11.4 Air Con & Cabin Pressure

• Aircraft pressurized air comes from engines, the auxiliary power unit (APU), or a ground source (air cart).
• The pneumatic system supplies pressurized air to the air conditioning packs, which pressurize the aircraft.
• Outflow valves in the forward and aft areas control the amount of air leaving the aircraft, maintaining cabin air pressure within operating limits.
• Compressing air raises its temperature, so pressurization systems often have a built-in cooling function (e.g., heat exchanger or air-cycle air conditioning system).

Air Supply from the Engine

• Turbine engines typically supply pressurized air from the engine compressor stage, bled off around the 5th and 9th stages.
• 5th-stage air is low-pressure air, supplied by a low-stage bleed port.
• 9th-stage air is high-pressure air, supplied by a high-stage bleed port.
• Low-pressure air is used during take-off, climb, and cruise.
• High-pressure air is used at low engine RPMs during descent or when the low-pressure supply is insufficient.
• Only one bleed port will open at a time, with automatic changeover.
• Bleed air from the engine compressor is usually free of contamination, but leakage can cause contamination from oils or fuel.
• Air pressure is adjusted by a Pressure Regulating Valve (PRV) or the Bleed Air Valve (BAV), which may also act as shutoff valves.

Engine-driven Compressors

• Bleeding air from the engine compressor reduces power output because less air is available for combustion.
• The compressor must work harder, increasing power demand and decreasing overall engine efficiency.
• Independent cabin compressors are designed to prevent increased fuel consumption.
• Compressors are driven through accessory drive gearing or powered by bleed air from an engine compressor.
• Compressors are divided into positive displacement compressors, centrifugal compressors and turbo compressors.

Positive Displacement (Roots Type) Blower

• A Roots-type blower takes a predetermined volume of air, compresses it, and delivers it to the cabin duct.
• Rotors mounted in an airtight casing on parallel shafts rotate at the same speed.
• Air travels around the outside of the case and is deposited into the plenum at the exit.
• Pressure builds up because the blower delivers more air than the system can use, and is controlled by outflow valves.
• The blower housing is finned for cooling.

Centrifugal Cabin Compressor

• A centrifugal cabin compressor is essentially an air pump.
• Outside air at atmospheric pressure enters the supercharger, where it is compressed by the high-speed impeller and delivered to a distribution system.
• Engine-driven cabin superchargers are mounted in the engine nacelle, connected directly or by a drive shaft.
• A variable-ratio drive mechanism adapts to fluctuations in engine RPM, ensuring optimal airflow.

Turbo Compressors

• The compressor of a turbine engine is a reliable source of air used for pressurizing the cabin.
• Bleed air removed from the engine reduces engine power and efficiency.
• Engine compressor bleed air may be used directly to pressurize aircraft, or to drive a turbo compressor.
• By driving a turbo compressor, less engine power is removed from the engine compressor.
• The turbo compressor method is more common in smaller aircraft applications.
• In the turbo compressor system, bleed air from the engine drives a turbine, which directly drives a compressor.
• Heated, compressed outside air is blended with cooled, decompressed bleed air to achieve the correct temperature and pressure before entering the cabin.

Cabin Air Compressor

• The B787 uses two electrically powered Cabin Air Compressors (CACs) to supply outside air to each of the air conditioning packs.
• The assembly includes a compressor, variable outlet diffuser, and a brushless DC motor actuated add heat valve.
• The CAC is powered by a three-phase motor.

Air Supply from the APU

• Turbine engine-powered aircraft require substantial amounts of air for starting and operation.
• Engine starting and ground air conditioning require high-volume and high-pressure pneumatic air.
• Large turbine engine aircraft are fitted with an APU to meet the demands for ground power when the engines are not running.
• A typical APU is a small turbine engine driving an electric generator, and used to supply bleed air for the air conditioning in the cabin.
• The APUs compressor supplies bleed air to a load compressor for heating, anti-ice and engine starting.
• APU systems use bleed air extracted from the compressor section, bleed air extracted from a separate load compressor driven by the turbine power section, or bleed air extraction from the compressor driven from the two-stage axial turbine.
• APUs are generally designed for bleed air extraction on the ground only.
• If an APU is designed for bleed air extraction in flight, then bleed air extraction is normally limited up to a specific flight altitude only, about 22 000 ft.
• The APU is located in the tail section of most aircraft and provides air supply through the bleed air duct.
• With the APU running, the APU compressor provides a sufficient volume of air to start the engines and supplies a flow of air to the Air Conditioning Pack (ACP) which then supplies nice conditioned, warm, or cool, air to the cabin environment.

Ground Cart Air Supply

• Aircraft without a running engine or serviceable APU will require the use of a pneumatic ground cart.
• A ground air cart consists of a small jet engine, contained within a soundproofed steel unit.
• It supplies sufficient air volume for engine starting and supports the operation of the air conditioning packs, through a duct to a dedicated connection on the outside of the aircraft.
• The cart is towed to the aircraft’s location on the ramp and connected to the aircraft’s pneumatic system ducting with a 4-inch diameter hose.
• Cart air is regulated to normal pneumatic system pressure, and can be used for pneumatic system troubleshooting, without the expense of running the APU or main engines.
• The air distribution system consists of a series of ducts carrying conditioned air from the packs to where it is required on the aircraft.
• The ground-based air conditioning unit can consist of a large air conditioner mounted on a truck, or a fixed type.

Air Conditioning

Introduction

• Air Conditioning Systems (ACS) must provide comfortable cabin temperatures, cool or warm air, regardless of the outside air temperature.
• The quality of the air supply must be clean and free of contaminants, fumes and odours that could affect the health or comfort of the passengers and flight crew.

ACS Physics

• To aid understanding of how ACS systems work, a review of physics terminology is provided for heat transfer:
  • Adiabatic - Temperature change without the addition or removal of heat.
  • Conduction - Transfer of heat from one object to another due to the physical contact of the two objects.
  • Convection - Process by which heat is transferred by bulk movement of a fluid.
  • Radiation - Transfer of energy between objects without the need for a medium.
  • Latent Heat - Heat that causes a substance to change its state with no change in temperature.
  • Sensible Heat - Heat, when applied causes a temperature change that can be sensed.
  • Super Heat - Heat energy added to a gas after complete evaporation.
  • Humidity - The amount of water vapour present in a volume of air.

Gas Law Application

• The General Gas Law applies directly to the operating principles of air conditioning systems.
• Air is compressed in the compressor and expanded in the turbine.
• The compression and expansion adds and removes energy from the air.
• The turbine extracts air energy to drive the compressor.
• The expansion of the air as it passes over the turbine creates a temperature drop and is the primary method of supplying cool air to the aircraft.

Air Conditioning System (ACS) Function

• The function of the ACS is to maintain a comfortable air temperature inside the aircraft.
• The ACS will increase or decrease the temperature of the air as required to obtain the desired temperature.
• The majority of ACS systems are capable of producing an air temperature range of 21 °C to 27 °C (70 °F to 80 °F).
• The temperature-controlled conditioned air is distributed to the various zones/areas of the aircraft.
• The ACS system controls humidity and prevents the windows from misting.
• A typical ACS system constantly measures the air temperature and compares it to the selected setting of the temperature controls.
• There are two types of ACS systems used on aircraft:
  • Air Cycle Machine – Used mainly in larger passenger aircraft
  • Vapour Cycle machines – Used mainly used in small and medium-sized aircraft
• The majority of ACS systems are designed to perform the following functions:
  • Supply ventilation air
  • Supply heated air
  • Supply cooled air
  • Cabin Pressurisation
  • Equipment Cooling

Air Cycle Air Conditioning System

• Air cycle air conditioning is used on most turbine-powered aircraft and prepares bleed air to pressurize the aircraft cabin.
• It makes use of engine bleed air or Auxiliary Power Unit (APU) pneumatic air during the conditioning process.
• The temperature and quantity of the air must be controlled to maintain a comfortable cabin environment at all altitudes and on the ground.
• The air cycle system is often called the air conditioning package or pack.
• It is usually located in the lower half of the fuselage or in the tail section of turbine-powered aircraft.
• Hot pressurised air, arriving from the aircraft Bleed System enters the primary heat exchanger at an average pressure of 30 to 40 psi, and a temperature of 150 ºC (300 ˚F), where ram air removes heat from the bleed air.
• The temperature drop of the air passing the primary heat exchanger is about 100 ºC (210 ˚F).
• The partially cooled bleed air goes to the compressor section of the air cycle machine, which is powered by the turbine section of the air cycle machine.
• The compressor section compresses the air which increases the pressure and temperature.
• The temperature rise of the air passing the compressor section is approximately 30 to 40 ºC (85 to 105 ºF).
• Subsequently, the high-temperature and high-pressure air goes through the secondary heat exchanger, to remove the heat added by the compressor.
• The temperature drop of the air passing the secondary heat exchanger is about 60 to 70 ºC (140 to 160 ºF).
• A water extractor duct removes some of the condensed moisture after it passes through and is cooled by the secondary heat exchanger.
• The secondary heat exchangers use the centrifugal effect of swirling the air through vanes to separate the water particles and direct the moisture to a water collector.
• After leaving the secondary heat exchanger the air is cooled prior to entering the condenser passing the re-heater.
• The re-heater is a cross-flow single pass air-to-air heat exchanger.
• After the re-heater air passes through the condenser.
• The condenser is a single cross-flow pass air-to-air heat exchanger.
• The core of the condenser consists of two smaller core modules arranged in a paralleled flow configuration, with a space between the modules.
• This allows a portion of the turbine outlet air to flow through even in excessive icing conditions.
• The cold air that flows across the condenser, coming from the outlet of the turbine section of the air cycle machine, cools the warm air going to the water extractors to below dew point.
• This allows the excess moisture to condense into water particles and can easily be removed from the subsequent water extractors of the system.
• This is achieved by using the centrifugal effect to separate the water from the air.
• Water collected from both water extractors is fed to spray nozzles into the ram air duct, to improve cooling efficiency of the heat exchangers.
• The dry air goes through the reheater to warm the air that goes to the turbine, to increase the turbine efficiency.
• The inlet temperature of the turbine section of the air cycle machine is at a level of 20 to 10 ºC (70 to 50 ºF).
• Rapid expansion, and energy extraction across the turbine, lowers the temperature of the air down to 10 ºC (14 ºF).
• This also rotates the turbine wheel, together with the wheels of the compressor, and the fan being on the same shaft to a speed of 40 000 to 50 000 rpm.
• The temperature and speed values apply when the cooling pack is operating at the maximum cooling capability, usually in hot and humid weather conditions on the ground.
• When the cooling demand is lower than the maximum in flight, or ice has built up in the condenser or in the exit of the turbine, some air is bypassed around the heat exchangers and the air cycle machine through a valve.
• This hot air bleed melts the ice and frees the exit of the system.
• At the same time the air passing through the heat exchangers and the air cycle machine is reduced, and subsequently the speed decreases, and the cooling pack delivers hotter air to the air conditioning system.
• At cruising altitude, the need for cooling is at a minimum because of the very low ambient temperature at that flight level.
• Most of the bleed air now passes only through the heat exchangers being cooled by ram air.
• As such the speed of the air cycle machine is at a minimum.

Air Cycle System – Components

Pack or Flow Control Valve

• The pack or flow control valve, is the valve that regulates bleed air from the pneumatic manifold into the air cycle air conditioning system.
• The valve is controlled with a switch on the air conditioning panel in the flight deck.
• Most pack or flow control valves are electrically controlled and pneumatically operated.
• They are also referred to as the supply shut-off valve.
• The pack or flow control valve opens, closes and modulates, to allow the air cycle air conditioning system to be supplied with a predetermined volume of hot, pressurised air.
• When an overheat, or any other abnormal condition is detected, the pack or flow control valve is commanded by a signal to close.
• The pack valve is a venturi-type butterfly valve controlled by a solenoid.
• It is pneumatically operated and spring-loaded to closed.
• The pack valve also contains a shut-off function which is controlled to close from the fire handle, during engine start and from the pack switch.
• When a pack switch is operated the pack valve solenoid is de-energised and if there is bleed air available the pack valve opens.
• Because of the fail-safe philosophy, the pack valve also opens when the electrical power supply is broken.
• If pneumatic power is not available, the pack valve closes because the actuator spring closes the valve.
• The pack valve has a built-in micro-switch which indicates if the pack valve is fully closed or is in the open position.
• The pack valve also has a manual override which allows the valve to be fixed in a closed position.

Pack Valve Function

• The main function of the pack valve is to control the airflow to the cabin which is achieved with a regulating assembly.
• The regulating assembly receives the airflow signal from a venturi tube, which consistently measures a mass flow.
• Unfortunately, with a constant mass airflow to the cabin there is an increase in air velocity when flying higher because of the decreasing cabin pressure.
• To ensure passenger comfort, a constant velocity of airflow is required, which means the volume of airflow must be constant.
• To enable this, the valve gets a correction signal from the cabin pressure to allow the regulation of the constant volume of airflow.
• Air conditioning systems are currently able to save energy, by adjusting the airflow according to the number of passengers.
• When the aircraft is fully loaded the total capacity of conditioned air is necessary.
• If the aircraft is not fully loaded, all of the conditioned air is not required, so the pushbutton is set to low flow and the pack valve closes partially.
• If the aircraft is fully loaded, all of the conditioned air is necessary, so the pushbutton is set to hi flow and the pack valve is more open.
• When the pack flow selector is in the normal position, this means that the cabin is receiving 100% airflow.
• When the pack flow selector is in the high position, then the pack valves supply the cabin with more than 100% airflow.
• When the pack flow selector is in the LO position then the pack valves supply the cabin with 80% airflow.
• To alter the airflow, there is an air conditioning panel in the flight deck, where the required airflow is selected.

Primary and Secondary Heat Exchanger

• The primary and secondary heat exchanger are devices that allow the heat transfer between two fluids, in this case air.
• Cold ram air (outside air) is used to cool the hot bleed air coming from pneumatic sources, the engines or APU.
• As the cold ram air passes over the heat exchanger fin-like tubes, the hot bleed air passing through the tubes is cooled.
• The flow of ram air over the heat exchanger is controlled with a modulating inlet and exit doors.
• This modulates in flight to provide the required cooling effect.
• The air supply from the primary heat exchanger is controlled to a constant temperature, by a temperature sensor controlling the flow of air from the pack, or flow control valve.
• On the ground there is insufficient air passing through the ram inlet doors, so a fan is used to induce an adequate airflow, to cool the heat exchangers.
• If required, the fan can be driven by engine bleed air, mechanically by the ACM drive shaft, or by an electric motor.
• The addition of the secondary heat exchanger is to further cool the air to a more acceptable temperature.
• This occurs prior to the air entering the compressor of the ACM, which can raise the pressure and temperature of the air passing through it.

Air Cycle Machine (ACM)

• The ACM consists of a centrifugal compressor and an expansion turbine (some systems utilise 2 expansion turbines).
• Air is delivered from the primary heat exchanger to the compressor, where it is reheated.
• It then flows through to the secondary heat exchanger for additional cooling.
• The cooled high pressure air flows into the expansion turbine, where a large percentage of its remaining energy is used to drive the compressor.
• As this air expands across the turbine, there is a large decrease in pressure.
• The decrease in pressure, along with the energy extracted to drive the compressor, provides a very large decrease in temperature.
• There are two types of cooling used in this system. -One is by transferring heat to the ram air, but most of the heat is removed by expansion, and converting it into work to drive the compressor. -This type of cooling system is called a bootstrap system.
• There are 2 types of ACMs used:
  • Oil Lubricated - A dipstick for checking the oil level is attached to the filler cap. An oil slinger is mounted outboard of each of the bearings which carry the shaft. The slingers pump an oil/air mist through the bearings to provide lubrication. Air/oil seals are provided between each slinger and the adjacent wheel.
  • Air Lubricated - Are oil free, use air bearings, and are becoming more common. They eliminate oil contamination of the air conditioning system, but are noisier, and require careful handling during removal and installation.

Reheater/Condenser

• The condenser/reheater has two functions:
  • To cool the air in the air conditioning pack before it goes through the water collector.
  • To heat the air conditioning pack air to add energy to the air.
• Air comes into the condenser/reheater from the secondary heat exchanger and flows over both heat exchanger cores.
• The heat exchangers absorb heat from this air.
• The decrease in air temperature condenses water, so that the water collector can remove it from the air.
• The water collector expels the water into the ram air duct, providing additional ram air cooling, to the primary and secondary heat exchangers, alternatively, the water can be vented to the outside of the aircraft.

Water Separator

• Separators are used in the cabin air conditioning system to remove excessive moisture from the air to prevent freezing.
• In addition, water in the cabin can end up as fog or water droplets, causing passenger discomfort, or corrosion in the cabin and distribution lines.
• The warmer the air, the more water it carries.
• Therefore, air nearer the ground has a larger amount of water dissolved in it which condenses in the cooling cycle.
• At high flight altitudes the air is very cold and very dry, so there is no water to condense in the cabin.
• Cool air from the air cycle machine no longer holds the quantity of water it has when warm.
• A water separator is used to remove the water from the saturated air before it is sent to the aircraft cabin.
• The separator operates with no moving parts.
• Foggy air from the ACM enters and is forced through a fibreglass sock that condenses and combines the mist into larger water drops.
• The intricate interior structure of the separator swirls the air and water.
• While the dry air passes through, the water collects on the sides of the separator and drains down and out of the unit.
• A bypass valve is incorporated in case of blockage.

Air Cycle Machine Bypass Air

• Some air can be bypassed around the ACM to keep the temperature of air exiting the ACM from becoming too cold and ice forming at its exit and/or in the condenser.
• This air is sometimes taken direct from the engine bleed air supply after the pack or flow control valve, or bled off after the secondary heat exchanger.
• The bypass air is controlled by a valve.
• The function of maintaining the air temperature at around 35 °F (2 °C) by feeding warm bleed air into the outlet of the ACM.
• They all perform the same function of maintaining the air temperature at around 35 °F (2 °C) by feeding warm bleed air into the outlet of the ACM and mixing it with the output air.
• The valves are electrically controlled, and pneumatically operated.
• The operation relies on a signal from the temperature sensors, controlled through the temperature control system of the air conditioning system.
• Repositioning is accomplished by the action of the coil in varying the amount of pressure bled from the pressure chamber.
• The closed position is the fail-safe position for the valve.

Trim Air Valves

• Trim air valves mix with conditioned air and provide accurate temperature-controlled air to the cabin and flight deck areas.
• They are located in the distribution ducting and use engine bleed air directly fed via the trim air valve to a mixing manifold.

Mix Chamber/Manifold

• A mix chamber/manifold collects the temperature-controlled, clean and conditioned air from the packs.
• Then distributes the warm/cool air to the flight compartments in the aircraft.

RAM Air Door/Valve

• The ram air inlet is a small scoop or door on the underside of an aircraft, also frequently located on the ‘wing to body fairing.’
• The majority of aircraft use a modulating door, controlled by actuators on the ram air inlet.
• These control the quantity of the outside ambient air that flows through the primary and secondary heat exchangers.
• When bleed air goes through the primary and secondary heat exchangers, the purpose of the ram air is to remove some of the heat.
• The ram air system helps control the air cycle machine (ACM) compressor discharge temperature.
• Maximum ram air is available on the ground and during take-off.
• During flight the doors/louvers modulate to reduce both the ram air flow and drag.
• To increase ram air recovery, modulating vanes are used on the ram air exhaust.
• A ram air fan provides ram air flow across the heat exchangers when the aircraft is on the ground.
• The majority of modern fixed-wing aircraft use a fan on a common shaft with the ACM, powered by the ACM turbine.

Vapour Cycle Machine Air Conditioning System

• A vapour cycle air conditioning system is typically used in aircraft powered by reciprocating (piston) engines,
• It is the only practical solution for air-conditioning piston engine-powered aircraft since such engines do not produce bleed air.
• The system only acts to cool the cabin, by transferring heat from inside the cabin to outside the cabin.
• These systems work on the principles of thermodynamics, specifically the vapour compression refrigeration cycle,
• The heat is transferred to the refrigerant liquid which then turns to vapour as it gains extra (heat) energy.
• The vapour then undergoes compression which makes it very hot.
• The heated vapour releases the gained heat energy to the outside air and then returns to the cabin as a liquid.
• The cycle continues.

Theory of Refrigeration

• Before getting into the fundamentals of refrigeration, a few basic definitions should be considered:
  • Heat is a form of energy transferred by a temperature difference.
  • Cold is a relative term referring to the lack of heat in an object, substance, or area.
• The boiling point of any substance varies directly with pressure.
• When pressure on a liquid is increased, its boiling point increases, and when pressure on a liquid is decreased, its boiling point decreases.
• The refrigeration, or cooling process, is the removal of unwanted heat from a selected object, substance, or space and its transfer to another object, substance, or space.
• Refrigerants are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapour or gas as they are pumped through the mechanical refrigeration system to cycle.
• The refrigeration cycle is based on the long-known physical principle that a liquid expanding into a gas extracts heat from the surrounding substance or area.
• Refrigerants evaporate or "boil" at much lower temperatures than water, which permits them to extract heat at a more rapid rate than the water on your finger.

System Operation

Components of a Vapour Cycle Machine

• The Refrigerant: A working fluid that undergoes repeated phase transition from a liquid to a gas and back again.
• The Compressor: This component pressurizes the refrigerant gas, causing its temperature to rise significantly.
• The Condenser: In this part, the high-temperature, high-pressure refrigerant releases heat to the external environment, often aided by the flow of ambient air or a separate cooling system.
• The Expansion Valve: The high-pressure liquid refrigerant passes through an expansion valve, causing it to rapidly expand and decrease in pressure. This results in a drop in temperature.
• The Evaporator: Inside the cabin, the low-pressure, low-temperature refrigerant evaporates, absorbing heat from the surrounding air.

The Refrigeration Process

• Step 1: Compression (Compressor)
  • The process begins with the compressor, which takes in low-pressure, low-temperature refrigerant vapour from the evaporator.
  • The compressor increases the pressure of the refrigerant, compressing it into a high-pressure, high-temperature gas.
  • This compression is an adiabatic process, meaning it occurs without the transfer of heat.
• Step 2: Condensation (Condenser)
  • The high-pressure, high-temperature refrigerant gas exits the compressor and enters the condenser.
  • In the condenser, the refrigerant releases heat to the surrounding environment (typically through external airflow or a separate cooling system).
  • As the refrigerant loses heat, it undergoes a phase change from a gas to a high-pressure liquid.
  • This liquid is now at high pressure and temperature.
• Step 3: Receiver-dryer
  • The receiver-dryer acts as a reservoir to store any of the liquid refrigerant that is not passing through the system at any given time.
  • If any refrigerant is lost from the system, it is replaced from that in the receiver-dryer.
• Step 4: Expansion (Thermal Expansion Valve)
  • The high-pressure liquid refrigerant from the condenser passes through the expansion valve.
  • As it rapidly expands, its pressure and temperature drop significantly.
  • This causes the refrigerant to become a low-pressure, low-temperature liquid-vapour mixture.
• Step 5: Evaporation (Evaporator)
  • The low-pressure, low-temperature refrigerant enters the evaporator located in the aircraft cabin.
  • A blower assists in passing the warm cabin air over the evaporator.
  • As the refrigerant absorbs heat from the warm cabin air, it evaporates back into a low-pressure vapour.
  • This heat absorption lowers the cabin temperature and makes it more comfortable for passengers and crew.
• Step 6: Repeat the Cycle
  • The low-pressure vapour from the evaporator is then returned to the compressor to begin the cycle once again.
  • The vapour compression cycle is a continuous process, maintaining the desired temperature and humidity levels in the cabin.
  • The heat absorbed from the cabin air during evaporation is released to the external environment during condensation, allowing for effective temperature control inside the aircraft.

Vapour Cycle Machine System – Components

Refrigerant

• A commonly used vapour cycle refrigerant was dichlorodifluoromethane (R12), some systems still use this.
• R12 was found to have a negative effect on the environment, in particular the Earth’s ozone layer and has now been replaced by tetrafluoroethane (R134a), which is classed as much safer for the environment.
• R12 and R134a should not be mixed, nor should one be used in a system designed for the other.
• R134a is a halogen compound (CF3CFH2), it has a boiling point of approximately –15 °F (-26 ˚C).
• It is not poisonous to inhale in small quantities, but it does displace oxygen; suffocation is possible if breathed in mass quantity.
• Caution - when handling any refrigerant:
  • Due to the low boiling points, liquid refrigerants boil violently at atmospheric temperatures and pressures.
  • They rapidly absorb heat energy from all surrounding matter.
  • If a drop lands on your skin, it freezes, resulting in a burn.
  • Similar tissue damage can result if a drop gets into one’s eye.
  • Gloves and other skin protection, as well as safety goggles, are required when working with refrigerant.

Receiver Dryer

• The receiver dryer acts as the reservoir of the vapour cycle system.
• It is located downstream of the condenser and upstream of the expansion valve.
• When it is very hot, more refrigerant is used by the system than when temperatures are moderate.
• Liquid refrigerant from the condenser flows into the receiver dryer.
• It passes through filters and a desiccant material which remove any foreign particles in the system and captures any water in the refrigerant.
• Water in the refrigerant causes two major problems:
  • The refrigerant and water combine to form an acid.
  • If left in contact with the inside of the components and tubing, the acid deteriorates the materials from which these are made.
  • Water in the system could form ice and block the flow of refrigerant around the system causing the system to become inoperative.
• Vapour can sometimes find its way into the receiver dryer, especially when the gaseous refrigerant does not completely change state to a liquid in the condenser.
• A stand tube is used to remove refrigerant from the receiver dryer.
• It runs to the bottom of the unit to ensure any liquid is removed and returned to the expansion valve.
• At the top of the stand tube, a sight glass allows the Engineer to see the refrigerant to determine that there is sufficient refrigerant in the system.

Thermal Expansion Valve

• Refrigerant exits the receiver dryer and flows to the expansion valve.
• The thermostatic expansion valve has an adjustable orifice which allows the correct amount of refrigerant through to obtain optimal cooling.
• This is achieved by monitoring the temperature of the gaseous refrigerant at the outlet of the next component in the cycle, the evaporator.
• The expansion valve should only permit the amount of refrigerant spray into the evaporator that can be completely converted to a vapour.
• If too much refrigerant is released by the expansion valve into the evaporator, some of it remains liquid when it exits the evaporator and could be dangerous.
• Vapour cycle air conditioning systems that have large evaporators experience significant pressure drops while refrigerant is flowing through them.
• Externally equalised expansion valves use a pressure tap from the outlet of the evaporator to help the superheat spring balance the diaphragm.

Evaporator

• The majority of evaporators are constructed of copper or aluminium tubing coiled into a compact unit.
• Fins are attached to increase surface area, facilitating rapid heat transfer between the cabin air blown over the outside of the evaporator with a fan and the refrigerant inside.
• The expansion valve located at the evaporator inlet releases high-pressure, high-temperature liquid refrigerant into the evaporator.
• The evaporator is situated in such a way that cabin air is pulled to it by a fan.
• The fan blows the air over the evaporator and discharges the cooled air back into the cabin.
• A remotely located evaporator may require ducting from the cabin to the evaporator and from the evaporator back into the cabin.

Compressor

• The compressor is the centre of the vapour cycle air conditioning system that that circulates the refrigerant around the vapour cycle system.
• It receives low-pressure, low-temperature refrigerant vapour from the outlet of the evaporator and compresses it.
• As the pressure is increased, the temperature also increases.
• The refrigerant temperature is raised above that of the outside air temperature.
• The refrigerant then flows out of the compressor to the condenser where it gives off the heat to the outside air.
• The compressor is the dividing point between the low side and the high side of the vapour cycle system.
• fittings are required for servicing, which can be accomplished with fitting upstream and downstream of the compressor.

Condenser

• The condenser is a radiator like heat exchanger situated so that the outside air flows over it and absorbs heat from the high-pressure, high-temperature refrigerant received from the compressor.
• A fan is usually included to draw the air through the compressor during ground operation.
• On some aircraft, outside air is ducted to the compressor, or the condenser is lowered into the airstream from the fuselage via a hinged panel.

Service Valves

• All vapour cycle air conditioning systems are closed systems but, access for servicing required.
• This is usually performed using two service valves which one valve is located in the high side of the system and the other in the low side.
• This is usually performed using two service valves.
• One valve is located in the high side of the system and the other in the low side.
• A common type of valve used on vapour cycle systems that operate with R12 refrigerant is the Schrader valve.
• It is similar to the valve used to inflate tires.
• All service valves should be capped when not in use.
• As a safety device to prevent inadvertent mixing of refrigerants, R134a valve fittings are different from Schrader valve fittings and do not attach to Schrader valve threads.
• A compressor isolation valve is sometimes used on aircraft which permits servicing and can isolate the compressor so the oil level can be checked and replenished without opening the entire system and losing the refrigerant charge.

Distribution Systems

Distribution

• The distribution of conditioned air into the cabin of an aircraft is achieved with a system of ducts leading from the pressurisation source into and throughout the cabin.
• The conditioned air exits through ceiling vents, where it circulates and flows out through floor level vents.
• The air then flows aft and exits the aircraft through the outflow valve(s) mounted on, or near the aft pressure bulkhead.
• On the majority of turbine-powered aircraft, temperature-controlled conditioned air from the air conditioning system is also used to pressurise the cabin as well.
• Mixing conditioned air with bleed air in a duct or a mixing chamber allows the flight crew to select the exact temperature desired for the cabin.
• The valve for mixing is controlled in the flight deck or cabin by a temperature selector.
• Centralised manifolds from which air can be distributed are common.
• Large aircraft are divided into zones for air distribution as well as contain a gasper air system and cooling air to electronics equipment bays.
• Medium size and larger turbine powered aircraft are fitted with a receptacle in the air distribution system which allows for a ground source of conditioned air to be connected directly to the cabin to limit operating time on the engines and APU.
• A check valve is used to prevent ground source air from flowing upstream into the air conditioning system.

Flow, Temperature and Humidity Control Systems

Cabin Temperature Control System

• The majority of cabin temperature control systems operate in a similar way.
• Temperature is monitored in the cabin, flight deck, conditioned air ducts, and distribution air ducts.
• Values are input into a temperature controller, normally located in the electronics compartment.
• A temperature selector in the flight deck can be adjusted to increase or decrease the desired temperature.
• The temperature controller compares the actual temperature signals received from the various sensors with the desired temperature input.
• An output signal is sent to a valve in the air cycle air conditioning system.
• Cabin temperature pick up units and duct temperature sensors used in the temperature control system are thermistors.

Humidity Control

• On long distance flights the cabin air as a tendency to become very dry after being continually recycled through the water separators.
• Onboard humidifiers are designed to help passengers stay hydrated, aiding better taste, improving quality of sleep, and reducing the effects of jetlag.

Air Recirculation

• Normally, up to 50% of the cabin air is filtered and recirculated.
• Recirculation of the cabin air saves on fuel because the engines only need to supply as little as 50% of the cabin air.
• Less engine compressor bleed means more power; therefore, the engines can be throttled back to maintain cruise speed, thereby saving fuel.
• Recirculation fans are electric motor-driven and ventilation is continuous through high-efficiency filters that remove airborne particles, bacteria and odours.

Air Flow

• Air enters the passenger cabin from overhead distribution outlets that are designed to create carefully controlled circular airflow patterns in the cabin.
• Air is