Heat Pipe Design Heat Pipe Working Fluids Because a heat pipe is an evacuated For electronics cooling device, the working fluid in a heat applications, copper/water heat pipes pipe will boil at any temperature are typically used. If the application above its freezing point. The heat requires heat pipe operation below pipe will begin to operate as soon 0˚C (32˚F), copper/acetone or copper/ as heat is applied to the device. The methanol heat pipes are often used. internal pressure of a heat pipe is the Under most conditions, water is saturation pressure of the working thermodynamically superior to other fluid at the corresponding heat pipe fluids and is the fluid of choice within operating temperature. For example, its operating temperature range. the internal pressure of a heat pipe          The typical operating temperature with water as the working fluid is          range for a copper/water heat pipe 0.29 psi at 20˚C, 14.7 psi at 100˚C,          is 10˚C to 200˚C (50˚F to 392˚F), but 224.9 psi at 200˚C, and 1,245 psi at          it can be extended to temperatures 300˚C. Within a heat pipe, working          above 300˚C (572˚F) using alternative fluids, including water, maintain their          envelope and wick structure normal freezing points. A properly          materials. designed copper/water heat pipe will not be damaged by the freeze/thaw cycle found in many commercial and military applications.      Advanced Wick Structures The heat flux capability of a heat pipe is determined by the boiling limit in the wick structure. Grooved, screen and sintered wick structures provide capillary pumping to return liquid to the evaporator and also serve as extended         surfaces   for evaporation or boiling heat transfer enhancement.