Radial vs Axial

tinybubbles

I have recently seen the Meg Radial but not yet tried one. I can certainly see how WOB would be less (surprisingly not much I notice). It would seem to me that the gas would take the path of least resistance. Since there would be a pressure gradient down the length of the canister, why wouldn’t the top of the scrubber be used up first, the bottom used less (like the cone of used sofnolime in an axial) and a higher chance of breakthrough. Maybe I’m missing something. I kind of expected the radial to have baffles of something to ensure the gas had to travel some distance through the scrubber agent. I was expecting rocket science but see two screens. Is the radial relying on the used sofnolime offering more resistance? Sorry if this has been discussed before. Thanks

PaulTG2

Hello,

I expect that the answer is that there isn't a meaningful pressure gradient within the canister. The scrubber material provide enough resistance, new or old, that the air pressure difference gets equalized out by normal airflow.

Sincerely,.

Paul

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ibbo

scary isn't it. it obviously works, but when i've asked before i've never got a straight answer as to whether wet, used or new lime flows better.

wedivebc

I figure that as the scrubber gets used up and the moisture content increases the resistance to flow will increase as well. What you are saying is probably happening but to a very small degree so for the most part the usage appears even.
I am sure there are scrubber gurus out there who will blow holes in my theory

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Dave....

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zzzzzzzz

There are several ways to look at what might be happening inside a scrubber. The gas moving bit is pretty easy to get, the chemical reaction and how it happens is a bit more subtle.Starting with gas velocities, for equivalent volume of absorbent, it follows that the axial design has more resistance and higher average gas velocity than the radial scrubber. If the tube diameter were large enough in an axial scrubber, it would behave almost the same as a radial scrubber, notwithstanding the increasing or decreasing velocity gradient in the radial scrubber. For the axial scrubber to behave more closely to a radial scrubber, a conical section would better approximate the other.The difference bewteen the two is really about geometry and how fast gas will flow and whether or not there is acceleration or decelaration in the gas flow. These geometric dependencies are added to the complexity of the respiratory cycle, starting and ending slowly with maximum flow somewhere in the middle and with a lot of variation.How does this affect the reaction in the scrubber?Scientific types will state that "dwell" time is the driving factor. The reason being that gas must diffuse through the various media and react chemically. This takes time. One considerable variable affecting dwell time is Rebreather architecture and design. Another is scrubber volume. Scrubber volume also drives geometry. Gas velocity is yet another factor in this chain since it affects the ability for the gases to get to the critical areas and react with the medium. Gases tend to naturally flow to areas of lower concentration. Concentration gradients are often the primary drivers in getting the CO2 to where it will react. Thus, the CO2 will diffuse to the reactive material because there is much less of it there (where there is active material). The nature of the flow is a critical element in this process.With all this in mind, low gas velocity will allow the diffusion forces to dominate. This is why a radial scrubber can be advantageous because it favors the low gas velocity approach. The main reason for this reality is due to packaging. Radial scrubbers can be easily built to provide large surface areas in small volumes and connecting plumbing. Reducing gas velocity is therefore achieved through the implementation of a high surface area perpendicular to the gas flow.An axial scrubber may be percieved to be more "reliable" in getting the CO2 to the reactive material because the same gas will traverse a lot of active material, but it generally comes with a higher gas velocity and therefore less opportunity for the gas to diffuse to the reactive chemicals. More importantly, such a design creates higher gas velocity profiles and this reduces the likelyhood of diffusing to the active portion of the chemical structure. Remember that the granules do not necessarily constitute a pure environment, the gas has to get into the granules. An axial scrubber guarantees that the gas will encounter active material, the question remains as to whether the CO2 will be able to react with the chemicals because the diffusion process may not dominate.The higher the gas velocity, the greater the resistance because drag grows to the square of velocity. This also means that power required to move gas increases with the cube of velocity. More power means, you guessed it, more CO2.All the above must be taken with a grain of salt since it is a statistical process and subject to a lot of other concerns and factors. Granule size is one such factor, temperature and much more. It does reflect some of what is going on in there and what is being traded in each design.

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z

jkaterenchuk

Read my thread and look at the pictures as it sure supports that we really do not know with certainty what the CO2 asborption profile looks like and how to control its uniformity.

http://www.rebreatherworld.com/megal...rb-grains.html

John