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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