1) As altitude increases air density decreases.
The consistent of composition are oxygen (21%) Nitrogen (78%) & other gases (1%) remains unaffected as altitude increases but the amount decreases.
This means that there is less oxygen at high altitudes.
At sea level with standard barometric pressure 14.7 oxygen pressure will be just over 3psi.
While at 25,000 ft. standard barometric pressure is 5.45 psi & oxygen pressure is just above 1 psi.
2) Secondly to function efficiently, human lungs requires air pressure in order to be able to transfer oxygen to the rest of the body.
Pressure also decreases as altitude increases, hence the pressure is needed so that a safe cabin altitude is maintained at all times relative to the A/c operating altitude.
An altitude increases lack of oxygen (O2) in blood cells occur affecting human performance.
From sea level to 8000 ft. there is sufficient oxygen content in the air for the human body to remain unaffected.
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PRESSURIZATION
Therefore pressurization is required to give more comfort to the passengers and crew by providing artificially created low altitude pressure within the pressurixed cabin.
The optimum cabin pressure to maintain to give maximum passenger comfort would to be maintain sea level pressure but in that case a cabin full of heavy air at sea level pressure will be heavy comparatively & will also enact a greater expensive force on the A/c structure due to the positive pressure differential.
The differential pressure is positive when the internal pressure (Inside the cabin) as compared to outside is more.
An acceptable compromise is made & a height of 8000 feet is maintained when operating at high altitudes.
Modern commercial transport Aircrafts are designed to be able to accept a maximum differential pressure of 9psi.
Such systems will also be able to maintain / control the rate of charge of cabin altitude, progressively in order to maintain passenger comfort during the climb to, & the descent from, cruising altitudes.
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DEFINITION OF PRESSURIZATION
Absolute Pressure – Measured along a scale with zero defining a complete vacuum.
A/c Altitude – Real height. of A/c above sea level.
Ambient pressure – Pressure in area surrounding the object under discussion.
Standard Barometric pressure – Weight of gases in atmosphere, 29.22″ H.g. or 14.7 psi.
Cabin Altitude – is the Cabin pressure in terms of equivalent altitude above sea level.
Differential pressure – is the Difference between cabin pressure & atmospheric pressure.
Positive differential – is when cabin pressure is higher than Ambient pressure
Negative Differential – is when cabin pressure is lower than Ambient pressure
Gauge Pressure – is used to Measure pressure in a vessel, containers or a pipeline.
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For pressurization Air conditioning is required.
A/c can not have pressurization without Air conditioning.
Air is constantly fed to the cabin by Air Conditioning system & is controlled by outflow values.
1) Cabin pressure controller – controls outflow valves
If outflow valve closes, cabin pressure increases
If outflow valve opens, cabin pressure decreases
The cabin pressure controller is set by crew so that as A/c climbs the rate of decrease of pressure in the cabin is such that the required cabin altitude will be reached at approximately the same time when A/c arrives at cruising altitude.
Cabin pressure controller has two knobs
1) Attitude knob – we select the altitude at which we went the cabin altitude to be maintained.
While turning the altitude knob, altitude figures appear in the window of pressure. controller which indicates maximum altitude up to which the maximum cabin differential can be maintained.
These figures should be adjusted to an altitude at which the A/c is going to fly.
At the same time a needle moves over a calibrate scale which is normally calibrated from – 1000 ft to 10,000 ft. (1000’ft below sea level to 10,000 ft above see level).
The needle scale indicates that cabin altitude will be maintained at that height.
The rate of change knob controls the rate of climb or descent of the cabin, irrespective of the actual rate of climb or descend of the Aircraft.
Normally the cabin rate of change should not be selected more than 500 ft. per minute, as sudden change of pressure is harmful for ear drums, but during emergency when quick depressurization of the cabin is required greater rate of change can be selected, there is normally a gauge in the cockpit which indicate the actual rate of change of the cabin.
2) Discharge valves – gets pressure signals from the pressure controllers & thus regulate their openings to maintain the required pressure inside the cabin, they are also vented to the static pressure. They are also called outflow valve.
3) Cabin safety valves – comes into operation, if the discharge outflow valve fails to maintain the required differential pressure & when the differential pressure increases to a value slightly higher than normal pressure at which it is set to relieve. It relives the extra pressure to the atmosphere & maintain the cabin differential at that setting.
4) Inward Relief valve – is normally installed in the case the pressure fails at higher altitude. & a/c descents to a lower altitude, a time may come when the pressure inside the cabin is less than the outside atmospheric pressure. Any time the cabin pressure is less by 0.5 psi or more than outside atmospheric pressure, the Inward relief valve opens & vents the cabin to the atmosphere & then onward maintains the atmospheric pressure inside the cabin.
5) Dump valve – The purpose of Dump valve is to reduce any residual pressure. Inside the cabin after the a/c has landed.
It may be operated manually, or automatically through the oleo micro-switches fitted on the landing gears.
