737 aircon recirculation fans (fuel consumption?, hold temp?)

[cosmo kramer]

Looking in the MEL, there is no performance penalty if the recirculation fan is inoperative. That despite the system description says it lowers the bleed demand and hence fuel consumption.

For lack of better, the 737.org.uk website has the following information:
A pack in HIGH flow will produce more cold air than normal, but has a 25% higher bleed air demand ..... The recirc fan will switch off if either pack is in high flow, giving a net reduction in the ventilation rate of 15%, so best cooling is achieved with pack(s) AUTO and recirc fan(s) ON.

So the big question is does it lower the fuel consumption during flight at all?

Or does it only increase the cooling capability and possibly fuel consumption when a high cooling demand (normally on ground) is needed? Consequently, during flight with a low cooling demand, does it make a difference if it's on or not?

The other question is how the temperature in the forward hold (for our four-legged friends) is influenced by the recirculation fan? Would it be warmer, colder or no difference with the fan off?

Any sources for reading, experience with the matter or logical reasoning appreciated...

[STBYRUD]

The packs are unaffected in their output and operation by the recirculation fans and if they are on or off - the system has been designed for a certain air flow to the cabin which is supplied either by two packs and the recirc fans or packs in high flow, if the recirculation fans are inop there will be no change in fuel consumption but a reduction in overall air flow through the cabin.

[FCS Explorer]

The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand.

and

The recirculation fan system circulates air from the passenger cabin around the
lining of the forward cargo compartment. When the overboard exhaust valve is
closed, exhaust air from the equipment cooling system is also diffused to the lining
of the forward cargo compartment for additional inflight heating.


what the book doesn't say:

in addition to the already very poor cockpit ergonomics the recirc fan will deafen you and the uncontrollable air streams might give you a stiff neck.
:ugh:

thats why i like to switch it off. and the increased fuel flow? hey, this poor design is not my fault!
by the way: it's allegedly 20kgs/hour. just one of those numbers you're supposed to comply with - but it's not written anywhere and you can't look it up......

[cosmo kramer]

if the recirculation fans are inop there will be no change in fuel consumption but a reduction in overall air flow through the cabin.
But that is not what they write in the systems description. This is what the FCOM says:

The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand.

FCS, anymore info on the source of the alleged 20 kgs/hour? I completely agree that it sucks, and that is the reason for my inquiries, I would like to be able to defend turning it off.

The quote with "heating" in bold that refers to the exhaust air for the equipment cooling is obvious. But it still remains unanswered how the recirculation fan(s) contribute to the temperature of hold 1?

[HAWK21M]

Recirculation fan will only increase the motive force of the existing Airflow & NOT alter bleed requirements.

[FCS Explorer]

The recirculation fan system circulates air from the passenger cabin around the
lining of the forward cargo compartment. When the overboard exhaust valve is
closed, exhaust air from the equipment cooling system is also diffused to the lining
of the forward cargo compartment for additional inflight heating.

and

The lower cargo compartments are designed and constructed to satisfy FAA
category Class C compartment requirements. This means the compartments are
designed to completely confine a fire without endangering the safety of the airplane
or its occupants. The compartments are sealed and pressurized but do not have fresh
air circulation and temperature control as do the upper passenger compartments.


which brings us to my next question concerning the reliability of cargo hold pressurization in relation to the survival of AVIHs.

if there is no airflow into those compartments and they have to rely on those often-patched and battered cargo doors to hold the pressure....

[cosmo kramer]

Yes, but the recirculation fan(s) move air from the lining back to the cabin. So if the air in the lining is heated by the exhaust from the equipment cooling and we are then move it back to the cabin, are we then also removing heat in the process for the benefit of the passenger cabin?

Obviously, if we remove air from the lining to the cabin, more air will flow from the cabin to the lining as well. So the question is which air source is warmer. The air coming from the exhaust from the equipment cooling or air from the passenger cabin.

Hence, would the air in the lining be warner if it wasn't recirculated.


Regarding pressure:
A pressure equalization valve is in the aft bulkhead of each compartment. The valves let only enough air flow into or out of the cargo compartments to keep the pressures nearly the same as the cabin pressure.
It was just a few lines lower than what you quoted above.
So even with a leaking seal in the door, air should flow through the pressure equalization valve.

[FCS Explorer]

thanks:ok: