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Sean,
Thanks again for taking the time to compose all that. There's a lot
of value there. I intend to measure cabin temp, relative humidity, and dew
point from the time I begin garage dives, and conduct some experiments in
the garage on managing the sub environment. In that situation I can
artificially create the adverse conditions, then see how successful I am at
mitigating them and at what cost.
Best regards,
Jim
In a message dated 3/15/2011 10:05:08 P.M. Central Daylight Time,
cast55@telus.net writes:
Not
being a sub owner, all I can offer is conjecture, but I will agree that
vessels with small energy budgets will have a difficult time with this, as
heating and cooling tend to constitute large power draws.
If you
are operating in cold water, the problem is a bit simpler to address,
as
it is relatively easy to insulate the hull with the exception of the bilge
in order to purposefully condense water on that surface and lower the RH,
or
to set up some sort of heat exchanger / keel cooling arrangement with a
condensate drain to do the same. In warmer water, this is much less
effective.
There are only two practical ways of removing moisture from
the air:
1) cool the air, reducing its moisture carrying capacity and
forcing it to
condense, or
2) Use hygroscopic materials /
desiccants to absorb the moisture.
I don't have much familiarity with
water absorbing materials, but certainly it
would be possible to set up a
moisture "scrubber" using desiccant media which
is replaceable just like
your CO2 scrubber. Of course, you run the risk of
having cabin air
which becomes uncomfortable because it is too dry, so it
would need to be
carefully designed to match the vessel's size and crew
complement.
Note also that using extremely dry oxygen in your life support
system will
help to reduce RH as it is added to the cabin air.
If you cool the air,
you can condense out some water, but air in a passive
condensing apparatus
is still at 100% RH until it is subsequently heated
beyond the
condensation temperature. This will happen as it mixes with the
other air in the cabin, or when heated by other sources (i.e. CO2 scrubber
is
exothermic, electronics will give off heat, and occupants give off
heat). You
need to maintain the cabin air at a comfortable
temperature for the occupants,
so the net heat gain/loss in the cabin
needs to be carefully designed. As far
as preventing viewport
fogging, if you are going to direct airflow against the
windows for this
purpose, it should ideally have low relative humidity.
Condensation
occurs when the air in close proximity to the window cools and
dumps its
moisture, so you can avoid this by maintaining a constant airflow
across
the window, but then you increase convective heat loss and cool the air
in
the cabin. The drier the air flow against the window, the slower it can
be
and still prevent condensation, and the slower it is, the less cooling
effect
it will have on the cabin air. In a high humidity cabin, one
possible flow
path would be through your condenser to remove moisture,
then through your
scrubber where the slight heat from the exothermic
reaction will keep RH below
100%, but the still relatively high RH is
actually advantageous to the
reaction, then to your electronics etc. to
hopefully gain heat and lower RH,
and then into the cabin via defog ports
near your windows.
Even if you don't painstakingly design to keep RH
around 50%, at minimum it is
important to try to keep it below 100%, not
only because this is generally
uncomfortable for the occupant(s), but also
because condensation causes
corrosion, and certainly may lead to failure
of electronic components. The
only liquid water in your sub should
be water that was designed to be there.
-Sean
On Tuesday
15 March 2011 11:51:32 you wrote:
> Sean,
>
> To pick up on
humidity control in general which you mentioned: This is a
>
topic which affects a number of things in a sub other than scrubber
>
performance and it's worthy of its own thread. In a small one or two man
> sub powered completely by batteries it seems to be difficult to
manage.
> I'd be very interested in what anyone has to offer on
the topic. We've
> had a prior discussion on a portable
cooling unit which would condense
> some of the cabin
moisture.
>
> Thanks,
> Jim
>
>
> In a
message dated 3/15/2011 12:25:50 P.M. Central Daylight Time,
>
cast55@telus.net writes:
>
> In a submersible, the problem
is more easily avoided, since it takes some
> time
> for the cabin
air to get close to 100% RH, and there are ways a designer
> can
condense out or otherwise remove moisture from the cabin air to
push
> it down.
> ABS standard is a relative humidity
between 30 and 70% RH, and this is
> probably what you should
shoot for. (50% RH is often quoted as a desirable
> target for
human comfort in terrestrial
buildings).
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