because the operation uses electricity for fuel, it makes a sustained environment so there is sustainable gases offered with all. - High efficiency filters get introduced at various airports and airliners. They sustain standards of security, allergen, moisture of particles. - Pressured air along with the same air recirculated with new air at around 50%. - In order for HEPA (High Efficiency Particulate Air) to have effectiveness with airplanes and their boarding abilities, it would have to give sufficient quality with the air used.

More on HEPA Filters

• Filter makes take away with particle matter with three means that operate, these are the three air operations. This interception involves inertial air and then has diffusive interception as well.
• An air stream gets sized out to its opening for filtration but some air is removed. Direct interception removes some sized matter.
• Particles smaller than the opening make a division of the air but some particles are captured with fiber making smaller ones through direct.
• Little particles coming up to below the 0.2 micron measurement have brownie force and also a interception process. There is an affect with this particles in rapid movements which then adhere to pores. Air systems are meant to have this adherence.

The air cycle air conditioning system, common in turbine-powered aircraft, uses engine bleed air or APU pneumatic air to control cabin temperature and pressure. Located in the lower fuselage or tail section, it prepares bleed air for pressurization, ensuring a comfortable environment at all altitudes. The pneumatic system supplies high-pressure air for air conditioning, anti-icing, and engine starting.