Radial Scrubbers

[Brian Garner]

Does anyone know which way the gas flows in a radial scrubber.
It seems to me that a flow from out to the centre would make the scrubber fade fast at the end of its life. This would be because there is a smaller mass of material near the centre. Thinking of concentric circles here.
If the gas flow path was from center to the edge then there will be a rapid use of the lime near the center pipe but a slowing of the wave front as it spreads outwards. But would this give more chances of channeling as the active area moves towards the outer edge. If you think that there may be say one inch of the outer layer unused, the surface area as tou get to the edge is much larger and the chances of a channel appearing must be greater.
As far as I am aware radial scrubbers are supposed to be the dogs danglies but thinking about the failure options makes me wonder.

Anyone who knows more care to comment.

Brian Garner

[Padowan]

I've had some interesting thoughts on this, I think most designs from from the centre outwards, which means that as the scrubber is used up, the theoretical "depth" of the reaction front gets thinner as the diameter expands.

However, consider this. For a typical dive provile, you go deep at the start, and end up shallow at the end, so it makes sence to have your scrubber working most efficiently* at the beginning of it's life. Deeper = higher gas density, which in an axial scrubber would mean "deeper" reaction front, but in a radial, working from the outside in, because of the larger diameter, the "depth of the front" might be thinner.

* by most efficiently, I mean "most tolerant to high gas densitys and flowrates and minimal breakthrough potential"

This is all theoretical straight from my head, so it's likely that there are people out there with much more useful/empirical evidence.....

[Brian Garner]

I wasnt thinking about the depth of the reaction area but this will certainly make a difference. Same volume of "active" lime but over differing areas.
If I understand you then you are saying that an outside to in would be better . I do agree with you. It seems to suit the dive profile better. Especially on deco, not a lot of work and a lower gas density because of the reduced depth..

More to think about. Hopefully someone who knows will be along on a minute.

Brian

[Simon A]

Brian,
I'm of the opinion that from the middle to the outside is the best way.
As you move away from the centre the gas velocity drops considerably. Due to this when you are using the centre part of the scrubber you have a reaction front that moves faster and a deeper depth of "reacting" lime. as you get to the outside this reaction front moves slower and reacts over a much shallower depth.
From this I would also summise that whilst a radial scrubber makes more efficient use of the lime, a partial breakthrough will escelate in severity much faster than on a convential axial or co-axial scrubber.

Simon A

==Edit==
I should have hit refresh, Padowan has described what i was trying to say in a far more eloquent manner.
=end edit=

[Brian Garner]

Well it seems the consensus is in to out. I dont suppose it really makes a massive difference when you think about it. It does seem that a radial scrubber is much more at risk from channeling though.The length of the gas flow path is that much shorter.

Brian Garner

[apitkin]

One other consideration is thermal. Assuming that the scrubber is immersed in cold UK water, passing the freshly expired gas down the centre (inside to out) helps retain the heat better than if it were passed down the outside next to the cold canister wall (outside to in). As heat contributes to the lime's ability to absorb, this helps keep the scrubber as efficient as possible.

An added benefit of having the very moist but scrubbed gas from the lime hitting the cold canister wall on its way out is that moisture will tend to be condensed there rather than other cool surfaces, like O2 sensors.

Andy

[Brian Garner]

Thanks Andy

That makes a lot of sense. Nice and hot in the middle where the reaction is.
This is probably one of the characteristics which makes radial scrubbers so efficient.


Brian

[schford]

Originally Posted by Brian Garner

Well it seems the consensus is in to out. I dont suppose it really makes a massive difference when you think about it. It does seem that a radial scrubber is much more at risk from channeling though.The length of the gas flow path is that much shorter.

Brian Garner

Depends on the type of Radial Scrubber you use....

If you use a canister scrubber such as the classic and IMHO not beaten CIS-LUNAR scrubber design which is a tower radial design then theoretically a scrubber that is not packed well could channel quicker compared to an axial design. Although compare those 6 to 8 hours run time with the same weight of slime.

The alternative to the Tower Radial is the doughnut Radial cannister - minimal chance of channeling but increased WOB.

Stuart

[Decodiver]

Oh, and by the way guys, the Cis scrubber goes from the outside to the inside........

[Brian Garner]

Quote

If you use a canister scrubber such as the classic and IMHO not beaten CIS-LUNAR scrubber design which is a tower radial design then theoretically a scrubber that is not packed well could channel quicker compared to an axial design. Although compare those 6 to 8 hours run time with the same weight of slime.

End Quote

I dont understand how this works. Surely a kilo of lime will absorb so many moles, ounces, bushels of CO2. This must be a fixed value. However the gas flow through it, we still have a kilo of lime. What am I missing here?

Brian