Fanless Heatpipe CPU Cooling System by FMAH

Do-It-Yourself Systems
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October 5, 2003 by Fred Mah

This fanless CPU cooling project is an exercise in logical design and simple execution using available technologies. It is far from simplistic. Fred Mah's fanless heatpipe cooled CPU system shows us a new approach to integrated system design that opens up all kinds of interesting possibilities. The cooling power of his system is nothing short of impressive. His project is a valuable contribution to the SPCR information "coffers".

Fred, btw, is a mechanical engineer who designs and manufactures various types of products and components, mainly for the audio industry. This probably helps to explain the resources accessed to have the various parts fabricated. Thank you, Fred! - Mike Chin, Editor / Publisher

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Much effort has been made in recent years to minimize noise generated by CPU cooling fans, a fact that has been demonstrated by the popularity of variable and low speed fans coupled with efficient CPU heatsink designs. Even with the adjustable fans generating lower noise at lower speeds, the main noise sources in a computer system are fans and hard drives. Therefore, the best way to eliminate the noise is to remove these sources. As it is impractical to get rid of the hard drives, it seems like a good idea to cool the CPU without a fan. After looking at products based on heatpipe technology, such as Zalman's graphics card coolers, I felt it would be a good idea to try passive CPU cooling utilizing heatpipes.

HEATPIPES

Heatpipes are capable of transferring a large amount of heat per a given volume of working fluid due to the phase change that takes place. Inside of a heatpipe is a liquid under low pressure (or vacuum) that boils into vapor when it absorbs heat. This vapor then condenses back into liquid at the cooler surfaces of the heatpipe and releases the heat. So the concept here is to draw the heat from the CPU into one end of the heatpipe, while putting the other end of the heatpipe in contact with a larger heatsink to expel the heat into the air. (Editor's note: In layman's terms, it's sort of like watercooling without the pump. Here is a thorough, accessible explanation of heatpipes, by Thermacore.)

DESIGN

After studying what was done with other passive CPU cooling projects by experimenters like bluehat1 and numano3, I determined that placing the heatsink on the opposite side of the motherboard would be the best configuration:

  • This would allow the creation of a case, or modification of an existing case, that could support the motherboard and heatsink in a way that wouldn't obstruct the motherboard components.
  • It would also not limit the size of the heatsink that could be used.
  • This was also optimal for placement of the heatpipes, since they would be used as the bridge between the CPU and the large heatsink.
  • Additionally, a good amount of heat would be carried to the outside of a case design with this configuration.

PARTS

At the beginning of the project, I purchased the following components for the heart of the system, chosen mainly on the basis of cost:

-- AMD Athlon 1700+ (Thoroughbred) - about $50

-- Shuttle AN35N Ultra motherboard: The least expensive nVidia nForce2 motherboard with the four heatsink mounting holes that I came across. It also had passive chipset cooling and cost around $80.

-- Heatsink: I happened to have a sample piece of aluminum heatsink extrusion that seemed large enough for the passive cooling job. The dimensions are 9 7/8" wide by 12" long by 1 5/16" tall. In order to see if this would do the job, I had to look at the range of heat that the CPU would be generating.

From the highly useful Processor Electrical Spec web page by Chris Hare, it can be seen that AMD's Athlon XP family of processors go up to 2.2 GHz in the 3200+ model which generates approximately 80 W of heat maximum. The 1700+ on hand dissipates about 50 W maximum.

The thermal resistance for natural convection of the heatsink was known to be 0.91 °C/W for a 3 inch piece. According to a technical document at Wakefield Engineering's website, the thermal resistance would decrease by half if the length of the part were increased by four times. Therefore, the 12 inch long extrusion should have a natural convection thermal resistance of 0.455 °C/W. In an ideal situation with no thermal losses between the CPU and the heatsink, this would mean that dissipating 80 W of heat would leave the CPU at a temperature of 36.4°C higher than ambient. So for a 25 °C (77 °F) room temperature, this would be a CPU temperature of 61.4 °C (142.5 °F). The maximum allowable CPU die temperature for a 3200+ Athlon is 85 °C, so the thermal resistance of this heatsink is theoretically sufficient for any Althlon XP processor available at this time.

A design concept was drawn up in Solidworks, and from this the dimensions for the heatpipes, copper blocks, large heatsink mounting holes, and test rig were determined. The idea was to have a minimalist test rig to quickly build and test the performance of this cooling system. I had the sheet metal stand portion fabricated and spray painted it with white primer to prevent rust.


Solidworks drawing of the proposed test rig

-- Heatpipes: Two 6 mm heatpipes were obtained from AVC America as an initial design sample. (AVC's main web site: http://www.avc.com.tw/index2.html). The idea was to evaluate the performance of the heatpipes for commercial applications. I did not know what kind of performance I would be getting from two pieces.

-- Copper Blocks: Copper bar stock of 1/2" thickness was purchased and designed to fit over the CPU using the four mounting holes on the motherboard. The blocks were machined with suitable mounting holes and had half-pipe grooves to fit the pipes. The large block/small block pairs were identical at both ends, to make the machining work easier. One large block attached to threaded holes in the big heatsink, while springs were used to apply approximately 16 lbs of load onto the large block mounted on the CPU die. The maximum load force for a heatsink mounted on a Athlon XP is given as 24 lbs in AMD documentation. In the picture, very long screws are visible in the mounting. These happened to be what I had available and are a little longer than necessary. The small blocks are secured to the larger blocks by threaded holes in the larger pieces.


Raw copper blocks and heatpipes

Before applying thermal grease and mounting the blocks, I lapped them down to about a 600 grit paper at the surfaces of contact. The edges of the block were not cleaned up too much, so they look a little rough. The area on the large heatsink that contacts the copper block was also polished to a sufficient flatness.


Lapped copper block surfaces


Surface of large heatsink polished to a sufficient smoothness


The pipes were not snug, but not too loose either, and after tightening the screws they were pretty well fixed in place. The heatpipes were a little shorter than I was expecting, so they don't quite stick out of the top of the copper block, as seen in the pictures below. Before tightening the blocks down, Artic Silver 3 was applied at the interface between the CPU die and the large copper block. Then generic silicon grease was put on all other contact surfaces. I didn't use AS3 since there was a fair amount of area to cover, and it would have used a lot of the AS3.



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