You are right Skyjob, to anyone reading this thread please accept my apologies of being sucked into discussing with that moron...
For the benefit of other reading the thread, a bellows is a small pressure sensitive bag. The 737 is a crude and much mechanical aircraft. And so is a bellows. Where exactly these are located, I won't play wise and say that I know. They could be on the pack in the pack bay and use outside air pressure to deduct the expected cabin altitude, or the could be remote from the pack (doubt it as according to DeFacto's quote, the bellows is connected with the servo spring).
Even if we were to assume that they are somehow sensing the pressure in the cabin, like DeFacto claims, it doesn't change a damn thing.
Since recirculation fans have
nothing to do with pressurization, which I mentioned several posts back already. On or Off, the pressure remains the same in the cabin, outflow valve won't change position. Because the recirculation fans are not adding any air mass to the inside of the aircraft, neither are they removing anything when off.
In the most basic terms, the recirculation fans are like buying two standing fans, and placing them in the aisle of the aircraft. They are just there to keep the air in motion in the cabin.