Industry Developments in District Cooling Systems

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

(This article will be featured in an upcoming 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. To read other stories from Norman Quesnel, visit https://www.qats.com/cms/?s=norman+quesnel.)

District cooling is the centralized production and delivery of cooling energy to collective regions of office, public or domestic structures. In a typical district cooling scheme, a central plant chills water from a contained reservoir or taken from an ocean or lake. The chilled water is delivered via underground, insulated pipelines to select buildings in a district. The buildings contain pumps and tubing systems that circulate the cold water within the living areas.

Air is forced past the circulating cold water to produce an air conditioned environment. The resulting warmed water in the tubes is returned to the central plant for re-chilling and recirculating.

District cooling can use either regular water or seawater and can be powered by electricity or natural gas. The output of one district cooling plant is enough to meet the cooling-energy demands of dozens of buildings. [1]

Nowhere is advanced district cooling being developed more than in the Middle East, particularly in its wealthier – and hotter – countries like those in the Gulf Cooperation Council (GCC): Saudi Arabia, Kuwait, United Arab Emirates (UAE), Qatar, Bahrain and Oman. Air conditioning is responsible for about 70 percent of the GCC’s electricity demand during peak summer months.

District Cooling

Figure 1. CAD Image of District Cooling in a High-Rise Building in Lusail City, an Urban Development Planned for Qatar. [2]


One district cooling example is Qatar’s very smart Lusail City. Still largely in planning, Lusail will use a state-of-the-art system to provide cool environments in its modern business and residential buildings. In typical fashion, the Lusail system will use chilled water in pipes feeding to different localities via an extensive system of underground tunnels and local substations. [2]

High Cooling Performance

In many ways, district cooling is a superior alternative to conventional, localized air conditioning. It helps reduce costs and energy consumption for both customers and governments alike, while also protecting the environment by cutting carbon dioxide emissions.

District Cooling

Figure 2. District Cooling Systems can Store 30% of Potential Cooling Output by Holding Water in Reserve for Seasonal Requirements. [3]

Some of the advantages district cooling has over traditional air conditioning includes 50 percent less energy consumption with better accommodation of peak cooling power demands. There are substantially lower maintenance costs than for individual, localized units. District cooling’s equipment has, on average, a 30-year working life, just about as long as conventional urban air conditioning systems.

District cooling systems reduce CO2 emissions because of their lower energy consumption. The centralized systems also free up useable space in individual buildings, including rooftops and basements where local cooling systems were formerly installed. [3]

District Cooling

Figure 3. District Cooling Layout for King Abdullah Financial District (KAFD) Under Construction Near Riyadh, Saudi Arabia. Total Capacity is 100,000 Tons of Refrigeration. [4]

District cooling is measured in tons of refrigeration, TRs, equivalent to 12,000 BTUs per hour. A refrigeration ton is the unit of measure for the amount of heat removed. It is defined as the heat absorbed by one ton (2,000 pounds) of ice causing it to melt completely by the end of one day (24 hours). In Qatar and Saudi Arabia, the district cooling systems being developed will contribute a combined 4.5 million tons of refrigeration.

District Cooling

Figure 4. District Cooling at the Nation Towers Area of Abu Dhabi is Managed by Tabreed, Which has 71 District Cooling Plants Throughout the Gulf Cooperation Council, GCC. [5]

Nation Towers is the site of two skyscrapers near the southern end of the ocean-bordering Corniche in Abu Dhabi, the capital of the UAE. The towers, 65 and 52 floors tall respectively, are joined by a sky bridge and together offer nearly 300,000 m2 of usable space.

The towers and the surrounding structures are air conditioned by a district cooling plant managed by Tabreed, the largest name in district cooling in the GCC. In 2015, per Tabreed, the company’s UAE-based district cooling systems reduced the amount of energy used in air conditioning by 1.3 billion kilowatt hours – the equivalent use of 44,000 UAE homes. [6]

Northeast from Abu Dhabi, the UAE city of Dubai is home to the sprawling WAFI Mall. The site uses Siemens Demand Flow technology to optimize the chilled water system that keeps its stores and restaurants at comfortable temperatures. Siemens Demand Flow technology uses specialized algorithms to optimize the entire chilled water system of a cooling plant, delivering energy savings of between 15 and 30 percent.

By simplifying operations, increasing the cooling capacity and improving efficiency, the system is able to reduce flow in periods of lesser demand, lowering operation and maintenance costs and significantly lowering energy use. [7]

But the Middle East is not the only part of the world employing district cooling. In another warm country, India, a new business district is being constructed on nearly 900 acres in the state of Gujarat. This is the Gujarat International Finance Tec-City, whose district cooling will provide a total cooling capacity of 1,800,000 TR. [8]

In Europe, Copenhagen is home to a successful district cooling operation. The city may not be thought of as in much need of air conditioning; summer high temperatures rarely exceed the mid-70s Fahrenheit. But even in Denmark, there is a need for indoor cooling inside buildings with large server rooms or where many people work or shop. The northern city already had a district heating system and harnessed much of that infrastructure to add cooling.

District Cooling

Figure 5. Copenhagen’s District Cooling System Reduces Carbon Emissions by Nearly 70% and Electricity Consumption by 80% Compared to Conventional Cooling. [5] (Pictured: Heat pipes running under Copehagen/Wikimedia Commons)

At times Copenhagen’s ocean water is so cold it doesn’t need to be chilled, which saves energy. The district cooling is targeted for co-located buildings (department stores, commercial buildings, hotels, and facilities with data centers) with cooling demands of 150 kilowatts (kW) or more. [9]

And in the U.S., Thermal Chicago provides the country’s biggest district cooling system. It includes five interconnected plants providing cooling to more than 100 buildings in the Windy City. During peak time of air conditioner use, the Thermal Chicago cooling system has reduced energy demand by more than 30 megawatts.

The facility’s also uses a different water-chilling technology that includes an ice-based thermal storage tank for faster cooling and return of chilled water to the infrastructure needing cooling. A YouTube video explains how Thermal Chicago water cooling is set up. [10]

District Cooling

Figure 6. Ice-based Cooling Section Within the Thermal Chicago District Cooling System, from YouTube Video. [10]

Recapping the basic steps of district cooling:

• A central plant chills water.
• A primary water circuit then distributes the chilled water to buildings through an underground insulated pipes network.
• A secondary water circuit in the customers’ building circulates the cold water.
• Air is then forced past the cold water tubing to produce an A/C environment.
• The warmer water of the primary circuit is returned to the central plant to be re-chilled and recycled.

District cooling is not a new technology, or even a new concept. Centralized production and distribution of temperature control has been in commercial use since the 19th century, mainly for heating purposes.

Today, for efficiency and environmental reasons – including rising global temperatures – district cooling is seeing a renaissance by being designed into many of the smarter cities being built around the world.

References
1. Tabreed, https://www.tabreed.ae/en/district-cooling/district-cooling-overview.aspx
2. Lusail City, http://www.lusail.com
3. CELCIUS Smart Cities, http://celsiuscity.eu/
4. Saudi Tabreed, http://www.fleminggulf.com/files/doc/DBUT09/Abdul_Jalil_Bakhruji.pdf
5. Tabreed, https://www.tabreed.ae/en/district-cooling/our-district-cooling-plants.aspx
6. The National UAE, http://www.thenational.ae/uae/environment/tabreed-reduces-carbon-emissions
7. Siemens, http://www.middleeast.siemens.com/me/en/news_events/news/news-2016/siemens-smart-building-tech-can-cut-gccs-cooling-bill-by-40.htm
8. Gujarat International Finance Tec-City, http://giftgujarat.in/district-cooling-system
9. Forbes, https://www.forbes.com/sites/justingerdes/2012/10/24/copenhagens-seawater-cooling-delivers-energy-and-carbon-savings/#1f8d40d74245
10. Thermal Chicago video, https://www.youtube.com/watch?v=ziEbY0oLf-o

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.

How Do Heat Sink Materials Impact Performance

By Michael Haskell, Thermal Engineer
and Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication produced by Advanced Thermal Solutions, Inc. (ATS) 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.)

Heat Sink Materials

This article examines the difference in thermal performance between copper, aluminum, and graphite foam heat sinks. (Advanced Thermal Solutions, Inc.)

Introduction

As thermal solutions for today’s electronics grow more challenging, demand rises for novel cooling ideas or materials. As a result, the proven methods of analytical calculations, modeling and laboratory testing are sometimes bypassed for a quick “cure-all” solution. Evolutionary progress is required of the thermal industry, of course. But, despite the urgency to introduce new ideas and materials, thorough testing should be performed in determining the thermal performance of a solution before it is implemented.

This article addresses the impact of material choice on heat sink performance. First, an evaluation of different materials is made in a laboratory setting, using mechanical samples and a research quality wind tunnel. This testing compares a constant heat sink geometry made from copper, aluminum, and graphite foam. Next, an application-specific heat sink study is presented using computational fluid dynamics (CFD) software.

In this study, a heat sink was designed in 3D CAD to cool a dual core host processor. The performance of both an aluminum and copper design was then evaluated using CFD.

Laboratory Tests of Copper, Aluminum, and Graphite Foam

The stated thermal properties of engineered graphite foams have enhanced their consideration as heat sink materials. Yet, the literature is void of a true comparison of these materials with copper and aluminum. To evaluate graphite foam as a viable material for heat sinks, a series of tests were conducted to compare the thermal performance of geometrically identical heat sinks made of copper, aluminum, and graphite foam respectively.

Testing was conducted in a research quality laboratory wind tunnel where the unducted air flow was consistent with typical applications.

(The results for ducted and jet impingement flows, though similar to the unducted case, will be presented in a future article along with a secondary graphite foam material.)

Test Procedure

Earlier foam experiments by Coursey et al. [1] used solder brazing to affix a foam heat sink to a heated component. The solder method reduced the problematic interfacial resistance when using foams, due to their porous nature. Directly bonding the heat sink to a component has two potential drawbacks. First, the high temperatures common in brazing could damage the electrical component itself.

The other issue concerns the complicated replacement or rework of the component. Due to the low tensile strength of foam (Table 1) a greater potential for heat sink damage occurs than with aluminum or copper [2]. If the heat sink is damaged or the attached component needs to be serviced, direct bonding increases the cost of rework.

Table 1. Thermal and Mechanical Properties of the Heat Sink Materials. (Advanced Thermal Solutions, Inc.)

To avoid these problems, the foam heat sink can be soldered to an aluminum or copper carrier plate. This foam-and-plate assembly can then be mounted to a component in a standard fashion. The carrier plate allows sufficient pressure to be applied to the interface material, ensuring low contact resistance.

In this study, the heat sinks were clamped directly to the test component without a carrier plate as a baseline for all three materials. Shin-Etsu X23 thermal grease was used as an interface material to fill the porous surface of the foam and reduce interfacial resistance. Five J-type thermocouples were placed in the following locations: upstream of the heat sink to record ambient air temperatures, in the heater block, in the center of the heat sink base, at the edge of the heat sink base, and in the tip of the outermost fin.

Heat Sink Material

Figure 1. Test Heat Sink Drawing. (Advanced Thermal Solutions, Inc.)

A thin film heater was set at 10 watts during all testing, and the heat source area was 25 mm x 25 mm, or one quarter of the overall sink base area, as shown in Figure 1. Both cardboard and FR-4 board were used to insulate the bottom of the heater, The estimated value of Ψjb is 62.5°C/W. Throughout testing, the value of Ψjb was 36–92 times greater than that of Ψja.

Results

As expected, the traditional copper and aluminum heat sinks performed similarly. The main difference was due to the higher thermal conductivity of copper, which reduced spreading resistance. During slow velocity flow conditions, the lower heat transfer rate means that convection thermal resistance makes up a large portion of the overall Θja.

Heat Sink Materials

Table 2. Test Heat Sink Geometry. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 2. Experimental Heater and Measurement Setup. (Advanced Thermal Solutions, Inc.)

As flow speed increases, the convection resistance decreases, and the internal heat sink conduction resistance is more of a factor in the overall Θja value. This behavior is evident in the table below, and when comparing the different heat sink materials. The graphite heat sink’s thermal performance was only 12% lower than aluminum at low flow rates. However, the performance difference increased to 25-30% as the flow rate increased (Table 3).

Heat Sink Materials

Table 3. Specific Thermal Test Results. (Advanced Thermal Solutions, Inc.)

Due to the lack of a solder joint, the foam heat sink experienced a larger interfacial resistance when compared to the solid heat sinks. This difference can be seen when comparing ΨHEATER-BASE in Table 3. To decouple the effect of interfacial resistance ΨBASE-AIR can be calculated. When ignoring interfacial resistance in this manner foam performs within 1% of aluminum at 1.5 m/s, and within 15% at 3.5 m/s.

Heat Sink Materials

Figure 3. Heat Sink Thermal Resistance as a Function of Velocity. (Advanced Thermal Solutions, Inc.)

Graphite foam-derived heat sinks show promise in specific applications, but exhibit several drawbacks in mainstream electronics cooling. Due to the frail nature of graphite foam, unique precautions must be taken during the handling and use of these heat sinks. When coupled to a copper base plate, graphite foam can perform with acceptably small spreading resistances.

However, the foam’s lower thermal conductivity reduces thermal performance at high flow velocities compared to a traditional copper heat sink.

The mechanical attachment needed to ensure acceptable thermal interface performance without soldering or brazing also hinders foam-based heat sinks from being explored in mainstream applications. Despite these challenges, the thermal performance-to-weight ratio of foam is very attractive and well-suited to the aerospace and military industries, where cost and ease of use come second to weight and performance.

Thermal Software Comparison of Aluminum and Copper Heat Sinks

A challenging thermal application was considered. This involved the use of a dual core host processor on a board with limited footprint area for a heat sink of sufficient size. A heat sink with a stepped base was designed to clear onboard components. It provided sufficient surface area to dissipate heat (Figure 4).

Due to the complexity of the heat sink, machining a test sample from each material was not practical. Instead, CFD was used to predict the performance difference between the two materials and determine if the additional cost of copper was warranted.

Heat Sink Materials

Figure 4. Stepped Base maxiFLOW™ Heat Sink (ATS). (Advanced Thermal Solutions, Inc.)

Because of the stepped base and a long heat conduction path, spreading resistance was a major factor in the overall thermal resistance. The effect of copper in place of aluminum due to its higher thermal conductivity (400 and 180 W/m*K respectively) is shown in Table 4. The CFD software predicted a 21% improvement using copper in place of aluminum. More importantly, it reduced the processor case temperature below the required goal of 95°C.

The performance improvement with copper is due to the reduced spreading resistance from the processor die to the heat sink fins. This effect is shown in Figure 5, where the base temperatures of both heat sinks are obtained from the CFD analysis and plotted together. The aluminum heat sink shows a hotter center base temperature and a more pronounced drop off in temperature along the outer fins. The copper heat sink spreads the heat to all fins in a more even fashion, increasing the overall efficiency of the design. This temperature distribution can be seen in Figures 6 and 7, which were created using CFDesign software.

Heat Sink Materials

Figure 5. Effect of Heat Sink Material on Temperature Distribution. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 6. Aluminum Stepped Base maxiFLOW™ Heat Sink Simulation. (Advanced Thermal Solutions, Inc.)

Heat Sink Materials

Figure 7. Copper Stepped Base maxiFLOW™ Heat Sink Simulation. (Advanced Thermal Solutions, Inc.)

Conclusion

Design engineers have many materials at their disposal to meet the challenging thermal needs of modern components. Classic materials such as aluminum and copper are joined by new technologies that bring improvements in cost, weight, or conductivity. The choice between a metallic, foam or plastic heat sink can be difficult because thermal conductivity provides the only available information to predict performance.

The first method for determining material selection is a classic thermodynamics problem: what effect does conductivity have on the overall thermal resistance in my system? Only once this is answered can the benefits of cost, weight, and manufacture be addressed.

References

1. Coursey, J., Jungho, K., and Boudreaux, P. Performance of Graphite Foam Evaporator for Use in Thermal Management, Journal of Electronics Packaging, June 2005.
2. Klett, J., High Conductivity Graphitic Foams, Oak Ridge National Laboratory, 2003.

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.

Case Study: Thermal Comparison of Copper and Aluminum Heat Sinks

Advanced Thermal Solutions, Inc. (ATS) engineers were tasked by a client to find a more cost-effective and lighter solution for a custom-designed copper heat sink that dissipated heat from four components on a PCB. ATS engineers compared the thermal performance of the copper heat sink to custom aluminum heat sinks embedded with heat pipes.

Aluminum Heat Sinks

ATS engineers worked on a comparison of a copper heat sink with an aluminum heat sink that had embedded heat pipes running underneath the components. Analysis showed that the aluminum heat sink nearly matched the thermal performance of the copper and was within the margin required by the client. (Advanced Thermal Solutions, Inc.)

Using analytical modeling and CFD simulations, the ATS engineers determined that switching to an aluminum heat sink with heat pipes that run underneath the components yielded case temperatures that were greater than 4.35%, on average, of those achieved with the copper heat sink. The largest difference between the two heat sinks was 9.2°C, over a single component.

Challenge: The client wanted a redesign of a custom copper heat sink to an equivalent or better aluminum heat sink with embedded copper heat pipes.

Chips/Components: Two Inphi (formerly ClariPhy) Lightspeed-II CL20010 DSPs at 96 watts and two Xilinx 100G Gearboxes at 40 watts each.

Analysis: Analytical modeling and CFD simulations determined the junction temperatures between the four components when covered by a copper heat sink (Design 1), by an aluminum heat sink with heat pipes that stop in front of the components (Design 2), and by an aluminum heat sink with heat pipes that run underneath the components (Design 3). The analysis demonstrated the difference between the heat sink designs in relation to thermal performance.

Test Data: CFD analysis showed an average component case temperature of 158.8°C with the original copper heat sink design, 158.3°C with Design 2, and 152°C with Design 3. The average difference in temperature between Design 1 and Design 2 was 0.5°C and the average temperature difference between Design 1 and Design 3 was 6.8°C.

Here is a CFD simulation from the top of the aluminum heat sink with the air hidden, showing the temperature gradient through the heat sink. (Advanced Thermal Solutions, Inc.)

Solution: The client was shown that aluminum heat sinks with heat pipes provided nearly the same thermal performance as the original copper heat sink design and at much lower cost and weight. The component junction temperature differences between Design 1 and Design 3 were well within the margin set by the client.

o The simulated air velocity is lower and the airflow cross section is larger than in the actual application, meaning absolute temperatures are higher than the customer has seen in their testing.

Net Result: Despite using conservative thermal conductivity calculations, aluminum heat sinks with heat pipes were shown to be a more cost-effective solution for achieving the client’s thermal needs than copper.

CLICK HERE FOR A TECHNICAL DISCUSSION OF THIS 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.

Analysis of Fan Curves and Fan Laws in Thermal Management of Electronics

This is the second installment in a two-part series examining the use of fans in the thermal management of electronics. Part one, which can be found at https://www.qats.com/cms/2017/03/06/utilizing-fans-thermal-management-electronics-systems, took a closer look at the common types of fans and blowers and the factors that engineers should consider when picking a fan.

In part two, basic fan laws will be explored, as well as using fan curves to analyze fan performance in a system. These standard calculations can help engineers establish boundary conditions for air velocity and pressure drop and ensure that these will meet the thermal requirements (e.g. ambient and junction temperature) of the system.

Fan Laws

CFD simulations of air velocity in a system with fans drawing air across high-powered components. Utilizing fan curves and fan laws enabled ATS engineers to establish the parameters for a successful use of fans for cooling this system. (Advanced Thermal Solutions, Inc.)

FAN LAWS

As noted by Mike Turner of Comair Rotron in “All You Need to Know About Fans,” the primary principle for determining whether or not a fan work within a particular system is that “any given fan can only deliver one flow at one pressure in a particular system.” Each fan has a specific operating point that can be discovered on the fan curve at the intersection of fan static pressure curve and the system pressure curve. Turner advises, “It is best to select a fan that will give an operating point being toward the high flow, low pressure end of the performance curve to maintain propeller efficiency and to avoid propeller stall.”

Before getting to the fan curve though, engineers must run through basic calculations to understand the conditions of the systems in which the fans will be placed. The three basic fan laws, according to Eldridge USA, are as follows:

Fan Laws

While those fan laws will tell you about the specific fans, it is also critical to examine the system in which the fans will be operating. Among the equations that can be used to characterize a system are Volumetric Flow Rate, Mass Flow Rate, Pressure, Power, and Sound (equations are listed below).

Fan Laws

A Qpedia Thermal eMagazine article entitled, “How to Use Fan Curves and Laws in Thermal Design,” added:

“Published fan laws apply to applications where a fan’s air flow rate and pressure are independent of the Reynolds number. In some applications, however, fan performance is not independent and thus the change in Reynolds number should be incorporated into the equation. To determine if the Reynolds number needs to be considered, it must first be calculated.

“According to AMCA specifications, an axial fan’s minimum Reynolds number is 2.0×106. When the calculated Reynolds number is above this value, its effects can be ignored.”

The equation to calculate the Reynolds number is as follows:

Fan Law

In an “Engineering Letter” from The New York Blower Company, it was explained that fan laws only work “within a fixed system with no change in the aerodynamics or airflow characteristics of the system.” In the case of electronics cooling, in which the system requirements will be mostly consistent (with margins for error in case of max power usage), these laws will govern the capabilities of the fans to provide the necessary forced convection cooling for the components in the system.

The Engineering Letter continued, “During the process of system design, the fan laws can be helpful in determining the alternate performance criteria or in developing a maximum/minimum range.” A Qpedia article entitled, “Designing Efficient Fans for Electronics Cooling Applications,” added, “As a general rule, fan efficiency increases with blade diameter and rotational speed.”

There are tools that can assist engineers in the calculation of these basic fan laws, including fan calculators, such as the one provided by Twin City Fans & Blowers.

ANALYZING FAN CURVES AND FAN PERFORMANCE

The aerodynamics of a fan can be charted in a fan curve, which displays the static pressure of the system dependent on the amount of air flow. As Turner noted, fan curves are read from right to left, beginning “with healthy aerodynamic flow and follow it through to aerodynamic stall.” Turner continued, “It is best to select a fan that will give an operating point being toward the high flow, low pressure end of the performance curve to maintain propeller efficiency and to avoid propeller stall.”

Fan Laws

An example of a basic fan curve with static pressure on the Y-axis and airflow on the X-axis. Fan curves are read from right to left beginning with healthy airflow.

There are means for testing fan curves, such as the FCM-100 Fan Characterization Module (pictured below) from Advanced Thermal Solutions, Inc. (ATS). The FCM-100 is specially designed with flow restriction plates that allow the user to control pressure drop across the system during testing. Used in conjunction with pressure and velocity measurement equipment, it verifies manufacturer performance data.

Fan Laws

The ATS FCM-100 Fan Characterization Module is a specialized unit designed to test and characterize fans of various sizes and performance outputs. (Advanced Thermal Solutions, Inc.)

The Qpedia article on fan curves explained, “During a typical fan test, a dozen or more operating points are plotted for pressure and flow rate, and from this data a fan curve is constructed.”

Once a fan curve is determined, it is possible to examine the data and find the operating range for the fans that will meet the thermal requirements of a system. It is also important to note a section in the fan curve, often referred to as the knee of the curve in which the relationship between flow rate and static pressure is no longer easy to predict. There is no longer an easily recognizable, calculable relationship between how a change in one will affect the other.

ATS field application engineer Vineet Barot explained how he analyzed fan curve data, particularly the knee of the curve, in a recent project:

“This is flow rate versus pressure. The more pressure you have in front of a fan, the slower it can pump out the air and this is the curve that determines that.

Fan Laws

Fan operating points on the board, determined by CFD simulations. (Advanced Thermal Solutions, Inc.)

“This little area here is sometime called the knee of the fan curve. Let’s say we’re in this area, the flow rate and pressure is relatively linear, so if I increase my pressure, if I put my hand in front of the fan, the flow rate goes down. If I have no pressure, I have my maximum flow rate. If I increase my pressure then the flow rate goes down. What happens in this part? The same thing. In the knee, a slight increase in pressure, so from .59 to .63, reduces the flow rate quite a bit.

Stratix 10 FPGA

CFD simulations showed that the fans were operating in the “knee” where it is hard to judge the impact of pressure changes on flow rate and vice versa. (Advanced Thermal Solutions, Inc.)

“So, for a 0.1 difference in flow rate (in cubic meters per second) it took 0.4 inches of water pressure difference, whereas here for a 0.1 difference in flow rate it only took a .04 increase in pressure. That’s why there’s a circle there. It’s a danger area because if you’re in that range it gets harder to predict what the flow will be because any pressure-change, any dust build-up, any change in estimated open area might change your flow rate.

Fan Laws

CFD analysis of flow vectors across high-powered components on a PCB. This simulation was part of an examination of fan performance in a system. (Advanced Thermal Solutions, Inc.)

CONCLUSION

While it is important to know the types of fans on the market and manufacturers provide data about the power and operating ranges of each product, it is important for there to be a basic understanding of the laws that govern how fans operate in a system and an ability to examine fan curve data in order to optimize performance.

“Bulk testing of electronics chassis provides the relationship between air flow and pressure drop and determines the fan performance needed to cool a given power load. The fan rating is often a misunderstood issue and published ratings can be somewhat misleading. Knowledge of fan performance curves, and how they are obtained, allows for a more informed decision when selecting a fan. Continued and ever shortening product design cycles demand a ‘get it right the first time’ approach. The upfront use of system curves, fan curves and fan laws can help meet this goal.”

Read more and see examples of fan laws and curves in practice at https://www.qats.com/cms/2013/07/24/how-to-use-fan-curves-and-laws-in-thermal-design.

CLICK HERE FOR PART I

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Utilizing Fans in Thermal Management of Electronics Systems

Fans in Thermal Management

There are different types of fans that are used in thermal management of electronics with tube axial fans being the most common. (Wikimedia Commons)


The ongoing trend in the electronics industry is for increasingly high-powered components to meet the ever-growing demands of consumers. Coupled with greater component-density in smaller packages, thermal management is more and more of a priority to ensure performance and reliability over the life of an electronics system.

As thermal needs have grown, engineers have sought out different cooling methods to supplement convection cooling. While options such as liquid cooling have grown in popularity in recent years, still one of the most common techniques is to add fans to a system.

Through the years, fan designs have improved. Fan blades have been streamlined to produce great flow rate with less noise and fans have become more power-efficient to meet the desires of customers trying to use less resources and save costs.

While much has changed in the presentation of fans, there are many basic concepts that engineers must consider when deciding how to implement fans in a project.

This is part one of a two-part series on how to select the best fan for a project. Part one will cover the types of fans that can be used. Part two, which can be found at https://www.qats.com/cms/2017/03/10/analysis-of-fan-curves-and-fan-laws-in-thermal-management-electronics, will cover fan laws and analyzing fan curves.

COMMON TYPES OF FANS AND BLOWERS

As described by Mike Turner of Comair Rotron in an article for Electronics Cooling Magazine, “All You Need to Know About Fans,” fans are essentially low pressure air pumps that take power from a motor to “output a volumetric flow of air at a given pressure.” He continued, “A propeller converts torque from the motor to increase static pressure across the fan rotor and to increase the kinetic energy of the air particles.”

In a white paper from Advanced Thermal Solutions, Inc. (ATS) entitled, “Performance Difference Between Fans and Blowers and Their Implementation,” it was added that fans are at their core, dynamic pumps. The article added, that in dynamic pumps “the fluid increases momentum while moving through open passages and then converts its high velocity to a pressure increase by exiting into a diffuser section.”

The biggest difference between a fan and a blower is the direction in which the air is delivered. Fans push air in a direction that is parallel to the fan blade axis, while blowers move air perpendicular to the blower axis. Turner noted that fans “can be designed to deliver a high flow rate, but tend to work against low pressure” and blowers move air at a “relatively low flow rate, but against high pressure.”

The three types of fans are centrifugal, propeller, tube axial, and vane axial:

• In centrifugal fans, the air flows into the housing and turns 90 degrees while accelerating due to centrifugal forces before being flowing out of the fan blades and exiting the housing.
• Propeller fans are the simplest form of a fan with only a motor and propellers and no housing.
• Tube axial fans, according to Turner, are similar to a propeller fan but “also has a venture around the propeller to reduce the vortices.”
• Vane axial fans have vanes trailing behind the propeller to straighten the swirling air as it is accelerated.

The most common fans used in electronics cooling are tube axial fans and there are a number of manufacturers creating options for engineers. A quick search of Digi-Key Electronics, offered options such as Sunon, Orion Fans, Sanyo Denki, NMB Technologies, Delta Electronics, Jameco Electronics, and several more.

Fans in Thermal Management

A fan is added to a heat sink on a PCB in order to increase the air flow and heat dissipation from the board component. (Advanced Thermal Solutions, Inc.)

FACTORS TO CONSIDER WHEN PICKING A FAN

When selecting a fan, engineers must consider the specific requirements of the system in which they are working, including factors such as the necessary airflow and the size restrictions of the board or the chassis. These basic factors will allow engineers to search through the many available options to find a fan that fits his or her needs.

In addition, engineers may look towards combining multiple fans in parallel or in a series to increase the flow rate across the components without increasing the size of the package or the diameter of the fan.

Parallel operation means having two or more fans side-by-side. When two fans are working in parallel, then the volume flow rate will be increased, even doubled when the fans are operating at maximum. Turner added. “The best results for parallel fans are achieved in systems with low resistance.”

In a series, the fans are stacked on top of each other and results in increased static pressure. Unlike parallel operations, fans in a series work best in a system with high resistance.

The ATS white paper noted, “In real situations, the fans may interfere with each other. The end results is a lower than expected performance.” Turner warns that in either parallel or series configurations there is a point in the combined performance curve that should be avoided because it creates unstable and unpredictable performance, but analyzing fan performance and fan curves will be covered in more detail in part two of the blog.

Efficiency is a major factor when selecting a fan. As noted in an article from Qpedia Thermal eMagazine, “A large data center contains about 400,000 servers and consumes 250 MW of power. It has been estimated that about 20% of the total power supplied to a high end server is consumed by fans.”

Clearly, finding a fan that can work efficiently with lower power will save a considerable about of resources. The article details several methods for creating efficiency in designing a system that includes fans:

“Fan power consumption is traditionally reduced by controlling the motor speed to produce only the airflow required for adequate cooling, rather than operating continuously at full speed. Significant energy savings can be achieved beyond this technique through fan efficiency increase. Optimizing the motor and electronic driver, increasing fan aerodynamic efficiency through careful redesign, and optimizing fan-system integration are three ways of achieving this.”

Read more about the techniques for achieving efficiency at https://www.qats.com/cms/wp-content/uploads/2015/03/Designing_Efficient_Fans_for_Electronics_Cooling
_Applications.pdf
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CLICK HERE FOR PART II.

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.