Air Conditioning Flashcards
Light a/c requirements for enclosed cabins
Light a/c non pressuriesd only requires a very basic system for heating and cooling cabin air.
~Hot air is taken from a heat exchanger wrapped around the exhaust.
~cool air is taken directly from outside ram air.
Max altitude 10000ft with no pressurisation
To fly at high altitudes must be pressuried
Air conditioning large a/c
Humidity is generally not controlled
Large a/c use bleed air to supply air conditioning packs
Aircondingng requirments
~provide 1 lb of air per SEAT per minute in normla operation
~at least 0.5 lb of air per seat per minute following failure of one conditioning pack
~temp range between 18-24 c
~ensure carbon monoxide is less than 1 part per 20000
ECS(environmental control system) is the component required for conditioning and pressursation
Principle operation OF A BOOTSTRAP COOLING
• In the Air Conditioning Pack
• One entering the Pack, Bleed Air from the engine is compressed and goes passes through a pre cooler
• It is then passed through a heat exchanger which sits in a Ram Air Duct
•The partially cooled bleed air then enters a turbine. This air drives the turbine and
“so loses considerable energy. It leaves the turbine at about 0°C
• This cold air is then mixed with other hot bleed air to provide air just right the tractor temperature to the cabin
• The first stage of cooling, between compressor and turbine, causes condensation.
A water separator is fitted just before the entry to the turbine section
The compressor is driven by the turbine
• This type of air cycle cooling is known as a bootstrap cooling system
——
• The combined compressor and turbine section are known as the
cold air unit
• The ram air duct is equipped with a fan to draw the air through it when
the aircraft is stationary
• The volume of hot bleed air which bypasses the cold air unit is
regulated by the temperature control valve
• The entire pack is, itself, cooled by a pack cooling fan
What pack cooling fan doesn in a bootstrap cooling system?
It sucks more air from the inlet when there is not enough ram air
Air distribution
Conditioned air flows through under floor pressure ducts ,then up behind the cabin wall trim and out of vents at cabin roof level
It then flow mostly downwards and is extracted at floor level by recirculation fans driven ac electric motors
About 50% is then re circulated after passing through HEPA filters.
HEPA(high efficient particular arresting ) trap 99% of bacteria
Principle of operation of cabin pressuraisation
~negative pressure relief valve 0.5 psi
~emergency differential pressure reliever (opens if the pressure controller/outflow valve fails)
The controller calculates a pressurisation schedule
This pressurisation schedule must comply with some parametres
~maximum differnetial pressure
~maximum rate of change of cabin pressurasation
~minimum cabin pressure
~for structural reasons ,maximum differential pressure is about 8-9 psi
~max differntial pressure occurs at a/c max operating altitude
~cabin pressure is expressed as a cabin altitude
~max cabin altitude is the same as minimum cabin pressure
~rate of change of cabin pressure is cabin vertical speed
~max cabin altitude is normally limited to 8000ft
~if the cabiun altitude exceeds 10000ft an audio alarm will sound
Pressure hull
Pressure hull extend from forward pressure bulk to rear pressure bulk
When an error is detected in cabin pressure
The pressure controller commands the outflow valve
The controller brings the cabin to the selected altitude at a rate not greater than 1800ft per minute
If the cabin reaches 11000ft the controller automatically closes the outflow valve to increase
~on descent cabin depressurises until it reaches pre set landing altitude . Usually just before touch down (500 ft -1000ft)
Max differential pressure
Differential pressure usually between 7.76PSI and maximum of 8.06
Cabin pressure should be greater and never negative than outside pressure
Safety valve
Usually two of them
Protetcs the structure from over pressurisation
~opens if cabin press rises to more than 0.5 psi above max differntial pressure
~then regultes pressure at max differntial plus 0.5psi
For negative differential pressure
The negative pressure relief valve opens at 0,5 psi in difference
