>Vapor-phase cooling is even more efficient: you
actually get the water up to boiling, and _LOTS_ of energy is carried off
in those steam bubbles. "
With vapor-phase cooling, a rise in H2O temperature near the boiling point
is fine, but heat transfer efficiency begins to greatly suffer at the point
of hot-spot boiling. Steam bubbles create an insulation property and
diminish the heat transfer efficiency. As the heat transfer efficiency
creases, H2O temperature rises, creating even more bubbles until thermal
runaway results.
Typical heat absorption with vapor-phase cooling is roughly 600
calories/gram of H2O while vaporizing. By contrast, in a closed-loop H2O
system, approximately 35 calories of heat can be absorbed per gram of H2O.
Using these numbers, vapor-phase cooling requires a flow of only ~ 6% of the
same H2O volume as does a closed H2O system.
It would be interesting to compare the efficiency of a closed-loop ATF oil
solution to these two H2O methods. As to maintenance of a typical
vapor-phase cooling architecture in amateur service, it's very minimal. I
purge my distilled H2O at 12 month intervals and during the year, I never
see leakage rise to more than about 2 mA of current at 4KV. Nor have I
experienced any algae build-up in the Florida heat & humidity.
Last year, I rebuilt the entire plumbing system of an Alpha Seventy
amplifier. It was the first re-build done to the amp in 40 years. Total
cost for all the original OEM parts was less than $10 + an additional $40
for a custom bellows section and about 12-inches of Polypropylene line. The
condenser is about the size of a motorcycle radiator and uses soft-soldered
joints. It would probably take a very long time before it fails and repair
should be easy.
Paul, W9AC
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