Classic AC operation

[gnelson22]

Many times during hot wx ops I notice a large difference in L/R duct pressures. I am under the immpression this indicates a pack valve is freezing up (the one with the higher duct pressure) and restricting flow and the mix valve should be driven warmer. Anyone have any input on this?

[PlaneTruth]

gnelson22 ,

Are you referring to the 737 "Classic"?

First, the 737 Classic should not use both packs on the ground except for heating. (The NG aircraft will freeze your socks off with both packs on in summer. It be much mo-bettuh. ;))

The pack valves on the 737 are electronically controlled and pneumatically operated. The bleed air that these valves control is 400+ degrees. By "Freezing" I assume you mean sticking. I have heard of this happening on virtually all the pneumatic valves in the 737 as age, coking and wear take it's toll.

The side with the pack on will show 15-25 PSI depending on airport elevation. The side with no pack on should show 30 PSI - 1.2 PSI per 1000 feet above Sea Level. If it is showing less than the other side when both packs are off there is a leak in the manifold structure or through the inoperative pack valve (as you said, it's stuck partially open and some pressure is leaking out) Play with the ISOLATION VALVE. Isolating the two sides will point toward the side with the leak.

Hope this helps!

PT (The Banned One)

:ok:

[mono]

Hmmm,

I'm assuming that both engines are running and the isolation valve is closed. If the engines are at idle then I think it could either be a leak as PT states or quite simply that the packs are supplying a different demand.

I'm not sure which variant you are refering to (-300/400/500) but the left pack supplies in the main the flight deck direct and recycled left and right supplies the cabin (fresh air for the drivers the rest get the dregs:= )

As the cockpit is a smaller area demand temperature is reached a lot quicker so, assuming hot OAT, for the left pack the mix valve will be partly open (some bleed air used to operate the air cycle machine AND some bypassing to achieve an output temp close to the desired temp). The right pack however is more likely to have its' mix valve closed and the ACM working harder as the combination of a high OAT, large area and lots of heat producing passengers. This air path is likely to produce more resistance and thus less mass flow.

Therefore it is possible that the left duct pressure will be lower than the right due to the different flow rates through the packs.

Conversely when operating when OAT is very cold the opposite may be the case.

If you get a little time on the ground play with the temp control knob for each pack and watch the duct pressure rise and fall as pack demand varies.

I have operated packs on the ground with OAT at -25 and have been lucky to get 10 psi duct pressure from the APU and not much more from the engines!! Shut the packs off however and it will climb to 30 psi.

This may be what you are seeing. It could be a load of :mad: of course.

:D