|Quoting N231YE (Reply 2):|
Interesting that Boeing is making a move with fresh air ventilation (not really mentioned here). Remember, it was Boeing who introduced the recirculated air system starting with the 757, because it saved fuel. If the 787 is all about efficiency and fuel savings, I wonder why Boeing would revert to the old way of doing things?
|Quoting Stitch (Reply 4):|
The HVAC systems on the 787 are electric instead of pneumatic, so fuel consumption may not apply (or the fuel needed to drive the generators is less then the fuel needed to recirculate pneumatic air).
In general you can put it pretty simple:
The pressurization of the 787 will be achieved by electrically driven compressors (?). In case they are shaft driven from the engine gear box it will be the same principle, some degree of power is taken off the rotating engine.
The recirculation fans are electrically driven. So there is no difference in how the power for them is produced.
Nowadays they use compressed air taken out of the engine before it gets into the combustion chamber. That however increases the required flow of air quite a bit and reduces the engines effectivity far more because the stoichiometric (ideal) air / fuel ratio in the combustion chamber is negatively affected. With no bleed air taken it will be possible to have this ratio and thus the engine performance always at the optimum level for the required power / thrust output.
So if you then have to decide whether to recirculate air or whether you supply only fresh air and both would happen with the same amount of energy, why not dump the old air and get fresh air only? Performance is not affected since it's the same amount of (electrical or electrical in combination with mechanical) power requirement.
The really critical question will be how much the engine manufacturer will be able to optimize the engine with no bleed air and how reliable the compressors will be. I can only assume that they use a principle that guarantees pressurized air as long as any engine is running with a kind of fail safe idea. Perhaps radial compression?
In general I assume that it will be hard to tell the difference right away as a passenger on board but in combination with the lower cabin altitude I think that you will feel a lot fresher at the end of your journey, not necessarily knowing what you owe it to.
And then, just not to let you think I know all this for sure: That's all just an educated guess. Details are to be asked directly from Boeing...