Structure and Principle
The heat pipe is hollow with the space inside evacuated, much the same as the solar tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside. Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100℃, but if you climb to the top of a mountain the boiling temperature will be less that 100℃. This is due to the difference in air pressure.
Based on this principle of water boiling at a lower temperature with decreased air pressure, by evacuating the heat pipe, we can achieve the same result. The heat pipes used in solar collectors have a boiling point of only 30℃. So when the heat pipe is heated above 30℃ the water vaporizes. This vapour rapidly rises to the top of the heat pipe transferring heat. As the heat is lost at the condenser (top), the vapour condenses to form a liquid (water) and returns to the bottom of the heat pipe to once again repeat the process.
At room temperature the water forms a small ball, much like mercury does when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard rattling inside. Although it is just water, it sounds like a piece of metal rattling inside.
This explanation makes heat pipes sound very simple. A hollow copper pipe with a little bit of water inside, and the air sucked out! Correct, but in order to achieve this result more than 20 manufacturing procedures are required and with strict quality control.
Structure and Principle Heat-pipe vacuum tube