- THE MAGAZINE
Powders can flow like water through any crack or crevice—even through clearances in rotary air locks—creating process instabilities, hazardous spills or serious housekeeping issues. Known as powder flooding or flushing, this problem affects a number of ceramic manufacturing facilities. Fortunately, the flow of even very fine powders can easily be controlled with the right types of batching equipment.
Causes of Air EntrainmentPowder flooding results from excess air that is trapped in voids between the individual powder particles. There are three common causes of air entrainment in powders: 1) back pressure from a baghouse, 2) a rathole flow pattern, and 3) uncontrolled air injection or gas generation.
Back pressure from a baghouse often occurs with a typical pressure pneumatic system (see Figure 1). This pressure, which usually amounts to only about 10 in. of water pressure, can cause severe flooding from feeders at the hopper bottom when it is coupled with air entrained in the conveyed solids.
Another more frequent cause of air entrainment is a rathole-type flow pattern that occurs whenever a hopper’s walls are neither smooth enough nor steep enough to result in flow at the walls. A typical conical hopper usually requires 70- to 80-degree slope angles to accomplish an even powder flow. If the rathole caves in when the hopper is partially or completely emptied, or if a high rate of powder flow is introduced into a hopper with a rathole, the incoming solids can entrain a bubble of pressurized air that fluidizes the powder. In this instance, flooding through belt, vibratory or screw feeders is almost certain to occur.
Uncontrolled air injection or gas generation from a chemical reaction will also fluidize a fine powder. The only appropriate use for uncontrolled air injection is in a pneumatic conveying system transporting powders. Even then, wide fluctuations in the feed rate are probable.
Flooding can also occur as a result of powder freefall. Voids between the powder particles entrain air, which also accelerates along with the powder. When the powder and entrained air reach the material surface, air is trapped within and fluidizes the product. The product can then flow from the bin’s outlet at an uncontrolled rate. This problem is especially relevant when the hopper is small relative to the flow rate and the retention time is short. A short retention time prevents air from escaping and allows the powder to maintain its fluidized condition.
Controlling Excess AirThe typical approach for controlling unwanted excess air is to use a rotary valve at the hopper outlet. Unfortunately, the rotating vanes of the valve tend to pump air into the hopper outlet, actually adding to the air supply instead of reducing excess air. As a result, flooding can occur through the valve when wear reduces the valve’s normally close tolerances. The blowback from a rotary valve can be reduced by using a rotary valve transition, which is designed to deliver powder to the rotary valve while venting away the blowback gas (see Figure 2).
Using a screw feeder can also provide continuous powder feed without introducing additional air. The screw compacts and seals against air pressure in the hopper, and it will also squeeze out excess air if it’s placed above the hopper.
As the powder drops into the hopper, it often re-aerates. Using a letdown chute that protrudes into the surface of the powder layer can dispel entrained air and allow the powder to discharge around the chute as the powder level changes. A letdown chute will also reduce the segregation of superfines by reducing entrained air.
Another way to eliminate unwanted excess air is by removing the air before the powder discharges. A deaeration cylinder that is vented or connected to a dust collector can be used to accomplish this goal. Since the perforated cylinder is self-cleaning, the air release from the cylinder will be unimpeded.
While excess air can cause problems, powder that is completely deaerated might result in flow rates that fall far below the required level. The key is eliminating enough air to prevent flooding or flushing while maintaining the right amount of air to produce consistently high flow rates. This can be achieved through an air permeation system that controls both the air pressure and the rate of injected air (see Figure 3).
Finally, a hopper with a one-dimensional convergence design can be used to minimize flooding potential even without air injection. This type of hopper works by increasing the solids contact pressure at the outlet. The increased pressure allows powder flow without excessive void expansion, thereby reducing the air vacuum created when voids expand. A reduced vacuum can increase the limiting flow rate by a factor of two or more.
Adding a vertical section between the hopper outlet and feeder, can further enhance the hopper’s effectiveness. Power in the vertical section provides suction at the hopper outlet and, in some cases, can eliminate the vacuum effect at the outlet.