Nov 17, 2009

Something for Nothing...

Log Entry 1975 - B727 Second Officer-in-training - CYUL - Departure Gate


When I was training as a Second Officer on the Boeing 727 just a few years ago, I learned a neat thing about airflow entrainment. The Trusty Trijet has two air conditioning systems, called "packs." In flight pack 1 draws bleed air from engine 1 and pack 2 bleeds engine 3. Then using the black magic principles of expansion and compression and so on these packs pump air of the desired temperature into our cabin. So far so good.

But what if we're on the ground and the engines are not running? Then we use the auxilliary power unit's (APU) bleed air to do the job. Trouble is one little APU cannot match the flow of two big JT8Ds - the APU just can't produce that much bleed air. So, (now this is the neat part) Boeing engineers equipped the system with a Flow Multiplier. They use the high velocity bleed air from the APU to entrain ambient air from the wheel well and Voila! We now get enough "bleed" air to run two packs. Amazingly, the bleed load on the APU with both packs running is less than with just a single pack.

One surprisingly hot spring day early in my training I was sent on a fam-flight. I had just finished the technical "ground school" part of the 727 course and was awaiting to pass through the bottleneck at the flight simulator portion. Fam-flights were great. We would tag along for a day with an operating crew and see how it all works. Of course not all the line pilots were equally good examples of how the job is to be done. But it sure gave a student the context for how all the parts should finally fit together.

This afternoon we're on the ramp in Montreal and it's a scorcher. The outside temperature is in the mid thirties Celcius. We board the aircraft just ahead of the passengers to discover the interiour is like a suana. Our winged aluminum tube has been sitting in the sun for over an hour baking because the unexpected hot weather caught everyong by surprise. Most of the airport's ground air conditioners are still in winter heating mode wating to be changed over to summer cooling. Whatever the excuse the cabin air is unbreathable.

The pilots shed their formal "blues" and get down to the ramp checks so we can depart on time. I try to be inconspicous as I settle into the observer's seat behind the captain's chair, across from Second Officer Milo. His first priority is starting the APU so he can then get the packs running. The procedures for checking the APU fire protection systems seem to take forever but finally the APU is started and stabilized. Milo flips on the first air conditioning pack watching the electrical load surge as the pack's cooling fan kicks on, followed by the APU exhaust gas temperature needle rising. There is a weak gush of hot air from the overhead grills and a slight change in the background noise. But suddenly the flow stops and the flight deck gets quiet again. The EGT has rapidly spiked to its top limit and the APU's self-protection circuit has tripped the pack off to prevent damage. Milo looks puzzled and goes through the reset drill and tries again. Same results.

Meanwhile the passengers were boarding, the cabin is getting hotter still, and the purser is calling out from the flight deck door - "Air! We need air back here fellas!"

Someone suggests as diplomatically as possible to SO Milo that he might try throwing on the second pack immediately after the first, "So the flow multiplier will kick in and reduce the bleed load..."

"Really?" he asks looking puzzled. He is reassured that this should work. So this time Milo flips on the first pack ... again a weak flow of hot air trickles from the vents and the EGT needle rockets towards the redline. As soon as the electrical surge of the first cooling fan subsides Milo tentatively flips on the second pack. And almost miraculously as the demand for even more bleed air awakens the flow multiplier, the APU EGT slows, stops its climb, and actually reverses down to sit in the middle of the green band. A welcome whoosh of air pours from the vents along with the happy roar of contented packs. In just a few minutes the icy air is having the desired effect. Everyone regains their true cool and we depart on time.

Since then I've always wondered what a flow multiplier looks like and how it works. Today I came across a new consumer product which employs the same principle. I'm sure the ole B-727 flow multiplier did not look nearly so elegant, but it sure did do the trick.

Something for nothing. Now there's a bargain you don't see everyday.

3 comments:

Curt Sampson said...

Are you sure that the B727 flow multiplier and this work on the same principle?

My guess would be that "flow multipler" is a generic term, by the use of it to describe the Rolls Royce LiftSystem designed to power VTOL jets.

From the system description you posted, which mentions "a turbine and compressor assembly called a flow multiplier," and references on this list of B727 SBs that mention a "FLOW MULTIPLIER COMPRESSOR OUTLET" and "FLOW MULTIPLIER BEARING COOLING", it sounds to me as if the flow multiplier is another, fairly normal compressor driven by the APU turbine. The issue I would guess is that the APU has plenty of power to push enough air for both packs, but the bleed air system alone can't supply enough volume of bleed air.

That said, the new Dyson fan looks extremely cool. I wonder how noisy it is?

Aluwings said...

Hi Curt,

In fact I never saw the mechanical workings of a flow multiplier. I was told that it was a method of using a low volume, high velocity stream to entrain a much higher volume of airflow, albeit at a reduced velocity.

I could imagine this involving a mahineless air-compression expansion technique, but some sort of mechanical turbine would also work. Or even a combination of the two?

Thanks for your insights.

Curt Sampson said...

Well, as I said, flow multiplier appears to apply to many things. If that particular one does indeed use entrainment, it seems unlikely it's just another turbine driven by a shaft, as with the LiftSystem.

I'm rather curious about this now, but some searching on the net came up with no more details, so I suppose we'll have to wait for someone more familiar with the system comes along, or for someone to go find the paper documents describing this that must exist somewhere.

Thanks for a quite interesting article!