PMN
Topic Author
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Cabin Pressurisation

Wed Jul 13, 2005 10:08 pm

Hi All,

Could any of you technically minded people please explain the process of cabin pressurisation? My understanding is that the cabin is pressurised in stages as the aircraft climbs. Does the cabin stay pressurised at around 8-9psi until the wheels touch the runway or does the pressure decrease during descent?

Also where exactly does the cabin air come from, and what kind of filtration is it passed through? My limited understanding is that it is fed from a stage in the engine, but where exactly?

Finally are the systems that control cabin pressure essentially the same for all pressurised aircraft? Are the systems on an ERJ-145 basically the same as on an A310?

This is something I've been wondering for a while, and A.Net seems as good a place as any to ask!

Thanks in advance
Paul
Edith in his bed, a plane in the rain is humming, the wires in the walls are humming some song - some mysterious song
 
air2gxs
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RE: Cabin Pressurisation

Wed Jul 13, 2005 11:20 pm

Quoting PMN (Thread starter):
My understanding is that the cabin is pressurised in stages as the aircraft climbs. Does the cabin stay pressurised at around 8-9psi until the wheels touch the runway or does the pressure decrease during descent?

The presurrization controller follows a schedule. It isn't really pressurized in stages, it's something that follows the climb (descent) profile of the aircraft, or more precisely, the ambient pressure. The controller senses the outside pressure and follows a schedule to maintain internal pressure. I don't know what the schedule is, but I'd imagine it varies slightly from one manufacturer to the other. The aircraft is usually at or very close to 0 PSID when the aircraft lands.

Quoting PMN (Thread starter):
Also where exactly does the cabin air come from, and what kind of filtration is it passed through? My limited understanding is that it is fed from a stage in the engine, but where exactly?

The air comes from the engine. Where in the engine depends on the manufacturer. You have high and low pressure from the high pressure spool or rotor. At high power settings the air comes from the lower stages of the engine and at low power settings from the higher stages. The air isn't so much filtered as processed.

In a nutshell; The air goes through a pressure/temperature regulating valve, then through a precooler (this air coming from the engine can be upwards of 450F). From there it enters the aircraft pnuematic system and onto the packs. The packs are the airconditioning/heating/pressurization system on an aircraft. Basically the air then passes through heat exchangers, a compressor, a turbine and a water seperator. The end result is air that can be as low as freezing. So, now we add hot air straight from the hot side of the system to "trim" the air and heat to comfortable level.

Now we allow it into the cabin. The cabin is a pressure vessel. It should be as airtight as possible and the only exit should be from the outflow valve. The outflow valve is controoled by the controller and meters air out of the aircraft according to the schedule set in the controller. The controller monitors cabin pressure and ambient pressure to maintain the schedule.

Clear as mud, right?

I'm sure others will fill in some of the gaps. I left a few, because chapter 21 theory is way in my past and to tell you the truth, without the book in front of me I can't remember all the details.

Most aircraft that use an ACM system should operate this way or something similar.
 
A/c train
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RE: Cabin Pressurisation

Thu Jul 14, 2005 12:28 am

pressurisation is achieved by feeding air into the cabin, and controlling the rate at which its allowed to outflow to atmosphere, the valve which controls this outflow is called an 'outflow valve', usually seen towards the back end of the aircraft on the lower half of the fuselage.
As has already been said. Air is bled off a compressor stage on the engine determined by, power setting/manufacturer, this is hot and high pressure, sent through a pressure regulater and shutoff valve, then through a pre cooler and ducted down too the packs, here it is either sent through an air cycle machine and cooled or bypassed and mixed by a temperature control valve downstream with warm air again to trim the temperature, this mixing is a function of temperature control from the flight deck. This conditioned air is then fed into the zones in the cabin through manifolds, this feeding of air pressurises the cabin, controlling of this pressure is done by modulating the outflow valve to achieve desired cabin pressure.
The outflow valves position is controlled by a cabin pressure controller, this can either be done automatically or manually. the target is too keep the cabin at a nominal 8.9psid(differential), cabin pressure is expressed on aircraft as cabin altittude which is the cabin altitude in relation too sea level, usually around 8,000ft, this will give a safe psid.
Look at it as a flight profile, aircraft starts its flight at the airfield altitude, as the aircraft climbs, the ambient pressure decreases, the higher the altitude the lower the pressure, so the altitude inside the aircraft also increases but at a LOWER rate than the aircraft itself, this is cabin altitude, controlled on climb descent by cabin vertical speed in 1000's Feet per minute, so the outlfow valve, controlled by the CPC's controls cabin altitude and cabin v/s too maintain a safe cabin psid.
 
SlamClick
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RE: Cabin Pressurisation

Thu Jul 14, 2005 12:32 am

The problem: Airplanes fly at altitudes where the air is too thin to breath. It must therefore, be pressurized for passengers and crew to get enough oxygen.

The solution: Jet engines take in more air than they need to operate, they scoop it up by the cubic mile at speed. The excess air is "bled" off the compressor stages and ported overboard, but some of this air is put to use, including pressurization of the cabin.

From the engine it is conditioned, that is cooled, water removed, filtered and finally hot air added back in to get the temperature just where you want it. (more art than science) This conditioned air is ducted throughout the cabin and some or all of the cargo compartments. It is then drawn out of the cabin and a portion of it is recycled and fed back into the supply duct. The rest of it flows over the radios etc. to cool them, around the sidewalls of cargo compartments to insulate them from the cold outside, then then overboard through a regulated "outflow" valve.

It is the position of this outflow valve that sets the pressure inside the cabin.

Atmospheric air pressue is the result of the column of air standing on top your location on earth, from ground level all the way up to the top of the atmosphere. At sea level this is roughly (and is "standardized" to) 14.7 pounds per square inch. As you go up, there is less air above you weighing down on you so the pressure decreases. At about 18000' the pressure is only about half that of sea level and at about 35000' it is about half that. Our problem is that we need the oxygen, which only comprises about 21% of the air, or, put another way, at sea level the "partial pressure of oxygen" is about 3 PSI. We needs something like that.

So you are flying from San Diego to Denver. You are taking off from sea level and landing at about 5000' above it. On the way the plane will cruise at 37000'.

At sea level you are getting the full 14.7 PSI of air.
At Denver you will be getting roughly 12.3 PSI, or a partial pressure of oxygen of about 2.5 PSI. That is adequate.
At 37000 feet however, the air pressure would only be about 3.1 PSI and the oxygen you'd get not enough to keep you conscious or alive.

What we do is pressurize to a PSI Differential which equates to a "lower" level in the atmosphere, or a "cabin altitude."

Our theoretical airplane can give us 8.0 PSI Differential, so at 37000' it is taking in air at about 3.1 PSI and compressing it, thereby adding about 8.0 PSI for a total internal pressure in the cabin of about 11.1 PSI. This equates to the natually occurring pressure at about 7500' above sea level. You can live with that.

The "schedule" goes something like this. You are going to take off from sea level and climb to 37000' and that is going to take you about 25 minutes. When you get the plane to altitude, you want the cabin to be at maximum differential of 8.0PSID. So the pressure controller programs a cabin climb from sea level to about 7500' cabin altitude over the same time period.

When you take off, you are already at sea level, so this means that you are actually reducing the cabin pressure as you climb. You are just reducing it at a slower rate than the atmospheric pressure outside the plane is decreasing as a function of your climb. So at about halfway from sea level to 37000' the cabin should be about halfway between sea level and 7500'.

On arrival in Denver, we will descend from 37000' to a landing at about 5000'MSL and that will take about twenty minutes. Over that time the pressure in the cabin will be increased from approximately 11.1 PSI to the field elevation equivalent of roughly 12.3 PSI. You should not even feel that happen.

A couple more notes:

Before takeoff most aircraft will pressurize to about 200' below field elevation. It seats all the plug-type doors and windows, stiffens the fuselage and reduces the noise level.

After landing most aircraft will drive the outflow valve full open or you would not be able to open the doors.

Lastly, I said this was scheduled over "time" and this might not be strictly the truth. The engineers who design the system almost certainly use inputs other than time to make the system regulate itself.

Sorry about the long-winded response.
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highflyer9790
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RE: Cabin Pressurisation

Mon Aug 01, 2005 12:32 am

Quoting PMN (Thread starter):
Does the cabin stay pressurised at around 8-9psi until the wheels touch the runway or does the pressure decrease during descent?

It is depressureized curing the descent i think.
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undehoulli
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RE: Cabin Pressurisation

Mon Aug 01, 2005 12:56 am

In a sense the cabin is depressurized during the descent, but actually the cabin pressure is increasing as the aircraft descends, eventually matching the landing altitude (or close to it) thus eliminating the differential pressure.
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Jetlagged
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RE: Cabin Pressurisation

Mon Aug 01, 2005 6:22 am

The cabin is pressurised relative to the altitude the aircraft is at, not relative to airfield pressure. So although a pressure differential exists to ambient, the cabin pressure is almost always lower than airfield pressure.

Quoting PMN (Thread starter):
Finally are the systems that control cabin pressure essentially the same for all pressurised aircraft? Are the systems on an ERJ-145 basically the same as on an A310?

The essentials are the same for all aircraft types (driven by the physiology of the human body). However the way cabin pressure is controlled will vary according to aircraft type, as will the maximum pressure differential. Some systems are more sophisticated than others. Older aircraft types tend to need more crew intervention than newer technology aircraft. For example a 747-200 will require the Flight Engineer to control and monitor pressurisation fairly closely. A modern airliner pressurisation system is almost entirely automatic, with inputs coming from the FMC flight plan.
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