Heat pipe

A heat pipe is a heat-transfer device that employs phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe, a volatile liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid, releasing the latent heat. The liquid then returns to the hot interface through capillary action, centrifugal force, or gravity and the cycle repeats. Due to the very high heat transfer coefficients for boiling and condensation, heat pipes are highly effective thermal conductors. The effective thermal conductivity varies with heat pipe length and can approach 100kW/(m⋅K) for long heat pipes, in comparison with approximately 0.4kW/(m⋅K) for copper. A typical heat pipe consists of a sealed pipe or tube made of a material that is compatible with the working fluid such as copper for water heat pipes, or aluminium for ammonia heat pipes. Typically, a vacuum pump is used to remove the air from the empty heat pipe. The heat pipe is partially filled with a working fluid and then sealed. The working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range. The stated/recommended operating temperature of a given heat pipe system is critically important. Below the operating temperature, the liquid is too cold and cannot vaporize into a gas. Above the operating temperature, all the liquid has turned to gas, and the environmental temperature is too high for any of the gas to condense. Thermal conduction is still possible through the walls of the heat pipe, but at a greatly reduced rate of thermal transfer. In addition, for a given heat input, it is necessary that a minimum temperature of the working fluid be attained ; while at the other end, any additional increase (deviation) in the heat transfer coefficient from the initial design will tend to inhibit the heat pipe action.