# Tag Archives: cold plates

## How to Achieve Localized Cooling with Cold Plates

Many applications in electronics cooling require a cold plate to remove heat from discrete components laid out on a board. In these circumstances, it is more efficient that the liquid does not completely fill the cold plate, but is only transferred to areas that need to be cooled.

With this kind of design, the required volumetric flow rate of the coolant will be significantly lower than if the entire cold plate was filled with liquid. The schematic for a typical example of this scenario is given in Figure 1. [1]

Figure 1. Schematic of a Board with a Localized Area of Heat Dissipation . [1]

In Figure 1, areas A, B, C and D must be cooled for the components dissipating from 5 to 15 W/cm2. The other areas, designated as open, have components that interfere with the cold plate and must be avoided in the design. Two designs were considered for this case: a drilled hole and a press-fit tube. Figure 2 shows the drilled hole concept.

As can be seen, there are multiple small holes around the heat dissipating components under the cold plate surface. Large holes are machined to interconnect the smaller holes. A technique called gun drilling was used for machining the long holes. The entire cold plate was made from a copper block.

Figure 2. Schematic of the Drilled Hole Cold Plate Design. [1]

Figure 3 shows the press-fit tube design. In this approach, a copper tube with high thermal conductivity is routed through the areas of heat transfer and either brazed or epoxied to the aluminum cold plate base. This design is considerably lighter and cheaper than the drilled hole design.

Figure 3. Schematic of the Press-Fit Tube Cold Plate Design. [1]

To analyze the performance of this cold plate configuration, simple analytical tools can be used for a standard cold plate design. A brief summary of the equations is described here. To analyze the problem, we first have to calculate how much flow is going through the cold plate, and evaluate the pressure drop of the flowing fluid.

Pressure drop is calculated from:

Where
Um = bulk mean fluid velocity (m/s)
f = fanning friction factor
Awet = wetted surface area of the tube
Ac = cross section of the tube
K = loss coefficients related to turns, sudden expansion and contraction, etc.

The friction factor was obtained from the following equation which is in satisfactory agreements for the laminar, turbulent and transition regimes [2]

Where

Where ν is the kinematic viscosity of the fluid (m2/s) and P is the wetted perimeter of the tube.

For the heat transfer calculation, the Nusselt number can be calculated from standard correlations in the literature for fully developed flow. The Nusselt number is related to the heat transfer coefficient as:

Where
Kf = fluid conductivity

For thermally developing flow the following correlation can be used: [3]

Where
Num = mean Nusselt number
Nu = fully developed Nusselt number
L = duct length

Then the convective resistance can be calculated as:

Where
hm = mean heat transfer coefficient

For the tube fitted design the overall thermal resistance is made of four components: convection, tube conduction resistance, epoxy conduction resistance and the cold plate. It is stated as:

Where
Rh = convection resistance
Rtube = conduction resistance of tube walls
Repoxy = conduction resistance of the epoxy
Rcoldplate = conduction resistance of the cold plate

For the drilled design the overall thermal resistance can be written as:

If the heat transfer coefficient is based on the local fluid temperature, then a caloric resistance must be added based on the fluid mass flow rate. The effective heat transfer coefficient is then:

Where
ṁ = mass flow rate (kg/s)
Cp = fluid heat capacitance (kJ/kg·K)

Figure 3 shows the pressure drop of the two designs as a function of water flow rate. It can be seen that with a water flow rate up to 1.89 l/min (0.5 GPM) the pressure drop between the two designs is almost the same, but at higher flow rates the drilled design’s pressure drop exceeds the tube design. The sharp 90-degree turn of the drilled holes, which lead to a higher loss coefficient, is the major contributor to the higher pressure drop.

Figure 4. Total Pressure Drop of the Drilled Design and the Tube Design as a Function of the Volumetric Flow Rate. [1]

Figures 5 and 6 show the effective heat transfer coefficient of the two designs as a function of flow rate. The bend and sharp increase of the curves around 0.95 l/min (0.25 GPM) is due to the flow transitioning from laminar to turbulent. The drilled hole design shows effective convection heat transfer between 7,000 and 27,000 W/m2K for the range of flow between 0 and 7.56 l/m (2.6 GPM). The press-fit tube design on the other hand shows a lower effective heat transfer coefficient of between 6,000 and 17,000 W/m2K.

This is mostly due to the interfacial resistance and tube wall conduction. In the drilled design example, these two resistances do not exist. In a real application, the pumping of fluid is constrained by the pump and its characteristic curve. Even though the drilled hole shows a higher heat transfer coefficient for the same flow rate, the extra pressure drop caused by the drilled design may have a lower flow rate hence lowering the heat transfer coefficient.

Figure 5. Effective Heat Transfer Coefficient of the Tube Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 6. Effective Heat Transfer Coefficient of the Drilled Hole Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 7 shows another cold plate design, this one by Lytron. [4] In this design, the extended-surface cold plate material and micro-channel aluminum extrusion are sandwiched between aluminum sheets. The entire assembly is welded using vacuum brazing. It is all aluminum, which makes it very light weight. The flexibility of this design allows the placement of cooling channels in different positions to enable localized cooling.

Figure 7. Lytron Vacuum Brazing of a Cold Plate for Localized Cooling. [4]

The above analytics show that the performance of a cold plate for localized cooling can be calculated using a simple analytical tool. The designer then has to consider such factors as weight, manufacturing, cost and thermal performance to decide the best option for his or her design. The characteristic of the pump has a paramount effect on the design and cannot be neglected.

References:

1. Seaho, S., Moran, K. and Rearick, D. (IBM Corporation) and Lee, S. (Aavid Engineering), Thermal Performance Modeling and Measurements of Localized Water Cooled Cold Plate, http://www.aavidthermalloy.com/technical/papers/pdfs/water.pdf
2. Churchill, S., Comprehensive Correlating Equations for Heat, Mass and Momentum Transfer in Fully Developed Flow in Smooth Tubes, Ind. Eng. Chem. Fundam., Vol.16, 1977.
3. Al-Arabi, M., Turbulent Heat Transfer in the Entrance Region of a Tube, Heat Transfer Eng., Vol. 3, 1982.
4. http://www.lytron.com

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

## Thermal Performance of Macro and Microchannel Cold Plates in Electronics Cooling

In recent years, intense activity has been gone into improving the capabilities of cold plates. Specifically, the use of microchannels has provided great improvements in cold plate thermal performance. Regardless of a cold plate’s channel size, the following equations can be used for heat transfer coefficients when determining thermal performance. [1]

Where,

= Nussselt number

Dh = hydraulic diameter

= Reynolds number

ν = kinematic viscosity
Pr = Prandtle number

The pressure drop can also be calculated as:

Where,
P = density
f = friction factor

In recent years, microchannel cold plates have gained popularity due to their high performance. Webb shows that the best results can be achieved when the channel aspect ratio is about 7.4, and with a fin aspect ratio of 8. [2] Figure 1 shows a Fin-H copper microchannel with a channel hydraulic diameter of 0.49 mm. Due to the small size of the channels, the flow is generally considered to be laminar. The optimization resulted in a 25-mm wide and 20-mm long microchannel cold plate. [2]

Webb considered both single-pass and two-pass designs on the water side. The two-pass version was made to determine if there was any mal-distribution of the water from the single-pass case.

Figure 1. Copper Microchannel Fin-H Used in a Cold Plate. [2]

Figure 2 shows the thermal resistance of the Fin-H for the 1-pass and 2-pass designs as a function of flow rate.

Figure 2. Thermal Resistance of a Fin-H Cold Plate as a Function of the Water Flow Rate. [2]

This figure shows that the 1-pass version has a much better thermal resistance than the two-pass model for the same flow rate. It also shows that the flow has been distributed relatively uniformly. Figure 3 shows the pressure drop of the cold plate as a function of flow rate for the Fin-H and the Thermaltake Bigwater 735 cooler. [3] The figure shows the pressure drop of the 1-pass design is only 38% of the 2-pass design.

Figure 3. Pressure Drop of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]

Figure 4 shows the thermal resistance of the Fin-H cooler in the 1-pass design compared to the Thermaltake cooler [3]. At 2.28 l/min the Thermaltake’s thermal resistance is 0.106 K/W. The balance point of the Fin-H for 1-pass is with a thermal resistance of 0.07 K/W at a flow rate of 0.361 l/min. This is only 16% of the flow rate for the Thermaltake cooler.

Referring to Figure 3, the pressure drop is almost the same for both coolers. The major implication is that the microchannel cold plate requires a smaller pump compared to macrochannel cold plates, and provides a 50% increase in thermal performance.

Figure 4. Thermal Resistance of a Fin-H Cold Plate and a Thermaltake Cooler as a Function of the Water Flow Rate. [2]

Another innovative approach is the concept of forced-fed boiling (FFB). [4] Figure 5 shows a schematic of this process. It consists of a micro-grooved, thin copper surface with alternating fins and channels. The microgrooves have a hydraulic diameter of 28 microns, an aspect ratio of 15, and a fin density of 236 fins per cm.

There are feed channels on top of the micro-grooved surface. The fluid is forced through these channels into the microgrooves, which are located on top of the heated surface. The fluid vaporizes in the microgrooves and moves upward, while the liquid flows beneath the escaping vapor. This keeps the surface wet, resulting in an increase of the critical heat flux (CHF).

Figure 5. A Force-Fed Boiling Cold Plate. [4]

Figure 6 shows the heat transfer as a function of the temperature difference between the inlet fluid and the surface for various values of the flow rate for R245fa, a non-aqueous fluid for low pressure refrigeration applications. The figure shows that for heat fluxes of about 200 W/cm2 or less, heat transfer is independent of the flow rate, but this is not the case at higher heat fluxes. It also shows that the slope of the heat flux decreases with increasing temperature difference.

Figure 6. Heat Flux as a Function of the Temperature Difference for the FFB Cold Plate. [4]

Figure 7 shows an interesting trend for the heat transfer coefficient as a function of heat flux for the same fluid. At first, the heat transfer coefficient increases with the increase in heat flux. This indicates that by increasing the heat flux, a phase change process takes place which changes the single-phase flow to two-phase heat transfer. After reaching an impressive peak at 300 KW/m2K, the heat transfer coefficient starts to decrease. This is attributed to local dryouts from bubble generation, which also blocks the microchannels.

Figure 7. Heat Transfer Coefficient as a Function of Heat Flux for the FFB Cold Plate. [4]

While advances in cold plate performance have been incremental, their technology is still evolving. Improvements in microchannel manufacturing will open more opportunities in this field. Microchannel cold plates provide tremendous heat transfer coefficient capacities, but limitations prevent their broad deployment.

Fouling, dryout, and fabrication issues have been major negating factors for microchannel deployment in the broader market. Microchannel cold plates may have particular value in such applications as military, space, and high capacity computing, where service and maintenance are part of the deployment.

However, from the design and problem-solution standpoint, microchannel cold plates can be an effective part of a closed loop liquid cooling system.

References
1. Dittus, F. and Boelter, L., Publications on Engineering, University of California at Berkley, 1930.
2. Webb, R., High-Performance, Low-Cost Liquid Micro-Channel Cooler, Thermal Challenges in Next Generation Electronic Systems II, Millpress Science Publishers, Rotterdam, The Netherlands, 2007.
3. Thermaltake Company, 2006.
4. Cetegen, E., Dessiatoun, S., and Ohadi, M., Force Fed Boiling and Condensation for High Heat Flux Applications, VII Minsk International Seminar: Heat Pipes, Heat Pumps, Refrigerators, Power Sources, Minsk, Belarus, 2008.

Learn more about Advanced Thermal Solutions, Inc. (ATS) standard and customized, high-performance liquid cold plates by visiting https://www.qats.com/Products/Liquid-Cooling/Cold-Plates.

For more information about ATS thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

## What Fluids Can Be Used With Liquid Cold Plates in Electronics Cooling Systems

By Norman Quesnel
Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc. (ATS)

Liquid cooling systems transfer heat up to four times better than an equal mass of air. This allows higher performance cooling to be provided with a smaller system. A liquid cooled cold plate can replace space-consuming heat sinks and fans and, while a liquid cold plate requires a pump, heat exchanger, tubing and plates, there are more placement choices for cold plates because they can be outside the airflow. [1]

One-time concerns over costs and leaking cold plates have greatly subsided with improved manufacturing capabilities. Today’s question isn’t “Should we use liquid cooling?” The question is “What kind of liquid should we use to help optimize performance?”

Figure 1. A Liquid Cooling System for a Desktop PC with Two Cold Plates. [2]

For liquid cold plates, the choice of working fluid is as important as choosing the hardware pieces. The wrong liquid can lead to poor heat transfer, clogging, and even system failure. A proper heat transfer fluid should provide compatibility with system’s metals, high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability. [3]

Today, despite many refinements in liquid cold plate designs, coolant options have stayed relatively limited. In many cases, regular water will do, but water-with-additives and other types of fluids are available and more appropriate for certain applications. Here is a look at these coolant choices and where they are best suited.

Basic Cooling Choices

While water provides superior cooling performance in a cold plate, it is not always practical to use because of its low freezing temperature. Additives such as glycol are often needed to change a coolant’s characteristics to better suit a cold plate’s operating environment.

In fact, temperature range requirements are the main consideration for a cold plate fluid. Some fluids freeze at lower temperatures than water, but have lower heat transfer capability. The selected fluid also must be compatible with the cold plate’s internal metals to limit any potential for corrosion.

Table 1 below shows how the most common cold plate fluids match up to the metals in different cold plate designs.

Table 1. Compatibility Match-ups of Common Cold Plate Metals and Cooling Fluids [1]

The choices of cold plate coolants will obviously have varied properties. Some of the differences between fluids are less relevant to optimizing cold plate performance, but many properties should be compared. Tables 2 and 3 show the properties of some common coolants.

Tables 2 and 3. Comparisons of Properties of Typical Electronic Coolants. [4]

An excellent review of common cold plate fluids is provided by Lytron, an OEM of cold plates and other cooling devices. The following condenses fluid descriptions taken from Lytron’s literature. [5]

The most commonly used coolants for liquid cooling applications today are:

• Water
• Deionized Water
• Inhibited Glycol and Water Solutions
• Dielectric Fluids

Water – Water has high heat capacity and thermal conductivity. It is compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Facility water or tap water is likely to contain impurities that can cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance.

If you determine that your facility water or tap water contains a large percent of minerals, salts, or other impurities, you can either filter the water or can opt to purchase filtered or deionized water. [5, 6]

Deionized Water – The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids, deionized water has a high resistivity. Deionized water is an excellent insulator, and is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended. [5, 7]

Inhibited Glycol and Water Solutions – The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications, and can also be used in enclosed spaces. [5, 8]

Dielectric Fluid – A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water. However, it has a much lower thermal conductivity and much higher price. PAO is a synthetic hydrocarbon used for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid-cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO-compatible recirculating chillers are available. Like perfluorinated carbons, PAO has much lower thermal conductivity than water. [5, 9]

Conclusion

Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications.

It is important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

For information on ATS Cold Plates, visit our Cold Plate page at https://www.qats.com/Products/Liquid-Cooling/Cold-Plates

## In the ATS Labs – Where Thermal Solutions Advance to Meet Industry Demands

Thermal management innovations need to match the rapid pace at which the electronics industry is advancing. As consumers demand new and more powerful devices or greater amounts of information at faster speeds, cooling solutions of the past will not be enough. Today’s cooling solutions must be smaller, lighter, and offer higher performance, but also need to be cost-effective, meet demanding project specifications, and be reliable for many years.

Advanced Thermal Solutions, Inc. (ATS) understands the importance of creating cutting-edge thermal solutions for its customers and has geared its thermal design capability and its research and development to match the innovations taking place in electronics design.

An ATS engineer assembles a rig for testing cold plates in one of ATS’ six state-of-the-art labs. (Advanced Thermal Solutions, Inc.)

To meet the need for innovative solutions, ATS engineers are hard at work in the company’s six state-of-the-art laboratories at the ATS headquarters, located in Norwood, Mass. (south of Boston). Thermal issues of all kinds are recognized, broken down, and resolved and cooling solutions are designed, simulated, prototyped, and rigorously tested in these research-grade facilities.

When someone thinks of a research lab, the initial picture is scientists in white coats working for major corporations, such as IBM, Microsoft, or Google, but the development of new ideas is an essential tool for any company in the technology field. Working with empirical tests in a lab environment pushes concepts from the white board or the computer screen to reality. There comes a time when engineers need to produce tangible data to ensure that a design works as planned.

ATS thermal engineers are no different. They use state-of-the-art instruments and software in each of the six labs to conduct a long list of characterization, quality-assurance, and validation tests. In addition to finding custom cooling solutions for customers, ATS engineers produce thermal management products for commercial uses, including a variety of next generation heat sink, heat pipe, vapor chamber, and liquid cooling designs.

Engineers test ATS instruments using a wind tunnel and sensors in the Characterization Lab. (Advanced Thermal Solutions, Inc.)

Among the most common tests performed in the ATS labs are:

• Measurements of air velocity, direction, pressure and temperature;
• Characterization of heat sink designs, fans and cold plates
• Flow visualization of liquid and air flow
• Image visualization characterization using infrared and liquid crystal thermography.

Many of the instruments that these tests are performed on were designed and fabricated by ATS. That includes open-loop, closed-loop, and bench-top wind tunnels; the award-winning iQ-200™, which measures air temperature, velocity, and pressure with one instrument; and the thermVIEW™ liquid crystal thermography system. Engineers also use specially-designed sensors, such as the ATS Candlestick Sensor, to get the most accurate analysis possible.

Smoke flow visualization tests run in ATS wind tunnels demonstrate how air flows through a system. (Advanced Thermal Solutions, Inc.)

Heat pipes and vapor chambers are increasingly common cooling solutions, particularly for mobile devices and other consumer electronics, and ATS engineers are working to expand the company’s offerings for these solutions and to develop next generation technology that optimizes the thermal performance of these products. This research involves advanced materials, new fabrication methods, performance testing, and innovative designs that are ready for mass production.

ATS engineer Vineet Barot sets up a thermal imaging camera for temperature mapping studies in the lab. (Advanced Thermal Solutions. Inc.)

ATS has also developed products to meet the growing demand across the electronics industry for liquid cooling systems. From new designs for recirculating and immersion chillers to multi-channel cold plates to tube-to-fin heat exchangers, ATS is continuing to expand its line of liquid cooling solutions to maximize the transfer of heat from liquid to air and researching new manufacturing methods, advanced materials, and other methods of enhancing the technology.

As liquid cooling technology has grown, ATS has met this demand with new instruments and lab capabilities, such as the iFLOW-200™, which measures a cold plate’s thermal and hydraulic characteristics, and full liquid loops to test ATS products under real-world conditions.

ATS engineer Reza Azizian (right) works with intern Vladislav Blyakhman on a liquid cooling loop in the lab. (Advanced Thermal Solutions, Inc.)

The labs at ATS are up to even the toughest electronics cooling challenges that the company’s global customers present. Thanks to its extensive lab facilities, ATS has provided thousands of satisfied customers with the state-of-the-art thermal solutions that they demand.

For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.

## #JustChilling: ATS Recirculating and Immersion Chillers for Liquid Cooling Systems

Advanced Thermal Solutions, Inc. has a line of recirculating and immersion chillers for conditioning the coolant in liquid cooling systems. (Advanced Thermal Solutions, Inc.)

ATS offers a variety of chillers, including the CHILL V and CHILL iM series, for conditioning the coolant in liquid cooling systems. The ATS-CHILL V series, including the ATS-Chill150V, ATS-Chill300V, and ATS-Chill600V, are re-circulating, vapor compression chillers that offers precise coolant temperature control using a PID controller. The ATS-CHILL iM is an immersion chiller for precise control of the bath temperature by immersing the evaporator in a fluid bath.

Learn more about ATS recirculating and immersion chillers in this recent blog post or in the video below:

ATS Liquid Cooling Products

In addition to chillers, ATS has a complete product offering for Liquid Cooling Closed Loop Systems, including flow meters, leak detectors, heat exhangers, and cold plates. ATS can also design off-the-shelf or custom liquid cooling systems to meet the thermal needs of a project.

ATS 3-Core design approach identifies the type of cooling required at the analysis level and informs the client of its options, saving cost and time on design iteration and simulation verification. Once it is determined that liquid cooling is the option to pursue, the ATS design team identifies all the required components of the liquid loop, as well its packaging requirements and integration in the system.

ATS offers a complete array of off-the-shelf liquid loop components that can be readily deployed or custom-designed to meet the thermal requirements of the system. Subsequent integration of the liquid loop into the system provides the customer with a turn-key option for thermal management of their system.

Don’t get burned! Take advantage of ATS expertise in liquid and air cooling to ensure proper thermal management for your project.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.