Bleed air sources

[Hardy Heinlin]

Good evening,

I have a question for the engineers again.

We have 6 sources of bleed air:

- 4 engines
- APU
- External

Are there any graphs that show how much pressure each source can provide? Not taking into account the consumers, but just what they can provide.

For the engines, that would include factors like RPM, pressure altitude and HP/LP valve switching. Just what they can provide -- no consumer connected.

If you have any hints -- thank you! :-)


Cheers,

|-|ardy

[Jeroen D]

I'm sure somebody will dig up these graphs.

Just some thoughts: Pressure, although relevant for a pneumatic system and it's performance, the volume of air going into the system tends to be even more relevant. Because it must be able to keep up with the demand of all simultaneous consumers.

I would think that the volumes of air coming from the six sources of bleed air vary substantially.

Jeroen

[Hardy Heinlin]

I've found some data for the APU (Pratt & Whitney PW901A).

Max load is 62 psia at -31°C, decreasing to 43 psia at 16°C, then rising again to 56 psia at 38°C and decreasing again ...

Minimum load is always ca. 32 psia.

I can imagine the compressibility of air plays a big factor in the long pipelines across the wings. But as a start for the model I just wanted to know some sort of start value. The rest is then based on that ...


Cheers,

|-|ardy

[Hardy Heinlin]

Does the external air supply at a nice noble airport provide pressures similar to those of the APU?

(Are the costs about the same?)


|-|

[Jeroen Hoppenbrouwers]

62 psi is about 4 bar. That is twice the pressure of a regular car tyre. The long, hoola-hooped air hoses that dangle off some air bridges certainly cannot take 4 bar, so I presume that if these are to pump cooled air into the plane, they for sure don't match the bleed air pressure (but possibly they match the pressure of the conditioned air ducts at slightly above cabin pressure).

I did never see the type of hose that a jet starter cart uses.


Jeroen

[Hardy Heinlin]

I don't mean the external air conditioning but the external bleed air which can pressurize the ducts and which are shown by green flow bars on the bleed air synoptic and by the word "EXT AIR".


|-|

[dutch57]

Hardy, was away a few weeks,

Will give you a reply tomorrow.

sincerly,
Bob

[dutch57]

Hardy

External air , can be provided by jetstarters, max pressure is about 46 psi with an airflow of around 200lbs/min. you need that flow for a starter to crank and start and engine. Normal is 2 jetstarters for engine start.

Apu is a different story, The figures you give are correct, but please note the air pressure supplied by the apu depends on what various systems demand.
For instance are any of the pack switches on , start valve open or is an airdriven hyd pump commanded on. so an exact figure for air pressure you can not tell.

Also note the APU is n-2 reulated to maintain 400hz freq for the generators , these commands also influences air pressure during max air load.

[dutch57]

Jeroen,

The high pressure hoses look the same except much stronger and about 4 inch diameter

[dutch57]

Hardy,

For the engines the PRSOV regulates the pressure to 46 psi, at all times independent of load.

Are you looking for the pressure relation PRV HPSOV and their pressures and switching moments?

sincerly Bob

[Hardy Heinlin]

Hi Bob, thank you! Your answer has helped.

Another question:

If I have only one PRSOV open somewhere at FL300-whatever, and have all consumers on, like NAI, WAI, 3 packs, 4 hyd DEM pumps, flaps in transit ... - can this single engine keep the ducts above 11 psi?


Cheers,

|-|ardy

[dutch57]

Hardy, nice scenario  

I do not think so, yes the engine can produce the air volume needed.
But the pneumatic duct diameter is the limiting factor.

And the smallest is the PRV i thought it is 8inch by hearth.

the problem is you have the airsource capable, but the duct can not handle the volume needed, i suspect pressure will drop to next to nothing.
Not sure but that is what i feel.

Example, by us in the hanger B744 , ext air connected via hangar ground system, one hose 4 inch , trying to retract Le Flaps.
Air pressure drops from 46 to about 8 psi.

Hope this sheds some light for you,

Bob

[Hardy Heinlin]

Example, by us in the hanger B744 , ext air connected via hangar ground system, one hose 4 inch , trying to retract Le Flaps.
Air pressure drops from 46 to about 8 psi.

Good that you mention this :-) I see similar things in my model. Do the LE flaps switch to electrical mode at 8 psi? In my current model they do. And the result is a running flip-flop: When the flaps switch to electrical mode, bleed air rises above 8 psi. That makes the LE again switch to pneumatic mode causing the pressure to drop below 8 psi and the flaps switch again to electrical mode etc.


Cheers,

|-|ardy

[dutch57]

Hardy goodmorning  

In my example and experience is that the Le-drive units do not like this they make a whaling sound and almost stop, i never had a reversion to primairy electric.
Response time of the Le units is fast even with the above situation.

There is no pneumatic pressure regulator in the Le-drive unit, so i feel that the pressure when this units stops depends on the unit(new/old) and the position in the aircraft(drive system resistance).

Reversion to prim electric is controlled by the FCC`s normally and they use position as guide line.

And if the FCC decides to revert to prim electric it will stay there.
I remember a few cases where i solved complaints and needed to reset the FCC`s

Hope this will help you a bit

sincerly Bob

Ps please keep asking i like this it is as a refresher course

[Hardy Heinlin]

Good morning Bob,

yes, I sort of expected this: Once the FCU has chosen the electrical mode, they keep moving in electrical mode until the commanded position is reached. I read that in several manuals. I guess I'll have to add this mode-change-inhibit feature.


Cheers,

|-|ardy

[Hardy Heinlin]

Does the PRSOV open-close mechanism work like a toggle switch when passing the 10 psi limit?

We know it may happen that it closes only 99%, let's ignore this effect for a moment. The question is only this:

When the pressure drops towards 10 psi, does the PRSOV gradually move to the closed position -- or does it snap to the closed position?

If it snaps it can generate a multivibrator as each time it closes it'll regain enough pressure to re-open etc. click, clack, click, clack ... *

* ... assuming the pressure to open the PRSOV can be provided on either side of the valve, i.e. on the engine side as well as on the duct side. Or does this depend on whether the PRSOV is in reversed position? Like this: To open a reversed PRSOV, the duct pressure must be above 10 psi. To open a normal PRSOV, the engine's  pressure must be above 10 psi.

Edit: OK, as far as I see, to open the PRSOV, only the engine pressure must be above 10 psi, not the duct. This will avoid the flip-flop. The PRSOV stays open when the LE flaps stop using air.


Cheers,

|-|ardy

[Hardy Heinlin]

Back to the LE flaps again: I think the flip-flop problem is not really solved if the PRSOV closes gradually instead of abruptly. Even if it closes gradually (between 30 and 10 psi) there will be a point where the flaps simply stop using air. When they have stopped using air, the duct pressure will re-increase, as far as I understand it. To me, it sounds just logical that the duct pressure rises when a big sucker has stopped sucking. And this re-increase of pressure will re-invite the big sucker to use this air ... if there's no inhibit in the FCU of doing so -- independent of the PRSOV technology.


Cheers,

|-|ardy

[Jeroen Hoppenbrouwers]

If there is no valve that stops the LE flaps sucking air, they will not stop. The flow may reduce to near zero, and they may not run, but they still suck. Therefore there is no difference in the duct and the PRSOV won't feel a temptation to open again.


Jeroen

[dutch57]

Hardy Jeroen,

On the Le drive unit you have a shutoff valve opened by command extend/retract

this one in its turn allows a butterfly vlv to open and it will release the inernal pneumatic brake  and the airmotor will turn.

There is no air pressure regulator in the unit.
There is a overpressure switch which protects the airmotor for too high pressures.

In case a drive unit runs out of air it simply stops pneumatic brake will be set and butterfly vlv will close due to lack of air.

Think in duct rupture(happens in realtime) FCC senses a drive unit or group is not extending/retracting and will revert to prim electrik and latches the condition.

It does this by removing elec signals to the drive unit(s).

sincerly Bob

[Jeroen Hoppenbrouwers]

The question now becomes: if the duct pressure drops below the level that the butterfly valve closes, will this increase the duct pressure again so high that the butterfly valve opens again, causing some intermittent pressure variations in the duct, even if the LE flaps don't move because the pressure never reaches high enough?

This shuttling may stop when the FCC switch to primary electric and close the command valve.


Jeroen