Dry ice blasting is a form of abrasive blasting, where dry ice, the solid form of
carbon dioxide, is accelerated in a pressurized air stream and directed at a surface
in order to clean it.
The method is similar to other forms of abrasive blasting such as sand blasting,
plastic bead blasting, or soda blasting but substitutes dry ice as the blasting medium.
Dry ice blasting leaves no chemical residue as dry ice sublimates at room temperature.
Dry ice blasting involves propelling pellets at extremely high speeds. 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. The sublimation process absorbs a large volume of heat from the surface,
producing shear stresses due to thermal shock. 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 ice used can be in solid pellet or shaved ice block forms. The shaved ice block
produces a less dense ice medium and is more delicate than the solid pellet system.
Dry ice blasting generally uses one of two delivery systems to combine compressed
air with ice and accelerate particles out of the delivery nozzle:
Single-hose technology was developed by Cold Jet, LLC in 1986, and uses a single
hose to deliver air blasts and dry ice. The single hose system can use a longer hose
than the double-hose counterpart without a significant drop in pressure when the
ice leaves the hose. The additional power comes at the cost of increased complexity.
Single hose systems are used when the surface to be cleaned has a heavier build-up
or when the surface to be cleaned is vertical or at a greater height than the hopper
and gas compressor.
Two-hose dry-ice blasting was developed before the single-hose system. Compressed
air is delivered in one hose, and ice pellets were sucked out of a second hose by
the venturi effect. Compared to a single-hose system, the two-hose system delivers
ice pellets with less force (approximately 5% for a given air supply) than a single-hose
system. Theoretically two hose systems have a limit to the vertical distance between
the machine and applicator. This limit however is well in excess of 25 feet. Two
hose systems are generally less costly to produce due to a much simpler delivery
system and they allow finer particles of ice to be delivered with lower velocity
as the late combination of warm air with cold ice results in less sublimation in
the hose. The latter properties allow for more delicate surfaces to be cleaned.
Dry ice blasting can be used to clean food processing equipment to effectively decontaminate
surfaces of Salmonella enteritidis, E. coli, and Listeria monocytogenes such that
these microorganisms are not detectable using conventional microbiological methods.
It may also be used to clean some equipment without disassembly and without producing
fire or electrical hazards. The EPA recommends dry ice blasting as an alternative
to many types of solvent-based cleaning.[dead link] Dry ice blasting can clean
numerous objects with differing, complex geometries at once.
Though not toxic, carbon dioxide can 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 (−108.4 °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 blasting medium.