Wednesday, September 28, 2016

Heat Pipes

Heat pipes are used in lots of modern computer hardware, they are the copper pipes that are threaded though and around the heat sinks and heat generating devices on motherboards and graphics cards. Heat pipes look like regular copper tubing until you try to change its temperature, an empty pipe will heat at one end and very little at the other, over time the tube will reach a steady state with a steep temperature gradient from one end to the other.

Applying heat to a heat pipe will quickly cause a rise in temperature along the entire length of the tube with a temperature gradient of only a few degrees. If you cool the far end of the pipe while it is hot, its temperature will drop along the whole length of the pipe, even the part where heat is being applied. With a heat sink at one end and a CPU at the other is is possible to make the effective radiating surface of the CPU hundreds of times greater than its actual one or two square cm.

Making a heat pipe is relatively simple: take a tube, and add a very small volume of water, then pull as hard a vacuum as possible inside the tube before sealing it. Pulling a vacuum lowers the boiling point of the water until it is boiling at room temperature, once sealed the liquid water will always be just on the verge of boiling, stopped only by the pressure of the water vapor in the upper part of the tube. When heat is applied the balance is disturbed and the liquid water starts to boil, water vapor from the boiling water travels up the tube carrying the heat with it. Once the vapor touches the cool sides of the tube it condenses back into liquid water where it can run back to the hot end and absorb more heat. Provided that all the water doesn't boil off at the same time the heat pipe can conduct head hundreds of times better than any solid material.

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