Rotating Heat Pipes (RHPs) and Rotating High Conductivity (HiK™) Shafts can both be used to remove heat from motors and other rotating systems, such as gears. In a rotating heat pipe, the interior is in the shape of a conical frustrum so that the centrifugal force returns the condensate to the evaporator. These heat pipes can be relatively expensive to maintain. In addition, the fluids cannot be allowed to freeze, so a low melting point, lower performance working fluid must be chosen.
The rotating HiK™ shaft is a less expensive, higher performance alternative to a rotating heat pipe. As shown in Figure 1 and Figure 2, a series of freeze-tolerant copper/water heat pipes are embedded in the shaft that requires cooling. The heat pipes are angled, so that centrifugal forces help return condensate from the condenser to the evaporator. The major difference between a rotating HiK™ shaft and an RHP is that wicked heat pipes can be used in the HiK™ shaft. This wick can be designed so that the heat pipe can withstand numerous freeze/thaw cycles. Performance is improved compared with a methanol RHP, since water is one of the best heat pipe working fluids.
The benefits of the rotating HiK™ shaft for rotor cooling are shown in Figure 3. In a conventional system, the heat generated by the rotor needs to be transferred through air gaps, leading to high temperatures in the rotor. In contrast, heat is removed more directly and efficiently with a rotating HiK™ shaft.
Rotating HiK™ shafts were tested in ACT’s rotating heat pipe test apparatus, shown in Figure 4. Tests were conducted at 1000, 2000, 3000, 4000 and 6000 RPM. Good agreement with theoretical limits was obtained at RPMs under 3000 RPM, see Figure 5. At higher speeds, the operating limit exceeded the capabilities of the IR Heater, and dryout could not be reached. When compared with conventional rotating heat pipes, the Rotating HiK™ shaft had:
- Improved Performance, with reduced thermal resistance
- Improved reliability – multiple heat pipe design provides redundancy
- Reduced cost, since standard copper/water heat pipes heat pipes can be used
- Potential Weight Savings