saoool
Posts: 9
Joined: Tue Sep 25, 2001 5:02 am

Recirculation Fans?

Mon Jan 06, 2003 5:37 am

What is the purpose of having recirc fans ? Seems to me they will recirculate the cabin air through the air conditioning plumbing and then back into the cabin and if passengers have a cold, the contaminated air will be recirculated also for all other passengers and crew to breath. What are the benefits of recirc fans ?
 
LMP737
Posts: 4800
Joined: Wed May 08, 2002 4:06 pm

RE: Recirculation Fans?

Mon Jan 06, 2003 5:47 am

The purpose of recirculation fans is decrease the use of engine bleed air. If you didn't recirculate some of the air would have to use engine bleed air for all of your cabin air needs. That would increase fuel burn which drives up your costs. The recirculated air goes through HEPA filters to remove airborne particulates.
Never take financial advice from co-workers.
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Recirculation Fans?

Mon Jan 06, 2003 6:01 am

The recirc fans move the air through the cabin , other wise it would be kinda stale without any air motion. Something has to drive the air through the punkah louvres. Also, not all a/c have filters for recirc air.
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: Recirculation Fans?

Mon Jan 06, 2003 7:23 am

The air passing through the pressurisation outflow valves and lav & galley vent system will create airflow.

The flight deck does not receive re-circulated air is is 100% fresh from the packs.
 
BR715-A1-30
Posts: 6525
Joined: Thu May 30, 2002 9:30 am

RE: Recirculation Fans?

Mon Jan 06, 2003 8:35 am

If you want a plane with 100% fresh air, fly on the 717. Just before takeoff, you can hear fresh air rushing above the overhead bins going through the A/C packs. This happens just as the pilot throttles up. Sounds cool, and the air suddenly gets COLD!!!
Puhdiddle
 
ScooterTrash
Posts: 553
Joined: Thu Aug 02, 2001 10:39 am

RE: Recirculation Fans?

Mon Jan 06, 2003 11:03 am

VC-10:

It is not true that all flightdecks do not receive recirculated air from the cabin. The Dash 8 I fly does, as did the ATR42/72 and so did every other turboprop or jet who's environmental system I was ever familiar with. I'd love to know which aircraft have flightdecks have 100% fresh air... It would be SO NICE not to have to smell that lav when it reeks!

As far as the packs creating airflow, that depends on the airplane. The flow from allot of packs is pretty darn weak (which is why flying during the summer with MEL'd recirc fans sucks so bad!). It sure is nice to have that extra kick form the recirc fan(s)!

BR715:

I am not familiar with the B717 environmental system, but I would imagine the surge you feel is due to increased bleed air output from the engines during a bleeds on takeoff (engines spin faster = more bleed air = more cold air from packs).
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: Recirculation Fans?

Mon Jan 06, 2003 11:35 am

Scootertrash,

A/c with fresh air to Flt Dk - VC10, 1-11, DC-10, 747, A340....I could go on.

I didn't say the packs create airflow, look again
 
airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: Recirculation Fans?

Mon Jan 06, 2003 1:00 pm

The recirc fans move the air through the cabin , other wise it would be kinda stale without any air motion. Something has to drive the air through the punkah louvres.

This is a erroneous statement unless you are talking about an HS-748 or similar. The 748 has a ground circulation fan that isn't related at all to modern recirc fans.

The recirc fans allow for less airflow from the air conditioning packs by recycling air from the cabin and cockpit (some aircraft may vary) and adding it to fresh pack being delivered by the pack. This increases the efficiency of the system by decreasing the demand of bleed air from the engines. Furhtermore, depending on the aircraft

The result is that the air is somewhat "stale" compared to older aircraft that don't utilize recirculated air.

As far as health is concerned, a 727 used to replace the air in the cabin every 5 minutes. A 757 does it every 10 minutes. That means that you have to re-breath some guys sneeze about twice as long in a 757 as a 727. Of course some filtering helps out. This sounds bad until you compare it to the average bus. I don't think the air gets changed in a bus or a house or an office building anywhere near as often!




 
ScooterTrash
Posts: 553
Joined: Thu Aug 02, 2001 10:39 am

RE: Recirculation Fans?

Mon Jan 06, 2003 1:44 pm

VC-10:

Interesting... so in those aircraft you mentioned, NO recirculated air makes it back to the cockpit? In the aircraft I have been associated with, air from the packs is mixed in a plenum with recirc air, then routed to the gaspers/vents in the cockpit.

True you did not say that the packs created the airflow. I was thinking about how anemic the airflow is with only packs and the outflow valve(s) causing movement of the air. I'm sure we can agree that without recirc fans the air would feel stale, at best. It is positively miserable in the Dash!
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Recirculation Fans?

Tue Jan 07, 2003 2:00 am

Airplay, my statement about recirc air and punkah louvres comes from the dash 8, where the only air supply to the "fresh air vents" is recirc air. Air from the packs goes to the floor or ceiling vents, then into the cabin, through the radio racks for cooling purposes, and then is dispensed as "fresh" air to the punkah louvres. Air from the recircs is also fed to the condenser mixer to augment flow from the packs. As Scootertrash says, a dash with the recirc MEL'd is pathetic. Especially since a Dash in perfect working order still has a poor air conditioning system. I'm sure each a/c has some differences, for sure the wondrous Dash 8 has several variations itself!
 
747Teach
Posts: 176
Joined: Wed Nov 29, 2000 3:05 am

RE: Recirculation Fans?

Tue Jan 07, 2003 2:18 am

Saool: For the 747-200 aircraft, the recirculation system "provides a method for supplementing the airflow to passenger and flight compartments to improve ventilation." 747-200 AMM, Chapter 21-25-00, page 1. In the 747, there are four recirculation fans, 3 in the overhead above the main deck passenger compartment for Zones 2, 3, and 4, and 1 in the left sidewall lining of the forward cargo compartment for Zone 1, the flight compartment. All of these fans discharge into their respective plenum or duct system. So the air in all zones, including the flight compartment, is a mixture of air conditioned air from the packs, and recirculated air from the cabin. The fans have large filters to remove particulates. The normal airflow through the cabin in the 747-200 is about 10,000 cubic feet of air per minute, of which 8000 is fresh air and 2000 is recirculated. And this airflow is changed out about every 5 minutes. Regards,

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