Re: Vsma Calculations: Mesh Vs Grain Size.
Dear Mr. Diego,
I would like to draw your attention to the general practice about specifying the size of product in context of screening and crushing. Often, the size referred is nominal size wherein it is understood that eighty percent of product will be less / equal to this size. 20% of product could be somewhat bigger than the mentioned nominal size, depending upon the nature of material and its cleavage lines. So, in such situations, the difference between nominal size and screen aperture size or crusher crushing gap would be somewhat large.
Sometimes, buyer also mentioned that in no case product should be larger than certain size (or chances of product larger than the mentioned size to be bare minimum or rare). In this kind of specifications, the difference between product size indicated in this manner and the aperture / gap dimension would be less. So, it is essential that one has to put his requirement about the size while defining the meaning of size, he is indicating. Even in this case, the aperture size would be marginally larger than product size specified in this manner.
The aperture size selection is also affected by the size gradation specified by the buyer. Because often he will say, fines / under-sizes should not be more than certain percentage. In this situation the decision becomes a compromise on buyer’s budget.
This reply covers the first part of your query.
Regards,
Ishwar G Mulani.
Author of Book : Engineering Science and Application Design for Belt Conveyors.
Author of Book : Belt Feeder Design and Hopper Bin Silo
Advisor / Consultant for Bulk Material Handling System & Issues.
Email : parimul@pn2.vsnl.net.in
Tel.: 0091 (0)20 25882916 ■
Vsma Clarification On Factor A
the explanation of factor A .....in specific regards to your question is as follows:
A = the predetermined STPH (PASSING or going thru) a specific opening. In this case, it means specifically passing the opening in the wirecloth (1/8" x 1/8" clear or square opening) OR your metric equivalent.
WHEN you pick it off the VSMA FACTOR A chart, it means passing 1/8" clear in this instance or 3.17mm nominal equivalent.
IF you are attempting to make a spec product and the spec says 100% or 95% passing 1/8" this is the number you pick off the VSMA chart.
UNDERSTAND...the angle assumed on an incline screen for this formula is 20 degrees. If your screen is steeper.....you can go to a bigger opening.....and still stay in spec.....DUE to foreshortening of the opening at a steeper angle...so the material does not see the full opening (whatever it happens to be) on the steeper slope. ■
VSMA calculations: mesh vs grain size.
Hello,
I have one question. When you want a grain size of 2 mm, then the mesh size should be 3.15 aprox (this is what the manufacturer of the screen says). So for industrial applications the mesh size should be a little bigger than the wanted grain size (aggregates).
If this is true (question that I transmit to this forum), then the VSMA capacity chart Factor A, what value gives ?. The mesh 1/8'' aprox 3.17 mm gives 0.95 STPH per sq.feet. So this 0.95 STPH is material less than 2 mm, or material less than 3.17 mm ??
Thanks for your attention and best regards, ■