Test for success

The best hopper is the least expensive one that consistently provides the required solids flow. A few simple indices can pinpoint potential solids flow problems and help optimize design.

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Conical Hopper

Figure 1. This hopper’;s simple construction
often makes it the most economical choice.

  Chisel-shaped hopper. The advantage of the chisel-shaped hopper (Figure 2) is that the slide angle can be HI plus 10° from the vertical with the end wall angle (HI) the same as that of a cone. This can increase the chisel hopper’;s capacity while decreasing its overall height. The outlet width must be AI/2 or greater with a slot length at least 2.5 times the width. The maximum flow rate without air injection is evaluated by multiplying the FRI by the ratio of the outlet area, i.e., a 12-in.-diameter circle.

Chisel Hopper

Figure 2. This design allows greater hopper capacity
at a lower overall height than a conical hopper.

  Cone-in-cone hopper. A cone-in-cone hopper (Figure 3) saves headroom because the outside cone angle measured from the vertical is 2HI. The inside cone angle is HI. The inside cone outlet must exceed the AI; so, the outer cone outlet must be 2AI minimum. Using air injection to increase flow rates is not recommended for cone-in-cone hoppers because it creates non-symmetric flow. Often, flow will occur only in a small localized channel when the FRI limit is exceeded, even though the AI and HI criteria are met.

Cone-in-Cone Hopper

Figure 3. This design saves headroom,
but doesn’;t suit air injection.

  Diamondback Hopper. This is a one-dimensional convergence hopper that combines the advantages of a chisel hopper and the circular outlet of the simple cone. However, unlike the cone, the outlet diameter in a Diamondback Hopper can be as small as AI/2 (the same as the chisel slot width). This allows for a much smaller feeder. One set of Diamondback Hopper sides must be vertical and the other must be HI plus 20° from the vertical (Figure 4). This makes the hopper very effective for solids with small HI values. The limiting rate in a Diamondback Hopper is somewhat larger than the area-adjusted FRI. Air injection works well to accelerate flow rates.

Diamondback Hopper

Figure 4. This hopper combines the advantages
of a chisel hopper with a circular outlet.

Achieve solid success
Bulk materials are composed of individual particles that can vary in size, shape, surface roughness, moisture content and chemical composition. You cannot extrapolate the behavior of one material to other materials, even those that have similar chemical compositions.
The key to success is knowing a material’;s flow properties and understanding the conditions that affect them. This knowledge, coupled with the ability to easily and accurately interpret the property data, will provide a sound basis for process improvements and equipment design or selection.

Kristin Dudley is a consulting engineer for Diamondback Technology Inc., San Luis Obispo, Calif., a firm that specializes in equipment and consulting related to solids handling. E-mail her at kdudley@diamondbacktechnologies.com.  Lee Dudley is president of Diamondback Technology. E-mail him at ldudley@diamondbacktechnology.com.



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