Control & Instrumentation
Going with the flow This article, by Mike Page of Rospen Industries, examines the process of
controlling the flow of powders into a process at a prescribed rate
Whatever means are employed to actually control the powder, certain basic fundamentals need to be assessed and understood before a solution can be arrived at.
Firstly the terminology used to describe aspects of the equipment are fairly universal and can be summarised as follows:-
Diameter of Screw – This is the nominal outside diameter of the screw not the diameter of the tube in which it runs. Pitch of Screw – The dimension from leading face to the next leading face of the screw.
Box Loading – The level of material in the tube. Usually quoted as a fraction when referring to the screw pitch, i.e.1/2, 1/3 etc. and usually quoted as a % when referring to the material loading within the screw tube/casing as 50%, 30% etc. Volumetric Feeding – Strictly based on a screw of a known diameter and pitch, metering powder at a specified speed. Any variation in the powders bulk density will cause a direct effect on the feed rate.
Gravimetric or Loss-in-Weight – A feeder usually identical in design to the volumetric feeder but mounted to a weigh platform to measure the weight lost from the feeder at intervals of time. The main benefit is that any changes in bulk density are detected and the screw speed varied to maintain the set rate.
Principles of metering screw feeding These can be summed up as follows:- 1. Proper filling of the screw – Otherwise known as entrainment. With this goes correct discharge from the hopper above the screw to ensure starvation does not occur. Each powder has an ideal speed range through which it will not adversely react to the process of changing direction and filling the pitches of the screw.
Usually this is determined by trials with the conclusion that the higher speeds will cause more problems to the extent that some capacities will tail off and not produce straight line graphs or accuracy will suffer due to bad entrainment.
Rospen feeders have a particular characteristic in usually being linear in that increases in screw speed are generally followed by corresponding identical increases in output.
Material flow from the hopper is assisted by two four blade or spiral agitators with intermittent or continuous running along with vibrator motors for extreme cases.
In the more difficult materials the standard taper hopper shape is replaced with vertical sided hoppers to ensure consistent flow to the screw.
With larger hoppers above 150 litres and up to 0.5 m3 ,
consider the use of a vibrating discharge cone along with air evassers arranged asymmetrically about the hoppers sides to promote an unstable material bridge in the hopper. In the case of assisted flow by either the above or larger
34 Solids & Bulk Handling • August 2010
vibrating cone bin activators fitted to larger hoppers, care should be taken not to mis-match the output from the discharger and the screw by too big a factor. Compaction between the two can result causing material which cannot get away to compact and fill the feeder with the result the screw bores or tunnels a hole without conveying smoothly.
Otherwise scale down the problem by batching from large hopper or silos into the feeder hopper using high and low level probes to control the refills from the silo.
2. Screw Geometry and Metering Zone – Once the screw size and pitch have been decided it is important to realise that to be effective the pitch of the screw selected must enter the metering zone, namely the discharge tube for the first 3 full pitches of the screw. On extended length screws the pitch is opened out to reduce the box loading. This has two effects, it lowers the stress on the drive shaft of the screw and also the consumed power taken to drive it. An added benefit is to alleviate compression of the powder if this is a problem material a) Screw Design – Feeders are supplied in single and twin screw configuration. The majority of applications use single screw with variations on pitch and diameter to suit the application. Twin screw feeders are generally used where extreme flushing of powders can occur or the complete opposite with cohesive powders like pigments or those with a high resin content which require a large entrainment area to flow properly. b) Solid and Wire Screws – Solid screws take the form of a continuous Archimedean spiral with a shaft running throughout the screw. The vast majority of cases use this type of screw. Even with the flavour powders usually associated with the snack industry solid screws are used, the only additional feature given
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