Brad Nichols | Syntron Material Handling
This article explains how hopper and hopper-to-feeder transition design can help offset material flowrate problems.
Peat, rock, flour — no matter which material you process, your hoppers must move it to feeders at an optimal flowrate without damaging the material. To accomplish this, both the hopper and the feeder need to work together. Sub-optimal design of either piece of equipment — or of both of them — can adversely affect processing.
This article focuses mainly on the hopper and hopper-to-feeder transition. The benefits of an ideal hopper design include uniform flow patterns, maximum capacity, optimized feeder size and performance, and reduced potential for material buildup and spillage.
Particle size distribution, bulk density, shear properties, and cohesive strength are main material factors that dictate the ideal hopper transition design.
- Particle size distribution. Material particle size and the range of particle sizes dictate the minimum openings of transition gates and throat dimensions. Improper sizing of these openings can promote bridging and improper flow from both the front and the rear of the hopper transition.
- Bulk density. Bulk density affects capacity and flow because of material-per-unit volume. Heavier materials tend to flow faster due to a higher momentum.
- Shear properties. Materials that are soft and spongy tend not to flow well from hoppers. Such types of material particles tend to pack into each other, causing bridging and promoting poor flow to the feeder.
- Cohesive strength. Material with enough cohesive strength, for instance due to stickiness or jagged edges, can cause slow movement through the hopper transition and possible bridging at the discharge. This may require a large opening or unique hopper design to allow for adequate flow.
These material factors, in conjunction with feeder stroke, stroke angle, and feeder vibrating frequency, can affect how well material flows from the hopper to and through the feeder.
Hoppers: From ideal to acceptable
Generally speaking, a properly designed hopper produces uniform material flow to the feeder trough, with material at the front of the hopper moving slightly slower than at the rear of the hopper. In addition, a properly designed hopper produces a bed depth of discharged material that’s slightly lower than the height of the hopper gate. This varies based on material particle size and cohesion. Uniform flow is achieved when the ratio of the hopper throat (T) and the gate height (H) is 0.6. (T = 0.6 x H or T/H = 0.6.)
While 0.6 is ideal, a ratio range of 0.5 to 1.0 also is acceptable. A T/H ratio below 0.5 or above 1.0 means material flow patterns can become distorted, which significantly reduces a hopper’s feedrate. Excess throat or improperly angled hopper walls and tapered skirt boards can result in non-uniform flow patterns, reduced capacity/bed depth, reduced material velocity, material buildup at the inlet, spillage at the back and sides, higher amperage draw, and an increased material load, possibly resulting in collapsed suspension springs. Compared to an ideally designed hopper, an improperly designed hopper might mean a larger feeder would be required to meet desired processing capacities.
To maximize your hopper’s effectiveness at moving material into a feeder, keep in mind the following recommendations. These pertain to free-flowing material. For non-free-flowing materials, contact your equipment supplier for specific design and specification advice.
- A hopper’s rear wall angle must be steep enough to permit material flow. A rear wall angle of 60 degrees, plus or minus 2 degrees, is recommended. A hopper’s front wall angle must be just steep enough to permit material flow, but the flowrate on the front wall should be slightly less than the rear wall’s flowrate. A front wall angle of 55 degrees, plus or minus 2 degrees, is recommended.
- The gate opening height must be at least two times the diameter of the largest material particle and should increase in proportion to the bed depth required to achieve the desired capacity. As stated earlier, the most economical feeder is selected when T/H = 0.6. If the T/H ratio is outside the range of 0.5 to 1.0, the material flow pattern is disturbed, which results in non-uniform flow.
- Gate location and type. When using an adjustable gate, its desired location is parallel to the hopper’s front wall. The gate must be as close to the front wall as possible and can’t be more than 2 inches away. The gate is acting as an adjustable front wall. Leveling blades and downstream gates need not be used to adjust the material flow. Horizontal shutoff gates can be used to perform feeder maintenance but mustn’t be used to regulate flow.
- For random-sized material, the inside width (between skirts) should be a minimum of 2.5 times the largest particle’s diameter. For near-size material, the width should be at least four times the largest particle’s diameter.
- The minimum length of the feeder is determined by projecting the angle of repose for the specific material from the gate point to the feeder pan and adding 4 to 6 inches to prevent a free-flow condition.
- The feeder mustn’t be in contact with any adjacent structures because it must be free to vibrate. Placement of the feeder in the processing line must include enough space around the feeder to allow its elevation to be decreased by approximately 2 inches to accommodate material load while the feeder isn’t operating. In addition, a 1-inch minimum clearance at the sides and a 1.5-inch clearance on the bottom and the rear wall of the feeder pan must be maintained under both loaded and unloaded conditions.
- Feeder skirts must taper in the direction of the material flow from inlet to discharge. In other words, skirts must diverge from the conveying surface. This is necessary to keep material from jamming and causing potential spillage and buildup. Additionally, skirts must run parallel to the feeder trough’s sides and must be reinforced to resist bulging outward against the trough.
For further reading
Brad Nichols (662-869-5711) is the engineering manager for Syntron Material Handling and is responsible for the development and design of the company’s material handling products. Brad holds a BS in electrical engineering from Mississippi State University and has more than 25 years of manufacturing and fabrication experience.
Syntron Material Handling • Saltillo, MS
800-356-4898 • www.syntronmh.com
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