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 first hopper flow pattern is what we'll discuss next. And the flow pattern is the key
to minimizing or exacerbating, making things worse, for flow problems.
The first flow pattern is called funnel flow, where the name funnel flow means
we can observe powder funneling through itself, overcoming its own internal friction,
rather than flowing along the walls of a sloping hopper.
Some of the material is moving, while the rest is stagnant, and it is often the root cause of
most flow problems, whether it be arching, more problematic could be ratholing,
flooding issues, segregation issues, can all be directly attributed back to a funnel flow
discharge pattern from a hopper. Why is this so common? Because essentially it is the
default flow pattern out there in industry. And many are not educated on the fact that there's
an alternative flow pattern that has many benefits to it and advantages over funnel flow.
It is often the default flow pattern because hopper angles of 45° and 60° are most commonly built.
Funnel flow geometries are shown. Both of these hoppers are funnel flow, because
material in blue remains stagnant and material in red flows through the central core.
The cohesiveness of a powder directly affects that flow channel development, and if a
rathole will form or not. On the right hand side, that flow channel in red is quite
extensive, and as the red material discharges it is likely that the blue material will cascade
off the flow channel that's dead, and let that material flow down out of the vessel.
This would happen with something like a dry sand or maybe gravel.
On the left hand side we have a steeper flow channel in red. And in this case here, once
that red material discharges, the large dead zone of blue material remains, leaving a
stable open rathole. Imagine if you're at a plant and you're receiving a delivery of flour.
That flour being blown into the silo is going to go right down those red flow channels that
are empty, and not stay with the blue material that's stagnant. That will induce flooding
problems or flushing. Any fresh material that's added to this vessel could have an issue with
uniformity or quality because of that segregation problem. Material in blue could
cake over time. It all comes down to the material preferring not to flow along the walls
of a hopper because the hopper is not steep enough, nor is sufficiently low in boundary
friction to allow particles to flow down against it.
In the next slide we see features of funnel flow. Probably the only good feature of a
funnel flow hopper is that it does not take up a lot of height. That is, the height taken up by
the sloping wall section of this vessel is not that large because the angle is relatively
shallow. That's the only good feature that I can see for a funnel flow hopper.
You get some negative features, such as a first-in, last-out flow sequence. The material
that comes in first into this hopper settles on the walls of the hopper, is the last to go out
because it's not moving along the wall boundary of the vessel.
Ratholes may develop, depending on the cohesiveness of the material.
The material that is acting like dry sand may discharge out without forming a stable
rathole, even though some of the material is dead on the walls. That actually could be a
good thing if you're handling a highly abrasive material like sand or glass cullet.
However if you're handling wet material, like a wet sand or a clay, that cohesiveness will
allow a very stable vertical rathole, open channel to form in the vessel, introducing
additional problems. FIne powders can flood.
Any of that rathole material that is collapsing could induce flooding with a fine powder.
And also fresh material that is added to the top of this vessel with go down through that
fast flow channel rather than deaerating and settling in the vessel and not flooding.
Lastly segregation, whereby fines are in the center, course particles are at the periphery,
is going to be made worse, because of that sequence of fines initially, followed by course product.
Imagine if this were a blended formulation of a powdered ingredient for a pharmaceutical,
and you had the fine active drug initially coming out, and then eventually the excipients
which are the course materials, the sugar, starch, cellulose, lactose, that material is
coming out at the end. Even though the powder formulation was well blended
upstream, filling of the bin it segregated, and then discharging out in funnel flow
exacerbated that segregation.
Funnel flow is suitable for material that meets all four of the following criteria.
All four requirements listed must be met in order to achieve reliable funnel flow discharge.
The first requirement is that of course particles, about 1/8 of an inch or 3 mm size average.
When you have coarse particles you don't run into flooding issues.
Air moves rapidly through course materials, like catalyst beads or plastic pellets,
and there is no chance of aerating the solids causing flooding.
Second, materials must be free flowing. Free flowing means that there is no bridging or
ratholing problems. Bridging and ratholing are prevelant in funnel flow, so your material
must be free flowing like a dry sand or plastic pellets.
Third, degradation of solids must not be an issue, or caking or clumping of materials.
If you think of some things like cyanuric acid, or fertilizer that you put on your grass,
those materials are highly prone to clump and form solid masses in bins or bags.
So handling those in funnel flow hoppers is not a good idea because that will just drive
the caking mechanism. And lastly, segregation must not be an issue. We know
with funnel flow we get a first-in, last-out flow sequence, preferential flow from the center
of the hopper, thus solids which segregate with fines in the center, course to the periphery,
will impart a fines-out-first sequence, eventually followed by course material.
Again, all four requirements must be met to reliably achieve funnel flow discharge for a bulk solid.