Steve Thompson wrote:
> I recommend http://www.david-kirkby.co.uk/ham/fanpaper.pdf as bedtime
> reading. The mathematical treatment might appear offputting, but at the end
> it distills out to some basic equations which give a guide to blower
> performance from their physical characteristics.
>
> I've not done experiments to verify the results, but I did have one blower
> that looked the part but wouldn't cool a 4CX250. Plugging the numbers in
> produced a result that it would be short on pressure.
>
> Steve
>
> _______________________________________________
>
There is an excellent article titled "Forced-air cooling of tubes" by William
Orr in the September 1967 issue of QST Page 20.
He discusses air requirements for various tubes and blower sizes. Very worth
while to read if you are doing any blower work.
Here are a couple of examples:
4CX250B requires 6.4 CFM of air at 1.12" of pressure. That takes a # 2 1/2
blower at 6000 rpm.
4CX1000 requires 22 CFM of air at .3" of pressure. That takes a # 3 blower at
3100 rpm.
So even though the 4CX1000 is a much larger tube it is much easier to cool
because of much less back pressure.
Hi back pressure requires a very high rpm on a smaller wheel. Larger wheel
blowers (4 to 6") can be run at lower speeds and produce enough pressure if
designed properly. I think that the KWS1 used a rather large blower wheel for
the 4CX250's in it.
The 4CX250 is probably one of the hardest tubes there are to come up with a
small quiet blower for. If you have ever seen any military gear with those
tubes, they often had small (1 to 1 1/2") blowers that ran at 10000 rpm or so.
Worked good but you could not stand to be near them.
73
Gary K4FMX
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