Aircraft Air Conditioning System

Air Cycle Machine

• The turbine cools dramatically due to work performed and de-compression of bleed air, resulting in cold air at the output.
• Compressor compresses outside air, heating it significantly.
• Heated compressed air is blended with cooled, decompressed bleed air to achieve correct temperature and pressure.

Jet Pump

• Jet pump flow multiplier increases air taken into the cabin.
• Nozzle blows high-velocity compressor bleed air into a venturi, producing a low-pressure region that draws in outside air.

Reservoirs

• Storage bottles provide reservoir of compressed air for pneumatic systems.
• Bottles are made of steel and may be wire-wound for maximum strength.
• Compressors build up system pressure when it falls below normal level.

Compressor Types

• Independent cabin compressors are designed to overcome disadvantages of bleed air sources.
• Compressors can be driven through accessory drive gearing or powered by bleed air from an engine compressor.
• Two main types of compressors: positive displacement and centrifugal.

Positive Displacement Type

• Roots-type blower takes a predetermined volume of air, compresses it, and delivers it to the cabin duct.
• Rotors mounted on parallel shafts rotate at the same speed, building up pressure.

Multi-Stage Compressor

• Check valves allow air to flow in only one direction.
• Three check valves are used in the diagram.

Load Compressor

• Load compressor can react to pneumatic demands of the aircraft more efficiently.
• Ability to offload the load compressor when full demand is not required makes it fuel-efficient.

Ground Supply

• Ground pneumatic supply is not designed to cope with demands of the air conditioning system.
• Use is normally restricted to main engine starting and short duration maintenance checks.

Ground Supply Cart

• Provides a source of pneumatics for ground operations when an APU is not serviceable.

Pneumatic and Vacuum Systems

• Pneumatic and vacuum systems are used to power aircraft instruments and systems
• Venturi systems are used to produce suction to drive certain instruments
• Venturi tubes are rated by the amount of vacuum they will produce at 120 mph or 104 knots
• Common problems with venturi systems include:
  • Formation of ice in the throat
  • Damage or deformation to the tube assembly
  • Foreign object blockage
  • Blocked filter cartridge
  • Leaking plumbing

Vacuum Systems and Pumps

• Air-driven gyro instruments use an engine-driven vacuum pump to evacuate the instrument case
• There are two main types of positive-displacement, vane-type vacuum pumps:
  • Wet-type pump
  • Dry-type pump
• Wet pumps use oil lubrication, which can contaminate the instrument
• Dry pumps use self-lubricating vanes and rotors, eliminating the need for oil lubrication
• Filter maintenance and plumbing condition are crucial for dry pumps

Low-Pressure Systems

• Some aircraft use low-pressure, air-driven gyro instruments as primary or backup instruments
• Turbine-powered aircraft use engine compressor bleed air, while reciprocating engines use vane-type air pumps
• The air is regulated and filtered before being directed to the instruments

Vane-Type Air Pump

• Vane-type air pumps are used on aircraft with reciprocating engines to provide low-pressure air
• A variable orifice enables the variable pneumatic restrictor to be set for a wide range of airflow

Bleed Valves and Pressure Regulating Valves

• Bleed valves are used to relieve compressor load in the event of engine lubricating oil pressure loss
• Pressure regulating (unloading) valves maintain a system pressure of between 2900 psi and 3300 psi
• Relief valves are used to prevent damage from excessive pressures

Indications and Warnings

• Pneumatic duct pressure indicators display left- and right-hand pneumatic duct pressure
• Overheat detectors monitor the pneumatic distribution system ducts for overheat conditions
• Warnings are displayed in the flight compartment in the event of an overheat condition

Aircraft Pressure and Vacuum Systems

• Many aircraft use compressed air or pneumatic systems for various purposes, including:
  • Pressurization
  • Air conditioning
  • De-icing
  • Anti-icing
  • Engine-starting
  • Hydraulic power generation
  • Jet pump operation
  • Water and hydraulic system pressurization
  • Instrument operation
  • Landing gear
  • Flaps
  • Brakes
  • Other forms of mechanical actuation

Pneumatic Systems Categories

• Pneumatic systems fall into four basic categories:
  • Vacuum systems
  • Low-pressure systems (1-10 psi)
  • Medium-pressure systems (50-150 psi)
  • High-pressure systems (1000-3000 psi)

Advantages of Pneumatic Systems

• Advantages of using compressed air over hydraulic or electrical systems:
  • Universally available in an inexhaustible supply
  • System components are reasonably simple and lightweight
  • Compressed air is lighter than hydraulic fluid
  • No return plumbing system is required
  • Minimised fire hazard
  • Temperature problems are minimised

Venturi System Vacuum Supply

• Used in early light aircraft and some simple home-built aircraft
• System is extremely inefficient and limited in its capacity to drive instruments
• Works by accelerating air through a narrowing section, producing a lower pressure at the throat
• Common problems with venturi systems:
  • Ice formation in the throat
  • Damage or deformation to the tube assembly
  • Foreign object blockage
  • Blocked filter cartridge
  • Leaking plumbing

Vacuum Systems and Pumps

• Air-driven gyro instruments use an engine-driven vacuum pump to evacuate the instrument case
• Two main types of positive-displacement, vane-type vacuum pumps:
  • Wet-type pump
  • Dry-type pump
• Wet-type pump:
  • Lubricated from the engine low-pressure oil system
  • Oil has a one-way passage through the pump and is captured by the oil separator
• Dry-type pump:
  • Incorporates carbon into the vanes and rotor
  • Self-lubricating, no oil is needed
  • Filter maintenance and plumbing condition is crucial

Low-Pressure Systems

• Some aircraft use low-pressure, air-driven gyro instruments as either primary or backup instruments
• Turbine-powered aircraft use bleed air from the engine compressor section
• Aircraft with reciprocating engines use vane-type air pumps driven by electric motors or the aircraft engine
• Air is regulated and filtered before it reaches the instrument

Vane-Type Air Pump

• Used to provide low-pressure air on aircraft with reciprocating engines
• Pump is driven by electric motors or the aircraft engine
• Air is pulled into the pump and moved out as the chambers decrease in size

Combined Vacuum and Low-Pressure Systems

• Used in aircraft with rubber de-icer boots on the leading edges of the aerofoils
• Inflatable boots are constructed with several integrated air tubes
• Tubes are attached to plumbing from an air control valve, enabling some tubes to be inflated while others are deflated

Medium-Pressure Systems

• Usually does not include an air bottle
• Compressed air is taken from the turbine engine compressor section or a separate compressor device
• Air is routed to a pressure-controlling unit to reduce the pressure to about 40 psi

High-Pressure Systems

• Air is usually stored in metal bottles at pressures ranging from 1000 to 3000 psi
• Bottle has two valves:
  • Charging valve
  • Control valve
• System is limited by the small supply of bottled air
• Cannot be used for continuous operation of systems like landing gear or brakes

Pneumatic Sources

• In flight:
  • Engine compressors
  • Airborne Auxiliary Power Unit (APU) (if approved for bleed air supply in flight)
• On the ground:
  • Mobile starter units or ground pneumatic networks
  • APUs
  • Engine compressors

Engine Bleed Air Supply

• Pressurised air is taken from the engine compressor
• Bled off the compressor in one or two places
• Air from the fifth stage is considered low-pressure air
• Air from the tenth stage is considered high-pressure air
• Both bleed ports are never open at the same time
• Changeover is automatic

APU Supply

• Two basic types of APUs:
  • Single compressor
  • Dual compressor
• Compressed air is delivered to the aircraft pneumatic system via a load control valve
• Most APUs are designed for bleed air extraction on the ground only

Ground Supply (External Supply)

• Used for main engine starting and short duration maintenance checks
• Not designed to cope with the demands of the air conditioning system

Compressors

• Type of unit used to provide air for the pneumatic system is determined by the system's air pressure requirements
• Independent cabin compressors have been designed to overcome the disadvantages of bleed air sources
• Compressor types:
  • Positive displacement type (Roots-type blower)
  • Centrifugal compressor
• Roots-type blower:
  • Takes a predetermined volume of air, compresses it, and then delivers it to the cabin duct
  • Pressure is built up because the blower can deliver more air than the system can use
• Centrifugal compressor:
  • Has a centrifugal impeller similar to a turbo supercharger
  • Outside air is admitted to the supercharger and is then compressed by the high-speed impeller
  • Delivered to a distribution system

Turbochargers

• Used to provide air for the pneumatic system
• Driven by a smaller amount of high-pressure air drawn from the engine compressor stage (bleed air)
• Bleed air drives a turbine, which directly drives a compressor
• Hot compressed bleed air driving the turbine cools dramatically
• Act of compressing the air heats it significantly
• Heated, compressed outside air is blended with the cooled, decompressed bleed air to achieve the correct temperature and pressure

Jet Pump

• Used to increase the amount of air taken into the cabin
• Essentially a special venturi inside a line from the outside of the aircraft
• Nozzle blows a stream of high-velocity compressor bleed air into the throat of the venturi
• Produces a low-pressure region that draws air in from the outside

Reservoirs

• Storage bottles provide the reservoir of compressed air which operates all services

• Compressors are used to build up system pressure when it falls below the normal level

• Volume of the actuators and pipelines determines the size of the bottles required for normal and emergency operation of the pneumatic services### High-Pressure Storage Bottles

• Bottles are mounted upright and have a fitting at the bottom with a supply connection, pressure gauge connection, and a drain valve to remove moisture or sediment.

• Stack pipes are provided at the connections to prevent contamination.

• Pressure testing is required at specified periods, and the date of testing is stamped on the neck of the bottle.

Pressure Control and Regulators

• A regulator valve is used to maintain a stable airflow of about four inches of mercury in a vacuum system.
• The valve is adjustable and relieves excessive vacuum by allowing atmospheric air into the system through its own filter assembly.
• Some vacuum systems have a suction reducer or restrictor to operate at different vacuum levels.
• The suction reducer has a variable orifice to set a wide range of airflow.

Bleed Valves

• Bleed valves are used in pneumatic systems and are normally kept closed by engine oil pressure.
• When the engine stops or is shut down, the valve opens and relieves compressor load to prevent air stored in the system from leaking into the compressor case.

Pressure Regulating (Unloading) Valve

• The unloading valve maintains a system pressure of between 2900 psi and 3300 psi.
• When the system pressure rises to 3800 psi, an internal relief valve dumps pump output pressure overboard.
• When the system pressure drops to 2900 psi, the output of the pump is directed back into the system.

Relief Valves

• Relief valves are used in pneumatic systems to prevent damage from excessive pressure.
• They act as pressure-limiting units and prevent excessive pressures from bursting lines and blowing out seals.
• At normal pressures, a spring holds the valve closed, and air remains in the pressure line.
• If pressure grows too high, the force it creates on the disk overcomes spring tension and opens the relief valve.

Engine Bleed Air Pressure Control

• The Pressure-Regulating and Shutoff Valve (PRSOV) regulates bleed air pressure and contains a latching solenoid valve, an overpressure switch, and a pressure relief valve.
• The PRSOV actuator opens the valve against spring force and pressure in chamber B to regulate the downstream pressure to 42 psi (nominal).

APU Bleed Air Pressure Control

• The Auxiliary Power Unit (APU) load compressor supplies the APU bleed air.
• The quantity of air changes with the different bleed air demands of the pneumatic system.
• Bleed air supply to the aircraft pneumatic system is controlled by a load control valve.
• The load control valve is controlled by the Electronic Control Box (ECB) and opens when the APU bleed switch is set to the on position.

Distribution and Ducts

• The pneumatic manifold collects compressed air from the engines, APU, or pneumatic ground cart and supplies it to the user systems.
• Valves in the manifold control the flow of bleed air into the manifold, isolate the manifold into left and right sides, and control the flow of manifold air into the user systems.
• Pneumatic manifold ducts are made of strong, corrosion-resistant, light-weight tubing.
• Some ducts have a gold coating to protect them from hydrocarbon contamination and decrease heat transfer.

High-Pressure Distribution System

• High-pressure systems store air in metal bottles at pressures ranging from 1000 to 3000 psi.
• The high-pressure storage cylinder has a charging valve and a control valve.
• Although the system cannot be recharged during flight, operation is limited by the small supply of bottled air.

Emergency Backup Distribution Systems

• Many aircraft use a high-pressure pneumatic back-up source of power to extend the landing gear or actuate the brakes if the main hydraulic braking system fails.
• The nitrogen is not directly used to actuate the landing gear actuators or brake units, but instead applies pressurised nitrogen to move hydraulic fluid to the actuator.