Design Considerations When Using Heat Pipes (Pt. 1)
This blog post offers some intermediate level heat pipe design guidance for the most prevalent types of electronics applications: mobile to embedded computing and server type applications with power ranging from 15-150 watts using processor die sizes of between 10x10mm to 30x30mm. I’m using these constraints because throughout the article I’ll mention some simple tips or guidelines that don’t necessarily apply when talking about power electronics applications. I’ll also be focusing on the most ubiquitous type of heat pipe – copper tube with sintered copper wick using water as the working fluid.
As you might have read in past blogs, heat pipes should be considered when the thermal design is conduction limited or when non-thermal goals such as weight can’t be achieved with other materials such as solid aluminum and/or copper. So, if you’re too the point where heat pipes may be an option, keep the following things in mind.
Thermal Conductivity of Heat Pipes Rarely Reach Published Limits
Regularly published heat pipe thermal conductivity range from 10,000 to 100,000 W/mK. That’s up to about 250 times that of solid copper and 500 times that of solid aluminum. But don’t count on those figures for typical electronics applications. Unlike solid metal, the effective thermal conductivity of copper heat pipes varies tremendously with heat pipe length, and to a lesser degree with other factors.
Figure 1 illustrates the effect of length on heat pipe thermal conductivity. In this example, we used three heat pipes to transport heat from a
Heat Pipe Designs Are Like Automobiles
Performance data for heat pipes are usually of the Honda Accord variety: adequate for most things, but horrible if you have a specific purpose in mind – like track racing or off-roading. Even when limiting our conversation to copper/water/sintered wick versions, heat pipe customization can markedly affect operational and performance characteristics. And just like cars, design changes to a heat pipe range from the simple to the exotic.
As we’ll see later, flattening and bending will negatively affect heat pipe performance but are an easy customization that
Changes to the internal structure of the heat pipe design, most notably wick porosity and wick thickness, allow heat pipes to be tuned to meet specific operating parameters and performance characteristics. For instance, when a given diameter heat pipe is required to operate at higher power loads or against gravity, the capillary pressure in the wick needs to increase. For higher Qmax, this means a larger pore radius. For effectively working against gravity (condenser below evaporator), this means a smaller pore radius and/or increased wick thickness. Additionally, it is possible to vary both wick thickness and porosity along the length of a single tube. Suppliers who specialize in heat pipe customization will regularly use custom formulated copper powders and/or unique mandrels to ensure the final product meets applications requirements.
Mechanical Engineering Choices Affect Heat Pipe Performance
Once the internal structure of the heat pipe design has been determined, it’s good to know how outward physical characteristics affect performance. With heat pipes, size generally matters most. However, changes to
Figure 2 shows power handing capacities (Qmax) for the most common heat pipe sizes. As noted earlier, Qmax may change amongst vendors for standard (Honda Accord type) heat pipes, but I’m taking some data from recent projects we’ve done – without getting too exotic.
Typically, sintered copper heat pipes can be flattened to a maximum of between 30-60% of their original diameter. I know
Simply put, there are two performance limits important for terrestrial heat pipe applications: the wick limit and the vapor limit. The wick limit is the ability of the wick to transport water from the condenser back to the evaporator. As mentioned, we can tune the porosity and thickness of the wick to specific applications, allowing for changes to Qmax and/or ability to work against gravity. The vapor limit for a particular application is driven by how much space is available for the vapor to move from the evaporator to the condenser.
The orange and blue lines in Figure 3 plot the respective limits for the various heat pipe sizes shown in Figure 2. It’s the lesser of these two limits that determine Qmax and as shown the vapor limit is above the wick limit, albeit only slightly for the 3mm heat pipe. As heat pipes are flattened, the
In this example, we’ve chosen to flatten the heat pipes to the specifications in Figure 2. As seen by the gray line, the vapor limit is below the wick limit, reducing the Qmax. Flattening the 3mm by a mere 33% causes the vapor limit to become the determining factor whereas we had to flatten the 8mm by over 60% for this to happen.
Bending the heat pipe will also affect the maximum power handling capacity. Here
For more technical information on bending both heat pipes and vapor chambers, please visit a past blog on how bending affects heat pipe performance.
Working Against Gravity
Let me start this section by saying that we’re using the same fairly ‘standard’ heat pipes that we’ve shown in prior examples so they haven’t been optimized to work against gravity (-90o). Figure 4 illustrates how the relative position of the evaporator to the condenser can affect both Qmax and heat pipe selection. In each case, Qmax is reduced by roughly 95% from one orientation extreme to the next.
In situations where the condenser must
Heat Pipe Design Calculations: A Practical Example
Let’s use an example to illustrate how different heat pipe designs might be used to solve a thermal challenge. Assume the following as know constraints: 70W heat source with dimensions 20 x 20mm and a single
be attheir most effective, heat pipes need to fully cover the heat source, which in this case is 20mm wide. From Figure 2, it appears we have two choices: three round 6mm pipes or two flattened 8mm pipes. Remember the three 6mm configurationwill be placed in a mounting block with 1-2mm between the heat pipes.
- Heat pipes can be used in conjunction to share the heat load. The 6mm configuration has a Qmax of 114W (3x38w), while the flattened 8mm configuration has a Qmax of 104W (2x52w)
- It’s just good heat pipe design practice to build
ina safety margin. You wouldn’t go bungee jumping with a rig only capable of supporting exactly your weight, would you? Same for heat pipes. I typically use 75% of their maximum capacity. So we’ve got 85.5W for the 6mm (75% x 104W) and 78W for the 8mm (75% x 104w)
- Now we need to factor in bending. A
90 degreebend will cause Qmax of each configuration to be reduced by another 5%. The resulting Qmax for the 6mm configuration is now just over 81W and for the 8mm configurationit’s now 74W, both of which are higher than the 70W heat source we’re trying to cool.
As can be seen, both heat pipe design configurations are adequate to move heat from the evaporator to the condenser. So why choose one over the other? From a mechanical
Please check back for Part 2 of this blog where I’ll be discussing condenser types and sizing as well as methods to attach the heat pipe assembly to the heat source.