Hi Sean,
when I go to open your emails outlook express asks me to install an Arabic
language package, will that help me translate some of your technical
terms???
You were looking at building a deep diving suit. Am wondering what sort of
life support system they have. Regards Alan
----- Original Message -----
From: "Sean T. Stevenson" <cast55@telus.net>
To: <personal_submersibles@psubs.org>
Sent: Wednesday, March 16, 2011 4:04 PM
Subject: Re: [PSUBS-MAILIST] Humidity control
> 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).