The bleed air is the "source of gain" Claus. Agree with Joe, the exterior air are heat exchangers only.

 

The bleed air source is like a river and you only need a creek to keep the cabin pressurized. The "little" turbochargers on our TSIO-520s pressurize the intake manifold of the engines and our cabin and we have a heat exchanger that of course we block on cool nights since our bleed air is much cooler than that coming off of the CFM-56.

So that is what I would have called it if I was a manufacturer... B)

(replacing "pneumatic" with "pressurization" and "cycle" with "conditioning" though)

 

  Now that's understatement!

 

 

 

Correct me if I am wrong but:

the variable air inlets of conditioning packs are used for (cold) outside air supply and so actually "pump up" the cabin in flight, while the outflow valves just control the overall cabin pressure (delta PSI). So IMO PACKs (or Packs) are the initiators of pressurization actually...

(And engine bleed air (about 200°C?) is just taken/added for hot air supply/mix to regulate the temperature)

 

All the air for pressurization comes from bleed air. There is no ram air for pressurization on the 737. You take some bleed air and cool it with the packs and mix it with the hot bleed air to get the temp you want.

Well, you may try to regulate your cabin pressure via the outflow valve only, but I would suggest to stay below FL100, because otherwise you will get a CABIN ALT WARN.

If the cabin is not supplied with "fresh outside air" via the PACKs, the cabin climb rate will maintain to be approx.100 to 150 feet per minute.

So from T/O to FL360 CRZ you will get the 10,000ft CAB ALT ALERT after approx. 30 minutes, and the "OXYGEN MASK light will illuminate" at 14,000ft CAB ALT after one hour in flight.

 

It is all logical: one cannot regulate the pressure in an unclosed system just by regulating the loss. There also must be some source of gain...

 

BTW: when at CAB ALT 14,000ft turning the PACKs on will give you a CAB descent rate of approx. 1,500ft per minute bringing your PAX back to a safe life environment in about 2 minutes...

 

B)

The packs are part of the air conditioning system. They provide more than enough air for the pressurisation system to control cabin pressure. Pressure is controlled by the outflow valves.

 

The packs don't pump any air. All the air comes under pressure from engine bleeds. Of course if you shut the packs off there is no air supply to the cabin but the same happens if you close the engine bleeds. Are they part of the pressurisation system too?

The bleed air is the "source of gain" Claus.

 

All the air for pressurization comes from bleed air.

 

All the air comes under pressure from engine bleeds.

Thanks gents!  Now I got it...  B)  

 

There is no ram air for pressurization on the 737.

The two blue signs "RAM DOOR FULL OPEN" above the 737 PACKs panel probably mislead me here in thinking they are fed with variable ram air inlets for pressurization ("input" source). Maybe the DC-8 did it like this?

 

Of course if you shut the packs off there is no air supply to the cabin but the same happens if you close the engine bleeds.

Are they part of the pressurisation system too?

I checked it in PMDG NGX and T7: FL360 CRZ with PACKS off leads to approx. "150 ft/min cabin climb rate" in the T7 and "500 ft/min cabin climb rate" in the NGX. Maybe the T7 cell is more thight than the one of the 737NG?

In both planes, if the outflow valve is fully closed and PACKs are off, the cabin will lose its pressurization completely (delta=0 PSI ) because of a "built in leakiness".

 

The engine bleed air actually feeds the packs: no bleed air, no cabin ("re-") pressurization via the packs (that is what I've learned here).

Consequently IMO the bleed air system (though a separate system in itself) is a part of the pressurization system: bleed+packs+outflow valve. But maybe that is a matter of taste.

 

Thanks to all for clearing this up. Was interesting to "see all the oxygen masks dropping"... :blink: