Zorg-Alien scrubber theory.

paulraymaekers

I always try the dual explication: why reduced absorbtion capacity when going deeper, and why better absorbtion when using more helium in the mix

for me the answer is (and I don't know if that is correct physically, but it explains well the behaviour of the scrubber):

gas has to go INTO the sorb grains, not just flowing around the grains, since most of the active part of the sorb is inside, and not at the surface (the surface of the grains will be saturated quickly)
so the sorb must be porous
when gas is flowing though the scrubber part of the gas passes inside the grain
the deeper you go, the smaller the fraction of CO2 in the breathing gas is, and the denser the gas is.
so the deeper you go, the gasflow into the grains will slow down do to the increased density of the gas, and more gas will flow around the grains to sorb that is further away form the inlet side of the scrubber, more to the end of the scrubber, where eventually it will enter there in the grains..

so now by adding helium in the mix, the density of the gas is much less, and for the same depth, gas with a higher helium content will more easy flow into the grains that oxygen-nitrogen mixtures.

this is show by the tests done by the US navy, comparing heliox and air in their rebreathers

regards
paul

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www.rEvo-rebreathers.com

.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

nigelh

I think the key words are 'Mean free path', Einstein-Bose statistics, Brownian motion and the concept that the motion through the scrubber is actually irrelevant and we are dealing with a particle migrating towards a wall in a passageway.

I think.

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nigelh

AD_ward9

I have seen the Zorg analogy before, and it is a good one, IMHO. You get a thicker hot area the deeper you go because there is no reaction front in the first place - there is simply a hot centre, which at higher pressure is more easily conducted away.

We posted a model of the scrubber thermo-dynamics on another thread (a Matlab & Simulink model). The approach we take is if someone proposes Hypothesis X, then one models Hypothesis X mathematically and produces a set of predictions for an experiment. One then does the experiment and if the results are as predicted, then Hypothesis X may be true, but if the results are not as predicted then Hypothesis X is definitely false. Unfortunately, the experiment proved temperature fronts in small scrubbers, (small = a scrubber that is filled with one breath), are not a function of slice A of the scrubber working then slice B of the scrubber, but changing the model to use just one slice (i.e. no front), did match the experimental results.

Further evidence that there is no "front" is that no-one found any correlation between the temperature measurements and the CO2 figures. I still have the UKP 100, I offered anyone who could post the correlation.

So, why is the heated area wider? I will explain:

1. Scrubbers are really good insulators. We developed a scrubber heater, but the time constant is so big (hours), that we relocated the heater to the inhale and exhale bags to heat the gas going into the scrubber rather than try to heat the scrubber mechanically. So another Hypthosis hit the dust.

2. Gas flow is heated by the scrubber reacting with CO2. The gas flow carries the heat to different regions. The ability of the gas to carry heat increases roughly linearly with the density of the gas (pressure). It therefore moves the heat across the scrubber, and carries it to the scrubber stick better. This conclusion is based on gas mechanics and modelling using a single slice model of that used to determine if "fronts" exist.

Incidentally, the longer you run a scrubber, the further the heat travels.

Also, if you use the blue stuff, it shows you where the scrubber is drying out.

Alex

granso

BINGO.
Moreover, the reaction probability is said to be concentration, but in case of gases it would be more appropriate to talk about partial pressure.
Now PpCO2 does not change (I mean keeping same effort we can consider it constant).
And the rest of the gas is an inert for the absorbant.
The "demons" see just the CO2, the other molecules are invisible.

I think