Answers to those Doggone Thermal Design Questions
By Tony Kordyban
Copyright by Tony Kordyban 2004
to: T. Kordyban
re: Blue sky, how hot it is
I am analyzing an electronics “hut”. The hut is a small building that protects our electronics from rain, snow, animals, vandals and such. It is not heated or air conditioned, and so the temperature inside is greatly affected by the outside weather conditions.
I am using a thermal simulation program to calculate the air temperature inside the hut. I think I know what to put in when it asks for power dissipation, ambient temperature, wind speed and solar radiation. But it asks for something called “T sky” in the section about radiation. My guess is that this is the temperature of the sky. Supposing my guess is right, how do I find T sky? I don’t see it listed in the weather data for my site.
Up In The Air from Mt. Airy
Dear Up,
Your guess is right. Tsky does mean “sky temperature.” The software Help function probably tells you that much (and not much more.) But what is the temperature of the sky? Is it the average air temperature of the atmosphere? How would you find that average? After all, temperature varies tremendously as you go up in altitude. It varies from ground ambient (at the ground), to maybe -70 degrees C in the stratosphere. Assuming you had the data for the temperature at all altitudes for the particular time and location you care about, should you just take an arithmetic average? Should you discount temperatures that are farther away? And what about density? Should low density air count less in the average than normal density air?
Sounds like a lot of work, and the truth is, you don’t need to do it. The temperature data is not available anyway, so even if you knew the right method, you couldn’t do it.
Sounds like a lot of work, and the truth is, you don’t need to do it. The temperature data is not available anyway, so even if you knew the right method, you couldn’t do it.
Here is the backwards definition of Tsky. Tsky is the “effective” temperature of the sky that gives you the correct value of radiation exchange between your object and the atmosphere using this equation:
Radiation = s e A (T object 4 – T sky 4)
where
s is the Stefan-Boltzman constant
e is the emissivity of the surface of your object (such as your hut)
A is the surface area of the object exposed to the sky
Temperatures in this equation are in absolute units, such as Kelvin.
If your hut were on the moon, where there is no atmosphere, you wouldn’t need the Tsky term in this equation. Your object would radiate outward, and there would be no atmosphere to radiate back. That is what the two terms in the equation are for. The Tobject term is for how much radiation the object emits, and the Tsky term is for how much radiation the object absorbs from the atmosphere. Yes, that’s right, the sky is radiating heat down on us all the time.
Here is the problem: the sky is not made of just one material. It is a mixture of gases, and the mixture is always changing with the weather. Oxygen and nitrogen are pretty much transparent to infrared radiation. But carbon dioxide and water vapor are not. When the air is clear and dry, Tsky is different from when there is a lot of humidity and/or a lot of cloud cover.
Every book I looked at has a different way of estimating Tsky, and I have no idea which method is the best. For that reason I am not going to give you any of them. I know that when I put something in this column, you readers just grab it and run with it. “But Kordyban said it was right!” you whine on the witness stand.
One source I found seemed to have a sensible position on Tsky. I will give it here because it is approximate, but good enough for engineering results: When there is cloud cover, Tsky is roughly the same as ambient temperature (expressed in absolute units.) If the weather is clear and dry, Tsky can be about 10 to 20 degrees K lower than ambient. One thing to note from this approximation is that Tsky is NOT absolute zero for Earth-bound conditions. At most it is 20 degrees K ( or C, remember that Kelvin and Celsius degrees are the same size) less than ambient. Even at the South Pole that might give a sky temperature of -70 C, which is still 200 degrees above absolute zero.
This range of sky temperature should allow you to “bracket” your calculations. Run your simulations assuming Tsky equal to ambient on the first iteration, and Tsky 20 degrees lower on your second iteration. Considering that radiation to the sky is only one of your heat flow paths, this variation in sky temperature may not have a big affect on your internal temperature anyway.
Now that I have answered the question about the temperature of the sky, I am ready to think about what type of cheese the moon is made of.
O Advisor to the Heatstricken,
Our outfit is too small to have our own thermal expert on staff. The guy who does mechanical CAD also runs the solder line when we have a second shift, and is also pretty good with the bubble wrap down in the shipping area.
Like most electronics companies, we have occasional thermal stumpers. So far we have been able to fake our way through them with advice from heat sink and fan vendors, but I’m afraid that someday we are going to have a thermal problem that will require a real expert.
Sure, there are consultants in electronics cooling that could help us. Can thermal problems really be solved long distance, by working over the phone and through e-mail? There are no local thermal experts in our area — we would have to work with somebody hundreds of miles away, and the expense of bringing somebody here for days or weeks at a time would break our budget. Is it practical to do electronics cooling by “remote control?”
Jack of All Trades
Dear Jack,
The short answer is: “What choice do you have?”
It is entirely possible to work with a remotely located thermal consultant and get reasonably good results. Telephone, e-mail, mailing hard copy drawings and shipping sample hardware back and forth can usually give a consultant the information he or she needs to answer your thermal question.
I have worked on many projects this way, and I think I have given my clients their money’s worth most of the time. The key to success in any “remote control” engineering is how good the communication is between you and the consultant.
A consultant will give you a good answer to the question you pose, whether the distance between you is a mile or a hundred miles. The trick is that the consultant will answer only the question you ask. What do you suppose happens if you don’t ask the right question?
Suppose you send me a detailed set of drawings of you electronics chassis, and a specification for the fan you intend to use. You ask me to determine the air flow rate through the chassis. This problem is well-defined, and in a few hours I give you a pretty good estimate for the air flow. All this is easily and accurately done via e-mail. You are happy with the numbers, and the project goes along smoothly. Eventually I get a nice check in the mail and file away your project.
So what’s wrong? Maybe the product doesn’t need a fan at all. Maybe it could be cooled by natural convection. Maybe the product could be smaller, simpler, cheaper and more reliable without a fan. But nobody thought to ask me that question. Some non-expert in thermal design just assumed that a fan should be used way back at the beginning of the project, and nobody questioned it. It wasn’t their job to question it.
Because I am remote, I only see the problems you tell me about. I don’t eat lunch with the other team members. I can’t accidentally hear about the trade-offs they are making in the design. Because I am not integrated into the design process, you don’t get the full value of what I know.
Here are a few other examples of what can happen because the thermal expert isn’t fully involved with the rest of the development team:
Electromagnetic Compatibility (EMC). The engineer who is stuck making the product meet FCC and other electromagnetic emission requirements will play it safe and have you seal up the chassis so tight that no air can get in or out. By the time the thermal consultant is brought on board there is no chance to request any air vents for cooling, even though a sealed chassis may be overkill for EMC.
Software. The software engineer knows that the processor is used only intermittently by the customer’s application software, and decides that in those “idle” moments it would be good to run diagnostic routines, just in case. What’s the harm in keeping the processor working all the time? The harm could be that continually running unnecessary diagnostics might actually be the worst case thermal load on the processor, causing the cooling system to be bigger than the customer needs. With no thermal expert around to notice this happening, the cost of heat sink and fan for the processor might be higher than it has to be.
Reliability. Adding a fan to a system can conceivably increase its overall reliability by reducing the temperature of critical components. But if the system operation depends on a single fan, it now has the reliability of a fan, which is usually much worse than the electronics alone. A reliability expert, working hand-in-hand with a thermal expert, can come up with the proper trade-off between temperature and component reliability. It might turn out that the hotter electronics is more reliable without the fan and its high failure rate.
Industrial design. Industrial designers are wonderful at coming up with great looking products that have good human interfaces. But if the thermal expert is not around to gently remind them, they often come up with boxes intended for natural convection that are horizontally oriented, or they put in lots of inlet vents and forget to include and equal amount of exit vents.
Safety. Product safety engineers are always trying to eliminate all the cooling vents in the chassis, for perfectly valid reasons. If nobody is around to argue for air holes, they will close them up to maximize the chances of meeting any conceivable safety requirement. Safety engineers often need to measure component temperatures under various fault conditions, and once in a while they can benefit from the assistance of a thermal expert in getting those measurements right.
Mechanical packaging. The thermal consultant designs a fan cooling system for your rack of electronics, and then moves on to another project. Two months later the packaging engineer is looking for space to stow large bundles of fiber optic cables, and that big, empty plenum between the fans and the electronics looks like the perfect spot. Nobody is around to object, so the plenum gets stuffed full of flow-obstructing cable bundles. Months later nobody can figure out why the cooling system doesn’t work nearly as well as the consultant’s test report claims.
Do these examples sound familiar to you? Or do they sound like I am trying to overstate the importance of having a thermal engineer around full time? That is the question you need to answer for yourself when you are trying to decide if a thermal consultant can do a good job for you.
Here is my recommendation: Hire a shipping person to do your shipping and a soldering person to run your soldering machine. They probably cost less than your mechanical engineer’s time, and they sure as heck cost less than a thermal consultant. Then get your mechanical engineer trained in the fundamentals of electronics cooling, to be your on-site thermal expert. That expertise doesn’t need to be of Ph.D. quality. But you need someone on the team full time asking the right thermal questions. When you run into tough technical thermal problems, then call in a consultant.
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Isn’t Everything He Knows Wrong, Too?
The straight dope on Tony Kordyban
Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years. Maybe that doesn’t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to cool electronics. He does have some book-learnin’, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit— no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn’t Nobels count more than Rose Bowls?)
In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney. He has run into so much nonsense in the field that he has found it easier to just assume “everything you know is wrong” (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.
Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down. These have been published recently by the ASME Press in a book called, “Hot Air Rises and Heat Sinks: Everything You Know About Cooling Electronics Is Wrong.” It is available direct from ASME Press at 1-800-843-2763 or at their web site at http://www.asme.org/pubs/asmepress, Order Number 800741.