air conditioning and cabin pressurisation
what does a simplified vapour cycle refrigeration system consists of?
- a liquid receiver - an expansion valve - an evaporator - a compressor - a condenser
the basic air cycle system consists of?
- a source of pressurised air - a pair of air to air heat exchangers - a high speed turbine - a fan/ compressor
what is choke heating?
- a type of heating that is used with positive displacement blower air supply systems - it consists of a butterfly valve assembly installed in the ducting which can partially restrict the airflow from the blowers causing a back pressure increase and so an increase in temperature
what does the air cycle machine consists of?
- an expansion turbine - a compressor - a flow augmenting fan all of these are mounted on a single shaft
what happens in an ACM?
- as the pressurised air expands in the turbine, mechanical energy is extracted at the turbine - this energy drives the compressor, which draws the air coming off the primary heat exchanger and raises its pressure and temperature
what are the 3 common variations of the air cycle cooling system?
- bootstrap system - evaporative air cycle system - regenerative air cycle system
the air condition system of a modern airliner can have what subsystems?
- cabin and cockpit temperature control system - mass flow control system - ventilation systems - humidity control system - cargo compartment system - distribution system - re circulation system - water extraction system
what are the airconditioning requirements?
- cabin temp: 65F to 75F - provision of fresh air: 1lb/min/pax - cabin humidity minimum: 30%
what is the device used to extract water in a water extraction system?
- coalescer bag type water separator - high pressure water separator with swirl vanes
what does the ventilation subsystem consists of?
- cockpit and cabin ventilation - cargo compartment ventilation - avionics ventilation - galley lavatory ventilation
the process of air cycle system consists of 3 steps. what are they?
- compression - heat transfer temperature gradient - expansion accompanied by work extraction
the cabin and cockpit temperature control system consists of?
- cooling system - heating system
which location would be suitable for water extraction?
- for low temperatures, at the turbine exhaust - for highest pressure, at the point upstream of the turbine
what is the function of the air conditioning system?
- it ensures that all compartments and equipment are maintained at certain environment conditions with respect to temperature, humidity and ventilation. - this is for survival and comfort of the occupants as well as the efficient functioning of various aircraft systems
what is the definition of BTU?
- it is the amount of heat required to raise the temperature of 1 lb of water by 1F - for metric is 1kg of water by 1C
what is the function of the pressurisation system?
- it isolates the cabin from the extremely low atmospheric pressure surrounding the aircraft - and maintains a pressure inside the cabin that is compatible with human survival, comfort and aircraft structural limitation
what is the purpose of the water extraction system?
- it provides a means of reducing the moisture content of the air supply to prevent precipitation in the ducting and in the cabin at high altitudes. - at high altitudes, the ambient air has appreciable amount of water content due to the decrease in capacity of the air to hold water vapour due to the reduced temperature
vapour cycle system can be divided into 2 types. what are they?
- open circuit vapour cycle system - closed circuit vapour cycle system
cockpit and cabin temperature control system can be actually split into 2. what are they?
- primary temperature control - secondary temperature control/ trim air control
why are foil air bearings used to support the ACM shaft?
- the bearings are more reliable - it requires fewer parts to support the rotating assembly - it needs no lubrication - zero wear - silent and smooth operation - higher damping and high acceleration
what are the 2 main types of cooling system?
- vapour cycle system - air cycle system
what is the principle behind water extraction?
- water needs to be precipitated before it can be extracted - precipitation occurs most effectively when the temperature of air is at its lowest
in the event of a failure of any duplicated fresh air supply system, the system shall be capable of supplying?
0.5 lbs of air per person per minute
the air supply to the cockpit is usually maintained at _ even if the ECS may contain a re-circulation system
100% purity and freshness
what is the legislation on the amount of oxygen to be supplied per person in the cabin?
1lb of fresh air per person per minute
what is the amount of humidity considered sufficient for operation of aircraft and passenger comfort?
30%
an ECS system requires air in the low pressure ranging from?
40-50 psi
what is the ideal amount of humidity for passenger comfort?
60%
what is the standard lapse rate?
Constant decrease in temperature (1.98C per 1,000 feet) as altitude increases.
the foil air bearings are coated with _ for low friction during lift off and touch down?
a solid film lubricant
what is referred to as hot or cold air?
air that is discharged from a heating or cooling system respectively
the re-heater and condensers in the high pressure water extraction loop are both?
air to air heat exchangers
why is it at higher altitudes, the pressure experienced by the aircraft is less than when it travels at a much lower altitude?
at higher altitudes, the air is rarefied and has less weight and thus exerts less pressure
what does BTU mean?
british thermal unit
how is primary temperature control achieved?
by modulating the discharge temperature of the packs
how do we remove the unpleasant order from the galley and lavatory?
by using either an extraction fan or outside differential pressure (with the assistance of the cabin)
the delivery from the packs is delivered into a unit called the?
cold air plenum
trimmed air may also be referred to as?
conditioned air
where are come locations where supplementary heating (electric heating) is supplied?
crew rest compartment of the A330
does humidity decreases or increases with an increase in altitude?
decreases
why does modern large aircraft utilise air-cycle cooling?
due to its simplicity, freedom from troubles and economy
when is IP bleed used?
during cruise operations
foil air bearings can handle severe environmental conditions such as the ingestion of sand and dust thus eliminating the need for?
filters in the airflow
what is the definition of charles law?
for a given mass of gas at constant pressure, a given increase of temperature always produces the same fractional increase in volume
when is HP bleed used?
for low rpm operation of the engine
is compressed gas hot or cold?
hot
what does the secondary temperature control system makes use of?
hot air taken from the supply duct to trim the pack discharge supply in accordance with the selected temperature
what is the function of an expansion valve?
it allows HP liquid freon at controlled rate (at the entrance to the evaporator) into the evaporator under a much lower pressure
what is the purpose of cabin and cockpit temperature control system?
it ensures that the crew and cabin compartments are maintained at the temperatures necessary to provide comfort and efficient operation
what is the characteristics of fresh air?
it has a high oxygen to carbon dioxide ratio
what is the disadvantage of freon 12?
it has a limited temperature range over which it can be used
what is an aspirator?
it is a jet pump or flow inducer which utilises conditioned air to operate the jet, drawing cabin air into the aspirator and returning the mixture back to the cabin
where is the air that is used in the cabin exhausted to?
it is exhausted through the air exit grills and outflow holes in the sidewall just above the floor
what is the source of air that is widely used by the ECS system on modern jetlners?
it is from the low pressure engine/ APU bleed air supplied by the pneumatic system
what is the direction of heat flow?
it is from the region of higher to lower temperature
why must the amount of air supplied for cabin use needs to be controlled?
it is so that a constant mass flow passes through the cabin in order to prevent pressure surges and also to maintain an adequate amount of air to meet normal ventilation, conditioning, pressurising and de-icing requirements
closed circuit systems are suitable for what type of aircraft?
it is suitable for aircraft flying at speeds in the region of mach 2/ 2.5 because the air cycle system becomes inefficient at these speeds
what is latent heat?
it is the heat that changes the form of a substance without changing its temperature
what is sensible heat?
it is the heat that changes the temperature of a substance with no change in the form of the substance
what is the principle purpose behind a refrigerator?
it is to extract heat from cold space
what is the principle purpose of a heat pump?
it is to supply heat at an elevated temperature
what does the word bootstrap refer to?
it is used to refer to a system that is self sustaining and is independent of external power
what is the principle behind the vapour cycle system?
it is where heat is absorbed by the evaporation of a liquid refrigerant at the unit called the evaporator and rejected into a heat sink (usually the atmosphere) at a unit called the condenser
what is the function of the ECS (environmental control system)?
it isolates the occupants from the extreme environmental conditions
what is the principle operation behind a coalescer type water separator?
it makes use of the action of passing through the fabric which causes the small droplets to combine to form large particles of water
what is the function of a humidity control system?
it provides a means of increasing the moisture content of the air supplied to the cabin at low humidity conditions
what is the advantage of using partial recycling of the used air?
it reduces the air load placed by the air conditioning system on the pneumatic system thus reducing the bleed load demand and improving the fuel efficiency of the engine
what is ventilation?
it refers to the constant removal of used air and renewal of the same with a continuous supply of fresh or unused air
vapour cycle systems are suitable for what types of aircraft?
large and very fast aircraft because of the following advantages - system can be designed to require minimum power input for a given amount of cooling - they allow the cooling to be applied very accurately at the required points - they cater for very important case where humidity reduction is required on the ground
when the vapour is condensed, there is heat released in converting the vapour to liquid. what is this process called?
latent heat of condensation
when liquid boils, it turns into vapour. what is this heat absorption without changing temperature process called?
latent heat of vapourisation
the re-circulation air accounts for nearly 35% of the supply air and is mized with the pack discharge in a unit called the?
mixer unit
does radiation depend on the existence of any intervening medium?
no
is the load compressor supply modified by the pneumatic system?
no, it is used directly
in engine bleed supplied air conditioning, the airflow is delivered through a valve called the?
pack flow control valve
when the passenger load is less, the setting of the PFCV can be reduced for the purpose of?
reducing the air load demand placed by the air conditioning system on the engine thus improving the fuel efficiency of the engine
a kind of refrigerant does a practical vapour cycle system employ?
refrigerant 12 aka freon 12
cold air from the cold air plenum will then be routed to?
the air conditioning zones
what is the principle difference between air cycle cooling and vapour cycle cooling systems?
the air is not reduced to liquid
each aircraft zone has a conditioned air distribution system and an individual air distribution system also known as?
the gasper system
what happens inside the evaporator?
the heated air from the main air supply system passes across the evaporator, releases heat to boil the liquid refrigerant and passes to the cabin at a much lower temperature
what happens inside an open circuit vapour cycle system?
the liquid refrigerant is evaporated direct to the atmosphere
what happens in a closed circuit vapour cycle system?
the liquid refrigerant is recycled for reasons of economy of operation and saving of operation and weight
what does IP bleed mean?
the tapping of air from the earlier stage of the compressor
what does HP bleed mean?
the tapping of air from the later stage of the compressor
what is the principle operation behind a high pressure type water extraction loop?
the water extraction occurs upstream of the turbine before the air suffers considerable pressure loss at the turbine of the ACM - water extraction is done when the pressure of air is still high so as to improve extraction efficiency
how are the various valves on the temperature control system operated?
they are all motor operated to allow for control by electronic controllers
where are the gasper system located?
they are located on the passenger service unit (PSU)
how do we control the pack discharge temperature?
through the 2 valves called the ram air louvers and the turbine bypass valve on the pack
where is the air that is supplied to the cockpit exhausted to?
to the avionics compartment through the floor level grill where it is used for electrical and electronic rack cooling
supply air taken upstream of the PFCV supplies the hot air manifold (plenum) which air is used for the purpose of?
trimming
both refrigerator and heat pump remove heat from a low temperature level and rejects it at a higher temperature. true or false?
true
cargo compartments are either ventilated, heated or both. true or false?
true
cooling air temperature rises due to ram effect of the air entering the ducting and consequently the effectiveness of the heat exchanger is reduced as the speed of the aircraft increases. true or false?
true
gasper unit only allows the control of the flow of air. adjustment of the temperature of air is not available. true of false?
true
hot air tapped from a hot air plenum would be supplied through dedicated trim air valves into each of these ducts that carry cold air to the dedicated zone. true of false?
true
in a high pressure water extractor, the water collects at the lowers point of the body and then drained to the ducts which leads to the water injectors. the injectors will then spray the water into the ram airflow. this process improves the efficiency of heat exchanging across the heat exchangers. true or false?
true
packs are usually made to discharge to the temperature requirement of the zone with the highest cooling demand. true of false?
true
the boiling point of any liquid depends only upon the pressure exerted on it and is not influenced by any outside the flow of heat. true of false?
true
how do we increase the airflow across the heat exchangers?
we couple a flow augmenting fan to the end of the turbine compressor shaft which induces an airflow over the dual heat exchangers thus improving the cooling contribution from the heat exchangers
