SMI Scrubber

SMI Scrubber
By Pete Readey


Focus on the Scrubber

The PRISM rebreather system, including the scrubber, is founded upon a long engineering history of creative and innovative breathing system designs. The PRISM scrubber is designed into a complete breathing system; therefore it is unlikely that using a PRISM scrubber in another unit will give the same performance if the system is not identical. In this excerpt I will endeavor to bring out a few salient points.


Materials

1. Plastic is the material of choice for SMI because it has lower thermal conductivity than stainless steel and aluminum, i.e. does not dissipate heat quickly. Maintaining a higher temperature is more efficient for your chemical reaction, which contributes to better duration.
2. The use of a plastic cage with an elastic plastic screen and open cell foam pads top and bottom, reduces grinding and dusting of grains by damping most of the vibration; this is a benefit of a more elastic structure.
3. A flexible plastic structure also reduces channeling by providing continuous compression and minimizing particle movement. Slight loss of material due to dusting, is compensated by the pressure of the screen and pads taking up small voids and reducing dead space.
4. The overall cost of molding components long term, versus the cost of fabricating them with stainless steel is significant especially in a small niche market. The increase in scrubber performance has made the cost advantages worth the initial investments of design and tooling for a plastic unit.

Function

Gas on the PRISM passes at fairly high speed directly from the diver’s exhale counter lung into the head and straight down the center of the scrubber. As it diffuses out its velocity slows as the cross sectional area in the scrubber increases (long dwell time); the gas then flows out of the scrubber at slow speed. Combining the lower velocity with a temperature drop at the bucket wall (the dew point) condensation appears on the bucket all the way up to the head ring (even with a clear, plastic, insulated bucket there is a steep temperature gradient on the bucket wall). The gas then passes through slits in the head, increasing in velocity as the cross sectional area decreases; the gas moves quickly across the sensors, out of the head into the inhale counter lung, returning to the diver’s lungs for a another cycle.

Reversing this flow on the PRISM resulted in the gas entering the scrubber at a reduced temperature (lowering scrubber efficiency), as the gas lost most of its heat through the bucket wall to the outside water. This was especially true below 65oF. This was one of the factors in our decision to choose the flow direction.

Addition -13th September 2006

This is a thermodynamic simulation for various materials with a hypothetical radial flow canister. The object was to see the temperature of the inner canister core while surrounded in cold water. The gas used is Air at 250 lt/min and an initial temp of 50degC. Thermodynamic properties were taken from the CosmosflowworksPE library and Nika EFD. Heat is supplied by the incoming gas not through chemical reaction.



Pete

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