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JAR 66 CATEGORY B1
MODULE 11.04
AIR CONDITIONING AND CABIN PRESSURISATION
uk engineering
CONTENTS
1 AIR CONDITIONING AND CABIN PRESSURISATION ...............
1-1
1.1 INTRODUCTION
.............................................................................
1-1
1.2 AIR SUPPLY
.................................................................................
1-1 1.2.1 Ram Air
.........................................................................
1-1 1.2.2 Engine Bleed Air
........................................................... 1-1
1.2.3 Compressors or Blowers.
.............................................. 1-1 1.2.4 Auxillary
Power Unit (APU) ........................................... 1-1
1.2.5 Ground Power Trolley
................................................... 1-2
1.3 AIR CONDITIONING SYSTEMS
........................................................ 1-2 1.3.1
Combustion Heating
...................................................... 1-2 1.3.2
Engine Exhaust Heating
................................................ 1-3 1.3.3
Compression Heating
.................................................... 1-3
1.4 AIR CYCLE AND VAPOUR CYCLE MACHINES
.................................. 1-4 1.4.1 Air Cycle Cooling
System .............................................. 1-4 1.4.2 The
Turbo Compressor .................................................
1-5 1.4.3 The Brake Turbine
........................................................ 1-6 1.4.4
The Turbo Fan
.............................................................. 1-7
1.4.5 Vapour Cycle Cooling System
....................................... 1-8 1.4.6 The Compressor
........................................................... 1-10
1.4.7 The Receiver Dryer
....................................................... 1-12 1.4.8
Thermostatic Expansion Valve ......................................
1-13
1.5 DISTRIBUTION SYSTEMS
...............................................................
1-14 1.5.1 Recirculation Air System
............................................... 1-18
1.6 FLOW, TEMPERATURE AND HUMIDITY CONTROL SYSTEMS .............
1-18 1.6.1 Coalescer Type Water Extractor
................................... 1-19 1.6.2 Bag Type Coalescer
...................................................... 1-20 1.6.3
Swirl Vane Type Water Separator .................................
1-21
1.7 PRESSURISATION SYSTEMS
.......................................................... 1-22
1.7.1 Control And Indication
................................................... 1-25 1.7.2 The
Un-Pressurised Mode ............................................
1-25 1.7.3 The Isobaric Mode
........................................................ 1-26 1.7.4
The Constant-Differential Pressure Mode ..................... 1-26
1.7.5 Cabin Air Pressure Regulator
........................................ 1-26 1.7.6 Isobaric
Control System ................................................
1-27 1.7.7 Differential Control System
............................................ 1-28 1.7.8 Safety
Valves
................................................................
1-30 1.7.9 Cabin Pressure Controllers
........................................... 1-30
1.8 SAFETY AND WARNING DEVICES
.................................................. 1-32 1.8.1
Overheating
..................................................................
1-32 1.8.2 Duct Hot Air Leakage
.................................................... 1-32 1.8.3
Excess Cabin Altitude
................................................... 1-33
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JAR 66 CATEGORY B1
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AIR CONDITIONING AND CABIN PRESSURISATION
uk engineering
1 AIR CONDITIONING AND CABIN PRESSURISATION 1.1 INTRODUCTION
Air conditioning systems control both the temperature and
humidity of the air within the cabin, cockpit and freight areas as
well as heating or cooling it as necessary. It should also supply
adequate movement of air through the aircraft for ventilation as
well as provide a means of removing smoke (if permitted) and
odours. A typical system comprises of five principle sections: a.
Air supply b. Heating c. Cooling d. Temperature control e.
Distribution
1.2 AIR SUPPLY The source of air supply and arrangement of the
system components depend on the aircraft type and system employed
but in general one of the following methods may be used:
1.2.1 Ram Air This is used in some unpressurised aircraft using
either combustion heating or warm air heating from an exhaust gas
heat exchanger. The ram air supply is from an intake directly in
the airflow either on the nose, wing or at the base of the tail
fin. The air after circulating 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 to the cabin.
Before the air enters the cabin it is passed through a temperature
control system 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 or blowers
being either engine driven via an accessory drive, by bleed air or
electric or hydraulic motors.
1.2.4 Auxillary Power Unit (APU) This provides an independent
source of pressurised air. It is basically a small gas turbine
engine that provides air and other service whilst the aircraft is
on the ground with its main engines stopped. It is usually a self
contained unit located in the tail section of the aircraft where it
can be run safely.
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1.2.5 Ground Power Trolley For use in extreme climates on
aircraft which do not have an APU or the APU is unserviceable. This
is a self contained air conditioning unit and can be connected to
the aircrafts cabin, to either heat or cool it depending on the
climate. This unit will run until the 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 system used.
Each system uses different methods for heating and cooling. In
general there are 3 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 and depends for
its operation on the combustion of a fuel air mixture within a
cylindrical combustion chamber. Ram air is augmented with an air
blower and fuel is metered from the aircraft fuel system through a
solenoid valve. The fuel air mixture is ignited in the combustion
chamber and the burnt gases swirl through the transfer passages of
the cylinder before being exhausted to atmosphere. This gas swirl
not only aids combustion but ensures that the gases impart against
the chamber and passage walls to allow maximum heat transfer. The
ram air flows over the outside of the combustion chamber where it
is absorbs the heat before it enters the cabin.
Typical Combustion Heater System
Figure 1
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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The temperature is controlled manually by setting a control
valve which is located downstream of the combustion chamber. This
controls the amount of air flow over the combustion 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/off switch.
1.3.2 Engine Exhaust Heating A typical exhaust heater is shown
in Figure 2. This also is used with a ram air ventilation system
but the heating of the supply air is much more simple and direct. A
heater muff surrounds the exhaust pipe of a piston engine aircraft.
The ram air enters this muff and extracts the heat from the hot
exhaust. This heated air is then passed into a chamber where it is
mixed with a separate cold air supply. Mechanically operated valves
are provided 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 when exposed to dangerous carbon dioxide levels. They are
sited in view of the pilots.
Exhaust System Heater
Figure 2
1.3.3 Compression Heating A typical compression heating sytem is
shown in Figure 3. This system relies on the principle whereby the
air temperature is increased during compression and is used by air
supply sytems utilising either engine driven compressors and
blowers and engine bleed air. 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 to a primary heat exchanger where ram air passes through the
exchanger matrix to cool the air.
CONTROL VALVE
EXHAUST MANIFOLD
HEATER MUFF
CONTROL
LEVERCLOSED
OPEN
RAM
AIR
TO CABIN
OVERBOARD DUMP
FLAP
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From the primary heat exchanger the cool air then goes to a
compressor where it is compressed and heated before going through a
secondary heat exchanger, again being cooled 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 valve where it is mixed with the hot air before being
distributed to the cabin. Adjustable flow control and temperature
control valves control the cabin temperature.
Typical (Compression) Bleed Air System
Figure 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 by doing or receiving work. If it does work
(expanded) its temperature will fall if it receives work
(compressed) its temperature will rise. The primary component in an
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 TO
CABIN
NRV
WATER SEPARATOR
COUPLED COMPRESSOR TURBINE
RAM AIR
PRIMARY HEAT
EXCHANGER
SECONDARY HEAT
EXCHANGER
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1.4.2 The Turbo Compressor In a typical system the turbine
drives a coupled compressor (Figure 4). A secondary heat exchanger
is located in line between the compressor outlet and the turbine
inlet. A ram air supply is provided to the primary and secondary
heat exchangers. For operation of the system on the ground an
induction fan or blower may be used to augment the air supply and
there may also be a ground air conditioning unit connection.
Turbo Compressor
Figure 4
The hot air supply initially air passes through a primary heat
exchanger where it is pre-cooled before entry to the compressor. It
is then compressed and heated by the compressor before being cooled
again as it passes through the secondary heat exchanger. The cooled
air then drives the coupled turbine where it does work and becomes
even colder. As this air cools, moisture condenses out of it and is
collected in a water separator. The water is centifuged out in the
seperator where it collects on the outer case and is then allowed
to drain overboard. To prevent this water from freezing warm air is
mixed with it via a temperature control valve when it reaches a
certain temperature. A typical turbo compressor is shown in Figure
5.
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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The cold air from the turbine then enters the mixer unit where
it is mixed with the pre-cooled air supply via the temperature
control valve to allow a variable warm air supply to the air
distribution system.
Turbo Compressor Cold Air Unit
Figure 5 1.4.3 The Brake Turbine A typical brake turbine is
shown at Figure 6. When cold air is selected the bleed air is
directed to the turbine of the cold air unit. As the air drives the
turbine the gas expands as work is being done resulting in a drop
in pressure and temperature.
BLEED AIR
TO INTERCOOLER
FROM
INTERCOOLER
TO
DISTRIBUTION
SYSTEM
COMPRESSOR
DIFFUSER
NOZZLE BLADES
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Brake Turbine Cold Air Unit
Figure 6 To prevent the turbine from rotating too quickly and
affecting the cooling efficiency, the turbine is coupled to the
compressor. As the compressor rotates ambient air is used as a
braking medium to slow the turbine. This system is an improvement
on the turbo compressor system as only one heat exchanger is
required
1.4.4 The Turbo Fan The turbo fan is mechanically similar to the
brake turbine cold air unit. In the turbo fan the turbines drives a
coupled centrifugal compressor which induces a capacity of air,
large enough to create a cooling flow of ram air through a heat
exchanger, cooling the bleed air. It also acts as acting as a
braking fan to control the turbine speed. A typical turbo fan is
shown in Figure 7. The major advantage of this system is that the
air conditioning system can be operated on the ground with engines
running without the need for ram air.
RAM AIR
TO
CABIN
MIXER
UNITHEAT EXCHANGER
CONTROL VALVE
AMBIENT AIR INLET
COMPRESSOR TURBINE
BLEED
AIR
AMBIENT AIR OUTLET
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Turbo Fan Cold Air Unit
Figure 7 1.4.5 Vapour Cycle Cooling System The vapour cycle
cooling system is used to control and reduce the temperatures
generated by electronic equipment used in modern aircraft. This
system works on the principle of the ability of a refrigerant to
absorb heat through 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. This is
because the volatile liquid starts to evaporate as it draws the
heat away from your hand. Liquids with low boiling points have a
stronger tendancy to evaporate at normal temperatures than those
with higher boiling points. Furthermore pressure affects the state
of a liquid substance. A reduction in pressure will cause a liquid
to change state into a gas or vapour. A typical vapour cycle system
operates with 2 distinct integrated systems, a sealed recirculating
refrigerant system and an air system. A typical system is shown at
Figure 8.
MIXER UNIT
BLEED AIR
RAM
AIR
HEAT
EXCHANGER
CONTROL VALVE
COMPRESSOR
TURBINE
RAM AIR OUTLET
TO CABIN
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Schematic Vapour Cycle System
Figure 8
Refrigerant System The system has 2 sides a high pressure side
and a low pressure side. Mixed with the refrigerant is a specified
amount of lubrication oil which lubricates and seals the
compressor. The liquid refrigerant passes from the receiver to the
thermostatic expansion valve for controlled release into the matrix
of the evaporator. Heated air from the main supply passes over the
evaporator matrix and by induction transfers heat into the liquid
refrigerant which on heating becomes a low pressure vapour. From
the evaporator the LP vapour feeds into a compressor which
pressurises the refrigerant to a high pressure. This HP refrigerant
then enters the condensor where it is cooled to a liquid by ram air
(or by induction fan air) passing through the matrix where it then
returns as a liquid to the liquid receiver, to repeat the
cycle.
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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Air System The main hot air supply drives the turbine, which is
directly coupled to the compressor. The air is also fed directly
downstream of the system to the temperature control valves. As the
air passes through the turbine it does work which reduces the airs
temperature which is then fed to the evaporator. This air then
passes through the evaporator where it is further cooled (as the
refrigerant absorbs the heat) and is then fed to the temperature
control valves. These valves controls the air temperature being fed
to the air distribution system. All components of this type of
system are usually mounted on a single removable quick release
panel (Figure 9) to allow complete pack changes when a fault
arises, instead of changing individual components. Some aircraft
use this type of system to air condition avionics bays as well as
the cabin.
Typical Vapour Cycle System
Figure 9
1.4.6 The Compressor The compressor pulls the low pressure
refrigerant vapour from the evaporator and compresses it. When the
vapour is compressed its pressure and temperature both rise.
FILTER
RAM AIR
COOLANT IN
COOLANT OUT
GROUND SERVICE
POINT
RECEIVER DRIER
TEMPERATURE BULB
EVAPORATOR
CONDENSER
QUICK RELEASE
PANEL
THERMAL EXPANSION
VALVE
COMPRESSOR
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Some compressors are engine driven by a v belt (Figure 10)
through an electromagnetic clutch assembly (Figure 11) which can be
engaged or disengaged as required. The clutch drive plate is keyed
to the compressor shaft and when the clutch is disengaged there is
clearance between the drive plate and the engine driven pulley. The
pulley rotates but the compressor is at rest. When the system calls
for cooling the electromagnet energises and pulls and locks the
drive plate to the drive pulley and therefore drives the
compressor.
Engine Driven Compressor
Figure 10
Electromagnetic Clutch Assembly
Figure 11
V DRIVE BELT
DRIVE PLATE
PULLEY
ELECTROMAGNETIC
CLUTCH COIL
COMPRESSOR
OUTLET TO
CONDENSER
INLET FROM
EVAPORATOR
DRIVE PLATE PULLEY ELECTROMAGNETIC
COIL
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Some compressors are driven by a hydraulic motor whose pressure
is supplied form an engine driven hydraulic pump. A solenoid valve
is fitted to the hydraulic manifold. When no cooling is required
the solenoid valve is de-energised allowing fluid to bypass the
motor and return to the reservoir. When cooling is required the
solenoid valve energises, closing off the bypass, allowing the
hydraulic fluid to drive the compressor 1.4.7 The Receiver Dryer
High pressure high temperature refrigerant leaves the condenser and
flows into the receiver dryer (Figure 12). This acts as a reservoir
to hold the supply of refrigerant until it is needed by the
evaporator. As the hot liquid enters the receiver dryer it first
passes through a filter which removes any solid contaminants. It
then passes through a layer of silica gel or activated alumina
which removes any water moisture from the liquid. It also acts as a
separator as some traces of vapour may be in the liquid. The
moisture is removed to prevent the system form freezing and
becoming blocked and to prevent the moisture from acting with the
refrigerant which would form 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 the pick up
tube. Some receivers include a sight glass that allows the checking
of the refrigerant. If bubbles are seen then the system requires
re-charging.
Receiver Dryer Figure 12
DESICCANTFILTER PADS
SIGHT GLASS
PICK UP TUBE
FROM CONDENSER TO THERMOSTATIC
EXPANSION VALVE
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1.4.8 Thermostatic Expansion Valve The thermostatic expansion
valve (TEV) is a metering device that controls the amount of
refrigerant that is allowed to flow into the evaporator by
measuring the temperature of the evaporator discharge. All of the
refrigerant should evaporate by 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 a liquid 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 the valve locates against push rods
that act against a superheat spring. This action controls the
position of the needle valve. The superheat spring tension is
factory pre-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 is insulated to allow
only the outlet temperature to be sensed. The bulb and capillary
tube is filled with a highly volatile fluid which reacts readily
with temperature changes. When the bulb senses a rise in
temperature the bulb liquid expands and exerts a force against the
diaphragm, the superheat spring and the evaporator inlet pressure
acting underneath the diaphragm. The amount of force exerted is
directly related to the temperature of the vapour at the evaporator
discharge. A needle valve is located between the inlet and outlet
of the TEV and its position is determined by the balance of the
forces acting above and below the diaphragm including the pre set
tension of the superheat spring. When the system is started the
evaporator is relatively warm and the bulb pressure above the
diaphragm is high. This acts against the push rods and overcomes
the superheat spring tension, to open the needle valve to allow
maximum flow to the evaporator. As the refrigerant evaporates the
evaporator outlet temperature decreases and the pressure above the
diaphragm also decreases. The superheat spring overcomes this drop
in pressure and closes the needle valve to a new position which
restricts the amount of refrigerant that flows into the evaporator
to ensure that it all evaporates by the time it reaches the
evaporator outlet.
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Thermostatic Expansion valve
Figure 13 1.5 DISTRIBUTION SYSTEMS The air distribution system
on most aircraft takes cold air from the air conditioning packs and
hot air bleed from the engines and mixes the 2 in a mixer unit to
the required temperature. The air is then distributed to side wall
and overhead cabin vents. 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 of
maintenance. ( 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 cooling
packs. Each passenger or crew can control the amount of air
received by controlling the position of the air outlet. This outlet
could be a rotary nozzle or a louvre.
VALVE BODY
SUPERHEAT SPRING
TEMPERATURE SENSING BULB
DIAPHRAGM
INLET
OUTLET
CAPILLARY TUBE
PUSH RODS
NEEDLE VALVE
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Air Conditioning Distribution Manifold Figure 14
Conditioned air systems dispense temperature controlled air
evenly throughout the cabin and crew areas. One duct system
supplies the cockpit (Figure 17) while another supplies the cabin.
The cabin ducting is then divided into 2 systems, the overhead
(Figure 15) and the sidewall systems (Figure 16). The overhead
system releases air into the cabin from outlets in ducting running
fore and aft in the cabin ceiling. The sidewall duct system takes
air through ducting between the sidewall and cabin interior linings
and releases it through cove light grills and louvres. A cockpit
controlled selector valve located on the main distribution manifold
allows all overhead, side wall or any combination of the two
systems to be used and varies the flow between the two.
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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Overhead Panel Figure 15
Duct sections throughout both the cabin and cockpit are joined
together with clamps or clips. Means of equalising the duct
pressures and balancing the air flows are designed into each
system. The systems are protected from excess pressures by use of a
spring loaded pressure relief valve usually located in the main
distribution manifold. The main manifold is located immediately
downstream from the mixing units in the air conditioning bay. On
large aircraft a cockpit controlled dual selector valves divides
the air between cockpit and cabin areas. These butterfly valves are
interlinked. When one is fully open the other is fully closed and
vice versa. Air is exhausted from the passenger cabin through
grills and outflow valves in the sidewalls above the floor. This
air can then be directed around the cargo compartment walls where
it assists in compartment temperature control. Some air then flows
to the cargo heat distribution duct under the compartment floor and
is then discharged overboard through the outflow valves.
GASPER FAN
FLOOR EXHAUST DUCT
ADJUSTABLE AIR OUTLETS
DUCTING
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Sidewall Ducting Figure 16
Below each floor air exhaust outlet is a flotation check valve.
This valve is a plastic ball held in a cage. If the cargo
compartments become flooded the balls float up the cage and seals
off the floor to help prevent water from entering the cabin.
Cockpit Air Distribution
Figure 17 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 a remote heat exchanger and
fan assembly in the vapour cycle system, to allow cooling to
specific areas such as avionics bays, fed from one of the zone
packs.
SILENCER
FAN ASSY
FAN ASSY PRESSURE SWITCH
COOLING FANS
FLIGHT DECK
TEMPERATURE SENSOR
AIR VENT
CABIN TEMPERATURE SENSOR
WINDOW DEMISTER
FLOOR EXHAUST VENTS
WALL FEEDER DUCTS
DISTRIBUTION BOXES
DISTRIBUTION DUCT
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1.5.1 Recirculation Air System To improve cabin ventilation and
supplement airflow the cabin air is recirculated back to the main
distribution manifold where it is mixed with conditioned air form
the cooling packs. The use of re-circulated air improves airflow
and offloads the air supply system. This off loading of the air
conditioning packs is converted into a fuel saving. The
re-circulation fan will draw air from the cabin area, through a
check valve and filter assembly to remove any smoke and noxious
odours before passing it to the mixer unit for re-distribution. The
check valve prevents any reverse flow through the 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 water moisture is in the air conditioning air
within the cabin (Figure 18). This is to ensure that passengers do
not suffer from the low humidity levels at higher altitudes and
that excessive moisture is removed at lower altitudes.
Typical Humidity Control System
Figure 18
CABIN HUMIDITY SENSOR
OVERFILL DRAIN
WATER SEPARATOR
DRAIN
COLLECTOR TANK
WATER PUMP AND
CONTROLLER
SPRAY NOZZLE
TO CABIN
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Humidity can be controlled in 2 ways:
Water Separation This is the removal of excessive moisture from
the conditioned air normally using a water extractor or
separator.
Water Infiltration This is the addition of water into the
conditioned air as it enters the cabin using a water pump and spray
nozzle. Water Extraction Water extraction is carried out by an
extractor or separator and there are differing designs, 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 heating effects of the air in the air cycle system. The
extractor is located in the air conditioning ducting prior to entry
into the cabin. There are 3 main types of water separator in use:
1.6.1 Coalescer Type Water Extractor
Coalescer Water Extractor Figure 19
PRESSURE RELIEF
VALVE
DRAIN
DIFFUSER
COALESCER
COLLECTOR SHELL
CONDENSER
TUBES
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The coalescer (Figure 19) consists of layers of monel gauze and
glass fibre cloth sandwiched between layers of stainless steel
gauze. It is supported by the diffuser cone and held in place by
the relief valve. As the conditioned air is passed through the
coalscer the moisture in the air is converted into water droplets.
These droplets then enter the collector shell and deposited into
the collector tubes where they drain down into the collector box.
This water is either drained overboard or passed to a water tank
where it can be stored and used to infiltrate the system if
required. The purpose of the relief valve is to open if the
coalescer becomes blocked to allow conditioned air into the cabin.
1.6.2 Bag Type Coalescer The bag is fitted over a support shell
within the extractor. A swirl is imparted into the conditioned air
as it passes the support shell. The fabric bag converts the
moisture to water droplets and the centrifugal effect of the swirl
on the droplets forces the droplets onto the outer shell where it
collects and then drains from the component. There is usually a bag
indicator which protrudes when the coalescer becomes dirty or
blocked. A relief valve is fitted in case the coalescer becomes
totally blocked. A typical bag coalescer is shown at Figure 20.
Bag Type Water Extractor
Figure 20
BLOCKAGE INDICATOR
BAG
PRESSURE RELIEF VALVE
WATER DRAIN
OUTLET SHELL
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1.6.3 Swirl Vane Type Water Separator This extractor (Figure 21)
uses either a rotating or fixed vane within the conditioned
airflow. The vane rotates at high speed or rotates the airflow at
high speed as the air passes through it and imparts a centrifugal
force on the air impinging it against the exit shell. This impact
converts the moisture into water droplets where it collects and
falls into the sump area where it is then drained away.
Swirl Vane Type Water Separator
Figure 21 1.6.4 Water Infiltration As aircraft increase in
altitude the moisture content of the outside air reduces to a level
that may cause discomfort to passengers. To counteract this, water
must be added to the conditioned air. This is done by pumping water
through a spray nozzle into the ducting downstream of the
extractor. The action of the spray nozzle and velocity of the
conditioned air converts the water droplets into a moisture. The
water used in this sytem is usually the water that is collected and
stored in a tank from the water extraction systems. This tank can
also be replenished from ground services if required. The tank has
an overboard drain in case it becomes overfull.Humidity sensors
located in the cabin automatically turn on the humidity controller
water pump to maintain cabin humidity at a certain level.
DRIAN
SWIRL VANE WATER SUMP
SEPARATOR SHELL
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1.7 PRESSURISATION SYSTEMS As aircraft became capable of
obtaining altitudes above that at which flight crews could operate
efficiently, a need developed for complete environmental systems to
allow these aircraft to carry passengers. Air conditioning could
provide the proper temperature and supplemental oxygen could
provide sufficient breathable air. The problem was that not enough
atmospheric pressure exists at high altitude to aid in breathing,
and even at lower altitudes the body must work harder to absorb
sufficient oxygen through the lungs to operate at the same level of
efficiency as at sea level. This problem was solved by pressuring
the cockpit/ cabin area. Cabin pressurisation is a means of adding
pressure to the cabin of an aircraft to create an artificial
atmosphere that when flying at high altitudes it provides gives an
environment equivalent to that below 10000 feet. Aircraft are
pressurised by sealing off a strengthened portion of the fuselage.
This is usually called the pressure vessel and will normally
include cabin, cockpit and possibly cargo areas. Air is pumped into
this pressure vessel and the pressure is controlled 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
pressure tight area. All panels and large structural components are
assembled with sealing compounds. Access and removable doors and
hatches have integral seals. Some have inflatable seals.
Pressurisation systems do not have to move large volume of air.
Their function is to raise the pressure inside the vessel. Small
reciprocating engine powered aircraft receive their pressurisation
air from the compressor of a coupled turbocharger. Larger
reciprocating engine powered aircraft receive air from engine
driven compressors and turbine powered aircraft use compressor
bleed air 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 fed to the engine inlet manifold to increase manifold
pressure which allows the engine 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 through a flow
limiter (or sonic venturi) and then through an inter-cooler before
being fed into the cabin. A typical system is shown at Figure
22.
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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 shock wave is formed which
limits the flow of air to the pressurisation system
Small Reciprocating Engine Aircraft Pressurisation System
Figure 22 Large Reciprocating Engine Powered Aircraft These
aircraft use engine driven compressors driven through an accessory
drive or by an electric or hydraulic motor. Multi engine aircraft
have more than one air compressor. These are interconnected through
ducting but each have a check valve 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 SHUT
OFF VALVE
COUPLED TURBO
COMPRESSOR
INTERCOOLER
SONIC VENTURI
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+3Turbine Powered Aircraft The air supplied from a gas turbine
engine compressor is contamination free and can be suitably used
for cabin pressurisation (Figure 23). Some aircraft use an
independent 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 Compressor
Figure 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 intake ducting. High velocity air from the
engine flows through this nozzle. This produces a low pressure area
around the venturi which sucks in outside air. This outside air is
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 Pump Figure 24
1.7.1 Control And Indication There are 3 modes of
pressurisation, un-pressurised, the isobaric mode and the
constantdifferential pressure mode. In the un-pressurised mode the
cabin altitude remains the same as the flight altitude. In the
isobaric mode the cabin altitude remains constant as the flight
altitude changes and in the constant-differential pressure mode,
the cabin pressure is maintained at a constant amount above the
outside ambient air pressure. The amount of differential pressure
is determined by the structural strength of the aircraft. The
stronger the aircraft structure the higher the differential
pressure and the 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 same as the outside ambient air
pressure. This mode is usually from sea level up to 5000` 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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1.7.3 The Isobaric Mode In this mode the cabin pressure is
maintained at a specific cabin altitude as flight altitude changes.
The cabin pressure controller begins to close the outflow valve as
the aircraft climbs to a chosen cabin altitude. The outflow valve
then opens or closes (modulates) to maintain the selected cabin
altitude as the flight altitude changes up or down. The controller
will then maintain the selected cabin altitude up to the flight
altitude that produces the maximum differential pressure for which
the aircraft structure is rated. At this point the constant
differential mode takes control.
1.7.4 The Constant-Differential Pressure Mode Cabin
pressurisation puts the aircraft structure under a tensile stress
as the cabin pressure expands the pressure vessel. The cabin
differential pressure is the ratio between the internal and
external air pressures. At maximum constant-differential pressure
as the aircraft increases in altitude the cabin altitude will
increase but the internal/external pressure ratio will be
maintained. There will be a maximum cabin altitude allowed and this
will determine the ceiling at which the aircraft can operate.
1.7.5 Cabin Air Pressure Regulator
The pressure regulator maintains cabin altitude at a selected
level in the isobaric range and limits cabin pressure to a pre-set
pressure differential in the differential range by regulating the
position of the outflow valve. Normal operation of the regulator
requires only the selection of the desired cabin altitude and cabin
rate of climb the adjustment of the barometric control. 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 cabin altitude 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 and the base with the outflow valve and diaphragm. The
balance diaphragm extends outward from the baffle plate to the
outflow valve creating an air chamber between the baffle plate and
the outer face of the outflow valve. Cabin air flowing into this
chamber through holes in the side of the outflow valve exerts a
force against the outer face of the valve which tries to open it.
This force is opposed by the force of the spring around the valve
pilot which tries to hold the valve closed.
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Cabin Pressure Regulator
Figure 25 The actuator diaphragm extends outward from the
outflow valve to the head assembly creating an air chamber between
the head and the inner face of the outflow valve. Air from the head
and reference chamber exert a force against the inner 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 allowed to flow from the pressure vessel and this controls the
cabin pressure. The position of the outflow valve is determined by
the amount of reference chamber air 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 26 incorporates an evacuated capsule, a rocker arm, valve
spring and a ball type metering valve. One end of the rocker arm is
connected to the valve head by the evacuated capsule and the other
end of the arm holds the metering valve in a closed position. A
valve spring located on the metering valve body tries to move the
metering valve away from its seat as far as the rocker arm
allows.
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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When the cabin air pressure increases enough for the reference
chamber air pressure to compress the evacuated capsule the rocker
arm pivots around its fulcrum and allows the metering valve to move
away from its seat an amount proportional to the compression of the
capsule. When the metering valve opens reference pressure air flows
form the regulator to atmosphere through the atmospheric
chamber.
Isobaric Control Operation
Figure 26 When the regulator is operating in the isobaric range,
cabin pressure is held constant by reducing the flow of reference
chamber air through the metering valve. This prevents a further
decrease in reference pressure. The isobaric control responds to
slight changes in reference pressure by modulating to maintain a
constant pressure in the chamber throughout the isobaric range of
operation. Whenever there is an increase in cabin pressure the
isobaric metering valve opens which decreases the reference
pressure and causes the outflow valve to open which then decreases
the cabin pressure. 1.7.7 Differential Control System
EVACUATED BELLOWS
ISOBARIC METERING VALVE
OUTFLOW VALVE
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The differential control system of the pressure regulator
(Figure 27) incorporates a diaphragm a rocker arm, a valve spring
and a ball type metering valve. One end of the rocker arm is
attached to the head by the diaphragm which forma a pressure
sensitive face between the reference chamber and the atmospheric
chamber.
Differential Pressure Mode
Figure 27 Atmospheric pressure acts on one side of the diaphragm
and reference chamber pressure acts on the other. The opposite end
of the rocker arm holds the metering valve in a closed position. A
valve spring located on the metering valve body tries to move the
metering valve away from its seat as far as the rocker arm allows.
When reference chamber pressure increases to the system
differential pressure limit set above the decreasing atmospheric
pressure it collapses the diaphragm which is set at differential
pressure and opens the metering valve. Air flows from the reference
chamber to atmosphere through the atmospheric chamber, which causes
a reduction in the reference pressure. This reduction in reference
pressure causes the outflow valve to open to reduce the cabin
pressure to maintain the system pressure differential.
METERING VALVE
OUTFLOW VALVE
ATMOSPHERIC CHAMBER
DIAPHRAGM
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1.7.8 Safety Valves Cabin Air Pressure Safety Valve The pressure
relief valve prevents cabin pressure from exceeding the
predetermined cabin to ambient pressure differential. A negative
pressure relief valve and pressure dump valve may also be
incorporated into this valve assembly. Negative Pressure Relief
Valve A pressurised aircraft is designed to operate with the cabin
pressure higher than the 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 enter the
cabin to ensure that this does not happen. It is basically an
inward pressure relief valve. Dump Valve This valve is normally
solenoid actuated by a cockpit switch. When the solenoid is
energised the valve opens dumping cabin air to atmosphere. Cabin
pressure will decrease rapidly until it is the same as the outside
air pressure and cabin altitude will 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 most aircraft this altitude is controlled and
maintained to around 5000`
Cabin Altitude Gauge
Figure 28
01
2
3
4
56
7
8
9
10 CABINALTITUDE
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The cabin rate of climb indicator (Figure 29) tells the pilot
the rate that the aircraft is either climbing or descending. The
normal climb rate is 500` per minute and the decent rate is 400`
per minute. The control can be automatic or manual depending on
aircraft type
Cabin Rate Of Climb
Figure 29 The differential pressure gauge (Figure 30) reads the
difference in pressure between the cabin and the outside air
pressures. This differential pressure is normally controlled and
maintained to around 7psi. This depends on the aircraft type and
the operating ceiling of the aircraft. The differential pressure
gauge may be combined with the cabin altitude (Figure 31).
Differential Pressure Gauge Dual Gauge Figure 30 Figure 31
01
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
01
2
3
4
56
7
8
9
10 01
2
3
4
56
7
8
9
10
PX DIFF
PSI
CABIN
ALTITUDE
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1.8 SAFETY AND WARNING DEVICES Both air conditioning and
pressurisation systems use safety and warning devices to protect
the aircraft from possible catastrophic failures. Some of the
protection devices may be inhibited in certain stages of flight,
landing or take off where the extra distractions caused by such
warnings may be too much for the crews to deal with safely. With
the air conditioning system the main concerns are with overheating
of the air conditioning packs and extraction and ventilation fans,
as well as hot air leaks from ducting which could damage
surrounding structure or components. 1.8.1 Overheating Most packs
systems are protected from overheating by a thermal switch
downstream of the pack outlet. If the outlet temperature reaches a
pre determined 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 aural chimes and to
illuminate a fault light on the pack selector switch. 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 fault in the automatic temperature control system in which
case the pilot may be able to control the system manually via a
manual selector switch on the cockpit controller. Extraction or
ventilation fans will be protected in much the same way. An
overheat will signal the central warning panel with associated
caution/warning lights and aural chimes. The fan may be isolated
automatically or manually. Once the 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 an effect on the system
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 central warning panel with associated
caution/warning lights and aural chimes. 1.8.2 Duct Hot Air Leakage
Any ducting that includes joints is liable to leak under abnormal
conditions. A duct protection system will include fire-wire
elements around the hot zones such as engine air bleeds, air
conditioning packs and auxillary power units if fitted.
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The sensing elements will be the thermistor type. As the
temperature around the wire increases the resistance decreases
until an electrical circuit is made. When the circuit is made a
warning signal is sent to the cockpit central warning panel with
associated caution/warning lights and aural chimes. The leaking
duct may be isolated automatically or may require the pilot to take
action to close off the air valves. 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 passengers
could unknowingly suffer the effects of hypoxia. This dangerous
condition is obviously undesirable especially for the aircrew. Most
aircraft give a warning on the CWP with associated audio and visual
warnings when the cabin altitude reaches 10000`. 1.8.4 Smoke
Detection Smoke detectors may be fitted within the cabin, avionics
bay and cargo areas to monitor systems which if become faulty may
generate smoke on overheating or are may be liable to catch fire.
These detectors will send a signal to the the CWP with associated
lights and audio warnings. They may also automatically switch on
extractor fans which will remove the smoke overboard and away form
the cabin and 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 the aircraft to prevent
any smoke from getting to the cockpit.
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