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Originally Posted by pHaestus
But if only transport is involved then shouldn't C/W remain fixed? It has to serve as another repository for CPU heat to lower the C/W, correct? Or does the C/W have a different meaning now as you have basically "lowered the thermal resistance of copper"?
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I'll jump in. Please correct me if my understanding is wrong.
C/W does not remain fixed because the transport mechanism in a heat pipe only works for a defined range of input power. This is why (in my opinion) heat pipes stink for general purpose usage. They have to be made for a specific heat load, which is fine for a manufactured system, where you match the heat pipe to a known CPU with known heat output. For consumers, you'd have to select a heat pipe for your particular CPU and then swap it out if you overclocked it differently.
Heat pipes work by depending upon a pool of liquid that evaporates from the hot end, travels to condense on the cool end, and then uses capillary action to migrate back to the hot end (repeat ad nauseum). This mechanical action of moving the phase change liquid (I doubt it is water, it is probably an alcohol or cheap refrigerant) depends upon several things.
If the hot end isn't hot enough to vaporise the liquid, then no heat pipe action occurs (no phase change).
If the hot end becomes too hot, then it "dries out", as Bill mentioned. The cool end may be able to recondense the liquid, but the liquid may re-vaporise before it reaches the hot-side.