Gas Turbines - Fuels — Page 367, Lesson 467

Let's start with the fuel heater, because that's the first thing the fuel meets on its way into the engine. In a gas turbine system, the fuel is passed through a heat exchanger that's powered by compressor delivery air. The whole point of that is to remove any ice crystals which may have formed while the fuel was exposed to the very low temperatures you get at high altitude. So the compressor air is hot, it warms the fuel, and any ice crystals that have formed in the fuel melt away before they can cause trouble. Now, some systems also use a fuel-cooled oil cooler. And here's the nice part — the oil has to be cooled anyway, and the fuel benefits by being warmed. So you get two jobs for the price of one. The oil gets cooled, the fuel gets warmed, and you've done both with a single piece of equipment. Next, let's talk about atmosphere exclusion. Once the fuel is in the aircraft fuel tanks, the main source of water contamination is the atmosphere that remains within the tank. If the tanks are topped up to full, then the atmosphere is excluded together with the moisture it contains. That minimizes the likelihood that the fuel will be contaminated. But there's a caution here — filling the tanks right up can be an embarrassment the next day if the ambient temperature rises. Because the volume of fuel in the tank will increase, and there's a danger it may spill out of the vent system. So you have to be careful about that. Now, waxing. Waxing is the depositing of heavy hydrocarbons from the fuel at low temperatures. The deposits take the form of paraffin wax crystals, and those crystals can clog the fuel filter and interfere with the operation of the fuel control unit. The effects of waxing can be minimized in two ways. First, the refinery can keep the levels of heavy hydrocarbons low. Second, the inclusion of a fuel heater in the engine fuel system helps as well. Then there's boiling. The temperature at which a fuel boils will vary with the pressure on its surface. As an aircraft climbs, the pressure on the surface of the fuel reduces, and with that reduction comes an increased likelihood that the fuel will boil and form vapour. The vapour locks that this effect causes will effectively cut off the fuel supply to the engine, with the inevitable result that the engine will stop. Fuel booster pumps fitted inside the tanks can overcome this problem by pushing fuel towards the engine, rather than engine-driven pumps sucking fuel from the tanks. So the booster pumps push, rather than suck, and that prevents the vapour locks. Finally, let's look at the effects of specific gravity. The specific gravity of a liquid varies inversely with its temperature. The heat release from the fuel is directly related to its specific gravity, so changes in fuel density can change the power output of an engine. On modern aircraft this usually makes little difference, because modern fuel control units will automatically compensate for the change in density of the fuel. But it should be appreciated that a change in specific gravity will also change the weight of the aircraft. And specific gravity is also known as relative density.