Cooling and insulating fluid

[Robert Baer]

I would like maximum thermal transfer (due to thermal circulation in
an enclosed tube) as well as good electrical insulation to (say) 25KV
minimum.

The tube is 1.5" diameter OD by about 6" long.
Two PCBs will be stacked end-to-end, and centrally located; their
dimension is 1.17 wide by 2.850 tall.
There are 168 zeners on each board for about 25KV zener regulation;
at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

Would the Cargill FR3 transformer oil be the best
(www.cargill.com/fr3fluid/) to use; better than mineral oil-based
transformer oils made from either naphtha or paraffin?

All comments and suggestions are appreciated.

 

[Bill Sloman]

   I would like maximum thermal transfer (due to thermal circulation in
an enclosed tube) as well as good electrical insulation to (say) 25KV
minimum.

   The tube is 1.5" diameter OD by about 6" long.
   Two PCBs will be stacked end-to-end, and centrally located; their
dimension is 1.17 wide by 2.850 tall.
   There are 168 zeners on each board for about 25KV zener regulation;
at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

   Would the Cargill FR3 transformer oil be the best
(www.cargill.com/fr3fluid/) to use; better than mineral oil-based
transformer oils made from either naphtha or paraffin?

   All comments and suggestions are appreciated.

De-ionised water would work. I've seen that used to cool the floating
anode of an X-ray tube running at that kind of voltage. It's got a
higher heat capacity and a lower viscosity than most oils.

An alternative would be a heat-pipe, if you could guarantee that the
fluid being evaporated could be spread evenly over all your zeners.

Plugging them all into a block of a alumia - which does have a
respectable thermal conductivity - 29 W/m/K - could work. Aluminium is
better - at 201 - as is copper at 385 but both conduct. Diamond is
brilliant (pun intended) at 900, and it is a good insulator - a vapour-
deposited layer might be worth the trouble and expense if you were
really pushing the state of the art.

 

[Jasen Betts]

I would like maximum thermal transfer (due to thermal circulation in
an enclosed tube) as well as good electrical insulation to (say) 25KV
minimum.

some sort of boiling liquid is going to do better than an unforced circulating
liquid. build a heat-pipe.

The tube is 1.5" diameter OD by about 6" long.
Two PCBs will be stacked end-to-end, and centrally located; their
dimension is 1.17 wide by 2.850 tall.
There are 168 zeners on each board for about 25KV zener regulation;
at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

25W is a lava-lamp scale energy flow. I don't think there's any need to
chase the maximum

 

[miso]

Will they sell you a small quantity? That is always the problem with
"cool stuff". Often it is easier to sweet talk a rep than to buy the stuff.

That said, I've always seen mineral oil used, but that doesn't mean FR3
isn't better. It is just a matter of availability.

I know zip about regulating 25kV, but meditating on such a string of
zeners, the first thing that comes to mind is a string of devices just
needs one device to fail for the whole string to fail. Is failure
tolerable for your application. I means, if nothing catches fire or
blows up, that might be OK. If a zener failing means you have a IED, I'd
consider a back up safety scheme.

Failure is an option as long as lawyers don't get involved.

I saw the post about using water, but you might want to consider
expansion. I don't know what it takes to boil water in a small tube.
Well actually I could compute it, but well you know...that would be work.

 

[Bill Sloman]

How long would the water remain de-ionised when in contact with
components, leads etc?

Depends on the metals involved, and whether the surfaces had been
varnished/potted before being exposed to the water.

Dutch plumbers fill regular central heating systems with clean water,
throw in an flexible expansion chamber to take thermal expansion and
contraction, and leave them sealed, with no corrosion inhibitor. It
works fine if you aren't silly enough to add aluminium radiators to
the copper pipes and the normal pressed steel radiators.

 

[Syd Rumpo]

I would like maximum thermal transfer (due to thermal circulation in
an enclosed tube) as well as good electrical insulation to (say) 25KV
minimum.

The tube is 1.5" diameter OD by about 6" long.
Two PCBs will be stacked end-to-end, and centrally located; their
dimension is 1.17 wide by 2.850 tall.
There are 168 zeners on each board for about 25KV zener regulation;
at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

Would the Cargill FR3 transformer oil be the best
(www.cargill.com/fr3fluid/) to use; better than mineral oil-based
transformer oils made from either naphtha or paraffin?

All comments and suggestions are appreciated.

This is going to be running at a high temperature, so perhaps something
which is solid or waxy at room temperature would make sense from the
mess/handling point of view?

As you're relying on density change with temperature to provide
circulation, maybe this characteristic is the most important
characteristic to consider. Does transformer oil circulate? You'll need
some expansion room too.

Cheers

 

[Syd Rumpo]

Diamond is
brilliant (pun intended) at 900, and it is a good insulator - a vapour-
deposited layer might be worth the trouble and expense if you were
really pushing the state of the art.

I wonder how well a diamond paste would work?

Cheers

 

[notbob]

All comments and suggestions are appreciated.

3.2 beer is an insulting liquid

 

[Bill Sloman]

Not long. The HV would force some current through even the cleanest
water. The current will tear apart metal surfaces and fill the water
with metal ions, and the process will quickly run away.

That didn't seem to happen with the water-cooled floating-anode X-ray
source that I was (marginally) invovled with at Nijmegen University,
back in the late 1990's. there certainly wasn't enough current flowing
to mechanically damage any of the surfaces involved. I'd have expected
the charge carriers to be (hydrated) hydrogen and hydroxyl ions -
which form spontaneously in water at any temperature about absolute
zero, at a concentration of about 10^-7M at room temperature. CO2
diffusing in from the atmosphere will push this up a bit, and regular
distilled water has a conductivity of the order of 1uS/cm due to the
carbonic acid content from the CO2 in the air.

 

[George Herold]

3.2 beer is an insulting liquid

Darn don't mention beer at ~4:30 on Friday.
I might have to go home and 'insulate' myself right now.

George H.

 

[Martin Riddle]

I would like maximum thermal transfer (due to thermal circulation in
an enclosed tube) as well as good electrical insulation to (say) 25KV
minimum.

The tube is 1.5" diameter OD by about 6" long.
Two PCBs will be stacked end-to-end, and centrally located; their
dimension is 1.17 wide by 2.850 tall.
There are 168 zeners on each board for about 25KV zener regulation;
at 1mA this would dissipate 25 watts (this is near max of guesstimate use).

Would the Cargill FR3 transformer oil be the best
(www.cargill.com/fr3fluid/) to use; better than mineral oil-based
transformer oils made from either naphtha or paraffin?

All comments and suggestions are appreciated.

ATSM1816 is a 0.100" gap.
This is similar to Shell Diala AX which is about 32kv for ATSM 1816.

It will be fine, as long as you have the right spacings and creepage
distances.
You need a expansion thingy there as it will probably expand 8% or so
from 25c to 40c.
and make sure your seals are Buna N.

Oh, and water is only good for making resistors
<http://home.earthlink.net/~jimlux/hv/rwater.htm>

Cheers

 

[Tim Williams]

De-ionised water would work. I've seen that used to cool the floating
anode of an X-ray tube running at that kind of voltage. It's got a
higher heat capacity and a lower viscosity than most oils.

Yeah, with ten feet of looped hose running back from the anode (maybe
less, but always some length, even if the DI is fresh).

At mA, inside a case, without pump, filter or replacement, you're nuts.

DI goes bad after a while, because there are always ions to dissolve, even
from nonobvious sources. Analytical chemistry texts go into grotesque
detail when ultrapure water is desired: even quartz glass apparatus
contains enough traces of sodium to foul up a sensitive ICAA test. The
solubility of silica alone becomes noticable.

As oil goes, how well it cools depends on how well convection works. In a
tight case, even water will end up significantly worse than potting
compound. A good epoxy or silicone potting compound, advertised for high
thermal conductivity ("high" meaning ~1 W/(m.K) or thereabouts), will do
better than stagnant water.

Tim

 

[Bill Sloman]

Robert is proposing 25 KV over about 6 inches. That's 350 times the
voltage gradient of the RCA transmitter, so assuming linear scaling
Why?

he's have to flush it about every day. And I don't think he'd include
a circulating system or purity testing. It wouldn't last long.

Relax. You don't know what you are talking about, and it's painfully
obvious. Don't go to the trouble of reminding us about this - it
wastes bandwidth and makes you look silly.

 

[Bill Sloman]

ATSM1816 is a 0.100" gap.
This is similar to Shell Diala AX which is about 32kv for ATSM 1816.

It will be fine, as long as you have the right spacings and creepage
distances.
You need a expansion thingy there as it will probably expand 8% or so
from 25c to 40c.
and make sure your seals are Buna N.

Oh, and water is only good for making resistors
<http://home.earthlink.net/~jimlux/hv/rwater.htm>

You seem to have stopped paying attention in your chemistry class at
about the same point that John Larkin did. Your "water" resistors are
filled with salty water.

 

[George Herold]

Hey that's cool. Would someone sell me a saltwater resistor?
(Price ~$1 each)
It'd be nice to show that it has the same noise as a metal resistor.

George H.

 

[Bill Sloman]

Hey that's cool.  Would someone sell me a saltwater resistor?
(Price ~$1 each)
It'd be nice to show that it has the same noise as a metal resistor.

Top posting? Too many insulating beers? With salt water resistors I'd
always be a bit worried about the processes going on at the
electrodes.Warburg impedances can be disconcerting.

http://en.wikipedia.org/wiki/Warburg_element

 

[Bill Sloman]

Yeah, with ten feet of looped hose running back from the anode (maybe
less, but always some length, even if the DI is fresh).

At mA, inside a case, without pump, filter or replacement, you're nuts.

DI goes bad after a while, because there are always ions to dissolve, even
from non-obvious sources.  Analytical chemistry texts go into grotesque
detail when ultrapure water is desired: even quartz glass apparatus
contains enough traces of sodium to foul up a sensitive ICAA test.  The
solubility of silica alone becomes noticeable.

This isn't trace element analysis. The 0.1uM concentration of hydrogen
and hydroxyl ions you get in pure water at room temperature would be
embarrassingly high for a trace element ion.

As oil goes, how well it cools depends on how well convection works.  In a
tight case, even water will end up significantly worse than potting
compound.  A good epoxy or silicone potting compound, advertised for high
thermal conductivity ("high" meaning ~1 W/(m.K) or thereabouts), will do
better than stagnant water.

Water is rarely stagnant - convection is pretty much inevitable, and
because water is less viscous than pretty much every oil, it convects
faster, and it's higher heat capacity shifts more heat per unit
volume. It's also got a relatively high thermal conductivity at 0.591
W/m/K which is about a factor of four higher than paraffin oil - 0.15
- and more than twice that of paraffin wax - 0.25.

A good - heavily loaded - potting compound should do better than
stagnant water, but only if it actually does fill the volume involved.
You've got to evacuate the volume you want to fill and de-air the
potting compound before you let it into the - evacuated - volume.
Water does have a conveniently low viscosity.

Farnell does silicone potting compounds with a thermal conductivity of
0.9 W/m/K which isn't all that much better than water

http://www.farnell.com/datasheets/1633577.pdf

http://www.farnell.com/datasheets/515246.pdf

There's an epoxy which does appreciablyt better at 1.25 W/m/k, but
it's not cheap.

http://www.farnell.com/datasheets/1484800.pdf

 

[Fred Abse]

Maybe a DOT 5 brake fluid?

Have you seen what that does to paint? God only knows what it does to FR4
and component encapsulants longterm.

 

[Fred Abse]

3.2 beer is an insulting liquid

"Having the promise of intoxication, without actually delivering"
(Garrison Keillor)

 

[Jasen Betts]

Have you seen what that does to paint? God only knows what it does to FR4
and component encapsulants longterm.

dot 5 is paint safe, it's a different formulation, mostly silicone.