B1 Mod 11.17 (5)

Issue 1 - 20 March 2001

JAR 66 CATEGORY B1

MODULE 11.04

AIR CONDITIONING ANDCABIN PRESSURISATION

ukengineering

CONTENTS

1 AIR CONDITIONING AND CABIN PRESSURISATION............... 1-11.1 INTRODUCTION .............................................................................1-11.2 AIR SUPPLY .................................................................................1-1

1.2.1 Ram Air.........................................................................1-11.2.2 Engine Bleed Air ...........................................................1-11.2.3 Compressors or Blowers. ..............................................1-11.2.4 Auxillary Power Unit (APU) ...........................................1-11.2.5 Ground Power Trolley ...................................................1-2

1.3 AIR CONDITIONING SYSTEMS ........................................................1-21.3.1 Combustion Heating......................................................1-21.3.2 Engine Exhaust Heating................................................1-31.3.3 Compression Heating....................................................1-3

1.4 AIR CYCLE AND VAPOUR CYCLE MACHINES ..................................1-41.4.1 Air Cycle Cooling System..............................................1-41.4.2 The Turbo Compressor .................................................1-51.4.3 The Brake Turbine ........................................................1-61.4.4 The Turbo Fan .............................................................. 1-71.4.5 Vapour Cycle Cooling System.......................................1-81.4.6 The Compressor ...........................................................1-101.4.7 The Receiver Dryer .......................................................1-121.4.8 Thermostatic Expansion Valve ......................................1-13

1.5 DISTRIBUTION SYSTEMS ............................................................... 1-141.5.1 Recirculation Air System ...............................................1-18

1.6 FLOW, TEMPERATURE AND HUMIDITY CONTROL SYSTEMS .............1-181.6.1 Coalescer Type Water Extractor ...................................1-191.6.2 Bag Type Coalescer......................................................1-201.6.3 Swirl Vane Type Water Separator .................................1-21

1.7 PRESSURISATION SYSTEMS ..........................................................1-221.7.1 Control And Indication...................................................1-251.7.2 The Un-Pressurised Mode ............................................1-251.7.3 The Isobaric Mode ........................................................1-261.7.4 The Constant-Differential Pressure Mode .....................1-261.7.5 Cabin Air Pressure Regulator........................................1-261.7.6 Isobaric Control System ................................................1-271.7.7 Differential Control System............................................1-281.7.8 Safety Valves ................................................................ 1-301.7.9 Cabin Pressure Controllers ...........................................1-30

1.8 SAFETY AND WARNING DEVICES ..................................................1-321.8.1 Overheating ..................................................................1-321.8.2 Duct Hot Air Leakage ....................................................1-321.8.3 Excess Cabin Altitude ...................................................1-33

B1 Mod 11.04 _34B192C.doc Issue 3 Feb 2002

JAR 66 CATEGORY B

MODULE 11.04


ukengineering

PAGEINTENTIONALLY

BLANK

Issue 1 - 20 March 2001 Page 1-1

JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

1 AIR CONDITIONING AND CABIN PRESSURISATION

1.1 INTRODUCTION

Air conditioning systems control both the temperature and humidity of the airwithin the cabin, cockpit and freight areas as well as heating or cooling it asnecessary. It should also supply adequate movement of air through the aircraftfor ventilation as well as provide a means of removing smoke (if permitted) andodours. A typical system comprises of five principle sections:

a. Air supplyb. Heatingc. Coolingd. Temperature controle. Distribution

1.2 AIR SUPPLY

The source of air supply and arrangement of the system components depend on theaircraft type and system employed but in general one of the following methods may beused:

1.2.1 Ram Air

This is used in some unpressurised aircraft using either combustion heating or warm airheating from an exhaust gas heat exchanger. The ram air supply is from an intakedirectly in the airflow either on the nose, wing or at the base of the tail fin. The air aftercirculating through the cabin is exhausted to atmosphere.

1.2.2 Engine Bleed Air

This is used in turbo jet aircraft in which hot air is bled of from the engine compressors tothe cabin. Before the air enters the cabin it is passed through a temperature controlsystem which reduces its pressure and temperature and is then mixed with ram air.

1.2.3 Compressors or Blowers.

This is used by some turbo jet, turbo prop or piston engine aircraft, the compressors orblowers being either engine driven via an accessory drive, by bleed air or electric orhydraulic motors.

1.2.4 Auxillary Power Unit (APU)

This provides an independent source of pressurised air. It is basically a small gas turbineengine that provides air and other service whilst the aircraft is on the ground with itsmain engines stopped. It is usually a self contained unit located in the tail section of theaircraft where it can be run safely.

2 B1 Mod 11.04 _34B192C.doc Issue 3 Feb 2002

JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.2.5 Ground Power Trolley

For use in extreme climates on aircraft which do not have an APU or the APU isunserviceable. This is a self contained air conditioning unit and can be connected to theaircrafts cabin, to either heat or cool it depending on the climate. This unit will run untilthe aircraft is independent of the trolley.

1.3 AIR CONDITIONING SYSTEMS

The method of air conditioning depends on the type of aircraft and the air supply systemused. Each system uses different methods for heating and cooling. In general there are3 types of heating systems used.

1.3.1 Combustion Heating

A typical layout is shown in Figure 1. This is usually associated with a ram air supply anddepends for its operation on the combustion of a fuel air mixture within a cylindricalcombustion chamber. Ram air is augmented with an air blower and fuel is metered fromthe aircraft fuel system through a solenoid valve. The fuel air mixture is ignited in thecombustion chamber and the burnt gases swirl through the transfer passages of thecylinder before being exhausted to atmosphere.

This gas swirl not only aids combustion but ensures that the gases impart against thechamber and passage walls to allow maximum heat transfer. The ram air flows over theoutside of the combustion chamber where it is absorbs the heat before it enters thecabin.

Typical Combustion Heater SystemFigure 1

FUEL SOLENOID VALVE

COMBUSTION HEATING AIR CONDITIONING SYSTEM

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


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

The temperature is controlled manually by setting a control valve which is locateddownstream of the combustion chamber. This controls the amount of air flow over thecombustion chamber. The slower the flow the hotter the air becomes and vice versa.The blower operation, fuel supply and ignition is normally controlled by a single on/offswitch.

1.3.2 Engine Exhaust Heating

A typical exhaust heater is shown in Figure 2. This also is used with a ram air ventilationsystem but the heating of the supply air is much more simple and direct. A heater muffsurrounds the exhaust pipe of a piston engine aircraft. The ram air enters this muff andextracts the heat from the hot exhaust. This heated air is then passed into a chamberwhere it is mixed with a separate cold air supply. Mechanically operated valves areprovided to control the flow of air supplied and therefore regulate the cabin temperature.

Carbon monoxide detectors may be used within the cabin to check for levels of the gas.These are usually indicators filled with bright coloured crystals which turn black whenexposed to dangerous carbon dioxide levels. They are sited in view of the pilots.

Exhaust System HeaterFigure 2

1.3.3 Compression Heating

A typical compression heating sytem is shown in Figure 3. This system relies on theprinciple whereby the air temperature is increased during compression and is used by airsupply sytems utilising either engine driven compressors and blowers and engine bleedair. Hot air is drawn in from either an engine bleed or air blower where it is then split.Some air goes directly to the distribution mixer control valves and the remainder goes toa primary heat exchanger where ram air passes through the exchanger matrix to coolthe air.

CONTROL VALVE

EXHAUST MANIFOLD

HEATER MUFF

CONTROLLEVER

CLOSED

OPEN

RAMAIR

TO CABIN

OVERBOARD DUMP

FLAP

SIMPLE EXHAUST SYSTEM HEATER


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

From the primary heat exchanger the cool air then goes to a compressor where it iscompressed and heated before going through a secondary heat exchanger, again beingcooled by the ram air. This cold air then passes through the turbine where it “does work”driving the compressor becoming even colder, before going to the mixer control valvewhere it is mixed with the hot air before being distributed to the cabin. Adjustable flowcontrol and temperature control valves control the cabin temperature.

Typical (Compression) Bleed Air SystemFigure 3

1.4 AIR CYCLE AND VAPOUR CYCLE MACHINES

1.4.1 Air Cycle Cooling System

This system works on the principle of the air dissipating or absorbing heat bydoing or receiving work. If it does work (expanded) its temperature will fall if itreceives work (compressed) its temperature will rise. The primary component inan air cycle system is the cold air unit. There are a number of types in use:

ECU

NRV

AUXILLARY POWER UNITNON RETURN VALVE

SHUT OFF VALVES

FLOW CONTROLLER

TEMPERATURE CONTROL VALVE

MIXER UNIT TOCABIN

NRV

WATER SEPARATOR

COUPLED COMPRESSOR TURBINE

RAM AIR

PRIMARY HEATEXCHANGER

SECONDARY HEATEXCHANGER


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

1.4.2 The Turbo Compressor

In a typical system the turbine drives a coupled compressor (Figure 4). Asecondary heat exchanger is located in line between the compressor outlet andthe turbine inlet. A ram air supply is provided to the primary and secondary heatexchangers. For operation of the system on the ground an induction fan orblower may be used to augment the air supply and there may also be a groundair conditioning unit connection.

Turbo CompressorFigure 4

The hot air supply initially air passes through a primary heat exchanger where it ispre-cooled before entry to the compressor. It is then compressed and heated bythe compressor before being cooled again as it passes through the secondaryheat exchanger.

The cooled air then drives the coupled turbine where it does work and becomeseven colder. As this air cools, moisture condenses out of it and is collected in awater separator. The water is centifuged out in the seperator where it collects onthe outer case and is then allowed to drain overboard. To prevent this water fromfreezing warm air is mixed with it via a temperature control valve when it reachesa certain temperature. A typical turbo compressor is shown in Figure 5.

TEMPERATURECONTROL VALVE

COMPRESSOR TURBINE

SECONDARY HEAT EXCHANGER

RAM AIR

TOCABIN

MIXER UNIT

PRIMARYHEATEXCHANGER

HOT AIR INLET

WATER SEPARATOR


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

The cold air from the turbine then enters the mixer unit where it is mixed with thepre-cooled air supply via the temperature control valve to allow a variable warmair supply to the air distribution system.

Turbo Compressor Cold Air UnitFigure 5

1.4.3 The Brake Turbine

A typical brake turbine is shown at Figure 6. When cold air is selected the bleedair is directed to the turbine of the cold air unit. As the air drives the turbine thegas expands as work is being done resulting in a drop in pressure andtemperature.

BLEED AIR

TO INTERCOOLER

FROMINTERCOOLER

TODISTRIBUTIONSYSTEM

COMPRESSOR

DIFFUSER

NOZZLE BLADES

TURBO COMPRESSOR


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Brake Turbine Cold Air UnitFigure 6

To prevent the turbine from rotating too quickly and affecting the coolingefficiency, the turbine is coupled to the compressor. As the compressor rotatesambient air is used as a braking medium to slow the turbine. This system is animprovement on the turbo compressor system as only one heat exchanger isrequired

1.4.4 The Turbo Fan

The turbo fan is mechanically similar to the brake turbine cold air unit. In the turbofan the turbines drives a coupled centrifugal compressor which induces acapacity of air, large enough to create a cooling flow of ram air through a heatexchanger, cooling the bleed air. It also acts as acting as a braking fan to controlthe turbine speed. A typical turbo fan is shown in Figure 7.

The major advantage of this system is that the air conditioning system can beoperated on the ground with engines running without the need for ram air.

RAM AIR

TOCABIN

MIXERUNITHEAT EXCHANGER

CONTROL VALVE

AMBIENT AIR INLET

COMPRESSOR TURBINE

BLEEDAIR

AMBIENT AIR OUTLET


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

Turbo Fan Cold Air UnitFigure 7

1.4.5 Vapour Cycle Cooling System

The vapour cycle cooling system is used to control and reduce the temperaturesgenerated by electronic equipment used in modern aircraft.

This system works on the principle of the ability of a refrigerant to absorb heatthrough a heat exchanger in the process of changing from a liquid into a vapour.A refrigerant is a substance that absorbs heat through expansion or vaporisation.For example if you drop some methylated spirit onto your hand it feels cold. Thisis because the volatile liquid starts to evaporate as it draws the heat away fromyour hand.

Liquids with low boiling points have a stronger tendancy to evaporate at normaltemperatures than those with higher boiling points. Furthermore pressure affectsthe state of a liquid substance. A reduction in pressure will cause a liquid tochange state into a gas or vapour.

A typical vapour cycle system operates with 2 distinct integrated systems, asealed recirculating refrigerant system and an air system. A typical system isshown at Figure 8.

MIXER UNIT

BLEED AIR

RAMAIR

HEATEXCHANGER

CONTROL VALVE

COMPRESSOR

TURBINE

RAM AIR OUTLET

TO CABIN

TURBO FAN COLD AIR UNITTURBO FAN COLD AIR UNIT


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Schematic Vapour Cycle SystemFigure 8

Refrigerant System

The system has 2 sides a high pressure side and a low pressure side. Mixed withthe refrigerant is a specified amount of lubrication oil which lubricates and sealsthe compressor.

The liquid refrigerant passes from the receiver to the thermostatic expansionvalve for controlled release into the matrix of the evaporator. Heated air from themain supply passes over the evaporator matrix and by induction transfers heatinto the liquid refrigerant which on heating becomes a low pressure vapour.

From the evaporator the LP vapour feeds into a compressor which pressurisesthe refrigerant to a high pressure. This HP refrigerant then enters the condensorwhere it is cooled to a liquid by ram air (or by induction fan air) passing throughthe matrix where it then returns as a liquid to the liquid receiver, to repeat thecycle.

THERMOSTATICEXPANSION VALVE

RECIEVER DRYER

CONDENSER

EVAPORATORTURBO COMPRESSOR

TEMPERATURECONTROL VALVES

AIR SUPPLY

RAM AIR

AIR DISTRIBUTION

TEMPERATURE SENSOR

CAPILLARY TUBE


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

Air System

The main hot air supply drives the turbine, which is directly coupled to thecompressor. The air is also fed directly downstream of the system to thetemperature control valves. As the air passes through the turbine it does workwhich reduces the airs temperature which is then fed to the evaporator. This airthen passes through the evaporator where it is further cooled (as the refrigerantabsorbs the heat) and is then fed to the temperature control valves. These valvescontrols the air temperature being fed to the air distribution system.

All components of this type of system are usually mounted on a single removablequick release panel (Figure 9) to allow complete pack changes when a faultarises, instead of changing individual components. Some aircraft use this type ofsystem to air condition avionics bays as well as the cabin.

Typical Vapour Cycle SystemFigure 9

1.4.6 The Compressor

The compressor pulls the low pressure refrigerant vapour from the evaporatorand compresses it. When the vapour is compressed its pressure and temperatureboth rise.

VAPOUR CYCLE COOLING SYSTEMVAPOUR CYCLE COOLING SYSTEM

FILTER

RAM AIR

COOLANT IN

COOLANT OUT

GROUND SERVICEPOINT

RECEIVER DRIER

TEMPERATURE BULB

EVAPORATOR

CONDENSER

QUICK RELEASEPANEL

THERMAL EXPANSIONVALVE

COMPRESSOR


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Some compressors are engine driven by a v belt (Figure 10) through anelectromagnetic clutch assembly (Figure 11) which can be engaged ordisengaged as required. The clutch drive plate is keyed to the compressor shaftand when the clutch is disengaged there is clearance between the drive plate andthe engine driven pulley. The pulley rotates but the compressor is at rest. Whenthe system calls for cooling the electromagnet energises and pulls and locks thedrive plate to the drive pulley and therefore drives the compressor.

Engine Driven CompressorFigure 10

Electromagnetic Clutch AssemblyFigure 11

V DRIVE BELT

DRIVE PLATEPULLEY

ELECTROMAGNETICCLUTCH COIL

COMPRESSOR

OUTLET TOCONDENSER

INLET FROMEVAPORATOR

DRIVE PLATE PULLEY ELECTROMAGNETICCOIL


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

Some compressors are driven by a hydraulic motor whose pressure is suppliedform an engine driven hydraulic pump. A solenoid valve is fitted to the hydraulicmanifold. When no cooling is required the solenoid valve is de-energised allowingfluid to bypass the motor and return to the reservoir. When cooling is required thesolenoid valve energises, closing off the bypass, allowing the hydraulic fluid todrive the compressor

1.4.7 The Receiver Dryer

High pressure high temperature refrigerant leaves the condenser and flows intothe receiver dryer (Figure 12). This acts as a reservoir to hold the supply ofrefrigerant until it is needed by the evaporator. As the hot liquid enters thereceiver dryer it first passes through a filter which removes any solidcontaminants. It then passes through a layer of silica gel or activated aluminawhich removes any water moisture from the liquid. It also acts as a separator assome traces of vapour may be in the liquid.

The moisture is removed to prevent the system form freezing and becomingblocked and to prevent the moisture from acting with the refrigerant which wouldform hydrochloric acid which would corrode the pipelines and galleries internally.The liquid falls to the bottom of the receiver dryer where it is picked up via thepick up tube.

Some receivers include a sight glass that allows the checking of the refrigerant. Ifbubbles are seen then the system requires re-charging.

Receiver DryerFigure 12

DESICCANTFILTER PADS

SIGHT GLASS

PICK UP TUBE

RECEIVER DRYER

FROM CONDENSER TO THERMOSTATICEXPANSION VALVE


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

1.4.8 Thermostatic Expansion Valve

The thermostatic expansion valve (TEV) is a metering device that controls theamount of refrigerant that is allowed to flow into the evaporator by measuring thetemperature of the evaporator discharge. All of the refrigerant should evaporateby the time it exits through the evaporator coils.

The Term Superheat

Superheat, is heat energy that is added to a refrigerant to change it from aliquid into a vapour. Superheated refrigerant is very cold, not hot.

A TEV is shown at Figure 13. The TEV outlet attaches to the evaporator inlet.The TEV inlet comes from the receiver dryer. A diaphragm situated on top of thevalve locates against push rods that act against a superheat spring. This actioncontrols the position of the needle valve. The superheat spring tension is factorypre-set.

A temperature sensing bulb connects above the diaphragm via a capillary tube.The bulb is located in the vapour flow at the evaporator discharge outlet. It isinsulated to allow only the outlet temperature to be sensed. The bulb andcapillary tube is filled with a highly volatile fluid which reacts readily withtemperature changes. When the bulb senses a rise in temperature the bulb liquidexpands and exerts a force against the diaphragm, the superheat spring and theevaporator inlet pressure acting underneath the diaphragm. The amount of forceexerted is directly related to the temperature of the vapour at the evaporatordischarge.

A needle valve is located between the inlet and outlet of the TEV and its positionis determined by the balance of the forces acting above and below the diaphragmincluding the pre set tension of the superheat spring.

When the system is started the evaporator is relatively warm and the bulbpressure above the diaphragm is high. This acts against the push rods andovercomes the superheat spring tension, to open the needle valve to allowmaximum flow to the evaporator. As the refrigerant evaporates the evaporatoroutlet temperature decreases and the pressure above the diaphragm alsodecreases. The superheat spring overcomes this drop in pressure and closes theneedle valve to a new position which restricts the amount of refrigerant that flowsinto the evaporator to ensure that it all evaporates by the time it reaches theevaporator outlet.


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

Thermostatic Expansion valveFigure 13

1.5 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 aircrewand 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.

VALVE BODY

SUPERHEAT SPRING

TEMPERATURE SENSING BULB

DIAPHRAGM

INLET

OUTLETCAPILLARY TUBE

PUSH RODS

NEEDLE VALVE


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Air Conditioning Distribution ManifoldFigure 14

Conditioned air systems dispense temperature controlled air evenly throughoutthe 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.

WATER SEPARATOR

GASPER FAN

MANIFOLD RELIEF VALVE

MIXER VALVES

TO OVERHEADDUCTS

TO SIDEWALLDUCTS

TO GASPEROUTLETS

TO SIDEWALL DUCTS

TO COCKPIT

CONTROL VALVESELECTOR LINKAGE

CONTROL VALVES


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

Overhead PanelFigure 15

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.

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 cargocompartment 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.

GASPER FAN

FLOOR EXHAUST DUCT

ADJUSTABLE AIR OUTLETS

DUCTING


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Sidewall DuctingFigure 16

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 17

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

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


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.5.1 Recirculation 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.

1.6 FLOW, TEMPERATURE AND HUMIDITY CONTROL SYSTEMS

Humidity control is the means of ensuring that the correct amount of watermoisture is in the air conditioning air within the cabin (Figure 18). This is toensure that passengers do not suffer from the low humidity levels at higheraltitudes and that excessive moisture is removed at lower altitudes.

Typical Humidity Control SystemFigure 18

CABIN HUMIDITY SENSOR

OVERFILL DRAIN

WATER SEPARATORDRAIN

COLLECTOR TANK

WATER PUMP ANDCONTROLLER

SPRAY NOZZLE

TO CABIN


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Humidity can be controlled in 2 ways:

Water Separation

This is the removal of excessive moisture from the conditioned air normally usinga water extractor or separator.

Water Infiltration

This is the addition of water into the conditioned air as it enters the cabin using awater pump and spray nozzle.

Water Extraction

Water extraction is carried out by an extractor or separator and there are differingdesigns, but its function is the same, to remove moisture from the conditioned air.Water is produced into the air conditioning system due to the cooling and heatingeffects of the air in the air cycle system. The extractor is located in the airconditioning ducting prior to entry into the cabin. There are 3 main types of waterseparator in use:

1.6.1 Coalescer Type Water Extractor

Coalescer Water ExtractorFigure 19

PRESSURE RELIEFVALVE

DRAIN

DIFFUSER

COALESCER

COLLECTOR SHELL

CONDENSERTUBES


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

The coalescer (Figure 19) consists of layers of monel gauze and glass fibre clothsandwiched between layers of stainless steel gauze. It is supported by thediffuser cone and held in place by the relief valve. As the conditioned air ispassed through the coalscer the moisture in the air is converted into waterdroplets. These droplets then enter the collector shell and deposited into thecollector tubes where they drain down into the collector box.

This water is either drained overboard or passed to a water tank where it can bestored and used to infiltrate the system if required. The purpose of the relief valveis to open if the coalescer becomes blocked to allow conditioned air into thecabin.

1.6.2 Bag Type Coalescer

The bag is fitted over a support shell within the extractor. A swirl is imparted intothe conditioned air as it passes the support shell. The fabric bag converts themoisture to water droplets and the centrifugal effect of the swirl on the dropletsforces the droplets onto the outer shell where it collects and then drains from thecomponent. There is usually a bag indicator which protrudes when the coalescerbecomes dirty or blocked. A relief valve is fitted in case the coalescer becomestotally blocked. A typical bag coalescer is shown at Figure 20.

Bag Type Water ExtractorFigure 20

BLOCKAGE INDICATOR

BAG

PRESSURE RELIEF VALVE

WATER DRAIN

OUTLET SHELL


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

1.6.3 Swirl Vane Type Water Separator

This extractor (Figure 21) uses either a rotating or fixed vane within theconditioned airflow. The vane rotates at high speed or rotates the airflow at highspeed as the air passes through it and imparts a centrifugal force on the airimpinging it against the exit shell. This impact converts the moisture into waterdroplets where it collects and falls into the sump area where it is then drainedaway.

Swirl Vane Type Water SeparatorFigure 21

1.6.4 Water Infiltration

As aircraft increase in altitude the moisture content of the outside air reduces to alevel that may cause discomfort to passengers. To counteract this, water must beadded to the conditioned air. This is done by pumping water through a spraynozzle into the ducting downstream of the extractor.

The action of the spray nozzle and velocity of the conditioned air converts thewater droplets into a moisture. The water used in this sytem is usually the waterthat is collected and stored in a tank from the water extraction systems. This tankcan also be replenished from ground services if required. The tank has anoverboard drain in case it becomes overfull.Humidity sensors located in the cabinautomatically turn on the humidity controller water pump to maintain cabinhumidity at a certain level.

DRIAN

SWIRL VANE WATER SUMP

SEPARATOR SHELL


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.7 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 in breathing, and even at lower altitudes the body must work harder to absorbsufficient oxygen through the lungs to operate at the same level of efficiency as atsea level. This problem was solved by pressuring the cockpit/ cabin area. Cabinpressurisation is a means of adding pressure to the cabin of an aircraft to createan artificial atmosphere that when flying at high altitudes it provides gives anenvironment equivalent to that below 10000 feet.

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 the pressure iscontrolled by an 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. Somehave inflatable seals.

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


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

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

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.

OUTFLOW VALVE SAFETY VALVE

RAM AIR

HEATING AIR

PRESSURISED AIR

EXHAUST GASES

COMBUSTION HEATER

RAM AIR SHUTOFF VALVE

COUPLED TURBOCOMPRESSOR

INTERCOOLER

SONIC VENTURI


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

+3Turbine 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 thecoupled 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


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Jet PumpFigure 24

1.7.1 Control And Indication

There are 3 modes of pressurisation, un-pressurised, the isobaric mode and theconstant–differential 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.

1.7.2 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


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.7.3 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.

1.7.4 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.

1.7.5 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.

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


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

Cabin Pressure RegulatorFigure 25

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.

1.7.6 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.

ACTUATORDIAPHRAGM

OUTFLOW VALVE

BAFFLE PLATE

BASE

REFERENCECHAMBER

HEAD

PILOT

DIAPHRAGM

ISOBARIC METERING VALVE

ADJUSTER CONTROL

BAROMETRIC CAPSULE

STATIC ATMOSHERE CONNECTION

ADJUSTERCONTROL

DIFFERENTIALMETERING VALVE

SOLENOIDDUMP VALVE

RESTRICTOR


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

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

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.

1.7.7 Differential Control System

EVACUATED BELLOWS

ISOBARIC METERING VALVE

OUTFLOW VALVE


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

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.

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 CHAMBERDIAPHRAGM


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.7.8 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 theoutside 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.

1.7.9 Cabin Pressure Controllers

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

The cabin altitude gauge (Figure 28) measures the actual cabin altitude. On mostaircraft this altitude is controlled and maintained to around 5000`

Cabin Altitude GaugeFigure 28

0 1

2

3

4

56

7

8

9

10 CABINALTITUDE


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

The cabin rate of climb indicator (Figure 29) tells the pilot the rate that the aircraftis either climbing or descending. The normal climb rate is 500` per minute and thedecent rate is 400` per minute. The control can be automatic or manualdepending on aircraft type

Cabin Rate Of ClimbFigure 29

The differential pressure gauge (Figure 30) reads the difference in pressurebetween the cabin and the outside air pressures. This differential pressure isnormally controlled and maintained to around 7psi. This depends on the aircrafttype and the operating ceiling of the aircraft. The differential pressure gauge maybe combined with the cabin altitude (Figure 31).

Differential Pressure Gauge Dual GaugeFigure 30 Figure 31

0 1

2

3

4

56

7

8

9

10DIFF PX PSI

UP

DOWN

CLIMB

1000 FT PER MIN

.51

2

1.5

.51

1.5

2

0 1

2

3

4

56

7

8

9

10 01

2

3

4

56

7

8

9

10

PX DIFF

PSI

CABINALTITUDE


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

1.8 SAFETY AND WARNING DEVICES

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.

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

Once the system has cooled down sufficiently the crew may have an option toreselect 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 also be protected from over or under speeding which will also have aneffect on the system temperatures. Speed sensors on the fan will indicate a faultwhen over or under speed limits are reached and a warning signal is sent to thecockpit central warning panel with associated caution/warning lights and auralchimes.

1.8.2 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 auxillary power units if fitted.


JAR 66 CATEGORY B1

MODULE 11.04


ukengineering

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.

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

1.8.4 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 the CWPwith associated 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 tooperate a valve to isolate the cockpit air conditioning ducting from the rest of theaircraft to prevent any smoke from getting to the cockpit.


JAR 66 CATEGORY B

MODULE 11.04


ukengineering

x

PAGEINTENTIONALLY

BLANK

B1 Mod 11.17 (5)

Documents

Transcript of B1 Mod 11.17 (5)