{"id":490,"date":"2012-12-24T10:56:48","date_gmt":"2012-12-24T16:56:48","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=490"},"modified":"2012-12-24T10:56:48","modified_gmt":"2012-12-24T16:56:48","slug":"everything-you-know-is-wrong-june-2003","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=490","title":{"rendered":"Everything You Know Is Wrong June 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

Dear Tony,<\/em><\/p>\n

Our electronics go in a box that can be mounted on the outside of a building, or even the mast of a ship at sea.\u00a0 The customer requirement says we must prove by testing that it can work when the outside air is 40 degrees C, the wind is calm, and the sun is beating down with an intensity of 1,000 Watts per square meter.<\/em><\/p>\n

I think our product can meet this requirement.\u00a0 But how can we prove it with a test?\u00a0 Our environmental chamber can control the air temperature and the relative humidity, but how do we duplicate the solar load?\u00a0 We could take our box toAlbuquerquein July, but the odds of getting the right solar intensity,\u00a0 air temperature and wind speed all at the same time seem pretty long.<\/em><\/p>\n

Can we just increase the air temperature in our chamber to simulate the extra heat load from the sun?\u00a0 How many degrees of ambient temperature are the equivalent of 1,000 Watts per square foot?<\/em><\/p>\n

Overworked from Beaverto<\/em>n<\/p>\n

 <\/p>\n

Dear Beav,<\/p>\n

You, like many in the outdoor electronics business, are the victim of an untestable test requirement.<\/p>\n

The requirement seems to be a fairly clear.\u00a0 But it has been left entirely up to you to figure out how to conduct the test.\u00a0 There is a reason your customer left it up to you.\u00a0 They don’t know how to do it either.<\/p>\n

\"\"<\/a>This is the thermal environment that your box will see, according to the worst case requirements.\u00a0 Your box is suspended on a pole or the side of a building, exposed to the sun and the air.\u00a0 The electronics inside your box generate some amount of heat that has to get out.\u00a0 I assume the box is sealed to keep out moisture and insects, so the only way for the heat to get out is by heating up the walls of the box, and then that heat goes to the environment by convection and radiation.\u00a0 The temperature of the electronics is tied to\u00a0the temperature of the walls of the box.\u00a0 The hotter the walls end up, the hotter the electronics get.<\/p>\n

The walls of the box see heat coming in and going out in a variety of ways.\u00a0 There is the solar radiation, at 1,000 W\/m2<\/sup>.\u00a0 That can be a lot of heat.\u00a0 But if you have done your homework designing your box, perhaps most of it never even gets absorbed by the walls.\u00a0 If you coated the surfaces white paint with low absorptivity in the solar spectrum, or made it highly reflective, or made it a double-wall construction so that the outer wall acts as a parasol for the inner wall, then perhaps only a small fraction of the solar radiation gets absorbed.<\/p>\n

Then we need to consider that the walls of the box are hotter than the surrounding air, so there will be convective heat transfer.\u00a0 Even if there is no wind, there is natural convection.\u00a0 The bigger the temperature difference between the box wall and the air, the more natural convection there will be.<\/p>\n

Again, because the walls of the box are hotter than the surroundings, it is possible that they can give off radiation to the surroundings.\u00a0 They can even radiate to the portion of the sky not occupied by the sun (the sun looks pretty much a point source, so you can assume radiation to the whole sky, at least what is not obstructed by the surroundings.)\u00a0 Surfaces like grass and trees are near air temperature, so the box radiate to them.\u00a0 Surfaces like asphalt parking lots\u00a0 may actually be hotter than the air, or reflect the sun directly toward the box, and add more radiation.\u00a0 That is why I show a double-ended arrow for the exchange of radiation with the surroundings.\u00a0 There is no way to tell if radiation will be a net loss or gain from the surroundings unless you know what they are.<\/p>\n

Let me sum up the situation:\u00a0 the box wall temperature depends on a lot of complicated heat transfer going in and out by different mechanisms at the same time, and a lot depends on the radiation characteristics of your box design.
\n<\/em><\/p>\n

\"\"<\/a>But this is the way you propose to test the thermal performance of your box.<\/p>\n

You will put it in an environmental chamber that has no radiation load.\u00a0 To compensate for the missing solar load, you will increase the ambient temperature higher than the required 40C.\u00a0 Now all you need to know is how high to set the ambient so that this picture looks just like the previous one.\u00a0 Should it be 65C?\u00a0 70C?\u00a0 100C?\u00a0 Or maybe it should be adjusted for the surface area?
\n<\/em><\/p>\n

There are a couple of reasons why this approach should never be accepted as proof that your box meets the customer requirement.<\/p>\n

1.\u00a0 The convection is wrong.\u00a0 Every environmental chamber has a robust circulating fan to keep the air temperature uniform.\u00a0 This is hardly “still air”.<\/p>\n

2.\u00a0 The radiation is wrong.\u00a0 You are not testing the most important feature of the box the feature that might make it work in the most most intense solar radiation — the ability of the walls to absorb or reflect solar radiation!\u00a0 Since there is practically no radiation exchange in this test (chamber walls tend to be bare stainless steel with low emissivity), this test proves nothing about how it will perform in direct sunlight.<\/p>\n

Let’s say you did some pretty good calculations, either by hand, or using any of the numerical simulation tools available, and you determined that your box wall would be 25 degrees C hotter than the air, based on the radiation properties of the box coating, and some assumptions about the surroundings.\u00a0 Can you then just boost the ambient in your environmental chamber until the box wall temperature hits\u00a0 65C, measure your component temperatures, and say that you have proved it meets the customer requirement?<\/p>\n

I don’t think so.\u00a0 You have proved that the electronics work when the box wall temperature is 65C.\u00a0 But you haven’t proven anything about the radiation.\u00a0 That was all done by calculation.<\/p>\n

A valid test, I think, would have to be done in a very large chamber, with a radiation source capable of producing the required intensity in the proper, sun-like position.\u00a0 It would have to be far enough away so that your box could still exchange radiation with ambient temperature surroundings in a realistic way, and the air would have to be approximately still.\u00a0 If the sun lamp were too close, which it would have to be in a normal-size environmental chamber, it would block out too much of the “sky”, and be an unfair test.<\/p>\n

I don’t know of such a solar test chamber.\u00a0 It sounds like it would be very expensive to accomplish such a realistic test.\u00a0 That is why the customer left the test definition up to you.<\/p>\n

I would go back to the customer and propose your simple environmental chamber test.\u00a0 If they don’t like it (and there is no reason they should), then you can ask them to give you a written procedure for a test they would believe in.\u00a0 That is only fair.\u00a0 That way, all potential vendors\u00a0 would have to conform to the same test.\u00a0 The requirement writers might have to admit they don’t know how to perform such a test either.<\/p>\n

Going to Albuquerque and waiting for a hot, windless day to run a test doesn’t sound so complicated now, does it?<\/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 Whom It May Concern:<\/em><\/p>\n

I’d like to learn how to calculate the heat spread angle in metal.\u00a0 I think it depends on the ratio of heat conduction in the metal and heat conduction in the surface it is joined with.<\/em><\/p>\n

Al in Albuquerque<\/em><\/p>\n

Dear Al,<\/p>\n

The so-called heat spreading angle is not mentioned in heat transfer textbooks.\u00a0 I had a hard time figuring out what you were asking about, until I found a mention of it in a textbook on circuit design.\u00a0 There was a short chapter on heat sinks.\u00a0 It correctly advised that if you mount a small component on the base of large heat sink, it might not perform as well as expected because of spreading resistance in the heat sink.\u00a0 Then it went on to show a figure of heat spreading out from the source at a 45 degree angle.
\n<\/em>
\n\"\"<\/a>A component is mounted on a block of aluminum.\u00a0 Does the heat shoot straight down through the metal like a laser beam?\u00a0 Or does it spread out at some fixed angle, i ?\u00a0 (The letter i is derived from the fact that the triangular temperature distribution was purely the product of somebody’s imagination.)\u00a0 The actual temperature distribution in the block depends heavily on the shape of the block and the thermal conditions at the edges of the block.<\/p>\n

Electronic engineering texts require students to analyze circuits using some of the most complex math I have ever seen, including differential equations.\u00a0 But when it comes down to describing something as simple and intuitive as heat conduction, math is completely abandoned in favor of Rules of Thumb.\u00a0 The spreading angle is a Rule of Thumb.<\/p>\n

Here is the equation for two-dimensional conduction in a solid.\u00a0 It looks pretty simple.<\/p>\n

\"\"<\/a><\/p>\n

Unfortunately, the solution depends a lot on the geometry and boundary conditions.\u00a0 The conductivity of the material is only one factor.\u00a0 Boundary conditions are the temperature and\/or heat transfer conditions at the edges of the solid.\u00a0 For example, are the surfaces isothermal, do they have uniform heat flux, or some complex combination of temperature distribution and heat flux?<\/p>\n

For most practical geometries and sets of boundary conditions, like a typical heat sink, there are no closed form solutions.\u00a0 The problem is most commonly solved numerically, using a finite difference solver.\u00a0 If the geometry you have in mind is not too complicated, you can probably do it yourself using a spreadsheet.<\/p>\n

Seri Lee wrote on article on spreading resistance in heat sink bases in the January 1998 issue of Electronics Cooling<\/a>.\u00a0 It gives equations for that type of geometry for small heat sources.\u00a0 They aren’t partial differential equations, but they don’t give you a simple spreading angle either.\u00a0 Good luck.<\/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 Dear Tony, Our electronics go in a box that can be mounted on the outside of a building, or even the mast of a ship at sea.\u00a0 The customer requirement says we must prove by testing that it can work […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"footnotes":""},"class_list":["post-490","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/490","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=490"}],"version-history":[{"count":4,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/490\/revisions"}],"predecessor-version":[{"id":499,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/490\/revisions\/499"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=490"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}