A type of passive heat exchanger (requiring no external energy source) that draws heal from a heat source and gives up heat to a heat sink, the atmosphere in most cases. In its basic form a heat pipe consists of a closed tube (the shell) two to six inches in diameter and as long as need be. The shell has a porous wick made of fine metal mesh in the inside circumference extending from top to bottom. The shell also contains a quantity of working liquid which may be anhydrous ammonia, liquid metals, glycerine, methanol, or acetone. Heat taken in or absorbed at the lower end of the boat pipe, the end in contact with the heat source, cause,, the liquid to evaporate and move up the pipe as a vapor. The dissipation of the heat at the upper end condenses the vapor which, as a liquid, moves back down the pipe in the wick by gravity or capillary action. The continuous cycle of vaporization and condenation within the closed pipe makes the heat pipe an efficient, natural-convection heat transfer loop. On the trans-Alaska pipeline, thousands of heat pipes were installed along the big line to maintain the frozen soil around the vertical support members.
