On 6/8/2017 5:08 PM, Bob kb8tq wrote:

Exactly right Bob.  The 10811 guys used to go nuts
about keeping water out of their vacuum system.
There were certain temperatures known as "water
points" at which some water was released.
The retained water was in spite of the temperature
already being above 100 degrees C (boiling).
It has something to do with monolayers of
water molecules not boiling away.

Rick N6RK

Bob wrote:

The insides of mains alternators are almost entirely metal -- tons and
tons of copper tubing, and the casings and rotor shaft are steel.  And
the alternators must operate at a relative humidity of absolute zero.
The problem is far from insurmountable, even at that huge scale.
Dealing with it should be very much easier at a scale of cubic centimeters.

They use condensers to remove the water during the hydrogen purge cycle,
which (for mains alternators) lasts several weeks.  I watched the
procedure several times 25 years ago, but I don't recall the particulars
now.

Best regards,

Charles

The difficulty with hydrogen is to keep it where you want it. It does not
take very much for it to leak out (or in, as the case may be)

On Jun 8, 2017 4:58 PM, "Alan Melia" alan.melia@btinternet.com wrote:

On 6/8/17 1:19 PM, Bob kb8tq wrote:

basically, when the mean free path gets to be shorter than the distance
to the wall, the thermal conductivity drops off.

MFP = 65 nm at 1013 hPa = 760 torr

So at 10-4 Pa/0.75E-3 micron (start of very high vacuum) the MFP is 65 cm

High vacuum usually starts around 0.1 Pa (close to 1 micron), where the
MFP is 65 mm - this is where the MFP is comparable to the size of the
stuff you're pumping down, and where you can't use a "pump", but rather
you need something that flings the air molecules toward the exit
(diffusion or turbo molecular pump) or something that is like flypaper
for molecules (sorption, cold finger, etc.)

If you've got a "refrigeration" vacuum pump, they pull down to about
30-40 microns - MFP is a few millimeters

Hi

My guess is that crystals are more sensitive to moisture than the alternators…. The level of “clean”
you need in a precision crystal enclosure is way beyond what is required in a number of other areas.
One layer of water molecules is way to much in a modern crystal.  can and do use crystals as detectors
of low levels of “stuff” for this very reason.

Bob

Hi

Cold traps and vac-ion pumps were very common on precision crystal seal setups 50 years ago.
They have gotten better since then….

Bob

In message 593A4677.5080509@yandex.com, Charles Steinmetz writes:

This is close to to being "not even wrong".

There is no requirement for "a relative humidity of absolute zero",
which is theoretically impossible if there is any iron around.

There are however stringent requirements for non-corrosion, dielectric
potential, cooling, corona behaviour etc. etc.

Cooling the stator is a no-brainer, it is almost always water-cooled.

If they could afford to use helium to cool the rotor, they would
jump on it instantly, but the cost is prohibitive at their leakage
rates.

Pure water would also be workable, but its high density means
it cannot be used to cool the rotor without a serious hit to
efficiency.

Next down the line is hydrogen, which comes with a shitload of issues.

Apart from all the obvious issues, lube-oil degradation, polymer
degradation, fire-risk, risk of explosion, health-risk etc.  hydrogen
has "interesting" solubility in metals.

If you want to purify hydrogen, you press it trhough a filter
consisting of a solid slab of palladium, and that takes a lot less
pressure than you would expect.

For iron in particular, hydrogen means embrittlement, so a
major focus in rotor design is to keep the hydrogen away from
the iron.

If you Google "generator hydrogen seal" and and you will find
little love for hydrogen cooling.

In 1993 Siemens put the first 170MVA air-cooled generator optimized
by computer simulations of the flow-fields and since then the
hydrogen cooling has been confined to an ever-decreasing top tier
of name-plate power.

Today fan cooling will take you to approx 400MVA and pressured air
cooling will take you to about 600MVA.

Above that, you are, almost by definition, in a nuclear power plant,
and all problems from hydrogen cooling of your single huge generator
pale in comparison to having a handful of smaller generators in
parallel.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Poul-Henning wrote:

No argument there.  But other methods all have their own shitloads -- it
just depends on which shitload the designers dislike the least.

Certainly there are other ways, and hydrogen cooling may have decreased
in popularity in the decades since I was paying attention (I consulted
for a turbine generator manufacturer in the late '80s). But there are
lots of hydrogen-cooled alternators in service, and it appears that they
are still being built.

Best regards,

Charles

My memory of high vacuum work is that you need to pump for 4 hours
at 300C to remove the water monolayer from glass.
On top of the that water monolayer is another water monolayer that comes off more easily,
and on top of that another………..

cheers,
Neville Michie

Hi

You can’t quite process a crystal at 300C, but you can get close.

Bob