Heatpipes: The Investigation Begins
Phoronix: Heatpipes: The Investigation Begins
Since breaking open bottles of beer with heatpipes and other hardware last month at the Intel Developer Forum in San Francisco, we have been cutting open a number of different heatpipes. In this article we have some new details to shed on heatpipes from a numbers of manufactures, including Thermalright, Thermaltake, OCZ, and Abit. These cooling mechanisms are supposed to keep our beloved PCs from overheating, but how does their manufacturing quality differ? With this article, we have plenty of pictures and videos showing you the differing qualities in heatpipes.
You might have to look harder...
A typical heat pipe contains a mixture of alcohol and water at less than atmospheric pressure. The pressure is to adjust the boiling point of the liquid and give it somewhere to go when it does boil. Maybe some people have figured out a mixture that can be put in at atmospheric pressure, but it is still important to exclude air from the inside of a heat pipe. Air doesn't boil and condense, so it shouldn't be there any more than it should be in your brake lines or air conditioner's freon piping.
The walls are lined with a wicking material that holds the liquid and carries it back to the hot part, where it boils and enters the lumen of the pipe, travels to the cold end, condenses, and repeats the cycle.
But the important thing is that, in a cold state, ALL of the liquid is held in the wick material. Any excess would be pointless. And it's held quite firmly; remember it's the capillary action that sucks the liquid the whole length of the heat pipe back to the hot end.
If you want to test the build quality of heat pipes, find someone who can introduce a micro pressure sensor into a sealed pipe and check for a consistent temperature/pressure curve. But that's a little more complicated than hacking one open and shaking wet copper braid to see if any drops fall off.
Might Want to Heat the Heatpipe
Heatpipes can also contain paraffin wax due to its low melting point and high thermal conductivity. Paraffin wax is solid at room temperature so it wouldn't simply flow out of a broken heatpipe unless the heatpipe was hot enough to melt the wax. This could lead to the false assumption that a heatpipe is empty when it really isn't.
no fluid visible is expected
Where to start? The heat pipes shown in the investigation use a sintered copper powder for the wick structure. This can be seen in the cross section of the pictures. A properly designed heat pipe that uses this type of wick structure will not have any excess water inside and there for no water will run out when opened. All of the water is held in the wick by the capillary action of the fluid. Very much like a wet paper towel.
There is evidence of fluid if you look closely. 1) the slight noise you heard when you cut them open is air rushing back into the heat pipe. The heat pipe is a vacuum device so when the seal is broke it will suck in air. 2) Now that the water is exposed to atmospheric pressure it will not vaporize until it reaches 100 degrees C. 3) If you carefully heat the opened heat pipe to over 100 degrees C you will see the water in the wick vaporize. 4) there is a very simple test for checking if a heat pipe is functioning. Stick one end in boiling water about 2 inches deep and hold the other end. See how long you can hold it? Should be a very short time, about 5 to 10 seconds. Try this with a heat pipe where you have cut the end off.
If indeed these parts functioned as cooling devices before they were cut open it is almost certain the heat pipes were functioning properly.
FYI, I have been in the heat pipe business since the 70's and have learned one or two things about them. Any questions, I would be happy to answer them..