W7CW Jay E Ostrem wrote:
> Has anyone experimented with using a furnace blower for cooling their
> tubes? I was curious if anyone had actually measured available back
> pressure for any particular size wheels. The handbooks list ratings
> for
> different size Dayton blowers, and wondered if the furnace type would
> be
> worth pursuing for cooling some of the larger triodes.
> 73 Jay W7CW
>
> 1AB Ranch
> Gillette, Wyoming
>
I've spent some considerable time playing with centrifugal blowers of
various sizes to cool big tubes, and looking at the theory to some
extent  without wishing to do a PhD in the subject. I don't know what
blowers are used in furnaces, but if they are centrifugal, the following
might be of use.
The cut off back depends mainly on the speed and diameter, as:
Cutoff back pressure = BPco= 9.5 x 10^ 9 N^2 d^2
where cuttoff back pressure is in inches of water, N is the speed in
revs/min and d is the diameter in inches.
The free air flow rate (cfm) is roughly:
Ffa=3.11 x 10^4 N w d^3,
where w is the wheel width in inches.
Once you know the BPco and Ffa, and some data on the tube, you can
estimate the flow the blower will produce through your tube.
See "Predicting the Performance of Centrifugal Fans for Valve Cooling",
D. R. Kirkby, (G8WRB), QEX, October 1997, pages 2731. I can send you a
copy if you wish.
I would add that I got so frustrated trying to find a suitable used
blower for a 3CX5000A7, that I spent #400 on a new one.

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