Measuring Heatpipe Efficiency

Cooling
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TEST RESULTS

I again ran the same tests as before. This time the heat would be removed from the CPU Simulator via the heatpipes, then dissipated into the air by the Thermaltake HSF as before. In essence, the only real change is the addition of the heatpipes in the heat transfer path, although it is far from perfect; there are small unavoidable losses in the interfaces at both the evaporator (where the CPU heat evaporates the coolant in the heatpipes) and at the condenser (where the HSF condenses the coolant back into liquid form).

mCubed 4-pipe HES w/ Thermaltake Pipe 101 fan on CPU Simulator
Fan
Sound
Heat
Max Temp.
Ambient
Temp. Rise
°C / W
12V
Loud
59W
48°C
21°C
27°C
0.4576
5V
Very quiet
59W
58°C
21°C
37°C
0.6271
12V
Loud
11W
25°C
21°C
4°C
0.3636
5V
Very quiet
11W
27.5°C
21°C
6.5°C
0.5909
Off
Silent
11W
37°C
21°C
16°C
1.4545
Reference Results: Thermaltake Pipe 101 directly on CPU Simulator
12V
Loud
59W
32.5°C
19°C
13.5°C
0.2288
5V
Very quiet
59W
43.5°C
19°C
24.5°C
0.4153

In comparing the results between direct HSF cooling and heatpipe-coupled HSF cooling, there is degradation of 13.5 °C for the HSF in high cooling mode (fan at 12V) and 12.5°C with the HSF in silent mode (fan at 5V). This drop in performance was greater than I expected. Perhaps there was too much thermal loss in the mechanical interfaces between the pipes and the blocks? Just to double check, I reassembled the plates, heatpipes and HSF and ran the test again, but there was no change.

At this point, I wondered what the effect of less power would be. As can be seen in the table above, with the CPU simulator at 11W, the heatpipes easily transport almost all the heat to the HSF.

mCubed claims that with the Borg system, "4 heatpipes can transport up to 120W to the heatsink". Although my four-heatpipe test results suggested the heatpipes were near capacity, this spec indicated that two heatpipes could actually carry the test-rig's 59W. To test this, I removed the two outer heatpipes and retested. Here is a photo of the new configuration:


Just two pipes this time.

mCubed 2-pipe HES w/ Thermaltake Pipe 101 HSF on CPU Simulator
Fan
Sound
Heat
Max Temp.
Ambient
Temp. Rise
°C / W
12V
Loud
59W
>80°C
17°C
>63°C
n/a
5V
Very quiet
59W
n/a
17°C
n/a
n/a
12V
Loud
11W
25°C
21°C
4°C
0.3636
5V
Very quiet
11W
28°C
21°C
7°C
0.5909
Off
Silent
11W
37°C
21°C
16°C
1.4545
Notes: Arctic Silver 5 TIM was used at the heatsink / simulator interface. Temperatures were recorded after the simulations had run for over 36 hours

The temperature kep climbing past 80°C with the HSF at 12V at 59W heat load. There was no point in testing cooling performance with the HSF at 5V. I also tested with reduced CPU-Simulator power (by reducing the source voltage to 5V). It's pretty obvious that a two-heatpipe configuration is not up to moving ~60Watts efficiently. I would estimate from these results that the two-heatpipe configuration is good for 40W maximum. *(See Final Editor's Note below.)

CONCLUSIONS

From my experience in working with and testing the Borg Heatpipes, I came to the following conclusions:

  • The heatpipes are very soft and easily bent into position (see photo below).
  • Adding an additional right-angle bend to each of these two heatpipes had absolutely no effect on the efficiency of their heat transport.
  • These heatpipes work! I would not however try to move 30W of energy per pipe that the specs indicate as their capability. Rather, I would suggest halving the heat to 15W per pipe in order to keep the CPU temperature to a reasonable level. The heatpipes and blocks must be well "connected" mechaniically to both the CPU and cooling mechanism (eg: HSF or Radiator Block).
  • Given their form and flexibility, these heatpipes are great for redistributing CPU heat.
  • For a slim low-profile case, I expect the best configuration would be the standard 1U HSF, but with cooling assistance via heatpipes and a secondary cooling system - this will be my next project!


The extra bend had absolutely no effect on the efficiency of either heatpipe's heat transport.

* * *


*Final Editor's Note

When thinking about heat transfer, it's useful to consider thermal pathways, points of constriction, and the speed of heat transfer. The fact is that a heatpipe or any thermally conductive path can transfer even thousands of watts — if the pathway is narrow, it will just take longer, much like dial-up versus DSL. This is a key point. If the heat is transferred too slowly, then it will build up at the source and cause temperature rise at the source, in this case, in the CPU. If the heat builds up too much, the CPU will overheat. So mCube's assurance that their 4-heatpipe system will transfer up to 120W is rather misleading, because it's subject to what happens outside the heatpipe and probably applicable only in the context of their own cases.

From a practical standpoint, there are several issues around the use of heatpipes to cool a CPU:

  • The heatpipes can only transfer heat from one end of the pipe to the other. You have to get the heat into one end and get it out of the other end. So there are three points of potential thermal loss: At the hot end, through the pipe itself, and at the cooling end.
  • Assuming that ALL of the heat from the source is effectively transferred to one end of the heatpipe, the speed of the heat transfer depends not only on the efficacy of the heatpipe's internal vapor-change action, but also on what happens at the other end. How big a cooling surface area that end is connected to, how well it's joined, and how much airflow there is — all of these factors matter.
  • Thermal transfer can be bottlenecked at the heat source, through the heatpipe, and at the condenser end. Any of these can hurt cooling performance.
  • In Brendan's test, both the heat source and the Thermaltake heatsink/fan are intrinsic to the test. Changing the HSF could have a significant effect on the measured °C/W value of the system and the apparent heat transfer of the heatpipes, just like lowering the heat at the source did. Because a heatsink with greater heatsink area (say, the Scythe Ninja or Thermalright Ultra-120) was not tried, we can't know for sure whether the heatpipes or the HSF was the greater bottleneck. It seems logical to believe that a bigger HSF would have given better results with the heatpipes. The test results, then, are most pertinent when the cooling surface area and the airflow aross it at the cooling end is about the same as the Thermaltake HSF.

The author's conclusion about keeping the thermal load to 15W per pipe still seems prudent. DIY PC silencers should generally exercise a bit of caution about cooling.


Other SPCR articles of related interest

Fanless Heatpipe CPU Cooling System by FMAH
Boxing & Watercooling to Silence
Zalman TNN-300 Fanless PC Enclosure System
Fanless Ultra Powerhouse PC by EndPCNoise

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Comment on this article in the SPCR forums.

About the Author

Location: Auckland, New Zealand

Day Job: Account Manager - Diagnostic Medical Imaging (eg: CT, MR, X-ray, etc)

Hobbies: Breakaway PC's. By this I mean anything that is outside the current mainstream. Once I had built a few 'normal' PCs, I started overclocking > which lead me to cooling and silencing > which lead me to SFFs > which lead me to cooling and silencing > which lead me to HTPCs > which lead me to slim-HTPCs > which lead me to cooling & silencing (again!).

Personal Spreel: I started life as an Aviation Technician which is where I gained my quals and experience in electronics. Although I consider myself very young ;-) back when I was learning the ropes, computers were room sized for commercial/security applications and we were calling PC's an AT or 286. After Aviation, Diagnostic Imaging (X-ray machines back then) found me and since then I have been either fixing these systems or more lately selling them. At one point I started a PC company in order to buy from PC components from distributors/agents and to fund my hobby. This went ok but at tax time I had to spend too much time at the books, so when we left for Australia (job promotion) I left the company behind with a mate.

In the last two years, I have been following HTPC's (with all the associated hype) and somewhere along the way decided my HTPC should be no larger than my Tivo or DVD player. This brought me back to PC cooling and silencing which has always interested me. By combining slim-HTPCs with my passion for cooling and silence I have a challenge akin to shoving a model ship into a glass bottle (there are similarities: they both have impossibly small "containers", drive you crazy trying to get it right, and look great in the living room!).

Currently I am waiting for Hiper Group to release their new Media Chassis. Whilst this happens I have delved back into - yep - cooling & silencing! And this has of course brought me to the good folks of SPCR!!



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