Let's talk about ice blasting – a technology that may have a future in the heat-treatment industry.
What is ice blasting?
Ice blasting is a cleaning process using dry ice (CO2). It is most commonly associated with cleaning of industrial equipment such as heat exchangers, tanks, and gas and steam turbines (Fig. 1). One of its key advantages is that it can typically be performed without major disassembly of components while avoiding fire or electrical hazards. Ice blasting is commonly used to clean food-processing equipment to effectively decontaminate surfaces to avoid such bacterial agents as salmonella enteritidis, E. coli and listeria.
In fact, the EPA recommends dry-ice blasting as an alternative to many types of solvent-based cleaning. Dry-ice blasting can clean multiple objects with differing, complex geometries at the same time. Due to the blast media sublimating without residue, dry-ice blasting is used as a maintenance tool in a variety of industries, including:
- Food manufacturing/processing
- Power generation
- Plastics manufacturing
- Printing and packaging
- Chemical manufacturing
- Fire restoration, mold remediation and historical preservation
- Petroleum refining (catalytic reduction units)
- Beverage and bottling
How it Works
Carbon dioxide (CO2) blasting works by combining three primary factors: pellet kinetic energy, thermal-shock effect and thermal-kinetic effect. Optimized cleaning is a result of controlling the various process parameters, including:
- Compressed air pressure
- Blast nozzle type (velocity distribution)
- Pellet size and density
- Pellet mass rate and flux density (particles per unit area per second)
- Pellet kinetic energy
Pellet Kinetic Energy
At high impact velocities and direct head-on impact angles, the kinetic effect of solid CO2 pellets is minimal when compared to other media (e.g., grit, sand, glass). This is due to the relative softness of a solid CO2 pellet, 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 ice-blasting process is considered to be nonabrasive.
Instantaneous sublimation (phase change from solid to gas) of CO2 pellets 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 microlayers within the coating. This sharp thermal gradient produces localized high shear stresses between the microlayers. The shear stresses produced are also dependent on the coating’s 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 microcrack 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 “microexplosion” at the point of impact.
The microexplosion 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.
Heat-Treat Case Study
A company in Texas with a cold-wall horizontal vacuum furnace contracted a local ice-blasting service to expedite the cleaning of their furnace interior. The hot zone had been removed so that all surfaces of the cold wall and the front door were easily accessible. A thick buildup of braze residue and soot coated these surfaces.
Using low pressure to avoid removing any of the interior’s epoxy-painted surface, the removal process began. It was observed that some exposed areas showed signs of failing paint coverage (flakes and bubbles). The process was able to remove the damaged paint and leave behind a clean surface that could be recoated with epoxy. Altogether, the blasting produced only a dust-like residue at the entrance floor of the furnace. This was easily swept up with a broom and dustpan.
The entire cleaning process took about five hours of blasting and used a minimal amount of carbon dioxide. The result was a pristine interior. In fact, the process worked so well that the decision was made to attempt to clean a number of other internal parts on the hot zone itself – from graphite to steel – and even the copper power feedthroughs.
The heavy buildup came off of all parts with no damage introduced. Regular high-velocity nozzles and high ice flow were used on the copper and steel. For the graphite, a fragmentation nozzle (makes the CO2 pellets similar to the consistency of table sugar) was used. The process required another 5-6 hours of blast time to remove all contaminants from the individual components that made up the hot zone. Thus, the entire process could be completed in a 10-hour workday.
Reasons for Using Dry-Ice Blasting
Users have provided the following list of reasons why they feel dry-ice blasting should be considered to simplify and reduce the time involved in common maintenance practices.
- It has fewer cleaning cycles and more effective cleaning cycles (reduced equipment downtime).
- Machines can be cleaned in place with minimal, if any, disassembly/reassembly (reduced equipment downtime).
- It is a dry process with no residue (environmentally friendly).
- It is a quicker cleaning method than alternatives (reduced equipment downtime).
- It is nonabrasive, nonflammable and nonconductive: dry ice blasting won’t damage most substrates and can be used safely on electrical components provided the power is turned off and remains off until the components have returned to room temperature.
- There is minimal secondary waste cleanup and disposal (reduced labor costs – temporary containment areas can possibly be reused).
- It can get into tight spaces that many other methods can not.
- It meets USDA, FDA and EPA guidelines (environmentally friendly).
- There is minimal exposure to chemicals or grit media (operator safety).
- It is not as labor intensive as traditional cleaning methods (reduced equipment downtime, lower cost).
The cost, speed and efficiency of ice blasting make it an option that can possibly reduce maintenance time for heat treaters.
1. Brandon Kelly, Global Ice Blasting (www.globaliceblasting.com), technical contributions and private correspondence