rebreather scrubber design lecture-short version

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 this material 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 fixed unit 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 the absorbent 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 testing frequently 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 allowing scrubber 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 the results 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 beneficial concept, 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 a carefully 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 and shape (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 radial scrubbers 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 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. A drop in scrubber temperature of 10 degrees celsius halves the reaction rate (approximately). 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 rate with CO2 at depth by other mechanisms as well. Loss of efficiency at depth with Helium is well recognized so an expanded capacity scrubber and low CO2 production by the diver are 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 shake down, 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 could differ 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 a scrubber. 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 with lower 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 scrubbers use 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 lecture-short version

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