Union-Bulletin

WALLA WALLA -- Gum be gone.

City and downtown officials got a look at a possible new way to get rid of old chewing gum, grime and other gunk Wednesday.

In front of a small group of onlookers gathered at Land Title Plaza, Dave Tolliver of Oxarc demonstrated how tiny pellets of dry ice driven at near-supersonic speed could provide a no-mess method to clean city sidewalks and other surfaces.

"Think of it in a sense as sandblasting without the sand," Tolliver said. "When the dry ice gets under something, it sublimates and blows it off."

Another advantage is the rice-grain size pellets quickly evaporate, leaving nothing behind. "All you have left is what you've removed," Tolliver said.

Among those watching the demonstration were Jim Dumont, Walla Walla Parks and Recreation director, and Elio Agostini, Downtown Walla Walla Foundation executive director. "It works on gum, that's for sure," Agostini observed.

Although dry ice blasting has been around since the 1970s, it used to need large and bulky machinery, Tolliver said. But technology has enabled delivery systems to become smaller and easier to use. The machine Tolliver used Wednesday, an IceTech model KG-50 Pro, is a compact cube that one person could wheel around easily. It did, however, require a large trailer-mounted compressor for its air supply and a 110-volt outlet for power.

While the demonstration showed dry ice blasting works, questions remained as to cost-effectiveness and whether it would be preferable to other methods. Another small issue was whether the dark-colored spot left after the gum had been removed would remain. "I doubt it, but we'll see," Tolliver said. "The proof will be in the pudding."

The most expensive parts of the system are the blaster itself (the model used Wednesday cost about $20,000) and the air compressor, although the blaster can be rented by the day, week or month. The cheapest component is the dry ice pellets.

"That costs about 55 cents a pound," Tolliver said.

HOW DOES DRY ICE BLASTING WORK?

• Dry ice pellets are accelerated in a jet of compressed air and strike the coating to be removed at velocities up to the speed of sound. Cleaning results from three effects:
• Kinetic effect-- When dry ice pellets strike a surface at the speed of sound, any coating on the surface is cracked and loosened.
• Thermal effect -- The low temperature of dry ice pellets makes the coating brittle, cracks it and loosens it as a result of reduced bonding between the coating and the underlying surface. This allows dry ice to permeate the coating.
• Explosive effect (Sublimation) -- Dry ice penetrates the coating and immediately sublimes (passes directly from solid to vapour state). This results in a 700-fold increase in volume, an explosive effect that lifts the coating off the surface.