Chapter 2.9.5 Carbon Dioxide (DryIce) Blasting（2.9.5章二氧化碳(干冰)爆破）.pdf

Basic Principles of OperationDry-ice particle blasting is similar to sandblasting, plastic bead blasting, or soda blasting wherea media is accelerated in a pressurized air stream (orother inert gas) to impact the surface to be cleaned orprepared. With dry-ice blasting, the media that impactsthe surface is solid carbon dioxide (CO2) particles. Oneunique aspect of using dry-ice particles as a blastmedia is that the particles sublimate (vaporize) uponimpact with the surface. The combined impact energydissipation and extremely rapid heat transfer betweenthe pellet and the surface cause instantaneous subli-mation of the solid CO2into a gas. The gas expands tonearly eight hundred times the volume of the particle ina few milliseconds in what is effectively a “micro-explosion” at the point of impact that aids the coatingremoval process. Because of the CO2vaporizing, thedry-ice blasting process does not generate anysecondary waste. All that remains to be collected is theremoved coating.As with other blast media, the kinetic energyassociated with dry-ice blasting is a function of theparticle mass density and impact velocity. Since CO2particles have a relatively low density, the processrelies on high particle velocities to achieve the neededimpact energy. The high particle velocities are theresult of supersonic propellant or air-stream velocities.Unlike other blast media, the CO2particles have a verylow temperature of –109°F (–78.5°C). This inherentlow temperature gives the dry-ice blasting processunique thermodynamically induced surface mecha-nisms that affect the coating or contaminate in greateror lesser degrees, depending on coating type. Be-cause of the temperature differential between the dry-ice particles and the surface being treated, a phenom-enon known as fracking, or thermal shock, can occur.As a material’s temperature decreases, it becomesembrittled, enabling the particle impact to break-up thecoating and sever the chemical bond that is weakenedby the lower temperature. The thermal gradient ordifferential between two dissimilar materials withdifferent thermal expansion coefficients can serve tobreak the bond between the two materials. Thisthermal shock is most evident when blasting anonmetallic coating or contaminate bonded to ametallic substrate.Historical DevelopmentIn the early 1930s, the manufacture of solidphase carbon dioxide (CO2) became possible. Duringthis time, the creation of “dry ice” was nothing morethan a laboratory experiment. As the procedure formaking dry ice became readily available, applicationsfor this innovative substance grew. Obviously, the firstuse was in refrigeration. Today, dry ice is widely usedin the food industry for packaging and protectingperishable foods.In 1945, the U.S. Navy experimented with dryice as a blast media for various degreasing applica-tions. In May 1963, Reginald Lindall received a patentfor a “method of removing meat from bone” using“jetted” carbon dioxide particles.In November 1972, Edwin Rice received apatent for a “method for the removal of unwantedportions of an article by spraying with high velocity dryice particles.” Similarly, in August 1977, Calvin Fongreceived a patent on “sandblasting with pellets ofmaterial capable of sublimation.”The work and success of these early pioneersled to the formation of several companies in the early1980s that pursued the development of dry-ice blast-ing technology.Dry-ice pelletizers and blast machines enteredthe industrial markets in the late 1980s. At that time,the blast machines were integrated with dry-iceproduction machines and therefore physically largeand expensive, and they required high air pressure(greater than 200 psi [1.4 MPa]) for operation.As the CO2blast technology advanced, theblast machines were separated from the production ofdry ice and the size and cost dropped. Improvednozzle technology and production of high-density dryice pellets has made blasting effective at shop airpressures (80 psi [550kPa]).Chapter 2.9.5Carbon Dioxide (Dry-Ice) BlastingRobert W. Foster162Figure 1. Dry ice blasting machine. Courtesy Cold JetInc.Equipment/MaterialsThere are two general classes of blast ma-chines as characterized by the method of transportingpellets to the nozzle: two-hose (suction design) andsingle-hose (pressure design) systems. In eithersystem, proper selection of blast hose is importantbecause of the low temperatures involved and theneed to preserve particle integrity as the particlestravel through the hose.In the two-hose system, dry-ice particles aredelivered and metered by various mechanical meansto the inlet end of a hose and are drawn through thehose to the nozzle by means of vacuum produced byan ejector-type nozzle. Inside the nozzle, a stream ofcompressed air (supplied by the second hose) is sentthrough a primary nozzle and expands as a high-velocity jet confined inside a mixing tube. When flowareas are properly sized, this type of nozzle producesvacuum on the cavity around the primary jet and ca。