ARE SCRUBBERS AND WHAT DO THEY DO?
We live in the best scrubber ever designed. It took nature several
million years to develop the optimal mechanisms that remove the
carbon dioxide from our planets atmosphere. So far it has worked
well, even with the increased human population and activities. Animals
are a major source of CO2. Average human activity produces 500 ml
of CO2 per minute which represents 4% of the exhaled gas.. That
adds up to a real problem if you put us in a place with no ventilation,
such as underwater.
Two things are most important for optimal operation of a scrubber.
One; is that the gas must come in contact with the absorbent material
and two; the gas must be near the absorbent material long enough
for a chemical action to occur. Traditional design has been to place
the absorbent materials in a cylindrical housing "can"
and pass the gas from one side to the other. If it did not work
well you got a bigger can until it removed a sufficient amount to
make the gas breathable, less than 0.5% by volume. This worked if
you did not mind having to deal with 8 to 10 pounds of absorbent.
For a two to four hours dive. Most of these "coffee" can
devices are less than 30% efficient as one pound of absorbent will
reduce the CO2 produced by a swimming diver for sixty minutes. A
two hour dive should require only two pounds of absorbent if the
device is optimal.
How Do Scrubbers Work?
Carbon dioxide is an acid gas. Acid gases can be reduced by oxides,
peroxides and hydroxides. Hydroxides are used in most scrubbers
because of their stability and ease of handling.
The hydroxides most commonly used are:
The Dewey Alma division of Grace
Chemical Company makes a trade name product "Sodasorb"
which is a mixture of all three hydroxides with the major component
being calcium. Other companies make similar compounds with other
additives such as barium and lithium.
The process. The reaction occurs in three phases;
gas, liquid and solid.
- Gas; the CO2
must come in contact with the pours of the granules.
- Liquid; The pours
of the granule hold water containing dissolved hydroxides. Optimal
moisture content of the granule is 12 to 19%. More or less moisture
will slow the reaction.
- Solid; The undissolved
hydroxides structures the matrices.
The absorption of CO2 is a chemical process. The
CO2 comes in contact with the moist film containing the dissolved
hydroxides and is taken into solution forming carbonic acid which
reacts with the hydroxides to form sodium carbonate. This process
regenerates the water and the sodium carbonate reacts with the hydrated
lime to form calcium carbonate, caustic soda and caustic potash.
The process produces excess water and heat. The process works best
when the three conditions of gas, liquid and solid are maintained,
the liquid being the major concern. To little liquid will not allow
the first step to occur. Too much liquid will dissolve the granules
and form a paste like composition which closes the pores in the
· High gas velocities. The gas will not be in
the granule bed long enough for the reaction to occur.
· Cold temperature; The water in the granules
will freeze into a solid state at low temperatures, below 29 d.F.
· Moisture retention on the canister surfaces:
The hot gasses within the scrubber will transport moisture to any
cooler surface within the canister. If moisture condenses on these
surfaces they can dissolve the granules into a paste and restrict
the gas passage.
· Heat; The granule bed will generate temperatures
up to 130.d.F. This is good for underwater use but bad for surface
· Channeling; This is a condition in which the
gas flowing through the scrubber bed finds a hole in the bed or
flows along the wall of the canister. The bypassing gas is not scrubbed
and the CO2 is passed through the scrubber to the user.
· Back pressure: The granules are packed so tight
that they are crushed into small sand like bits and restrict the
passage of the gas.
· Water entering the scrubber: The granules will
melt together into a slush like paste which will create back pressure,
loss of reaction and CO2 passing through the system to the user.
· Retention of water in the scrubber canister:
The process of reducing 5% CO2 from the breathing mixture for two
hours will generate app., eight ounces of water. This is excess
water that must be conducted out of the granule canister. A properly
designed canister will allow the hot gas to exit the canister before
condensation occurs. Water traps are used to reservoir the excess
water or drains can be installed to "bleed" the system.
· Contamination; The scrubber canister must be thoroughly cleaned
after each use. Biological contaminants can crosscommunicate infectious
agents from one person to another. The use of "disposable"
scrubbers and counter lungs help reduce this problem.
The "TEN COMMANDMENTS"
1. Removing carbon dioxide from the breathing gas is a "chemical
reaction". The canister is not a "filter".
2. Heat from the canister is a positive indication that the
absorber is working. A cold canister is a bad indication.
3. Do not reuse materials as the chemical action depletes
4. Excessive water will "melt" the absorbent materials.
Remove all excess water from the canister.
5. Quality of the absorbent materials can not be determined
in the field. Depleted materials may give an immediate reaction
to carbon dioxide. But depleted materials will stop reacting within
minutes of use. Insure a quality material by knowing your supplier
and keeping the materials sealed air tight prior to use. Pre packaged
absorbers in a disposable cartridge is the best way to insure quality
6. Using rebreathing equipment with scrubber monitoring systems
is desirable if available.
7. "Old hands", at rebreather use may tell you
that you can smell and taste the buildup of carbon dioxide in the
breathing gas. This is true. But, the content must be above 0.08%
by volume to do this. The maximum allowed to be safe by the US Navy
is 0.05% by volume.
8. Do not use a canister that is below 20 deg, Fahrenheit
temperature. For cold water operation the canister must be pre heated
to above freezing to insure the chemical process will start.
9. Never use a rebreather with a used canister as:
a. You do not know how long the unit will last.
b. The previous user may be infectious.
10. Be aware of the "9" and stay safe
Scrubber shown here inside counter lung ready to install