GEK Wiki / Understanding Mass Flow of Solids in Hoppers
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Understanding Mass Flow of Solids in Hoppers

Page history last edited by jim mason 10 years, 1 month ago

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Problem Description

Solid material transfer is a complex problem and can cause a number of issues in a gasifier and related systems. When fuel stops flowing in a part of the system, it can cause a number of problems downstream, including overheating of parts and reduction of gas quality. Therefore, knowing how to keep materials flowing smoothly or diagnose a problem if it occurs is important.

 

Different materials have different properties. Materials with a steep angle of repose will tend have more problems flowing than materials with a lower angle of repose. Smaller particles will often flow through a given diameter element more easily (except at small, powdery sizes where cohesive forces come into play). Wood pellets are easier to handle in an auger and hopper than wood chips, and cubes are very tricky to handle.

 

Flow Modes

Within a hopper, materials can move at different velocities. The two main velocity profiles, or flow modes, are mass flow and funnel flow. Mass flow is generally preferred, since materials all travel through the hopper at essentially the same rate. In funnel flow, materials at the edges and corners don't flow and leave the hopper, which may cause them to rot or cause other problems. Funnel flow can also cause material to segregate into areas of finer and coarser particles.

Mass Flow

Mass flow occurs when all areas of the hopper have a constant flow. Mass flow is generally preferred.

Funnel Flow

 

In funnel flow, the center material moves fastest, and the outside walls do not move at all (stagnant).

Expanded Flow

 

 

Hopper Flow Problems

Material can stop flowing through a hopper for a number of reasons. Below are the common structures you may encounter.

Ratholing

 

Ratholing occurs when the sides of the material are stable enough that they remain supported while the central material exits the hopper

Arching/Bridging/Doming

 

 An arch forms when the material is cohesive enough to support the material above it, spanning the hopper opening and stopping flow

Interlocking Arch

 

 

Cohesive Arch

 

 
   

 

Resources

“10 Key Steps for Achieving Reliable Flow From Hoppers.” SHAPA Technical Bulletin, 2001. http://www.shapa.co.uk/pdf/techdata31.pdf.

Chase, George G. “Solids Notes 10.” The University of Akron. http://chemical.uakron.edu/fclty/chase/Solids/SolidsNotes10%20Hopper%20Design.pdf.

Mehos, G., and E. Maynard. “Handle Bulk Solids Safely and Effectively.” http://www.aiche.org/uploadedFiles/Publications/CEPMagazine/090938_public.pdf.

Purutyan, H., B. H. Pittenger, and J. W. Carson. “Solve solids handling problems by retrofitting.” Chemical Engineering Progress 94, no. 4 (1998): 27-40. http://www.jenike.net/TechPapers/solve-probs-retrofit.pdf

 

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