Snow Cleaning systems rely on the expansion of either gaseous or liquid carbon dioxide. The output stream is usually a high velocity solid and gas mix and focused at the surface for cleaning. Cleaning is accomplished by a combination of momentum transfer and solvent action of the dry ice with surface contamination. Cleaning mechanisms are discussed elsewhere on the site. CO2 Snow Cleaning can remove particles of all sizes, even submicron sized particles, and also hydrocarbon based deposits and films.
The most common commercial approach to the snow cleaning technology involves single expansion nozzles with high velocity outputs. The goal within the orifice and nozzle design is to have a constant enthalpy expansion and a high velocity stream. The asymmetric venturi nozzles (supersonic nozzles) can yield these conditions. Other nozzle geometries give rise to high velocity snow streams but are less focused, may need nitrogen boosting, or can compromise organic removal abilities.
Carbon Dioxide Snow Cleaning units from Applied Surface Technologies use the asymmetric venturi nozzle design and generate a high velocity snow stream. With this selection, the snow spray systems can remove both particulates and organic residues and can be formed with either a liquid or gas CO2 source.
More details can be found at our snow formation page.
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CO2 Snow Cleaning removes both particulates and organic based residues. Particles of all sizes can be removed, from visible to submiron. To date, the smallest particles removed have been about 0.03 microns. This is shown in our Atomic Force Microscopy Section for the quartz example.
CO2 Snow cleaning also removes organics, especially hydrocarbon based contamination. Oils, fingerprints, facial grease, machine shops lubricants, and many others all come off quickly. Even silicone based contamination can be removed.
The particle and organic removal steps are simultaneous.
The cleaning process leaves no residues, is non destructive and there is no chemical waste to handle after cleaning.
A simple example of particle and organic removal from the exact same areas can be seen at Cleaning Examples of Particle and Organic Removal.
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When either liquid or gaseous CO2 is expanded from high pressures, such as cylinder pressure of 800 psi., the large pressure and temperature changes lead to solid CO2 nucleation. Factors that determine the effectiveness of the phase changes, the output velocities, organics removal, the nondestructive nature of the cleaning process, and details on the thermodynamics of snow formation can be found at Thermodynamics of Snow Formation. First year college Chemistry is useful to understand the page.
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Essentially, CO2 Snow Cleaning is a hybrid method, a mix of impact energy of pellets and solvency action of liquid CO2. It is the only CO2 cleaning process that removes particles and organic residues simultaneously without any other assistance. It is a continuous process, not batch like supercritical or liquid CO2. Further, it is non destructive to well bonded overlayers, opposite the abrasive power of pellets. Another major difference is the price, CO2 snow cleaning equipment starts are about $1700 for a full system, the other processes require a factor of 10 greater costs to get going.
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The simplest description of cleaning mechanisms are described as a combination of momentum transfer between the incident CO2 Snow and surface particle populations. The snow's collisions with surface particulates impacts momentum to the surface particulates and frees them from the surface. The blowing gas just sweeps them away.
Organic removal is by two possible mechanisms, solvency and freeze fracture. When the dry ice snow strikes the surface, the pressure increases between the surface and the dry ice. Once this pressure exceeds 78 psi., the triple point pressure, the solid phase reverts back to a liquid phase. Liquid CO2 is an excellent solvent and can absorb large quantities of hydrocarbons readily.
CO2 Snow cleaning can also remove silicones, and flux residues under certain conditions. In this process, the desired species are not soluble in liquid CO2 and thus a another mechanism is present. Here, the removal is by freeze-fracture, just by freezing the debris off.
There are more details to the cleaning mechanisms and the reader can get the details at Cleaning Mechanisms.
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The debris and snow just fly away. Since it does, the user has to make sure that proper venting or capture is done and no clean regions get re-contaminated.
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Proper cleaning using CO2 Snow Cleaning requires attention to the set up and details. Proper cleaning using CO2 Snow Cleaning requires attention to the set up and details. A proper CO2 Snow cleaning set up has methods to:
