What is this copper thing?

Originally Posted by sawmilleng

Wouldn't the heat pipe originally have had some low boiling point liquid sealed inside of it? Where the liquid would boil at the CPU chip end, the vapor travel up the pipe to the cool end, condense, (giving up heat) and then run back down the pipe to the CPU as a liquid to start the cycle all over again?

I would guess that the strands of fine copper wire inside the tubes acted to improve the heat transfer into the liquid.

Can anyone offer any insight if this is the way they worked?

Saw! How are you? I meant to post this when you asked your question, but I was having browser problems and gave up.

Yes, that's how a true heat pipe works; it's part of a closed-loop evaporative cooling system. In CPU cooling applications, vapor travels from the hot end of the system, where the CPU is, through the heat pipe to the cold end (heat sink), which typically consists of an array of cooling fins, possibly coupled with a fan. Condensation then returns the vapor to the liquid phase, and the liquid travels back to the heat source. However, these systems don't necessarily rely on gravity to get the condensate back to the hot end; they also use capillary action. Most CPU heat pipes include a capillary structure (or wick) on their inner wall. Vapor condenses on the pipe wall at the cold end and migrates through the capillary structure toward the hot end at a rate sufficient to replenish the liquid that evaporates at the heat source. The most common types of capillary structure used in CPU applications are sintered metal powder, metal mesh, and grooves. Here are some heat pipe cross sections showing each type, respectively:



The iMac G4 actually used its case as a heat sink. The two heat pipes that ran from the CPU terminated in thermal contact with a big conductive metal dome that rested just beneath the outer casing. In this picture you can see the two metal "pads" (circled in red) where the heat pipes in the base attached to the upper metal case:



And this is the metal dome itself, which functions as a heat sink and as a Faraday cage for the iMac:





To achieve adequate heat transfer, the specifications called for thermal paste to be applied at these joints and at the interface with the CPU. The thermal design tolerances must have been pretty tight, because problems with overheating occurred if the outer ends of the heat pipes didn't make full contact with the case due to slight misalignment, under-tightened bolts, or even too little or too much thermal paste.

I've never dissected one of these machines, so I don't know from firsthand experience what's inside these pipes. However, the Apple manual calls them "thermal pipes," which in this context is synonymous with heat pipes, and technicians usually refer to them as heat pipes. so by definition they should contain a working liquid. If you cut open a heat pipe, you find the fluid in the wick lining, where most of it is absorbed.

The coolest application (both figuratively and literally) of this type of system is in the pipeline support structures used for the elevated portions of the Trans Alaska Pipeline System. The vertical support members (pilings) include heat pipes to prevent melting of the permafrost in which they are anchored. There are approx. 420 miles of elevated pipeline and over 124,000 heat pipes.