What is mass flow?
With mass flow the whole contents of the silo is moving, as soon as product is extracted from the silo.
This type of flow is characterised by:
- first in - first out;
- little segregation;
- steady flow and a well controllable discharge-capacity;
- no risk of ageing, decay, or contamination;
- possibility of 'following' product batches with a specific composition.
Disadvantage of mass flow can be that in certain cases silo quaking can occur. When dealing with abrasive products, the silo wall will wear quicker. In general this will not be problem, because of the low flow velocities in a silo.
How to design for mass flow ?
It may be clear, that in by far the most cases mass flow is desired. Design of the silo must be such, that this type of flow is guaranteed for the product handled.
The flow pattern in a silo depends on:
- the slope angle of the hopper;
- the friction between product and wall;
- the shape of the hopper;
- the internal friction of the product.
Here in general can be stated that mass flow is sooner achieved by a steeper and smoother hopper. The finish of walls and corners must be smooth. Furthermore a hopper with a slot shaped opening is better than a rectangular or round hopper.
Mass flow design
The procedure for the design of a mass flow hopper is as follows:
- The internal friction of the product is measured.
- The wall friction of the product on the proposed wall material is measured.
- On the basis of these data the appropriate hopper shape is determined, and the mass flow angle is calculated.
- When in step 3) no practical solution is found, a coating, lining or other wall material with a lower wall friction might be an option.
- If this is not feasible, vibration of aeration can be a solution.
- If this does not work, then application of a hopper must be abandoned.