[PSUBS-MAILIST] co2 scrubber

Mon Feb 13 07:45:39 EST 2017

Well, given the drastic impact on energy budgets that active air conditioning can have, I would definitely focus on what you can do passively first.  Moisture will preferentially condense on any surface which is cooler than the air temperature, and in a hull submerged in cold water, it is likely to be wet everywhere if the cabin air is humid. I like your idea of blowing air under the floor against an uninsulated hull, but in conjunction with completely insulating the hull above the floor, which will act to increase the temperature difference by maintaining a warmer cabin and lowering the relative humidity of the returning air.

I would not specifically feed dry air into your scrubber. As I mentioned in that post, moisture is actually part of the reaction (both as a solvent and as a reaction product), and it will lose effectiveness in dry air.

Just let the scrubber do its thing, and manage humidity separately.  That could be as simple as starting with a dry cabin (don't wear wet clothes when you get in) and allowing humidity to rise over the course of a dive. Dessicants can help too, and a relatively cheap one is calcium chloride (sidewalk ice melter).

Diving in cold water makes moisture removal easier. If you're in the tropics,  you're more likely to need an active solution.

Keep in mind that 30 - 70 % is the ABS recommendation, but your electronics are generally fine as long as they never get wet. Most specifications will state a maximum of 95% non-condensing for this reason. Human comfort is a different issue entirely. The greater the humidity, the greater the reported "comfort" temperature.

I might look at doing what I could passively through design first and foremost (incidentally, if you have any heat-generating electronics, try to avoid heat sinking them through the hull, as that heat is potentially useful), and then look as the most power efficient active dehumidifier I could find if that isn't sufficient to keep RH below 100% (or ideally below the ABS 70% limit if the power budget allows).

An important thing to keep in mind is that warm air holds more moisture,  so as long as your hull is colder than the other surfaces in the boat, condensation will preferentially occur there. This is a potential problem for someone who travels with their boat in the winter to submerge it in water which is warmer than the air temperature, because if the hull warms up faster than the installed equipment in the boat, the condensation will occur there. This is why I'm a fan of conformal coatings on the components of marine electronics.

Sean

On February 13, 2017 12:30:29 AM MST, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,
>in this email you are advising keeping humidity between 30 & 70%.
>Any ideas on the best way of doing that? I am going to use a humidity
>sensor & PLC, so have the option of automating functions.
>  Have thought of having some sought of valve that channels air through
>a filter prior to the scrubber, when humidity is high. 
>Another thought was blowing air under a raised floor so that moisture
>laden air was forced between the floor & hull, & condensed against the
>cold hull.  I have thought about peltier coolers also, as they have the
>dual
>option of being heaters by reversing the current. They are a bit
>expensive
>on power though.
>Alan
>
>Sent from my iPad
>
>> On 5/02/2017, at 2:31 PM, Sean T. Stevenson via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>> 
>> I am skeptical of the utility of the colour indicating absorbent. The
>reaction front in an axial scrubber is unlikely to be planar (unless
>perfectly diffused), but rather conical, so you could have breakthrough
>in the center before the indicating edge approaches the end of the
>canister, for example. I think actually measuring the CO2 accurately
>and using the rise in level as the indicator of scrubber breakthrough
>is the better course of action. One thing to note is that the colour
>change in the indicating absorbent is usually subtle, and submarine
>interiors are usually subdued light environments. In some media, the
>colour is readily apparent immediately after reaction, but the colour
>fades when left to stand for some time.  Also, even if you have an
>acrylic or polycarbonate canister, dust from the scrubber media can
>accumulate on the inner wall and make visual inspection difficult.
>> 
>> If I were me, I'd be inclined to design a scrubber that would permit
>relatively easy media or canister changes in situ, with a designed
>capacity equal to the mission time + reserve time (or split between
>multiple scrubbers), and then keep one or two additional sealed
>canisters or media changes tucked away on board.  That way, you can
>make effective use of the media by exhausting it right to breakthrough.
>If you make short dives, you can do several on the same canister, and
>if you reach breakthrough (arbitrarily set but below the permissible
>limit), you change it out. I would choose a radial flow design, with
>airflow from outside to inside for maximum efficiency, and consider
>addressing the operating humidity of the sub to keep it between 30% and
>70% RH.  Some moisture is actually required for the scrubber to
>function, as the chemical reaction actually occurs in aqueous solution
>on the surface of the media granules. I would definitely make a point
>of ke! eping a partially used scrubber sealed off from atmospheric air
>when not in use. In the absence of a sealed container, simply keeping
>your hatch closed would suffice, as once the CO2 in the cabin is all
>reacted it is inert. Atmospheric air has 400 ppm CO2 in it, which will
>consume a scrubber over time if left exposed.
>> 
>> Sean
>> 
>> 
>>> On February 4, 2017 5:45:26 PM MST, Brian Cox via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> I'd like to do a radial scrubber design but I also would like to use
>the indicating SodaSorb, so I can see the color change.  Being able to
>see the color change with a radial design would be a challenge !
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: "Sean T. Stevenson via Personal_Submersibles"
><personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] co2 scrubber
>>> Date: Sat, 04 Feb 2017 14:53:27 -0700
>>> 
>>> Yes. For the chemical reaction to work properly, the air needs some
>dwell time against the scrubber media. This may be specified in the
>media datasheets. Regardless, if your fan is too strong and you're
>blowing tons of air through it, you may just dry it out while scrubbing
>really inefficiently.  If you! slow it down, you give the CO2 time to
>react and the bed gets more efficient. This is one of the reasons why
>radial flow scrubbers have an advantage - the surface area is huge, so
>the actual flow velocity through the bed is minimal in comparison to an
>axial flow design. You only need enough airflow to remove CO2 faster
>than you produce it, and ideally to process the entire air volume in
>your cabin a few times an hour. Slow, large diameter fans will be more
>effective than a high speed but small fan, and probably easier on your
>batteries too.
>>> 
>>> Sean 
>>> 
>>> 
>>> On February 4, 2017 2:38:32 PM MST, Brian Cox via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>>> Is there such a thing as to TOO much air flow through a scrubber ??
>>>  
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: Cliff Redus via Personal_Submersibles
><personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] co2 scrubber
>>> Date: Fri, 3 Feb 2017 09:34:40 -0600
>>> 
>>> Brian, see
>http://www.ntz-filter.com/Radial_Filtration_versus_Axial_Filtration for
>picture which is worth at least a hundred words.
>>> 
>>> Cliff
>>> 
>>> 
>>> 
>>> On Thu, Feb 2, 2017 at 11:36 PM, Brian Cox via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>>> What is the difference between a Axial scrubber and a radial ?
>>>  
>>> Brian
>>> 
>>> --- personal_submersibles at psubs.org wrote:
>>> 
>>> From: via Personal_Submersibles <personal_submersibles at psubs.org>
>>> To: Personal Submersibles General Discussion
><personal_submersibles at psubs.org>
>>> Subject: Re: [PSUBS-MAILIST] co2 scrubber
>>> Date: Thu, 2 Feb 2017 16:29:34 -0600
>>> 
>>> Started life with axial scrubber but now use radial design.  Has !
>worked much better for me.
>>> 
>>> Cliff
>>> 
>>> Sent from my iPad
>>> 
>>> On Feb 2, 2017, at 1:06 PM, hank pronk via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>>> 
>>> Thanks' guys, that helps.  
>>> Cliff, I assume you have an axial scrubber
>>> Hank
>>> _______________________________________________
>>> Personal_Submersibles mailing list
>>> Personal_Submersibles at psubs.org
>>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> _______________________________________________
>Personal_Submersibles mailing list Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> 
>>> _______________________________________________
>>> Personal_Submersibles mailing list
>>> Personal_Submersibles at psubs.org
>>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> 
>>> 
>>> _______________________________________________
>Personal_Submersibles mailing list Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> 
>>> Personal_Submersibles mailing list
>>> Personal_Submersibles at psubs.org
>>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> _______________________________________________
>Personal_Submersibles mailing list Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>> 
>>> Personal_Submersibles mailing list
>>> Personal_Submersibles at psubs.org
>>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>> _______________________________________________
>> Personal_Submersibles mailing list
>> Personal_Submersibles at psubs.org
>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
>
>------------------------------------------------------------------------
>
>_______________________________________________
>Personal_Submersibles mailing list
>Personal_Submersibles at psubs.org
>http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.whoweb.com/pipermail/personal_submersibles/attachments/20170213/6367b01b/attachment-0001.html>