We have two large tumble-style wheel blast machines of similar capacity and horsepower, but one machine consumes almost twice as much abrasive as the other. Both machines use S-330 steel shot. The vendor did a screen analysis of the separator discharge, recommended new door seals on the offending machine, assured us that the separators on both machines were properly set, and set the abrasive flow to both machines for maximum efficiency. Both machines produce equal work, but one continues to use almost twice as much shot. What have we missed? A.K.
If the useable shot is contained in the system with good seals, is not being discharged from the separator, and is not carried out with the parts, there is only one place left for it to go. That is the dust collector. You did not state whether or not the vendor checked the dust collector fines. Good shot can get to the dust collector by means of the cabinet and elevator ventilation system. The first place to check is the vent on the elevator. Most of these machines use a bucket elevator to return abrasive from the process chamber to the separator. The buckets are scooping up hundreds, even thousands, of pounds of shot, fines, and dirt every minute. This area needs ventilation, and most machines have at least one dust collector vent on the elevator housing. A wide-open vent on the elevator can pull several pounds of good shot every hour. Another likely conveyance of shot to the dust collector is the cabinet vent system. All cabinet and elevator vents should have adjustable air flow valves. Set these valves for the minimum flow necessary to prevent dust emissions.
After adjusting the vent system it is necessary to re-adjust the separator vents since more air will now be pulled through the separator, increasing shot consumption.
The dust collector system probably has a plenum chamber separator in the main line going to the collector. This chamber is designed to re-capture good abrasive pulled out by the vent lines. There will be a one-way air valve at the bottom of the discharge line from the plenum. If that valve is not functioning properly, this last chance to save good shot will be lost.