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AIR CONDITIONING ANDCABIN PRESSURISATION

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4 AIR CONDITIONING AND CABIN PRESSURISATION

4.1 INTRODUCTIONThe atmosphere above10,000ft is too thin and cold for normal breathing.Passenger carrying aircraft, operating above this height need an air conditioningand pressurisation system. The temperature of the air passing through thepassenger cabin, flight deck and other compartments must be strictly controlled,as well as flow rate and level of humidity.Cabin temperature will normally be maintained between 15 and 30 degreesCelsius. Additionally, a controlled amount of pressurisation is necessary, so thatthe air pressure in the passenger cabin and adjacent areas does not exceed theequivalent of the ambient air pressure at 8000ft.Air conditioning is also essential for un-pressurised aircraft types.

A typical air conditioning and pressurisation system comprises eight principlesub-systems:

Air Supplies (Pneumatics ATA 36)

Cooling

Heating

Temperature Control

Humidity Control

Mass Flow Control

Distribution Pressurisation

4.2 AIR SUPPLY

The source of fresh air supply and arrangement of essential components will varybetween aircraft type and each air conditioning system, but in general one of thefollowing methods described in the following paragraphs will be adopted:

4.2.1 Engine Bleed Air (compression)

This method is the most common and is installed on the majority of modernaircraft types. Very hot air is tapped from the main engine compressor stages andsupplied to the cabin, flight deck and other areas. Before the air enters the cabin,it is passed through a temperature control system, which reduces its temperatureand pressure. Additionally, a means of flow control is utilised and in some aircraft,humidity control forms part of the system. (See Fig 1)In pressurised aircraft, the discharge of the conditioned air is regulated tomaintain the cabin pressure at the selected pressure altitude.

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Typical (Compression) Bleed Air SystemFigure 1

4.2.2 Air Compressors or Blowers

This method is used on turbo-prop, piston engine or even turbo-jet aircraft wheremain engine compressor bleed is unavailable or unsuitable.

Normally the compressor or blower will be mechanically driven from theaccessory gearbox of the main engine and its air supply routed via a temperaturecontrol system, in a similar manner to the engine bleed method.

4.2.3 Auxiliary Power Unit (APU)

The APU is a small gas turbine engine, which can be connected into the main airsupply system and provide an independent means of air conditioning andpressurisation, either on the ground or in flight, when the main engines cannotsupply. It will utilise the engine bleed air principle outlined above.

ECU

NRV

AUXILIARY POWER UNITNON RETURN VALVE

SHUT OFF VALVES

FLOW CONTROLLER

TEMPERATURE CONTROL VALVE

MIXER UNITTO

CABIN

NRV

WATER SEPARATOR

COUPLED COMPRESSOR TURBINE

RAM AIR

PRIMARY HEAT

EXCHANGER

SECONDARY HEAT

EXCHANGER

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4.2.4 Ram Air

This method is normally found as the primary ventilation system on un-pressurised aircraft. A ram air scoop placed directly into the airflow, will providethe means of air supply as the aircraft moves forward.Since the air at altitude will be cold, the temperature control system throughwhich it passes before entering the cabin, will normally be a form of heater.A self-contained combustion type heater will be employed, or the some form ofexhaust gas heater. The air conditioning ducting will be routed around thecombustion heater casing or around engine exhaust duct to obtain convectionheating.On pressurised aircraft, a ram air system can be used as a means of emergencyventilation, following a complete loss of the main system.

Typical Combustion Heater SystemFigure 2

4.2.5 Ground Cart

This will be an independent means of heating or cooling the passenger cabin onthe ground. It can be used on aircraft that do not have an APU. The trolley will beconnected externally to the aircraft, via a purpose built inlet into the airconditioning system and normally employs a combustion type heater and the

means to control the output of the air temperature from a control panel the cart.

FUEL SOLENOID VALVE

FUEL SUPPLY

OFFOFF ONON

WARM AIR OUTLETS

COLD AIR OUTLETS

RAM AIR

EXHAUST

COMBUSTION CHAMBER

DEMISTER

FLOW CONTROL VALVE

ENGINE DRIVEN AIR BLOWER

AIR SUPPLY

ONOFF

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4.3 COOLING

When bleed air is used as the air supply, the air tapped off the enginecompressor can reach a temperature in excess of 300 degrees Celsius.This is obviously far too hot to be fed directly into the air-conditioned areas, so itmust first be cooled down to around 20 degrees Celsius.There are two main methods of cooling;Air Cycle and Vapour Cycle cooling systems.

4.3.1 Air Cycle Cooling

Air cycle cooling relies on three basic principles; surface heat exchange,

expansion and energy conversion.

Surface heat exchange, provides cooling by passing the air tapped from theengine compressor (charge air) across some form of heat exchanger. The chargeair is subjected to the effect of a colder cross flow, normally ambient air, scoopedby an intake and passed across the heat exchanger as the aircraft moves forward(ram air). Although 90% of heat is given up in this way, the charge airtemperature can never be reduced below the ram air temperature by this methodalone.

Expansion, provides cooling when the pressure of the charge air is reduced by

increasing its velocity and expanding it across the turbine of a so-called Air CycleMachine (ACM) or Cold Air Unit (CAU). In this way, the temperature of the chargeair can be rapidly lowered to zero degrees Celsius, irrespective of the ram airtemperature

Energy Conversion, cools by making the hot air do work. This is achieved byusing the charge air to drive a turbine, which is connected by a shaft to thecompressor or fan within the cold air unit, thus converting heat energy into kineticenergy. This method will also help to reduce the charge air to zero degreesCelsius.

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Turbo CompressorFigure 3

4.3.1.1 HEAT EXCHANGERS

These are components within the air conditioning system that transfer heat fromone gas stream to another. Ram air is used as the cooling medium to cool thevery hot charge air ducted from the engine compressor or the gearbox mounted

air compressor or blower.Depending on where they are placed within the air conditioning system, heatexchangers are often described as;

A Pre-cooler or Primary Heat Exchanger

An Inter-cooler or Secondary Heat Exchanger

The basic construction is a sealed unit containing a series of cooling passages;through which the charge air flows and over which the ram air is directed.Between these passages are thin corrugated strips, that also serve to dissipateheat as the ram air passes over them.

TEMPERATURE

CONTROL VALVE

COMPRESSOR TURBINE

SECONDARY HEAT EXCHANGER

RAM AIR

TO

CABIN

MIXER UNIT

PRIMARY

HEAT

EXCHANGER

HOT AIR INLET

WATER SEPARATOR

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4.3.1.2 AIR CYCLE MACHINE (ACM) OR COLD AIR UNIT (CAU)

The ACM/CAU is the primary component in an air cycle cooling system. Anumber of different types can be found including;The turbo-compressor, the brake turbine and the turbo-fan.All three use the charge air to drive the turbine and the major differences betweeneach type, relates to the overall weight for a given mass flow, the size andmethod of dissipating the power output of the turbine.

Turbo Compressor Cold Air UnitFigure 4

The turbo-compressor type consists of a turbine driving a centrifugal compressorand operating in conjunction with an inter-cooler connected between thecompressor and turbine stages.Its basic construction consists of two main casings, the turbine volute andcompressor volute casings. The two casings are connected together and enclosea bearing housing with two bearing assemblies, supporting a shaft upon which

the turbine and compressor wheels are mounted.

BLEED AIR

TO INTERCOOLER

FROM

INTERCOOLER

TO

DISTRIBUTIONSYSTEM

COMPRESSOR

DIFFUSER

NOZZLE BLADES

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The turbine wheel revolves within a nozzle ring and the compressor wheel rotateswithin a diffuser ring. The very hot charge air from the engine compressor bleed

and routed via the pre-cooler, enters the eye of the ACM/CAU compressor. Itbecomes compressed on passing through the diffuser ring, increasing itstemperature and energy.From the compressor, the hot air is directed across the inter-cooler matrix overwhich ram air passes and is then directed into the turbine volute nozzle ring,where it drives the turbine. The resultant expansion and energy conversion,rapidly lowers the air pressure and temperature.It is then directed towards the passenger cabin. (See Fig 3)

The ACM/CAU compressor and turbine wheels rotate at extremely high speeds,often in excess of 80,000 rpm, so efficient bearing lubrication is essential to

ensure smooth and trouble-free running.Two lubrication methods are used; Integral wet sump arrangements, orpressurised air bearings that need no oil lubrication.The wet sump type normally has a sump containing oil and a means of meteringit to the bearings usually by the use of integral wicks or with an oil slinger thatpumps an optimum oil/air mix to the bearings. This ensures the correct amount ofoil at the bearings at all times. Oil replenishment is critical however, as too muchoil will lead to the charge air being oil contaminated and too little oil, may result ina premature seizure of the rotating shaft.The air bearing type uses a pressurised air supply to support the shaft in a similarmanner to the hovercraft principal. As the rotor floats on a thin layer of air, it isessential that this type is kept clean and dry and completely free from oil andgrease.

Brake Turbine Cold Air UnitFigure 5

RAM AIR

TOCABIN

MIXERUNITHEAT EXCHANGER

CONTROL VALVE

AMBIENT AIR INLET

COMPRESSOR TURBINE

BLEEDAIR

AMBIENT AIR OUTLET

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The brake-turbine type of ACM/CAU, has its charge air routed directly from the

pre-cooler to drive the turbine. The air expands across the turbine as before,resulting in a large temperature and pressure drop. Since this layout dispenseswith the need for an inter-cooler, it results in a greater efficiency due to weightsaving. To safeguard against the turbine rotating too fast, it is coupled with acompressor, which rotates in ambient air and consequently acts as a brakingmedium. Additionally, the slower rotation of the shaft further improves turbineoutput efficiency. (See Fig 5)

Turbo Fan Cold Air Unit

Figure 6

The turbo-fan type is mechanically similar to the brake-turbine arrangement. Inthis case however, the turbine drives a large centrifugal fan instead of a normalcompressor. The fan is draws a large quantity of ambient air over the pre-cooler,which cools the incoming charge air.The major advantage of this type over the other two, is that with the fan-inducedairflow over the pre-cooler, it can be used with the aircraft stationary on theground with the aircraft engines running. It does not need to rely solely on ram airas the cooling medium for the pre-cooler.

MIXER UNIT

BLEED AIR

RAM

AIR

HEATEXCHANGER

CONTROL VALVE

LARGE FAN

TURBINE

RAM AIR OUTLET

TO CABIN

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4.3.2 Vapour Cycle Cooling

The vapour cycle cooling system can be used as an alternative to the air cyclecooling system. Although not commonly used these days for air conditioningsystems, the system may be used as the means to remove heat from electricaland electronic equipment.The system relies on the principle of the ability of a refrigerant to absorb heatwhen changing from a liquid to a gas, through the process of vaporisation orexpansion.For example, if you were to put a drop of a highly volatile liquid such asmethylated spirits or petrol on the back of you hand, it will feel cold. This isbecause the liquid starts to evaporate and draws the heat necessary forevaporation from your hand. Liquids with a low boiling point have a stronger

tendency to evaporate at normal temperatures than those with a high boilingpoint.Furthermore, the amount of pressure acting on a liquid substance will affect itsstate. A sufficient reduction in pressure will cause any liquid to change state intoa vapour or a gas. Conversely, a corresponding increase in pressure will reversethe process.

Schematic Vapour Cycle SystemFigure 7

THERMOSTATIC

EXPANSION VALVE

RECIEVER DRYER

CONDENSER

EVAPORATORTURBO COMPRESSOR

TEMPERATURE

CONTROL VALVES

AIR SUPPLY

RAM AIR

AIR DISTRIBUTION

TEMPERATURE SENSOR

CAPILLARY TUBE

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The major components of a typical system are a liquid receiver, a thermostaticexpansion valve, an evaporator, a turbo-compressor, a condenser and acondenser fan. Often these components are mounted close together to form aline-replaceable refrigeration pack or vapour cycle cooling pack.The liquid receiver acts as a reservoir and provides storage for the refrigerant,normally a highly volatile chemical such as Freon. The refrigerant will pass fromthe liquid receiver to a thermostatic expansion valve where it is metered andreleased into the evaporator. The very hot charge air from the main engine bleedflows across the evaporator, releases heat that vaporises the liquid refrigerantand passes into the passenger cabin at a much lower temperature.Meanwhile, the now vaporised refrigerant gas is directed towards the turbo-

compressor. It is drawn into the compressor wheel, the coupled turbine of whichis driven by the main engine bleed air. (Note: In some cases, an independentmeans instead of a turbo-compressor may be used to compress the refrigerantgas, such as an electric motor, as in a domestic refrigerator).The refrigerant gas leaves the compressor at a high pressure and temperatureand passes across the matrix of the condenser. The gas is cooled by the ram air,flowing across the matrix and so condenses back into a liquid once again. It thenreturns to the liquid receiver to repeat the refrigeration cycle once again.The condenser fan is used to induce air across the condenser matrix when theaircraft is stationary on the ground and no ram air is available.

Typical Vapour Cycle SystemFigure 8

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4.4 HEATING

Un-pressurised aircraft use a ram-air system for ventilation. At altitude, the ram-air passing through the cabin would be very cold, so a heating system is required.Heating systems can be generally divided into two types:

Exhaust heating systems

Combustion heating systems

4.4.1 Exhaust Heating Systems

In its simplest form, this type of heating system employs a heater muff thatsurrounds the exhaust pipes coming from a piston engine, or the jet pipe of a

turbo-jet. A ram air scoop at the forward end of the heater muff allows some ofthe cold air to go to directly to a mixing valve.The remainder, enters the muff and surrounds the exhaust/jet pipes. Heat fromthe pipes is transferred into the ram air and carried to the mixing valve. Theheated air joins the cold air at the mixing valve and the combined flow is directedinto the passenger cabin.Some form of control lever, operated from within the aircraft and connected to themixing valve, allows the proportion of hot and cold air to be modulated in order tosuit the cabin heating requirements.To cater for the possibility of the ventilation air becoming contaminated from theexhaust pipes, some aircraft will be fitted with carbon monoxide detectors withinthe cabin area. These are indicators filled with brightly coloured crystals, whichturn black if exposed to dangerous levels of carbon monoxide.

Exhaust System HeaterFigure 9

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4.4.2 Combustion Heating Systems

This system uses a purpose built combustion chamber heater assembly toprovide the heat source, rather than the previously described exhaust heatingmethod. Fuel is directed from the aircraft fuel system, through a pressureregulating and shut off valve that ensures the fuel is at the correct pressure foratomisation. Other components include a fuel filter, a fuel pump and spray nozzle,where it is atomised and ignited with an igniter plug. The combustion chamberassembly heats up the ram air that passes around it.

4.5 TEMPERATURE CONTROL

In order to operate the aircraft in an infinite number of climatic and operatingconditions, the temperature in the passenger cabin, flight compartment and otherareas needs to be regulated for comfort.Temperature regulation for the majority of aircraft that employ the engine bleedair method is usually accomplished by controlling the proportion of hot and coldair coming from the air supply system. An electric motor driving a double butterflytype air mixing valve, regulates the cabin temperature, by allowing a controlledamount of hot air to by-pass the air cycle system. This air is then recombined inproper proportions with the cold air that has been directed through the air cyclesystem at a down stream mix chamber. The position of the air-mixing valve isdetermined by signals from the temperature control system.

The temperature control system is normally operated automatically or as amanual system, if the automatic temperature controller should fail.

Typical Combustion Heater System

Figure 10

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During automatic operation, the temperature controller continually monitors cabintemperatures and repositions the air mixing valve if necessary to keep the

temperature at the selected level.In order to achieve this, the controller receives signals from temperature selectoron the flight deck (the temperature requested) and from temperature sensors inthe passenger cabin, flight compartment and supply ducts (the actualtemperature). If a difference between the requested and actual temperaturesoccurs, the controller will send an output signal, to re-position the air mixing valveuntil parity exists once more.During manual operation, the temperature control circuit bypasses the controllerand connects the temperature selector on the flight deck, directly to the air-mixingvalve. Other sensors in the system transmit compartment temperatures toindicators on the flight deck overhead panel, so that the actual temperatures and

the position of the air-mixing valve can be monitored.

Temperature ControlFigure 11

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4.6 HUMIDITY CONTROL

Humidity control is the means to ensure that the correct amount of water moisturecontent is in the air conditioning air within the aircraft cabin. This is necessary toensure occupants do not suffer from low humidity levels that are experienced withhigh altitude flight.

Humidity control can be achieved two ways;

Water Separation

Water Infiltration

Water separation is the removal of excessive moisture from the charge air,normally by a water extractor or separator.

Water infiltration is the addition of moisture into the conditioned air as it enters thecabin using a water pump and spray nozzle.

4.6.1 Water SeparationWater Extractor

Water can be introduced into the air conditioning system due to the compressionand expansion of the air in the ACM/CAU and other areas of the air cycleprocess.There are three types of water separator in general use; the coalescer/diffusertype, the coalescer/bag type and the swirl vane type.

4.6.1.1 COALESCER/DIFFUSER TYPE

This type consists of a coalescer constructed from layers of monel metal gauzeand glass fibre cloth sandwiched between layers of stainless steel gauze. It issupported by the diffuser cone and held in place by a relief valve housing. As theair leaves the diffuser and passes over the coalescer, moisture in the air isconverted into water droplets. The droplets enter the collector shell and aredeposited into collector tubes where they drain down to a collector box fromwhere the water is ejected overboard.

COALESCER WATER EXTRACTORFIGURE 12

PRESSURE RELIEFVALVE

DRAIN

DIFFUSER

COALESCER

COLLECTOR SHELL

CONDENSERTUBES

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4.6.1.2 COALESCER/BAG TYPE

A porous bag, supported by a shell is fitted within the extractor to convertmoisture into water droplets. A swirl is imparted into the conditioned air and thecentrifugal effect forces the droplets to the outlet shell where it collects and drainsfrom the component. A bag visual indicator operated by back pressure, will showwhen the coalescer bag becomes dirty or blocked. In this case, a relief valve willopen to ensure flow is still available.

Bag Type Water ExtractorFigure 13

BLOCKAGE INDICATOR

BAG

PRESSURE RELIEF VALVE

WATER DRAIN

OUTLET SHELL

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4.6.1.3 SWIRL VANE TYPE

This type uses centrifugal force to spin the moisture-laden air outwards againstthe exit shell. The swirl vane, either fixed or rotating imparts the swirl by rotatingthe airflow at high speed. The action, separates the heavier water droplets in themoisture and collects them in a sump, to be drained away.

Swirl Vane Type Water SeparatorFigure 14

DRAIN

SWIRL VANEWATER SUMP

SEPARATOR SHELL

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4.6.2 Water Infiltration

Humidity control can also include the addition of water into the air conditioningsystem. As an aircraft climbs to high altitude, the moisture level in the air reducesto a much lower amount than at lower levels of altitude. The reduction in moisturemay cause discomfort to the aircraft occupants. To counteract this, moisture isadded into the conditioned air, by pumping water from a tank to a spray nozzlepositioned at the cabin air inlet. Humidity sensors will detect low humidityconditions and automatically turn on the controller water pump to restore thehumidity to acceptable levels.

CABIN HUMIDITY SENSOR

OVERFILL DRAIN

WATER SEPARATOR

DRAIN

COLLECTOR TANK

WATER PUMP AND

CONTROLLER

SPRAY NOZZLE

TO CABIN

Typical Humidity Control SystemFigure 15

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4.7 MASS FLOW CONTROL

Legislation requires that a minimum amount of fresh air be supplied topassengers and crew. In addition stale air must be removed and odourseliminated. Most pressurisation systems rely on the fact that air is delivered at aconstant rate under all conditions of flight in order to function correctly.Mass flow control systems constantly monitor the velocity and density of the airsupply by either increasing or decreasing the demand upon the source of supply,or by spilling excess supply air overboard.The mass of air must be controlled at a constant value regardless of aircraftaltitude or cabin pressure. It must also adjust for changes in main enginecompressor speed in bleed air systems, or changes in rotor speed when aseparate air supply from an accessory gearbox driven blower is incorporated.

4.7.1 Mass Flow Controller

This type automatically caters for changes in air density, cabin back pressure andengine compressor supply pressure. At ground level and during take off and theearly stages of flight, the pressure available from the main engine compressoroutlet is high. As altitude increases or when the engines are set to cruisingspeeds, the supply pressure drops.The amount of pressure from the engine compressor bleed acting on an altitude-compensated piston valve, determines the position the valve will adopt whenopposed by a spring and back pressure from the cabin. The pressure drop acrossthe valve, will vary the size of outlet ports and will thus determine the valvesdegree of opening and closing. This will result in a constant mass flowdownstream of the valve at all times.

Mass Flow Control

Figure 16

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4.7.2 Spill Valve Flow Controller

This type receives the charge air supply through a metering duct, which sensesvariations in the velocity and density of the air. The metering duct on sensingthese variations, transmits the information to a mass flow controller, whichconverts the air pressure signals into electrical signals. The electrical signals inturn control the position of spill valves. They will move towards a more open orclosed position, to vary the amount of air spilled overboard, thereby ensuring aconstant flow rate into the cabin.At sea level, with the engines at low power, the absolute capsule D will becompressed by atmospheric pressure. The contacts A, B and C will be in theposition shown and the spill valve will be towards closed.

With the main engines at take off power, the air velocity through the ventureincreases, causing a pressure differential across the controller diaphragm. Thiswill cause contact B to move towards contact C and when they touch, the spillvalves will be driven towards the open As the aircraft climbs, the static pressurein the metering duct and controller will decrease. The absolute capsule will nowexpand and the position of contacts A and C, will be adjusted in relation tocontact B. When contact B is touched, the spill valves will move towards closedonce more and once again the mass flow to the cabin will remain constant.

Mass Flow Controller OperationsFigure 17

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4.8 DISTRIBUTION SYSTEMS

The air distribution system on most aircraft takes cold air from the air conditioningpacks and hot air bleed from the engines and mixes the 2 in a mixer unit to therequired temperature. The air is then distributed to side wall and overhead cabinvents. On some aircraft the cabin air is then drawn back into the mixing unit by re-circulating fans where it is mixed with new air and then re-distributed.

All major components are usually located together in a designated bay for ease ofmaintenance. ( Figure 14).

A gasper fan provides cold air to the individual overhead air outlets for the aircrew

and passengers. This air can be drawn direct from outside or from the coolingpacks. Each passenger or crew can control the amount of air received bycontrolling the position of the air outlet. This outlet could be a rotary nozzle or alouvre.

Air Conditioning Distribution ManifoldFigure 18

WATER SEPARATOR

GASPER FAN

MANIFOLD RELIEF VALVE

MIXER VALVES

TO OVERHEAD

DUCTS

TO SIDEWALL

DUCTS

TO GASPER

OUTLETS

TO SIDEWALL DUCTS

TO COCKPIT

CONTROL VALVE

SELECTOR LINKAGE

CONTROL VALVES

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Conditioned air systems dispense temperature controlled air evenly throughout

the cabin and crew areas. One duct system supplies the cockpit (Figure 17) whileanother supplies the cabin. The cabin ducting is then divided into 2 systems, theoverhead (Figure 15) and the sidewall systems (Figure 16). The overhead systemreleases air into the cabin from outlets in ducting running fore and aft in the cabinceiling. The sidewall duct system takes air through ducting between the sidewalland cabin interior linings and releases it through cove light grills and louvres.

A cockpit controlled selector valve located on the main distribution manifoldallows all overhead, side wall or any combination of the two systems to be usedand varies the flow between the two.

Overhead PanelFigure 19

Duct sections throughout both the cabin and cockpit are joined together withclamps or clips. Means of equalising the duct pressures and balancing the airflows are designed into each system. The systems are protected from excesspressures by use of a spring loaded pressure relief valve usually located in themain distribution manifold. The main manifold is located immediately downstreamfrom the mixing units in the air conditioning bay.

On large aircraft a cockpit controlled dual selector valves divides the air betweencockpit and cabin areas. These butterfly valves are interlinked. When one is fullyopen the other is fully closed and vice versa.

GASPER FAN

FLOOR EXHAUST DUCT

ADJUSTABLE AIR OUTLETS

DUCTING

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Air is exhausted from the passenger cabin through grills and outflow valves in thesidewalls above the floor. This air can then be directed around the cargo

compartment walls where it assists in compartment temperature control. Some airthen flows to the cargo heat distribution duct under the compartment floor and isthen discharged overboard through the outflow valves.

Sidewall DuctingFigure 20

Below each floor air exhaust outlet is a flotation check valve. This valve is aplastic ball held in a cage. If the cargo compartments become flooded the ballsfloat up the cage and seals off the floor to help prevent water from entering thecabin.

Cockpit Air DistributionFigure 21

SILENCER

FAN ASSY

FAN ASSY PRESSURE SWITCH

COOLING FANS

FLIGHT DECKTEMPERATURE SENSOR

AIR VENT

CABIN TEMPERATURE SENSOR

WINDOW DEMISTER

FLOOR EXHAUST VENTS

WALL FEEDER DUCTS

DISTRIBUTION BOXES

DISTRIBUTION DUCT

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Aircraft may be separated into zones each with its own air conditioning system

and controls for that zone located in a distribution bay. Some areas may have aremote heat exchanger and fan assembly in the vapour cycle system, to allowcooling to specific areas such as avionics bays, fed from one of the zone packs.

4.8.1 Re-circulation Air System

To improve cabin ventilation and supplement airflow the cabin air is recirculatedback to the main distribution manifold where it is mixed with conditioned air formthe cooling packs. The use of re-circulated air improves airflow and offloads theair supply system. This off loading of the air conditioning packs is converted into afuel saving.

The re-circulation fan will draw air from the cabin area, through a check valve andfilter assembly to remove any smoke and noxious odours before passing it to themixer unit for re-distribution. The check valve prevents any reverse flow throughthe fan and ducting when the fan is not in use.

4.9 PRESSURISATION SYSTEMS

As aircraft became capable of obtaining altitudes above that at which flight crewscould operate efficiently, a need developed for complete environmental systemsto allow these aircraft to carry passengers. Air conditioning could provide theproper temperature and supplemental oxygen could provide sufficient breathableair.

The problem was that not enough atmospheric pressure exists at high altitude toaid breathing in and even at lower altitudes the body must work harder to absorbsufficient oxygen, through the lungs, to operate at the same level of efficiency asat sea level. This problem is overcome by pressurising the cockpit/ cabin area.Cabin pressurisation is a means of adding pressure to the cabin of an aircraft tocreate an artificial atmosphere that when flying at high altitudes it provides givesan environment equivalent to that below 10000 feet. The minimum quantity of

fresh air supplied to each person on board must be at least 0.5lb/ minute.

Aircraft are pressurised by sealing off a strengthened portion of the fuselage. Thisis usually called the pressure vessel and will normally include cabin, cockpit andpossibly cargo areas. Air is pumped into this pressure vessel and is controlled byan outflow valve located at the rear of the vessel.

Sealing of the pressure vessel is accomplished by the use of seals around tubing,ducting, bolts, rivets, and other hardware that pass through or pierce the pressuretight area. All panels and large structural components are assembled with sealingcompounds. Access and removable doors and hatches have integral seals. Some

have inflatable seals.

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Pressurisation systems do not have to move large volume of air. Their function isto raise the pressure inside the vessel. Small reciprocating engine powered

aircraft receive their pressurisation air from the compressor of a coupledturbocharger. Larger reciprocating engine powered aircraft receive air fromengine driven compressors and turbine powered aircraft use compressor bleedair

Small Reciprocating Engine Powered Aircraft

Turbochargers are driven by the engine exhaust gases flowing through a turbine.A centrifugal compressor is coupled to the turbine. The compressors output is fedto the engine inlet manifold to increase manifold pressure which allows theengine to develop its power at altitude. Part of this compressed air is tapped off

after the compressor and is used to pressurise the cabin. The air passes througha flow limiter (or sonic venturi) and then through an inter-cooler before being fedinto the cabin. A typical system is shown at Figure 22.

Sonic Venturi

A sonic venturi is fitted in line between the engine and the pressurisation system.When the air flowing across the venturi reaches the speed of sound a shockwave is formed which limits the flow of air to the pressurisation system

Small Reciprocating Engine Aircraft Pressurisation SystemFigure 22

OUTFLOW VALVE SAFETY VALVE

RAM AIR

HEATING AIR

PRESSURISED AIR

EXHAUST GASES

COMBUSTION HEATER

RAM AIR SHUT

OFF VALVE

COUPLED TURBO

COMPRESSOR

INTERCOOLER

SONIC VENTURI

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Large Reciprocating Engine Powered Aircraft

These aircraft use engine driven compressors driven through an accessory driveor by an electric or hydraulic motor. Multi engine aircraft have more than one aircompressor. These are interconnected through ducting but each have a checkvalve or isolation valve to prevent pressure loss when one system is out of action.

Turbine Powered Aircraft

The air supplied from a gas turbine engine compressor is contamination free andcan be suitably used for cabin pressurisation (Figure 23). Some aircraft use anindependent compressor driven by the engine bleed air. The bleed air drives the

coupled compressor which pressurises the air and feeds it into the cabin

Turbo CompressorFigure 23

Some aircraft use a jet pump to increase the amount of air taken into the cabin(Figure 24). The jet pump is a venturi nozzle located in the flush air intakeducting. High velocity air from the engine flows through this nozzle. This producesa low pressure area around the venturi which sucks in outside air. This outside airis mixed with the high velocity air and is then passed into the cabin

BLEED AIR

ENGINE

PRESSURE VESSEL

(CABIN/COCKPIT)

OUTFLOW VALVE

FLUSH AIR INTAKE TURBO COMPRESSOR

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Jet PumpFigure 24

4.9.1 Control And Indication

There are 3 modes of pressurisation, un-pressurised, the isobaric mode and theconstantdifferential pressure mode. In the un-pressurised mode the cabinaltitude remains the same as the flight altitude. In the isobaric mode the cabinaltitude remains constant as the flight altitude changes and in the constant-differential pressure mode, the cabin pressure is maintained at a constant amountabove the outside ambient air pressure.

The amount of differential pressure is determined by the structural strength of theaircraft. The stronger the aircraft structure the higher the differential pressure andthe higher is the aircrafts operating ceiling.

4.9.2 The Un-Pressurised Mode

In this mode the outflow valve remains open and the cabin pressure is the sameas the outside ambient air pressure. This mode is usually from sea level up to5000` but does vary from aircraft to aircraft.

ENGINE

FLUSH AIR INTAKE

PRESSURE VESSEL

(CABIN/COCKPIT)

JET PUMP

BLEED AIR

OUTFLOW VALVE

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4.9.3 The Isobaric Mode

In this mode the cabin pressure is maintained at a specific cabin altitude as flightaltitude changes. The cabin pressure controller begins to close the outflow valveas the aircraft climbs to a chosen cabin altitude. The outflow valve then opens orcloses (modulates) to maintain the selected cabin altitude as the flight altitudechanges up or down. The controller will then maintain the selected cabin altitudeup to the flight altitude that produces the maximum differential pressure for whichthe aircraft structure is rated. At this point the constant differential mode takescontrol.

4.9.4 The Constant-Differential Pressure Mode

Cabin pressurisation puts the aircraft structure under a tensile stress as the cabinpressure expands the pressure vessel. The cabin differential pressure is the ratiobetween the internal and external air pressures. At maximum constant-differentialpressure as the aircraft increases in altitude the cabin altitude will increase butthe internal/external pressure ratio will be maintained. There will be a maximumcabin altitude allowed and this will determine the ceiling at which the aircraft canoperate.

4.9.5 Cabin Air Pressure Regulator

The pressure regulator maintains cabin altitude at a selected level in the isobaricrange and limits cabin pressure to a pre-set pressure differential in the differentialrange by regulating the position of the outflow valve. Normal operation of theregulator requires only the selection of the desired cabin altitude and cabin rate ofclimb the adjustment of the barometric control.

Cabin Pressure RegulatorFigure 25

ACTUATOR

DIAPHRAGM

OUTFLOW VALVE

BAFFLE PLATE

BASE

REFERENCE

CHAMBER

HEAD

PILOT

DIAPHRAGM

ISOBARIC METERING VALVE

ADJUSTER CONTROL

BAROMETRIC CAPSULE

STATIC ATMOSHERE CONNECTION

ADJUSTER

CONTROL

DIFFERENTIAL

METERING VALVE

SOLENOID

DUMP VALVE

RESTRICTOR

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The regulator shown in Figure 25 is a typical differential pressure type regulator

that is built into the normally closed air operated outflow valve. It uses cabinaltitude for its isobaric control and barometric pressure for the differential control.A cabin rate of climb controller controls the pressure change inside the cabin.

There are 2 main sections to the regulator, the head and reference chamber andthe base with the outflow valve and diaphragm. The balance diaphragm extendsoutward from the baffle plate to the outflow valve creating an air chamberbetween the baffle plate and the outer face of the outflow valve. Cabin air flowinginto this chamber through holes in the side of the outflow valve exerts a forceagainst the outer face of the valve which tries to open it. This force is opposed bythe force of the spring around the valve pilot which tries to hold the valve closed.

The actuator diaphragm extends outward from the outflow valve to the headassembly creating an air chamber between the head and the inner face of theoutflow valve. Air from the head and reference chamber exert a force against theinner face of the outflow valve helping the spring to hold the valve closed.

The position of the outflow valve controls the amount of cabin air that is allowedto flow from the pressure vessel and this controls the cabin pressure. Theposition of the outflow valve is determined by the amount of reference chamberair pressure that presses on the inner face of the outflow valve.

4.9.6 Isobaric Control System

The isobaric control system of the pressure regulator shown in Figure 26incorporates an evacuated capsule, a rocker arm, valve spring and a ball typemetering valve. One end of the rocker arm is connected to the valve head by theevacuated capsule and the other end of the arm holds the metering valve in aclosed position. A valve spring located on the metering valve body tries to movethe metering valve away from its seat as far as the rocker arm allows.

When the cabin air pressure increases enough for the reference chamber air

pressure to compress the evacuated capsule the rocker arm pivots around itsfulcrum and allows the metering valve to move away from its seat an amountproportional to the compression of the capsule. When the metering valve opensreference pressure air flows form the regulator to atmosphere through theatmospheric chamber.

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Isobaric Control OperationFigure 26

When the regulator is operating in the isobaric range, cabin pressure is heldconstant by reducing the flow of reference chamber air through the meteringvalve. This prevents a further decrease in reference pressure.

The isobaric control responds to slight changes in reference pressure bymodulating to maintain a constant pressure in the chamber throughout theisobaric range of operation. Whenever there is an increase in cabin pressure theisobaric metering valve opens which decreases the reference pressure andcauses the outflow valve to open which then decreases the cabin pressure.

4.9.7 Differential Control System

The differential control system of the pressure regulator (Figure 27) incorporatesa diaphragm a rocker arm, a valve spring and a ball type metering valve. One endof the rocker arm is attached to the head by the diaphragm which forma apressure sensitive face between the reference chamber and the atmosphericchamber.

EVACUATED BELLOWS

ISOBARIC METERING VALVE

OUTFLOW VALVE

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Differential Pressure ModeFigure 27

Atmospheric pressure acts on one side of the diaphragm and reference chamberpressure acts on the other. The opposite end of the rocker arm holds themetering valve in a closed position. A valve spring located on the metering valvebody tries to move the metering valve away from its seat as far as the rocker armallows.

When reference chamber pressure increases to the system differential pressurelimit set above the decreasing atmospheric pressure it collapses the diaphragmwhich is set at differential pressure and opens the metering valve. Air flows fromthe reference chamber to atmosphere through the atmospheric chamber, whichcauses a reduction in the reference pressure. This reduction in referencepressure causes the outflow valve to open to reduce the cabin pressure tomaintain the system pressure differential.

METERING VALVE

OUTFLOW VALVE

ATMOSPHERIC CHAMBER

DIAPHRAGM

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4.9.8 Safety Valves

Cabin Air Pressure Safety Valve

The pressure relief valve prevents cabin pressure from exceeding thepredetermined cabin to ambient pressure differential. A negative pressure reliefvalve and pressure dump valve may also be incorporated into this valveassembly.

Negative Pressure Relief Valve

A pressurised aircraft is designed to operate with the cabin pressure higher thanthe outside air pressure. If the cabin pressure were to become lower than the

outside air pressure the cabin structure could fail. Outside air is allowed to enterthe cabin to ensure that this does not happen. It is basically an inward pressurerelief valve.

Dump Valve

This valve is normally solenoid actuated by a cockpit switch. When the solenoid isenergised the valve opens dumping cabin air to atmosphere. Cabin pressure willdecrease rapidly until it is the same as the outside air pressure and cabin altitudewill increase until it is the same as the flight altitude.

Ditching valve

If any of the cabin control valves were situated below the water level and theaircraft ditch in the water, the cabin would quickly flood. To prevent thishappening, either a mechanical or electrical ditching selection, can be made bythe crew to seal off all pressurisation valves and inlets.

4.10 ELECTRONIC PRESSURISATION CONTROL

Most modern airliners have the means to electronically control the cabin pressure

automatically for the entire flight, from settings made by the flight crew beforetake off.

The pressure control system consists:

a flight deck control panel

an automatic pressure controller with pressure sensing inputs and outputs tomonitoring indicators

an electrically-driven gate-type outflow valve

inward and outward safety relief valves

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4.10.1 Flight Deck Control Panel

This provides a means for the flight crew to control the cabin pressure bypositioning the outflow valve. There are three mode selections available; Auto,Standby or Manual.

The desired mode will normally be Auto, where all settings such as intendedcruise (flight) altitude and destination airfield (landing) altitude are made beforeflight. This will allow automatic control of cabin pressure for the whole of thatflight.

This is called the fully automatic mode.

Alternatively, Standby or back up mode can be selected, where a cabin altitudesetting must be made for each desired cabin pressure change. The input settingis then controlled automatically as before.This is called the semi-automatic mode.

If neither the fully or semi-automatic modes are available, (i.e.: the pressurecontroller fails), the outflow valve can be positioned directly from the flight deck byoperating the electric torque motors to drive the valve.This is called the manual mode and a choice of an ac or dc electrical supply is

available.

Figure 28

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4.10.2 Automatic Pressure Controller

The pressure controller provides output control signals to the outflow valves ac ordc torque motors. The motors position and modulate the valve to establish andcontrol actual cabin pressure in accordance with the controllers pre-programmedclimb, cruise or descent schedules. This will ensure that for every aircraft altitudethere will be a particular cabin altitude.Input signals to the controller are from the flight deck control panel, cabin andambient pressure sensors, barometric correction and air/ground sensing.

4.10.3 Outflow Valve

The valve has a moving gate designed to cover or uncover an aperture in thefuselage skin. An increase in the aperture size will cause cabin pressure to fall(cabin altitude to ascend), whereas a decrease in the aperture size results in anincrease in cabin pressure (cabin altitude to descend). The gate is driven by oneof two electrically driven motors, the choice of ac or dc motor being determinedby flight crew input.Motor input signals come from the controller when in the auto or standby modes,or directly from a control panel switch when in the manual mode.

Auto Mode Flight Profile

Figure 29

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4.10.4 Inward and Outward Safety Relief Valves

Fuselage frames are designed to accept tensile loads associated with andoutward force from within the pressure cell. Their ability to withstand compressionloads that would occur if the pressure outside the aircraft were higher than withinthe pressure cell is poor. Therefore an inward relief valve will open and equalisethe pressure if the inward or negative differential exceeds about 0.5 psid.Two outward relief valves are fitted to prevent the maximum outward differentialpressure from exceeding the structural limit. This will typically be around 8.5psid.Even though the main pressure control is electronic, the safety relief valves aremechanical operated and are completely independent of any automatic controlsystem.

Outflow ValveFigure 30

Pressurisation System ValvesFi ure 31

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4.11 CABIN PRESSURE INDICATION

Most pressurisation systems have three basic cockpit indicators cabin altitude,cabin rate of climb and the pressure differential indicator. The cabin altitudegauge measures the actual cabin altitude.

Cabin Altitude GaugeFigure 32

The cabin rate of climb indicator tells the pilot the rate that the cabin is eitherclimbing or descending. (I.e. the rate at which the cabin loses or gains pressure)A typical maximum climb rate is 500ft per minute and the maximum descent rateis 300ft per minute. The control can be automatic or manual depending on aircrafttype.

Cabin Rate of ClimbFigure 33

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The differential pressure gauge (Figure 34) reads the difference between thecabin and the outside air pressures. This differential pressure is normally

controlled and maintained to a structural limitation around 7psid. This depends onthe aircraft type and the operating ceiling of the aircraft. The differential pressuregauge may be combined with the cabin altitude (Figure 35).

Differential Pressure Gauge Dual GaugeFigure 34 Figure 35

4.12 SAFETY AND WARNING DEVICES

To ground test the pressurisation system with the engines running, at least threemen are required inside the aircraft for safety reasons.

Both air conditioning and pressurisation systems use safety and warning devicesto protect the aircraft from possible catastrophic failures. Some of the protectiondevices may be inhibited in certain stages of flight; landing or take off where theextra distractions caused by such warnings may be too much for the crews todeal with safely.

With the air conditioning system the main concerns are with overheating of the airconditioning packs and extraction and ventilation fans, as well as hot air leaksfrom ducting which could damage surrounding structure or components.

4.12.1 Overheating

Most packs systems are protected from overheating by a thermal switchdownstream of the pack outlet. If the outlet temperature reaches a predetermined figure the switch will operate causing the pack valves to shut,preventing air from getting to the packs, as well as sending a warning signal to

the cockpit central warning panel with associated caution/warning lights and auralchimes and to illuminate a fault light on the pack selector switch.

01

2

3

4

56

7

8

9

10DIFF PX PSI

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Once the system has cooled down sufficiently the crew may have an option to

reselect the overheated system. The overheat may have been caused by a faultin the automatic temperature control system in which case the pilot may be ableto control the system manually via a manual selector switch on the cockpitcontroller.

Extraction or ventilation fans will be protected in much the same way. Anoverheat will signal the central warning panel with associated caution/warninglights and aural chimes. The fan may be isolated automatically or manually. Oncethe fan has cooled down it may be possible to re-select if required. Fans may alsobe protected from over or under speeding, which will also have an effect on thesystem temperatures. Speed sensors on the fan will indicate a fault when over or

under speed limits are reached and a warning signal is sent to the cockpit centralwarning panel with associated caution/warning lights and aural chimes.

4.12.2 Duct Hot Air Leakage

Any ducting that includes joints is liable to leak under abnormal conditions. A ductprotection system will include fire-wire elements around the hot zones such asengine air bleeds, air conditioning packs and auxiliary power units if fitted.

The sensing elements will be the thermistor type. As the temperature around thewire increases the resistance decreases until an electrical circuit is made. Whenthe circuit is made a warning signal is sent to the cockpit central warning panelwith associated caution/warning lights and aural chimes. The leaking duct may beisolated automatically or may require the pilot to take action to close off the airvalves. The faulty system will then remain out of use.

4.12.3 Excess Cabin Altitude

If the cabin altitude was allowed to increase unchecked the crew and passengerscould unknowingly suffer the effects of hypoxia. This dangerous condition isobviously undesirable especially for the aircrew. Most aircraft give a warning on

the CWP with associated audio and visual warnings when the cabin altitudereaches 10000`.

4.12.4 Smoke Detection

Smoke detectors may be fitted within the cabin; avionics bay and cargo areas tomonitor systems, which if become faulty may generate smoke on overheating, orare may be liable to catch fire. These detectors will send a signal to the CWP withassociated lights and audio warnings. They may also automatically switch onextractor fans, which will remove the smoke overboard and away form the cabinand cockpit areas. In this event, the pilot may have a switch or control lever to

operate a valve to isolate the cockpit air conditioning ducting from the rest of theaircraft to prevent any smoke from getting to the cockpit.

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