How does Dry Ice Blasting Work?Dry ice blasting works because of three primary factors: pellet kinetic energy, thermal shock effect and thermal-kinetic effect. Hailstorm Dry Ice Blasting optimizes blast performance for each application by combining these forces and adjusting:
- Compressed air pressure
- Blast nozzle type (velocity distribution)
- CO2 pellet size and density
- Pellet mass rate and flux density (particles per unit area per second)
- Pellet Kinetic Energy
The Hailstorm Dry Ice Blasting process incorporates high velocity (supersonic) nozzles for surface preparation and coating removal applications. Since kinetic impact force is a product of the pellet mass and velocity over time, the Hailstorm Dry Ice Blasting delivery system achieves the greatest impact force possible from a solid CO2 pellet by propelling the pellets to the highest velocities attainable in the blasting industry.
Even at high impact velocities and direct head-on impact angles, the kinetic effect of solid CO2 pellets is minimal when compared to other media (grit, sand, PMB). This is due to the relative softness of a solid CO2, which is not as dense and hard, as other projectile media. Also, the pellet changes phase from a solid to a gas almost instantaneously upon impact, which effectively provides an almost nonexistent coefficient of restitution in the impact equation. Very little impact energy is transferred into the coating or substrate, so the Hailstorm Dry Ice Blasting blasting process is considered to be nonabrasive.
Thermal Shock Effect
Instantaneous sublimation (phase change from solid to gas) of CO2 pellet upon impact absorbs maximum heat from the very thin top layer of surface coating or contaminant. Maximum heat is absorbed due to latent heat of sublimation.
The very rapid transfer of heat into the pellet from the coating top layer creates an extremely large temperature differential between successive micro-layers within the coating. This sharp thermal gradient produces localized high shear stresses between the micro-layers. The shear stresses produced are also dependent upon the coating thermal conductivity and thermal coefficient of expansion / contraction, as well as the thermal mass of the underlying substrate. The high shear produced over a very brief expanse of time causes rapid micro-crack propagation between the layers leading to contamination and/or coating final bond failure at the surface of the substrate.
The combined impact energy dissipation and extremely rapid heat transfer between the pellet and the surface cause instantaneous sublimation of the solid CO2 into gas. The gas expands to nearly 800 times the volume of the pellet in a few milliseconds in what is effectively a “Micro-explosion” at the point of impact.
The “Micro-explosion,” as the pellet changes to gas, is further enhanced for lifting thermally-fractured coating particles from the substrate. This is because of the pellet’s lack of rebound energy, which tends to distribute its mass along the surface during the impact. The CO2 gas expands outward along the surface and its resulting “explosion shock front” effectively provides an area of high pressure focused between the surface and the thermally fractured coating particles. This results in a very efficient lifting force to carry the particles away from the surface.
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What is Dry Ice?
CO2 is a natural medium, which serves many life sustaining purposes. It is a key element involved in the carbon cycle; it is the only source of carbon for the carbohydrates produced by agriculture; it stimulates plant growth; and it helps to moderate the overall temperature of the earth. Animal respiration is believed to add 28 million tons of carbon dioxide per day into the atmosphere. By contrast, the U.S. CO2 industry can supply only 25,000 tons per day and 95% of this amount is from by-product sources, or less than 0.04% of the other sources combined.
With a low temperature of -78° C, dry ice solid has an inherent thermal energy ready to be tapped. At atmospheric pressure, solid CO2 sublimates directly to vapor without a liquid phase. This unique property means that the dry ice blast medium simply disappears, leaving only the original contaminant to be disposed of. In addition, cleaning in water sensitive areas (e.g. in the vicinity of electrical cabinets) is now practical.
The grade of carbon dioxide used in blasting is the same as that used in the food and beverage industry and has been specifically approved by the FDA, the EPA and the USDA. Carbon dioxide is a non-poisonous, liquefied gas that is both inexpensive and easily stored at work sites. Of equal importance, it is nonconductive and non-flammable.
CO2 is a natural by-product of several industrial manufacturing processes such as fermentation and petrochemical refining. The CO2 given off by the above production processes is captured and stored without losses until needed. When the CO2 is returned to the atmosphere during the blasting process, no new CO2 is produced. Instead, only the original CO2 by-product is released.
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