Re: [PSUBS-MAILIST] Humidity control

["Sean T. Stevenson" <cast55@telus.net>]

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).



************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003.  Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.

If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
	removeme-personal_submersibles@psubs.org

Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.

PSUBS.ORG
PO Box 53
Weare, NH  03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************