OK, I have just got back in from experimenting with a 737-700. The aircraft was on the ground with the engines stopped and the APU running. Both packs were in AUTO, isolation valve OPEN and APU bleed ON. Note for non-NG pilots: You are supposed to run both packs from the APU when on the ground on an NG, unlike the classics where you are only supposed to run a single pack from the APU.
With both engine bleeds ON (normal config on a turnaround) the packs were running at the normal flow, APU EGT was 460C. I switched one engine bleed off – no change. Only when both engine bleeds were switched OFF did the packs go to high flow, APU EGT now 490C. Now, switching either engine bleed back ON made the packs return to normal flow.
I suspect that the Vol 2 (Page 2.10.4) is either wrong or badly worded when it says “When operating one pack from APU (both engine BLEED air switches OFF), regulates to high flow.” Because from the experiment it is also the case if operating both packs from the APU.
I think the EGT rise is also significant, because it is only a 30C rise for both packs to go from normal to high flow. I suspect that this is why the increase to high flow is scheduled (allowed) to happen because it is well within the capability of the APU. Remember that the Allied Signal APU on the NG’s is much more powerful than the classics and has no EGT limits.
Finally, bear in mind that the scheduling of flow from high to normal is dependant upon many other factors ie ground/air sensors; pack switch position; APU bleed valve; engine bleed switches; flap position and the position of the other pack switch. These factors can give many combinations, not all of which give predictable results in the flow rate. As a pilot I am happy to have the luxury of any AUTO switches in my flightdeck which make the necessary adjustments to bleed demands or whatever as required to save me from having to remember to do them, especially at times of high workload eg bleeds off take-off, EFATO etc.
S & L