Heat Pipe Technologies
The modern heat pipe technology was originated from the Los Alamos Scientific Laboratory in 1963. A good description of the history of the heat pipe technology can be found in an early article published by Yale Eastman (currently a Director of ACT) in Scientific American in May 1968.
A typical tubular heat pipe is illustrated in the figure below.
It is a vacuum tight device consisting of a working fluid and a wick structure. The heat input vaporizes the liquid working fluid inside the wick in the evaporator section. The vapor, carrying the latent heat of vaporization, flows towards the cooler condenser section. In the condenser, the vapor condenses and gives up its latent heat. The condensed liquid returns to the evaporator through the wick structure by capillary action. The phase change process and two-phase flow circulation continue as long as the temperature gradient between the evaporator and condenser is maintained.
In addition to the common tubular form, heat pipes can be made into annular and planar configurations to fit each application. Other relevant variations include loop thermosyphons and loop heat pipes. Both loop thermosyphons and loop heat pipes have condensers that are located far away from the evaporators and use the evaporation and condensation heat transfer mechanisms to absorb and reject heat. In a loop thermosyphon, the condensed liquid is returned from the condenser to the evaporator by gravity. In a loop heat pipe, the driving force for the liquid return is capillary forces developed in the evaporator wick structure. Both the loop thermosyphon and the loop heat pipe are suited for heat transport over relatively long distances.
The most commonly used heat pipe working fluid today is water, which is suitable for temperatures between 20°C and 250°C. Heat pipes operating at lower temperatures use methanol, ammonia, propylene, ethane, nitrogen, oxygen and hydrogen as working fluids. For higher temperature applications, alkali metals such as cesium, potassium, sodium and lithium are used.
ACT has designed and manufactured heat pipes having various wicks of various geometries, and operating at various temperatures from -150°C to 1,100°C. These heat pipes have been used in diverse applications including spacecraft thermal control, medical surgery temperature control, catalytic reactor cooling, and electronics thermal management.
Over the past several years, ACT has been actively developing high-temperature water heat pipes and intermediate temperature heat pipes that are potentially useful in thermal management applications requiring operation between 150 and 475°C. Traditional copper/water and superalloy/alkali metal heat pipes are ineffective at these temperatures. ACT's development efforts were initially driven by spacecraft thermal control applications and have more recently expanded into a number of consumer product and industrial process applications. The development efforts include extensive materials evaluation, life test, high performance wick design, and integration of heat pipes in assemblies.
Below is a slide show of photos of various heat pipes manufactured by ACT.
(operating between 20 and 200°C)
(operating between 20 and 200°C)
(operating down to -105°C)
(CCHPs, operating between -40 and 60°C)
(VCHP, operating between -40 and 60°C)
(operating between 20 and 275°C)
in a radiator panel
(operating between 200 and 475°C)
(operating up to 275°C)
(operating between 300 and 600°C)
(operating between 400 and 1,000°C)
(operating at 750°C)
(operating between 20 and 120°C)