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[AMPS] Blowers for finned tubes

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Subject: [AMPS] Blowers for finned tubes
From: (Rich Measures)
Date: Wed, 11 Mar 98 02:17:16 -0800
>>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
>For finned air cooled tubes, you need to supply the recommended CFM of air
>volume through the anode fins which will give you a back pressure. Your
>blower needs to be capable of supplying that CFM, into that pressure. 
However, measuring cfm requires elaborate test equipment.  Measuring 
pressure in inches or cm of water column [using a manometer] is not 
difficult.  It has been my experience that the mfg. ratings of blowers 
tend to be enthusiastic.  There is no substitute for measuring the 
pressure drop with a manometer.  I cut holes in a suitable cardboard 
box., insert the tube, chimney and socket, and measure the pressure 
differential produced by the blower in question.  
>if you live at high elevation or use 50 Hz power, remember to derate things
>to accomidate these reductions in air volume. If you overload a blower, it
>will fall way off on performance (CFM), and be hard on the motor.
The greatest load is free delivery.  
>Once you have selected a blower, you really should do a little homework to
>prevent meltdown in the future, when you key down into a tough load, and it
>is a hot day, and there's dead bugs and dirt on the airfilter blocking it a
>If you play with a furnace blower, you should either know the curves on the
>blower/motor combination, or do your own tests to prove this. You will need
>a long piece of duct connected to the blower, some Dwyer airflow measuring
>pitot tubes (the red fluid in them), and a flap you can shut on the far end
>to load up the blower. I can refer you to a paper on the subject of testing
>blowers when the Broadcast Electronics model FM-30A 30 KW FM transmitter
>was developed. It was written by Jeff Steinkamp of Quincy, Illinois, and
>can be gotten by contacting BE at (217) 224-9600 and asking for a reprint.
>I believe it was presented at a National Assoc of Broadcasting convention
>technical presentation, but cannot remember the year (sometime around
>The furnace type may be big volume movers, but not such high pressure
They work pretty ok for 8171s.
>Duct work does present back pressure, as do filters and floor
>grates for heating. I suspect you will find one that will work, but it may
>be serious overkill. If noise, power consumption, and size are no concern,
>then try it!
>Another tip, use a tiny spot of Omegalaq or Tempilaq paint on the tube,
>right below the anode seal on the ceramic, or above the seal on the anode
>metal. For glass tubes, you have to be very careful due to making a hot
>spot with your temperature paint on the glass and causing a suck in.
>Besides, if your glass tube is that hot, it will break!  The Tempilaq range
>I use is from 150 -250 deg C, in increments of about 25 deg C. If your
>paint melts and changes color all the way up to 250, at any spot around the
>seal, then you can expect that the tube life may be short in your Amp. Oh,
>one more thing, on the bigger tubes with handles  one has to also cool the
>stem, the lower contact of the filament in the socket. Same drill as above.
>Perhaps these tips are a bit overkill, but then again, you were talking
>about furnace blowers, so I assume that your tubes have handles at least.
The way to tell when you have too much blower is if it blows the tube out 
of the socket.  
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R. L. Measures, 805-386-3734, AG6K   

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