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Author Topic: Tube heat dissipation  (Read 27648 times)
KE3WD
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Posts: 5688




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« Reply #45 on: August 17, 2013, 09:56:29 AM »

So hand me some heat. 

Bottle it up and ship it to me.

73
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K7KBN
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« Reply #46 on: August 17, 2013, 11:55:09 AM »

There's the challenge!  Dewar die!
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73
Pat K7KBN
CWO4 USNR Ret.
KB1LKR
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« Reply #47 on: August 17, 2013, 03:24:29 PM »

If you are referring to the old metal-cased vacuum tubes brought out around the WWII era, the coating was black on them more likely due to available coatings than anything having to do with black body radiation.  Besides, those tubes were not high power transmitting tubes, most of them were signal power or rather low level beam tetrodes such as the 6L6, 6V6, etc. - and heat from those was not anywhere near the situation encountered with the RF output tubes. 

It should be noted for emissivity (radiation), unlike absorption, the color of the coating does not matter. That it is not a mirrored/polished surface increases the emissivity though.

The suggestion to put a dark/dull barrier, e.g. black anodized aluminum, between the two tube is an interesting one, it might slightly increase radiant heat transfer to something that could than be force convection cooled. It would be an expensive solution in small volumes however. The interesting question is: how much of the plate radiation is absorbed by the glass envelope, where convection would than take over. For glass, the absorption may be significant for wavelengths longer than a couple um.
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W0BTU
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« Reply #48 on: August 17, 2013, 04:19:22 PM »

It should be noted for emissivity (radiation), unlike absorption, the color of the coating does not matter.

I have always believed otherwise from reading physics books; for one thing, that's why some heat sinks are black anodized.

However, it has always fascinated me as to why a dull black surface should both radiate and absorb heat better.
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G3RZP
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« Reply #49 on: August 18, 2013, 05:33:21 AM »

I have seen some red metal 6SJ7s - I was told they were a special selection for some parameter.
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KE3WD
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« Reply #50 on: August 18, 2013, 08:39:33 AM »

I have seen some red metal 6SJ7s - I was told they were a special selection for some parameter.

I also remember some certain black metal can tubes that touted a Red Dot on them that indicated something "special" about them as well.  Forgot what that something special was, exactly, though.  As I recall, the spec designation for such was, "Selected"...


73
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KC8OJU
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« Reply #51 on: August 18, 2013, 01:27:01 PM »

Hey OM
Everybody brandishing around the word 'black' like it was a color.
Black is not a color, you show me black and I will show you  a color in the light spectrum even though you can't see it, 'cause we can only see certain light.
You know the ole' saying "if it ain't broke don't fix it".
73 OM
de n8zu
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W8JX
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« Reply #52 on: August 18, 2013, 03:31:56 PM »

It should be noted for emissivity (radiation), unlike absorption, the color of the coating does not matter.

I have always believed otherwise from reading physics books; for one thing, that's why some heat sinks are black anodized.

However, it has always fascinated me as to why a dull black surface should both radiate and absorb heat better.

You are correct to believe otherwise. It does matter on radiation too. Flat black it best here.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
W6EM
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« Reply #53 on: August 18, 2013, 03:51:23 PM »

There's the challenge!  Dewar die!
Aha!!  Great play on words.  The elegance of English.

Where I once worked, "surplused" dewars were frequently used as beverage cups for obvious reasons.  However, distinctly marked, for obvious reasons.

Pass around some heat........ or lack thereof.

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KB2WIG
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« Reply #54 on: August 18, 2013, 03:51:47 PM »

" There's the challenge!  Dewar die! "

P,

You beat me to it.

klc
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EXTRALight  1/3 less WPM than a Real EXTRA
NJ1K
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« Reply #55 on: August 19, 2013, 07:05:27 AM »

Ok, so I guess I need to chime in here.

First off, the term “heat” does indeed have a useful purpose in the engineering field.  Having worked with thermodynamics for many decades now, it would be hard to imagine professional engineering reference manuals defining thermal energy transfer without using the word “heat”.  Obviously, heat isn’t a substance in and of itself, rather it is a form (or state) of energy.  It can be transferred by conduction, convection or radiation.  Yes, one can argue that it is molecular activity within a physical material, and that can be correct for energy contained within the material, but that’s not radiated energy.  For radiated energy to be defined as “heat” its wavelength would fall into the infrared spectrum.  I know there will be some here that will argue with that, but so what.  These are all valid points.

Now, warmer air vs. cooler air, yes, warmer air is lighter and will naturally rise above the cooler air however, in a forced air system, whether the forced air is up-flow or down-flow, one would be hard pressed to be able to measure any difference in cooling ability of the flow.  In the case of ceramic tubes, the intended air flow is from base to anode.  If the tube were inverted and the forced airflow is now down-flow, with the same CFM you would have the same amount of cooling.  Gravity will not affect this.

Now, whether better results are obtained by pressurization or exhaustion (not evacuation), little difference can be measured between the two assuming the airflow path is the same and the enclosure has no placed for infiltration.  A squirrel cage blower placing an inch water column negative pressure on an enclosure will not effectively change the density of the air enough to affect the cooling ability and you will not be able to measure a change in cooling capacity.  At least not with typical field measuring instruments.  What CAN change cooling capacity is the routing of the air as well as infiltration through small cracks and other entry points in the chassis.  In a negative pressure system, air entering the enclosure through infiltration can bypass components that need cooling.  This is the main reason pressurization systems are preferred.  Not because of the perceived effect of the air being less dense on a negative pressure system. 

Now onto the glass envelope: Most glass blocks most infrared radiation.  Therefore, most of the “heat” will stop at the glass envelope and conduct through the glass and then to the air and be convected away with the air.  Yes, some will be radiated through the glass but will be a small percentage.  So, painting the interior of the enclosure black won’t really help a whole lot.  What’s most important on glass tubes is that there is appreciable air velocity against the glass tube wall, and across the tube entry points AKA the pins and anode connection. 

Hope this helps with some understanding but I am sure there will be many that will argue these points.

 
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KE3WD
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« Reply #56 on: August 19, 2013, 08:43:40 AM »


A totally empty Dewar can hold no "heat".  (totally empty = vacuum)

Gotta be something that exhibits the properties of being able to have what we call Thermal Mass inside that Dewar before you can attempt to keep this "heat" in there. 

And that is because the "heat" is actually the molecular motion in that thermal mass, the molecules being forced into faster vibrations due to the application of energy. 


73
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NJ1K
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« Reply #57 on: August 19, 2013, 11:27:36 AM »


A totally empty Dewar can hold no "heat".  (totally empty = vacuum)

Gotta be something that exhibits the properties of being able to have what we call Thermal Mass inside that Dewar before you can attempt to keep this "heat" in there. 

And that is because the "heat" is actually the molecular motion in that thermal mass, the molecules being forced into faster vibrations due to the application of energy. 


73

This is all ya got?

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KE3WD
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Posts: 5688




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« Reply #58 on: August 20, 2013, 08:17:52 AM »

It ain't all I got, duh, but it may be all I'll share. 
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KH6AQ
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« Reply #59 on: August 20, 2013, 09:28:07 AM »

Quote

KB1LKR...
It should be noted for emissivity (radiation), unlike absorption, the color of the coating does not matter. That it is not a mirrored/polished surface increases the emissivity though.


Aren't emissivity and absorption reciprocal as stated by Kirchhoff's law of thermal radiation? That is, if we paint a surface black and its absorption is increased, its emissivity is increased by the same amount. 

http://en.wikipedia.org/wiki/Kirchhoff%27s_law_of_thermal_radiation
« Last Edit: August 20, 2013, 09:30:52 AM by WX7G » Logged
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