[ARC5] An Interesting Failure Mode

[Richard Knoppow]

    This brings up a whole question about how tubes are cooled. 
Excepting tubes with external anode radiators or water cooled types, the 
heat must be dissipated mostly by radiation. Some heat is generated by 
the heater but most is plate dissipation. The envelope gets hot because 
the glass must absorb and re-radiate some of the heat. In some tubes the 
glass may also be heated by electron bombardment. So, an air flow around 
the tube can reduce the heat of the glass envelope but does nothing for 
the heat directly radiated from the tube electrodes. Also, since the 
tube contains a high vacuum the envelope acts something like a thermos 
bottle. Again, heat from electrodes must be radiated and heating of the 
glass is from absorbed radiation. It would be interesting to know just 
how transparent tube glass is at the low IR wavelengths of heat. I am 
sure this data exists somewhere. I think this may get complicated since 
the glass is radiating heat back toward the electrodes as well as to the 
outside. In addition to radiation there is some cooling by convection. 
Someone said the tube in question was covered with a screen held in 
place with some sort of conformal coating. Certainly this would reduce 
radiation cooling. It would also damp the vibrations of the glass quite 
a lot.

On 12/13/2016 1:31 PM, WA5CAB--- via ARC5 wrote:
> Although that might be true, it doesn't necessarily follow from what
> very little we know..
>
> (1) The heated air around the tube must still be removed in order to
> remove the heat.  The tube may be in a compartment without any natural
> air flow through it such that it still reaches an excessively high
> temperature.
>
> (2) although the air is colder, it is also thinner and natural flow may
> still not remove enough heat.
>
> In a message dated 12/13/2016 14:11:59 PM Central Standard Time,
> wrcromwell at gmail.com writes:
>> The temperature at 20,000 feet is far below zero. I don't think there
>> were any pressurized (heated cabin) aircraft before the B-29. The
>> mosquito was a plywood, twin engine aircraft. If the thin air (due to no
>> pressurization) affected the fan then the fan also wouldn't be needed in
>> the frigid air. No beer cooler required, too -evil grin-.
>>
>> 73,
>>
>> Bill  KU8H
>
>
> Robert Downs - Houston
> wa5cab dot com (Web Store)
> MVPA 9480
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL