The oil to fuel heat exchanger is a really smart system invented to save energy and prevent complexity in aircraft.
Aircraft very commonly fly at thirty to forty thousand feet where temperature can reach as low as -57°c, now considering that Jet A-1 freezes at roughly -47°c, we know we have to create a system that keep the fuel warm and therefore liquid.
Now we could install electric heaters but they would add extra load on the accessory gearbox of the engines and thus increase fuel burn. We could also use bleed air but that would again decrease the effectiveness of the engine requiring a higher fuel flow.
At the same time, we have oil flowing through an engine that reaches temperatures of 1700°c that needs to be cooled. So we could install radiators around the engine to exchange the heat from the oil with the cold outside air but that would create extra drag and therefore increase fuel burn.
Instead, we could connect both systems and prevent any waste of energy and prevent the operators of the aircraft to have to car for another component/system in an already complex aircraft.
What we are left with is a system that cools the oil and heats the fuel at the same time. On a jet aeroplane, fuel heaters are located in the engines LP fuel system and are heated using engine lubricating oil.
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