What happens to fuel when it freezes???

[islandhopper]

Hi,
Just wondering if anyone could tell me what happens to fuel when it starts to freeze? ie if the freeze temp is -47c what kind of state is it in at -46c?
If it freezes (say -49c) can it be still be used?
Just interested to know as it's pretty bl@@dy cold through europe/Russia.

[the dean]

hi islandhopper,

as i understand it, fuel turns to jelly and clogs tanks...

thats why jets have heaters to keep the fuel warm...and not just jets...our 172 has a diesel engine and the jet A1 is circulated to keep it warm.if indications are that the fuel is getting cold we switch tanks because as fuel is used some of it is pumped back from the engine into the fuel tank to keep the unused fuel there up to a warmer than otherwise level.

some smaller jets that do'nt have fuel heaters use and additive put into the tank at the time of refuelling to prevent freezing.

its not so much the cold temperatures you are talking about in other countries that is the problem...its the extreme temperatures at altitude.the temperature drops by 3 degrees every 1000 feet you go up.

i'm sure others will give you more details...:ok:

dean.

[Panama Jack]

Think of candle wax, but a wax that melts at temperatures that are warmer than -47 Celcius.

[john_tullamarine]

The Dean,

Do you use stock A1 or do you put a bit of oil in to bring it back closer to the diesel fraction ?

[the dean]

hi john,

no we use regular Jet A1....can use diesel but there are temperature restrictions.

dean.

[Re-entry]

Freezing Point Because it is a mixture of more than a thousand individual hydrocarbons, each with its own freezing point, jet fuel does not become solid at one temperature the way water does. As the fuel is cooled, the hydrocarbon components with the highest freezing points solidify first, forming wax crystals. Further cooling causes hydrocarbons with lower freezing points to solidify. Thus, the fuel changes from a homogenous liquid, to a liquid containing a few hydrocarbon (wax) crystals, to a slush of fuel and hydrocarbon crystals, and, finally, to a near-solid block of hydrocarbons. The freezing point of jet fuel is defined as the temperature at which the last wax crystal melts, when warming a fuel that has previously been cooled until wax crystals form. Thus the freezing point of fuel is well above the temperature at which it completely solidifies.
Coefficient of Thermal Expansion
Liquids increase in volume as their temperatures increase. The coefficient of thermal expansion is a measure of the rate of volume increase with temperature. A typical value for the coefficient of thermal expansion of kerosene-type jet fuel is 0.00099 per degree Celsius [(ºC)–1] [(0.00055ºF)–1]. At this rate, one gallon of jet fuel will expand 4.0 percent for an increase in temperature of 40ºC [1.000 gallon at 0ºC (32ºF): 1.040 gallon at 40ºC (104ºF)].
Of course, the relationship is reversible. For example, as jet fuel cools in the aircraft’s tanks during flight, it will occupy a smaller volume than it did on the ground. The coefficient of thermal expansion can be used to calculate the volume decrease.


The primary criterion for fuel system performance is pumpability – the ability to move fuel from the fuel tank to the engine. Pumpability is influenced both by fuel fluidity and fuel system design. In lieu of a fuel system flow simulation test, the industry uses freezing point as an indicator of a fuel's low-temperature pumpability. Jet fuel typically remains pumpable approximately 4ºC to 15ºC (8ºF to 27ºF) below its freezing point.1

Found the above on a google search.

[Floppy Link]

as i understand it, fuel turns to jelly and clogs tanks...
thats why jets have heaters to keep the fuel warm
no "tank heaters" as such, at least on the 757 or 767 (if I remember correctly - it's been 2 years since I flew one, open to correction) - the fuel would pass through the oil cooler on its way to the engine. It was called the Fuel Cooled Oil Cooler (FCOC) but could just have easily been called the Oil Warmed Fuel Warmer.
.
Have just looked up the Embraer 135/145 and they also have no heaters, just a FCOC.
.
Fokker F27 Friendship has fuel heaters but in the lines, not in the tanks - they would come on for a spell before landing to ensure no ice crystals would clog the filters and remove the power just when you needed it

...the temperature drops by 3 degrees every 1000 feet you go up.

2 degrees/1000ft for the Jet Standard Atmosphere
1.98 degrees/1000ft for ISA

[chornedsnorkack]

its not so much the cold temperatures you are talking about in other countries that is the problem...its the extreme temperatures at altitude.the temperature drops by 3 degrees every 1000 feet you go up.

Only in unstable air. In stable air, the temperature drops slower or increases with height.

Anyway, at altitude the fuel is heated by stagnation and friction heating. Which are not available at standstill on ground.

[BANANASBANANAS]

At altitude it's the TAT as opposed to OAT that you are interested in. As the previous post quite rightly said, there is friction heating and I find the easy way to think of it is that TAT (Total Air Temperature) is effectively a True Temperature to the aircraft structure which, of course, includes the fuel tanks.

It has been a problem on N Atlantic flights for us recently - but it does provide a great excuse to fly faster.:ok:

[Re-entry]

Actually flying faster doesn't do much for you.

At OAT -60 deg C, increasing from say M.82 to M.85 only increases ram rise from 28.5 to 30.7 deg.

Better to descend.

[alexban]

Floppy,I may be incorrect,but the fuel heater /oil cooler is installed in the fuel tank.The oil passes trough it and cools,while warming the fuel.
It seems to have more sense,cause if the fuel in the tanks cools too much it won't be able to pass through pumps intakes in order to reach the heater afterwards.
I'll check the books when possible.

[om15]

The Cessna 680 Sovereign has hydraulic oil/fuel heat exchangers in the inner wing tanks, the spoiler system hydraulic pressure lines route through the fuel in the fuel tank to create a heat exchanger, heat from the hot oil is transfered to the cold fuel, bleed orifices downstream provide a continous flow to circulate the hot oil.
Also the PWC 306 engine fitted to the 680 has a fuel/oil heat exchanger to further heat the fuel before the engine fuel filter and HMU.

regards,
om15

[BANANASBANANAS]

Re Entry,

Descending isn't always beneficial or even an option due to traffic or fuel burn constraints. Increasing speed will only provide (as you say) a small ram rise, but it is usually enough. The object of the exercise is to keep the TAT gauge at least 3 deg warmer than the freeze point of the fuel. And if you have just refuelled in USA that means -37 deg C. Once in warmer air you can slow down again, but if you have descended you may not get your original level back. Better (imho) to speed up first, then consider descent/reroute etc.

Always willing to be corrected though.:ok:

[chornedsnorkack]

So, fuel freezes at -37 Celsius and a plane is screwed at TAT of -34 Celsius?

Then descending to landing is dangerous. While temperatures at a couple of kilometres altitude would be in the region of -30...-40 in winter in the inland continental areas of Canada and Asia, on surface the temperatures of -50, -60 could be found. How does one land in winter in Yellowknife, Klondike, Fairbanks or Yakutsk? Which planes function with everything cooled to -60 Celsius on ground?

[alexban]

In very cold areas the fuel has an additive in order to lower the freezing point.The normal jet A1 in Europe has a freezing temp below -40 C (stand to be corrected though ) ,but in Russia I think it's lower than that.I don't remember though,but it was something below -56 C :confused:
With -60 C on ground? well,you may ask the russians also,but I've heard of some flights done on a very remote,and cold location ,where it was necessary to keep the engines running...otherwise the oil would've become so dense that the engine won't start again. :hmm:

[BANANASBANANAS]

chornedsnorkack,

Jet Fuel we use in USA freezes at -40 deg C, so our operating limit is -37 deg C. We even get a pretty amber Eicas message to let us know when the fuel temp falls to -37 deg C. I think you need to differentiate between OAT and TAT. I have seen below -60 deg C OAT (or SAT if you prefer) in CRZ but a TAT of about - 35 deg C. Remember, its also a function of time. 100+ tonnes of fuel at +20 deg C on the ground does not instantly fall to the TAT temp in the CRZ. It can take several hours for the "cold soak" to take effect.:ok:

[chornedsnorkack]

100+ tonnes of fuel at +20 deg C on the ground does not instantly fall to the TAT temp in the CRZ. It can take several hours for the "cold soak" to take effect.
Sure. And arriving from a cruise at -35 C TAT, turning around a plane at an airport where air temperature is, say, -45 Celsius in Fairbanks, which is in USA, might do something ugly. (The record in Fairbanks is -55 Celsius).

[Re-entry]

The lowest OAT (SAT) I have witnessed is -73 deg C.

Any advances?

[TURIN]

B737
Hydraulic fluid heat exchangers in the wing fuel tanks. There to cool the hydraulic fluid, not heat the fuel.
Generic
Fuel cooled oil coolers primarily there to cool the engine/IDG oil.
However, as the heat has to go somewhere then warming the fuel is no bad thing...
unless it's an Airbus in which case they installed a complex return system that, would you believe, stops the fuel in the wings from getting too warm.:rolleyes:
Ok, ok I know it's all clever stuff and designed for optimum performance, but bl00dy hell, AIRBUS!:{

[BANANASBANANAS]

chornedsnorkack

There are procedures in place at some airports to keep the fuel "warm" (less cold!) in storage but I cant speak about Fairbanks specifically. As a general point please remember that it is the temp of the fuel rather than the OAT that is critical.

Maybe someone with experience of Fairbanks can enlighten both of us?

[Fly3]

Flying the polar routes from the USA, hence refuelled with Jet A, we have the ACTUAL fuel freeze temperature tested for the fuel uplift. This is done by analyising three samples. One at the start of refuelling, one half way through and one at the end. It cost a bundle I believe but the results are quite useful. The worst freeze temp I have been given for Jet A (generic FP -40C) was -45.6C.

[160knots]

So the stuff they spray into the Lear during refuelling is that for preventing an ice build up or prevent the gel from forming. The old Citation used the same procedure.

[Re-entry]

Why you should use a fuel additive
All turbine aircraft fuels contain some dissolved water. It cannot be extracted because it does not exist as particulate water. When an aircraft rises to flight altitude, the fuel cools and its capacity to retain dissolved water is reduced. Some of the dissolved water separates out as discrete water that can form into ice crystals or remain as a supercooled liquid. When supercooled water strikes a tubing bend or a filter, it can freeze quickly and block a fuel line or filter. If suspended ice crystals are present, they can also block a filter. Prist Hi-Flash anti-icing aviation fuel additive controls icing in aircraft fuel by depressing the freezing point of water.


Controlling Fuel System Icing
Prist Hi-Flash additive has limited solubility in jet fuel, but is completely soluble in water. When dissolved water separates from the fuel, some amount of Prist Hi-Flash additive quickly leaves the fuel and preferentially dissolves in the water. This depresses the water's freezing point. As the fuel gets colder, and more water particles appear, more Prist Hi-Flash additive leaves the fuel and enters the water, and your aircraft's fuel lines stay clear.


This is a separate issue from the actual jet fuel freezing.

[160knots]

Just looking at some Stats:

AVTUR – Freezing point -47C

Aviation Turbine Kerosene AVTUR- Freezing-50C

JET A- Freezing point -47C

I am told the fuel temp at high altitude drops to the minus 20’s, so why does one fuel need PRIST and the other not? Surely all fuels contain some dissolved water including JET A.