Axial Scrubber Design Comparison

jeff h

There seems to be two basic axial designs. The first is a solid fill with no tube in the middle and the second is the doughnut fill with a tube in the middle.

I’m interested in material usage for each design in relation to the cone shaped flow front or usage front. In the solid fill the cone comes to a single point in the middle of the scrubber. In the doughnut fill design the cone point is actually circular and forms a circumference in center of the scrubber material. Therefore the gas exits over a larger area of material each cycle.

The height/amplitude of the cone would also be different which may effect material usage and time to breakthrouh.

I'm also curious if there are other benefits to this design like non-clumping of material from moisture, channeling or packing consistency center to edge, etc.

teksimple

I am not sure I understand you, but let me try. From what I see, your cone front is DIFFERENT than that employed in standard axial designs. I do not know of a design that uses such a "cone" front, but let me know otherwise. Yes, there it appears there is more surface area of sorb exposed on one side of the gas path, but are you sure that this greater specific surface area on one side of the scrubber equates to greater gas dwell over the total sorb inside the scrubber?

I expect that there is a reason why egress and ingress areas faces ("fronts") are equivalent in area on every scrubber I am aware. I propose that it is because that arrangement is the most efficient for CO2 removal and use of space in a rebreather. Wouldn't the surface area of the sorb grain itself be more important in a rebreather loop than the increased surface area of just one face? Smaller grain equates to more exposed sorb surface area for a given mass of sorb.

Increased surface area of the sorb to gas exposure is the reason why radial (example: Cis Lunar) and annular axial (example: Mk 15.x) scrubbers are more efficient that elongated axial scrubbers. But what about cone fronts on BOTH sides of the scrubber? As you suggest, a cone would give you more surface area than a flat face. I think a cone shaped rebreather would be inefficient because gas would tend to follow the path of least resistance which is where the path through the scrubber is shortest.

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Scubascooby

I think what Jeff is saying is that the reaction front in the scubber is conically
shaped rather than the scrubber is conically shaped.

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jeff h

Yes, thank you, the "reacton front" is what I was referring to, but the flow is also in the same shape.

Most of the axial scrubbers are a solid fill including the meg, inspo, dolphin, ida, sport kiss, etc., however the classic kiss has the doughnut design (maybe some others?).

In the end it probably doesn't matter. All things equal it seems like you might get a more consistent pack out of the CK design (assuming there's a gradient in packing density from center to edge).

richc

Is it? Why do you think the flow or reaction front forms that shape?

jeff h

This was covered in my Mod 1 course and I thought that was a well known phenomenon, but maybe not. I think it is also covered in Jeffrey Bozanic's book and in Joseph Dituri/Brian Greene/Richard Pyles video.

Others could better explain this but I'll give it a shot. I believe it is caused by the shape of the scrubber, location and size of the gas inlets/outlets and the resulting gas velocity through the scrubber material. For instance, in the Meg the inlet opening is the diameter of the scrubber, however the outlet is a 1.7” hole in the middle. Therefore you have lower velocities at the inlet and a larger surface area (ie the base of the cone).

Beyond that there maybe other gas flow dynamics that would cause the gas to naturally neck down in the middle. This could be caused by things like laminar flow at the wall with increasing friction at the interface or turbulent edge flow, etc. It’s been a long time since I looked into this stuff.

Maybe what I was taught was wrong. I do know when I dump my scrubber the hard clump in the middle is usually conical.

-Jeff

nigelh

I keep hearing this and when I sit down and try an model it I don't get more efficient and I don't get a cone. With granular media any attempt at modelling flat walls seems to get more flow so the exact opposite effect to a cone but it all washed out a few particle sizes from the edge.

Is there any published work on the subject or is it just something people say to prove their rebreather is better? The only thing I can see that would be wrong is to put a bend in the path.

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nigelh

Rebreathing Red

jeff h

Thank you for the excellent explanation.

Based on that explanation I still think the cone in the sketch is the right shape. If it is warmer in the center then the reaction will happen faster and the material in the center will get used up faster.

I wonder if this means the doughnut shaped scrubber would have a more uniform temperature distribution? In the long run it probably doesn't matter but I've been curious about this for awhile.

Thanks again,

-Jeff

York

That does sound reasonable, but I did the heat transfer calculations for the Dolphin canister several years back, and found that the heat loss through the wall is negligeable and not enough to cool the gas even by 1 degree as it flows through the scrubber. If it were, you would end up with with a slight gradient near the wall, and the main temperature of the bulk, which would be off-set by the slightly increased flow near the wall due to what is referred to as near-wall channeling (based on the lesser packing density next to the flat wall) - in effect the bulk temperature would be insulated by an envelope of slightly increased gas flow near the wall. This btw is insignificant in terms of break-through, as the volume-% of the near-wall flow is less than 3% of the total flow. As result, you don't have a radial temperature gradient in axial scrubbers.

As far as I understand it, the cone shape(s) that Jeff describes is due to the outlet of the canister. On the Meg, the flow through the whole width of the scrubber has to funnel into the hole that connects to the head. So it's a simple question of flow profile rather than having to do with the chemical reaction. Of course the reaction can only occur where gas flows.

The reason why radial scrubber are more efficient is due to the speed-of-flow and resulting ratio of reaction-front and bed length. In a radial canister, the flow is much less than on an axial, resulting in a sharply defined, short reaction zone. This means that either the reaction zone has not reached the end of the canister, or its has fully reached it, and only a very brief transition time between these two states. On an axial canister the reaction zone is wider and less defined, and therefore when it reaches the end of the canister resulting in a breack-through, there is still a lot of only partially-used scrubber material left. This would also mean that an axial canister is more forgiving when pushing beyond the initial break-through, whereas a radial canister would have an almost complete breakthrough immediately. I would expect that the increased efficiency of the radial scrubber over the axial scrubber would decrease with increased canister sizes.

Not sure if that makes any sense

Joerg