If your training and education is in chemical engineering, as is mine, you likely learned a lot about handling liquids and gasses. By comparison, the subject of bulk solids handling was rarely taught. Perhaps that’s why it is often easy to identify which vessels, lines, or equipment in a chemical plant handle solids – they’re the ones with all the hammer marks.
I knew that fluids could be readily classified as Newtonian, non-Newtonian, viscoelastic, etc., and I was always able to find viscosity and specific gravity data that I needed to tackle a project involving liquids. But what was the best way to characterize powders? Angle of repose? Tapped density? I found a great table in a book by Woodcock and Mason that classified bulk solids:
Fortunately, I located a copy of Andrew Jenike’s Bulletin 123, which described the fundamental principles of storing and handling bulk materials. His most memorable statement was as follows:
“The angle of repose… is only useful in the determination of the contour of a pile, and its popularity among engineers and investigators is due not to its usefulness but to the ease with which it is measured.”
For a powder, I learned that I’d need to measure its cohesive strength, wall friction, and bulk density using an instrument called a shear cell tester. Bulk density and cohesive strength measurements would permit me to determine the outlet size of a hopper, bin, or silo required to prevent flow obstructions. Wall friction would allow me to determine the hopper angle required to prevent ratholing.
My company purchased a shear cell tester, and I was soon able to gather my own solids flow data. I also learned that bulk solids handling is a lot like electricity – a little knowledge is sometimes more dangerous than none at all. I could calculate the outlet dimension that would guarantee discharge of the powder, but would it give me the desired flow rate? If I wanted a slotted outlet, what type of feeder would I need? My tester could only operate at room temperature, but my process operated at several hundred degrees – how would I get applicable strength and friction test results? Fortunately, Jenike & Johanson could conduct tests under actual process conditions, knew of tricks to increase discharge rates or reduce space requirements, and provide recommendations based on years of experience.
I enjoy the subject of bulk solids handling, and was fortunate to be hired as a project engineer at Jenike & Johanson. Find a job you like, and you’ll never have to work again.