- Dry ice blasting
Dry ice blasting is a process in which
dry iceis accelerated in a pressurized air stream and directed at a surface in order to clean it. The method is similar to sand blasting, plastic bead blasting, or soda blastingbut 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 energyto the surface on impact and producing minimal abrasion. The sublimation process absorbs a large volume of heatfrom 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 absorb 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, [ [http://patft.uspto.gov/netahtml/PTO/srchnum.htm Moore, David E., US patents#4,617,064 and #4,744,181] ] 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, and subsequent increased rate of failure. 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
hopperand gas compressor. This capability requires the use of specialty hose that is rated for both high pressure and low temperatures. Lengths sufficient to take advantage of this are only made on custom order and for most applications are prohibitively expensive.
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 is well in excess of the 25 foot length of hoses supplied with single hose machines. Two hose systems are less costly 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.
It should be noted that the stated differences between these two mechanisms should be treated with skepticism. Single hose delivery systems remain under patent while two hose systems are not. Manufacturers of single hose systems tend to exaggerate the differences in order to sell a more profitable product. There is no published, independent, research quantifying the difference between the systems.
Dry ice blasting can be used to clean food processing equipment. 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. [ [http://www.epa.gov/reg5rcra/wptdiv/wastemin/tcafinal.pdf EPA- TECHNICAL FACT SHEET FOR 1,1,1-TRICHLOROETHANE (TCA) HAZARDS AND ALTERNATIVES] ] Dry ice blasting can clean numerous objects with differing, complex geometries at once.
Though not toxic,
carbon dioxidecan displace oxygenresulting in asphyxiaif 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 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.
* The first patent regarding dry ice technology was US Patent # 2,421,753 issued in 1947
* The first patents regarding development and design of modern-day single-hose dry ice blasting technology were awarded to David Moore of Cold Jet, LLC in 1986, 1988 (US patents #4,617,064 and 4,744,181)
* [http://www.strahlverfahren.de/engl/ The International Conference on Dry Ice Blasting - has been held annually since 2002 in Berlin, Germany]
* [http://www.strahlverfahren.de/files/info/flyer_co2-blasting.pdf Blasting with Solid CO2 Flyer, from the Fraunhofer Institute]
* [http://www.messergroup.com/cz/_asopis__On_Air_/on_air_2_english.pdf Messer Group 2007 article on Dry Ice Blasting (pdf, pp 8-12)]
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