Tag Archives: thermal resistance

The Ultimate Heat Pipe Guide: Selection for Performance

What are Heat Pipes?

Heat Pipes have been called Heat Superconductors! In this engineering article we’ll talk about what a heat pipe, how they are made, compare them with heat sinks, and talk about performance in various thermal management applications.

Diagram of a basic heat pipeFigure 1 Schematic View of a Heat Pipe [1]

Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.

The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.

Sections of A Heat Pipe

A heat pipe has three sections: the evaporator, the adiabatic, and the condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with a liquid such as water. When the evaporator section (Le) is exposed to a heat source, the liquid inside vaporizes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (Lc). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section of the heat pipe (La), the adiabatic portion, has a very small temperature difference.

Chart showing pressure drop distribution in a heat sinkFigure 2 Pressure Drop Distribution in a Heat Pipe [1]

Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.

Structure of a heat pipe

  1. Metallic pipe: The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.
  2. Working fluid: Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the operating temperature range.
  3. Wick: The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance.
    Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.

Different Wick Structures in a Heat PipeFigure 3 Different Wick Structures – From top to bottom: Sintered powder, fine fiber, wrapped screen, axial groove

Table 1 shows experimental data for the operating temperature and heat transfer for three different types of heat pipes [1].

Heat Pipes With Different Constructions and Operating ConditionsTable 1: Heat Pipes with Different Structures and Operating Conditions [1]

Certain factors can limit the maximum heat transfer rate from a heat pipe. These are classified as follows:

  1. Capillary Limit: Heat transfer is limited by the pumping action of the wick
  2. Sonic Limit: When the vapor reaches the speed of sound, further increase in the heat transfer rate can only be achieved when the evaporator temperature increases
  3. Boiling Limit: High heat fluxes can cause dry out.
  4. Entrainment Limit: High speed vapor can impede the return of the liquid to the condenser

A heat pipe has an effective thermal conductivity much larger than that of a very good metal conductor, such as copper. Figure 4 shows a copper-water heat pipe and a copper pipe dipped into an 80oC water bath. Both pipes were initially at 20oC temperature. The heat pipe temperature reaches the water temperature in about 25 seconds, while the copper rod reaches just 30oC after 200 seconds. However, in an actual application when a heat pipe is soldered or epoxied to the base of a heat sink, the effective thermal conductivity of the heat pipe may be drastically reduced due to the extra thermal resistances added by the bonding. A rule of thumb for the effective thermal conductivity of a heat pipe is 4000 W/mK.

Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper PipeFigure 4. Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper Pipe [1].

Heat pipe manufacturers generally provide data sheets showing the relationship between the temperature difference and the heat input. Figure 5 shows the temperature difference between the two ends of a heat pipe as a function of power [2].

Temperature Difference Between the Evaporator and the Condenser in a Heat PipeFigure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2]

Types of heat pipes

There are many heat pipe shapes in the market, but the most common are either round or flat. Round heat pipes can be used for transferring heat from one point to another. They can be applied in tightly spaced electronic components, such as in a laptop. Heat is transferred to a different location that provides enough space to use a proper heat sink or other cooling solution. Figure 6 shows some of the common round heat pipes available in the market.

Typical Round Heat Pipes in the Market.Figure 6. Typical Round Heat Pipes in the Market.

Flat heat pipes (vapor chambers) work conceptually the same as round heat pipes. Figure 7 shows a flat pipe design, they can be used as heat spreaders. When the heat source is much smaller than the heat sink base, a flat heat pipe can be embedded in the base of the heat sink, or it can be attached to the base to spread the heat more uniformly on the base of the heat sink. Figure 8 shows some common flat heat pipes.

Conceptual Design Schematic of a Flat Heat Pipe

Figure 7. Conceptual Design Schematic of a Flat Heat Pipe

Commonly Used Flat Heat Pipes

Commonly Used Flat Heat Pipes

Figure 8. Commonly-used Flat Heat Pipes

Although a vapor chamber might be helpful in minimizing spreading resistance, it may not perform as well as a plate made from a very high conductor, such as diamond. A determining factor is the thickness of the base plate. Figure 9 shows the spreading resistance for 80 x 80 x 5 mm base plate of different materials with a 10 x 10 mm heat source. The vapor chamber has a spreading resistance that is better than copper, but worse than diamond. However the price of the diamond might not justify its application. Figure 9 also includes the spreading resistance from the ATS Forced Thermal Spreader (FTS), which is equal to that of diamond at a much lower cost. The FTS uses a combination of mini and micro channels to minimize the spreading resistance by circulating the liquid inside the spreader.

Thermal Spreading Resistances for Different Materials

Thermal Spreading Resistances for Different Materials

Importance of an Heat Pipe

Heat pipes have a very important role in the thermal management arena. With projected lifespans of 129,000-260,000 hours (as claimed by their manufacturers), they will continue to be an integral part of some new thermal systems. However, with such problems as dry out, acceleration, leakage, vapor lock and reliable performance in ETSI or NEBS types of environments, heat pipes should be tested prior to use and after unsatisfactory examination of other cooling methods have been explored.

Have you got a question on heat pipes or their application? How about an interest in bringing ATS’s team of experienced thermal engineers into one of your projects?  Reach us by visiting ATS Heat Pipe Page  or email us at ats-hq@qats.com or give us a call at 781-769-2800

References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid

The New Qpedia Thermal eMagazine is Out

Qpedia Thermal eMagazine, Volume 7, Issue 4, has just been released and can be downloaded at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues.

Featured articles in this issue include:

Dropwise Condensation in Vapor Chambers
Considerable attention has been devoted in the past to the evaporation process taking place in a vapor chamber. However, increased heat fluxes at the condensation end have prompted efforts to improve the condensation performance of the vapor chambers. This article presents a review of a novel method for improving the thermal performance of a vapor chamber condensing section by using special surfaces promoting dropwise condensation.

 

Heat Sink Manufacturing Using Metal Injection Molding

Using Metal Injection Molding It is only in the last few years that metal injection molding (MIM) has gained a foothold in the thermal community and its salient advantages have become more evident. The MIM process allows intricate features to be added into the heat sink design to boost thermal performance and its production process is very scalable compared with machining. Injection molding enables complex parts to be formed as easily as simple geometries, thereby allowing increased design freedom.  This article explore the merits of copper material in the MIM process.

 

Industry Developments: Thermoelectric Modules and Coolers

Thermoelectric modules (TEMs) are rugged, reliable and quiet devices that serve as heat pumps. The real heat-moving components inside TEMs are thermoelectric coolers or TECs. These are solid-state heat pumps and are designed for applications where temperature stabilization, temperature cycling, or cooling below ambient, are required. Today, TEMs are used in electro-optics applications, such as the cooling and stabilizing of laser diodes, IR detectors, cameras (charge coupled device), microprocessors, blood analyzers and optical switches. This article explores some of the latest developments in these devices.

 

Technology Review: Reducing Thermal Spreading Resistance in Heat Sinks

In this issue our spotlight is on reducing spreading resistance in heat sinks. There is much discussion about how this phenomenon can be achieved, and these patents show some of the salient features that are the focus of different inventors.

 

Cooling News featuring the latest product releases and buzz from around the electronics cooling industry.

 

Download the issue now.

 

Not a Qpedia subscriber? Subscribe Now for free at: http://www.qats.com/Qpedia-Thermal-eMagazine/Subscribe-to-Qpedia and see why over 18,000 engineers read Qpedia.

New maxiFLOW DC-DC Brick Heat Sinks Ideal for Military-COTS Applications

ATS has recently launched a new product line of maxiFLOW heat sinks, specially designed to cool DC-DC converters. The new line of heat sinks can be used with Vicor’s DC-DC converter Bricks, including their military-COTS applications.

Vicor’s Maxi, Mini, and Micro series DC-DC converters are relied upon by over eight thousand OEMs for their proven performance, broad coverage of input and output voltages, ease of mechanical mounting and thermal management flexibility. These converter modules use advanced power processing, control, and packaging technologies to provide the performance, flexibility, and ruggedness expected in a Military COTS product. High frequency ZCS/ZVS switching, advanced power semiconductor packaging, and thermal management provide high-power density with low noise and high efficiency.

maxiFLOW Heat Sink for Half Brick DC-DC Converters

 

ATS’ patented maxiFLOW technology cools millions of BGAs and other PCB components. The same technology is now available for cooling eighth, quarter, half and full brick modules, such as the Micro, Mini, and Maxi series from Vicor. Unlike other converter heat sinks, the patented maxiFLOW heat sink design reduces air pressure drop and provides greater surface area, increasing thermal performance by 30-200%.

Vicor’s Micro, Mini, and Maxi DC-DC Converters

Vicor’s offering of full, half, and quarter-brick modules feature a patented low noise design with the highest reliability and power density available. Fully encapsulated, Maxi, Mini and Micro series DC-DC converters utilize a proprietary spin fill process that assures complete, void free encapsulation making them suitable for the harshest environments. Two grades (H & M) are available with temperatures to -55°C operating and -65°C storage. H & M-Grade modules are qualified to the stringent environmental tests of MIL-STD-810 and MIL-STD-202 and undergo 100% Environment Stress Screening.

By combining technology from industry leaders Vicor and ATS, it can be ensured that DC-DC converters will have superior performance in the harshest environments, which is vital for military and aerospace applications.

To learn more about maxiFLOW Brick DC-DC converter heat sinks, please visit our Power Brick Heat Sink Page or email ats-hq@qats.com, or call us 781-769-2800.

New Qpedia Thermal eMagazine Published!

Qpedia Thermal eMagazine, Volume 6, Issue 11, has just been released and can be downloaded at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues. Featured articles in this month’s issue include:

Honeycomb Heat Sinks for LEDs

LEDs, or light-emitting diodes, are a form of solid-state lighting. An LED light is often made of a small piece of semiconductor, an integrated optical lens used to shape its radiation pattern, and a heat sink, used to dissipate heat and maintain the semiconductor at low operating temperature. LED lights present many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size and faster switching. This article examines Ma et al’s  findings with respect to the honeycomb heat sink design employed in LEDs, which has proven to be highly efficient.

Characteristics of Thermosyphons in Thermal Management

With the increase of heat fluxes and shrinking chip sizes in electronics applications, there is a need to spread the heat from the small chip to the larger heat sink or to transport the heat to a location where there is ample space to remove the heat. Heat pipes, vapor chambers and thermosyphons have been introduced to undertake this task and, in this article, we focus on some aspects of the design of thermosyphons. The advantage of thermosyphons is that they have no capillary limit and can transport large amounts of heat over long distances.

Industry Developments: Heat Pipes Providing High Performance

Heat pipes are increasing in type and use for the benefits they provide. Because of their lower total thermal resistance, heat pipes transfer heat more efficiently and evenly than solid aluminum or copper. Heat pipes contain a small quantity of working fluid (e.g. water, acetone, nitrogen, methanol, ammonia). Learn the conclusions of a recent study that focused on the best working fluid and another study of heat pipes in outer space.

Technology Review: Cold Plates, 2010 to 2012

Qpedia continues its review of technologies developed for electronics cooling applications. We are presenting selected patents that were awarded to developers around the world to address cooling challenges. After reading the series, you will be more aware of both the historic developments and the latest breakthroughs in both product design and applications.

Cooling News featuring the latest product releases and buzz from around the electronics cooling industry.

Download the issue now.

Not a Qpedia subscriber? Subscribe Now for free at: http://www.qats.com/Qpedia-Thermal-eMagazine/Subscribe-to-Qpedia and see why over 18,000 engineers read Qpedia.

The New iFLOW-200 Tests and Measures the Thermal and Hydraulic Performance of Cold Plates

Advanced Thermal Solutions, Inc. (ATS) has released a new thermal test instrument, the iFLOW-200, which assesses the thermal and hydraulic characteristics of cold plates in electronics cooling. It can be used to simulate a wide range of conditions to optimize a cold plate’s performance before it is commercialized or prior to its use in an actual application.

 

The iFLOW-200 measures coolant temperatures from 0-70°C with the high accuracy of ± 1°C. Differential pressure of the coolant in the cold plate is measured up to 103,000 Pa (15 psi), with the precise accuracy of ± 1%. Distilled water is used as the reference coolant. For test comparisons, the systems coolingVIEW software can also calculate thermal resistance and pressure drop as a function of flow rate for selected liquids.

 

The instrument system includes a pair of K-type thermocouples for measuring temperature changes on the cold plate surface. Temperatures are monitored on the coolingVIEW interface.

 

The iFLOW-200 system features easy set up and operation to save time when evaluating different cold plate models. Designed for accuracy and convenience, the iFLOW-200 simply requires setting the starting and ending coolant flow rates, and choosing the dwell time, pumping power and other parameters. These are easily done on any PC using the systemd user-friendly application program.

The iFLOW-200 system features separate controller and hydraulics enclosures with USB connections. The hydraulic package includes a fluid level indicator, coolant inlets and outlets from/to the cold plate under test, ports for surface temperature thermocouples, and a fluid cooling system for its internal heat exchanger. The iFLOW-200 is also ideal for testing alternative liquids.

 

More information about the iFLOW-200 Cold Plate Characterization System can be found at http://www.qats.com/Products/Temperature-and-Velocity-Measurement/Instruments/iFLOW-200