Mass flow vs funnel flow comes down to wall slip. Choose mass flow for cohesive, caking, flood-prone or segregating powders. Funnel flow suits only coarse, free-flowing, non-degrading solids where segregation does not matter and a stagnant zone is acceptable.
This comparison sits within powder and bulk solids handling. For the wider storage picture, see silo design for bulk solids.
Mass flow and funnel flow side by side
| Criterion | Mass flow | Funnel flow | Planning use |
|---|---|---|---|
| What moves | All material flows whenever any solid is withdrawn | Material flows through a channel within stagnant material | Decides whether any stored powder can sit unused |
| Hopper requirement | Steep and smooth enough for solids to slide on the walls | Hopper not steep and smooth enough to ensure wall sliding | Sets the wall angle and wall material you need |
| Discharge sequence | First-in first-out | Not first-in first-out, because stagnant material stays put | Matters when age or batch order matters |
| Cohesive powders | Recommended | Channel expands upward with cohesive solids, and a rathole can form | Default to mass flow for sticky or caking product |
| Segregation | Minimised by the first-in first-out sequence | Can be enhanced when a rathole forms | Choose mass flow if blend uniformity matters |
| Suitable solids | Cohesive, flood-prone and time-degrading materials | Coarse, free-flowing, non-degrading solids only | Funnel flow is the exception, not the default |
These contrasts follow the Solids Handling Technology guidance on funnel flow and mass flow and the Processing magazine article on feeders and flow pattern.
How each flow pattern moves material
The two patterns are distinguished by whether all the material moves during discharge. If it does, the hopper is in mass flow. If it does not, it is in funnel flow.
Mass flow
Mass flow needs a hopper steep and smooth enough that the whole contents slide along the walls. Every particle moves whenever any solid is withdrawn, so there are no dead zones. Fine powders also tend to deaerate, which avoids flooding because of the longer residence time.
Funnel flow
Funnel flow happens when the hopper is not steep and smooth enough to ensure sliding along its walls. Material flows toward the outlet through a channel that forms within stagnant material. With cohesive solids that channel expands upward, and the stagnant material around it is left to sit.
Choosing a pattern by material behaviour
The material’s behaviour should drive the choice, not the hopper’s initial price. Wall friction determines whether the solid slides on the wall (mass flow) or adheres to it, so the wall surface is part of the decision.
- Cohesive powders: Mass flow is recommended for cohesive materials and powders that can flood. Funnel flow lets cohesive solids build stable channels, so it is the wrong default for them.
- Segregating blends: Mass flow minimises segregation through its first-in first-out sequence. Funnel flow with a rathole can enhance particle segregation, so it only suits cases where segregation is unimportant.
- Caking or time-consolidating products: These need a residence time that is as even as possible. Dead zones in funnel flow let material rest and consolidate while the channel keeps discharging, which is the opposite of what these products need.
- Coarse, free-flowing, non-degrading solids: This is the narrow group where funnel flow is generally suitable, and only when segregation is unimportant.
One caution applies to conical mass flow hoppers. The velocity gradient in the converging section preferentially extracts material from the centre, which can matter for blends that segregate. Treat this as a reason to review the outlet and feeder design, not as a reason to abandon mass flow.
Hopper half angle for mass flow
The hopper half angle for mass flow is the steepness at which the solid slides on the wall instead of adhering. It depends mainly on wall friction, measured for your powder against your actual wall material, and on the powder’s own flow properties.
What sets the angle
Design charts relate the allowable half angle to the wall friction angle and the effective angle of internal friction. A rougher wall or a more frictional powder calls for a steeper hopper. A smoother wall allows a gentler one. The chart values themselves belong to the specific material test, so do not borrow a number from another powder.
Hopper shape changes the answer. The plane wall inclination needed to generate wall slip for mass flow is approximately ten degrees lower than for a conical hopper.
Finding it in practice
Wall friction properties are measured with the Jenike Shear Tester, using a sample of the real wall material. The same shear testing yields a cohesive-strength flow function, from which the opening size needed to prevent arching and ratholing is determined. Test the wall finish you will actually specify, because a change of liner or surface treatment changes the friction and therefore the angle.
Ratholing in funnel flow
Funnel flow causes ratholing because only the central channel is withdrawn. Even if the outlet is fully live, the preferential flow channel may empty out and form a stable rathole. The stagnant material then stays standing around an empty core.
Wall friction enters the rathole calculation through the coefficient u, equal to tan of the wall friction angle between the stored solid and the cylinder walls. In one published example, the critical rathole diameter was 18.3 ft at a 20 ft effective head after 3 days at rest. That shows how resting time and head both feed into the stable diameter, but it is an illustration, not a target for any other powder.
A real case shows the operating cost. In a granulation line studied by Jenike & Johanson, a rathole formed in both the portable container and the press hopper, so operators had to poke at the material to induce flow. The Jenike case study on granulation flow problems describes it. For remedies short of redesign, see how to prevent bridging in hopper.
Cost and height trade-offs
Mass flow hoppers cost more to design because they must be steep, smooth and matched to measured wall friction. Steeper walls generally take more height, so headroom is the practical limit in low buildings.
- Plane-flow option: Its walls can be about ten degrees lower than a conical hopper’s. The increased holding capacity allows equipment to be designed with less headroom.
- Feeder requirement: A plane-flow hopper requires a feeder that collects material from an extended length of outlet slot. Budget for that feeder along with the hopper.
- Whole-system cost: Plane flow offers advantages that can outweigh any difference in initial cost, so compare lifetime handling problems rather than the hopper price alone.
Frequently asked questions
Does mass flow ensure first-in first-out discharge?
Mass flow is associated with a first-in first-out sequence, and that sequence is what minimises segregation. Because a conical hopper’s velocity gradient draws preferentially from the centre, the sequence is not perfectly uniform in every design.
What is expanded flow?
Expanded flow is a compromise between the two patterns. It secures the flow benefits of mass flow at the outlet when the contents are stable and segregation is not an issue.
Is there evidence that redesigning for mass flow pays off?
In the Jenike granulation case, flow property data were used to design new components that would promote mass flow. The press speed was then doubled and consistent tablet weights were achieved.
