Heat pipe orientation significantly impacts its maximum power carrying capacity (Qmax) and must be carefully evaluated before selecting the heat pipe’s diameter and wick structure.
As seen in the chart below, Qmax diminishes by over 90% when a sintered heat pipe operates against gravity, with the evaporator positioned directly above the condenser (-90 degrees). For example, an 8mm heat pipe can carry up to 127 watts in a gravity-aided orientation, 68 watts in a horizontal (neutral) position, and just over 8 watts when working against gravity. This calculation is based on a heat pipe length of 200mm, evaporator length of 25mm, and condenser length of 100mm. Use our online heat pipe calculator to generate results.
Heat Pipe Qmax Shrinks as Vertical Orientation of Condenser to Evaporator Changes
Engineers should pick the worst-case angle of orientation before selecting heat pipe diameter and wick characteristics.
Heat Pipe Selection
Once basic parameters like heat pipe length and worst-case angle of orientation are determined, select a heat pipe with a diameter that ensures Qmax exceeds the thermal design power (TDP) of the IC. Heat pipe Qmax is additive when each pipe is positioned directly above the heat source.
For instance, an 8mm heat pipe operating completely against gravity can handle 8.3 watts of power. By using three 8mm heat pipes in the same configuration, the total power carrying capacity increases to 24.9 watts.
Heat Pipe Optimization
In the example above, with a TDP of 30 watts, three 8mm heat pipes fall slightly short of the target. If upgrading to a 10mm heat pipe is not feasible, the performance of the 8mm heat pipe must be enhanced by optimizing its internal wick structure.
Changes to Wick Thickness & Pore Radius Alter Qmax
The chart above compares a standard sintered heat pipe to one with a thicker wick and an optimized pore radius. These modifications raise the original Qmax from 8.3 watts to 11.9 watts per pipe in the -90 degree orientation, exceeding the 30-watt TDP when using three heat pipes.
In conclusion, selecting and optimizing heat pipes requires careful consideration of orientation, length, diameter, and wick structure to ensure they meet the thermal demands of the application. By understanding the impact of these parameters and leveraging design optimizations, engineers can achieve reliable and efficient heat pipe performance even in challenging conditions.
