{"id":466,"date":"2012-11-23T19:19:34","date_gmt":"2012-11-24T01:19:34","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=466"},"modified":"2014-04-13T20:26:46","modified_gmt":"2014-04-14T01:26:46","slug":"everything-you-know-is-wrong-march-2003","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=466","title":{"rendered":"Everything You Know Is Wrong March 2003"},"content":{"rendered":"

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

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

Copyright by Tony Kordyban 2003<\/p>\n

Hey Thermal Question Dude,<\/em><\/p>\n

Somebody asked a while back about finishes for heat sinks.\u00a0 Your answer about black anodizing dealt only with how it changes the radiation properties of the surface.\u00a0 But what about the convection?\u00a0 When you paint a heat sink black, or anodize the surface, aren’t you in effect covering up the surface with a blanket?\u00a0 I would think the paint or anodized metal are thermal insulators.\u00a0 So you might improve the radiation by making the surface black, but don’t you mess up the convection?<\/em><\/p>\n

Overheated in Bakersville<\/em><\/p>\n

 <\/p>\n

Dear Over,<\/p>\n

I hate to be picky — well, that’s not true.\u00a0 If I did not love to be picky, I wouldn’t be in the thermal article writing business.\u00a0 By adding a layer of insulation to the surface of my heat sink, I would not be messing up the convection heat transfer at all.\u00a0 I would be messing up the CONDUCTION heat transfer within the body of the heat sink.<\/p>\n

A quick review of the basic equation for convection will illustrate my picky point.<\/p>\n

Q convection<\/sub> = h A (T air<\/sub> – T surface<\/sub>)<\/strong><\/p>\n

where<\/p>\n\n\n\n\n\n\n\n
Q convection<\/sub><\/td>\nis the heat carried away by convection<\/td>\n<\/tr>\n
h<\/td>\nis the convective heat transfer coefficient, which depends on the air velocity and the temperature gradient in the thermal boundary layer<\/td>\n<\/tr>\n
A<\/td>\nis the surface area in contact with the air<\/td>\n<\/tr>\n
T air<\/sub><\/td>\nis the reference air temperature<\/td>\n<\/tr>\n
T surface<\/sub><\/td>\nis the surface temperature<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\"\"<\/a>

Anodizing a heat sink does create a layer on the surface that could act as a thermal insulator. But when you want something to be an insulator, doesn't it always need to be pretty thick?<\/p><\/div>\n

No place in the equation does it ask for the conductivity of the solid.\u00a0 The solid could be made completely out of Jello or anodized aluminum, and the convective heat transfer behavior would be exactly the same.<\/p>\n

Well, OK, not exactly the same.\u00a0 Your gut tells you that an aluminum heat sink has to work better than one made entirely of Jello, so the conductivity has to matter somehow.\u00a0 It does matter, indirectly, by changing the surface temperature term of the equation.\u00a0 If the heat sink were made of polystyrene instead of aluminum, then T surface<\/sub> would be different.<\/p>\n

Can we estimate how much an insulating blanket would change the temperature of a heat sink? Easy.\u00a0 It depends on the heat flux (the power per unit area) conducting through the heat sink, the conductivity of the blanket, and the thickness of the blanket.\u00a0 Then we just use the basic equation for 1-dimensional heat conduction, which you should know by heart now, but I will provide anyway.<\/p>\n

Q conduction<\/sub> = (k A )\/t\u00a0 (T aluminum<\/sub> – T surface<\/sub>)<\/strong><\/p>\n

where<\/p>\n\n\n\n\n\n\n
Q conduction<\/sub><\/td>\nis the heat conducted through the thickness of the anodized layer<\/td>\n<\/tr>\n
k<\/td>\nis the thermal conductivity of the anodized layer<\/td>\n<\/tr>\n
A<\/td>\nis the cross-sectional area of the heat sink<\/td>\n<\/tr>\n
t<\/td>\nis the thickness of the anodized layer<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The heat flux is just Q conduction<\/sub> \/ A, so if we rearrange the equation a little bit, we can see that<\/p>\n

(T aluminum<\/sub> – T surface<\/sub>) =\u00a0 heat flux x (t \/ k)<\/strong><\/p>\n

I have not been able to discover the thermal conductivity of the anodized layer.\u00a0 (It might even be a pretty good conductor for all I know!)\u00a0 It is an oxide of aluminum that is somewhat porous, with the pores sometimes filled with organic material, depending on the anodizing process.\u00a0 Let’s be very pessimistic and assume that it has conductivity similar to an insulator like plastic, say k = 0.1 W\/mK.<\/p>\n

One thing I was able to find out by surfing the web,\u00a0 was that the layer of anodized material is very thin, about 2 x 10 -5<\/sup> meter (about 0.0007 inches).\u00a0 You can specify it to be thicker, but it won’t make your radiation properties any better, so why would you?<\/p>\n

The only other number we need is the heat flux.\u00a0 Of course, that depends entirely on your application.\u00a0 In my fairly ordinary heat sinks for digital electronics I encounter\u00a0 fluxes in the range of\u00a0 100 to 1,000 W\/m2<\/sup>.\u00a0 You can figure out yours by dividing the power dissipation of your component by the surface area of your heat sink.<\/p>\n

Let’s take my heat sink dissipating a heat flux of 1,000 W\/m2<\/sup>.\u00a0 Given an anodized layer thickness of 0.00002 m and conductivity of 0.1 W\/mK, I get a temperature difference across the anodized layer of 0.2 degrees C.\u00a0 I consider that negligible.<\/p>\n

But don’t take away from this that the thermal insulating effect of anodizing is always negligible.\u00a0 For me, and for most electronic heat sink applications, it probably is.\u00a0 But not always.\u00a0 If your heat sink has a thicker coating,\u00a0 of paint, for example, or a very high heat flux, such as from a motor control power supply, then maybe the temperature difference would be important.\u00a0 This simple analysis is a tool you can use to decide if it will be important.\u00a0 I live by that old thermal aphorism, “Give an engineer an answer, and you feed his\/her curiosity for a day.\u00a0 Give an engineer a tool for generating answers, and he\/she stops nagging you endlessly.”<\/p>\n

If you find out you DO have to worry about the anodizing, then you’d really better start to worry about all the dirt that is going to collect on that heat sink over years of operation.\u00a0 That blanket of grease and dust and cat hair could easily get to be a lot thicker than the anodized layer.<\/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

To Mr. Kordyban,<\/em><\/p>\n

What about all these thermal conferences and symposiums?\u00a0 I subscribed to a couple trade magazines on electronic packaging and before I could start reading them, my mailbox was flooded with brochures for various meetings called SemiTherm and I-Therm and InterPack and IMAPS, not to mention\u00a0 the CoolingZone conference.\u00a0 Like lots of packaging engineers, I am responsible for sheet metal, injection molded plastic parts, stress analysis, cable harnesses, EMI, safety, shock and vibration, and even shipping containers.\u00a0 I do thermal analysis only part time.\u00a0<\/em><\/p>\n

Do these conferences have anything for engineers like me?\u00a0 Some of the topics sound interesting, but some seem really exotic.\u00a0 Is it just a bunch of professors and PhD candidates showing off for each other?\u00a0 Or can I learn something I can use on the job right away?\u00a0<\/em><\/p>\n

Metal-Bender from Pittsburgh<\/em><\/p>\n

Dear Bender,<\/p>\n

These things are always a mixed bag.<\/p>\n

My biggest gripe with technical conferences of every stripe is that there is often a disconnect between the label on the package and the product inside.\u00a0 They should have warning on every paper like “Some settling of contents may occur during shipping.”\u00a0 The abstracts and the bodies of the presented papers sometimes have little in common.\u00a0 For instance,\u00a0 you spot the title of a paper, “A simple method of determining the thermal conductivity of a printed circuit board,” which sounds like it could be very useful.\u00a0 So you travel across four time zones to hear the talk, and it ends up being the derivation of an equation for heat conduction from a single point source through a composite plate.\u00a0 It is almost like a printed circuit board, except the equation is only valid for circular sources on infinitely large plates, or some such extremely limited piece of the puzzle.\u00a0 It’s not what you thought it would be when you read the abstract.<\/p>\n

There are academic papers from universities, but there are also talks by people from industry who are working on real products, just like you.\u00a0 You will find some duds and some winners in both categories.\u00a0 If you attend conferences on a regular basis, what you will come to realize is that in thermal engineering, there are very, very few breaththroughs to report on.\u00a0 The technology is mature, and progress is made in baby steps.\u00a0 For a first-time attendee, that might be difficult to listen to.\u00a0 It is like trying to learn all about the history and culture of a foreign country by reading random stories from their daily newspapers.<\/p>\n

So maybe you shouldn’t count on gaining a lot of useful information from the presented papers at a conference.\u00a0 There are still three things that might pay off for you.<\/p>\n

1.\u00a0 Short courses.\u00a0 Almost every conference offers 1 or 2-day short courses.\u00a0 These can be very useful, especially if you have not had a lot of formal education in thermal engineering and have had to pick it up as you go along.<\/p>\n

2.\u00a0 Vendor exhibits.\u00a0 Here is your chance to pick up catalogs, samples, and nifty pens and flashlights.\u00a0 Plus you can quiz the vendors for hours without feeling guilty about not giving them a purchase order at the end of the day.\u00a0 This is a great opportunity to see lots of suppliers side-by-side, face-to-face, especially if your office is off the beaten track.\u00a0 And you get to hear the questions your competitors ask them, too.<\/p>\n

3.\u00a0 Which leads into perhaps the most valuable feature of any conference:\u00a0 you get to sit at big tables and eat lunch with a bunch of strangers.\u00a0 After about 20 awkward minutes, you will begin to swap stories, maybe about your plane trip at first, but eventually you will start sharing stories about your problems at work.\u00a0 And if you are lucky, or persistent, you will find somebody who has already solved a problem that has been bugging you for months.\u00a0 I think there is more productive exchange of technical information among the attendees in the halls and lobbies of the hotel than there is in the meeting rooms.<\/p>\n

So I recommend attending a conference once a year or so, if you can swing it.\u00a0 If you happen to nod off during a presentation, no big loss.\u00a0 Just make sure you’re awake during the coffee breaks!<\/p>\n

I can’t really endorse one conference over another, since I haven’t attended all of them to compare fairly.\u00a0 The only hint I can give you about guaranteed quality is if you see my name listed as one of the presenters.\u00a0 (Judge for yourself whether that is a sign of high or low quality.)<\/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<\/strong><\/em><\/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","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2003 Hey Thermal Question Dude, Somebody asked a while back about finishes for heat sinks.\u00a0 Your answer about black anodizing dealt only with how it changes the radiation properties of the surface.\u00a0 But what about the convection?\u00a0 When you paint a […]<\/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-466","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/466","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=466"}],"version-history":[{"count":3,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/466\/revisions"}],"predecessor-version":[{"id":470,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/466\/revisions\/470"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=466"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}