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Heat Pipe Design Guidelines Document
Heat pipes are a transport mechanism to move heat from the hot source to an area where the heat can be dissipated. Heat pipes do not actually dissipate the heat and are therefore incorporated into many different types of heat sinks as helpers. A heat pipe is a copper tube with an internal wick structure that is sealed on both ends with a small amount of water inside. As heat is applied to the pipe, the water will boil and turn to a gas, which then travels to the colder section of the heat pipe where it condenses back to a liquid. It is the evaporation and condensing of the water that forms a pumping action to move the water (and thus the heat) along the pipe.
\.There are many types of wick structures that can be used within the heat pipe and they are generally classified as grooved, mesh, powder, and hybrid. A grooved heat pipe is a copper tube with a series of shallow grooves around the internal perimeter of the heat pipe. While the water is a liquid, it travels in the grooves and while it is a vapor it travels in the open space of the pipe. Grooved pipes can be used in horizontal orientations, but are very limited in performance if used about 15° out of horizontal.
A mesh heat pipe is a smooth walled copper tube with a woven copper mesh installed along the interior of the pipe. The mesh is designed to remain in contact with the walls of the pipe in areas where the pipe may be bent or flattened. Mesh pipes can be used in horizontal and about 30° out of horizontal orientations.
A powder wick heat pipe can also be known as a sintered heat pipe. During the manufacturing process a mandrel is installed in the center of the pipe and copper powder is poured into the pipe around the mandrel. After the powder is sufficiently packed, the parts are placed into a sintering oven. Once at temperature, the copper powder will stick to the pipe and to itself, forming lots of internal pockets like a sponge. Because of the small pocket sizes, sintered pipes can efficiently move the water and can be used horizontally, vertically, and at all points in between, including upside down.
The performance (amount of heat that can be transferred) of a heat pipe is a function of its length, diameter, wick structure, and overall shape. The larger the diameter, the more power that can be transported, but the longer the length, the less capable the pipe. Sintered heat pipes are better than meshed, which are better than grooved. Heat pipes can be bent and flattened in order to move the heat where needed or to fit into smaller spaces, but these types of modifications impact the total heat that can be transported. In cases where the amount of heat needed to be transported is too great for a single pipe, multiple pipes can be used in parallel and series to move more heat over greater distances.