Dear Mr. Rekhawar,

It seems you want to screen out (-) 30mm size material (as a product of direct use). Now, you have got a feeding rate of 62.5 tph. Suppose in this feed material if you have 50 tph of (-) 30 mm size material. Suppose, in your screen you are able to screen out only 40 tph of (-) 30 mm material and rest is going as oversize. Then, your screening efficiency is 40/50 i.e. 80%.

Based on the above, I think, you will be able to work out the screening efficiency.

Refer screens designers and suppliers for more specific information.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

VSMA standard sizing efficiency formula plugs in 95% this assumes we are speaking of the TOP DECK POSITION of a one deck machine. Efficiency will decrease at the second deck and the third deck.

This will happen on a multiple deck screen as we become less efficient as we make the drop thru to the next deck below at part of the way down the next deck below. We have less length of machine to work with.......hence the efficiency decreases.

EFFICIENCY = LONGER SCREEN or length determination

TONNAGE = WIDTH of screen box

George Baker MODERATOR

Attachments

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The attached file may (or may not) be of interest regarding screening efficiency ■

Whoops .. did not want to attach .. however

Standard reference books give a number of ways to state efficiency, but for various reasons they all leave a lot to be desired as to completely indicate the performance of a vibrating screen.

In the first place, efficiency testing is generally done in a laboratory using test sieves of accepted standards of openings and wire diameters. These openings may be entirely different from those which the sample for testing was taken.

In the second place, a problem arises because efficiency is a simple measure of the amount of material removed by the screen as compared to the theoretical amount that should have been removed. It follows from this that if the amount of material to be removed is small as compared to the total amount of feed, then only a small amount of carry over can produce a very small efficiency figure even though the amount of offending material is minimal.

It is because of these sorts of practical reasons, most operators are not so much interested in overall efficiency of a screen but are mainly concerned that their screened product meets specification.

There are two common methods of measuring screen efficiency. The method selected should depend upon whether the desired product is the “overs” or the “throughs” from the screening deck.



WHEN THE “THROUGHS” ARE CONSIDERED TO BE THE PRODUCT,

the screen operator wants to recover as much as possible of the undersize material

and we look at this case in terms of the

EFFICIENCY OF UNDERSIZE RECOVERY

WHEN THE “OVERSIZE” IS CONSIDERED TO BE THE PRODUCT,

the screen operator wants to remove as much as possible of the undersize material

and we look at this case in terms of

EFFICIENCY OF UNDERSIZE REMOVAL



For identical conditions each of these methods will give very different answers, and so when making any statement of efficiency it is important to qualify which method has been used.

IF NOT STATED, IT IS USUAL TO ASSUME THAT EFFICIENCY HAS BEEN EXPRESSED AS “EFFICIENCY OF UNDERSIZE RECOVERY”. ■

John: Thanks for your imput......appreciated.

All of these figures we knock around are in fact assumptive and are to be utilized as general guidelines as your comments indicated.

This is historical testing data done by (in the case of VSMA) seven very well established Vibrating Screen Manufacturing Companies.

At the end of the day, it is and ART not a science to come to an accurate determination of screen efficiency. The math or equation only gets us part of the way there, the reality of site conditions gets you the rest of the way there.

Keep Smiling.........George ■

To perform the actual efficiency calculation you need to take accurate sieve analysis (gradation) samples of:

(1) the feed to the screen deck

(2) the oversize material after it has been screened

then efficiency of undersize removal = (a/b) x 100

where:

a = % of feed material that should pass over the deck (from sieve analysis to deck)

b = % of material actually passing over the deck (from sieve analysis of the ‘overs')

this is the general industry standard method for calculating screening efficiency.

95% is normally considered as commercially perfect, and 90% is often quite acceptable.

however remember that if there is relatively little undersize in the feed (say less than 30%) - even a small amount of carryover of marginal oversize pieces will make the screening efficiency low. ■

Dear Rekhawar,

We do prefer 4% of undersize in the product and 2% of oversize in undersize. Normally we get 90 to 95% screen efficiency.

With regards. ■