Fine powders often need air injection to increase flow rates to an acceptable level. With entrained or injected air, powders flow like water, but when deaerated, the extremely low flow rate may be confused with a hang-up or arching condition. Unfortunately, too much air injection can cause uncontrolled flow that oscillates from extremely slow or no-flow to sudden flushing that spews powder from belts and screw feeders, small holes in hoppers or around the blades of worn or poorly adjusted rotary valves. Too little air will limit the rate to unacceptable levels.
Cement powder is an excellent example of the bimodal flow rates of fine powders. One example of this involved a handling system for transporting cement from barges to shore-side silos. The company unloaded the cement to a surge hopper using a clamshell-equipped crane. The gravity surge hopper alternately fed twin blow tanks for the final dense-phase pneumatic lift to the silos. Because the surge tank was outside, dust was a major concern. Consequently, the crane operator was instructed to lower the clamshell to the top of the cement before opening it. This avoided any dusting associated with dropping the cement into the surge tank; however, this action also left the cement in a totally deaerated condition. As a result, when the cement tried to flow through the hopper to the outlet, it expanded and created a vacuum in the voids between the dry cement particles. The in-rush of air from the hopper outlet into these voids caused an extremely slow cement flow rate, even from a 12-inch-diameter outlet. The pneumatic conveying system could not achieve design capacity because of the extended blow tank fill time.
Ironically, the same powder that has a flushing tendency will also cause limiting rates if allowed to deaerate in the bin. In this case, carefully controlled air injection is essential.
The solution to this classic limiting rate problem is air injection in the hopper. The injected air pressurizes the void space and allows the cement to expand without air flow from the hopper outlet. Because the amount of air needed is only that required to expand the voids, there is no danger of dusting from the small amount of injected air. With controlled air injection, the cement flow rate is easily increased ten fold or more.
If a material has a flushing tendency, you should consider
▪ enlarging the bin's surge capacity and retention time
▪ running at a consistently higher level
▪ using a deaeration cylinder insert
▪ using a special compacting screw such as the Diamondback Arch-breaking Screw Feeder.
