Feeders are widely used for metering bulk solids and discharging the contents of hoppers and silos. Numerous attempts have been made to describe the process of feeding but quite often they only cover certain products and hopper construction. In this article the reader will find a more general approach to this field of problems.
Feeders are widely used for metering bulk solids and discharging the contents of hoppers and silos. At the smaller scale power requirements may not be economically significant, but for larger units they become very important, as underassessment can have expensive consequences. It can also be difficult to determine what power is actually necessary, on which to allow a safety factor, so possible gross over-assessments incur excess capital costs and long term inefficiencies. The topic has commanded considerable interest in recent years [1-7], but the subject is complex and general conclusions and formula have been derived from results and may only by valid in similar circumstances to the experimental conditions. A more accurate method of predicting feeder loads is clearly needed, , but there is also considered to be a complementary need for an understanding of the factors that influence the load and of good practice design features that minimise their value. Investigations with equipment that does not exploit the mechanics of the prevailing stress system will not yield optimum results.
The first task in an investigation is to define the conditions of interest. In the case of feeder loads the main objective is usually to determine the maximum extraction force that has to be provided. The worst case scenario is that of starting with a full bin after a period of time consolidation, taking into account any local vibrations, input loading during filling and any time-related effects on the product condition. However, practical steps can be taken to mitigate initial start and re-start conditions by both design and operating practice. Also, as running periods normally occupies most of the operating time and may be a small fraction of the starting load, steps should be taken to mitigate starting loads and it may be practical to accept a temporary drive overload on starting, provided this is within industrial tolerance.
The main operating conditions of interest that may apply are:
Initial start with full bin following maximum static storage time.
Initial start with full bin immediately after filling.
Re-start with full bin.
Continuous running with full bin. (In practice, running with lower contents tends to take similar power until nearly empty).
The geometry of the bin and feeder must then be considered. These can have different characteristics as below:
Mass flow with square or round outlet.
Mass flow with slot outlet. (Feeder must have progressive extraction).
Funnel flow with square or round outlet.
Funnel flow with slot outlet and progressive extraction feeder.
Funnel flow with slot outlet and local extraction feeder.
The size of outlet that is required to ensure reliable flow from a hopper depends on whether the flow regime generated during discharge is of mass flow or non-mass flow design, so the mode of flow prevailing will affect the size of opening, the shape of the stressed arch that forms over the outlet when flow has occurred and the degree of support given to this arch when flow is not taking place. A slot outlet with Mass flow is recommended, at least mass flow for the outlet section, as this offers predictable flow with a good compromise between holding capacity and small outlet size.