Density Variation
It is presumed that a check has been made to confirm there is no variation in consistency of ingredients due to segregation, which would seem a potential prospect with a cement/ rubber granule mixture.
Filling and emptying the volumetric device will cause pressure variations due to displacement if there is no venting, but this should be consistent and easily identified with the fill cycle. This aone would not account for batch to batch variations.
It would be useful to see a graph of the variations experienced and whether this can be related to the discharge of the mixer into the feed bin of the transfer screw. At fist glance one reason ma be that the mixer discharge is highly dilated and that the amount taken away by the screw increases as the powder settles. The bin is probably not mass flow so the extraction variation will be most pronounced, giving the lightest weights shortly following the mixer discharge. If this is the case a solution would be to expand the flow path in the bin by means of an insert to provide a more uniform residence time for settlement. See an article on bin inserts that I co-authored in the July edition of Chemical Engineering. ■
Re: Detecting Variation In Density / Aeration
Thanks for you reply.
I guess I can test for segregation by doing a rudimentry bulk density check. Though I (not bulk powder expert) would have expected that the results of segregation would be reasonably random. We have observed a cyclic nature with the weight variation.
The volumetric bowl is vented with a 50mm diameter riser pipe with a filter. This seems to be adequate in most other applications of the volumetric filler. (it's a standard type machine). We tried removing the filter and found the supply sock still 'sucked in' So the negative airflow must be greater than what can vent through a 50mm hole.
Is there a layman's way to identify if a bin is mass flow or not? ■
Density Variations
Variations in segregation are not random, but related to the operation that cause it to occur. For example, frequent filling and emptying of a bin will give cyclic variations, or stop/start discharges from a continuous supply. A clue is given by the frequency, which may be compared with the sequence of activities.
If the rubber particles can be screened out of a few samples it should show if there is segregation and also indicate any potential density variation
Mass flow can be verified by examining the upper surface behaviour of a bin during discharge if it is seen to move down coherently over the whole time that the parallel section is covered. Note that in a tall bin, the surface may move down over the whole cross section when it is well-filled, because a a 'core flow' channel may have expanded spread to the outer walls, but the key period to watch is when the level of the contents is just above the hip of the converging section. There may be a slight dip in the centre at this point, but if the material touching the walls is also moving, the bin is mass flowing. Another way, if there is a small connection on the side of the bin, such a for a level probe or rodding hole that can be safely removed, it may be practical to see movement of the bulk past the hole during discharge. Another way is to place a marker or dye on the side of the top of the filled bin and note when it discharges. This will be last out in a mass flow bin, but would emerge quite early in the discharge process if the bin was not mass flow. Any way to confirm slip on material lying against the converging walls will show that mass flow is taking place. ■
Detecting Variation in Density / Aeration
I have an installation where a packaging line is filling cement/ rubber particle mix into 20kg bags volumetrically. Simple proven technology for this application. In this instance we are experiencing large variations in final weight. It appears to be cyclic. The volumetric filler is fed from an 8" incline screw from a bin that is fed via a blender. There is a sock coupling between the 8" incline screw and the volumetric feeder bowl. This sock can be observed to be 'sucking in' and 'relaxing out' throughout the process. My theory is that the bulk density is varying due to this negative pressure at times. Can you suggest a method to measure the actual bulk density of the product as it is at any particular instant. Also, a method to measure the actual negative pressure.
What amount of negative pressure/ vacuum would be sufficient to effect the bulk density? ■