11.4 Air Conditioning Flashcards
How is the aircraft pressurised?
Engines
APU
Air Cart
What do the outflow valves do?
Control the amount of air that goes out of the aircraft.
Why do most pressurisation systems have a built in for cooling pressurised air?
So that air at any altitude can be cooled.
Where is the bleed air typically taken from?
Compressor stage 5 and 9
What is air from the 5th stage considered?
Low pressure
When is low pressure bleed air used?
Take off, climb and cruise.
When is high pressure bleed air used?
Low engine rpm speeds, during descent.
How is bleed air pressure adjusted?
Pressure regulating valve (PRV) or bleed air valve (BAV)
What are the independent cabin compressors designed to do?
Reduce the amount of bleed air required.
What are the three groups of cabin compressor?
Positive displacement
Centrifugal
Turbo
What is a positive displacement compressor also known as?
Roots type
How does a root type blower build pressure?
By supplying more air than the system needs.
What is a centrifugal cabin compressor?
Fundamentally an air pump, air is compressed by an impeller.
What do centrifugal compressors have a variable ratio drive for?
To offset engine fluctuations
How does a turbo compressor system work?
Bleed air from the engine drives a turbine which in turn drives a compressor.
What is a CAC?
Cabin air compressor.
How many phases is a CAC?
Three phase
When a ground air cart is not available, what can be used instead to supply high volume, high pressure starting air?
APU
What job has the greatest demand on an APU?
Supplying bleed air.
What three methods does an APU have of supplying bleed air?
Bleed air from the compressor
Bleed air from the load compressor
Bleed air from the two stage axial turbine
What is the limit of bleed air extraction on an APU?
22,000 feet
What does the APU master switch do?
Primes and opens the flaps
What is cart air pressure regulated too?
Normal pneumatic system pressure.
What is ACS?
Air conditioning system.
What is adiabatic?
Temperature change without the addition or removal of heat.
What is conduction?
Transfer of heat from one object to another due to contact.
What is radiation?
Transfer of energy without a medium.
What is convection?
Transfer of heat through a medium.
What is latent heat?
Causes a substance to change state without a change in temp.
What is sensible heat?
When applied causes a temp change that can be sensed.
What is super heat?
Heat energy added to a gas after complete evaporation.
What are basic gas laws?
Turbine- expands and cools
Compressor- squeezes and heats
What temp range are most ACS able to produce?
21 to 27 degrees c
What two types of ACS are there?
Air Cycle machine
Vapour Cycle machine
What ACS system is more common on turbine aircraft?
Air cycle
Where does hot bleed air first enter and at what pressure?
Primary Heat Exchanger
30-40psi @ 150c
When ram air cools bleed air in the primary heat exchanger what is the temp drop?
100 degrees c
What is the second section of the air cycle?
Travels to the compressor section, temp rises to approx 30-40 degrees c
What is the third stage of the air cycle?
Passes through a turbine to cool by 60 degrees.
Water
What follows stage 3?
Water extractor, as the water is removed it enters the water collector.
What is stage 4 of the air cycle?
Re-heater and into a condenser
What does the air that flows across the condenser coming from the turbine do?
Lowers the dew point of the air into the water extractor to enlarge the water drops.
How do water extractors extract?
Centrifugal force
What happens to the water extracted from the extractor?
It is used in spray nozzles for ram air, to improve cooling efficiency of heat exchangers.
What is the purpose of the temperature control valve?
When there is ice build up etc in the system bleed air is bypassed from before the heat exchangers.
When is the air cycle machine at its minimum?
High altitude due to lower ambient temps, most of the bleed air only passes through the heat exchangers.
What is the pack/flow control valve?
Valve that regulates bleed air from the pneumatic manifold.
What is another name for a pack/flow control valve?
Supply shut off valve.
What happens when an overheat is detected?
The pack/flow valve automatically shuts off.
What type of valve is the pack valve?
Venturi type butterfly valve.
What happens when the pack switch is operated?
The pack valve solenoid is de-energised and if there is bleed air available the valve opens.
Due to fail-safe philosophy what does the pack valve do when electrics drop out?
Open
What is the ICAO minimum airflow per person?
0.283m cubed
What is the calculation to work out if an aircraft meets the ICAO standard airflow?
Convert volume flow to minutes by x60 then divide by max pax.
What is volume flow measured in?
Seconds
When the LO position is selected how much air is the pack supplying?
80%
On the ground there is insufficient ram air to use in the heat exchanger, what is used to generate flow?
A fan
What is a bootstrap system?
Transfer heat to the ram air, but majority is removed by expansion and converting into work.
What are the two types of ACM?
Oil lubricated
Air lubricated
What do slingers do in an oil ACM?
Pump oil mist into the bearings.
What is the benefit of air lubricated ACM?
No oil contamination.
What is the negatives of air lubricated ACM?
Noisy and require careful handling.
What two functions does a reheater/condenser have?
Cool before water collection
Heat to help the turbine in ACM.
What are trim air valves?
Used in some systems to mix conditioned air and provide accurate temp controlled air.
What is the mix chamber?
The final part of the system in which the conditioned air is mixed with warm air and distributed to the desired areas.
On the ECAM A/C temp colours meanings?
Green- Duct temps
White- Actual zone temp
Blue- Selected temps
To increase ram air recovery what is used on the ram air exhaust?
Modulating vanes
How does a vapour cycle machine act?
Moves hot air from the cabin to outside the cabin.
What principle does a vapour cycle machine work on?
Vapour compression refrigeration cycle.
How does the refrigeration cycle work?
Heat goes to the liquid, which turns to vapour as it gains heat. Vapour is then compressed which makes it very hot. Vapour goes outside cooling the air inside
What happens to the boiling point if pressure is reduced?
Reduced
What are refrigerants?
Chemical compounds that are alternately compressed and condensed into a liquid and then allowed to expand into a vapour.
What is a refrigerant?
A working fluid that undergoes repeated phase transition from a liquid to a gas and back again.
What is the compressor in a vapour cycle machine?
This component pressurizes the refrigerant gas, causing its temperature to rise significantly.
What is the condensers role in the system?
Where the refrigerant releases heat to the external environment.
What is the expansion valve?
The high-pressure liquid refrigerant passes through an expansion valve, causing it to rapidly expand and decrease in pressure. This results in a drop in temperature.
What is the evaporator?
Inside the cabin, the low-pressure, low-temperature refrigerant evaporates, absorbing heat from the surrounding air.
What happens at the receiver-dryer process?
Acts as a reservoir to store any of the liquid refrigerant.
What is a commonly used refrigerant?
Dichlorodifluoromethane (R12)
Why is R12 not used anymore?
It has an impact on the ozone.
What has R12 been replaced with?
R134a
What can result if R12 and R134a are mixed up?
Damage to components
What is the boiling point of R134a?
-26 degrees c
Why does the receiver-dryer have desiccant in it?
To capture any water in the fluid.
What two problems does water in the refrigerant have?
Forms an acid
Could form ice
How do you know if the system is low on refrigerant?
Bubbles on the sight glass.
What does the expansion valve do?
Control the amount of refrigerant to make sure it is the amount that can be vapourised.
What are most evaporators made of?
Copper or aluminium
Why do evaporators have fins?
To increase surface area
Vapour air cycle machines are closed systems, how is servicing carried out?
Two service valves, one on the high side and one on the low.
What type of service valve is used on R12 systems?
Schrader valve
In what position does a service valve allow the system to be worked on?
Intermediate
Where does conditioned air enter and exit the cabin?
Ceiling vents into floor vents.
What is gasper air?
Individual pax air
Up to what altitude is the quantity of oxygen sufficient to support humans?
10,000 feet
What equivalent altitude do aircraft pressurisation systems produce?
8000 feet or less
At what height must pax be on emergency oxygen?
15,000 feet
What is atmospheric pressure measured in?
Hectopascals
What is 1013 hPa equal too?
14.7 psi
What is the rate of change in temp in the troposhere?
-2 degrees c per 1000 feet
What is cabin altitude?
Cabin pressure in terms of the equivalent above sea level.
What is cabin differential pressure?
The difference between air pressure inside and outside the cabin.
What is cabin rate of climb?
Rate of change of air pressure inside the cabin expressed in feet per minute.
What 3 modes of cabin pressurisation are there?
Isobaric
Differential
Manual (Emergency)
How does isobaric cabin mode work?
Maintain cabin altitude at a single pressure
How does differential cabin pressure mode work?
Constant pressure difference between inside and outside
What happens when isobaric mode is being used and the aircraft climbs beyond a certain altitude?
It automatically switches to differential mode.
An increase in cabin altitude is?
A decrease in cabin pressure.
What is a positive safety valve also known as?
Excessive differential pressure relief valve.
What does a positive safety valve do?
Outwards pressure relief.
When does a positive safety valve open?
Between 8 and 10psi over the maximum differential.
What is the trigger point for a negative safety valve?
If the pressure outside the aircraft exceeds the inside by 0.5 to 1 psi
When does a dump valve open?
When the aircon pack fails or emergency pressurisation is selected.
What three pressure indications are there?
Cabin altimeter
Cabin rate of climb
Cabin differential
On modern aircraft what page displays all the information?
ECS- Environmental control system
When the aircraft is in auto, what happens as the aircraft takes off?
Switches from Ground to proportional control.
When the aircraft is in auto, what happens as the aircraft reaches cruise?
Switch to isobaric
What is the maximum permissible rate of change of cabin pressure?
0.16 psi/min (1500ft/min)
During climb what is the rate of cabin climb?
500ft/min
During descent what is the rate of cabin climb?
300ft/min
When are system tests required?
Initial proof pressure test
AMM
System malfunction
After repairs
What should a good leak detector be able to detect?
1/2 ounce of refrigerant a year
What does a low pressure gauge read on a manifold set?
Up to 150psi
What is the CWP?
Central Warning Panel, provides information on system status.
What is a red indicating light?
Critical failure
What is an amber indicating life?
Caution
What is a green indicating light?
System is ready for operation
What is a white/blue indicating light?
System is in use
