Cleaning mechanism
Ice
particles change phase at normal working temperatures, therefore are
ideally suited as a blast cleaning agent. As a phase change solid, ice
particles moving at high speed can perform impact cleaning work before
phase change, during phase change and after phase change.
Before
Phase change,
ice particles are solids possessing momentum to displace contamination on
a target. Displacement results when ice particle momentum exceeds the
inertia of the contamination. Displacement is the “bulk removal”
components of cleaning. Figure 6 serves as a pictorial illustration.
Figure
6: Displacement work on impact.
This
shows that, for contamination deposited on a planar surface, maximum
displacement force is achieved at 0 degree [4].
In reality, surfaces have irregularities and the optimal displacement is
achieved at some non-zero angle.
During
phase change,
ice particles exert a pressure against the surface as it deforms,
providing a strong icesurface frictional interaction whereby minute
amounts of contaminations can be scrubbed away. Scrubbing represents the
“detail cleaning” component of cleaning. Figure 7 illustrates this
stage.
Figure
7: Frictional work during impact.
The
above shows that the maximum force is achieved at normal attack, 90
degrees from the maximum for displacement. In practice, optimal cleaning
can be achieved at between 30L and 60L. It is interesting to note that
there is no longer a mass or size dependence. The power of this scrubbing
force is substantial: at a blast pressure of about 10 bars, it has been
estimated to be approximately 300 bars.
After
phase change,
ice particles melt into water to rinse away removed contamination, as
shown below.
Figure
8: Rinsing work after impact.
The
fact that water is generated is important for cleaning applications. First
water is a solvent that removes all soluble salts. The blast mist also
encapsulates blast debris to control air-borne dispersion. This is
particularly important for worker safety in the removal of asbestos fibers
and radioactive contamination.
[4]
Timoshenko, S.T., Goodier, J.N., 1970, Theory of Elasticity, McGraw-Hill, NY,
pp. 398-402.