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As our economy continues to slow down companies are resorting to retrofitting their existing equipment as opposed to buying new equipment. This is not an entirely bad situation. Sometimes existing equipment that has been retrofitted can work quite reliably. A scientific approach is required to identify your problem areas, evaluate your material properties and rectify your problems with existing solids handling equipment. The following is a discussion of what is necessary to provide a properly designed retrofit:

Identify your problem areas

This requires knowledge of solids flow problems and their results. If you have read our column before, you are aware of the types of solids flow problems that can occur; namely, no flow (due to arching or ratholing), erratic flow, flooding, segregation and limiting discharge rate. Sometimes it is difficult to determine if your product is arching or ratholing, or both. Even if you look into the container from the top, you may not see a rathole, even though it is forming within the product. Segregation problems are somewhat more difficult to solve and typically require a site visit by an expert in the field.

Evaluate your material properties

If you read some of the articles archived on this website, you will definitely notice a common thread to most of them. In order to reliably identify a flow problem and handle any bulk solid, you need to evaluate the flowability of your bulk solid. This requires you to use a reputable testing laboratory capable of determining your materials':

• Cohesive properties to predict opening sizes to prevent bridging and ratholing, by determining your material's cohesive strength as a function of consolidation pressure.
• Wall friction properties to determine wall slopes for mass flow, by measuring the friction that develops between a bulk solid and any hopper wall material.
• Compressibility properties to determine a material's bulk density as a function of consolidation pressure, which is required in the analysis of bridging, ratholing, and hopper angles.
• Limiting discharge rate which predicts the limiting discharge rate of a solid by measuring its permeability to air or gas.

Once you have identified your flow problems and evaluated your material's flow properties, you can develop possible solutions to those problems and retrofit your existing equipment.

Modifications to existing equipment

Modifications to your existing equipment may be something as simple as installing a liner of stainless steel or TIVAR 88 (which is an ultra high molecular weight polyethylene supplied by Poly Hi Solidur). A liner that develops less friction may allow your material to flow in mass flow and eliminate ratholing.

Figure 1:  Modified Hopper

Perhaps you can "cut off" a portion of your funnel flow conical hopper and install a properly designed mass flow hopper to provide an expanded flow pattern. The mass flow pattern developed by the mass flow hopper will expand into the funnel flow hopper and, if designed properly, will cause a stable rathole to become unstable and allow material to flow reliably. See the Figure 1, which shows an expanded flow approach.

Sometimes it is easier to replace the entire funnel flow hopper with a properly designed mass flow hopper. Remember to keep in mind, that the feeder design is as important as the bin design. If the feeder is not designed properly, modifying your hopper for mass flow may be a waste of time and money.

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.