13.11.2 Air Conditioning Flashcards
What are the four sources of air supplies for the air conditioning system on an aircraft?
Gas turbine compressors
Engine driven blowers
APU compressors
Ambient ram air inlets
What is the first stage of cooling in an air conditioning system?
Primary heat exchanger
What is the alternative name for the air cycle machine?
Cold air unit
How can the mix valve be controlled in the event of failure of the temperature regulator?
Manually using the temperature selector switch
What causes a signal to drive the mix valve to change positions when the temperature selector switch is in the ‘auto’ position?
Control bridge circuit with reference resistance from switch on one arm and cabin temperature sensor on the other arm
When unbalanced, a signal is developed
What is the purpose of the anticipation and limit sensors in the ducting to the cabin?
Modulate any rapid changes as a result of unbalanced bridge
Prevents sudden blasts of hot or cold air
What happens when the heat in the duct reaches 90°C?
Moves the mix valve to fully cold
Duct overheat light illuminates
How can the system be returned to normal following a temperature of 90°C?
Pushing the reset button
Under what condition does the other thermal switch protect the air conditioning duct from overheating?
Should control electrical power be lost
In the air conditioning system, what does the 120°C switch operate?
Pack valve closing
Illuminating the pack trip off light
Under what other conditions, other than the 120°C overheat switch being activated, will the pack valve close due to overheat?
If the air cycle machine compressor discharge overheat switch is actuated
or
If the air cycle machine turbine inlet overheat switch is actuated
At what temperature will the air conditioning pack deliver air to the different aircraft zones?
Pack output temperature will be to the lowest zone temperature selected
On long haul flights, what is the relative humidity kept at using humidifiers?
30%
How does a humidifier operate?
Potable water jet impinges at right angles on the high pressure air jet
Water becomes atomised
Air becomes uniformly saturated
Why is the recirculating air entering the mixing chamber made to swirl?
Ensures that the recirculated air and the fresh conditioned air are thoroughly mixed
In the recirculation fan, what are the relative quantities of fresh air to recirculated cabin air?
60% fresh
40% recirculated cabin air
What are the symptoms of ozone exposure?
Chest pain
Coughing
Shortness of breath
Fatigue
Headaches
Nasal congestion
Eye irritation
How does an ozone converter work?
Separates ozone from the air and by catalytic action of the substance contained within the converter, converts ozone to oxygen
What type of noble metal is used in a ozone converter?
Palladium
What is the principle of vapour cycle cooling?
Latent heat
What are the basic components of a basic vapour cycle cooling system?
Receiver dryer
Thermostatic expansion valve
Evaporator
Compressor
Condenser
How is the amount of refrigerant put into a system determined?
By weight rather than volume
How is refrigerant normally put into the system?
In its vapour form after the system has been evacuated and is still under vacuum
What must be done when opening the refrigerant system?
All the refrigerant must be recovered using special equipment
Not permitted to release the refrigerant into the atmosphere
What must be done if a refrigeration system has been opened?
It must be evacuated before it is recharged
What properties must refrigerants used in aircraft installations have?
Non–flammable
Non–explosive
Non–toxic, both in pure state and when mixed in air
The refrigerant must not react unfavourably with lubricating oils used within the system
Ideally the lubricating oil should be miscible with the refrigerant
Non–corrosive
What refrigerant are aircraft systems being modified to use or designed for?
Refrigerant R–134a
What are the three principles of air cycle cooling?
Surface heat exchange
Expansion
Energy conversion
Where does the fuel in combustion heaters come from?
Aircraft fuel tanks
What is the medium that is heated in a combustion heater?
Ram air
Why is ram air kept at a positive pressure?
To prevent noxious gases from entering the cabin
What is the purpose of the ‘whirling’ flame produced in the combustion chamber?
Ensures even burning of the fuel and heating of the chamber walls
In a combustion heater, what is the purpose of the limit switch?
First line of defence against overheat
Closes fuel valve, opens it again once the overheat condition is removed
In a combustion heater, what is the purpose of the overheat switch and how does it differ from the limit switch?
If the temperature continues to rise past that which would normally operate the limit switch
Shut the combustion heater down and cannot be restarted in flight
What continues to operate in a combustion heater, even though the fuel valve is closed?
Spark plug and combustion blower
What does CAP 747 GR11 require about combustion heaters?
Intervals not exceeding 500 operating hours, or 2 years, whichever is sooner, combustion heaters and their exhaust systems must be completely dismantled and inspected
Which valve must open to allow engine bleed air into the air conditioning system?
Electro–pneumatic pack valves
Which valves allow hot bleed air to bypass the cold air unit?
Mix valve
1.7°C valve
What is the name of the system which allows passengers to be individually cooled?
Gasper outlets
How is cooling achieved through the heat exchangers when the aircraft is on the ground?
A turbo fan induces air to flow across the heat exchangers
How is the temperature in the duct between the compressor and the secondary heat exchanger controlled?
Ram air temperature sensor
State the two ways the turbine of the ACM achieves cooling?
Pressure drop
Heat energy converted to mechanical energy
How is the water extractor prevented from freezing?
Temp sensor control 35°F (1.7°C) valve
Valve will allow by–pass warm air past the ACM into the inlet of the water separator
Prevents moisture freezing
Which three sensors would trip the pack valve?
ACM compressor discharge overheat switch
ACM turbine inlet overheat switch
120°C thermal switch
What would cause the mix valve to motor to fully cold?
90°C duct overheat thermal switch
What might cause the compressor discharge overheat sensor to operate?
A problem with the ram air inlet which has an effect on the primary heat exchanger
What is the purpose of a three wheel air cycle machine?
Incorporates a fan which is used to induce airflow across the heat exchangers by its own action
What two things would happen if the water extractor became blocked with ice?
Indicator piston moves from the white area into the red coloured area of the indicator cap
Safety relief valve operates to ensure the flow of air into the cabin
In a displacement blower system, which two components control the amount of air in the system?
Excess air is spilled off by opening or closing a spill valve
Mass flow controller controls the spill valve
What is the purpose of the choke valve in the displacement blower system?
Restricts the flow, thus producing a pressure rise to increase the air temperature
What is latent heat?
Quantity of heat which causes a change in state of a substance without changing the temperature
Which components control the amount of liquid refrigerant entering the evaporator?
Thermostatic expansion valve
Temperature bulb
What state is the refrigerant in when it enters the evaporator?
Mostly in liquid form
Besides driving the refrigerant around the system, what other function does the compressor provide?
Lowers the pressure in the evaporator
Supplies refrigerant to the condenser at a pressure and temperature at which it can be easily condensed back into liquid
Raises the pressure of the refrigerant
What state is the refrigerant in when it enters the condenser?
Hot gas
What is the function of the receiver dryer?
Reservoir of the system
Within the manifold set, what are the colours of the hoses for the high side, the low side and the centre hose?
High side – red
Low side – blue
Centre hose – yellow
In a combustion heater, what would happen if the normal thermostat in the system failed?
There is a limit switch
Closes fuel valve until overheat condition is corrected
Why does the low side hose indicate a pressure below that of atmosphere?
It is the vacuum side
