The document summarizes current heat pipe technology and its limitations. It describes how heat pipes work by using capillary action in a wick structure to transport heat via phase change of a working fluid between an evaporator and condenser section. Key limitations are the wick capillary and entrainment limits. Heat pipes can currently transfer up to 10W/cm2 and handle heat loads up to 15W. Micro loop heat pipes are an emerging technology that can achieve higher heat transfer densities of up to 30W/cm2 and heat loads over 120W by addressing these limitations.

1. 5/11/2009 1
Fig. 1 Heat Pipe Operation.
Eq. 1
Evaporator Length(cm) 3
Condenser Length(cm) 5
Tranport Length(cm) 7
Total Length(cm) 15
Steam Temp [C] 65
Wick Thickness (micron) 150-180
Pore Diameter (micron) 500
Permeabilty (m^2) 2.3E-09
Example Heat Pipe Parameters
TABLE 1
Current and Future Heat Pipe Technology
Ahmed Shuja, PhD
A heat pipe is a hollow tube of enclosed structure; containing a fluid that transfers large
quantities of heat when it is evaporates and a wick that brings the fluid back to its starting point
when it condenses. The wick fills the entire cavity of the heat pipe. This entire process is
accomplished with no outside power, no mechanical moving parts, and no noise. The heat pipe
is an axial heat transfer device that typically requires a length to diameter aspect ratio of at least
greater than 20:1 and no more than 50:1. The aspect ratio is a complex relationship between
vapor core diameter and wick permeability.
The typical heat pipe as in Fig. 1 consists of a sealed
tube that has been partially evacuated, so that the internal
pressure is below the saturated atmosphere of 14.7 psia.
The inside walls of the tube are usually lined with a
capillary wick structure and a small amount of fluid,
which will vaporize. When heat is applied at one end of
the tube, the fluid within the pipe will vaporizes or boils.
This generates a force that drives the vapor to the
opposite end of the tube, where heat is removed.
Removing the heat forces the vapor to condense, and the
wick draws the fluid back to the starting point, where
the process is repeated.
Heat pipes are well understood and characterized devices and the maximum heat transfer
capacity in W/cm2
can be calculated in a straight forward manner. In Eq. (1) [1] the maximum
heat transfer is shown to be a function of the liquid properties, term one on the right side, the
wick properties, term 2 on the right side, and wick cross sectional area, term 3. The effective
length Leff, of the heat pipe on the left side of the equation is distance from the middle of the
evaporator and middle of the condenser.
Using water’s thermo-physical properties at 65o
C and
characteristic’s of a heat pipe shown in Table 1 the
maximum heat transfer is calculated. These values
represent values for heat pipes produced in high
volume that would be considered competitive to
MLHP. For example, 3mm OD heat pipe with length
of 15cm with an evaporator 3 cm, condenser 5 cm and
sintered copper wick having a thickness, w, of 0.015
thick the calculation show it can handle 12.6W. The
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2. 5/11/2009 2
Fig. 2 Heat Pipe LED Engine.
sintered copper wick gives better performance than radially grooved tubes, screens or cloth
wicks so it will be used for this example. The typical permeability, K (23.2E-10 m^2) and
effective radius, re (241E-6 m) was obtained from reference [2]. Given these parameters the
diameter of the heat pipe can be varied from 3 mm to 8mm and Table 2 is generated. Also in
Table 2 the calculated values are then compared to the measured values from a high volume heat
pipe manufacturer [3] and are found to agree closely. The usable area for heat transfer in an LED
engine application are shown in Table 2. There is not a lot of usable area. Alternatively, the heat
pipe can be flattened to use the full length of the evaporator, in this case the 3cm length and 4mm
width for a 8mm tube diameter, however flattening changes the maximum heat transfer
capability.
Heat Pipes and LEDs
TABLE 2
HEAT PIPE PERFORMANCE TODAY (15 cm length)
3mm 4mm 5mm 6mm 8mm
Max W Max W Max W Max W Max W
Flatten T=2.0mm 3.2 5.1 7.1 10.6 /
Flatten T=2.5mm 3.1 5.4 8.2 14.0 21.2
Flatten T=3.0mm 2.4 5.0 8.7 14.3 22.8
Round End Mount 0.1 0.2 0.3 0.4 0.6
Final Forming Thickness
Q-max Table Watt
Assumes end mount is only distal area of heat pipe.
The important wick properties are the permeability K, the pore size re , and porosity w. By
looking at Eq. (1) a wick with large permeability K and small pore size re will produce a high
heat transport. In the current example the wick has a pore diameter of 500 micron, and high
permeability due to having high porosity 92%.
Integrating multiple bends into heat pipes causes performance degradation of maximum heat flux
by 20% for a single 900
bend or 50% for two 900
bends. Additionally the heat pipe is has
gravitational orientation dependency. In the situation where the evaporator is placed above the
condenser section the maximum heat transfer is degraded ~20%.
Heat pipes are now being applied to cooling LED
light engine modules for potential increase in the
lifetime and luminous output of LEDs. For
example in Fig. 2 the heat pipe LED engine
shown is a 8mm OD x 15cm long heat pipe. In
this example the active area of the heat pipe only
occurs at the far end of the heat pipe where the
surface mount LED package is placed. The heat
density is determined by looking at the above calculation for a 8mm heat pipe and the area of
the evaporator used to obtain the measured data and is found the heat pipe is dissipating
9.37W/cm2
. Then by taking the active area of the heat pipe used for the LED engine as 1.5 cm2
the heat pipe can be shown to handle 14W, at 65C saturation tempeature and this agrees exactly
with the online data sheet from the manufacturer [4].

3. 5/11/2009 3
Fig. 3 Heat Pipe Heat Flux Limits.
In conclusion current heat pipe technology as applied to LED modules can operate in the range
of 10W/cm2
and provide reliable cooling in the range of 15W with 65C as the saturation
temperature. The design is limited by packaging area available because a saddle must be brazed
to the end of the heat pipe. Also the second major limitation is the wick design where the pore
diameter and wick permeability are limiting factors.
Heat Pipe Limitations
The maximum heat transfer rate that a heat pipe can transfer is limited by either the breakdown
of continuous recirculation of the fluid or a maximum circulation limit. There are five limits for a
heat pipe shown in Fig. 3 including the viscous
and sonic limit due to vapor flow, the wicking
limit due to liquid flow, the entrainment limit
due the cross flow of vapor and liquid and
finally the boiling limit due to nucleation in the
wick causing dry out. The relative position of
the heat pipe limits are shown in Fig. 2 with
total axial heat flow q versus the temperature of
the vapor. The heat pipe will operate in the
region below the curves and the curves vary
depending on the wick material and fluid. In the
above example the calculated limitation is the
wick capillary limit which is dominated by the
effective pore diameter reff and the permeability
K. Also in heat pipes since the vapor and liquid
flow in opposite directions a shear force exists at
the liquid-vapor interface. When their relative velocities are high, the interface becomes
unstable. The entrainment limit is dominated by the vapor core diameter thus the larger the
diameter of the heat pipe, the better. In the micro loop heat pipe design these two important
design limitation are moved further up the Y-axis. The capillary limit is increased because the
micro-pores used are two orders of magnitude smaller and the permeability is higher by creating
straight high aspect ratio pores. The entrainment limit is relaxed by separating the liquid and
vapor flow in the micro loop heat pipe into two separate lines. Also the wick material is only at
the evaporating interface and does not line the external tubing.
Additional limitations are encountered removing the heat from a heat pipe through the
condenser. The condenser length is limited by the heat pipes allowable overall length, which is
~30% of the overall heat pipe. Within the condensation section of the heat pipe the wick acts as a
thermal resistance for condensation. Thus the heat pipe overall heat transfer capability is also
limited to less than 40W by the usable length at the condensation section.
Heat Pipe Cost and R&D

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Heat Pipe MLHP
Wick Type Sintered Cu Silicon
Pore Diameter(micron) 500 5
Heat Density W/cm^2 10 30
Heat Flux W 15 120
Transport Length(cm) 15 100
TABLE 3
Heat Pipe vs. MLHP Comparison
It is a common belief that heat pipes cost merely a dollar or less, and this is quite true for the heat
pipe applied to laptop cooling with the aid of force air cooling. The cost is driven down by
utilizing lower performance cloth or radial grooved heat pipes that have larger effective radius
pores. An example of a cheaper heat pipe is a 8mm x 150 mm long heat pipe that is flattened to
a thickness 2.5 mm with a radially grooved wick. The better performing heat pipes with sintered
wicks are necessary for cooling LED light engine modules cost $15-$25 [5] for a 8mm x 150
mm sintered wick heat pipe modeled above.
Heat pipe and heat removal technology is a
hot are of research since waste heat is still a
major industry hurdle. Over half a century
ago the heat pipe was invented by Grover.
Current research trends include private
investments in development of metal sintered
bi-porous wicks [6]. In this approach porous
metal powders are packed together and
sintered at ~1/2 the melting temperature of the
metal. This enables small pores for high capillary pressure and large pores in between for higher
permeability. According to a patent application from Samsung [7] by applying such techniques
to small heat pipe 4 - 8 mm a ~30% increase in maximum heat flux was measured. This shows
heat pipe technology based on sintering is a well matured technology and incremental
improvements are being made but no substantial improvement in performance is to be expected.
The most cutting edge research has revolved around nano-material wicks by Intel & DARPA [8-
9]. The nano-material wicks still suffer from the lack of a cost effective method of deposition
ex. (microwave plasma deposition) and additional research is needed on the growth seeding
mechanism. The investment by DARPA of $35 million for thermal ground plane research [9]
may yield new wick materials with significant performance improvement, but at this point in
time it is purely conjecture. Any new wick materials may have use in a micro loop heat pipe
implementation. Historically it has taken in excess of 10 years for any DARPA funded research
to reach commercialization. No impact of DARPA research on PCS, Inc. is anticipated in the
foreseeable future.
REFERENCES
1. S.W. Chi, “Heat Pipe Theory and Practice-A Source Book”, Hemisphere, Washington
DC, 1976.
2. Tien C.L. Heat Pipes in “Handbook of Heat Transfer Applications”, eds. W.M.
Rohsenow, J.P. Hartnett and E.N. Ganic, Chapter 5, McGraw-Hill, New York, 1985.
3. Yeh-Chiang Technology Corp. http://www.yctc.com.tw/heat-pipe.htm
4. www.neopac-opto.com
5. Enertron Online Store http://www.enertron-inc.com/heatpipe.asp
6. Rockwell Reference Thermes II 2007
7. S.M. Oh, L. Vasiliev, “Heat Pipe and Method of Manufacturing the Same”, US
2003/0141045.

5. 5/11/2009 5
8. U. Vadakkan, G.M. Chrysler, J. Maveety, M. Tirumala, “A Novel Carbon Nano Tube
Based Wick Structure for Heat Pipes/Vapor Chambers” Semiconductor Thermal
Measurment and Management Symposium, pp.102-104 March 2007.
9. Y.C. Lee, V.M. Bright, R. Yang, S.M. George, “Flexible Thermal Ground Plane”
Presented at DARPA P.I. Meeting October 18, 2008.
10. http://www.darpa.mil/MTO/programs/tgp/index.html