Tag Archives: thermal management

Technical Discussion of ATS Telecom PCB solution

Posted on October 28, 2016 by | 1 comment

Last year, Advanced Thermal Solutions, Inc. (ATS) was brought in to assist a customer with finding a thermal solution for a PCB that was included in a data center rack being used in the telecommunications industry. The engineers needed to keep in consideration that the board’s two power-dissipating components were on opposite ends and the airflow across the board could be from either side.

Telecom PCB

The PCB layout that ATS engineer Vineet Barot was asked to design a thermal solution for included two components on opposite ends and airflow that could be coming from either direction. (Advanced Thermal Solutions, Inc.)

The original solution had been to use heat sinks embedded with heat pipes, but the client was looking for a more cost-effective and a more reliable solution. The client approached ATS and Field Application Engineer Vineet Barot examined the problem to find the best answer. Using analytical and CFD modeling, he was able to determine that ATS’ patented maxiFLOW™ heat sinks would provide the solution.

Vineet sat down with Marketing Director John O’Day and Marketing Communications Specialist Josh Perry to discuss the challenges that he faced in this project and the importance of using analytical modeling to back up the results of the CFD (computational fluid dynamics).

JP: Thanks for sitting down with us Vineet. How was the project presented to you by the client?
VB: They had a board that was unique – where it would be inserted into a rack, but it could be inserted in either direction. So, we faced a unique challenge because airflow could be from either side of the board. There were two components on either side of the board, so if airflow was coming from one side then component ‘A’ would get hot and from the other side then component ‘B’ would get hot. The other thing was that the customer, who is a very smart thermal engineer, had already set up everything and he was planning on using these heat sinks that had heat pipes embedded in them. The goal was to try and come up with a heat sink that would do the same thing, hopefully without requiring the heat pipes.

JO: Can we talk for a second about the application? You mentioned that airflow was from either side, the board was going to be used in a data center or a telecom node?
VB: It was for a telecom company.

JP: Was there a reason he didn’t want to use a heat pipe?
VB: I think probably cost and reliability. We use heat pipes embedded in the heat sinks too, so it’s not a something we never want to use, but the client wanted to throw that at us and see if we had alternatives.

JP: Can you take us through the board and the challenges that you saw?
VB: As you can see from this slide, there are four main components and two of the hottest ones are on the edge. Airflow can be from right to left or left to right, so which one would be the worst-case scenario?

Telecom PCB

JO: From right to left, I think?
VB: Correct. This one is a straightforward one to figure out because not only is the component smaller but the power is also higher. Even though [air] can go both ways, there’s a worst-case scenario.

This was the customer’s idea – a straight-fin heat sink with a heat pipe and he put one block of heat pipe in there instead of two or three heat pipes that would normally be embedded in there. You can clearly see what the goal was. You have a small component in here, you want to put a large heat sink over the top and you want to spread the heat throughout the base of the heat sink. All the other components are also occupied by straight-fin heat sinks.

JO: Okay, at this point in the analysis, this is the rough estimate of the problem that you face?
VB: This is a straightforward project in terms of problem definition, which can be a big issue sometimes. This time problem definition was clear because the customer had defined the exact heat sink that they wanted to use. It’s not a bad heat sink they just wanted an improvement, cost-wise, reliability-wise.

This is the G600, which is the air going from left to right. The two main components are represented here and we want to make sure that the junction temperatures that the CFD calculated is lower than the maximum junction temperatures allowed, which they were. These heat sinks work. As we always like to do at ATS, we like to have two, independent solutions to verify any problem. That was the CFD result but we also did the analytical modeling to see what these heat sinks are capable of and the percent difference from CFD was less than 10 percent. Twenty percent is the goal typically. If it’s less than 20 percent then you know you’re in the ballpark.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JO: Do you use a spreadsheet to do these analytical modeling?
VB: HSM, which is our heat sink modeling tool, and then for determining what velocity you have through the fins, the correct way of doing this is to come up with the flow pattern on your own. You go through all the formulas in the book and determine what the flow will be everywhere or figure out what CFD is giving you for the fan curve and check it with analytical modeling. You can look at pressure drop in there, look at the fan curve and see if you’re in the ballpark. You can also check other things in CFD, for example flow balance. Input the flow data into HSM and it will spit out what the thermal performance is for any given heat sink. HSM calculations are based on its internal formulas.

JO: We effectively have a proprietary internal tool. We’ve made a conscious decision to use it.
VB: To actually use it is unique. Not everybody would use it. A lot of people would skip this step and go straight to CFD. We use CFD too but we want to make sure that it’s on the right path.

JP: What do you see as the benefit of doing both analytical and CFD modeling?
VB: CFD, because it’s so easy to use, can be a tool that will lead you astray if you don’t check it because it’s very easy to use and the software can’t tell you if your results are accurate. If you do any calculation, you use a calculator. The calculator is never going to give you a wrong answer but just because you’re using a calculator doesn’t mean that you’re doing the math right. You want to have a secondary answer to verify that what you did is correct.

JP: What was the solution that you came up with for this particular challenge?
VB: We replaced these heat sinks with the heat pipe with maxiFLOW™, no heat pipe needed. One of the little tricks that I used was to off-set the heat sinks a little bit so that these fins are out here and so the airflow here would be kind of unobstructed. And I set this one a little lower so it would have some fins over here, not much, that would be unobstructed. The G600 configurations worked out with the junction temperatures being below what the maximum requirement was without having to use any heat pipes for the main components. There is also a note showing that one of the ancillary components was also below the max. Analytical modeling of that was within 10-11 percent.

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

As you noted, this was the worst-case scenario, going from right to left and you can see because it’s the worst-case scenario this tiny little component here that’s 14 watts that’s having all this pre-heated air going into it, it’s junction temperature was exactly at the maximum allowed. That’s not entirely great. We want to build in a little bit of margin but it was below what was needed.

The conclusion here was that maxiFLOW™ was able to provide enough cooling without needing to use the heat pipes and analytical calculation agreed to less than 20 percent. We would need to explore some alternate designs and strategies if we want to reduce the junction temperature even further because that close to the maximum temperature is uncomfortable. The other idea that we had was to switch the remaining heat sinks, the ones in the middle, which are straight fin, also to maxiFLOW™ to reduce pressure drop and to get more flow through this final component.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JP: If you have an idea like that, is it something that you broach with the customer?
VB: They really liked the result. If this was a project where the customer said, ‘Yep, we need this,’ then we would have said here’s the initial result and we have an additional strategy. At that point the customer would have said, ‘Yeah this is making us uncomfortable and we need to explore further’ or they would have said, ‘You know what? Fourteen watts is a max and I don’t know if we’ll ever go to 14 watts or the ambient we’re saying is 50°C but we don’t know that it will ever get to 50°C so the fact that you’re at max junction temperature at the worst-case scenario is okay by us.’

JP: Do you always test for the worst-case scenario?
VB: It’s always at the worst-case scenario. It’s always at the max power and maximum ambient temperature.

JP: Was this the first option that we came up with, using maxiFLOW™? Were there other options that we explored?
VB: Pretty much. The way that I approached it was doing the analytical first. You can generate 50 results from analytical modeling in an hour whereas it takes a day and a half for every CFD model – or longer. These numbers here were arrived at with analytical modeling; the height, the width, the top width, were all from analytical modeling, base thickness to measure spreading resistance, all of that was done on HSM and spreadsheets to say this will work.

JP: Do you find that people outside ATS aren’t doing analytical?
VB: No one is doing it, which is really bad because it’s very useful. It gives you a quick idea if it’s acceptable, if this solution is feasible.

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

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Posted in Analysis, CFD, Consulting, heat sink, PCB, Thermal Analysis

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Q&A: ATS Thermal Engineer Sridevi Iyengar

Posted on October 21, 2016 by | 10 comments

Sridevi Iyengar

ATS thermal and field application engineer Sridevi Iyengar does CFD modeling (like the one shown above) and on-site consulting for ATS from her location near Bangalore, India. (Advanced Thermal Solutions, Inc.)

Advanced Thermal Solutions, Inc. field application and thermal engineer Sridevi Iyengar recently spoke with Marketing Communications Specialist Josh Perry about her career in engineering and the work that she does for ATS. Iyengar works near her home in Bangalore, India and provides ATS with CFD simulations and on-site support for customers in the region.

In this Q&A, Iyengar speaks about why she became an engineer in the first place, how she came to work at ATS, the type of projects that she works on, the challenges that she faces as a woman in a male-dominated industry, and what it is like working halfway around the world from the engineers at ATS’ Norwood, Mass. campus.

JP: How did you get interested in engineering? How did it all start for you?
SI: I was a good student in high school and in college and my father is a metallurgical engineer. He was a professor in one of the premier institutes in India, the Indian Institute of Science. When we were at the crossroad, during 12th grade, honestly the bright students either went into medicine or engineering and since my math skills were pretty good and I’d been to the Indian Institute of Science a couple of times I had written the entrance examinations for both streams. For engineering, I got into a very good school.

Although I didn’t know about the different disciplines of engineering, I happened to go into chemical engineering because that’s what my rank got me into. I liked it because chemical is kind of a fusion between math and physical phenomena and so that’s where my engineering journey started.

After my Bachelor’s, I wanted to do higher studies. I got married and came to the United States and I wanted to continue in my field of study. I didn’t want to move into software like pretty much everybody else from India when they move to the U.S. I wanted to keep myself different and I had a lot of support for that from my family. The first place I set up home is Norwood, Mass. (in 1993). I was preparing for my GRE and contemplating whether I should take my AGRE but I got positive responses from a couple of schools that I was also keen on getting into. I had options. One was the University of Massachusetts – Lowell, one was Rutgers University and the University of California – San Diego. I chose San Diego.

I was actually accepted into the doctoral program, however at UC-San Diego I liked the fluid mechanics and heat transfer program but then I didn’t want to jump into a Ph. D. without really having real world experience. I wanted to finish my Master’s, work for a few years and then maybe come back if I was interested. Much to my disappointment of my dad, I dropped out of the doctorate program with my Master’s and entered the job scene.

My entry into thermal engineering was kind of by chance. My first job was with Structural Dynamics Research Corporation (SDRC) in San Diego. It was the advanced test and analysis group. I had a background in heat transfer and fluid mechanics and therefore I joined as an intern and they made me do a little bit of this and that. The software associated with the IDEAS master series for electronics cooling was MAYA-ESC electro-systems cooling and TMG (thermal model generator) and we did a project for Cisco Systems in the Bay Area. I worked for about a year and half at ATA-SDRC. SDRC was doing a lot of projects for defense and their core area was becoming more and more defense and I was not a U.S. citizen so it was very difficult for them to assign me to projects because I didn’t have security clearance. At that time I jumped ship and I joined Cisco Systems as a mechanical engineer.

JP: How did you hear about Advanced Thermal Solutions, Inc.? How did you end up working here?
SI: ATS, the company, I knew even when I was at Cisco back in 1999. I was with Cisco until 2005 and at that time I knew about Advanced Thermal Solutions because as a mechanical engineer my job was also to source heat sinks. Also, that it was based in Norwood kind of struck a chord and it remained in my mind. I had known a lot about [ATS CEO, President and founder] Dr. Kaveh Azar because a close colleague of mine had worked closely with Kaveh. And of course Qpedia Thermal eMagazine was/is a very useful online journal.

How I joined ATS was a very, very chance meeting. We moved back to India in 2009 and I was working for an aluminum extrusion company in their thermal management division. It’s a Swedish company called Sapa. Sapa opened an office in India and it was just the sales manager and myself in the Indian team when I started. I worked with Sapa for three years and I was working for their global application team, half working for Sweden and half trying to set up the market in India. At Sapa I did a little bit more than thermal management. Sapa acquired an extrusion facility and also had a machining/anodizing unit. I was exposed to various aspects of manufacturing with regards to aluminium extrusions, fabrication etc., and worked on several other projects, which needed someone who could work with the customers and the manufacturing team at Sapa – sort of like a liaison and the engineering hand of the sales person.

When I quit Sapa, I thought I would go freelance doing electronics cooling consulting and I met one of the sales channel partners for ATS and with him I went and met Dr. Kaveh and Shashwat Shashwat (ATS Product Realization Manager), who were visiting India. This was in May of 2014 and initially it was just supposed to be a ‘hello, how are you’ meeting, but then we started talking and having common professional contacts and interests made it a very interesting interaction. We had lunch and when I came back home that evening Shashwat called me and asked if I was interested in working for ATS. I had no doubts whether I would take this opportunity; I took it with both hands. It’s worked out very well for me so far.

JP: What kinds of projects are you working on for ATS?
SI: There were two things for me, the mandate. One was that we wanted to beef up our presence in India. We already had a sales presence and we were selling heat sinks through Digi-Key and if the engineers know what they want then it’s not a big deal, but it helps them so much to know that there is technical staff from ATS present in India and in Bangalore in the southern region. They call and they say, ‘We’re looking at this heat sink, do you think it’s okay?’ Otherwise they send an email and then they wait for Norwood to reply. So, my role was to support the local sales partners that we have. They do the initial sales call and everything, but then if there’s anything technical they can say, ‘You know, ATS has a presence here? We have this engineer who is in electronic cooling and she has experience.’ I’ve gone to several meetings with them.

Secondly, for the U.S. customers, when it comes to CFD simulations like FloTherm then I work very closely with Norwood. In fact, I’ve done quite a few projects with [ATS field application engineers] Greg Wong or Peter [Konstalilakis], Vineet [Barot] too. A lot of times there are CFD simulations, they face the customers, they get the answers and I run the simulation and build the models here, do the analysis, we discuss the results and they send it to the customer.

JP: Is there a lot of collaboration between yourself and the engineers here in Norwood?
SI: Almost daily. I am online pretty much every day from 6 and on Wednesdays and Fridays we have the team meeting. On other days, I usually chat up with my counterpart on the project and, if it’s a major project, then the discussion is fairly involved. A lot of times, I’ll have a lot of questions so I’ll contact my teammates during my evening and he’ll take it up with the customer, get all the questions answered and by the time morning rolls around everything is sent to me by email and I get through my day. There is a lot of collaboration.

JP: Looking at thermal engineering as a whole, where do you see the industry going?
SI: People realize the importance of up-front thermal design and these folks who are dealing with high-powered components are aware of the importance of up-front thermal design. However there are still a lot of projects in which the hardware engineers are still not zoned into thinking of up-front thermal management, it’s coming in as kind of a ‘Oh it’s too hot, let’s do something about it’ approach. However, I think that mindset is changing a lot and I think the next-gen heat sinks like vapor chambers, heat pipes, and nano-materials will really start making their appearance more and more in thermal solutions because we’re getting to a point where the run of the mill is not cutting it.

JP: Do you see that change coming fairly quickly? In this industry, it seems like things change every day.
SI: The mindset should change because there’s always an aversion towards liquid and PCB. The more we educate people and the fact that we see everything in liquid cooling systems working…It takes some time for them to know that, okay it is a fairly fail-safe method. It will take at least a year or two and it should be running at that time and then people will catch on. It’s not something that can be easily brought on, I think, because generally we know that liquids and electronic components don’t mix. To assure them that it will not mix and there’s no chance of it coming into contact, I think that’s the stumbling block.

It’s market education and also having systems out there functioning, so that we can show them it’s not just theoretical. You have systems in practice and I think that makes a difference. If we can show it in theory, it doesn’t help as much because in theory everything looks wonderful, so we need to show them in practice and all the possible problems that can come up have been addressed and it is working in the field not just in the test lab.

JP: As a woman in a predominantly male-dominated industry, has it been difficult at all?
SI: In India, even back in 1993, we had a lot of engineers who were graduating but a lot of them didn’t stay back in what I call hardcore engineering. People used to go into information technology because they thought somehow it was more suitable for the women in the workforce situation. But I personally, I’ve had a fulfilling time and it is good to distinguish yourself and be different. The work that we do at ATS is hardcore engineering and we have engineers to lead us. We have Dr. Kaveh Azar and Dr. Bahman Tavassoli who have years of engineering experience and yeah sometimes they come down hard on us but that’s because they know what they’re doing. They’ve been there, done that, and they want to extract the best out of you and they want you to think like an engineer always. That’s what is unique of working at ATS.

JP: Do you hope to inspire other women to not only join the field, but stick with the ‘hardcore’ engineering?
SI: Yeah, absolutely. There have been young women who have reached out to me, young engineers who graduated in India, and I tell them have patience and learn the skills needed to get a job. It’s very easy to learn a few programming languages and jump into IT, especially in India right now, but you’re going to be just like anybody else. If your heart really lies in engineering, you should stick on, network, upgrade your skills and you’ll definitely find a job. The first job is everything you need and after that, if you do well there, then the path is smooth.

JP: How has it been for you as a ‘distant worker’ in terms of not being located here in Norwood? We have a lot of great technology like Skype and GoToMeeting, how have you found it being a ‘distant worker’?
SI: Since I interact with the engineers on an almost daily basis it is not that different. ATS engineers and the customers are very understanding of the time difference and accommodate the meetings, if any, so that it is not totally at unearthly hours for me. I also have the freedom to have my own schedule and that is very helpful since I am a working mother. I’ve been to ATS once and so I have met most of the team there.

The only thing is that I don’t have that touch and feel. Sometimes the ATS engineers have the heat sinks/components on their desk and they’re looking at it. A lot of times they will look at it, turn it around and these are things that I will have to accomplish through video call on Skype or the engineers take pictures and send them to me. But it’s not the same. That’s the only drawback. And of course when you folks have your team lunches/picnics … I feel left out.

JP: From our conversation, it sounds like you really like challenging projects?
SI: I think we all like to be challenged once in a while. With involved models, one of the challenges was I’d have to remotely log in and run the model in the 12-core PC and ensure nobody is logged in and I used to run it through the night and post-process it via remote connection. I’d transfer the results over and make the PowerPoint. However I was given a super fast simulation computer locally so all I need is a VPN connection. Even if the VPN connection goes down, FloTherm will not cut off the simulation and it runs through the solve.

Every now and then I support local customers with their heat sink selection requests. Some local customers have asked for training sessions as well, which is something I would like to start fairly soon.

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

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Posted in ATS News, CFD, Engineering, Modeling, Simulation, thermal engineering

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ATS welcomes engineering students from Tufts

Posted on October 19, 2016 by | 1 comment

Tufts University

Dr. Bahman Tavassoli of Advanced Thermal Solutions, Inc. gives a demonstration of a wind tunnel to Dr. Marc Hodes (left) and a group of students from Tufts University. (Advanced Thermal Solutions, Inc.)


On Friday, Oct. 14, Advanced Thermal Solutions, Inc. (ATS) welcomed Dr. Marc Hodes and a group of six mechanical engineering students from Tufts University to its Norwood, Mass. campus. The students learned about the company, its products, and took a tour of two of ATS’ four laboratories to see some of the testing equipment utilized by ATS engineers.

After a welcome from ATS founder, President and CEO Dr. Kaveh Azar, the students enjoyed a brief introduction from Marketing Director John O’Day about the company, its products, and the importance of thermal management in the design of today’s high-powered electronics.

The lab tours were led by Dr. Bahman Tavassoli, ATS Chief Technologist. First, he took the students into the Characterization Lab to demonstrate the BWT-104 open-loop wind tunnel and the CLWT-067 closed-loop wind tunnel. The students learned how ATS engineers use Candlestick sensors, thermocouples and the iQ-200 to measure air velocity, temperature, and pressure across a PCB using one system. There was also a thermVIEW Liquid Crystal Thermography unit set up, in which ATS engineers use infrared (IR) cameras to examine hot spots on a cold plate.

Tufts University

Students take a closer look at ATS testing equipment. (Advanced Thermal Solutions, Inc.)

Dr. Bahman Tavassoli

Dr. Tavassoli answers questions from Tufts University students. (Advanced Thermal Solutions, Inc.)

The Tufts students learned more than simply how the testing processes worked. They also learned why thermal management is an important consideration in the early stages of a design. Dr. Tavassoli and Dr. Hodes spoke of their professional experiences in the field of thermal engineering and where projects had gone wrong when thermal issues were not considered in the planning stages.

Dr. Azar also joined the students in the lab to show them the wicking material being used by ATS engineers in state-of-the-art vapor chamber designs.

Tufts University

ATS CEO, President and founder Dr. Kaveh Azar speaks with the student from Tufts in the Characterization Lab. (Advanced Thermal Solutions, Inc.)

After the Characterization Lab, the students were taken into the Electronics Lab and were given a demonstration of the Water Flow Visualization equipment. ATS engineers use the equipment to test how air will flow through a system.

The students asked numerous questions of Dr. Tavassoli to get a better idea of the important concepts of thermal engineering that were presented in the 90-minute visit to ATS. Now, the students will have the real-world applications that they saw at ATS in mind when learning the concepts of thermodynamics, thermal fluids, and more in their Tufts courses.

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

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Posted in ATS News, Consulting, Engineering Student Visit, thermal engineering

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Industry Developments: Cooling Nuclear Power Plants

Posted on October 7, 2016 by | 1 comment

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.)

Most man-made electricity in the U.S. is provided by thermoelectric power plants. In these large scale installations, water is boiled to steam to spin the plant’s turbines and to ultimately generate electricity. To provide the heat necessary to produce this steam, a power plant could burn coal, natural gas or oil. But, in fact, most plants don’t burn anything. Instead, they use a very hot, but carefully controlled core of nuclear material to provide the thermal energy for continuous steam.

Most large power plants use pressurized water reactors (PWRs) with nuclear fuel as their power source. There are different cooling requirements inside these plants and they are typically achieved with primary, secondary and tertiary thermal solutions. First, heat must be managed inside their reactor vessels where the radioactive material is housed. Then, in the steam generators, hot water from the reactor vessels is cooled by transferring its heat to a separated water source, converting it to steam. Lastly, after the steam moves past the turbines, it is condensed back to liquid water, which then returns to the steam generator. An illustration of a nuclear power plant with a pressurized water reactor is shown in Figure 1. [1]

Nuclear Power Plant

Figure 1. Components of a Pressurized Water Reactor in a Nuclear Power Plant. [2]

Inside a PWR’s reactor core, the primary coolant, usually ordinary water, is heated by energy from atomic fission. Under high pressure to keep it from boiling, the heated water flows along a primary, closed-loop piping system into a steam generator. Here, the heat from the primary loop transfers into an isolated, lower-pressure secondary loop also containing water.

The water in the secondary loop enters the steam generator at a pressure and temperature slightly below that required to initiate boiling. Upon absorbing heat from the primary loop, it becomes saturated and slightly super-heated. The water changes phase to steam, which serves as the working fluid to push the turbine blades and generate electricity.

Finally, the steam is condensed back to water and re-enters the secondary loop. There are different ways to provide this tertiary level of cooling to cause this condensation. [3]

Fueling a Nuclear Reactor

A nuclear power plant’s reactor is most often fueled by U-235, a type of uranium that fissions easily. U-235 is a component of uranium hexafluoride fuel, which is made from mined or milled uranium oxide, called yellowcake. To make the uranium hexafluoride usable as a fuel, it is enriched to increase its U-235 content from 1 percent up to 3-5 percent. This is a low concentration and the enriched uranium is stable over a wide range of environmental conditions.

After the uranium hexafluoride is enriched, a fuel fabricator converts it into uranium dioxide powder and presses the powder into solid fuel pellets. The fabricator loads the ceramic pellets into long, pencil-thin rods made of a noncorrosive material, usually a zirconium alloy. These tubes, each about 4 meters long, are grouped by the hundreds into bundles that are called fuel assemblies. [4]

Figure 2. A Pressurized Water Reactor Includes Inlets and Outlets for Passing Water Coolant. [5]

Figure 2. A Pressurized Water Reactor Includes Inlets and Outlets for Passing Water Coolant. [5]

A single fuel rod assembly for a pressurized water reactor (PWR) is approximately 13 feet high and weighs about 1,450 pounds. [6]

Step 1. Cooling the Nuclear Core

During a nuclear fission chain reaction, fuel rods heat up to about 800°C. If they are left uncovered by water, they’ll reach temperatures well about 1,000°C and begin to oxidize. That oxidation will react with any water that remains in the vicinity, producing highly explosive hydrogen gas. So, fuel rods are kept submerged in demineralized water, which serves as the primary coolant. The water is kept in a pressurized containment vessel and reaches about 325°C. [7]

At the atomic level, continuous exothermic fission in the fuel rods releases heat into the water in the PWC’s reactor. Nuclear power plants manage this fission and its resulting heat with the use of control rods. The rate of fission can be controlled–even stopped–by inserting and removing the control rods in the reactor. The control rods are made with neutron-absorbing material such as cadmium, hafnium or boron. Their presence controls the rate of nuclear reaction by absorbing neutrons, which otherwise would contribute to the fission chain reaction.

Figure 3. Control Rods Manage the Fission Rate Inside Nuclear Reactor Cores. [8]

Figure 3. Control Rods Manage the Fission Rate Inside Nuclear Reactor Cores. [8]

A single uranium fuel pellet the size of a fingertip contains as much energy as 17,000 cubic feet of natural gas or 1,780 pounds of coal. This relatively clean energy property, along with its vast half-life (700 million years), makes U-235 a viable alternative to burning fossil fuels to turn power plant turbines. [6]

Control Rod Drive Mechanisms (CRDMs) lower, raise, and keep in position assemblies of control rods inside a nuclear reactor. The rods absorb free neutrons, limiting the number available to cause fission of nuclear fuel. [8]

Step 2: Heat Transfer in Steam Generators

In a PWC-style nuclear power plant, the primary coolant, carrying heat from the reactor core, flows through a looped system into and out of a steam generator. Inside the generator it transfers its heat to an isolated, secondary coolant, water, converting it to steam. This steam travels in a secondary loop to the turbines. The transfer of heat from the primary loop to the secondary loop is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive.

Figure 4. Illustration of a Steam Generator. [9]

Figure 4. Illustration of a Steam Generator. [9]

There are multiple generators in a nuclear power plant. Each can measure up to 70 feet in height and weigh as much as 800 tons. A generator has more than 10,000 tubes, adding up to hundreds of miles in total length. A steam generator’s tubes are in a U-shape formation and each tube is about 19mm in diameter. Coolant from the reactor enters the generator’s inlet nozzle and circulates through the U-tubes.

The secondary coolant flows upward by natural convection through the bundle absorbing heat from the tubes of primary coolant. As heat is transferred through the tube walls, the secondary coolant, water, is turned into steam that flows from the top of the generator.

The materials that make up the steam generators and tubes are specially made and specifically designed to withstand heat, thermal expansion, high pressure, corrosion and radiation. The tubes are an important barrier between the radioactive and non-radioactive sides of the plant. For this reason, the integrity of the tubing is essential in minimizing the leakage of water between the two sides. [9]

Figure 5. Steam Generator Tubes Transfer Heat from the Primary to the Secondary Loop. [8]

Figure 5. Steam Generator Tubes Transfer Heat from the Primary to the Secondary Loop. [8]

Step Three: Condensing the Steam

Once the steam has passed through a turbine, it must be cooled back into water by a third process and returned to the steam generator to be heated once.

Figure 6. Simple Illustration of Recirculation Scheme for Power Plant Steam. [10]

Figure 6. Simple Illustration of Recirculation Scheme for Power Plant Steam. [10]

There are three main methods of cooling a power plant’s steam and residual hot water:

Once-through systems take water from nearby sources (rivers, lakes, oceans), circulate it through condensers, and discharge the now warmer water to the local source. Once-through systems were initially popular because of their simplicity, low cost, and the abundant supplies of cooling water. But these systems can cause disruptions to local ecosystems, mainly from the large water withdrawals.

Wet recirculating systems reuse cooling water in a second cycle rather than immediately discharging it back to the original water source. Typically, wet recirculating systems use cooling towers to expose water to ambient air. Some water evaporates, but the rest is sent back to the condenser in the power plant. Because wet-recirculating systems only withdraw water to replace what’s lost through evaporation, these systems have much lower water withdrawals than once-through systems. Before being fed into the steam generator, the condensed steam (referred to as feed water) is sometimes preheated in order to minimize thermal shock.

More recently, plants have started using a third type of steam cooling system called dry cooling. Instead of using water to lower cooling water temperature, these systems use air passed over the cooling water by one or more large fans. Running those fans can require a significant amount of electricity, which makes this system less suited for large plants that require a lot of steam such as those powered by coal or nuclear energy. [11]

Three Integrated Cooling Systems

The illustration below is a simplified look at the main cooling loops in the Davis-Besse nuclear power station in Ohio. It features a pressurized water reactor in which uranium fuel is in long metal fuel rods (1) leading down to the reactor core (2). The reactor core is inside the reactor vessel (3) which is filled with purified water. Control rods (4) on top of the reactor start and stop the chain reaction that produces heat. When the rods are withdrawn, the nuclear chain reaction occurs, producing heat.

Figure 7. The Three Main Cooling Loops in a Nuclear Power Plant. [12]

Figure 7. The Three Main Cooling Loops in a Nuclear Power Plant. [12]

The water inside the Davis-Bessie PWR is under pressure so it won’t boil as its temperature rises by passing through the nuclear core. The water then travels along tubes through the steam generator (5) and back to the reactor. This constitutes the primary loop (green). After it has passed through the steam generator, the water has cooled down. The average temperature in this cycle is maintained at 582°F.

When the primary coolant water passes through the steam generator, its heat is transferred to the secondary loop (blue). Heat is transferred without the water in the primary loop and secondary loop ever coming in contact with each other. The water in the secondary loop boils to steam in the steam generator. This steam flows to the turbine generator (6). It is here that the steam’s energy is made into electricity.

When the steam leaves the turbine, it comes in contact with pipes carrying cooling water. As the steam cools, it changes back into water. The third loop (yellow) contains the water that is cooled by the large cooling tower (7). [12]

Among all of the power plants in the US, just over half reuse their cooling water. The rest are either dry systems or hybrid systems which can switch between dry and some sort of wet cooling depending on the temperature and availability of water.

References:
[1] Bright Hub Engineering, http://www.brighthubengineering.com/power-plants/2722-components-of-nuclear-power-plant-coolant/
[2, 3] http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/nuclear-power-reactors.aspx
[4] https://en.wikipedia.org/wiki/Nuclear_reactor_core
[5] https://www.britannica.com/technology/nuclear-reactor/Coolant-system
[6] Nuclear Energy Institute, http://www.nei.org/Knowledge-Center/Nuclear-Fuel-Processes
[7] http://energy.gov/sites/prod/files/2014/01/f7/csp_review_meeting_042313_martin.pdf
[8] Vallourec, http://www.vallourec.com/NUCLEARPOWER/EN/products/nuclear-island/Pages/crdm.aspx
[9] http://cdn.intechopen.com/pdfs-wm/14150.pdf
[10] Union of Concerned Scientists, http://www.ucsusa.org/clean_energy/our-energy-choices/energy-and-water-use/water-energy-electricity-cooling-power-plant.html#.V8i9Cs9ATct
[11] https://en.wikipedia.org/wiki/Pressurized_water_reactor
[12] http://www.co.ottawa.oh.us/ottawacoema/davisbesse.html

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Case Study: Thermal Management in Harvard Medical School Tissue Analysis Instrumentation

Posted on October 4, 2016 by | Leave a comment

Designers of today’s highly advanced medical diagnostic equipment must overcome many of the same thermal challenges common to telecommunications, industrial and information technology electronics.

In addition, medical diagnostic devices present unique design issues and boundary conditions that factor into thermal solutions. These include isothermal and cyclic temperature demands, precise test repeatability, and maintaining the patient’s safety and comfort.

These kinds of issues were presented by Harvard Medical School to the experts at Advanced Thermal Solutions, Inc. (ATS) when it needed a cooling solution for the diagnostic equipment it was relying on for the analysis and observation of human tissue samples in a controlled laboratory setting. This was the school’s Frozen Tissue Microarrayer System.

ATS engineers had to provide thermal solutions to meet a range of design goals:

• Provide long-term temperature control for tissue samples embedded in an optimum cutting temperature fluid.
• Create a cooling system to maintain tissue samples below -70°C for six hours.
• Ensure operator visibility of the samples.
• Eliminate humidity and frost within the system to prevent sample contamination.

ATS Cooling Solutions

ATS engineers developed highly effective thermal solutions to meet all the design requirements of the diagnostic equipment. A reservoir in the device holds the liquid cooling medium and tissue samples are loaded through an opening at the top. Through a duct, cool air is circulated over the top of the samples to maintain temperature and humidity requirements.

As seen in Fig. 1 (below), the diagnostic system consisted of:

• Frozen tissue coring machine (on the right in the photo)
• Tissue sample loading area at the top of the cooling system (seen on the left)
• Duct system (on both sides of system) to circulate cool air
• Ice/alcohol reservoir at the system’s bottom to contain the cooling medium

Harvard Case Study

Figure 1. Prototype system created by ATS engineers for Harvard Medical School laboratory. (Advanced Thermal Solutions, Inc.)

Conduction Cooling Design

In operation, tissue samples are loaded into removable aluminum cassettes that fit tightly into a metal receiver (top left, Figure 2). The receiver contacts the cassette on five sides which allows for cooling of the samples by conduction. The receiver is lowered into a reservoir containing a slurry of dry ice and ethyl alcohol. Here the receiver is maintained at a constant temperature until the dry ice evaporates. The reservoir is double-walled and insulated to extend the evaporation time of the dry ice.

The receiver also features integral fins that increase surface area for drawing heat downward from the base of the cassettes into the icy slurry (bottom left, Figure 2). These fins are based on the same ATS heat sink design principles used in the company’s high performance maxiFLOW™ heat sinks.

Using analytical modeling, ATS engineers determined that 10 fins were the optimal number for cooling the cassette receiver and its contents. CFD simulations also showed that the 10-fin concept resulted in an optimal design. The engineers validated their analytical and CFD results through empirical testing. It was determined that extending 10 fins into the slurry provided the cooling performance to maintain tissue sample temperatures below the -70°C threshold for 9.75 hours.

Further temperature testing using thermocouples showed only a 2.5°C difference between the coldest points at the bottom of the fins and the tissue samples in the cassette. This proved that the design overcame thermal conduction resistance and could effectively maintain the samples below their critical temperature.

Figure 2. Temperature testing with thermocouples demonstrated that the temperature difference between the bottom of the fins and the top of the cassette, through three intervening layers, was only 2.5°C. This proved that the thermal design was successful. (ATS)

Figure 2. Temperature testing with thermocouples demonstrated that the temperature difference between the bottom of the fins and the top of the cassette, through three intervening layers, was only 2.5°C. This proved that the thermal design was successful. (ATS)

Figure 3. Using a heat sink-specific thermal resistance network ATS determined that the optimal number of fins was 10. (ATS)

Figure 3. Using a heat sink-specific thermal resistance network ATS determined that the optimal number of fins was 10. (ATS)

Convection Cooling Design

The above conduction cooling design provided only part of the solution. There were additional needs to maintain the temperature at the top of the samples and to decrease the relative humidity of the cool air from the ambient air in the lab. ATS engineers designed a convection cooling system to fulfill these requirements.

A heat exchanger was installed with its fins in the dry ice/alcohol slurry and its other side extending into a duct to cool the air passing over it. This approach uses the same cooling medium for both convection and conduction to ensure there is no temperature differential throughout the sample and that the sample is as isothermal as possible.

Air is pushed by a counter-rotating fan through the duct and into the heat exchanger. The heat exchanger forms a thermal link between this air and the slurry mixture. The heat exchanger was designed with an optimum balance between its surface area and the resulting pressure drop to ensure the fan was operating with the most effectiveness.

Once the air passes the heat exchanger, it moves through the ducts and into a diffuser at the top of the system. The diffuser disperses the air over the sample creating a barrier between the tissue and the ambient environment of the lab so outside moisture and heat are not transferred in.

The ATS engineers tested this design using an array of thermocouples and ATS hotwire anemometer Candlestick Sensors connected to an ATS ATVS-2020, a temperature and air velocity scanner. They determined there was too much mixing between the air flowing over the samples and ambient air. The diffuser was redesigned with a new connection to the duct and an optimized outlet radius (see Figure 4).

In the ducts, a molecular sieve desiccant housed in a honeycomb structure was used to reduce the dew point of the air to -84.4°C, which was well below the -72°C air temperature in the duct.

Figure 4. Initial testing led to a redesign of the air diffuser to prevent ambient humidity from mixing with the air over the tissue samples. (ATS)

Figure 4. Initial testing led to a redesign of the air diffuser to prevent ambient humidity from mixing with the air over the tissue samples. (ATS)

Conclusions

ATS engineers performed a final series of tests of the Frozen Tissue Microarrayer System using Candlestick Sensors, thermocouples and the ATVS 2020 scanner. The tissue temperature stayed constant over the required six-hour period and well below the -70°C threshold. In fact, testing determined that the tissue temperature remained below the threshold for nearly eight hours before warming above a usable temperature (Figure 5). The multi-part cooling system was a success, meeting the original design objectives provided by Harvard Medical School.

Figure 5. Final testing showed that the ATS cooling design kept tissue temperature (shown in blue in the graph above) below the -70°C threshold for more than the required six hours. (ATS)

Figure 5. Final testing showed that the ATS cooling design kept tissue temperature (shown in blue in the graph above) below the -70°C threshold for more than the required six hours. (ATS)

The process of designing cooling solutions for the Frozen Tissue Microarrayer demonstrated that thermal design practices used throughout electronics cooling can be applied in the medical device industry. Fin efficiency, thermal resistance, and pressure drop calculations are standard regardless of the application. Thermal solutions should be considered early in the design process so they can be incorporated into the overall system as efficiently as possible.

The experts at ATS used traditional thermal calculations, CFD simulations, empirical testing, and its leading-edge heat sink technology to successfully design the thermal solution for this medical equipment application. The ATS design allowed Harvard Medical School to test tissue samples while meeting its strict requirements.

To learn more about the design, watch the video below:

Download a PDF of this case study at http://www.qats.com/cms/wp-content/uploads/Harvard-Medical-case-study.pdf.

Visit www.qats.com, call 781-769-2800 or email ats-hq@qats.com to learn more about ATS and its Thermal Management Analysis and Design Services.

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