W7CW Jay E Ostrem wrote:
>This may be an over simplification, but I think Rich is alluding to it;
>
>Let's say you are measuring back pressure on an installed tube, utilizing a
>manometer. It seems to make sense that if the back pressure is correct for
>the altitude, then the cfm should be correct also.
That's true, but it overlooks two points.
One is that the air pressure is often quite low, and can be difficult to
read accurately due to surface tension effects in the water tube. There
may also be errors due to the location of the manometer inlet - your
layout will not be the same as the layout Eimac used to take the
measurements for the datasheet. Therefore a direct volume flow
measurement can be much more accurate.
The second point is that the back pressure given in Eimac datasheets is
only for blowing from underneath the socket - and that's not the only
possible method. Many (probably most) VHF/UHF amps use the "K2RIW"
system of blowing into a sealed anode compartment, with an exhaust
chimney between the anode cooler and the outside world. There is no
chimney between the socket and the anode. After cold air has circulated
around the output tuned circuit, most of it goes up through the exhaust
chimney. A fraction of the air flow (typically one-quarter to one-third)
is diverted downwards through the socket to cool the base.
The "K2RIW" system has the big advantage that the airflow resistances of
the anode and the base appear in parallel, not in series, so the
required pressure drops dramatically (typically only 0.25-0.3 in wg for
4CX 250Bs, compared with 0.6in for the standard configuration). This
means that most blowers are operating in a much more comfortable part of
their characteristic curve, and can deliver a higher volume flow rate
and therefore better anode cooling than the standard configuration. Even
though there is no direct air blast onto the base seals, temperature
measurements (and many thousands of hours of experience) have shown that
this system does give effective cooling.
Because this system is not recognized by Eimac, and the base cooling is
regulated by the size of the exit hole from the sealed input
compartment, there is no substitute for direct measurement of volume
flow rates out of the anode exhaust and out of the input compartment.
It's interesting that HF and VHF/UHF amps have gone in different
directions about cooling. Obviously there are some good reasons (eg the
tendency to use resonant cavities at VHF/UHF that have to be sealed
anyway) but I don't believe this can be the whole story.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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