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Heatsinks move heat away from its point of origin by conducting the heat through metal, usually aluminum or copper. Heatpipes move heat through a process known as Vapor Phase. Vapor phase is the same process that keeps your work environment cool in the summer time, and keeps your frozen produce frozen.
What is Vapor Phase? Vapor phase is, simply put, a liquid evaporating into a gas, resulting in a cooling effect. Ever notice how the wind feels cold after getting out of the pool, even though it’s a warm and sunny day outside? The wind feels cold because it is evaporating the water that’s on your skin.
The heat source at the bottom of the heatpipe heats the liquid to its boiling point. When the liquid boils, it cools itself by evaporation. The liquid will remain at its boiling point, but will not exceed more than a few degrees above its boiling point, because the constant evaporation stabilizes its temperature.
Now that some of the liquid is a gas, it must be cooled in order to condense back to a liquid state. The surface area of the cylinder from the fluid level up performs this duty. The gas “passes” its heat off to the cylinder walls, which allows the gas to cool and return to its liquid state. The cylinder walls “pass” the heat off to the air on the outside of the cylinder. Many heatpipes have fins on the outer side of the walls to aid in “passing” the heat to the air.
OK, I know what you’re thinking: “You mean I have to heat the liquid to its boiling point?” Well, yes, you do. But pressure can be used to control the boiling point. Liquids with a high boiling point – like water or alcohol – will boil at a lower temperature when placed in a vacuum. Chemicals that are natively a gas, like R134A, a common refrigerant, can be liquefied when under pressure. The trick is to get the pressure just right so that the boiling point is just above room temperature. So if you build a heatpipe, you will need to do some experimenting to find the optimal pressure.
Of course, the most exciting use for heatpipe is using them in conjunction with other radical forms of cooling. I’ve heard from several guys who said that they are interested in using them in conjunction with peltiers. I myself plan to build one to use in conjunction with intercooling – cooling water with a peltier and using a heatpipe to remove the heat from the peltier’s hot side.
I’m sure that we will see many interesting uses for heatpipes over the coming months and years. Now that you know the basics, I encourage you to go out and find further information pertaining to building you own heatpipes, and start horsing around right away. Have fun folks, but play safely 🙂
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