In many cases, poor flow from a storage vessel can be caused by the feeder drawing material from the vessel and discharging it into the downstream process. Effective material discharge from storage requires that the feeder withdraws material uniformly through the entire cross-section of the hopper’s discharge outlet. Regardless of the hopper design, an obstructed outlet due to a poorly designed feeder or partially closed slide gate will result in funnel flow. The following guidelines can help prevent flow problems from a poorly designed screw, belt, or rotary valve feeder.
Mass-flow screw feeder. The key to a proper screw feeder design is to provide an increase in capacity in the direction of the material feed. This is critical when the screw is used under a hopper with a slot-shaped outlet. A standard screw with a constant-diameter shaft and constant-pitch flights will pull material preferentially from the back of the hopper outlet, as shown in Figure 1a, because the first screw flight is the only one that isn’t already filled with material.
A mass-flow screw feeder, as shown in Figure 1b, overcomes this problem by using a tapered (or conical) shaft in the screw’s first section and flights that increase in pitch in the following section. This allows material to enter the screw along the entire length of the hopper opening rather than just filling the first flight while the rest of the material remains stagnant.
Mass-flow belt feeder interface. As with a screw feeder, the key to a proper belt feeder design is to provide increasing capacity in the direction of the material feed. This is achieved by using a mass-flow belt feeder interface, as shown in Figure 2. The belt feeder interface tapers in both plan view and elevation view to allow the material pile on the belt to grow wider and taller in the direction of the feed. The interface also has a slanted nose (or front end) with an arch-shaped cutout to ease material flow onto the belt.
Rotary valve. A rotary valve feeder mounted directly to the hopper outlet may have a tendency to develop a preferential material flow channel on the side of the hopper outlet where the empty valve pockets are first exposed to the material. The valve pocket quickly fills with material from this channel while material stagnates over the remaining portion of the hopper outlet, potentially causing bridging, ratholing, and other flow problems. You can typically prevent this by installing a vertical pipe section with a height about one to two times the hopper outlet diameter between the hopper outlet and the rotary valve inlet. If the storage vessel is feeding into a pressurized conveying system, the rotary valve should also be vented to avoid gas counter flow, where conveying gas flows up through the rotary valve and impedes the flow of the discharging material stream. This can be a particular problem with fine powders.
Eric Maynard is vice president at Jenike & Johanson.