Heat pipes might seem like a new concept, but you are probably using them every day and don''t even know it. Laptop computers often using small heat pipes to conduct heat away from the CPU, and air-conditioning system commonly use heat pipes for heat conduction.
The principle behind heat pipe's operation is actually very simple.
Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100oC (212oF), but if you climb to the top of a mountain the boiling temperature will be less than 100oC (212oF). 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 AUDSUN solar collectors have a boiling point of only 30oC (86oF). So when the heat pipe is heated above 30oC (86oF) 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. This principle is used on not only the heat pipe collector but also the AUDSUN pressure system solar water heater.
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. By this way we can know the heat pipe is available.
Because the heat pipe is located within the evacuated glass tube, brief overnight temperatures as low as -20oC (14oF) will not cause the heat pipe to freeze. Plain water heat pipes will be damaged by repeated freezing. The water used in AUDSUN heat pipes still freezes in cold conditions, but it freezes in a controlled way that does not cause swelling of the copper pipe.
Water or antifreeze can be used for circulation between heat pipe collector and the heat exchanger inside the storage cylinder. Antifreeze is recommended for cold area. |
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