figure 1: heat pipe operation.

introduction

all electronic components, from microprocessors to high end power converters, generate heat and rejection of this heat is necessary for their optimum and reliable operation. as electronic design allows higher throughput in smaller packages, dissipating the heat load becomes a critical design factor. many of today's electronic devices require cooling beyond the capability of standard metallic heat sinks. the heat pipe is meeting this need and is rapidly becoming a main stream thermal management tool.

heat pipes have been commercially available since the mid 1960's. only in the past few years, however, has the electronics industry embraced heat pipes as reliable, cost-effective solutions for high end cooling applications. the purpose of this article is to explain basic heat pipe operation, review key heat pipe design issues, and to discuss current heat pipe electronic cooling applications.

heat pipe operation

a heat pipe is essentially a passive heat transfer device with an extremely high effective thermal conductivity. the two-phase heat transfer mechanism results in heat transfer capabilities from one hundred to several thousand times that of an equivalent piece of copper.

as shown in figure 1, the heat pipe in its simplest configuration is a closed, evacuated cylindrical vessel with the internal walls lined with a capillary structure or wick that is saturated with a working fluid. since the heat pipe is evacuated and then charged with the working fluid prior to being sealed, the internal pressure is set by the vapor pressure of the fluid.

as heat is input at the evaporator, fluid is vaporized, creating a pressure gradient in the pipe. this pressure gradient forces the vapor to flow along the pipe to a cooler section where it condenses giving up its latent heat of vaporization. the working fluid is then returned to the evaporator by the capillary forces developed in the wick structure.

heat pipes can be designed to operate over a very broad range of temperatures from cryogenic (< -243°c) applications utilizing titanium alloy/nitrogen heat pipes, to high temperature applications (>2000°c) using tungsten/silver heat pipes. in electronic cooling applications where it is desirable to maintain junction temperatures below 125-150°c, copper/water heat pipes are typically used. copper/methanol heat pipes are used if the application requires heat pipe operation below 0°c.

heat pipe design

there are many factors to consider when designing a heat pipe: compatibility of materials, operating temperature range, diameter, power limitations, thermal resistances, and operating orientation. however, the design issues are reduced to two major considerations by limiting the selection to copper/water heat pipes for cooling electronics. these considerations are the amount of power the heat pipe is capable of carrying and its effective thermal resistance. these two major heat pipe design criteria are discussed below.

limits to heat transport

the most important heat pipe design consideration is the amount of power the heat pipe is capable of transferring. heat pipes can be designed to carry a few watts or several kilowatts, depending on the application. heat pipes can transfer much higher powers for a given temperature gradient than even the best metallic conductors. if driven beyond its capacity, however, the effective thermal conductivity of the heat pipe will be significantly reduced. therefore, it is important to assure that the heat pipe is designed to safely transport the required heat load.

the maximum heat transport capability of the heat pipe is governed by several limiting factors which must be addressed when designing a heat pipe. there are five primary heat pipe heat transport limitations. these heat transport limits, which are a function of the heat pipe operating temperature, include: viscous, sonic, capillary pumping, entrainment or flooding, and boiling. figures 2 and 3 show graphs of the axial heat transport limits as a function of operating temperature for typical powder metal and screen wicked heat pipes. each heat transport limitation is summarized in table 1.

table 1: heat pipe heat transport limitations.

figure 2: predicted heat pipe limitations.

as shown in figures 2 and 3, the capillary limit is usually the limiting factor in a heat pipe design.

figure 3: predicted heat pipe limits.

the capillary limit is set by the pumping capacity of the wick structure. as shown in figure 4, the capillary limit is a strong function of the operating orientation and the type of wick structure.

figure 4: capillary limits vs. operating angle.

the two most important properties of a wick are the pore radius and the permeability. the pore radius determines the pumping pressure the wick can develop. the permeability determines the frictional losses of the fluid as it flows through the wick. there are several types of wick structures available including: grooves, screen, cables/fibers, and sintered powder metal. figure 5 shows several heat pipe wick structures.

it is important to select the proper wick structure for your application. the above list is in order of decreasing permeability and decreasing pore radius.

grooved wicks have a large pore radius and a high permeability, as a result the pressure losses are low but the pumping head is also low. grooved wicks can transfer high heat loads in a horizontal or gravity aided position, but cannot transfer large loads against gravity. the powder metal wicks on the opposite end of the list have small pore radii and relatively low permeability. powder metal wicks are limited by pressure drops in the horizontal position but can transfer large loads against gravity.

effective heat pipe thermal resistance

the other primary heat pipe design consideration is the effective heat pipe thermal resistance or overall heat pipe δt at a given design power. as the heat pipe is a two-phase heat transfer device, a constant effective thermal resistance value cannot be assigned. the effective thermal resistance is not constant but a function of a large number of variables, such as heat pipe geometry, evaporator length, condenser length, wick structure, and working fluid.

figure 5: wick structures.

the total thermal resistance of a heat pipe is the sum of the resistances due to conduction through the wall, conduction through the wick, evaporation or boiling, axial vapor flow, condensation, and conduction losses back through the condenser section wick and wall.

figure 6 shows a power versus δt curve for a typical copper/water heat pipe.

figure 6: predicted heat pipe δtt.

the detailed thermal analysis of heat pipes is rather complex. there are, however, a few rules of thumb that can be used for first pass design considerations. a rough guide for a copper/water heat pipe with a powder metal wick structure is to use 0.2°c/w/cm2 for thermal resistance at the evaporator and condenser, and 0.02°c/w/cm2 for axial resistance.

the evaporator and condenser resistances are based on the outer surface area of the heat pipe. the axial resistance is based on the cross-sectional area of the vapor space. this design guide is only useful for powers at or below the design power for the given heat pipe.

for example, to calculate the effective thermal resistance for a 1.27 cm diameter copper/water heat pipe 30.5 cm long with a 1 cm diameter vapor space, the following assumptions are made. assume the heat pipe is dissipating 75 watts with a 5 cm evaporator and a 5 cm condenser length. the evaporator heat flux (q) equals the power divided by the heat input area (q = q/a_evap; q = 3.8 w/cm²). the axial heat flux equals the power divided by the cross sectional area of the vapor space (q=q/a_vapor; q = 95.5 w/cm²).

the temperature gradient equals the heat flux times the thermal resistance.

it is important to note that the equations given above for thermal performance are only rule of thumb guidelines. these guidelines should only be used to help determine if heat pipes will meet your cooling requirements, not as final design criteria. more detailed information on power limitations and predicted heat pipe thermal resistances are given in the heat pipe design books listed in the reference section.

heat pipe electronic cooling applications:

perhaps the best way to demonstrate the heat pipes application to electronics cooling is to present a few of the more common examples. currently, one of the highest volume applications for heat pipes is cooling the pentium processors in notebook computers. due to the limited space and power available in notebook computers, heat pipes are ideally suited for cooling the high power chips.

fan assisted heat sinks require electrical power and reduce battery life. standard metallic heat sinks capable of dissipating the heat load are too large to be incorporated into the notebook package. heat pipes, on the other hand, offer a high efficiency, passive, compact heat transfer solution. three or four millimeter diameter heat pipes can effectively remove the high flux heat from the processor. the heat pipe spreads the heat load over a relatively large area heat sink, where the heat flux is so low that it can be effectively dissipated through the notebook case to ambient air. the heat sink can be the existing components of the notebook, from electro-magnetic interference (emi) shielding under the key pad to metal structural components. a notebook heat pipe is shown in figure 7.

figure 7: typical notebook heat pipe heat sink.

typical thermal resistances for these applications at six to eight watt heat loads are 4 - 6°c/watt. high power mainframe, mini-mainframe, server and workstation chips may also employ heat pipe heat sinks. high end chips dissipating up to 100 watts are outside the capabilities of conventional heat sinks. heat pipes are used to transfer heat from the chip to a fin stack large enough to convect the heat to the supplied air stream. the heat pipe isothermalizes the fins eliminating the large conductive losses associated with standard sinks. the heat pipe heat sinks dissipate loads in the 75 to 100 watt range with resistances from 0.2 to 0.4°c/watt, depending on the available air flow.

in addition, other high power electronics including silicon controlled rectifiers (scr's), insulated gate bipolar transistors (igbt's) and thyristors, often utilize heat pipe heat sinks. heat pipe heat sinks are capable of cooling several devices with total heat loads up to 5 kw. these heat sinks are also available in an electrically isolated versions where the fin stack can be at ground potential with the evaporator operating at the device potentials of up to 10 kv. typical thermal resistances for the high power heat sinks range from 0.05 to 0.1°c/watt. again, the resistance is predominately controlled by the available fin volume and air flow.

scott d. garner p.e.
thermacore inc. 
lancaster, pa.

references:

1. brennan, p.j. and kroliczek, e.j., heat pipe design handbook, b&k engineering, nasa contract no. nas5-23406, june 1979.
2. chi, s.w., heat pipe theory and practice, hemisphere publishing corporation, 1976.
3. dunn, p.d. and reay, d.a., heat pipes, 3rd. edition, permagon press, 1982.
4. eastman, g. yale and ernst d.m., heat transfer technology (heat pipe), kirk-othmer: encyclopedia of chemical technology, volume 12, john wiley and sons, inc., 1980.
5. peterson, g.p., an introduction to heat pipes modeling, testing, and applications, john wiley and sons, inc., 1994.