Heat exchangers are used in transport-type aircraft hydraulic systems to remove excess heat from the hydraulic fluid produced by the operation of hydraulic pumps and other system components. During normal operation, the hydraulic fluid absorbs heat as it passes through pumps, valves, and actuators. If this heat is not removed, the fluid can lose viscosity, oxidize more rapidly, and reduce the service life of seals, pumps, and other hydraulic components.
To prevent overheating, transport aircraft commonly use hydraulic heat exchangers installed inside the aircraft fuel tanks. These heat exchangers are typically located in the main or center fuel tanks near the hydraulic system return lines. Hydraulic fluid flows through aluminum finned tubes or passages inside the heat exchanger, while the surrounding fuel absorbs the heat. The large temperature difference between the warm hydraulic fluid and the cooler fuel allows heat to transfer efficiently from the hydraulic system to the fuel.
Using fuel as a cooling medium provides several advantages. It cools the hydraulic fluid before it returns to the reservoir, extends the service life of the hydraulic fluid and hydraulic pumps, and helps warm the fuel during high-altitude flight where fuel temperatures can become very low.
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| Hydraulic heat exchanger |
Because the fuel absorbs heat from the hydraulic system, the fuel level in tanks containing the heat exchangers must be maintained above a specified minimum. If the fuel level becomes too low, the heat exchanger may not be completely submerged, reducing its cooling effectiveness and possibly causing hydraulic fluid temperatures to rise above allowable limits. For this reason, aircraft operating procedures often specify minimum fuel quantities whenever hydraulic systems are operated on the ground or in flight.