Aircraft Cabin Air Supplies

[Sparweb]

I have been trying to get an answer to this question from the aircraft OEM, but the answers always miss the point. I am also not finding guidance in Advisory Circulars or the like. The airplane in question is a Canadair RJ 700, but perhaps the nature of the question isn't specific to this one type of aircraft.

We are carrying out an aircraft interior re-configuration in which several large monuments are being installed in the cabin. They are so large that they interrupt the overhead stowage bins. Since the aircraft design relies upon these stowage bins to carry fresh air supplies, and distribute it through the cabin, the interruptions of the bins also interrupt this flow. So that the passengers and crew still have fresh air delivered to their seats and workstations, I'm considering various ways of re-routing the air flows around the monuments. Most of the air is fed into plenums built into the overhead stowage bins, so at first, I planned to mount hose fittings to each bin's plenum, and run a hose between them. It won't work well, but it will be "just enough". Thankfully I only have to be concerned with the "gasper" air supplies (the little valves that each passenger can open or close over their seats). There is an existing path for the bulk air supply to get to the bins and air ducts that are separated from the others.

That's when I noticed that the plenum for the "gasper" air and the plenum for the bulk cabin air supply (which blows a constant stream of temp-controlled air through the whole cabin) are routed right beside each other in the bin. In fact, it would be a trivial task to just drill some holes through the interface between them, thus supplying the gasper plenum from the bulk air plenum, with no hoses to snake around the cabin walls.

But-

This leads me to wonder, and here is the question I want to ask: Why is the gasper air supply segregated from the bulk cabin air supply at all?

I am tempted to take gasper air straight from the main plenum of cabin air flow - both are conditioned at the same temperature and nearly the same pressure. It looks like it would work, but it is clearly designed NOT to work this way. So why does the OEM go to great lengths (and cost) to keep these air supplies separate, when it all ends up in the same place?

If there is a very compelling reason to NOT intermix these air flows, then I'm very eager to hear about it!

STF

 

[MikeHalloran]

According to:

http://en.wikipedia.org/wiki/Environmental_control_system_(aircraft)

... Typically, the gaspers are fed from the engine compressors when the engines are running, OR from the APU or a ground source when the engines are not running.

I don't know if it's a formal requirement, but I personally really like having gasper air available at all times.


Mike Halloran
Pembroke Pines, FL, USA

 

[Sparweb]

Hi Mike,
I like the gaspers, too. And I intend to maintain that feature in this aircraft. The Wiki article is pretty ambiguous, and may have misled you. There's no question that I want the gaspers to continue to work. Their supply line is interrupted by the cabin monuments: how to re-connect them is what I'm considering, so that they can work again.

The CRJ700 has 4 air circulation supplies coming out of its mixing chamber: one for the cockpit, one for the forward cabin, one for the aft cabin, and one for the gaspers. A separate line for the cockpit makes a lot of sense: redundancy and dealing with various contingencies is clear to me. The cabin is very long, so two supplies to the cabin is also reasonable. But WHY is there one for the gaspers? The gaspers would work just fine if they were plumbed directly to the other air supply lines. In fact, the air supply line to the cockpit DOES tee off to the gaspers in there!



STF

 

[MikeHalloran]

Rereading, it appears that the gaspers are not typically fed ANY recirculated cabin air, e.g. with smoke.



Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

SparWeb
Does the SRM have a ventilation circuit diagram in it ?
B.E.

You are judged not by what you know, but by what you can do.

 

[rb1957]

can't you run the existing gaspers behind the monuments ? maybe with a little tweeking and re-routing ? i thought the gaspers etc lived behind the bins ?

how about offering two designs ... one with monuments as planned and "poor" gaspers air, one with teeked monuments and good air delivery ?

i agree with mike ... i think it's a smoke-recirc issue that keeps gasper air separate from bulk cabin air.

Maybe you should learn French ? mind you, in a few years maybe we'll have to learn Mandarin ??

another day in paradise, or is paradise one day closer ?

 

[MintJulep]

ECS starts around page 300:

http://www.flywestwind.com/Hangar/Aircraft_Files/FOMs/CRJ-700 FOM.pdf

I didn't read carefully, but there appears to be some capability to change the distribution in the event one of the two air conditioning packages fails.

The segregated ducting would help accommodate that.

 

[Sparweb]

Yes, Berkshire, the air circulation system is well laid out in the AMM. This where I got the idea. When two lines start at exactly the same place and end at the same place, I have to wonder why.
The CRJ200 (previous generation) did not segregate the gasper air supply from the main cabin air supply. This was introduced in the CJ700. I have access to all operations and maintenance documents for the CRJ700. There is no explanation for the change in any of the manuals I have.

After a number of discussions with Bombardier about the entire ECS (I'm dealing with many changes apart from this one) this question is never answered. I wouldn't say evasive at this point, but when my formal inquiries have 3-4 questions, and they only answer 2... it's like sending e-mail to a teenager.

Anyway, I've ruled out air cycle pack failures - the two of them feed the mixing manifold directly, and all cabin distribution lines come from the mixing manifold. Failure of one ACM pack may degrade the output, but all distribution lines are affected.

RB,
Yes, the lines can be run around the monuments. Hoses, criss-crossing all over. It's messy, and there will be losses (flow and pressure) as these hoses will get quite long. In fact, it seems to me the "bad" way to do this is with long hoses going around the monuments. Lots of little points of potential failure, elbows here and there... I am considering the idea of offering two ways to the customer (let them decide).

MJ,
Good find - though I already have a copy of the AFM and FCOM here. For curiosity's sake, I went to the section on environmental controls. It has the same "oversimplification" as my copies of the manuals. If you flip to page 332 of that PDF, the diagram shows a hose coming out of the mixing manifold that goes to the "Aft Lavatory". Well actually that's the line that connects to the gaspers. Just above it there are other lines that are labeled "gasper supply" but they do not feed the gaspers at all - they feed the bulk air supply that flows through the storage bins. This diagram doesn't accurately represent the way the system actually works.
Also in the FCOM (in the AFM actually) is the expected emergency procedure for smoke in the cabin. There is no provision to isolate air distribution ducts. If the smoke is in the cabin and the air distribution ducts are suspected, the procedure only involves shutting down one ACM, then the other.

I was in the aircraft recently, running just on ground support cart with an air supply (ported into the side of the mixing manifold), and I felt fresh air in the cabin, from all of the ducts and the gaspers just fine. Neither of the main ducts or the gaspers seemed to get preferential treatment. I have also examined where these ducts go with the floor boards taken up. The paths are clear to see, just like the diagrams in the IPC. Separate lines for each of these supplies, but they start at the mixing manifold and end at the cabin, all of them.

The only reason I can come up with is redundancy. Maybe a system analysis came up with a failure mode that prevented enough fresh air getting to the cabin, and the only way to get it to that part of the cabin was to have separate distribution ducts. I'm not satisfied with that answer, because the cabin already has redundant supplies (front/back) without the gaspers.

STF

 

[aerok]

First post here after being a frequent reader for several years!

Not too familiar with these systems, but perhaps the second duct is intended to supply air at a specific constant pressure to the front of the cabin. If the gasper duct was to be used for this purpose, opening some or all of the gaspers would produce a significant pressure drop.

It may be possible that by supplying a consistently higher pressure airflow to the front of the cabin would tend to direct "used" air (as well as any smoke in the cabin) toward the rear of the aircraft where it can be exhausted.

Could you re-route the ducting around the monuments and install a booster fan if the pressure drop is too large? Might be more expensive and complicated than it's worth, though.

 

[SAITAETGrad]

1. Start by seeing if you can't correspond by voice communication rather than email! I know you've tried to do this but maybe you need to get more emphatic about that need.
2. Fresh gasper air would not appear to be a strict requirement. Since gasper flow is variable based on useage, the lack of recirc air makes sense as basic flow requirements and efficiency set with gaspers @ zero flow and recirc air @ maximum allowable for that condition to maximize fuel efficiency. Gasper is just plus plus (or depending on your perspective- loss loss) on the system, right? Fresh gasper air more good for gasping by anxious pax, but less good recirc air should be fine...unless you are hauling princesses, right?
3. Smoke in cabin? Wouldn't your pax be on chem oxygen?
4. Joining the plenums together will result in some minor losses. Likely you can live with these but will require test with some uncertainty.
5. Joining the plenums will likely increase cabin noise to some degree. Doubt this is a concern for your application.
6. Since all lines are joined at single point of failure at mixing valve, perhaps you can show in your SSA that your design is just as unsafe.
7. Presumably your # of occupants is greatly reduced. Seems to be you have considerable tolerance for losses in system output.

 

[dgapilot]

Normally the gasper air is only cool or cold air, and the bulk air is heated (or cooled) as needed. Ever try and breath with a hair dryer blowing in your face?

 

[Sparweb]

quote: ...Since all lines are joined at single point of failure at mixing valve, perhaps you can show in your SSA that your design is just as unsafe.

Yes that says it as succinctly as it can get.

In the big picture, the choice between keeping the distribution lines separate, or just combining them, is a relatively small affair. But it nags at me. I have many outstanding questions to discuss with the OEM (in person, by e-mail, by phone, semaphore, etc.) that they are already working on answering. Those questions are more important than this (and way beyond the scope of this thread), and getting them to answer one more just slows down the responses to the others. Particularly in this case where I, an outsider, want to ask them, the OEM, "Why did you do something so pointless?", (not in so many words, of course) because that's just not the spirit that would motivate them to answer my other questions.

Reading back up the discussion in this thread, I can see how counter-intuitive the air distribution system is. Most who answered don't believe my story that there are so many extra lines, or didn't catch on to what I was explaining. No worries, folks, because it makes no sense to me either.


STF

 

[bashdarov]

good post thank you


Graduated from Soran University with First Class Degree with Honours in Computer Science.

 

[chicopee]

The question " Why is the gasper air supply segregated from the bulk cabin air supply at all?" I think that the answer should be obvious. The cabin in which the pilot and copilot seat, is probably more important then the rest of the plane in the event of rapid depressurization, therefore, a separate fresh air system would be needed for breathable air, otherwise the breathable air may be depleted for the pilot cabin. Don't forget the cabin door is also kept closed during flight,so it would be more reliable to have two separate air system to maintain flight worthiness. Redundancy is the key.

 

[Sparweb]

Chicopee,
That's not really the question. Devil's in the details. The pilot's supply hose is segregated, and I think the reason for that is obvious. There is a second and third air supply hose, though, for passengers only. And a fourth, for gaspers. All four come from one mixing chamber. The one mixing chamber always receives air from the ACM packs, therefore redundancy in supply comes from dual ACM packs, not from the mixing chamber.

A diagram is necessary:

~       |________________________________________________________________________________Crown_Skin_|
        |                                                                                           |
  Rear Press Bkhd                                                                         Fwd Press Bkhd
        |        __________________________________      _______________    _________________       |
        |       |[u] [COLOR=#EF2929]~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~[/color][/u]|[COLOR=#EF2929]~~~~[/color]|[u][COLOR=#EF2929]~~~~~~~~~~~~~~[/color] [/u]|  |[u]~~~~~~~~~~~~~~~~~[/u]|      |
        |           Plenum                 ^    ^           ^    Plenum        ^    Plenum          |
        |                                  |    |           |                  |                    |
        |              Aft                 |    |           |   Fwd            |     Flight         |
        |              Cabin               |    |           |   Cabin          |     Deck           |
        |                                  |    |           |                  |                    |
        |_______________________________________________________________________________Cabin_Floor_|
        |                                  |    |           |                  |                    |
        |     _________                    |    |           |                  |                    |
ACM 1  ----->|         | --> to gasper ----^    |           |                  |                    |
        |    | Mixing  | --> to aft cabin  -----^           |                  |                    |
        |    | Chamber | --> to forward cabin   ------------^                  |                    |
ACM 2  ----->|_________| --> to Flight Deck -----------------------------------^                    |
        |                                                                                           |
        |________________________________________________________________________________Belly_Skin_|

The ACM packs are behind the aft pressure bulkhead. The Mixing Chamber is under the floor. It feeds 4 distribution lines. One goes to the flight deck, uninterrupted. Two distribution lines go under the floor, then rise up in ducts to the sidewalls, connect to plenums, and diffusers distribute the air to the cabin uniformly. Those plenums I have drawn as "____". The ends of each of these bulk air supply plenums do not connect, therefore the aft cabin only gets this uniform distribution of air from the riser that came from the one duct that feeds it. I haven't shown it, but the left side of the aircraft is also separate from the right. The flight deck is completely segregated from these ducts, too.

The gaspers are fed by one distribution line under the floor. It too rises up the sidewalls and enters a small plenum, which I've drawn in red as "~~~". As you can see, the ~~~ plenum runs parallel to the ____ plenum of the bulk air supply. In fact, the gasper plenum is inside the bulk air plenum. To get air from the gasper plenum to the little overhead valves, a hole is made in the gasper plenum wall, carefully not puncturing the bulk air plenum wall that it shares, and running a tube down. The plenum for the gaspers runs the full length of the cabin but stops before the flight deck. The flight deck has its own gaspers, which I won't mess with.

My cabin monument is located between the aft and forward cabin air supply risers in the walls, therefore, the bulk air plenum in front of it is still working. But the gasper plenum will be cut and the forward end isolated from its supply. Since the remaining gasper plenum is inside the bulk air plenum in the forward cabin, which is still getting air, the simplest way to get the gaspers to work is to drill holes in the little wall that keeps them apart.

I wouldn't have any reluctance to do this, but the designers of the aircraft seem to have gone to great lengths to keep them separate, never letting air for the gaspers mingle with air for the bulk distribution. Even though the distribution lines for gaspers and especially the aft cabin bulk distribution lines run in parallel, and are in intimate contact with each other throughout their entire journey. In my investigation, I've collected quite a bit of evidence to show that puncturing the membrane between the gasper distribution plenum and bulk air distribution plena would cause no adverse effect at all.

But I always allow for something I could be missing...

STF

 

[dgapilot]

Go back to my earlier post. The gasper air comes from the mixing chamber, but it is ONLY cool or cold air. The bulk supply air coming from the mixing chamber is heated (or cooled) as required by the temperature setting of the cabin control. That is why the gasper and bulk supply are segregated. They all come from the same source, the difference is the temperature that the air in the line is. If you cut the gasper for the forward part of the cabin, then tie those gaspers to the bulk air system, you will get hot air out the gasper if the system is calling for heat.

 

[Sparweb]

The ACMs set the temperature of air the mixing chamber. If recirculation is turned on, that comes in both left and right sides of the manifold and gets mixed with the ACM supply. There is no heat source dedicated to only passenger supply or the flight crew's supply. Certainly no supply of cooler air dedicated to the gaspers.

Although it isn't quite THAT simple.
The Left ACM pack supplies the left side of the mixing chamber, which mostly supplies the flight compartment, and the Right ACM pack supplies the right side of the mixing chamber, which mostly leads to the passenger compartment distribution. If the flight crew sets a different cockpit temperature from the passenger cabin, they can get it. One or both ACMs will decrease or increase their temperature accordingly.

But the segregation of left and right is not sealed, just baffled. In the case of failure of one ACM pack, all distribution lines will be supplied, just at a lower rate. And the gaspers take from both the right and the left. Your guess is as good as mine whether the left-hand gaspers get the cockpit temperature air, or if somehow the baffles in the mixing chamber are arranged to let the passenger temperature air flow from the right side of the mixing manifold to left for the left-hand gaspers.

None of this gets colder air into the gaspers. Any sense of "cold" from a gasper when you open it is just the fast stream displacing the warm boundary layer on your skin.

STF

 

[dgapilot]

ACM output is cold air, bleed air is hot. Your diagram shows ACM to the mixing plenum. The real question is how the ACM air temperature is controlled. Most systems I've seen have a bleed air bypass that will control the flow through the ACM. The mixing plenum usually has an ACM input, and a bleed air input, again modifying the air temperature. The mixing plenum usually isn't just a simple mixing box, but a series of chambers that will allow air to be distributed at temperatures based on valve positions controlled by the temperature control system. Again, the gasper air is ACM air without modification from bleed air. It's been a number of years since I did any work with CRJs, but that's how most other systems work.

Your statement about the gasper not being cold air doesn't hold up. I remember seeing ice crystals coming out the gaspers and hot air out the bulk air on some aircraft.

 

[SAITAETGrad]

If the gaspers and vents were connected, seems you would not have assurance of adequate vent air through the cabin. If for example, all the gaspers in the back were cranked wide open, the flow at the forward end may suffer. Also consider how the initial system validation was maybe simplified by segregating the gasper air.