Today I was looking at some heat sinks for my computer and I noticed that the latest trend in computer heat sinks is going toward the “heat pipe” technology. I never really paid attention to these and at a glance thought they were just using solid copper bars to conduct heat away from the processors. They aren’t.
I should have remembered from school that “heat pipes” are actually copper tubes with a working fluid in them. The operation of a heat pipe is simple in principle but to design one takes quite a bit of thought.
Everyone knows that water boils at 100 deg C. If you didn’t know that, stop reading now, there’s no hope for you. So boiling is a pretty cool thing. The reason is that as water boils it absorbs energy, but does not increase in temperature. All fluids are like this. So if you have a whole lot of heat to get rid of, and you know what temperature you want to keep the thing giving off heat, just find a liquid that boils at that temperature! If you can use a device to focus all that heat energy into the liquid, the temperature of the device won’t exceed the boiling point of the liquid until all the liquid is gone.
Don’t believe me? Next time you have a camp fire, fill a paper cup with water and place it in the fire. The water starts to boil, and as long as the cup has water, it doesn’t burn.
So how does this relate to a heat pipe? Well a heat pipe is a pipe made of metal, with a small amount of liquid in it. If you hold it upright, and heat the bottom, the liquid at the bottom will boil. The gas produced from the liquid gets “caught” at the top of the pipe, where it condenses on the colder part of the pipe. When the gas condenses, it dumps all the heat that was required to make it boil onto the surface of the pipe. So now you have a method to transport a giant amount of heat over a pretty good distance with no temperature gradient.
So if you compared that to a solid bar, which requires a temperature gradient to transfer heat through conduction, the top of the solid bar would be some temperature colder than the bottom of the bar in order to make the heat move through the bar. The heat pipe would be the same temperature at the top of the bar, since the liquid only boils/condenses at one temperature.
So this brings me to my rant. Hopefully it is obvious to you by now that the heat pipe has to be in a vertical, or at least inclined position so that the gas can rise to the top cold side and the liquid can run to the hot side. Well computer coolers are all shown in this vertical position, but when you install them they become sideways due to the position of the computer. There is no site so far that tests the effectiveness of the heat pipe coolers on their side vs right-side up. So if you are a computer component test person, I would like to see this done. If these style of heat sinks are as popular as they seem to be, then people are going to need to know this stuff pretty soon.