A320 apu
 

If you look at the APU schematic, portion of the ram air goes into the load compressor than is directed either for pneumatic supply if the bleed control valve is open or vented into the exhaust if the bleed control valve is closed. So essentially the APU always provides bleed pressure. The BCV only controls the direction to which it is going. What I am wondering is if there is supporting evidence that the APU would burn more fuel with the APU BLEED in the ON position. I would think it is the same since RPM and bleed pressure from the load compressor is constant. My airline claims it burns less fuel. They may certainly be right but it doesn't make sense to me. Any inputs/sources?

You're forgetting the variable IGV's. When APU bleed is selected off, the IGV actuator will move the IGV's toward closed (unloading the load compressor), so the beast doesn't have to work so hard. (the FCU won't need to schedule as much fuel to maintain rpm)

Very good point. I didn't think about that. Does anyone have an idea as to the fuel burn per hour with APU bleed ON versus OFF?

Saving is 23kg/hr with APU Bleed Off on the ground. A319.

Cheers
mcdhu

Your fcom 2 gives you all information you need

Michelda

By the shear reduction in sound i can tell that less energy is being used.

Apu fuel data
 

hey there ,as per what have heard from my company engineers and tech guys the fuel burn on the apu will depend on bleed demand and electrical load sharig ..now generally is nething between 50kgs to 150k/hr depending if we using ne of the bleed/electrical equiptment.

if we use the bleed demand from the apu then the load causes more fuel to be comsumed , hence we can either carry out a packs off t/o and save fuel and other thing we guys do is switch off the apu bleed pb when the a/c is parked on the ground for more that 2 hours of ground time at base stations and attach an external air cond cart or lp supply for cond....