Dry-ice blasting

Dry ice blasting is a process where a medium is accelerated in a pressurized air stream to impact a surface to be cleaned or prepared. This is similar to sand blasting, plastic bead blasting, or soda blasting. However, instead of using hard abrasive media to grind on a surface (and damage it), dry ice blasting uses rice grain-sized dry ice (solid carbon dioxide) pellets as the blasting medium. The pellets are accelerated at supersonic speeds and create mini-explosions on the surface (due in part to the sublimation process which expands the dry ice volume 800 times upon impact) to lift the undesirable item off the underlying substrate.^{[citation needed]}

Dry ice is non-conductive, chemically inert, non-poisonous and nonflammable. The key advantages of this technology are its ability to clean sensitive surfaces and the fact that there are no residues of the blasting medium after the blasting process since dry ice sublimes at room temperature.

Process

Dry ice blasting works because of three primary factors: pellet kinetic energy, thermal shock effect and thermal-kinetic effect.

Pellet Kinetic Energy

Kinetic impact force is a product of the dry ice pellet mass and velocity over time. Dry ice blasting achieves the greatest impact force possible from a solid CO₂ pellet by propelling the pellets at supersonic speeds.

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). This is due to the relative softness of 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 a nonexistent coefficient of restitution in the impact equation. Very little impact energy is transferred into the coating or substrate, so the dry ice 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 contaminant (or coating) final bond failure at the surface of the substrate.

Thermal-Kinetic Effect

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 due to 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.

Types of dry ice blasting machines

There are two basic dry ice blasting systems in existence:

single-hose (airlock) systems
dual-hose (venturi) systems

The basic difference is the resulting blasting aggression, although aggression can also be influenced by the dry ice media used (shaved dry ice block or dry ice pellet) and the applicator design (the effective blast power of an applicator of a given length is inversely proportional to the area of the blast opening). Single hose dry ice blasting systems are the predominant technology used as of 2007.

Single-hose systems

Single-hose technology was pioneered and introduced by Cold Jet, LLC in 1986^[1]. In single-hose systems there is one hose leading from the dry ice container (hopper) of the blasting machine to the applicator (hose nozzle) and there is a feeder system that feeds the dry ice particles and compressed air into that single hose. The dry ice particles are then accelerated by the compressed air stream through the entire length of the hose, dramatically increasing their kinetic energy and aggressiveness.

A key advantage of single-hose systems is the ability to use longer hose lengths, which allows an operator to be further from the machine with little reduction in blast performance. Single-hose system aggression is generally better for removing heavier build-up or for blasting at vertical elevations where the machine is at a lover level than the blasting surface.

single hose systems also have the following limitations:

The airlock used to allow ice pellets to be placed into the air stream will only provide intermittent operation. This causes in vibration at the applicator. This has been known to cause additional operator fatigue.
The additional complexity of the airlock reduces overall reliability of the system.
because dry ice is normally stored at its sublimation point it has a tendency to 'cake', a process similar to the formation of a snowball, within the airlock jamming the system.

Two-hose systems

Early dry ice blasting systems used dual-hose technology, a system that uses the Venturi effect to accelerate the dry ice particles. In two-hose systems compressed air is supplied to the blast applicator (hose nozzle) through one hose, while dry ice is supplied through a second hose running from the applicator to the dry ice container (hopper). The passage of compressed air through the applicator creates a suction on the second hose that pulls the dry ice particles from the hopper into the compressed air stream at the applicator. The dry ice particles and compressed air are then blasted together.

As the dry ice particles are only accelerated the length of the applicator by the compressed air, the dual hose system has a smaller kinetic effect and offers less aggression than single hose technology. In spite of FUD attempts by manufacturers of single hose systems the actual difference in cleaning power is negligible for the most applications. The reason for this is that no system can accelerate particles faster than the speed of the air in the applicator. Ice particles will begin to approach this speed in any applicator longer than a half meter.

Two hose systems also have the following limitations:

the length of the hose is limited by the two-hose suction capability, which then limits how far away the blasting can take place from the machine. The limit for effective use is typically the length of the supplied hose. Though, for certain applications (mold and fire remediation, hose lengths of two to three times this can be effective.
the limited aggression of the two-hose system will not allow for vertical blasting because the suction would also have to overcome gravity. The vertical distance from the machine to the applicator is typically limited to the length of the supplied hose.
the lower efficiency and impact velocity created by pulling particles into nozzle by suction results in lower blast power for a given air supply as compared to single hose machines.

Two hose systems also have the following advantages:

the venturi delivery system has no moving parts resulting in higher reliability.
the venturi technology, being simpler, is considerably less expensive than single hose systems.
the two hose system separates the hot compressed air (see ideal gas law) is separate from the flow of the cold dry ice. This separation allows the use of extremely fine granules that would otherwise sublimate in the blast hose. This in turn may be exploited allow a further decrease in aggression for particularly delicate applications.

Benefits

Dry ice blasting equipment manufacturers claim many unique and superior benefits over traditional blasting media and cleaning methods, including the following:

Dry ice blasting is non-abrasive, nonflammable and nonconductive
Dry ice blasting is environmentally-friendly and contains no secondary contaminants such as solvents or grit media
Dry ice blasting is clean and approved for use in the food industry
Dry ice blasting allows most items to be cleaned in place without disassembly
Dry ice blasting can be used without damaging active electrical or mechanical parts or creating fire hazards
Dry ice blasting can be used to remove production residues, release agents, contaminants, paints, oils and biofilms
Dry ice blasting can be as gentle as dusting smoke damage from books or as aggressive as removing weld slag from tooling
Carbon dioxide is a non-poisonous, liquefied gas, which is both inexpensive and easily stored at work sites

Comaprison with other industrial cleaning methods

Abrasive blasting

The abrasive blasting process results in some level of cleanliness and roughness with sand being the most common blasting media. Like all open blasting, sandblasting creates fugitive dust, and this dust can be toxic as it is contaminated with the removed substance. Even if not toxic it is a nuisance, creating mess and dramatically shortening the life of all nearby moving parts through wear.

Dry ice blasting uses nonabrasive dry ice that for most substrates will not wear away the surface being cleaned and won't create additional waste for disposal. In addition, many industrial applications allow machines to be cleaned in place without removal or disassemble when using dry ice blasting as opposed to typical media blasting.

Soda Blasting

Soda Blasting is generally an effective cleaning method. However, Soda Blasting, like all open blasting, creates a great deal of secondary waste. Often, the time spent blasting is matched, if not doubled by the time it takes to clean up the extra waste soda blasting creates. In fact, the residue and waste left behind by soda blasting can adhere to wood and other substrates being blasted.

There is also evidence that soda blasting can have a negative effect on the PH levels of the soil it comes into contact with after blasting, thereby killing surrounding vegetation. This is not the case with CO₂ blasting

High Pressure Water Blasting

While it is an effective method of cleaning, water blasting has limited applications. On steel surfaces, for example, it cannot create any specific surface profile, which is a key parameter in paint performance. Also, the use of water induces flash rusting, which makes paint or coating application more difficult and risky. Furthermore the use of water blasting on production equipment including automated welding lines, presses, motors and machine tools can result in severe electrical problems.

Dry ice blasting is a dry non-conductive process and can be used on any variety of materials as well as on or near electrical equipment.

Solvent Cleaning

Most solvent cleaning processes involve substances that are detrimental to the environment and worker safety. When solvents are used to dissolve unwanted surface materials, a subsequent flushing, rinsing or hand tool operation is frequently required to remove the dissolved materials. Equipment must often be disassembled or extensively prepared prior to the solvent cleaning to protect sensitive portions. Solvent management and disposal are also costly issues for businesses.

Dry ice blasting dissolves and blasts away unwanted material in one step. Like solvent baths, the dry ice blasting process can simultaneously clean numerous objects with differing, complex geometries. Plus, dry ice blasting systems provide safe, thorough, in-place cleaning for components, subassemblies and complete machines. The dry ice sublimates on contact with the surface, preventing the creation of any secondary waste for cleanup. The EPA recommends dry ice blasting as an alternative to many types of solvent-based cleaning.^[2]

Power Tool Cleaning

Power tool cleaning can provide a quick solution for flat, simple geometries - but it can also damage or wear down surfaces. Dry ice blasting provides the benefits of power tool cleaning without added wear on expensive molds and other production tooling.

Hand Tool Cleaning

Quick, easy tasks that would take too long to set up for more mechanized approaches are often accomplished using hand tools that may end up damaging equipment. Dry ice blasting can result in subtantially fewer labor hours required for cleaning when compared to cleaning by hand, and eliminate the potential for part damage.

Environmental impact

Dry Ice Blasting is a clean and safe industrial cleaning process. Key environmental indicators include:

Dry ice blasting is safe to use with food processing equipment^{[citation needed]}
Dry ice blasting does not generate secondary waste as does sand, soda, or water blasting which can leave toxic secondary waste to be cleaned up in addition to the toxic substrate
Dry ice blasting is safe and non-toxic (once pellets impact the surface they dissipate into the atmosphere) whereas soda blasting is believed to kill surrounding vegetation
Dry ice blasting reduces or eliminates exposure to dangerous chemical cleaning agents

In addition to being clean and safe, dry ice is also obtained as a bi-product of other industrial processes - i.e. it is made from reclaimed carbon dioxide. It does not produce or add carbon dioxide to the atmosphere and therefore does not contribute to the greenhouse effect.

Safety considerations

The temperature of dry ice at normal presesure is -78 °C. Insulated gloves should be worn to handle dry ice. Eye and ear protection should be worn at all times when operating (or in the vicinity of) dry ice blasting equipment. Even though carbon dioxide is non-poisonous, it does displace oxygen in the atmosphere so working spaces should be properly ventilated. Because carbon dioxide is 40% denser than air, placement of exhaust vents at or near ground level is recommended when blasting in an enclosed area. In an open environment, existing ventilation is sufficient to prevent asphyxiation by carbon dioxide buildup.

History

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)

References

External links

[1] Moore, David E., US patents#4,617,064 and #4,744,181

[2] EPA- TECHNICAL FACT SHEET FOR 1,1,1-TRICHLOROETHANE (TCA) HAZARDS AND ALTERNATIVES

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