All Hoppers Are Not Created Equal!
by Joseph Marinelli

In my previous two articles, I discussed measurement of wall friction angles and the determination of hopper angles required to ensure mass flow. Hopper shapes such as cones and wedges were discussed.

Typical wedge shaped hoppers are shown in Fig. 1. As I stated in my previous articles, wedge shaped hoppers are more forgiving than conical hoppers and can handle materials with a wide range of flowability. There are several distinct advantages (and some disadvantages) of wedge shaped hoppers over conical shaped hoppers, as follows:

Figure 1.  Wedge-Shaped Hopper Examples

Advantages

1. Wedge hoppers require less steep hopper walls to ensure mass flow. Typically, they can be about 11° less steep than cones and still promote flow along the walls -mass flow. As a matter of fact, the recommended angle can at times be exceeded without jeopardizing mass flow. This approach will obviously provide a hopper that is not as tall as a cone, with significant headroom and potentially significant cost savings.
2. Wedge shaped hoppers require a smaller width than conical diameters to prevent bridging. For example, say your material is cohesive enough to bridge over a 10-inch circular opening. You would probably use a 12-inch diameter opening to prevent your material from bridging. If you used a wedge configuration (with a slotted opening), the slot opening has to be about 6-inches wide (by about 18-inches long, to minimize end wall effects) to prevent bridging with the same material. Consider that this will allow you to use a smaller feeder.
3. Wedge shaped hoppers will allow material to flow at a higher discharge rate. This is simply because of the increased cross-sectional area afforded by the slotted opening.

Disadvantages

There are some disadvantages that have to be considered as follows:

1. Wedge shaped hoppers may cost more to fabricate than a conical geometry.
2. The feeder used to discharge material must be capable of discharging material over the entire cross-sectional area of the slotted outlet. This requires a specially designed screw or belt for use with the hopper. Keep in mind that this feeder may be more expensive than one used for a circular opening. Also consider that this feeder will discharge at its end. If you require discharging along the centerline of your bin, a conical configuration may be more advantageous.

The advantages and disadvantages stated above will allow you to make an educated decision as to which geometry to use for your material. Remember the following:

Know your material flow properties!  Be sure that your feeder will maintain a mass flow pattern!

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.