Measuring Heatpipe Efficiency

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August 8, 2006 by Brendan Wynn

A new contributor to SPCR reports from New Zealand on his efforts to determine the efficacy of the Borg 6mm HES (Heatpipe Extension Set) by mCubed, which is designed for use in their passive home theater PC cases. Brendan's aim is to design a secondary backup system to aid a 1U HSF in cooling the CPU in an upcoming extremely low profile Hiper Group media case. In the process, he created a testing rig that's worthy of the SPCR lab, one that allows the thermal loss factor of this heatpipe system to be easily seen. The results are applicable mostly to the mCube heatpipes and mounting system. We simply do not have experience with a big enough sampling of heatpipes and heatpipe mounting systems to generalize. However, the article gives insights about the kind of results that can be expected from DIY heatpipe setups.

- Mike Chin, Editor


This article was written after I became interested in ways to transport heat around SFF (small form factor) and slim cases.

My current project is a HTPC using the slimmest case I can find. At this time, the lowest profile case on the market is the Hiper Group media case, which is a mere 55mm in height! Hiper will be releasing a new version soon, and it is this media case that I have my eye on. While waiting, and after reading reviews of the current Hiper case, I decided to look into ways to augment the cooling of the 1U CPU HSF (heatsink fan) supplied as an accessory with the Hiper Media case.

Editor's Note: The author awaits the release of the Hiper HMC-2x53x.

Today's CPU air-cooling systems almost always use heatpipes to transport heat away from the CPU, normally into the case where it can be vented outside. Deciding this would be an effective mechanism to investigate further, I sourced some heatpipes to try them in various configurations. In scouring the Net for a source of heatpipes, I came across the mCubed Borg passive case heatsink system. Figuring I had time before the new Hiper case became available, I embarked on testing the effectiveness of the Borg system and perhaps making modifications/additions to incorporate it into a slim HTPC system.

The following is a pictorial rambling of my efforts to test and design a slim HSF heatpipe CPU cooling system.

Editor's Note: What is a Heatpipe? (explanation from wikipedia)

A typical heat pipe consists of a sealed hollow tube. A thermoconductive metal such as copper or aluminium is used to make the tube. The pipe contains a relatively small quantity of a "working fluid" or coolant (such as water, ethanol or mercury) with the remainder of the pipe being filled with vapour phase of the working fluid, all other gases being excluded.

On the internal side of the tube's side-walls a wick structure exerts a capillary force on the liquid phase of the working fluid. This is typically a metal powder sintered or a series of grooves parallel to the tube axis, but it may in principle be any material capable of soaking up the coolant.

Heat pipes contain no moving parts and typically require no maintenance, though non-condensing gases that diffuse through the pipe's walls may eventually reduce the effectiveness, particularly when the working fluid's vapour pressure is low.

The materials and coolant chosen depends on the temperature conditions in which the heat pipe must operate, with coolants ranging from liquid helium for extremely low temperature applications to mercury for high temperature conditions. However, the vast majority of heat pipes uses either ammonia or water as working fluid.

The advantage of heat pipes is their great efficiency in transferring heat. They are actually a better heat conductor than an equivalent cross-section of solid copper. The general principle of heat pipes using gravity dates back to the steam age.

Additional sources of information about heatpipes:


The first thing I needed was a test-bed. It had to include a heat-source (CPU Simulator) and method of measuring "CPU" temperature. So I made a CPU Simulator around a 100Watt, 2.2 ohm aluminium-encased resistor. I mounted it in a length of aluminium channel and encapsulated it in an epoxy potting-resin (photo is pre-potting).

CPU Simulator: 100Watt, 2.2 ohm aluminium-encased resistor

M3 screws to attach the CPU-Simulator to a HSF, in this case a Thermaltake Pipe101.

I also encapsulated a NTC-resistor to enable temperatures to be monitored. This NTC-resistor was mounted in the centre of the rig, which meant it measures the highest possible temps. I also soldered in a feedback cable so the voltage across the resistor could be measured here rather than at the PSU to avoid any potential voltage drop. [Editor's Note: A Negative Temperature Coefficient (NTC) resistor is a temperature-dependent resistor, commonly called thermistor, with a negative temperature coefficient. When the temperature rises, the resistance of the NTC resistor drops. They are often used in temperature detectors and measuring instruments.]

To make the CPU-Simulator useful I needed to make a test-bed. This was done using a laminated board, multimeter, PC PSU and cabling.

The test bed.

I wired the HS fan to a switch, enabling easy selection of 5V or 12V fan voltage. This meant I could test various configurations with different cooling capacities.

With the resistor connected to the PSU 5V rail, the 2.2 ohm resistor becomes an 11W heat source. With it connected to the 12V rail, it turns into a 59W heat source. The voltages were actually slightly less than 5 and 12 volts, as there was voltage drop across the supply cable — hence the odd wattage values.

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