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B737NG “PACK” NNC

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B737NG “PACK” NNC

Old 18th Sep 2014, 11:46
  #41 (permalink)  
 
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I'm surprised nobody has mentioned the Overboard exhaust valve.
You're at 10,000ft, doing 290kts, outflow valve open, cabin heating up, what would happen to the Overboard Exhaust Valve if the right Recirc fan was turned off?
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Old 18th Sep 2014, 12:59
  #42 (permalink)  
 
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But similarly it reduces fuel by reducing output from the packs.
Not a direct correlation but an indirect effect of the system.
The myth just won't die.

I'll make a video on my next flight. Absolutely nothing happens to the fuel flow when turning off the recirculation fans. It does however get quieter, because the air is no longer being recirculated!
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Old 18th Sep 2014, 13:41
  #43 (permalink)  
 
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what would happen to the Overboard Exhaust Valve if the right Recirc fan was turned off?
It will open,as the right recic fan as the function of smoke removal and the air pulled by the exhaust fan (cabin and E/E) will go out...instead of being directed around the forward cargo bay for heating..

I'll make a video on my next flight. Absolutely nothing happens to the fuel flow when turning off the recirculation fans. It does however get quieter, because the air is no longer being recirculated!
Please keep your video,the fuel consumption is so small it is useless.
Air is not being recirculated but is circulating....packs provide cabin conditioned air via the passengers center and side walls of the cabin.

Let's just see what boeing engineering has to say.
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Old 18th Sep 2014, 13:57
  #44 (permalink)  
 
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Please keep your video,the fuel consumption is so small it is useless.
So what exactly are we discussing here?
Then you agree?
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Old 18th Sep 2014, 14:06
  #45 (permalink)  
 
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Agree to what?
I said from the very beginning,as we were discussing about the MEL not reflecting it as a fuel penalty because of the negligeable amount.
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Old 18th Sep 2014, 16:49
  #46 (permalink)  
 
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Agree to what?
I said from the very beginning,as we were discussing about the MEL not reflecting it as a fuel penalty because of the negligeable amount.
Ok, from the AMM:
Recirculation
This part of the A/C system recycles approximately 50 percent of the cabin air for ventilation purposes. This reduces the quantity of fresh air from the pneumatic system for ventilation.
Do you really think a "negligible amount" of fuel, can compensate for 50 precent lack of ventilation?
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Old 19th Sep 2014, 10:40
  #47 (permalink)  
 
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B737HT,

Here is the answer from BOEING ENGINEERING on your PACK NNC question:
In the PACK NNC , the effect of the recirculation fans is optimized if only the right fan is operating because the fan motors are a heat source.

The left recirculation fan draws air from the area immediately around the mix bay while the right recirculation fan draws air from the "horseshoe duct" beneath the forward passenger cabin. Operation of the right recirculation fan promotes airflow through the grilles in the forward passenger cabin and better ventilation is achieved with the right fan than with the left.

Also,

The step to turn off the left pack may soon be removed as it already has been removed from the master QRH for the -800 that Boeing uses. Not sure when that will be sent to the operators.
Basically what i wrote in my post #3.

Now concerning the MYTH,non sense FCOM misinterpretation of extra fuel consumption by turning OFF the recirculation fans that Cosmo keeps on blabbing about,here what BOEiNG ENGINEERING via General ELECTRIC have to say:
According to GE and our flt ops engineering department there will be a slight increase in TSFC when the fans are turned off due to increased bleed air demand from the engines. Just as you expected.

Best regards,
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Old 19th Sep 2014, 15:09
  #48 (permalink)  
 
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Just out of idle (or morbid) curiosity, how many more pages do you guys plan on keeping this up?
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Old 19th Sep 2014, 17:34
  #49 (permalink)  
 
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What a credible source.. Just as credible as mine:
Dear Cosmo,
Just as you expected, there is of course no increase in fuel consumption with the recirculation fans off. They are only there to increase ventilation in the cabin. They do not add to the pressurization of the aircraft when ON, hence when turned OFF there is no change in the bleed demand.
Best regards
Your Boeing engineer
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Old 20th Sep 2014, 06:18
  #50 (permalink)  
 
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The difference being that the source of De facto's quotes are easily believable whereas it's hard to tell if you are purporting to present a real quote, or if you are being facetious and openly making up a pretend quote.
Which is it Cosmo? Have you been stumped by Boeing and GE saying there is an increase in TSFC and made up a "joke quote" in response or are you saying that your quote is genuine?
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Old 20th Sep 2014, 18:17
  #51 (permalink)  
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Got it. Additionally, can we monitor the BITE test page of APU fuel consumption with recircle fans on/off on ground to check out you guys' discussion ?
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Old 20th Sep 2014, 19:31
  #52 (permalink)  
 
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Good idea B738HT, but that does not take into account the mentioned pressurisation of the aircraft having an effect on the bellows...
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Old 20th Sep 2014, 23:57
  #53 (permalink)  
 
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There is no difference in APU Fuel Flow with recirculation fans off.

However, there is a significant difference with one pack off (which I believe is common knowledge). The FF and EGT of the APU is higher when running one pack, instead of both.
E.g. left pack off to reduce cockpit noise during briefing, telephone calls etc. For the same reason I demonstrate this by opening the APU maintenance page in the FMC, to teach my colleagues to switch off the left recirculation fan instead, which leads to a similar decrease in noise levels.

The reason for this discussion is there is a common misunderstanding, that there is a significant increase in FF if the recirculation fans are switched off during flight. As a example in the above case - if switched off on ground and forgotten to be turned back on. The argument against it is sometime: "But they are not of any checklist to be caught if we forget to turn it back on. We might be using a lot of extra fuel.

The misunderstanding lies in that people think the packs (in auto), recovers the lack of 50% ventilation: They do NOT.

50% ventilation will be missing in the cabin, leading to the stuffy air mentioned, cold air pockets, lack of oxygen (not deadly levels mind you, but enough to cause discomfort) etc etc. The total oxygen in the volume of air inside the aircraft is normal, but it won't be distributed evenly, because the air locally is not moving fast enough. There will be no increase in fuel flow on the FF indicators, as can be easily demonstrated by turning the recirculation fans off during flight. Outflow valve won't move either. There will be a slight pressure bump (as seen on the cabin vertical speed descending for a very brief moment), because the velocity of the airflow is changing. All other parameters remain the same through out the switching: Outflow valve position, bleed duct pressure indication and FF - not the slightest change!

Anyway, SUPPOSE we assume that DeFacto is right (which I do not think he is, for the record), but anyway lets ASSUME:

It doesn't change the fact of what I mentioned so far. Recirculation fans on, or off, there is no measurable change in fuel flow on the engines. The FF indicators measure to 10 kg. Hence, EVEN if DeFacto would be right, that it has a tiny effect on the packs, it would be:

1) In the order of 5 kg pr. engine pr. hour (more would likely cause a rounding up/down in the last digit).

2) This TINY amount of fuel/increase in bleed air, would have NO WAY of recovering a 50% loss of ventilation.

3) Hence: The recirculation fans were NOT installed to save this TINY amount of fuel! They are installed to save the HUGE amount of fuel that would otherwise have been required, had the aircraft NOT been equipped with recirculation fans.

If they hadn't been installed at all, the packs would have needed to supply significantly more air to the cabin for ventilation, which would have resulted in a significant increase in bleed requirement and hence fuel. Otherwise the aircraft would not have met certification requirements for cabin ventilation:
FAR 25 section 831:
For normal operating conditions, the ventilation system must be designed to provide each occupant with an airflow containing at least 0.55 pounds of fresh air per minute.
THIS is the point I have argued all along. Frankly, it doesn't matter if DeFacto is right (which for the record, I still believe he is not) if there is a tiny insignificant increase in FF. In any case, all my points still stand.
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Old 21st Sep 2014, 00:01
  #54 (permalink)  
 
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Seriously, nobody cares.
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Old 21st Sep 2014, 00:30
  #55 (permalink)  
 
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Yet, you continue to comment. If you don't care, you are free to click on another subject.
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