Development of Heatpipe-Based Remote Heatsinks
for Desktop PCs
by Takahiro Shimura * , Hajime Noda * , Mamoru Shimada * and Yuichi Kimura *Recently the heat generation rate of electronic devices continues to increase,reaching as high as 100 W for CPUs in desktop PCs. Since conventionalcooling methods are coming close to performance limits due to the restriction as to theouter dimensions allowed for heatsinks, there is a need for new cooling methods focusing
on high-heat generating devices —in excess of 100 W. Accordingly, we have investigated
the applicability of a new heatpipe-based remote heatsink (HP-RHS), where heatsinks with asufficient heat-dissipating area are installed in an open space, and the heat is transferred there
using heatpipes.
The experimental results show that, using an HP-RHS with a 120-mm square fan, a heat rateof up to some 150 W can be dissipated adequately while keeping the temperature difference
between the CPU case and the ambient below 30°C. This paper describes the successful
development of such a good heatsink, which is capable of cooling over-100W heat-generating
devices offering practicality together with silent operation.

ABSTRACT���INTRODUCTIONAs semiconductor devices such as CPU improve in
performance, the heat they generate during operation
continues to increase. Features of cooling technologies
required for modern CPUs are such that multiple devices
have to be cooled to cope with the graphic chips and thelike that grow in heat generation, and that not only large
heat generation but also high-density heat generation
have to be dealt with because basically the size of thedevices is decreasing. Thus there is a general requirement for CPU cooling technology to efficiently cool electronic devices that are high in heat generation rate as wellas in heat density.
It is to be noted that, as can be seen from Figure 1,
the heat generation rate of current desktop PCs is reaching as high as 100 W. In conventional cases where theheat generation rate was below 100 W, it was sufficient toadopt a cooling system in which, as shown in Figure 2,
a heatsink was placed in an open space just above theCPU, and a fan is used to send air. But such a system has
limitations in performance, because the open space justabove the CPU is obviously limited so that —even if the
fin array is densely packed— a sufficient heat-dissipatingarea of the heatsink cannot be assured when strict restrictions are posed on the outer dimensions of the heatsink.
The performance of a heatsink may be represented by* Components and Advanced Interconnect Technologies���� ���������� ���� ���

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Figure 1

Trends in heat generation rate for CPU in
desktop PCs.
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Figure 2

Conventional CPU cooling method.

Dept., Ecology and Energy Lab., R&D Div.

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Development of Heatpipe-Based Remote Heatsinks for Desktop PCs

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Figure 3

Thermal performance required for heatsinks.

thermal resistance Rca which is defined as the difference
between the CPU temperature Tc and the ambient temperature within the PC Tamb divided by the heat generation
rate Q. Using this definition, the performance limit of theconventional cooling method can be estimated, throughour prototype experiments for desktop PCs, as Rca =0.25~0.30°C/W.
In this context, Figure 3 shows the Rca values required
for sufficient heat dissipation. While the required R ca
values can slightly change depending on individual
designs, Figure 3 shows representative values which was
calculated based on an assumption that the allowabletemperature difference between the CPU case and theambient is 30°C. It can be seen from Figure 3 that the limit
for conventional cooling systems is equivalent to a heatgeneration rate of 100 W, and that a new cooling method
is required to replace conventional methods beyond thelimit of heat generation rate of 100 W.
One of the prerequisites to be taken into consideration
for designing a new cooling method is that in any designs
the final destination of heat dissipation is the air, and thatno solution can be reached unless heat dissipation fromthe heatsink to the air is suitably improved. Methods
for improving heat dissipation into the air may include:
increasing the speed of airflow; enhancing turbulent flow
through modification of the fins; and increasing the surface area of the fin array. The first two ideas are rather
unacceptable because these may eventually lead to anincrease in airflow noise.
In an effort to devise some measures to increase thesurface area of the fin array, we adopted a scheme ofremote heatsink (RHS) in which a heatsink with a sufficient surface area is installed in an open space of the PC
and the heat is transferred from the CPU to the heatsinkusing some means. The RHS method is advantageous interms of the noise mentioned above, because a larger fancompatible with a larger fin surface area can be used toassure a sufficient airflow while suppressing the fan rotation that controls the noise level. Moreover, an additional
fan may not be necessary if the ventilation fan can beshared with the RHS for cooling.
Transferring methods of heat to the RHS may comprise:circulating water using a pump —so-called water cooling;using heatpipes; and applying compressor refrigerators.
Among these, use of heatpipe is most practical for the following reasons.

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Figure 4���Schematic diagram of HP-RHS.

(1) It requires neither moving parts nor power source asis the case with pumps or compressors.
(2) It is low in cost because of its simple structure.
(3) High long-term reliability is assured.
(4) Because heat transfer is effected through phasechange, heat with greater densities can be efficiently
diffused.
(5) Because of its small amount of working fluid neededin addition to its excellent sealing performance, there
is no danger of liquid leakage.
This paper reports on the results of experiments wehave carried out with regard to the application of heatpipe-based remote heatsinks (HP-RHS) to desktop PCs,
together with some application examples.

2.

MAXIMUM HEAT TRANSFER RATE OFHEATPIPE

HP-RHS itself has already been in practical use innotebook PCs 1), 2), but the heatpipes for desktop PCs
obviously have to be longer due to the difference in thesize of the equipment. While notebook PCs typically use
heatpipes of 100~200 mm in length, the length may
be extended to some 400 mm for desktop PCs. Since
heatpipes have a tendency to decrease in the maximum
heat transfer rate Q max as the heat transfer distanceincreases, this presents a design problem to be solved.
On the other hand, while heatpipes are known to operate more efficiently in the bottom-heat mode where heatis transferred in the upward direction, it is advantageousthat desktop PCs can be expected, when compared to
notebook PCs, to be in an environment compatible withthe bottom-heat mode. Figure 4 shows a typical layout of
HP-RHS in bottom-heat mode, in which L-shaped heatpipes are used together with, e.g., a 120-mm square fan
for cooling the RHS. Below will be described the results
and some knowledge obtained of an experiment, in which
Qmax of an L-shaped heatpipe alone was measured as
an example of actively taking advantage of bottom-heatmode.

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Development of Heatpipe-Based Remote Heatsinks for Desktop PCs

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Figure 7

Example of thermal resistance measurements at various temperatures.

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Figure 5

Setup for measuring L-shaped heatpipe.

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Figure 8��������� �������

Qmax of L-shaped heatpipe under various conditions.

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Layout of thermocouples.

In the experiment, for the sake of comparison, thediameter and length of the heatpipe were fixed to 6-mm
φ and 400 mm, respectively. As Figure 5 shows, a heater
20 mm in length was attached on the horizontal portionof the L-shaped heatpipe to make an evaporator section, while a water-cooling jacket 120 mm in length was
attached on the vertical portion to make a condenser
section, and the other portions are covered with thermal
insulators to make an adiabatic section. The working
temperature Tv and the horizontal length Lhr were thought
to be most influential factors, excluding the fixed conditions, in determining Qmax, so that we decided to carry out
experiments using these two as parameters. As shown in
Figure 6, temperatures were measured at two points ofthe evaporator, adiabatic and condenser sections respectively using thermocouples, thereby letting the averaged
values represent the temperature of respective sections,
namely: evaporator section Te; adiabatic section Ta; andcondenser section Tc. In addition the thermal resistancewithin the heatpipe Rhp and the working temperature Tv
were defined as shown in Figure 6.
Figure 7 shows an example of thermal resistancemeasurements at various working temperatures for aheatpipe having a horizontal section length of 50 mm.
In every case of working temperatures the Rhp is seen to����� ��� ������������� �� ���������� �������� �� �������� �� ��������� �������� ��� ��

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Figure 9

Qmax·Lhr of L-shaped heatpipe.

rapidly rise at a certain temperature, indicating that dryout occurs within the heatpipe. Defining Qmax as the heatrate at the boundary where Rhp shows a rapid increase,
Qmax at various conditions were obtained namely, for horizontal lengths of 50 mm, 100 mm, 150 mm and 200 mm;
and for working temperatures of 30°C, 40°C, 50°C and 60°C. Figure 8 shows these Qmax values under various conditions, where the data are divided into groups according
to the working temperature and are plotted using X-axis
that is scaled by the reciprocal of Lhr —the four conditions
above mentioned correspond respectively to 20 m-1, 10
m-1, 6.7 m-1 and 5 m-1. It can be seen that the data of
the same working temperature can be fitted to a straight

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Development of Heatpipe-Based Remote Heatsinks for Desktop PCs

line, and that every straight line passes the origin of coordinates, meaning that the product Qmax·Lhr can neatly be
expressed by a constant —the slope of the straight line.
Accordingly, the Qmax of L-shaped heatpipes can be represented as shown in Figure 9.
Thus we have been able to clarify Qmax of L-shaped
heatpipes experimentally, and succeeded in describing
the performance using the index Qmax·Lhr.

3.

THERMAL RESISTANCE OF HEATPIPE

Let us now describe the thermal resistance of a heatsink
in which heatpipes are actually integrated. While a
cooling method using only RHS is shown in Figure 4,
it was thought that simultaneous use of conventionalheatsinks would be more effective for efficient cooling,so that we fabricated a prototype of combined heatsinkshown in Figure 10, where a conventional heatsink is
installed additionally just above the CPU, and carried out
performance evaluation. Meanwhile, the heatsink installed
just above the CPU is referred to as “direct coolingheatsink (DHS)” hereinafter in order to distinguish it fromRHS.

a Qmax of some 50 W per heatpipe is sufficient for dealing
with a heat generation rate in excess of 100 W.
On the DHS, 35 aluminum fins of 0.4 mm in thickness
were mounted by mechanical crimping at a pitch of 1.9
mm. Also for the RHS, 98 aluminum fins of 0.2 mm thickness were mounted mechanically at a pitch of 1.2 mm
to constitute a stacked fin array. The experiments wereconducted under the condition of inputting a heat rate of
200 W from the bottom of the DHS using a 20-mm square
heater, and cooling the DHS and the RHS individuallyusing a 12V-DC fan —60 mm x 60 mm x 15 mm for the
DHS and 120 mm x 120 mm x 40 mm for the RHS.
The heater and ambient temperatures were measured
for the two cases of fan-cooling both the DHS and the
RHS and fan-cooling only the RHS, and assuming thatthe heater temperature is equivalent to the CPU case temperature, respective values of Rca were obtained based onthese temperature measurements. The results show that
Rca = 0.193°C/W for the case of both DHS and RHS working, and Rca = 0.213°C/W for only RHS working. These
values were converted using the curve in Figure 3 to heatgeneration rates that would allow sufficient dissipation,
resulting in 155 W for both DHS and RHS working and
141 W for only RHS working. Thus it was demonstrated
that in each case the heatsink could cope with the heatgeneration rate in excess of 100 W.

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4.

DESIGN BY SIMULATION

Lastly we will describe a design example using
simulation. Photo 1 shows an HP-RHS product which was
employed in a desktop PC of the 2004 summer model.
Four heatpipes are used to transfer the heat to the RHS
with dimensions of 120 mm x 120 mm x 60 mm, which iscooled using a 120-mm square fan —25 mm thick, 37 dB
in noise level, virtually hidden in Photo in the rear of theheatsink.

Figure 10 Schematic of combined heatsink.

In the prototype heatsink, five L-shaped heatpipes
each with a horizontal length of 60 mm and a total
length of 400 mm were used. With the aim of allowing arbitrary design of the distance between the DHS
and the RHS while keeping the horizontal length
short, the heatpipes were bent in a two-step configuration —to rise up aslant from the horizontal
section then becoming vertical near the RHS. The
Q max of each heatpipe was estimated based on theelementary performance evaluation mentioned earlier to be about 50 W at a working temperature of 40°C.
Since the fin efficiency of RHS becomes higher when
multiple heatpipes are attached dispersedly on the fins, it
is not necessary for a single heatpipe to accommodate all
the heat rate. For a fin 120 mm in width, for example, it isconsidered preferable, from the point of fin efficiency, toattach three to five heatpipes dispersedly, and, therefore,

Photo 1

Appearance of an HP-RHS product for desktop PC.

The heatsink was modeled and thermo-fluid dynamics analysis was carried out to calculate the temperature
distribution in the heatsink. Figure 11 shows an exampleof the results obtained under the conditions of: both fin

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Development of Heatpipe-Based Remote Heatsinks for Desktop PCs

5.

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Figure 11 Temperature distribution of heatsink. (by calculation)and heat-receiving block is of aluminum; heat input is
103 W; airflow rate to the RHS is 1.67 m3/min; and ambient temperature is 35°C (308K). Physical properties of
the heatpipe itself obtained from the evaluated results ofheatpipe elements were used in the calculation model. As
can be seen from Figure 11, the highest temperature wasreached at the surface of the heat-receiving block corresponding to the center of the incident plane, resulting ina heat-dissipation relationship of Rsa = 0.116°C/W, whereRsa is defined as the thermal resistance due to the temperature difference between the maximum temperature
at the center of the heat-receiving block and the ambient
temperature. The calculated value approximately agrees
with the experimental result obtained using the same airflow, thus confirming the validity of the calculation model.
When pure copper is used for both the fin and theheat-receiving block aiming at improving the thermal performance, a relationship of Rsa = 0.102°C/W is obtained
through the use of the same model, thus indicating a
reduction of Rsa by about 12 %. While materials are to be
selected from the standpoint of both thermal performanceand weight, use of aluminum was found satisfactory inthe current design to achieve the required performance,
so that aluminum was employed in consideration of its
lightweight for both the fin and the heat-receiving block.
Incidentally, ease of integrating dissimilar metals, e.g.,copper heatpipe and aluminum fin, is one of the advantageous features of Furukawa Electric’s heatsink which
employs mechanical jointing.
Lastly, let us compare the thermal performance by
converting the values into Rca, into which Rsa can be converted by adding the thermal contact resistance for CPU.
The resistance is, although slightly subject to change
depending on the selection of thermal interface material (TIM), around 0.10°C/W. In the above mentioned case,therefore, Rca =0.216°C/W for aluminum and Rca =0.201°C/W for copper. Thus referring to Figure 3, it can be seenthat the design can deal with the heat generation rates of
up to 139 W and 149 W for aluminum and copper, respectively.

CONCLUSIONSAs the next-generation heatsink for desktop PCs capableof coping with a heat generation rate exceeding 100 W,
the HP-RHS scheme was investigated, in which a heatsink
with a sufficient surface area is installed in an open space
and the heat is transferred there using heatpipes. It has
been clarified that for an HP-RHS using a 120-mm squarefan, for example, Rca is about 0.20°C/W; in other words,
it is possible to maintain the temperature difference
between the CPU case and the ambient not higher than
30°C for heat generation rates of up to around 150 W.
Thus we have been successful in demonstrating that the
HP-RHS can cope with heat generation rates in excess of
100 W, and also in practically supplying the products tocommercial desktop PCs.
Since HP-RHS has practicality, in comparison with othercooling techniques like water cooling, in terms of reliability and cost it is expected to become widespread indesktop PC applications just like in notebook PCs. Futuretasks for us would be to build up basic data that are
applicable to many cases, and to make more efficient useof the data for design applications.

REFERENCES
1) J. Sotani et al.: “A Micro Heat-Pipe for Cooling Notebook PCs,”
Proceedings of the 9th International Heat Pipe Conference (1995)
877.
2) K. Namba et al.: “Heat-Pipes for Electronic Devices Cooling and
Evaluation of Their Thermal Performance,” Proceedings of Itherm’98 (1998) 456.

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