There is no difference in APU Fuel Flow with recirculation fans off.
However, there is a significant difference with one pack off (which I believe is common knowledge). The FF and EGT of the APU is
higher when running one pack, instead of both.
E.g. left pack off to reduce cockpit noise during briefing, telephone calls etc. For the same reason I demonstrate this by opening the APU maintenance page in the FMC, to teach my colleagues to switch off the left recirculation fan instead, which leads to a similar decrease in noise levels.
The reason for this discussion is there is a common misunderstanding, that there is a significant increase in FF if the recirculation fans are switched off during flight. As a example in the above case - if switched off on ground and forgotten to be turned back on. The argument against it is sometime: "But they are not of any checklist to be caught if we forget to turn it back on. We might be using a lot of extra fuel.
The misunderstanding lies in that people think the packs (in auto), recovers the lack of 50% ventilation: They do NOT.
50% ventilation will be missing in the cabin, leading to the stuffy air mentioned, cold air pockets, lack of oxygen (not deadly levels mind you, but enough to cause discomfort) etc etc. The total oxygen in the volume of air inside the aircraft is normal, but it won't be distributed evenly, because the air locally is not moving fast enough. There will be no increase in fuel flow on the FF indicators, as can be easily demonstrated by turning the recirculation fans off during flight. Outflow valve won't move either. There will be a slight pressure bump (as seen on the cabin vertical speed descending for a very brief moment), because the velocity of the airflow is changing. All other parameters remain the same through out the switching: Outflow valve position, bleed duct pressure indication and FF - not the slightest change!
Anyway, SUPPOSE we assume that DeFacto is right (which I do not think he is, for the record), but anyway lets ASSUME:
It doesn't change the fact of what I mentioned so far. Recirculation fans on, or off, there is no measurable change in fuel flow on the engines. The FF indicators measure to 10 kg. Hence, EVEN if DeFacto would be right, that it has a
tiny effect on the packs, it would be:
1) In the order of 5 kg
pr. engine
pr. hour (more would likely cause a rounding up/down in the last digit).
2) This TINY amount of fuel/increase in bleed air, would have NO WAY of recovering a 50% loss of ventilation.
3) Hence: The recirculation fans were NOT installed to save this TINY amount of fuel! They are installed to save the HUGE amount of fuel that would otherwise have been required, had the aircraft NOT been equipped with recirculation fans.
If they hadn't been installed at all, the packs would have needed to supply significantly more air to the cabin for ventilation, which would have resulted in a significant increase in bleed requirement and hence fuel. Otherwise the aircraft would not have met certification requirements for cabin ventilation:
FAR 25 section 831:
For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.
THIS is the point I have argued all along. Frankly, it doesn't matter if DeFacto is right (which for the record, I still believe he is not) if there is a tiny
insignificant increase in FF. In any case, all my points still stand.
