re from captain87
Hello everyone ! ...
7triple
The APU flow rate is not always LOW ... not for all the aircrafts ...
According Airbus system philosophy , the APU has been set to deliver always HI flow to the packs. Besides HI flow is automatically selected by the remaining BMC (Engine 2) in case of one engine failure. At this purpose I've to say that what you stated according the APU Flow is strictly relative. Obviously if we select the Pack Flow manually to HI, we've a flow greater than APU accordingly. I'd like to remind that "APU is a generator and the engine has a generator (IDG's)" ... the purpose of the APU is to reduce the engine workload and acting as a "backup" in such normal and abnormal situations. Said that, I suppose that there shouldn't be any substancial differencies in terms of efficiency.
The APU has some operating limitations for bleed and electrical power supply. The first one (bleed) is often more restrictive according altitude.
Best Regards
by captain87