Starlionblue From Greenland, joined Feb 2004, 16375 posts, RR: 66 Reply 1, posted (3 years 11 months 2 days 14 hours ago) and read 5319 times:
As I understand it, low temperatures can lead to fuel reaching its freezing point, which is when wax crystals form. This is probably not something you want flying around in the fuel system. Ice is not as big a problem apparently.
High temps do not occur (at least I don't think so). The air is cold even at a moderate altitude, even in the tropics.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 4, posted (3 years 11 months 2 days 10 hours ago) and read 5260 times:
Quoting Kimon (Thread starter): Ditto for high fuel temperatures when cruising or does this not occur.
I've never heard of this happening in cruise; even on a hot day in the tropics, cruise altitude temperatures are too low.
Hot fuel can be an issue on the ground since the vapour pressure goes up with temperature and, if it gets high enough, the fuel pumps will cavitate and the suction feed won't work. This is one of the reasons that you don't usually see new jets certified to use JP-4 anymore (JP-4 has a higher vapour pressure than Jet-A).
The major issues vis a vis fuel on TWA800 was that the center tank was empty, which provided a very large ullage space to accumulate vapour. A full tank has much less potentially flammable space. The issue with heating in that case isn't directly the fuel temperature, but the fuel vapour that forms in the ullage. Hot fuel will vapourize more, and more quickly. If you get enough vaporization you'll go above the rich limit and be safe again. Post TWA800, the primary defense is still minimizing ignition sources. Flammability reduction (nitrogen generation systems and the like) are something of a backstop.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 81 Reply 10, posted (3 years 11 months 2 days 6 hours ago) and read 5196 times:
Quoting BMI727 (Reply 8): I've always been under the impression that cooling the oil is more important than heating the fuel, and the fuel is just the most convenient heat sink.
For the FOHE, it's generally both...you need to cool the oil and the fuel is a convenient heat sink, but you also want to heat the fuel to melt any ice crystals before they get into the hydraulic bits. Some engines cool the oil using fan air and heat the fuel using other mechanisms.
The hydraulic oil cooler is the one where this issue usually comes up; in that case, it's all about cooling the hydraulic oil. You do happen to heat the fuel in the process, but fuel heating is not why you do it. In fact, you'd generally rather not heat the fuel in the main tanks. The FOHE heating all happens at the engine, except in a few special circumstances.
Quoting Kimon (Reply 9): So if the CWT were filled to the brim,the question of vapourization and related problems would not arise?
It's still there, but it's much smaller. With a full tank it takes a lot longer to heat up the fuel, so you tend not to have as much vapourization. In addition, the ullage volume is so small that you go through the rich limit quickly and aren't in the flammable zone for very long.