A technology that is often overlooked as a design option are heat pipes. Although it is widely used by many industries it is sometimes considered by the uninitiated to be a device that is difficult to design into a system. This is incorrect. Many devices, including most of today’s computers, use heat pipes successfully to cool processors or other components that need high conductivity path from the chip to a remote heat sink and fan.
How Heat Pipes work
As shown in Fig.1, heat pipes are passive devices that transfer heat by the evaporation and condensation of a two-phase working fluid. The internal portion of the pipe contains a porous wick structure that is saturated with liquid working fluid. When heat is applied at one end and creates a temperature difference within the heat pipe, the liquid evaporates, and then the vapor travels to the condenser at the opposite end of the heat pipe. In the condenser, a heat sink or heat exchanger removes heat by dissipation to the colder environment, condensing the vapor. Capillary forces in the wick return the condensate to the evaporator, where the cycle repeats.
Typical electronics cooling heat pipes have a copper envelope and wick, and use water as the working fluid. The envelope is evacuated, water is added, and then the heat pipe is sealed. Properly fabricated copper/water heat pipes have been operated for over twenty years without maintenance.
For a given heat pipe design, the maximum heat pipe power is a function of length, adverse gravitational heat (evaporator located above the condenser), and temperature. The issues of gravity and g-loading must be understood by the designer and the heat pipe thermal management scenario should understand the system requirement for orientation with respect to gravity as well as the potential for g-loads to be experienced during operation. (e.g., Flight environment for aircraft or UAVs). In a 1-g environment, water heat pipes can operate up to 10 inches (25 cm) against gravity[1], although the power decreases as the adverse elevation increases; see Fig.2. A free calculator to calculate heat pipe performance is located here.
Heat pipes are widely used for cooling of desktop and notebook computers. NASA has also used this technology on many programs. When the environmental conditions are well known and gravitational effects are accounted for, this technology can provide a very effective means of transferring heat at small ΔT’s. Such a method would be advantageous in laser crystal cooling, laser diode cooling, power electronics, and power supplies as well as other high temperature components. The heat pipe is utilized in wide range of industries these days which include injection molding, bioprocessing, temperature calibration,[2] and solar.
A heat pipe does not have a set operating temperature; heat pipes can theoretically operate from the triple point to the critical point, although the performance is low at both points. For water heat pipes, the heat pipe power is low below ~ 25°C, and reaches a peak around 150°C; see Fig.2. At temperatures below zero, heat transfer is by conduction only through the wick and envelope. (note that water heat pipes can survive thousands of freeze/thaw cycles when properly designed.). The upper temperature limit for a copper heat pipe is around 150°C, when the vapor pressure starts to become significant.
Heat pipes are able to achieve this high conductivity through two-phase heat transfer. The process involves the liquid-vapor phase change at the evaporator, and a vapor-liquid phase change at the condenser. Since both the boiling and condensation heat transfer coefficients are very high, the internal ΔT of the heat pipe is on the order of 2°C. Heat pipes have a much higher effective thermal conductivity than even copper or diamond. While solid conductors such as Aluminum 6063-T6 (~200 W/m-K); Copper (~390 W/m-K) and CVD Diamond (1,500 W/m-K), heat pipes have effective thermal conductivities that are completely beyond anything the design engineer may have envisioned before they discovered heat pipes. These effective conductivity values range from 5,000 W/m-K to as high as 200,000 W/m-K.
When customer requirements permit it, heat pipes may be used in other designs in order to reduce temperature rises from a thermal load to a heat exchanger. Other methods are available to do this including graphite impregnated metal and diamond substrates, although these have much lower effective thermal conductivities, on the order of 550 W/m K[3]
| Thermal path via Heat pipe | Standard thermal path |
| T-air 60 °C | T-air 60 °C |
| T- Hot side ~63 °C | T-T Hot side ~75 °C |
Table 1. Example Comparison of component temperature when heat is conducted to a remote heatsink via heat pipe vs. standard thermal path.
The heat pipe can be a valuable tool to any thermal engineer or mechanical design engineer. Understanding the function of the heat pipe and what it can provide will help any designer solve some of the toughest heat transfer and thermal management problems that they may encounter.
References
[1] Water Heat Pipe Parameters and Limitations, http://www.1-act.com/water-heat-pipe-parameters-and-limitations/
[2] http://www.1-act.com/products/isothermal-furnace-liner/heat-pipe-blackbody-cavity/
[3] http://www.1-act.com/when-to-use-heat-pipes-hik-plates-vapor-chambers-and-conduction-cooling/
