1. Definition of funnel flow and mass flow in silos

Hoppers / Silos can be classified in 2 types : mass flow silos and funnel flow silos. This classification is done according to the flow pattern of bulk solids in the silos.

In funnel flow, also called core flow, the product located at the center of the silo will move much faster than the product located at the sides.

In mass flow silos, in contrary, all particles will move at the same speed, which means that the distribution of residence time in the silo will be reduced, whereas such a residence time can be very large with funnel flow hoppers.

Related pages : powder properties, design of silos for good flow

Figure 1 : Bin discharge patterns

2. Pros and cons of each design

Table 1 : Pros and Cons of mass and funnel flow design [1]

Hopper type	Advantages	Disadvantages
Funnel flow	Can offer larger capacity with a reduced height (cone walls angles are flatter than with mass flow hoppers) Reduced abrasion of hopper walls since particles at wall do not move	Hoppers are sensitive to arching and ratholing making discharge unpredictable (issues can be mitigated with disharging aids) Risks of formation of archs or ratholes, then collapse of archs that can cause structural issues to large silos Collapse of arches can fluidize the whole silo and make the product suddenly flow very quickly Silos sensitive to segregation
Mass flow	First in - First out, particles have more or less the same residence time in the silo No arching, stable flow output Less segregation (density is stable at outlet, important for feeder design), such silos even give a certain degree of re-mixing right at the outlet of the silo [2] Flooding happen rarely since arches are not stable to form	Can require tall geometries that may not fit in the installation May not be possible for all materials - use of discharging aids is then to consider

When having designed a mass flow hopper, it is also very important to have a proper design of the equipment taking out the product from the hopper in order to keep the mass flow effect : gates (butterfly valves, gate valves) should be 100% opened, and feeders for instance screw conveyors) should be taking product on the whole length of the outlet opening of the hopper (use of a variable screw diameter).

3. How to chose in between mass flow and core flow ?

What do you need ? [2]

What you want or have...	What do you need ?
The powder process can easily segregate, demix	Mass flow As mass flow implies at the outlet a mixing of the different streams within the silo, risks of segregation are reduced
The powder spoils easily	Mass flow Mass flow will ensure that the residence time of the whole mass of powder is more or less constant avoiding dead area where the product can degrade
Have a constant density at the outlet of the silo	Mass flow As for segregation, mass flow will ensure a more constant product at the outlet of the silo over time
The powder can fluidize very easily	Mass flow Using mass flow will avoid that the powder fluidize too much at the center of the silo and flush out
The powder is cohesive	Mass flow Having a mass flow silo properly designed will help to avoid arching and bridging within the silo
No particular concerns...	You may consider core flow silos as their construction is usually cheaper and design more compact. But beware of all what is written above ! In any case, the silo must be designed carefully by 1st assessing the flow properties of the bulk solid to store

Note that V shaped hoppers (wedge hoppers) can present an interest vs cone shaped hoppers : they can provide a mass flow while being more compact.

4. Silo inspection and cleaning

Be it mass flow or funnel flow, a large silo of several dozens of m³ must be inspected and maintained on a regular basis.

In order to avoid that some material stay too long in the silo (in case of compaction for mass flow silos, or due to the design for funnel flow silos) it is advised to empty completely a silo on a regular basis (it is then practical to have 2 silos as 1 can take over while the other one is empty).

External inspection (especially for outside silos) and internal inspection should also be carried out yearly. It helps to detect structural problems, or issues that could lead to damages. For example, it is important to check if no material is hanging on the inside of the roof as some material buildup could dramatically increase the weight and lead the roof to collapse. If required, a cleaning of the inside can be done by specialized companies (this requires a specific training and very strict safety).

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Source [1] Principles of Powder Technology, M.J. Rhodes, 1990 [2] Feeder Design for Solids Handling, Chemical Engineering, Marinelli and Miller, 2017

[2] Ten key steps for achieving reliable flow from hoppers, SHAPA Technical Paper 3, SHAPA, 2001

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