Hi John,
Thanks so much for your input. This is exactly the kind of info I was
looking for.
I'm familiar with the Pex tubing, but never thought of using it. It
sure would hold it's round cross section better than the softer silicone.
I have a metal lathe, that I have used for winding wire on large coil
forms. (Unfortunately, its' slowest speed is way too fast to use the
machine under power. I turn the chuck by hand.) I was thinking of
doing something like a bifilar filament choke like you would find on a
grounded grid amp. Maybe on a diameter of 2 to 3 inches. 6 or 8 turns
on a 3 inch dia would be a bunch.
I will surely kick around the info you provided.
73 de Steve, NR4M
On 4/15/2018 1:36:PM, John Lyles wrote:
Steve,
You have 3600 VDC and want to keep the leakage current small, in the
uA, no mA. So if that is, say 100 uA, then R=3600/100e6, or 36 Megohms.
RCA recommended never lower than 10 Megohm path resistance to prevent
electrolysis in the water connections.
I use PEX tubing, used for home water pipes now, and it can be formed
into a coil by heating it carefully with one or two heat guns and
winding it around a wood dowel. Then it will retain it's form when
cooled off. If you screw up, just reheat and form it again. Amazing
stuff. I am not sure if it is available in 3/8 inch. Eimac recommends
against silicone tubing in their literature, although I am using red
siliconebased radiator hoses for megawatt amplifiers that I build for
work.
You have three variables, the ID of the insulated piping, the length
from HV to ground through it and the resistivity of the water. I use
resistivity not conductivity since the value is easy to understand and
the dimensional units cancel out correctly in the formula. The pipe is
a resistor of high value depending on what these variables are.
With 3/8 tubing chosen, I will assume 0.375 inch ID for this example,
but you can check the actual ID and correct if I guessed wrong.
Convert it to SI units, 0.95 cm. Also, pick a length of your pipes
that is convenient to fit in the cabinet, remembering that the hose
must be floated on insulator or an insulated form for most of the
length. I started with 12 inches, or 45.97 cm long per hose. 1
Megohmcm water chosen for this example. This is so short that coiling
might not be needed depending on the cabinet space.
Since the ID is 0.95 cm, area is 0.709 cm^2.
R = resistivity x (L/A)
L is 45.97 cm
A is 0.709 cm^2
resistivity is 1 Megohmcm
R= 64 Megohms
This is twice what we wanted, so you can use 500 Kohmcom water with
this length of tubing and diameter. If you double the length, the
current goes in half, etc. I would recommend not going to below 500
kohmcm water though.
Remember that this is per hose, so with two hoses (in and out) the
total current loading on the power supply from the water pipes is
twice what was calculated above. This is what a DC hipotter should
measure if the tube is off, no filament power applied.
Also, temp rise is dependent on flow and dissipation.
P (kW) = 0.2648 x (ToutTin) x Flow (GPM) for water
You don't want to create a very high linear water flow rate through
the tube and piping that is beyond acceptable practice as it is noisy
and can cause erosion as well as turbulent conditions in the anode. So
that weighs into your 3/8 tubing decision.
73
John
K5PRO
Date: Fri, 13 Apr 2018 21:40:20 0400
From: Steve Bookout <steve@nr4m.com>
Hello all,
...
My general plan was to use 3/8 inch silicone tubing, to and from, the
tube boiler.? In order to give myself some 'dielectric length', I was
going to wrap several turns of the tubing around a round form in a
single layer.? This would be kind of like coaxial choke some wrap on a
piece of PVC.? ??? I think I would rather have 3 or 4 feet of 'series
water', than have the HV only 10 inches of water from conductive
'stuff'.
Plan on putting a micro ammeter from a metal water fitting? to chassis
in order to measure the conductivity from the water to chassis, at some
point in the water circuit.
I know that really pure water is actually corrosive and will do it's
best to gain ions in order to get to some natural level of
minerals/contaminants.??? I built the coolers out of several pieces of
brass and hard silver brazed it all together.? I know the electrical
current will cause an etching or eroding effect of the materials in the
cooler, but I used what I had.?? At this point, I have no idea how often
I will changing? out the water.? Could be after only 10 hours of use, or
it may be 100 hours and the cooler may rot out in no time.
So, I need to know what the 'big boys' do in industry.? At what point,
in uS/cm, do I change out the water? The metering of the water
conductivity would be measure 3 or 4 feet from the tube boiler (@ 3600
volts)? How many inches (feet) of 3/8 inch dia distilled/low ion water,
in silicon tubing, do I need to have between 3600 volts and gnd (thru a
meter?)
...
73 de Steve, NR4M

Message: 4
Date: Fri, 13 Apr 2018 21:43:33 0400
From: Steve Bookout <steve@nr4m.com>
Subject: [Amps] more on water cooling an amp
I just realized that I could use my Hipot tester to test things and
help set it up.
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