An important goal of the powder processing industries is to predict the performance of processing equipment from measurements of powder properties.
This article, presents a new collaborative study from Freeman Technology and Gericke to establish predictive correlations for the volumetric flow rate delivered by screw feeders, using dynamic powder testing, which can be used to accelerate and optimize screw feeder selection.
Screw feeders are used on a regular basis in the powder handling industries to control the flow of material from a hopper into a process. The performance of the feeder is directly influenced by the properties of the powder, making it necessary to tailor the design of any given system to the product being handled.
Generally, a poorly matched feeder/powder combination will be associated with high screw torques, low feed rates, and the accumulation of powder on the tube walls. Both short and long term operating efficiency is decreased by these factors.
This article describes collaborative research conducted by Freeman Technology (Tewkesbury, UK) and Gericke (Zurich, Switzerland) to identify powder properties that can be reliably measured to predict feeder performance.
The study highlights the value of dynamic powder characterization within this context, concluding that the development of robust models enable the prediction of the feed rate through a screw feeder from flow properties such as aerated energy (AE), flow rate index (FRI), and specific energy (SE). These models make the selection of a feeder for a specific powder easier, predicting its likely performance with a high degree of confidence.
Specifying Screw Feeders
Screw feeders are equipped with one or more rotating augers or helixes that are mounted in an enclosed chamber (Figure 1). As the auger rotates, powder is transferred according to the Archimedes screw principle, which has been manipulated for material transport for centuries.
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Figure 1. A screw feeder transfers powder using one of more augers enclosed in a tube
Various industries use screw feeders, and Gericke has been routinely supplying machines for handling foods, chemicals, construction minerals, detergents, and plastics for several decades. Three factors directly influence the choice and specification of each feeder. They are:
- Installation constraints associated with the plant layout – the number of feeders, feed distances, available headroom, and mounting requirements
- Material properties – is the material being handled free-flowing, adhesive pr cohesive, fragile and prone to attrition, abrasive, fluidisable or compressible
- Process requirements – feed capacity, whether the plant is operating continuously or in batch mode, the accuracy required, operating pressure, and the extent of automation
The size of the feeder (length and diameter), the drive, geometry and pitch of the auger, and the accessories used to ensure consistent flow are the main design variables that can be manipulated to meet any combination of requirements.Vibrational feeders and fluidisation or agitation in the feed hopper are all possible options. Feed rate may be controlled on the basis of volume (volumetric or weight (gravimetric).
Specifying the optimal screw feeder for any given application is critical to operational success for Gericke. Any feeder that is not properly matched to the product it is handling is likely to be associated with poor long term operation, which can manifest in many ways.
For example, flow rate may be erratic or poorly controlled, which could directly impact the overall efficiency and performance of the process. However, if the powder is cohesive, material accumulation within the equipment may be problematic, particularly if there is frequent product changeover and/or sensitivity to batch-to-batch contamination.
Therefore, understanding how to characterize powders helps to predict their performance in different types of equipment. Powders are subjected to different environments inside the screw feeder, flowing gravitationally from the feed hopper into the forcing and potentially compacting flow regime inside the rotating auger(s) that drives the material forward.
The properties of the powder directly determine the way in which a powder responds to these conditions. Consequently, powder properties have a major impact on the choice of equipment and the need or otherwise for bespoke development.
Collaborative research between Gericke and Freeman Technology assessed whether properties measured using the FT4 Powder Rheometer® could be correlated directly with feeder performance. The main aim of the study was to assess the feasibility of predicting feeder performance from powder properties to speed up the identification of an optimal screw feeder solution for any given material.
Correlating Powder Properties with Screw Feeder Performance
To investigate correlations between the properties of five different powders and their performance in two different screw feeders, an experimental study was conducted. The following powders were tested:
- Maltodextrin
- Calcium Hydroxide
- Milk Protein
- Calcium Citrate
- Cellulose
In the first stage of the study the FT4 Powder Rheometer was used to conduct comprehensive testing of each powder sample. In each case, bulk, dynamic, and shear properties were measured for each powder with a high degree of repeatability (RSD<5%)...
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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
For more information on this source, please visit Freeman Technology.