[Carbon
dioxide (chemical formula CO2) is typically known as a gas, though it exists as a solid when it reaches
temperatures below −78.5 °C (−109.3 °F). This process is called deposition, and
goes directly from gas to solid. When temperatures warm up, it sublimates and
goes directly back into a gas]
Carbon dioxide is a colourless, odourless
gas, this chemical compound is composed of two oxygen atoms each covalently
double bonded to a single carbon atom. Using dry ice (a solid form of carbon
dioxide) is a form of abrasive blasting, similar to sand blasting, plastic bead
blasting, or soda blasting but substitute’s dry ice as the media, where it
involves propelling pellets at extremely high speeds in a pressurized air
stream and directed at a surface in order to clean it.
How does it work?
Dry ice pellets are accelerated in a jet
of compressed air and strike the coating to be removed at velocities up to the
speed of sound. Cleaning results from three effects:
Kinetic Effect - when dry
ice pellets strike a surface at the speed of sound, any coating on the surface
is cracked and loosened.
Thermal Effect - the low
temperature of dry ice pellets makes the coating brittle, cracks it and loosens
it as a result of reduced bonding between the coating and the underlying
surface. This allows dry ice to permeate the coating.
Sublimation - dry ice
penetrates the coating and immediately sublimes (passes directly from solid to vapour
state). This results in a 700-fold increase in volume, an explosive effect that
lifts the coating off the surface.
Cleaning with dry ice
Most system uses small rice size pellets
of dry ice ejecting them from a jet nozzle with compressed air. It works
somewhat like sandblasting, with superior results. The frigid temperature of
the dry ice -109.3°F (-78.5°C) "blasting" against the material to be
removed, causes it to shrink and lose adhesion from its sub surface.
Additionally when some of dry ice penetrates through the material to be
removed, it comes in contact with the underlying surface. The warmer sub surface
causes the dry ice to convert back into carbon dioxide gas. The gas has 800
times greater volume and expands behind the material speeding up its removal.
Paint, oil, grease, asphalt, tar, decals, soot, dirt, ink, resins, and
adhesives are some of the materials removed by this procedure. Only the removed
material must be disposed of, as the dry ice sublimes into the atmosphere.
The actual dry-ice pellets are quite
soft, and much less dense than other media used in blasting-cleaning (i.e. sand
or plastic pellets). Upon impact, the pellet sublimates almost immediately,
transferring minimal kinetic energy to the surface on impact and producing
minimal abrasion and without damage to the surface or its substrate. Dry-ice
leaves no chemical residue as it sublimates at room temperature.
This is assumed to improve cleaning as
the top layer of dirt or contaminant is expected to transfer more heat than the
underlying substrate and flake off more easily. The efficiency and
effectiveness of this process depends on the thermal conductivity of the
substrate and contaminant. The rapid change in state from solid to gas also
causes microscopic shock waves, which are also thought to assist in removing
the contaminant.
The EPA recommends dry ice blasting as
an alternative to many types of solvent-based cleaning. Dry ice blasting can
clean numerous objects with differing, complex geometries at once.
These type of systems are used in
semi-conductor clean rooms, as there is no moisture involved with Co2 as it
does not exist in a liquid form at room temperature and any moisture (condense)
will flash-off immediately.
Ideal for use on – grilles, wheel
surfaces, engine bay or intricate or limited space
Safety and PPE
Carbon dioxide is increasingly toxic
starting at concentrations above 1% and can also displace oxygen resulting in
asphyxia if equipment is not used in a ventilated area. In addition, because
carbon dioxide is heavier than air, exhaust vents are required to be at or near
ground level to efficiently remove the gas.
At normal pressure dry ice is −78 °C (−108 °F)
and must be handled with insulated gloves. Eye and ear protection are required
to safely use dry ice cleaning equipment. Compared to other blasting-cleaning
methods, dry ice blasting produces fewer waste products and does not require
clean-up of a the cleaning media
Compressed air requirement – 87 PSI (6
bar) pressure amount of air delivery would depend upon machine size.
polarjet.ch
Replaces Sandblasting
This method is superior to sandblasting
because the dry ice is soft enough not to pit or damage the underlying surface.
The dry ice sublimes quickly into the air and only the removed material must be
cleaned up. Dry Ice blasting eliminates equipment damage in two ways. First,
dry ice does not erode or wear away the targeted surface as traditional grit
media and even wire brushes do. This means that surface integrity and critical
tolerances are preserved and equipment will not have to be replaced due to
surface erosion common with sand, glass beads, and other abrasive media.
Second, with traditional cleaning methods, equipment is often damaged (bumped,
dropped, etc.) while in transit to or from the dedicated cleaning area. Instead
most equipment and machinery can be cleaned while in place.
Replaces Steam
1.
Dry ice blasting is also an
improvement over steam and water blasting for several reasons:
2.
Electrical parts and
generators can be immediately put back into service used without waiting to
dry.
3.
Radioactive contaminate
removal doesn't require large storage containers for contaminated water.
4.
Mildew and mould removal are
far more complete with less chance of regeneration because of water vapour or
moisture.
5.
Dry ice blasting removes
more algae, sea slime, and mussels on boat hulls, than water blasting, which
allows the organic matter to reattach sooner.
Replaces Damaging Solvents
Finally Dry Ice blasting is used in
place of many environmentally damaging solvents. These chemicals include
trichlorethane, methylene chloride, perchlorethane, orthodichlorobenzene,
cresylic acid, and caustic solutions. Since dry ice evaporates completely as a
gas it leaves no wastes. Only the material being removed must be disposed of.
When dry ice cleaning replaces hazardous chemical cleaners the disposal cost of
that chemical is eliminated. Tinker Air Force Base has reported it eliminated
hazardous waste disposal associated with 17,000 gallons of chemicals they no
longer need to use each year because of dry ice cleaning.
Pros
Dry ice is completely non-toxic. In
contrast to cleaning methods that use water or high-pressure hosing, there is
no emission of polluted water to the environment. The dislodged coating can be
swept or vacuumed up and disposed of as appropriate. Dry ice blasting produces
no toxic fumes from solvents and other chemicals during cleaning.
Replaces steam, solvents , sandblasting
Cons
1.
The loud noise produced
requires ear protection equipment and may cause irritation to other people
nearby although newer machines are much quieter.
2.
Effective cleaning can only
occur in a straight line of sight from the Dry Ice jet nozzle. Sometimes parts
can be dissembled to help.
3.
Large amounts of carbon
dioxide are released which can be harmful if not ventilated out of the space.
Typical equipment Vendors
PolarJet - http://www.polarjet.com/index.php/products
I would like to think that these articles become an
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industry experts who seek to advance their knowledge.
I hope the above article was informative. By having some
understanding of the ‘What’ and ‘Why’ as well as the ‘How’ along with a little
science to help you understand how the chemicals we use react, you can achieve
the results you desire.
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