AVSIG: JetA freeze concern? wwswsigarch.jpg (7236 bytes)

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AVSIG Discussion Sections >> Training & Proficiency

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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389339 - 04/11/14 12:29 PM Attachment (165 downloads)

Here's the fuel flow schematic I have. I don't see where fuel is recirculated right off.

Sorry, don't seem to be able to post the image directly.

Best,

Dave

Edited by Dave Siciliano (ADS) (04/11/14 12:31 PM)


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Mase Taylor
Top Gun


Reged: 04/29/04
Posts: 9446
Loc: SOCAL
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389340 - 04/11/14 12:43 PM Attachment (117 downloads)

Here ya go.



--------------------
Fly The Airplane As Far Into The Crash As Possible. - Bob Hoover 1922-2016 R.I.P.

Edited by Mase Taylor (04/11/14 12:44 PM)


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Jeff Hartmann CIC
Top Gun


Reged: 05/18/04
Posts: 7323
Loc: Chico,CA
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389343 - 04/11/14 01:10 PM

Dave,

Recirculation is a poor choice of words. From the 300/350 manual.

"A fuel-purge line is positioned at the output side of the high pressure pump, which constantly directs a small amount of fuel back to the gravity-feed line between the wing and the nacelle tank, to ensure the FCU stays clear of vapor and bubbles."

On the 300/350 if you have fuel in the aux and full wings, when you start it will automatically draw from the aux tanks, and the purge fuel returns to the wings. So, on a short hop the wings may overfill and vent overboard.

( you have read Tom Clements speak of pulling the aux breakers until you have flown a half hour or so, to prevent this. )

The medium blue line on your system. It goes through the purge valve and back to where it started from. On later planes they pick off the Fuel Flow just before the flow divider after the FCU.

Edited by Jeff Hartmann (EQY/SVH) (04/11/14 01:14 PM)


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Jeff Hartmann CIC]
      #389351 - 04/11/14 02:44 PM

Very helpful Jeff. Purge was what I was seeing. I'll take some time this weekend to look.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389427 - 04/13/14 11:01 AM

It turn out the blue line from my FCU is in fact a purge line and really doesn't return heated fuel to my tanks. Tom Clements was nice enough to provide this insight:

... In most 90s and 100s, it is only the start switch that activates this solenoid valve to open. Since the purpose of the purge is to make sure that the nozzle discharge of fuel is not interrupted with "air" bubbles -- more typically, fuel vapor -- during the start, and since someone realized that we can start a PT6 without using the start switch -- i.e., a windmilling airstart in which Auto-Ignition is used to activate the ignitors -- when the 200-series got designed Beech wisely also sent power to the purge valve when ignition OR starter was activated.

Best,

Dave


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Dave Siciliano (ADS)
Top Gun


Reged: 05/17/04
Posts: 8469
Loc: ADS (Dallas, TX)
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389428 - 04/13/14 11:03 AM

Since I have cold tanks, I was still puzzled as to why Jet A doesn't freeze at that temp. An engineer on Beechtalk added this:

The Latent Heat of Fusion is why it doesn't get that much colder, in any appreciable time.

Put a cup of water in the freezer. It will chill to liquid at 32F very quickly.
It takes 10x the heat removal to get the water from 32F liquid to 32F solid (ice).

Same idea at work, but the specific latent heat of fusion, in terms of kiljoules per kilogram (or BTUs per pound) is different than water, but the same general behavior is followed.

Best,

Dave


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Russell Holton
AVSIG Member


Reged: 07/07/05
Posts: 14136
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389432 - 04/13/14 02:43 PM

Yes, but percentage wise, how much of the fuel can you freeze before you run into trouble? The fact you've hit the freezing point says it's no longer a question of "if" but "when" before it gets ugly.

If it's like ice, what happens is the ice will form bit by bit as it's able to give up it's heat. It won't wait until you hit some jackpot and the whole thing flash freezes.

In the case of a fuel tank, I'd think it would start forming "ice" at the bottom (largest surface area connected to the outside air). But that's where the fuel intake is.


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
Posts: 1857
Re: JetA freeze concern? [Re: Dave Siciliano (ADS)]
      #389436 - 04/13/14 05:58 PM

>>Latent Heat of Fusion<< good point.

Mac


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Mac Tichenor (DAL)
Top Gun


Reged: 06/04/04
Posts: 1857
Re: JetA freeze concern? [Re: Russell Holton]
      #389437 - 04/13/14 06:01 PM

Russell -

I've been told that Jet A thickens starting a few degrees above its freezing point, theoretically giving a little advanced warning via fuel pressure readings.

Mac


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Randy Sohn
Gradn Exlated Ordre of teh Fyling Fingres


Reged: 08/31/01
Posts: 23187
Loc: Savage, MN - U.S.A.
Re: JetA freeze concern? [Re: Mac Tichenor (DAL)]
      #389440 - 04/13/14 06:41 PM

Quote:


I've been told that Jet A thickens




Mac, that's what I'd written above, the "gell point". Diesel truckers also encounter it often.

best, randy


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