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Fanless Heatpipe CPU Cooling System by FMAH

Submitted by Mike Chin on Mon, 2003-10-06 08:51. Do-It-Yourself Systems
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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 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 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 <0.6°C/W at the maximum overclock ~70W setting. The opposite effect occurs with a cooler CPU -- the airflow decreases, reducing the cooling effect. Hence the 0.73°C/W at the 1.25 Vcore 30W setting.

Discuss this article in the SPCR Forums.



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