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Thanks, Sean,
I had meant to ask if the scrubber reaction produces water, but then
completely forgot to. Thanks for addressing the possibility of sodium
hydroxide (NaOH) or lye being in the outflow from the scrubber as well as
residue from the Sodasorb or other material.
Would you consider it to be better to not have the scrubber intake too
close to the crew member's face in order to reduce the
concentration of the moisture entering the scrubber? Obviously that
would mean the CO2 would be more dilute as well since it would mix more with
cabin air prior to the scrubber intake. Would that make the scrubber's
operation more "evened out" with less chance of the undesirable products?
Thanks,
Jim
In a message dated 3/15/2011 12:25:50 P.M. Central Daylight Time,
cast55@telus.net writes:
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
>
>
> The contents of this
e-mail are intended for the named addressee only. It
> contains
information that may be confidential. Unless you are the named
>
addressee or an authorized designee, you may not copy or use it,
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> disclose it to anyone else. If you received it in error
please notify us
> immediately and then destroy it.
>
> 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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