Tag Archives: heat pipes

Making the Change

Posted on July 1, 2026 by | Leave a comment

Air cooling was once the dominant method for cooling processors on a PCB, employing heat sinks, fans, and blowers. These solutions are still popular, but faster-running, higher-powered devices, like most GPUs used for AI can’t be sufficiently cooled with air. Liquid cooling is needed. [1,7]

There isn’t a universal application point where engineers must switch from air cooling to liquid cooling. The decision mainly depends on allowable temperature and heat flux. Other considerations include higher costs and power needs, added hardware, space and weight, and required more monitoring. [1,3]

Fig 1 – Air Cooling Heat Sink vs. Liquid Cooling. [9,10]

In practice, once devices reach several hundred watts in a compact package, or when the required thermal resistance approaches 0.1°C/W or lower, liquid cooling often becomes the preferred solution.

Low Thermal Resistance

Engineers usually don’t switch to liquid cooling simply because a chip reaches a particular wattage. They switch when the required junction-to-ambient thermal resistance falls below what can be achieved economically and reliably with available airflow and heat sink models. [2,3,5]

Fig 2 – Typical Chip Temperature Location [11]

Consider an equation and example:

Tj = Tα+ P x RαJA                              [2]

Where:

Tj = junction temperature

Tα = ambient temperature

P = power dissipation

RαJA = thermal resistance from junction to ambient

If keeping the chip within its temperature limits requires an unrealistically low thermal resistance, air cooling may not be practical.

Example:

Ambient = 40°C

Maximum junction temp = 90°C

Chip power = 500 W

The allowable thermal resistance is:

RαJA = 90 – 40 /500 = 0.1°C/W

Achieving 0.1°C/W with air is extremely difficult. Liquid cooling is the practical solution. [2,3]

High Heat Flux

Heat flux (power per unit area) is often more important than total power. [3]

For example:

  • A 100 W device spread onto a large heat sink may be easy to air-cool
  • A 100 W device concentrated in a 1 cm² die can be much harder

Modern high-performance processors, GPUs, laser diodes, RF power amplifiers, and power electronics often reach heat fluxes that make air cooling difficult.

Fig 3 – Heat Flux Measures the Amount of Heat Energy Transferred from a Warmer Side to a Cooler Side. [12]

Typical industry guidance suggests:

  • Air cooling: comfortable below ~10–20 W/cm² [1,6]
  • Advanced air cooling: can sometimes handle 50–100 W/cm² with aggressive airflow [1,6]
  • Liquid cooling: often becomes attractive above ~100 W/cm² [1,6,7]

In basic terms:

  • Air cooling is usually limited by how much heat you can transfer from the heat sink to the air.
  • Liquid cooling is usually limited by how much heat you can get from the chip into the coolant.

Where Air Cooling Works Well

Fig 4 – Heat Sinks from Advanced Thermal Solutions, Inc. [9]

  • Total heat dissipation is relatively modest, up to a few hundred watts
  • There is enough surface area for heat sinks
  • There is sufficient airflow through the enclosure
  • Higher fan noise is acceptable  [1,3]

Air cooling is used in many consumer electronics products and networking devices.

Coolants Remove More Heat

Since water-based coolants have much higher heat capacity and thermal conductivity than air, they can transport large amounts of heat away with relatively small temperature rises. [3,9]

For example:

  • Air’s specific heat capacity is about 1 kJ/kg·K 
  • Water’s is about 4.2 kJ/kg·K   

And water is about 830 times denser than air, so a small coolant flow can carry away an enormous amount of heat compared with an equivalent volume flow of air. [9]

Fig 5 – Tubing and Ports on a Liquid Cooling System [9]

That’s why modern AI accelerators, high-power motor drives, and high-density data-center servers are increasingly moving toward liquid cooling: not because air cooling is impossible, but because the required heat removal density becomes impractical with fans and heat sinks alone. [1,7,8]

The Benefits of Faster Flow Rates

Air cooling performance increases when the rate of airflow increases. Thus, many heat sinks are supported with fans and blowers to enhance heat transfer into the passing air. [3]

Liquid cooling, similarly, is enhanced with higher flow rates. A common expectation is that a coolant that stays longer at the heat source (chip) will absorb more heat. But, at slower flow rates, the coolant heats up as it moves through a cold plate and downstream in the cooling loop. As a result, the returning coolant has less of a temperature difference from the chip, and heat transfer becomes less effective. [3,4]

With a faster flow of liquid coolant, the fluid temperature stays more uniform and keeps a steady, higher temperature difference from the chip. This makes heat extraction more efficient and improves the lowering of chip temperatures. [3,4]

Flow rates can be optimized per application. There are diminishing returns on performance as flow rates increase, as well as added costs. Your liquid cooling system provider should provide performance data and recommendations for proper flow rates. [3,4]

In-Between Air and Liquid Cooling

There are options that fall between air and liquid cooling methods. Engineers should look through these choices before designing in a liquid system. They could provide simpler, lower-cost, but reliably effective cooling       

The expanded options include:

  1. Better PCB thermal design (thermal vias, thicker copper planes)
  2. Larger heat sinks
  3. Forced-air cooling with optimized airflow paths
  4. Heat pipes or vapor chambers to spread heat
  5. Remote heat exchangers connected by heat pipes
  6. Liquid cold plates       [3,4]

Fig 6 – Heat Pipes May Meet Cooling Needs in Place of Air or Liquid Cooling [9]

Not a Tradeoff

Traditional air cooling is inadequate for many of today’s thermal management needs. Other methods, including enhanced air cooling  systems, may offer solutions. But for many applications, including fast-growing AI units and data centers, liquid cooling is essential. In this case, there is no tradeoff because its use is the only option. [1,6,7]

Consider This

Here are some analogies for comparing air to liquid (water) cooling:

Air is a ghost. Because air is so light and empty, a single cubic meter of it can only grab a tiny handful of heat before it gets too hot and gives up.

Water is a sponge. Because water is about 830 times denser and packed tight with mass, that same one cubic meter acts like a massive thermal sponge. It can swallow up a staggering amount of heat before its temperature rises even a single degree.

To cool a hot system, you either have to blow a hurricane of lightweight air past it, or gently glide a tiny, heavy stream of water over it.

References

  1. ASHRAE. Thermal Guidelines for Data Processing Environments, latest edition.
  2. JEDEC Solid State Technology Association. JESD51 Series: Methodology for the Thermal Measurement of Component Packages.
  3. Frank P. Incropera, David P. DeWitt, Theodore L. Bergman, and Adrienne S. Lavine. Fundamentals of Heat and Mass Transfer, Wiley.
  4. David A. Reay, Ryan McGlen, and Peter Kew. Heat Pipes: Theory, Design and Applications.
  5. Texas Instruments. Thermal Design by Insight, Not Hindsight (Application Report).
  6. Open Compute Project Foundation. Advanced Cooling Solutions documentation.
  7. NVIDIA. Data Center Liquid Cooling technical papers and deployment guides.
  8. National Institute of Standards and Technology (NIST). Thermophysical Properties of Fluids Database.
  9. Advanced Thermal Solutions, Inc., https://www.qats.com
  10. Chatsworth Products, https://www.chatsworth.com/en-us/resources/blogs/2026/5-misunderstood-facts-about-direct-to-chip-liquid-cooling/
  11. Advanced Thermal Solutions, Inc, https://www.qats.com/
  12. EngineerExcel, https://engineerexcel.com/flow-of-heat/

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October Webinar 2021 Cooling Methodologies for High Power Systems and Components, 10-21-21 2PM EST

Posted on October 19, 2021 by | Leave a comment

High power systems of various kinds have become even more widley deployed than in the past. From 15kW IGBT power systems, to 50kW high density power converters, to even higher wattage for computing racks. As the need for applications for systems with this power has grown, so too has the thermal management solutions that allows these systems to operate at their intended performance levels.

This webinar covers the newest strategies and technologies for these very high power applications. In this technology review webinar attendees will learn:

  • How the integration of the thermal architecture with electronic architecture can can significantly improve the effectiveness of thermal management.
  • The use of passive two phase thermosyphon loops for cooling data center racks
  • How to passively cool outdoor 7.2kW power supply electronics enclosures
  • The application of a closed rack cooling system based on pulsating heat pipes
  • The development of a loop heat pipe with kW-class heat transport capability
  • A new class of graphene enhanced heat pipe with heat dissipation characteristics 3.5x higher than copper
  • Using multi-stage heat pipe loops for cooling high heat density data centers
  • The application of pin finned cold plates for cooling high power density converters (HPDC)
  • Many more new and latest developments in thermal management for high power systems and components will be reviewed. This webinar, presented by thermal management expert Dr. Kaveh Azar, Ph.D., will address all these issues and more. The webinar is one hour in length with time for questions and answers afterwards online and after the webinar concludes.
Register for Webinar

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Thermal Performance of Heat Sinks with Heat Pipes or Vapor Chambers for Servers

Posted on May 1, 2019 by | 1 comment

Most blade servers for data and telecommunication systems use air to cool the high-power chips inside. As the power level of these chips keep increasing, the pressure is on thermal engineers to design ever higher performance air-cooled heat sinks. In recent years, advancements in manufacturing of thinner heat pipes and vapor chambers have enabled engineers to integrate the heat pipes and vapor chambers into the blade server heat sinks.

A heat pipe is a device with high thermal conductance that can transport large amounts of heat with a small temperature difference between its hot and cold ends. The idea of a heat pipe was first proposed by Gaugler [1]. However, only after its invention by Grover [2, 3] in the early 1960s, were the remarkable properties of heat pipes realized by scientists and engineers. It is now widely used to transport heat from one location to another location or to smooth the temperature distribution on a solid surface.

A heat pipe is a self-driven two-phase heat transfer device. A schematic view of a heat pipe is shown in Figure 1. At the hot section (evaporator), the liquid evaporates and turns to vapor. The vapor flows to the cold section (condenser) and liquefies there. The liquid is driven back from the cold section to the hot section by a capillary force induced by the wick structure. By using boiling and condensation, the heat pipes can transfer and spread the heat from one side to another side with a minimum temperature gradient.

Figure 1. Typical heat pipe. [4]

Vapor chambers are flat heat pipes with very high thermal conductance. They have flat surfaces on the top and bottom sides.  See Figure 2, which can be easily attached to a heat source and a heat sink.

Figure 2. Vapor chamber. [5]

Just like heat pipes, vapor chambers use both boiling and condensation to maximize their heat transfer ability. A vapor chamber generally has a solid metal enclosure with a wick structure lining the inside walls. The inside of the enclosure is saturated with a working fluid at reduced pressure. As heat is applied at one side of the vapor chamber, the fluid at locations close to the heat source reaches its boiling temperature and vaporizes. The vapor then travels to the other side of the vapor chamber and condenses into liquid. The condensed fluid returns to the hot side via the gravity or capillary action, ready to vaporize again and repeat the cycle.

In electronics cooling, heat pipes are generally used to move the heat from electronics to heat dissipation devices. For example, in a desktop computer, multiple heat pipes are used to transfer heat from a CPU to an array of cooling fins, which dissipate the heat to ambient environment through convection. Vapor chambers are generally used to spread heat from a small size device to a larger size heat sink, as it is shown in Figure 2. If used in server heat sinks, the heat pipes and vapor chambers are both used to spreading the heat due to the low profile and large footprint of the heat sinks.

Compared to copper heat spreaders, heat pipes and vapor chambers have the following merits.

First, they have a much higher effective thermal conductivity. The pure copper has a thermal conductivity of 401 W/m°C and the best conductive material of diamond has a thermal conductivity of 1000-2000 W/m°C. The effective thermal conductivity of a well-designed heat pipe and vapor chamber can exceed 5000 W/m°C, which is an order of magnitude higher than that of pure copper. Second, the density of the heat pipe and vapor chamber is much lower than that of copper. Due to its hollow structure, the heat spreaders made by vapor chambers are much lighter than those made of copper. These properties make them the ideal candidate for high heat flux and weight sensitive heat spreading applications.

Dynatron Corporation is an electronic cooling provider specializing in heat sink for servers. This article compares the thermal performance of its server heat sinks, some of which have integrated vapor chambers. Figure 3 shows the photos of two Dynatron 1U passive server heat sinks for Intel’s Sandy Bridge EP/EX Processors. The R12 is made of pure copper with skived fins. The R19 has a vapor chamber base and stacked copper fins. The heat sink specification is listed in Table 1. The R19 is 150g lighter than the R12.

Figure 3. Dynatron passive heat sinks R12 (left) and R19 (right). [6]
Table 1. Dynatron passive heat sink specification.

Figures 4 and 5 show the thermal performance of R12 and R19 at different flow rates. At 10CFM, both heat sinks have a thermal resistance of 0.298ºC/W. When the flow rate increases to 20CFM, the R19’s thermal resistance is 0.218ºC/W, which is 0.020ºC/W lower than that of R12.

Figure 4. Dynatron R12 heat sink performance. [6]
Figure 5. Dynatron R19 heat sink performance. [6]

Figure 6 shows the photos of two Dynatron 1U active server heat sinks for Intel’s Sandy Bridge EP/EX Processors. The R18 is made of copper with skived fins. The R16 has vapor chamber base and stacked copper fins. Both heat sinks use same blower. The heat sink specification is listed in Table 2. The R16 is 90g lighter than the R18.

Figure 6. Dynatron active heat sinks R18 (left) and R16 (right). [6]
Table 2. Dynatron active heat sink specification. [6]

Figures 7 and 8 show the thermal performance of R18 and R16 at different blower speeds. At 3000RPM, the R18 and R16 heat sinks have thermal resistance of 0.437ºC/W and 0.393ºC/W, respectively. When the blower speed increases to 6000RPM, the R18’s thermal resistance is 0.268ºC/W and the R16’s thermal resistance is 0.241ºC/W. The R16 is constantly able to outperform the R18 at different blower speeds and its thermal resistance is 10% lower than R18.

Figure 7. Dynatron R18 heat sink performance. [6]
Figure 8. Dynatron R16 heat sink performance. [6]

The comparison of the Dynatron heat sinks shows that heat sinks with vapor chambers have a slight thermal edge vis-a-vis its copper counterparts even though they are light. This is true for both passive and active heat sinks.

Glover et al., for Cisco, have tested different heat sinks either with embedded heat pipes or vapor chambers for their servers and published their findings [7]. They tested five different heat sinks from different vendors, who utilized different manufacturing technologies to fabricate the heat sinks. The five heat sinks are similar in size: 152.4 x 101.6 x 12.7mm. Table 3 summarizes the physical attributes of these five heat sinks.

Table 3. Cisco tested heat sink specification. [7]

Figure 9-11 shows the three vapor chamber heat sinks with different vapor chamber structures and fin designs. Heat sink A-1 is an extruded aluminum heat sink with a vapor chamber strip. The 40 mm wide vapor chamber strip is embedded in the center of the base. It is the lightest one among five tested heat sink. Heat sink B-1 and C-1 have full base size vapor chamber and aluminum zipper fins.

Figure 9. Heat sink with vapor chamber A-1. [7]
Figure 10. Heat sink with vapor chamber B-1. [7]
Figure 11. Heat sink with vapor chamber C-1. [7]

Figures 12-13 show the two heat sinks with embedded heat pipes. Heat sink C-2 has heat pipes embedded inside its aluminum base. It uses zipper fins and has a copper slug in the middle of the base. Heat sink D-1 has three flat heat pipes embedded in its base. It has a copper plate as base.

Figure 12. Heat sink with heat pipes C-2. [7]
Figure 13. Heat sink with heat pipes D-1. [7]

Glover et al. tested the five heat sinks at different mounting orientation and air velocity. Table 4 presents the summary results of the heat sinks at 3m/s approach air velocity. The tested heat sinks were mounted horizontally with heater sources underneath the heat sink bases.

Table 4. Heat Sink Performance at 3 m/s with horizontal mounting position and bottom heating. [7]

The C-1 heat sink has the lowest thermal resistance; thus, its values are used as the benchmark for other heat sinks. The performance of heat sinks is purely design dependent. For vapor chamber heat sinks, the thermal resistance value varies from 0.19 to 0.23°C/W for 30 W of power. For heat sinks with heat pipes, the C-2 heat sink has a thermal resistance of 0.23°C/W, which matched with that of A-1 and B-1.

The D-1 heat sink has the highest thermal resistance, which is the result of inferior design and manufacture. However, the D-1 heat sink still has relatively low thermal resistance when it is compared to a regular heat sink without a heat pipe and vapor chamber.

Figure 14 shows the thermal resistance of the five heat sinks for 60W of input power at different air velocities. The C-1 heat sink performs best for all velocities and the D-1 heat sink’s performance is the worst.

Figure 14. Heat sink thermal resistance at 60 W. [7]

The pressure drop across the heat sink at different air velocities was also measured and the results were plotted in Figure 15. The B-1, C-1 and C-2 heat sinks have similar fin structures. Therefore, their pressure drop is similar, too. The pressure drop of the A-1 and D-1 heat sinks are similar and higher than the other heat sinks. This is because the A-1 heat sink has thicker fins and the D-1 heat sink has a thicker base.

Figure 15. Heat sink pressure drop. [6]

Because the heat pipes and vapor chambers use capillary force to drive liquid back from the condensation section to the evaporation section, their thermal performance is prone to orientation variation. Glover et al. also investigated the effects of the mounting orientation on the performance of the five heat sinks. They found the effect of the orientation is design dependent and is the result of both the wick structure and the entire heat sink assembly construct.

The heat sink specification from Dynatron Corporation and the test results from Cisco, show that the server heat sinks with embedded heat pipes or vapor chamber have a better thermal performance than their copper counterparts. The heat sinks with embedded heat pipes or vapor chamber are also lighter than the pure copper heat sinks, which make them more suitable for applications which are weight sensitive. If the cost of such heat sinks is justified, they are definitely good candidates for server cooling applications.

References

  1. Gaugler, R. S., US Patent 2350348, Appl. 21 Dec, 1942. Published 6 Jun. 1944.
  2. Grover, G. m., US Patent 3229759. Filed 1963.
  3. Grover, G. M., Cotter, T. P., and Erickson, G. F., “Structure of Very High Thermal Conductance.” J. App. Phys., Vol. 35, P. 1990, 1964.
  4. http://www.lightstreamphotonics.com/technology.htm
  5. http://www.thermacore.co.uk/vapour-chamber
  6. http://http://www.dynatron-corp.com
  7. Glover, G., Chen, Y., Luo, A., and Chu, H., “Thin Vapor Chamber Heat Sink and Embedded Heat Pipe Heat Sink Performance Evaluations,” 25th IEEE SEMI-THERM Symposium, San Jose, CA, USA, 2009.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].

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Webinar on Heat Pipes and Vapor Chambers

Posted on November 7, 2018 by | Leave a comment

Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. This month’s webinar will be held on Thursday, Nov. 29 from 2-3 p.m. ET and will cover the role of heat pipes and vapor chambers in heat transfer. Learn more and register at https://qats.com/Training/Webinars.

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Technical Note: Effect of Vacuum and Fill Ratio on Performance of Heat Pipes

Posted on July 17, 2018 by | 7 comments

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

The discussion generally arises as to what would be the heat pipe performance as a function of liquid fill ratio and vacuum. Before we show some results, let’s see how a heat pipe works. The thermodynamic cycle of a heat pipe is shown in figure 1 in a T-S diagram. [1]

Fig. 1 – Thermodynamic cycle of a heat pipe. [1]

Liquid at state 1 enters the evaporator and after absorbing the heat vaporizes to a mixture at 2 or to a saturated vapor at 2’. This vapor travels through the length of the heat pipe and enters the condenser at state 3. This vapor after losing its heat in the condenser exits at state 4 which is saturated liquid and upon travelling through the wick loses its temperature until it reaches point 1, which the cycle begins. If one looks at the phase diagram for a liquid, for example water in figure 2, it is apparent that the state of the liquid should be very close to the liquid vapor line in order for the liquid to promptly changes phase from liquid to gas upon heating. The triple point of water is at 4.58 Torr at temperature of 0.0075℃.

Fig. 2 – Phase diagram of water.

The state of liquid pressure should be in the region below atmosphere (vacuum) and above the complete vacuum. To obtain lower operating temperature the heat pipe pressure should be decrease and vice versa. The state of liquid after the condenser has to be saturated liquid, so the wick can create the liquid motion.

Lin, et al. [2] have shown that maximum heat transfer in a heat pipe is an exponential function of vacuum pressure according to the following formula:

Heat Pipes

Where,
Qmax,0 = maximum heat transfer at 0 pressure (no condensable gas)
PNCG = pressure of the non-condensable gas
∆Pcg = pressure drop difference between capillary and gravity

This equation clearly shows that by decreasing vacuum pressure, Qmax increases.

Another important factor is the amount of liquid in the heat pipe which is commonly called the fill ratio or inventory. If there is too much liquid, evaporation will not happen or delayed, and if there is not enough liquid, the dry out condition will happen. The rule of thumb is the volume of the liquid should be higher than the volume of the pore volume of the wick.

Figure 3 shows that as the pressure decreases from 10 Torr to 1 Torr the Qmax increases. The graph also shows that as the inventory (fill ratio) increases from 0.7 ml to 1.1 ml, Qmax peaks at 0.8 ml. This corresponds to a fill ratio of 26.4%, which is the ratio of the liquid volume to total volume of the heat pipe when it is empty. This graph shows the importance of fill ratio. If the fill ratio is not optimized as is shown for example for 1 Torr, Qmax drops from 8 W to 4 W, a 50% drop that can be catastrophic for the application. Mozumder et al. [3], in their experiment, measured the thermal resistance of a heat pipe for different fill ratios and power.

Fig. 3 – Qmax as a function of vacuum pressure and fill ratio for a heat pipe. [2]

Figure 4 shows that as the fill ratio increases from dry run to 85% (in their experiment fill ratio is defined as the volume of liquid to the volume of the evaporator section), thermal resistance decreases, but then increase with further increase of fill ratio.

Fig. 4 – Heat pipe thermal resistance as a function of fill ratio. [3]

The aforementioned arguments demonstrate that the fill ratio and vacuum pressure are very important in the proper design of a heat pipe. There is not much data in the literature about the effect of these two factors on the performance of the heat pipe. And it appears that most heat pipe manufacturers either resort to a try and error procedure or use the information from past experience. This topic needs further research.

Advanced Thermal Solutions, Inc. (ATS) is an industry-leader in heat pipe technology and has recently expanded its offering of high-performance, off-the-shelf heat pipes to provide the broadest selection on the market. Use the heat pipe selection tool to find the right fit for your project and avoid the cost and time for custom solutions. Learn more at https://qats.com/Products/Heat-Pipes.

References
1. Ong, S., “Heat Pipes”, Jurutera, April 2008.
2. Lin, W., Chao, C., Calvin, Y., Hsu, G., Chou, S., “Effect of vacuum pressure and the Working Fluid Inventory to the Maximum Heat Loading (Qmax) of the Heat Pipe”, 10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011.
3. Mozumder, A., Akon, A., Chowdhury, M. Banik, S., “Performance of Heat Pipe with Different Working Fluids and Fill Ratios”, Journal of Mechanical Engineering, Vol. ME 41, No. 2 December 2010.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or [email protected].

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