rebreather scrubber design short version

7/22/2019 Rebreather Scrubber Design Short Version

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CO2 Scrubber Designs

Scrubbers come in many shapes and sizes but fall into primary

categories: axial, radial, hybrid-flow and pre-packed cartridges.Pendulum flow canisters are also a special case.

Subtypes are annular-axial, box-style, flat packs or flat cans,

recognizing that radials by their nature being annular or

cylindric. Some annular scrubbers are relatively flat rings

(aspect ratio) while others are deep cylinders. Some extreme

examples have been produced but the ones that have persisted

are the ones that work well and have good breathing

characteristics. Annular and radial scrubbers are also some

times referred to as toroidal. Toroidal describing a ring or

doughnut shape.

A basic axial scrubber is nothing but a tube filled with a quantity

of absorbent material (soda-lime) with screens at either end and

is ideally spring loaded or has some other means of keeping thismaterial lightly compressed. Exhaled breathing gas enters one

end, flows through the scrubber material following its axis, the

CO2 is absorbed and the gas exits the scrubber through a screen

ready to be breathed again (after O2 is added). This design has

stood the test of time despite issues and difficulties. Axial

scrubbers are also made in box form as well as in ring form

commonly called annular-axial or toroidal.

A basic radial scrubber may be visualized as a tube inside of a

can (a larger tube) with CO2 absorbent contained in the space

between these, where the exhaled gas enters via the smaller

Lecture Notes on Rebreather Design / Scrubber Design Topics-JFW



centered inner tube exits via perforations in its wall, passes

through the scrubber material and then passes back into the

breathing loop either directly or via a plenum consisting of a

larger cylinder or housing surrounding it, to be breathed again.Like other scrubber types, radial scrubber designs have

advanced and are adapted for specific requirements. Radial

scrubbers have been used in pendulum systems as well.

Hybrid flow or cross flow scrubbers designs are an attempt to

improve scrubber efficiency by directing gas flow through

portions of the device that are relatively unused or have only

relatively stagnant flow. The pendulum scrubber is often

described as inherently more efficient as the gas passes twice

through it in a to and fro fashion during the respiratory cycle.

Cross flow scrubbers attempt to use some of these principles to

improve efficiency. Excellent articles addressing these are

available at the Rubicon site. None of these units have proven

to possess sufficient improvement in effectiveness to justify

their complexity for rebreather use, but may be useful for fixedunit use on small submersibles. Some designs that increase

internal heat retention and dwell time may still prove useful as

long as they do not increase breathing resistance.

Total resistance to breathing is a fairly straight forward measure.

Total work of breathing of a rebreather system is probably more

important and is certainly more complex to measure. It is also positional, depth dependent and will be a topic for a separate

article. Adequate length of flow path (aka mean free-path or

thickness) through the absorbent is a key factor in efficiency and

resistance, as is type and grain size of absorbent. There are




some “rules of thumb that have developed for each type which

are detailed in the summary.

In general the longer the flow path the better e.g. the thicker theabsorbent the better (from a reaction path perspective), but the

longer the flow path the worse the breathing resistance.

The larger the grain size the lower the breathing resistance

The smaller the grain size the better the absorption per unit

volume. The better the packing the less likely the chance of

channeling and subsequent CO2 toxicity. So called self packing

or facilitated packing designs do reduce this risk. Over-packing

or excessive compression as may occur in longer axial scrubbers

can worsen the breathing resistance.

Efficiency is measured in several ways, but of more importance

to the diver is the duration of dive time allowed. This can also

be stated as the time until CO2 breakthrough or time until

scrubber failure, which is when the scrubber stops being

effective at removing the CO2 from the breathing gas in your rebreathers loop. A common and useful measure of efficiency is

made by comparing the total mass of scrubber material to the

time until breakthrough for a specific scrubber design at a

specified workload (and hence CO2 production), in a carefully

simulated breathing cycle. This is then validated with testing

using real divers during carefully controlled workloads.

Flat packs can be visualized for analysis as segments of an

imaginary much larger radial or annular axial scrubber.

Conversely radial and annular axial scrubbers can be broken up

into theoretical lamina to be mathematically analyzed. This is




actually an effective method to approach this problem.

Mathematical modeling of scrubber dynamics is inherently

problematic especially when trying to expand this to computer

simulation and real time analysis. Empiric real world testingfrequently yields results outside of the acceptable limits

predicted by simulation.

A flat pack shaped like a circular tablet is called a “tablet

scrubber”. These have not proven to be any more effective than

an annular axial with the “doughnut hole”. During empiric

testing for breakthrough, it is revealed that this additional mass

of absorbent material does not appreciably increase time till

scrubber failure compared with an annular axial. Using high

workload breathing it is actually revealed that a tendency

towards earlier breakthrough exists, despite an increased mass of

absorbent. Increasing the size further, slowing and

redistributing the gas flow and increasing the dwell time tends to

correct this tendency. This demonstrates the importance of

testing outside of the computer simulation environment.

The general improvement in scrubber material, especially in its

uniformity has markedly decreased the incidences of CO2

breakthrough. Careful training in packing and preparing

scrubbers and the use of spring loaded and adequate elastomeric

compression systems have also decreased channeling due to

packing defects. Dispelling old myths about reusing or allowingscrubber material to regenerate has also reduced the rate of

rebreather accidents.




The granular absorbent material can vary from prismatic to

small cylinders to tiny spheres. Individual granules of spherical

absorbent are often referred to as prill. Absorbents are

manufactured in different ways with different concentrations of alkaline hydroxides. This is very simple in concept but has been

refined slowly over years of use. There are also the newer

“solid fill” cartridges. The efficiency of a particular granular

absorbent may vary dramatically based on its chemical

makeup, size and porosity. The actual size of the granule has an

effect, as fewer large granules will fit in a given canister, hence

reducing the absorbent surface area and the absorbents duration.

Pre-packed canisters use the same chemistry in a slide in style

packaging. While they offer potential regarding ease of use,

currently they are not as efficient or cost effective as their loose

pack counterparts.

Scrubber material imbedded into tiny sintered polymer granules,

despite the hype (and issued patents) is neither a new idea or theresults of rolling (French) plastic absorbent CO2 curtains into a

can. What is new and remarkable is the tremendous research

and testing effort that went into producing a very useable

material. These have very predictable characteristics, clever

molded in pre-channeling and they recover well should a

flooded canister occur. The developers overcame many

challenges and have largely succeeded. It is a beneficialconcept, but adds cost and the potential for purposely engineered

incompatibility. This could be a discussion in itself so we will

not explore this further at this time.




Testing total CO2 absorption capabilities by titration is a useful

test for determining the total combining power remaining in a

used sample of absorbent but is not that useful in determining

the functional capabilities of a scrubber system, as well as acarefully performed series of tests using a breathing simulator

and carefully measuring changes in the CO2 that occur in the

breathing loop. I know, I have performed these tests and many

variations on them with several generations of equipment.

Modern equipment is so much more convenient and automatic

data logging and displays are a vast improvement.

All CO2 absorbent canisters for rebreathers are inefficient, so

acknowledge this and move on. Use a canister that is adequate

for the mission plus 30 %. For a critical or demanding dive,

having a scrubber capacity with double the anticipated need is

very reasonable. Be prepared to discard relatively large

quantities of unused scrubber material and buy it in bulk

whenever possible. Different manufacturers test to slightly

different standards, but all the products specifically for rebreathers are of good quality and similar. Some may be better

for your apparatus. Use what the manufacturer recommends or

what more experienced divers use with your unit.

A scrubber design must be chosen due to considerations of

duration, breathing resistance, breakthrough characteristics,

flood recovery and mission requirements including size andshape (fitting into the housing). Longer dwell time for expired

gas improves CO2 absorption. In a larger scrubber the gas

volume per expiration is relatively smaller in volume, spends

more time dwelling in the scrubber material as it moves slowly




through the scrubber and the relatively larger mass of scrubber

material retains heat from the reaction better. So a larger

scrubber is favored especially for deeper diving in colder water,

look at a cross section of the Mark 16 or the large radialscrubbers used in the PASC rebreathers for military clearance

divers.

Scrubbers are more efficient at shallower depths, they lose

efficiency below 20 meters, they really lose efficiency below 30

and 40 meters of depth. They are more efficient at lower CO2

production rates so take it easy. They lose efficiency at colder

temperatures, one pre-breathes the apparatus for several reasons,

an important one is to bring the scrubber up to operating

temperature. Cold water cools the scrubber and a cold scrubber

is less efficient. A frozen scrubber is nearly inert. Helium

carries heat away more efficiently, we use Helium at depth,

depth decreases efficiency. I can see that you are beginning to

understand the problem with the use of the term scrubber

efficiency. Inert gas compression may decrease the reaction ratewith CO2 at depth by other mechanisms as well. Loss of

efficiency at depth with Helium is well recognized so expanded

capacity and low CO2 production is recommended.

Each canister type has advantages and disadvantages but

common issues are:

Loss of volume due to settling or “packing down” of absorbent

produces increased risk of channeling. Breathing resistance is

always an issue and less is better. Duration must always exceed




mission requirements, ideally with a large safety margin. Flood

avoidance and recovery; the effects of and recovery from

flooding (aka Water handling) of the rebreather system and

scrubber.

Common axial canister have a longer bed length or amount of

absorbent in the breathing path. This not true of doughnut

shaped axial such as the USN MK15/16 series but is true of

all current recreational units. Radials generaly have a shorter

bed length (they have a less thick or shorter mean free path for

the gas molecules to travel). Axial scrubbers make for a longer

slimmer cylinder whereas radials scrubbers tend to make for a

fatter barrel shape, but can also be configured into a flatter torus

or carousel shape. Annular axial scrubbers are toroidal or

doughnut shaped, some look like a small tire.

Shorter radial canisters are easier to pack and not as sensitive to

minor packing variations but all canisters must be carefully

packed. Flat-Can scrubbers are easy to pack and tap or shakedown, they require compression just like other types.

Absorbent granules or prill when added to a canister will settle

or pack down. That is to say the granules will redistribute and

move into position as they are tapped and they fill up the gaps

between particles. In a long axial canister the difference in

column height between packed and unpacked absorbent coulddiffer 5% to be as much as 10% of the column length. If the

canister (column) is then not repeatedly tapped and topped off as

it is prepared the risk of channeling is increased. This risk is

somewhat less in a radial scrubber, which still must be carefully




prepared. In fairness this is much less bothersome in some

models, but for a serious diver preparing the scrubber is not

really the onerous task it is described as in some advertisements

and internet forums.

Radials are generally filled at right angles to the bed length

hence the settling or pack down height is relatively smaller

compared to a taller axial. The resulting small change due to

settling is unlikely to result in channelling when spring

compression is used to keep the absorbent in place. Spring

compression either by metallic springs or elastomers is a must to

decrease channelling risks.

In short if axial canisters are not topped up and tapped down

they are more prone to channelling than radials, but all scrubbers

must be carefully prepared. With compression plates radials

may often be packed to a prescribed level and the spring plate

takes care of the rest. The use of a long filler tube or long drop

tube attached to a funnel greatly increases the ease of packing ascrubber. Never breathe dust from the absorbent, remember it is

an alkaline. Longer bed lengths mean more resistance in the

breathing circuit. Simply put, axial canisters may have more

breathing resistance. Breathing resistance is also a function of

granule size. The smaller the granule the more resistance. The

trade off is that smaller granules are often more efficient.

The rule of thumb formula for estimating a simple axial

canister’s duration is that for scrubbers of greater than 1 kg,

approximately each additional kg of absorbent equals an hour of

life at a moderately low work rate such as slow swimming.




Radial canisters may generally give 20 to 25% longer duration

than axial scrubbers of the same mass of absorbent load.

Although there are many caveats such as this relationship is

more accurate when scrubber mass is greater than 2 kg and withlower work loads. A high diver work load and therefore a high

CO2 production is anathema to scrubber life.

Flood recovery or Water handling is the last issue. Better

rebreather designs have water traps to prevent scrubber flooding.

This is a fairly straightforward mechanical problem. Avoid

scrubber flooding and provide multiple pathways to clear water.

Avoid caustic cocktails. The use of water resistant hydrophobic

membranes are promising, but these can worsen breathing

resistance. Large radial scrubbers do tend to handle water better

and maintain an adequate breathing pathway. However it

usually takes serious flooding to make all the absorbent

unbreathable in any scrubber. There is nearly always a gas path.

Large radial scrubbers are potentially superior. Larger scrubbersuse more material than most recreational divers need. Smaller

scrubbers fulfill the needs of common dive profiles. Hence

matching the apparatus and the scrubber to the dive profiles

actually dived is a wise course.

DESIGN ELEMENTS: Important considerations when

evaluating CO2 scrubber designs:

Use a scrubber size appropriate for mission requirements.




Anticipated duration needed plus a minimum of 30 % excess for

light duty purposes. Double the anticipated need for deep

demanding dives and where risk is high, such as caves and

overhead environments.

Reduce breathing resistance whenever possible. This is

especially important where depths greater than 60 meters are

anticipated.

Larger scrubbers are generally better.

Radial scrubbers may be made larger for the same work of

breathing cost. Radial scrubbers are more efficient, but this may

only be of significance when they are larger and when higher

workloads are anticipated.

Large radial scrubbers do recover from floods better, but a

flooded scrubber is a good reason to cancel a dive when possible

or to change rebreathers in an expedition, cave dive or other situation where stopping is not possible.

An important element of rebreather design is to decrease the risk

of flooding by having water traps and diversion built into the

design, as hydrophobic membranes improve they should be

routinely included in the designs. I have dived for decades now

without a flooded scrubber primarily by using over the shoulder bags and eliminating as many water entry points as possible.

The back up water trap rarely has anything but condensation in

it. Anticipate failure points and try to eliminate them.




Baffling in scrubbers is a topic that was once considered much

more important. In general baffling should be kept to a

minimum. Complex baffling rarely improves efficiency.

Baffling often increases packing difficulties and therefore errors.The best example of baffling in a civilian radial scrubber would

be the Ouroburos scrubber which is an intelligent and effective

design. This radial scrubber is also noted for being easy to pack,

although I don’t use the term self-packing, this is often referred

to by that term. The concept of baffling can be extended to the

molded in channels used in plastic bonded scrubber designs.

These provide a predictable pathway for gas flow. Please check

the photos and diagrams and examine the examples of these on

the table during the break.

I still dive a Lt. Lund type SCR for fun in shallow water, so the

units complexity should match the demands of the dive profiles

it is used for.

High workload dives are best handled by SSA whenever possible. Rebreathers are a poor choice for high workload

situations.


rebreather scrubber design short version

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