This is Eric Maynard with Jenike & Johanson, Director of Education and Senior Consultant.
I will be your instructor for the Flow of Solids in Bins, Hoppers, Feeders and Chutes.
The alternative flow pattern to funnel flow is called mass flow, and Dr. Andrew Jenike,
who is renown as the founding father of the field of bulk solids handling, came up with mass
flow designs in the 1950s and 60s. And Dr. Jenike recognized that if we get flow of particles
along the walls of a hopper, all the material will be in motion whenever any is discharged.
No material is stagnant; no ratholing as possible.
We see a mass flow discharge pattern resulting. The material in red is flowing in the center of the
hopper as well as along the sloping walls of the hopper. All of the material is in motion during
discharge. Jenike recognized that if all the material is moving during discharge, then no
stable ratholing can form, and we get a first-in, first-out flow sequence, almost like a liquid level
flow moving through a tank. Now there may be some differential velocity between the material
moving in the center than at the walls. That's to be expected because of the boundary influence
at the walls, and we'll talk a little bit about that in Lectures 8 & 9 and its effects on segregation.
But in most cases, achieving that first-in first-out flow, that mass flow, is very good for processes.
The key to achieving mass flow, as shown by the graphic, is that particles must flow along the
walls of the hopper. The hopper slope is typically smoother and steeper than its alternative in
funnel flow. The material must flow along the walls to the hopper so you have to have a sufficient
angle to let gravity pull the particles down, and you must have a low enough value in boundary
friction so particles can slide against that surface. There are some cases that even very
smooth hoppers will require to be steep, upwards of 75-80° from horizontal in order to get mass flow.
So it's a combination of both factors, boundary friction and the angle of the hopper, to get mass flow.
Almost all 45° hoppers will not be mass flow. Some 60° from horizontal hoppers will be mass
flow, but most are not. Most 60° hoppers are funnel flow.
With mass flow we get a first-in, first-out flow sequence. The material that comes in first at the
base of the hopper is the first to exit, so minimizing caking problems, or settlement
issues with fine powders, can be fixed, can be prevented, by going to a mass flow discharge.
Ratholes cannot form. One of Jenike's biggest contributions to developing mass flow was that
stable ratholes, which are detrimental to many processes with funnel flow hoppers, these
cannot form with mass flow. Fine powders will deaerate. Material that comes in first to the bin
has a chance to deaerate before going out, and any new fresh material at the top surface of the hopper
up in the cylinder section, that is, when that comes in highly aerated and fluffy and low density,
by the time that moves down to the base of the hopper, it has a chance to deaerate and we
no longer have a flooding problem. Segregation by sifting, in this case, is minimized.
Fines in the center effectively recombine at the outlet, overcoming any type of that segregation effect.
So though the particles have separated upon going into the vessel, fines in the center, course
to the periphery, by the time those particles meet at the outlet, the fines and the coarse
particles are effectively re-blending together as those two streams combine at the base of the hopper.
We also get a consistent uniform density and feed. No longer do we have zones of bridge
material, rathole material, flooding material, high density, low density. We've got a very consistent
uniform bulk density of material discharging from the mass flow hopper.
So our screw feeders, belt feeders, rotary valves that are operating in a volumetric mode,
grabbing a volume of material per revolution or per stroke, have a very consistent mass flow
rate or weigh flow rate discharge because the density exiting the mass flow hopper is very consistent.
Mass slow a suitable for many materials. Whereas with funnel flow, we needed a course,
free flowing, non-caking, non-segregating material, mass flow opens up to a wide variety
of materials, such as many cohesive bulk solids, those which are very fine, those which are
prone to degradation or segregation. So whether we're handling titanium dioxide or
carbon black, brown sugar, or coal, these are all materials that are prone to bridge and rathole,
we can handle them in mass flow, eliminating or preventing the ratholing problem.
We can handle fine materials, like fumed silica, talc, cornstarch, that are prone to flood because
the first-in first-out flow sequence gives us a nice liquid level drawdown, so deaeration capability
at the top of the hopper is possible. Materials that are prone to cake or degrade like fertilizers,
cyanuric acid, can be reliably handled in mass flow because of that first-in, first-out flow sequence.
No stagnant zones of material, no ratholing with dead material.
And lastly, solids that are susceptible to segregation, separation of particles by size, shape,
or density into like zones in a bin, are effectively recombined with a mass flow discharge pattern.
We are going to learn about some particular mechanisms of particle segregation whereby
mass flow can actually make things worse. So it doesn't mean that we get rid of mass flow, we don't use it.
We actually can still use mass flow but we've got to keep those solids from segregating in the first place.
A very wide variety of bulk materials can be handled reliably in mass flow.
And that's why we're taking this course, to learn about an effective alternative to funnel flow
discharge, and we don't have to accept the default flow pattern and its inherent problems with it.
Here's an example of a conversion to mass flow. This is at a cement plant in Joppa, Illinois.
I worked with the engineer at this plant, writing a technical paper about this application, and more
importantly, I worked with this engineer on how to fix this problem.
They were experiencing arching and ratholing with gypsum. Gypsum, calcium sulfate, is an
additive added to cement clinker during grinding, and it controls the setting time of concrete
when you add it with aggregate and water. The gypsum flow difficulties were paramount in
the pyramidal shaped hoppers. You'll notice on the hoppers on the left hand side, how much bin rash
was put on them from operators beating on them with sledge hammers and mallets all day long.
This cement plant estimated the problem to be at $75,000 (U.S.) per year because of downtime,
lost product, not having the correct chemistry because of gypsum flow interruptions.
The solution was to achieve mass flow to eliminate the ratholing with both the synthetic
and natural gypsum. The return on investment was less than a year.
By replacing the pyramidal hopper with a transition hopper, and by extending the length of
the weigh belt feeder located beneath it. By putting a mass flow transition hopper in there,
we were able to achieve first-in, first-out, no ratholing, no caking and was gravity flow alone.
No more air cannons, vibration, the need to use sledge hammers; in fact this picture on the right
hand side was after many months of operating this hopper, and you'll notice there is no bin rash.
They don't have to hammer on the hopper; it just worked by gravity flow alone.
Certainly there would've been that much greater of a return on investment if they had done the
hopper design correctly from the start. This gets back to the lack of education in U.S. academia
where mechanical, civil, chemical engineers are not being taught about the ill effects of poor
solids handling, and the ramifications. If engineers were taught about mass flow, funnel flow,
when they see the downsides to funnel flow and the upsides to mass flow, what engineer would
be selecting funnel flow hoppers with materials that are not suitable for its design?
It's one thing if you have a material that meets all four requirements of course, free flowing,
non-segregating, and non-caking, to be reliably handled in funnel flow. But if not, and you violate
any of those requirements, the engineer should be selecting mass flow.