{"id":570,"date":"2013-06-07T20:30:13","date_gmt":"2013-06-08T01:30:13","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=570"},"modified":"2014-04-13T20:29:09","modified_gmt":"2014-04-14T01:29:09","slug":"everything-you-know-is-wrong-january-2004","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=570","title":{"rendered":"Everything You Know Is Wrong January 2004"},"content":{"rendered":"

Answers to those Doggone Thermal Design Questions<\/strong><\/p>\n

By Tony Kordyban<\/strong><\/p>\n

Copyright by Tony Kordyban 2004<\/p>\n

to:\u00a0 T. Kordyban <\/em>
\n re:\u00a0 Blue sky, how hot it is<\/em><\/p>\n

I am analyzing an electronics “hut”.\u00a0 The hut is a small building that protects our electronics from rain, snow, animals, vandals and such.\u00a0 It is not heated or air conditioned, and so the temperature inside is greatly affected by the outside weather conditions.<\/em><\/p>\n

I am using a thermal simulation program to calculate the air temperature inside the hut.\u00a0 I think I know what to put in when it asks for power dissipation, ambient temperature, wind speed and solar radiation.\u00a0 But it asks for something called “T sky” in the section about radiation.\u00a0 My guess is that this is the temperature of the sky.\u00a0 Supposing my guess is right, how do I find T sky?\u00a0 I don’t see it listed in the weather data for my site.<\/em><\/p>\n

Up In The Air from Mt. Airy<\/em><\/p>\n

\n

Dear Up,<\/p>\n

Your guess is right.\u00a0 Tsky<\/sub> does mean “sky temperature.”\u00a0 The software Help function probably tells you that much (and not much more.)\u00a0 But what is the temperature of the sky?\u00a0 Is it the average air temperature of the atmosphere?\u00a0 How would you find that average?\u00a0 After all, temperature varies tremendously as you go up in altitude.\u00a0 It varies from ground ambient (at the ground), to maybe -70 degrees C in the stratosphere.\u00a0 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?\u00a0 Should you discount temperatures that are farther away?\u00a0 And what about density?\u00a0 Should low density air count less in the average than normal density air?<\/p>\n

Sounds like a lot of work, and the truth is, you don’t need to do it.\u00a0 The temperature data is not available anyway, so even if you knew the right method, you couldn’t do it.<\/p>\n

Sounds like a lot of work, and the truth is, you don’t need to do it.\u00a0 The temperature data is not available anyway, so even if you knew the right method, you couldn’t do it.<\/p>\n

Here is the backwards definition of Tsky<\/sub>.\u00a0 Tsky<\/sub> is the “effective”\u00a0temperature of the sky that gives you the correct value of radiation exchange between your object and the atmosphere using this equation:<\/p>\n

Radiation = <\/b>s e<\/b> A (T object<\/sub> 4 <\/sup>– T sky<\/sub> 4<\/sup>)<\/b><\/p>\n

where<\/p>\n

s is the Stefan-Boltzman constant
\ne\u00a0 is the emissivity of the surface of your object (such as your hut)
\nA is the surface area of the object exposed to the sky<\/p>\n

\nTemperatures in this equation are in absolute units, such as Kelvin.<\/p>\n

If your hut were on the moon, where there is no atmosphere, you wouldn’t need the Tsky<\/sub> term in this equation.\u00a0 Your object would radiate outward, and there would be no atmosphere to radiate back.\u00a0 That is what the two terms in the equation are for.\u00a0 The Tobject<\/sub> term is for how much radiation the object emits, and the Tsky<\/sub> term is for how much radiation the object absorbs from the atmosphere.\u00a0 Yes, that’s right, the sky is radiating heat down on us all the time.<\/p>\n

Here is the problem:\u00a0 the sky is not made of just one material.\u00a0 It is a mixture of gases, and the mixture is always changing with the weather.\u00a0 Oxygen and nitrogen are pretty much transparent to infrared radiation.\u00a0 But carbon dioxide and water vapor are not.\u00a0 When the air is clear and dry, Tsky<\/sub> is different from when there is a lot of humidity and\/or a lot of cloud cover.<\/p>\n

Every book I looked at has a different way of estimating Tsky<\/sub>, and I have no idea which method is the best.\u00a0 For that reason I am not going to give you any of them.\u00a0 I know that when I put something in this column, you readers just grab it and run with it.\u00a0 “But Kordyban said it was right!” you whine on the witness stand.<\/p>\n

One source I found seemed to have a sensible position on Tsky<\/sub>.\u00a0 I will give it here because it is approximate, but good enough for engineering results:\u00a0 When there is cloud cover, Tsky<\/sub> is roughly the same as ambient temperature (expressed in absolute units.)\u00a0 If the weather is clear and dry, Tsky<\/sub> can be about 10 to 20 degrees K lower than ambient.\u00a0 One thing to note from this approximation is that Tsky<\/sub> is NOT absolute zero for Earth-bound conditions.\u00a0 At most it is 20 degrees K ( or C, remember that Kelvin and Celsius degrees are the same size) less than ambient.\u00a0 Even at the South Pole that might give a sky temperature of -70 C, which is still 200 degrees above absolute zero.<\/p>\n

This range of sky temperature should allow you to “bracket” your calculations.\u00a0 Run your simulations assuming Tsky<\/sub> equal to ambient on the first iteration, and Tsky<\/sub> 20 degrees lower on your second iteration.\u00a0 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.<\/p>\n

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.<\/p>\n

\n
\n<\/div>\n

O Advisor to the Heatstricken,<\/em><\/p>\n

Our outfit is too small to have our own thermal expert on staff.\u00a0 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.<\/em><\/p>\n

Like most electronics companies, we have occasional thermal stumpers.\u00a0 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.<\/em><\/p>\n

Sure, there are consultants in electronics cooling that could help us.\u00a0 Can thermal problems really be solved long distance, by working over the phone and through e-mail?\u00a0 There are no local thermal experts in our area — we would have to work with somebody hundreds of miles away, and the expense of\u00a0 bringing somebody here for days or weeks at a time would break our budget.\u00a0 Is it practical to do electronics cooling by “remote control?”<\/em><\/p>\n

Jack of All Trades<\/em><\/p>\n

 <\/p>\n

Dear Jack,<\/p>\n

The short answer is:\u00a0 “What choice do you have?”<\/p>\n

It is entirely possible to work with a remotely located thermal consultant and get reasonably good results.\u00a0 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.<\/p>\n

I have worked on many projects this way, and I think I have given my clients their money’s worth most of the time.\u00a0 The key to success in any “remote control” engineering is how good the communication is between you and the consultant.<\/p>\n

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.\u00a0 The trick is that the consultant will answer only the question you ask.\u00a0 What do you suppose happens if you don’t ask the right question?<\/p>\n

Suppose you send me a detailed set of drawings of you electronics chassis, and a specification for the fan you intend to use.\u00a0 You ask me to determine the air flow rate through the chassis.\u00a0 This problem is well-defined, and in a few hours I give you a pretty good estimate for the air flow.\u00a0 All this is easily and accurately done via e-mail.\u00a0 You are happy with the numbers, and the project goes along smoothly.\u00a0 Eventually I get a nice check in the mail and file away your project.<\/p>\n

So what’s wrong?\u00a0 Maybe the product doesn’t need a fan at all.\u00a0 Maybe it could be cooled by natural convection.\u00a0 Maybe the product could be smaller, simpler, cheaper and more reliable without a fan. But nobody thought to ask me that question.\u00a0 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.\u00a0 It wasn’t their job to question it.<\/p>\n

Because I am remote, I only see the problems you tell me about.\u00a0 I don’t eat lunch with the other team members.\u00a0 I can’t accidentally hear about the trade-offs they are making in the design.\u00a0 Because I am not integrated into the design process, you don’t get the full value of what I know.<\/p>\n

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:<\/p>\n

Electromagnetic Compatibility (EMC)<\/b>.\u00a0 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.\u00a0 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.<\/p>\n

Software<\/b>.\u00a0 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.\u00a0 What’s the harm in keeping the processor working all the time?\u00a0 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.\u00a0 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.<\/p>\n

Reliability<\/b>.\u00a0 Adding a fan to a system can conceivably increase its overall reliability by reducing the temperature of critical components.\u00a0 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.\u00a0 A reliability expert, working hand-in-hand with a thermal expert, can come up with the proper trade-off between temperature and component reliability.\u00a0 It might turn out that the hotter electronics is more reliable without the fan and its high failure rate.<\/p>\n

Industrial design<\/b>.\u00a0 Industrial designers are wonderful at coming up with great looking products that have good human interfaces.\u00a0 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.<\/p>\n

Safety<\/b>.\u00a0 Product safety engineers are always trying to eliminate all the cooling vents in the chassis, for perfectly valid reasons.\u00a0 If nobody is around to argue for air holes, they will close them up to maximize the chances of meeting any conceivable safety requirement.\u00a0 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.<\/p>\n

Mechanical packaging<\/b>.\u00a0 The thermal consultant designs a fan cooling system for your rack of electronics, and then moves on to another project.\u00a0 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.\u00a0 Nobody is around to object, so the plenum gets stuffed full of flow-obstructing cable bundles.\u00a0 Months later nobody can figure out why the cooling system doesn’t work nearly as well as the consultant’s test report claims.<\/p>\n

Do these examples sound familiar to you?\u00a0 Or do they sound like I am trying to overstate the importance of having a thermal engineer around full time?\u00a0 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.<\/p>\n

Here is my recommendation:\u00a0 Hire a shipping person to do your shipping and a soldering person to run your soldering machine.\u00a0 They probably cost less than your mechanical engineer’s time, and they sure as heck cost less than a thermal consultant.\u00a0 Then get your mechanical engineer trained in the fundamentals of electronics cooling, to be your on-site thermal expert.\u00a0 That expertise doesn’t need to be of Ph.D. quality.\u00a0 But you need someone on the team full time asking the right thermal questions.\u00a0 When you run into tough technical thermal problems, then call in a consultant.<\/p>\n

\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014<\/p>\n

Isn\u2019t Everything He Knows Wrong, Too?<\/strong><\/p>\n

The straight dope on Tony Kordyban<\/em><\/strong><\/p>\n

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.\u00a0 Maybe that doesn\u2019t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to\u00a0cool electronics.\u00a0 He does have some book-learnin\u2019, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit\u2014 no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn\u2019t Nobels count more than Rose Bowls?)<\/p>\n

\"\"In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney.\u00a0 He has run into so much nonsense in the field that he has found it easier to just assume \u201ceverything you know is wrong\u201d (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.<\/p>\n

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.\u00a0 These have been published recently by the ASME Press in a book called, \u201cHot Air Rises and Heat Sinks:\u00a0 Everything You Know About Cooling Electronics Is Wrong.\u201d\u00a0 It is available direct from ASME Press at 1-800-843-2763 or at their web site at\u00a0http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0\u00a0<\/em><\/strong>Order Number 800741.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2004 to:\u00a0 T. Kordyban re:\u00a0 Blue sky, how hot it is I am analyzing an electronics “hut”.\u00a0 The hut is a small building that protects our electronics from rain, snow, animals, vandals and such.\u00a0 It is not heated or air conditioned, […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-570","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/570","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=570"}],"version-history":[{"count":2,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/570\/revisions"}],"predecessor-version":[{"id":572,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/570\/revisions\/572"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=570"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}