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Re: [PSUBS-MAILIST] open source scrubber design
Remember the scrubber reaction:
CO2 (g) + Ca(OH)2 (s) -----> CaCO3 (s) + H2O (l)
in the presence of NaOH as a (net non-consumed) catalyst.
Not only is water evolved from the reaction, but excess water in the form of
condensation will prompt some of the NaOH to go into solution, so any
condensate drippings you get from the scrubber bed itself are caustic.
Dealing with this in a rebreather is problematic because the diver's exhaled
gas is at 100% relative humidity to begin with, so rebreather designers play
tricks with counterlung and scrubber water traps / condensing surfaces,
hydrophobic membranes, etc. The latter ensures that at worst, the air passing
through the scrubber is at 100% relative humidity non-condensing, but then
since the scrubber reaction is exothermic, the additional heat pushes the RH
down (sufficiently to account for the additional water produced by the
reaction). Where it does not, you get condensation, although this can happen
on a very small scale where the condensate is not large enough to coalesce
into drips and cause a problem, but merely "dampens" the scrubber bed and
reduces the effective surface area for scrubbing, so that the scrubber doesn't
last as long as it could under ideal conditions.
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). One thing to note is that
a scrubber will not function effectively in completely dry air (0% RH), since
the first stage of the reaction requires that the CO2 in the air first
dissolve into aqueous solution before it can react with the NaOH. Thus, the
higher end of the humidity range actually provides more opportunity for
scrubbing, provided you keep the air below the condensation threshold.
-Sean
On Tuesday 15 March 2011 07:29:16 you wrote:
> Alec,
>
> The comment about placing the fan near my feet was more about just getting
> the noise away from my head especially if the acoustics of the canopy turn
> out to be like a sound chamber. I'm expecting that the inlet for the
> scrubber would be somewhere near my head. I like the concept of having
> the fan downstream in the configuration to draw air through the
> scrubber(s) instead of pushing it through. I also want to consider
> electrical interference with other instruments and components in placing
> the fan.
>
> Since air exhaled during respiration contains moisture and the air in a
> sub is often humid to begin with, I'm hoping there would be some method of
> having the air as dry as possible prior to entering the scrubber, but I
> have no specific concept or design in mind for accomplishing that goal.
>
> Depending on the temperature and moisture content of the air leaving the
> scrubber system, it might be useful to duct it toward viewports to prevent
> fogging.
>
> I probably should drop out of discussions until after the April 15th tax
> deadline, so if you don't hear from me for a bit, that's why. I'll catch
> up later.
>
> Cheers,
> Jim
>
>
> In a message dated 3/15/2011 8:42:29 A.M. Central Daylight Time,
> Alec.Smyth@compuware.com writes:
>
> From prior scrubber experiments, the agent gets soggy and drips. There
> would be condensation on the scrubber insides for sure. But water would
> still not run uphill, so if the fan is mounted at the highest point I
> don't see a need for a water trap. Your comment about putting the fan at
> your feet sounds like the fan might be under the scrubber. Is that
> correct? In this present configuration, the scrubber is a vertical
> cylinder (actually two concentric cylinders) with the fan on top, drawing
> air through the center. The bottom end of the cylinder can hold water.
>
> BTW here's last night's update. My new computer fans didn't arrive
> yesterday but I poked around and found I had a squirrel cage fan on hand,
> just like the ones Cliff had referred to only a bit bigger. Cliff's
> references were 75mm by 75mm. The one I had was 120mm by 120mm, 23 cfm,
> 50db. I adapted it to the scrubber and tested with and without a
> Sofnolime load. BTW the scrubber capacity turned out to be a hair under
> 10 lbs of Sofnolime. I was very surprised by the result, compared to my
> earlier tests with computer fans. Cliff, in a word you were right!
> Although I was not measuring anything, seat-of-the-pants there is no
> noticeable decrease in airflow when the scrubber is loaded. The fan
> didn't even notice the resistance caused by drawing air through the
> Sofnolime, and was putting was putting out a gale. Probably too much so,
> I can't imagine I would need quite that much airflow. So last night I
> ordered a squirrel cage fan with about half the airflow for another test.
> The 50db noise level is tolerable but tiresome, and my goal is to find a
> fan that has sufficient airflow yet no more than what is needed, in order
> to minimize the noise.
>
> To be continued...
>
>
> Alec
>
>
> ____________________________________
> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org] On Behalf Of
> JimToddPsub@aol.com Sent: Monday, March 14, 2011 7:59 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] open source scrubber design
>
>
>
> Alec,
>
> I'm tending toward having the fans draw the air through the scrubbers
> rather than push it. I think that might make it easier to design for even
> airflow through the material. It also makes it easier for the fans to be
> located nearer my feet instead of near my head.
>
> I would think the scrubbers could be subject to degradation from moisture
> intrusion or some other causes I'm not aware of, so I'm planning
> redundancy there, too. The question I have is if it would be
> prudent/practical to install a filter/water trap upstream. Hopefully that
> could be done without significantly impeding airflow. This brings up the
> related topic of humidity control in the sub in general.
>
> Comments?
>
> Thanks,
> Jim
>
>
> In a message dated 3/14/2011 4:53:04 P.M. Central Daylight Time,
> Alec.Smyth@compuware.com writes:
>
> Excellent points gentlemen, thanks. I had initially mounted the fans to
> blow into the center cylinder, but will now reverse them based on Sean's
> suggestion. Note I say "fans" because I'm mounting two of them in
> series, for redundancy, although only one would normally be used. The
> scrubber itself has no moving parts, so I figure the fan is all that can
> fail.
>
> Cliff, I agree squirrel cage fans would be a better choice from a
> performance perspective. However, I'm so tight on space that I'm at
> least giving axial a try before discarding the idea. Also, Deep Worker
> uses axial. Note I'm not shooting for 72 hours, and because Snoopy has
> 12V main propulsion, the whole boat runs off one big battery bank so
> current draw should not be an issue. I'll have to see about the pressure
> and noise. I've convinced my daughter to join me in Snoopy for some
> hours, so we can make it a two person test. However, I am still awaiting
> delivery of the fans, which should be here any day.
>
> Jon, the photo you linked to looks functionally identical to the eBay
> filters, except the end pieces are plastic while these are aluminum. The
> thickness of scrubbing agent is about 2". The one I'm using is eBay item
> #400187881680.
>
> I'll report back post-test.
>
>
> Alec
>
>
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> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T.
> Stevenson
> Sent: Monday, March 14, 2011 4:01 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] open source scrubber design
>
> The scrubber is a radial flow design, with the flow direction from the
> outer diameter to the inner one. This makes most efficient use of the
> scrubber material, as the flow area is greatest at the outer diameter
> when the gas contains more CO2, and reduces as you approach the inner
> tube, as CO2 is removed. Also, the inlet flow area of the scrubber
> (outer diameter x pi x
> length) is huge in comparison to the minimum scrubber inlet diameter,
> making the velocity (assuming even pressure distribution) almost nil
> through the absorbent. One thing to be wary of when comparing
> submersible scrubbers to rebreather scrubbers, is that the gas flow
> through the rebreather device is cyclic - there is a very small pressure
> differential between the gas in the exhale counterlung and the gas in
> the inhale counterlung, which leads to slow gas movement through the
> scrubber - the "dwell time" of gas inside the scrubber is only
> interrupted when the inhale counterlung collapses and gas must be drawn
> through. Submersibles, on the other hand, must use steady flow and so
> the gas velocity through the material must be accounted for - of course,
> you can make up for higher speeds with more passes, but the least noise
> and power consumption solution is a slow flow through a large device.
>
> -Sean
>
> On Monday 14 March 2011 12:12:54 you wrote:
> > Alec's test results will be very interesting to follow. These
> > canister units, if I understand the way Alec intends to use it, are a
> > radial design. One of the potential problems I see is that there is
> > only 1-1.5 inches of material in the void between the canister walls
> > which makes me question whether that will provide adequate "dwell
> > time" for the CO2 to pass over the material and be absorbed
> > efficiently. It will also be interesting to hear from Alec how
> > difficult the units are to pack with material.
> >
> > Alec, depending upon your results I would also look a modifying the
> > can somewhat by adding a center tube, maybe 1 inch in diameter through
> >
> > the axial center of the can (like this
> > http://www.flickr.com/photos/billreals/2966584447/ ). Add sodasorb
> > into the interior (and between the canister walls if you want) and
> > feed the air flow through the center tube via a centrifugal fan. This
> >
> > would allow more dwell time through more material.
> >
> > Jon
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