RE: [PSUBS-MAILIST] open source scrubber design

["Smyth, Alec" <Alec.Smyth@compuware.com>]

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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