Radial Scrubber Design Questions

S. Marc

I read with great interest the article on the radial scrubber designed by Ken (Skipbreather), and have looked very closely at the pics of the scrubbers for the CIS and the one designed by Golem, and have a couple of quick questions as I am thinking of designing one similar to Ken's for one of my homebuilds.

On an "in to out design" radial scrubber, why do the perforations in both the scrubber outer and the exhale tube / inner screen (inside the scrubber itself) start very near the top. Would that not mean that a lot of the exhaled gas passes just through the top/upper portion of the sorb?

Keep in mind that I am a novice, but I was thinking that perhaps the first part of the exhale tube / inner screen ought not to have perforations so as to ensure that the exhaled gas travels further to ensure it has more dwell time when travelling through the sorb and that it passes through more sorb before exiting the scrubber, thus giving more duration and less chance of breakthrough in high workload situations.

Would this be more of a factor depending on the diver's position in the water, say vertical as opposed to horizontal, or does it make no difference?

If the diver is horizontal, would not the majority of the exhaled gas only travel through the upper part of the scrubber and not the lower part? As in the part of the scrubber above the exhale tube / inner screen due to the exhaled gas rising through the upper portion of the scrubber bed?

I was really impressed with Ken's design and plan on trying to replicate it in a Classic Kiss style homebuild of mine, using a much longer center section (I have machined a center section that is 6" ID x 15" long with a potential scrubber length of up to a maximum of 11" after space for the absorbent system and lid with sensors is fitted).

Anyone think there is anything to this or am I wasting my time?

Marc

webhead

I will admit that I had the same thought when I started seeing radial scrubbers for the Meg (Cis, ISC and Golum's stuff). I would think a radial scrubber for the Inspo/Evo would be more efficent since the gas enters on one end of the scrubber, passes thru the sorb and exits the canister on the other end. Not so on the Meg.

Curious to hear other's thoughts on this and an explaination as to why this is not a problem (i.e. top part of the sorb being only part that gets used).

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silent running

When emptying my in-out radial, I see a uniform pattern-used sorb clumped around the center tube-inlet from top to bottom.

You should remember that the open/no restrictions inner tube will fill with gas-lower pressure-before moving out as it fills top to bottom and then hits the sorb/more restrictive part-higher prerssure-then filling the stack and flowing through to the lower pressure on the outside of the scrubber. If anything, given gravity, the gas would sink more to the bottom of the inlet tube given that we are not usually perfectly horizontal or hardly ever upside down. That's the best explanation I have right now for why the flow is distributed roughly even from top to bottom: there is a fairly uniform pressure all along the surface of the inner tube during exhale. I will inquire more with those who know more when I can...

webhead

Thanks, SR. Interesting insight. I like the differential pressure idea pushing the as the full length of the inlet side of the sorb.

And the prism feeds gas into and out of the scrubber from the top (haven't seen that unit in peron yet)?

Does anyone make a wind tunnel for testing the fluid dynamics of a scrubber?

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whoizrob

WHen you have a full scuba tank is there more pressure at any given point in the cylinder? No, because gas will fill a container and exert the same pressure at all points. Granted the scrubber tube has holes in it, but so does an aluminum scuba tank the holes simply act as a restriction providing back-pressure which will act the same way more or less. I suppose at high flow rates (greater than you could provide) you might see somthing akin to what happens when you poke holes into a bottle of water at varying heights with head pressure being akin to high flow rates but I'd assume that with our desired WOB and low volume the differences would be immeasureable.

HTH,

Rob

welderbob

does anyone make a wind tunnel for testing the fluid dynamics of a scrubber? [/quote]



No, but Peter @ Stem Machines uses a very expensive engineering program that simulates what is going on inside the loop.

OceanOpportunity

I've been working on research regarding scrubber design with several students at Univ of Rhode island and MIT for the better part of a year now. A principle thing that should be understood is that bedlife is is a direct function of many variables. The amount of absorbant dictates the maximum CO2 that can be absorbed. The efficiency of the scrubber is another story. In a basic axial design, it is easy to calculate using well established TBL formulas, assuming the scrubber bed allows for a minimum 1sec dwell time for a full breath of gas. The idea with radial scrubbers is that they end up more efficient by reducing velocity across the scrubber bed, hence increasing dwell time, but also resistence through the bed for a comparable amount of material in an axial design. This is why pound for pound, a well designed radial will be more efficient, but in the end, both will absorb the same amount of CO2.

I guess my point is that arbitrarily designing a radial scrubber is a bit scary unless you can model the gas flow, and your specific design does what it is intended to do...being increase dwell time. Axial designs are well documented by the Navy, as far back as the 1950's, and offer a very good platform for a homebuilt design.

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