[AMPS] 3CV1500A7; what is vapor cooling?
Richard W. Ehrhorn
w4eto@rmii.com
Tue, 4 Aug 1998 14:14:15 -0600
Hi Jon & all...
Concerning your question, Jon, the best readily-available explanation of
vapor-phase cooling that I know of was a construction article by Jack Quinn
(of Eimac) in QST around 1968 give or take a year. Think its title was "The
Stanley Steamer." In a nutshell, more or less, this is why & how vapor
cooling is used:
Vapor-phase cooling is commonly used in very high power amps because the
heat absorbed in converting water to steam is 540 cal/gm (of water
vaporized, if I remember correctly), while in conventional liquid water
cooling the water absorbs only 1cal/gm/degreeC of temp rise. Physically,
the vapor- cooled tube's anode is surrounded by a small tank, or boiler, in
which pure water is vaporized into steam by anode heat. The steam is
condensed and the water recycles repetitively.
In typical closed-loop liquid-water-cooled systems, the maximum outlet
(hot) water temp must be held well below 100 degC to avoid hot-spot boiling
on the anode surface, which can and does create steam bubbles, which in
turn "insulate" the hot spot from the water so the hot spot gets even
hotter. This typically creates a temperature runaway and may lead to
destruction of tube and/or cooling components. Typical inlet (cool) water
from the chiller may be specified as </= say, 45 degC, and maximum outlet
water temp as 80 degC to avoid spot boiling. (I'm sure John Lyles can speak
with more experience on these issues.)
Anyway, in this example each gram of cooling water passing through the
tube's water jacket can absorb not more than (80-45) = 35 calories of heat.
Conversely, each gram of 45 degC water entering a vapor-cooled tube's
boiler absorbs approximately [540 + (100-45)] = 595 cal/gm while
vaporizing.
So vapor cooling requires passing only about 35/595 = 1/17 as much water
volume through the system as does water cooling.
Steam emanating from the boiler is converted back to water just as vapor is
condensed in a Kentucky still - generally with a copper condenser strongly
resembling an automobile radiator. The water returns to the reservoir and
continues to recycle by convection. At least in small systems like the
ALPHA SEVENTY, no pump is needed. Better yet, with the 6x10x1.5" condenser
used in the ALPHA 70V, to about 600W plate dissipation no FAN is needed to
cool the condenser - convection air does the job. If maximum power is run
for extended periods, all the steam can't be condensed by air convection
alone. Escaping steam is detected by a thermostat on the vent tube and
activates a "whisper fan," which supplies plenty of air to condense the
steam created by at least 1.5 kW of plate dissipation.
Vapor cooling is a joy to use for ham radio 'cuz it's virtually silent.
Downside is that the water supply must be very clean to keep leakage
current through the water in the supply tube (the anode is at +HV, the
reservoir at chassis ground) low enough to avoid heating to the boiling
point. If water in the supply tube boils, steam bubbles form & tend to
interrupt the water supply. Also, the water reservoir must be topped-off
occasionally, as a small amount of water is lost due to evaporation from
the vent.
Small impurities collect in the boiler, and at perhaps yearly intervals the
system must be taken down and cleaned. This isn't hard to do, and a
considerable number of ALPHA 70Vs (built in 1970-71) evidently are still
receiving regular TLC and use. But in a time when many of us never check
our cars' oil or water between 3000-5000 mile shop service intervals... air
cooling is the more practical approach for 1.5 to a few kW amps in today's
run-it-until-it-quits society!
Sorry to make this so long & rambling, guys, but maybe a few others besides
Jon will be interested enough to look up Jack Quinn's article.
73, Dick W0ID
-----Original Message-----
From: Jon Ogden [SMTP:jono@enteract.com]
Sent: Monday, August 03, 1998 3:36 PM
To: Richard W. Ehrhorn
Subject: RE: [AMPS] 3CV1500A7
>To the best of my recollection, Eimac decided to discontinue making the
>3CV1500A7 (actually a 3CX1000A7 with a boiler instead of air fins) some
>time in the late 70's.
Dick,
What is meant by "vapor" cooled as opposed to air cooled. And what do
you mean by a "boiler"?
Just curious as I was just a little knee biter in 1972 and don't know
much about tubes from back then.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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