We are considering a wheel blast machine to tumble-blast our parts. Some machines have “air wash separators” and some do not mention separators at all. Is it important to have a separator? And, will you explain what an air wash separator is and how it works? W.S.
You did not state what you will be blasting, and the answer depends on the application. Any wheel blast machine performs better with a separator. There are some light applications, however, that can get along without one. The extra capital outlay for a separator is large, and you have to weigh that against the benefit in your specific application.
The separator’s job is to remove the fines and contaminants from the abrasive stream. Fines result from the breakdown of the abrasive. Most wheel blast abrasive is steel shot or grit, although other more or less friable abrasives are used. The fines accumulate and they reduce the cleaning power of the blast. The contaminants are the burrs, scale, sand, paint or other material removed from the parts. As little as 2% accumulated sand or forging scale in the blast stream doubles the wear rate on the high wear wheel parts —impeller, control cage and blades.
In applications other than sand and heavy scale removal, the contaminants are not so damaging to wheel parts. The cost of a separator may not be justified if, for example, you are blasting for light deburring or to impart a decorative finish. Machines without separators generally have dust collector connections to one or more cabinet vents, turning over the cabinet air many times each minute. This approach saves initial investment and may be adequate. The problem is that a strong air stream for evacuating the cabinet may pull considerable quantities of usable abrasive from the system. In such systems, it is a good idea to include a plenum chamber in the duct system for recovering some of the usable abrasive that otherwise is lost.
Air blast systems designed for light abrasives usually use only cabinet ventilation to remove the fines. These systems generally control loss of usable abrasive by baffling and reducing the cabinet ventilation air flow.
An air wash separator works with the heavier metallic abrasives. The abrasive mass, including all the fines and contaminants, is poured onto an inclined plate, making a waterfall of abrasive. This waterfall is spread across the bottom of the plate to give a thin, uniform, curtain of abrasive. Spreading is accomplished by a spreader plate, or bar, that regulates the thickness of the curtain. Spreader bars are free-floating, and push against the abrasive waterfall. They are weighted, or spring loaded, so the pressure is adjustable for making a uniform spread across the width of the inclined plate.
The thin waterfall of abrasive drops into the hopper and is returned to the wheel. An adjustable stream of air is pulled through the waterfall, deflecting the lighter pieces —sand, scale, paint, small burrs, broken abrasive—so that they fall to the rear of the waterfall. Another baffle, located behind the waterfall, will skim off the fines that have been deflected. The air volume, and for some machines, the skimmer baffle can be adjusted.
Understanding an air separator and how to adjust one is important for controlling the cost of wheel blasting. An improperly adjusted separator will either dramatically increase your abrasive consumption, or your wheel maintenance. A valuable tool for adjusting the system is a stacked series of laboratory screens for analyzing the size distribution of the abrasive going through the wheel. It also is used for detecting useable abrasive removed by the separator or lost to the dust collector.
Blast equipment vendors can help you decide if you need an air wash separator. And, if you do purchase one, they can help you select the screen sizes for your analysis, and explain about analysis of the abrasive stream, the separator discharge and the dust collector refuse.