QANTAS747-438
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Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 4:45 am

With respect to the recent FAA reccommendations on the 777, what is the fuel/oil heat exchanger and how does ice buildup affect things?
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
Fly2HMO
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 5:01 am

Simply put, the fuel, which is usually pretty cold, takes heat away from the oil thereby cooling the oil, and in the process the fuel warms up. So its a win win situation since you're killing two birds with one stone.
 
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jetmech
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 5:03 am

Quoting QANTAS747-438 (Thread starter):
what is the fuel/oil heat exchanger and how does ice buildup affect things?

The Fuel Cooled Oil Cooler (FCOC) / Fuel Oil Heat Exchanger (FOHE) exchanges heat between the engine oil and fuel streams. Heat is exchanged in such a manner that the engine oil is cooled whilst the fuel is heated up.

The reason for cooling of the oil is reasonably self explanatory. The fuel needs to be heated up as inevitably, Jet A1 contains a small amount of suspended water. During cruise, the fuel temperature can easily go below the freezing temperature of water. When this happens, the suspended water will freeze into solid ice particles.

If enough of these ice particles come together, large chunks of ice can form in the fuel. These chunks of ice can accumulate in filters, engine controls and other fuel system components causing blockages. The heating of the fuel by the FCOC / FOHE is designed to prevent ice formation by melting any solid water particles.

Ironically, in the case of the BA-777, it appears that ice caused a partial blockage in the FOHE, which lead to restricted fuel flow. You can see the FCOC in this picture of an RB-211.

http://www.enginehistory.org/G&jJBro...tt/RAFCosford/Rr%20rb-211-22-c.jpg



The FCOC is the component that is located in a straight line above the vertical member of the yellow engine stand. The oil and fuel make two pases through the FCOC, which has a low pressure and high pressure side.

Regards, JetMech

[Edited 2009-03-11 22:18:39]
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BMI727
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 5:22 am

Like in all engines, the oil needs to be cooled. The fuel spends its time sitting in the wings and gets rather cold. The fuel is just used because it is a handy heat sink for the oil. Generally, it is more important to cool the oil than to heat the fuel, but heating the fuel is useful too. It would be possible to have a separate cooling system, but it would be heavy, complex and just stupid. The FOHE system is really a pretty nifty piece of engineering.
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QANTAS747-438
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 7:13 am

Thanks for the answers. So let's say that ice IS building up... how much could actually emerge to the point where it would be a blockage? Wouldn't the immense volume and speed of the fuel flow break it apart in or out of a filter?
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
Tristarsteve
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 10:04 am



Quoting JetMech (Reply 2):
You can see the FCOC in this picture of an RB-211.

That is a pretty old RB211!
It must be some sort of prototype. Look at the hot stream reverser.
I think these were all removed before the first TriStar entered service.
Also there are no reverser hand turning points or external lock out holes. When I worked for GF we sometimes loaned an engine from RR which was like this. If you had a reverser failure you had to climb in to the Nbr 1 and 6 gearboxes in the cold stream to hand wind it closed ( after removing a blocker door to get in) Very awkward! I did it once.

Quoting BMI727 (Reply 3):
Generally, it is more important to cool the oil than to heat the fuel, but heating the fuel is useful too. It would be possible to have a separate cooling system,

The Trent 800 has a separate cooling system as well as the FOHE. The FOHE is basically uncontrolled. All the oil passes all the fuel all the time, except during engine start at low temps when the oil is bypassed.
There is also an AOHE. This is controlled by the FADEC. As the oil gets hot, a valve opens and passes bypass air over the oil flow to cool it further. This valve modulates to control the oil temp, but if it fails, it fails open. You must ensure that the oil is cooled.
 
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jetmech
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 10:55 am



Quoting QANTAS747-438 (Reply 4):
Wouldn't the immense volume and speed of the fuel flow break it apart in or out of a filter?



Quoting BMI727 (Reply 3):
but heating the fuel is useful too.

It may need to be looked at more carefully according to this Flightglobal article.

http://www.flightglobal.com/articles...t-800-fuel-oil-heat-exchanger.html

Quoting TristarSteve (Reply 5):
If you had a reverser failure you had to climb in to the Nbr 1 and 6 gearboxes in the cold stream to hand wind it closed ( after removing a blocker door to get in) Very awkward! I did it once.

I do not envy the poor mechanic who had to do that! It felt more than dodgy enough climbing into a RR T/R when all the safeties were in place!

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
ex52tech
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 1:16 pm



Quoting QANTAS747-438 (Reply 4):
So let's say that ice IS building up... how much could actually emerge to the point where it would be a blockage? Wouldn't the immense volume and speed of the fuel flow break it apart in or out of a filter?

I would have to speculate and be able to answer your question due to the many variables and different makes of engine. It would depend on how cold the fuel is, and how much ice has accumulated in that fuel, type of engine and how it is equipped.

Most engine's fuel filters could go into "by-pass", and the icy fuel would go around the filter, only to possibly cause trouble down stream. Some engines have fuel heaters that supplement the fuel/oil cooler, those fuel heaters use high stage compressor air to heat the fuel, and a pressure differential system to alert the crew that they have ice in the fuel.

Most airplanes main fuel systems are a high volume, low pressure systems, and that volume is dependent on the power settings of the engine. The point in the flight where you will have the highest volume moving through the system would be when the fuel is warm, and has not been cold soaking at altitude.
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Tristarsteve
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 2:56 pm



Quoting JetMech (Reply 6):
It felt more than dodgy enough climbing into a RR T/R when all the safeties were in place!

We had an incident last year that brought it home to me.
B767 RB211-524H. Aircraft arrived on gate with a T/Rev defect.
The technician was not very familiar with the engine. He went in the flight deck and selected T/Rev. The reverser deployed. There was enough air trapped in the ducts to operate one cycle aft. He then selected fwd and nothing happened, no air left.
To operate the air driven T/Rev, you need to run the APU and open the PRSOV. I do this by taking the plug off the engine start valve and selecting engine start. So when I arrived at the aircraft, I did this and the reverser translated fwd again.
I had always worked on the premise that if the APU was shut down the T/Rev was safe. It is not!
 
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HAWK21M
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 4:00 pm

http://www.tpub.com/content/aviationmaintandmisc/TM-1-1500-204-23-3/img/TM-1-1500-204-23-3_92_0.jpg

A unit that has seperate passages for Oil & Fuel to be routed but pass over each other without mixing.
The Idea is to warm the fuel to prevent icing & simultaneously Cool the oil by the cold fuel.
regds
MEL.
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QANTAS747-438
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 7:54 pm



Quoting JetMech (Reply 6):
It may need to be looked at more carefully according to this Flightglobal article.

http://www.flightglobal.com/articles....html

Wow, the photo in that article is amazing! The filter is just as I pictured it to be like, but not that kind of ice accumulation. It looks like it hailed in there.
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
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jambrain
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Thu Mar 12, 2009 8:10 pm



Quoting QANTAS747-438 (Reply 10):
Wow, the photo in that article is amazing!

did you mean this
http://www.ntsb.gov/Pressrel/2009/FOHEface.jpg

IIRC this photo was from a rig test where fuel with excessive water content was cooled to below -22C, not a real engine removal.
Jambrain
 
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jetmech
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Fri Mar 13, 2009 6:20 am



Quoting TristarSteve (Reply 8):
I had always worked on the premise that if the APU was shut down the T/Rev was safe. It is not!

The one thing I always did if I had to get into a RR T/R with air on the ship, was turn the air supply shutoff lever on the T/R motor to the off position. This would take care of any issues with residual air pressure in the pneumatic ducts.

Of course, it was always much better to lockout the sleeve as well, but in cases where you were short for time and could not open the fan cowls to get to the pins, applying the air shutoff on the motor was at least better than nothing.

Quoting QANTAS747-438 (Reply 10):
The filter is just as I pictured it to be like,

I think that photo is of the core of the FOHE.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
CFMTurboFan
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Mon Mar 16, 2009 3:29 pm

JetMech,

I wouldn't be going anywhere near that thrust reverser with any air in the system! I have rigged and run alot of those RB211 thrust reversers in the engine shop before sending an engine to test. One of my colleagues told me of an incident in Saudi where someone was chopped in half by a thrust reverser during a function test.

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jetmech
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Tue Mar 17, 2009 4:04 am



Quoting CFMTurboFan (Reply 13):

I avoid them like the plague. Unfortunately, there are plenty of LAME's who are happy to tell you to just "quickly jump in the back" to check something out! What this really means is "forget about the safeties as it takes too long to fit them."

The lever on the air motor is better than nothing and quick to switch off however. I think the amount of air trapped in the motor would hopefully be too small to translate the sleeve  scared 

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
CFMTurboFan
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RE: Fuel/Oil Heat Exchanger - What Do They Do?

Tue Mar 17, 2009 6:19 pm

I am amazed that some guys jump in there without safety precautions.

Nothing is more valuable that your health. I want to be around for a long time to see my kids grow up, taking chances like some of these guys is insane!!!

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