Powder Characterisation Techniques for Hopper Design

In this and coming posts, I’m going to take a look at which powder properties are most relevant to specific powder applications. In recent years it has become increasingly evident that the value of measuring different properties is dependent on the extent to which they describe how a powder behaves in a given process or as a certain product. This understanding highlights the limitations of ‘single number’ powder testers and the enhanced value of instrumentation that offers multivariate characterisation. I’m starting with hopper design, the only powder handling process where equipment geometry can be designed from measurements of powder properties. For many, the shear testing methodologies required for hopper design are their first, and for some, only, introduction to the world of powder characterisation. However, despite their longevity, shear cell testing, hopper design and hopper operation continue to present a challenge.

The basics of hopper design

Getting powders to flow consistently from hoppers is an issue in many powder processing industries. Frequently encountered problems include: bridging, leading to no flow/erratic flow/stoppages; flooding (uncontrolled flow); segregation; and funnel flow/ratholing (flow through the core of the hopper with an outer stagnant layer).

Figure 1 - Flooding (left) and erratic flow/stoppages, as evidenced from hammer rash (right), are just two of the problems routinely associated with sub-optimal hopper design and/or operation.

Successful hopper operation relies on an efficient match between the in-process material and certain attributes of the hopper: material of construction; half angle (the steepness of incline of the hopper walls); and outlet size. Generally, smoother materials, steeper half angles and larger outlet sizes all tend to promote flow. 

Figure 2 - Hopper design methodologies lead to specification of the steepness of incline of the hopper wall and outlet size on the basis of a stress balance.

The hopper design methodology developed by Jenike in the 1960’s remains the standard today. For a more detailed discussion I’d refer you to ‘Modern Tools for Hopper Design’ but in summary it relies on calculating the flow function (FF) and flow factor (ff). FF depends purely on the shear strength of the powder, as determined from shear cell testing, while ff depends also on the characteristics of the hopper – material of construction, and shape. Hopper half angle and outlet size are calculated on the basis of these two parameters.

 Figure 3 - In hopper design the intersect between the ff and FF plots defines the no flow/flow transition point from which the geometric specifications for a hopper are established.

Because of these methodologies, many believe hopper design to be a relatively robust element of powder handling, and in relative terms it may be. However, operational problems are common, and at many companies hopper specification is an out-sourced expert task. Why is it that even with defined methods in place, robust hopper design remains challenging?

Hopper troubleshooting

What emerges from Jenike’s methods is that if the material of construction, shape or half angle of a hopper is different from that of another unit then a different outlet size might be needed to achieve mass flow, for the same powder. Equally importantly, using a hopper that works well with one powder to handle an alternative material, may not be successful. These points are relatively well-recognised, but what is less well-understood is that optimum values of FF and ff, and hence optimal hopper geometry, may change simply because of environmental conditions. For example, if the hopper is filled when the relative humidity is higher than normal, discharge behaviour may be compromised.

This observation suggests that conducting hopper design, and the associated testing in-house, may be advantageous. In-house capability makes it easier for engineers to fully scope the conditions over which the design may need to operate, and for troubleshooting teams to get to the root cause of a problem. The barrier is having the necessary expertise, but there are tools that can help. Automated shear testing, coupled with modern hopper design software guide the user through every step of the design process, from analysis through to computation. Using such tools helps powder processors to get the best out of existing hoppers (assessing whether new materials will work with existing equipment) and specify robust new units with confidence.

Further reading: Please refer to our white paper “Modern Tools for Hopper Design” for a more detailed discussion of hopper design and how to implement it efficiently.