{"id":502,"date":"2013-01-05T13:59:22","date_gmt":"2013-01-05T19:59:22","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=502"},"modified":"2014-04-13T20:26:19","modified_gmt":"2014-04-14T01:26:19","slug":"everything-you-know-is-wrong-july-2003","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=502","title":{"rendered":"Everything You Know Is Wrong July 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 TK,<\/em><\/p>\n

You preach that a person shouldn’t be allowed to practice cooling of electronics unless they can clearly explain the difference between heat and temperature to their mother-in-law.\u00a0 I tried last Sunday over a chicken and dumpling dinner and got stuck.\u00a0 I\u00a0 wrote the equation for conduction on the tablecloth and got slapped on the wrist.\u00a0 So then I launched into the old electrical analogy where voltage is temperature and current is heat, but it didn’t register with Ma.\u00a0 How do you explain the difference between heat and temperature to your mother-in-law?\u00a0<\/em><\/p>\n

Stuck washing dishes in San Jose<\/em><\/p>\n

 <\/p>\n

Dear Stuck,<\/p>\n

Analogy is a powerful teaching tool, but it has one crippling weakness — the student has to be familiar with the thing you compare heat\/temperature to, or it does no good at all.\u00a0 The voltage\/current analogy never did work for me, because Ohm’s Law was just one of those equations I memorized to pass a test.\u00a0 I never developed an intuitive sense for electricity.<\/p>\n

Even a really good analogy doesn’t work on everybody.\u00a0 It has to be tailored to the audience.\u00a0 Due to the complicated history of my wife’s family, I have three mothers-in-law.\u00a0 I had to develop a different story for each of them.<\/p>\n

In college I took a class that was team-taught by professors from every discipline in the engineering department.\u00a0 One day a EE prof spent half an hour trying to explain the relationship between speed and torque in a pair of gears.\u00a0 He said that the best way to understand how gears convert speed and torque was to think of them as a transformer converting voltage and current.\u00a0 I nearly pulled out my hair (yes, I had some in college.)\u00a0 Gears were obvious to me.\u00a0 If you mesh a little gear to a big gear, the little one has to spin faster than the big one.\u00a0 You can see it happening, even if it’s just in your head.\u00a0 How did talking about a bunch of invisible lines of magnetic flux make it any more clear?\u00a0 But as soon as he drew the chalk line connecting Gear Ratio to Turns Ratio, the EE students in the back of the room gasped, “Aha!”\u00a0 So his analogy was helpful to somebody.<\/p>\n

Here’s one that works for some of my mothers-in-law:<\/p>\n

What is the difference between money and being rich?<\/strong><\/p>\n

Heat is like money.\u00a0 It can flow from one body to another, and it can accumulate.\u00a0 Like heat, it can move around, or be converted to something else, but it isn’t ever lost if you do your accounting properly.<\/p>\n

If heat is money, then temperature is a measure of how rich somebody is, compared to somebody else.<\/p>\n

Illustration:\u00a0 Does a million dollars make one rich or poor?<\/p>\n

Town A has a thousand citizens and Town B has ten citizens.\u00a0 The federal government, being fair and benevolent, gives each town a grant of $1 million.\u00a0 Both towns have received the same amount of money.\u00a0 But which town now has richer citizens?<\/p>\n

On average, Town A folks have $1,000 each.\u00a0 Town B people, again on average, each have $100,000, so they are much richer than the people of Town A.\u00a0 (In this way I want to get across the idea that temperature is, roughly speaking, the average amount of heat energy per molecule.\u00a0 I say roughly, because molecules are obviously not all alike.\u00a0 But in my analogy, molecules are represented by people.)<\/p>\n

If you put the two towns in contact, which way will the money flow?\u00a0 (This is where the analogy breaks down.\u00a0 In real life, the rich people would find a way to get the poor people’s money away from them.\u00a0 Perhaps that is an analogy for refrigeration.)\u00a0 Being consistent with heat transfer theory, though, you have to assume that the richer you are, the more money you spend.\u00a0 So if you put Town A and Town B in contact, money tends to flow from Town B to Town A until all the folks are equally rich (on average).\u00a0 At equilibrium, each person would have $1980.<\/p>\n

The rate at which money flows from Town B to Town A would be limited by how well the two towns were connected.\u00a0 A poor connection would slow the flow.\u00a0 This is similar to thermal resistance.<\/p>\n

Try stretching the analogy further as your homework assignment.\u00a0 What is the money analog for radiation?<\/p>\n

Maybe the money\/wealth analogy doesn’t work for you.\u00a0 Maybe you have trouble balancing your checkbook and think money grows on trees.\u00a0 You might prefer the scenario of water flowing through a pipe under pressure.\u00a0 Or you could check out the exotic analogy in Chapter Four of my book<\/a>, in which temperature is likened to the population density of crowd-hating monkeys on a tropical island.\u00a0 Or temperature is like the Nielsen rating of a TV show, and heat is like … maybe not.<\/p>\n

The point is that you need to have a clear picture in your own mind about this concept of flux, resistance and potential, before you start applying it to cooling electronics.\u00a0 And the best way to learn something is to try to teach it to somebody else.\u00a0 Now, if heat were similar to chicken gravy, what feature of the chicken and dumpling dinner would be equivalent to a 70 degree C ambient rated component?<\/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

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

In engineering school, I thought that Murphy’s Law was an amusing exaggeration.\u00a0 All the homework problems had workable solutions, so how hard could being an engineer be in the real world?\u00a0 But after getting snagged into the job of cooling circuits, I soon learned that Murphy’s Law was just the beginning.\u00a0 It seems that for every trick you think of to make temperature go down, Mother Nature has a trump card up her sleeve.\u00a0 You know what I mean:\u00a0 you add a thermoelectric cooler to refrigerate a hot chip, but that makes the heat dissipation triple and everything ends up hotter than before.\u00a0 Are ALL the laws of nature stacked against us? <\/em><\/p>\n

Pessimist from Murphysboro <\/em><\/p>\n

Dear Pessimist,<\/p>\n

There are a couple of examples I can think of where nature tries to help the struggling electronics cooling engineer.\u00a0 One is natural convection.\u00a0 When you increase the power, natural convection works harder (because the air flow is driven by the heat itself.)\u00a0 In forced convection, if you double the power, the temperature rise doubles.\u00a0 But in natural convection, when you double the power, the temperature rise only goes up by a factor of 1.7.\u00a0 Don’t you feel lucky?<\/p>\n

If you work a lot with natural convection, you might be familiar with my second example.\u00a0 Because natural convection is a relatively weak form of heat transfer, radiation can play a large part.\u00a0 It can be responsible for more heat transfer than natural convection itself, under the right conditions.<\/p>\n

\"\"<\/a>

Figure 1. Radiation in a room.<\/p><\/div>\n

Radiation is friendly to the thermal engineer in certain circumstances.\u00a0 The one I am thinking of is when you have a sealed, heat-generating enclosure, such as a laptop computer, sitting on a bench in a large room.\u00a0 Let’s ignore the possibility of the sun baking down on our little thought experiment.\u00a0 In such a case, radiation is friendly, because as the ambient temperature goes up, radiation becomes more and more effective.\u00a0 Assuming the power of our box is constant, that means as the ambient increases, radiation increases, and the box temperature rise above ambient would actually decrease, partially offsetting the increase in ambient.<\/p>\n

As a pessimist, you probably assume the friendliness of radiation has got to be negligible.\u00a0 Let’s take a look at a numerical example.
\n<\/em><\/p>\n

Figure 1 is a hot electronic box with a surface area of 1 square meter.\u00a0 Let’s say it is painted with a really, really black paint so that its emissivity is 1.\u00a0 It is completely surrounded by the walls of the room, and the walls are at the same temperature as the air.\u00a0 The walls are also pretty good absorbers of infrared radiation, but that is not so important, as long as the room is much bigger than the box.\u00a0 The ambient temperature of the room is stable, but from day to day it can vary from 0 C up to 50 C.<\/p>\n

The net amount of heat given off by radiation from the box to the room is given by this equation:<\/p>\n

Q rad<\/sub> = S E A ( Tsurface<\/sub>4 <\/sup>– T ambient<\/sub> 4<\/sup>)<\/strong><\/p>\n

where:<\/p>\n\n\n\n\n\n\n\n\n
\n

Q rad<\/sub><\/p>\n<\/td>\n

\n

is the heat given off by radiation to the room in W<\/p>\n<\/td>\n<\/tr>\n

\n

S<\/p>\n<\/td>\n

\n

is the Stefan-Boltzman constant equal to 5.67e-8 W\/m2<\/sup> K4<\/sup><\/p>\n<\/td>\n<\/tr>\n

\n

E<\/p>\n<\/td>\n

\n

is the emissivity of the box surface (in our example = 1)<\/p>\n<\/td>\n<\/tr>\n

\n

A<\/p>\n<\/td>\n

\n

is the box surface area (1 m2 <\/sup>in this example)<\/p>\n<\/td>\n<\/tr>\n

\n

\u00a0Tsurface<\/sub><\/p>\n<\/td>\n

\n

is the absolute surface temperature of the box in degrees Kelvin (K)<\/p>\n<\/td>\n<\/tr>\n

\n

T ambient<\/sub><\/p>\n<\/td>\n

\n

is the absolute ambient temperature in degrees Kelvin (K)<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Everything in this equation is a constant except for the temperatures.\u00a0 Unlike conduction or convection, radiation depends on the fourth power of the temperatures.\u00a0 Not the temperature difference, but the difference of the fourth powers of temperature.\u00a0 That gives radiation a behavior that is unexpected by those used to dealing with convection and conduction.\u00a0 Radiation increases as the ambient goes up, even if the temperature difference between the two surfaces remains constant.<\/p>\n

Let’s assume that the box surface temperature is 10 degrees hotter than ambient.\u00a0 At 0 C ambient (273K), the box gives off 49 W by radiation.\u00a0 At the maximum ambient of 50C, radiation increases to 80W (assuming the box temperature is still 10 degrees hotter than ambient.)\u00a0 Over the operating ambient temperature range, radiation increases by about 60%.\u00a0 That is not exactly negligible, especially when you consider how unfriendly nature has been to us in so many other areas.<\/p>\n

\n
\n
\"\"<\/a><\/dt>\n
Radiation is not constant as the ambient goes up. Here is one of the rare instances in which the laws of physics are friendly to the thermal engineer.<\/dd>\n<\/dl>\n<\/div>\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 TK, You preach that a person shouldn’t be allowed to practice cooling of electronics unless they can clearly explain the difference between heat and temperature to their mother-in-law.\u00a0 I tried last Sunday over a chicken and dumpling dinner and […]<\/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-502","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/502","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=502"}],"version-history":[{"count":4,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/502\/revisions"}],"predecessor-version":[{"id":508,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/502\/revisions\/508"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=502"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}