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Fanless Heatpipe CPU Cooling System by FMAH

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

* * *

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 href="http://users.erols.com/chare/elec.htm">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 href="http://www.wakefield.com/pdf/thermal_tutorial.pdf" target="_blank">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 href="http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/23794.pdf">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.

THE SYSTEM



Test
fitting the parts



Close
up of blocks and heatpipes



Large
heatsink mounted on backside



FIRST TEST



After all the components were secured in place, I fired up the system
and went into BIOS. Initially I used a two-fan Antec Smartpower
power supply that I happened to have available. Later I put in a

Seasonic Super Tornado 300 (SS-300FB)

. The Seasonic is much
quieter.

When I turned on the system without a hard drive, I put
my head a few inches from the motherboard to see what I would
hear. I didn't really hear much but the power supply fan.
Ocassionally the motherboard generated various kinds of electrical hum
and crackles depending on the load, but this is likely to vary with
the motherboard design and CPU load. Such electrical noise was mostly apparent when
overclocking the CPU.

I sat in the BIOS screen with the
temperature readings. I was happy to see the temperature did not
ramp up quickly, it slowly went to a stable reading after many minutes.



I spent a fair amount of time trying different CPU speeds with various
overclocking settings. This was done to simulate different CPU
heat loads and see whether the CPU temperature would go out of
control. The highest speed Althon XP is a 3200+ at 2.2 GHz.
I was able to run the system for around 10-15 minutes at that speed, but
the system was unstable, because I received a system error in
Windows. The temperature was in the low
60s °C when the error occurred. I suspect the CPU/RAM combination is the main factor in the
overclock success/failure, not overheating. The system seemed to be fairly stable at
2 GHz. Perhaps PC3200 RAM instead of PC2100 Kingston "value
ram" would make a difference in the maximum overclocking tests.

TESTING & RESULTS

The
powered on system



I performed a clean installation of MS Windows 2000. I used Speedfan 4.08 to monitor temperatures, and CPUBurn with Runprio
with the following command line "runprio -x high burnk7" to perform a
AMD K7 class burn at high priority. This setting provides at least
5°C higher temperatures than Prime95. The Shuttle
motherboard has no thermal probe in the CPU socket, so the readings from
Speedfan were assumed to be from the onboard thermal diode in the CPU. I
did not make any thermocouple measurements since I don't have that kind
of equipment yet. The temperatures at load were taken after
CPUBurn had been running more than one hour. Room temperature was
taken with a digital multimeter with temperature probe that was placed
below the large heatsink.

Idle Temperatures
Athlon XP
Setting
Room
°C
CPU
°C
1.46 GHz
1700+ stock: 133 MHz FSB
x 11 multiplier, 1.60V
23
40
1.46 GHz
1700+ undervolted: 133 MHz FSB
x 11 multiplier, 1.25V
23
37
2 GHz
1700+ overclocked: 200 MHz FSB x 10 multiplier,
1.65 V
22
44

*

Temps with CPUBurn:
Full Load
Athlon XP
Setting Room Temp °C CPU Temp °C

Temp Rise °C

CPU
Power (W)

Thermal
Resistance (°C/W)
1.46 GHz
1700+ stock: 133 MHz FSB
x 11, 1.60V
22
52
30 49.4*
0.61
1.46 GHz
1700+ undervolted: 133 MHz FSB x 11, 1.25V 22
44
22 30** 0.73
2 GHz
Maximum overclock: 200 MHz FSB x 10,
1.65 V
23
64
41 68.3~71.6**
0.60~0.57
* From href="http://users.erols.com/chare/elec.htm">Processor Electrical Spec
web page by Chris Hare

** See Calculating CPU Power & Thermal Resistance at bottom of page for details.

The system works well enough to run
over a wide range of Athlon XP CPU speeds, keeping the CPU temperature
below the 85°C maximum. This is a very positive result. Even the current fastest XP, the Athlon XP-3200+ w/Barton core, can probably be cooled well enough with this setup: Its heat dissipation of 76.8W would mean a max temp rise of 47C. In a 22-23°C room, the max CPU temp would then be ~70°C -- still well below the 85 °C max.

(Editor's note: The 41°C temperature rise with a 68.3W CPU is obviously higher than the theoretical calculation, presented earlier in the article, of a 36.4°C rise for a 80W heat source cooled by this heatsink. This is due to unavoidable losses in the heat transfer between the CPU and the heatsink.)

The noise level of the CPU cooling system is virtually nil; the only significant noise in the system comes from the the power supply and hard drive. As expected, the hard drive is the biggest
offender, and I often turned the system on and off without the hard drive connected just to "hear" the system.



ANALYSIS & CONCLUSIONS



The only part of the cooling system that became too hot to touch
sometimes were the copper blocks mounted on the CPU. During the
load tests at 2 GHz, these could be touched for a few seconds, while the
rest of the components could be touched for longer periods of
time. The copper blocks at the cooler end of the heatpipes were
able to be touched during the testing, and were noted to be somewhat
cooler than the hotter end. The large heatsink was always warm,
but a hand could be placed on them almost all the time.

I also tried aiming a floor fan at the large heatsink to see what
effect this would have on the CPU temperatures. During a load
testing, the CPU temperature was approximately 5°C lower than
without the forced air blowing on the heatsink. This was a rough
estimate to gauge the performance of the heatpipes. If the
temperature had not dropped, then this would have indicated that the
heatpipes were a limiting factor in cooling. Therefore, the
temperature could be even lower if design changes were made. These
could include increasing the surface area of contact onto the large
heatsink, improving surface finish smoothness and fit, using only the
highest performance thermal compound, making a vertical tunnel for
airflow over the heatsink, and increasing the size of the large
heatsink. There are many ways to modify this system, but the
existing system has proven to function very effectively at both reducing
noise and cooling the CPU in a passive manner.

This design works well for normal usage, and most likely even for
extended gaming sessions. The only things necessary for a complete
quiet system would be to silence the hard drive, use a passively cooled video
card, and a very quiet or passively cooled power supply.
Use you own imagination about how to physically integrate drives and a power supply to this system.


** Calculating CPU Power & Thermal Resistance

1. For the 1.25Vcore at 1.47 GHz: It is known that at 1.6V and 1.47 GHz, the T-bred core XP dissipates 49.4W. Since CPU wattage is directly proportionate to Vcore:

  • 1.6V squared = 2.56
  • 1.25V
    squared = 1.5625

The latter figure is 39% lower that the former. Thus at 1.25Vcore, the dissipated power is 61% of 49.4W or ~30W. This result jibes with Kostik's nifty CPU Power calculator utility

2. For the 2 GHz overclocked state: According to the href="http://users.erols.com/chare/elec.htm">Processor Electrical Specs
web page, the Athlon XP-2400+ Thoroughbred is clocked at 2 GHz at 1.65V (the same settings as this overclocked XP1700+) and dissipates 68.3W. However, plugging in those settings for a XP1700+ in Kostik's CPU Power calculator utility yields the higher 71.6W figure. So both numbers are provided.

In any case, as Russ (Rusty075 in the SPCR forums) pointed out, the changes in thermal resistance with a hotter or cooler CPU are to be expected: As the temp of the heatsink goes up, the speed of the air moving across the HS increases, thanks to the "stack" effect. That will drop the °C/W - hence the


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