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Thread: Screen Efficiency?

  1. #1
    canuck1 Guest

    screen efficiency?

    I am sending 3/4" minus material (90% pass 1/2", 50% pass 1/4") over another screen with a 1/2" square (wire) opening on the top deck. How much 1/2" minus stone should be retained in the screen oversize (3/4" x 1/2")?

    I am only feeding a maximum of 170 (avg. 120) total tonnes per hour of 3/4" minus to a 6'x 16' deck. It has a 10.5 degree incline, a 3/8" circle throw, and is running at 850 rpm.

    I am getting almost 40% of the supposed 1/2" + that will pass a 1/2" lab sieve.

    Can you help me?

  2. Hello Canuck1,

    The subject pertains to screening efficiency. Certain value of screening efficiency is considered while designing / selection of the screen. The quantity of passing size retained on the screen deck will be according to this screening efficiency (in reverse order).
    Suppose, you are feeding 100 tph of 100% (-) 19 mm size on screen deck having opening corresponding to 19 mm passing size and if the screening efficiency is 75%, then you will have around 75 tph passing through the screen and 25 tph will be on screen deck for discharge as oversize.

    The screening efficiency affects the size and price of the screen. Therefore, if this issue is not taken care of during choosing the screen then although screen may appear to be economical, but it will have more proportion of the screen passing size getting discharged as oversize.

    The screening efficiency arises due to reason that all the material will not have opportunity to pass through the deck during course of its movement on the deck. Hence, bigger the screen, the material will have more opportunity to get through the screen deck. Also, more quantum of oversize in the feed material tends to reduce the screen efficiency because such oversize material will tend to cover the available area of the screen and thereby the passing size will have less opportunity to pass through.

    The subject matter is more suited to the consultancy / advisory services from the professionals in this field.

    Regards,
    Ishwar G Mulani.
    Author of Book : Engineering Science and Application Design for Belt Conveyors.
    Email : parimul@pn2.vsnl.net.in
    Tel.: 0091 (0)20 25882916

  3. #3
    George Baker

    George Baker

    Vice President- Corporate Communications

    Assinck Ltd. [eDir]

    Assinck Ltd.

    Professional Experience 39 Years / 9 Month George Baker has 39 Years and 9 Month professional experience

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    Screen Inefficiecy Problem 2nd attempt


    You are experiencing a TYPICAL carryover problem of fines in your overs. The question is WHY?

    Primary Reason: SLOPE is INCORRECT = TOO LOW:

    The SLOPE angle of decline should be minimum 20 degree decline by VSMA rules. If you apply this assumptive rule......the expected efficiency should be 95% efficient. This is pretty normal by math calculation and by practical in the field experience. 95% efficiency is also assuming your material is relatively DRY, FREE FLOWING vs WET or non-free flowing.

    In theory: you should only be seeing 5% of minus material in your overs by the rules....assuming we are operating at 95 percent efficiency.

    YOUR SLOPE: is 10.5 degrees.

    Here is what happens or DOESN'T happen more correctly:

    The foot travel which at 20 degree angle decline would be around 60-75 FPM is now TOO SLOW. The material gangs up on the screencloth, BED DEPTH or thickness of the material mat becomes too deep. The FINES or minus material you want to pass thru the opening CAN NOT. Why?

    BED DEPTH too deep: the fine material can not be separated or thrown out of the bed because it is too DEEP. So...it stays trapped inside the bed thickness....and carries over the deck when it should be passing.

    VSMA: WILL give you size of screen deck area needed......but, you must also double check the BED DEPTH calculation for efficiency to accomplish 95% EFFICIENCY.

    I took the liberty to recheck your application in the VSMA book:

    3/8" at 850 RPM is OK:
    running directional flow/ assuming the weight per cubic foot is 100 lbs/ and material is relatively free flowing to ensure the bed depth does not gain up too deeply

    GENERAL RULE THUMB: We must have enough STROKE or circle size to clear the top deck opening of wirecloth firstly...or we don't get to the bottom deck.

    FOR YOUR APPLICATION: The SLOPE ANGLE is defined in VSMA chart at 18 - 25 degree slope range as the pick.

    CONCLUSION:
    You are meeting most of the rules for efficiency BUT, your slope is TOO LOW.

    Troubleshooting: This is the problem.

    ACTION REQUIRED:

    RAISE the feed end of the machine up to 20 degrees and recheck your testing sieve results.....you will find this should correct your problem.

    Also: when you raise the angle....you can actually go 1/16" bigger opening on the wirecloth and still stay on your product spec...try one section and you will see. This is because at 20 degree angle the material will NOT SEE as big a hole due to foreshortening of the opening due to 20 deg slope.

    LASTLY: VSMA also recommends 5/16" circle or stroke at 900RPM
    which you could try if raising the angle is NOT AN OPTION.

    Sorry...for late reply........I have been out of country.

    HOPE THIS HELPS.

    PS: If you need on site help in ONTARIO CANADA......I will come to you. No problem

    George Baker - VIBRATING SCREEN CONSULTANT & MODERATOR

  4. #4
    George Baker

    George Baker

    Vice President- Corporate Communications

    Assinck Ltd. [eDir]

    Assinck Ltd.

    Professional Experience 39 Years / 9 Month George Baker has 39 Years and 9 Month professional experience

    Discussions 883 George Baker acceded to 883 discussions, Articles 0 George Baker wrote 0 articles, Publications 0 George Baker Baker released 0 publications

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    SCREEN INEFFICIENCY

    CANUCK 1

    You may also want to check:

    WIRECLOTH wire diameters are not too thick

    WIRECLOTH is NOT pitting which will slow down material


    George

  5. #5
    Hi canuck 1
    I would gladly work out your problem, but would need the following information.
    1. Density of material.
    2. Wet or dry screening.
    3. Wire or poly decking.
    4. Most important is a gradiation analysis of your material.
    5. Cut point sizes of your deck, square or long slot.
    Forward this imformation when you have time.
    I will be in Canada next week, and could probably arrange a meeting if you stay in Ontario.
    Regards
    Ian McCallum

  6. #6
    canuck1 Guest
    Thank you all for your help. I suspected the slope of the screen deck was some of the problem, but I am not observing excessive bed depth at the discharge end of the screen.

    I am working in a quarry environment, 100% crushed, extremely dense (160-180 lbs/ft) material, dry screening only.

    I was able to upsize the (std. diameter) wire cloth from 1/2" square opening to 9/16", but only got a negligible improvement.

    (feed) gradation as follows:
    (percent passing)
    3/4" 100.0%
    5/8" 99.6%
    1/2" 88.4%
    3/8" 71.1%
    1/4" 51.6%
    No. 4 43.7%
    No. 8 28.6%
    No. 16 19.7%
    No. 30 13.3%
    No. 50 10.1%
    No. 100 7.8%
    No. 200 5.8%

    I would like to screen out as much 1/2" minus as I can and minimize the volume of 1/2" - carryover on the top deck.

    My bottom screen is slotted (1/4" x 1 1/2" long) and I have no carryover between the 1/2"and the 1/4" screens.

    sorry, but I am on the west coast.

  7. #7
    George Baker

    George Baker

    Vice President- Corporate Communications

    Assinck Ltd. [eDir]

    Assinck Ltd.

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    SLOPE adjusted or NO?

    can you confirm if you raised the angle?

    and if so............What actual slope are you at now?


    If bed depth is not a problem: can you actually tach the speed of the screen and advise that?

    and we still have the question: ARE YOU BLINDING or plugging the hole openings on the 1/2" openings?

    You percentage of NEAR SIZE material is consecutive sieves is very high and this will also create a problem of passing or splitting out the NEAR SIZE from the total feed mix.

    YOU MAY have to actually really tighten up the top deck to HELP LOAD RELIEVE prior to getting to the second deck.

    WE SHALL RECTIFY THIS !!!

    Regards, Are we having fun yet? George Baker MODERATOR

  8. #8
    canuck1 Guest

    no change in slope

    No, I am unable to change slope of the screen in its current position. I do not see much more than 1 1/4" of bed depth at the discharge end of the screen. Plugging & blinding does not seem to be a problem on this screen.

    I have replaced all the springs (they were sagging) with little effect, I even increased the screen opening to 9/16" with only a 5% improvement over earlier numbers.

    I am confident about the screen rpm I have provided, but not exactly sure whether or not the throw is 3/8" or 7/16" circle.
    I have a tough time reading my circle throw gauge. any tips?

    I thought about your suggestion of more speed and less throw, but became concerned that I would end up with the earlier problem you described. If material travels less distance with each motion, won't it then gang up at some point on the deck and cause the bed depth problem you described earlier? Or is a 50 rpm increase in speed enough to counteract this?

    I can see why VSMA recommends a 20 degree angle since being stuck at 10.5 gives me very few options (like counterflow) to increase my efficiency.

    Thanks again

  9. #9
    George Baker

    George Baker

    Vice President- Corporate Communications

    Assinck Ltd. [eDir]

    Assinck Ltd.

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    Hi again.......

    Thankyou for the update.

    HONESTLY: you problem is mainly WRONG INCLINE or decline or slope (you pick).

    ANGLE is the demon here....in actual fact the foot travel is too slow....if you were to flourescent paint a stone and put on right at the feed end and time with your watch til it drops off the end of the screen deck discharge lip....this will confirm FOOT TRAVEL rate is too slow due to low angle.

    So you are landlocked so to speak....tight within the area you are installed and no room to raise up. A very typical problem unfortunately.

    Is the machine grease lube? If it is easily raised without a detremental effect on bearings. If splash oil or oil in the end covers......we must adjust and be careful we do not GANG UP oil on low side and starve bearings of oil on the high side...IF WE RAISE THE SCREEN BOX AT THE FEED END...... MORE LATER.......on this little prob.

    IS it two bearing or four bearing........whose mfr.......?

    __________________________________________________

    Reading the CIRCLE: A tip or trick for you. Most guys have a magnetic plate with actual circle sizes on it. They show 1/2" 3/8" 1/4" 3/16" actual circles on them. When you put it on the side of the machine when vibrating..........HERE IS WHAT YOU DO.

    You look at all circles.....and ONE of them.......and only one of them will show a nice little DOT in the dead centre when vibrating. THIS IS THE "Actual" circle or stroke size you are operating that box at.

    The other circles will show.......BIGGG shaded areas in the middle and these are NOT the correct or actual circle size operating at.

    TRICK: We have sticky back paper we put on magnetic card - use a sharp pencil - touch ever so lightly........like real light touch and just one revolution needed not ten......just messes up the stroke or circle and makes it unable to read. OR just tape a piece of white paper onto magnetic card.......mark FEED END RIGHT, FEED END LEFT, DISCH LEFT RIGHT ETC, CENTRE. and do stroke check in all those areas.

    NOW, take a tape measure and carefully measure across width of actual circle....and determine if it is 1/4" or 3/8" line to line and now you know how to do it.

    IF too small........check with factory and increase offset weights of the wheel to increase to proper size.......or decrease weight and increase speed........BUT, always advisable to check with factory first.....THEY KNOW the limits.

    LAST again: You really should try.....some FLEXMAT self cleaning wirecloth for the crushed material....more open area and better efficiency. THIS STUFF IS by MAJOR WIRE......PQ


    George Baker Vibrating Screen Consultant

  10. #10
    canuck1 Guest
    George,

    Thank you for the circle throw measurement tips. My machine is a Pioneer 6x16, grease lubrication, older model 2 bearing machine.

    I have spoken with manufacturer and they said it is possible to remove all the shaft weights (bolt on weights as well as the "pork chop"counterweight itself. They claim there is enough eccentricity in the shaft to provide some throw, but couldn't tell exactly how much. They will allow 900 rpm, but don't have any advice on raising the screen.

    This deck is a little different than others I have seen (besides the slope), The first 12 ft of the 2nd deck sits above the shaker mechanism as per normal, but the last 4 ft of the screen is a kind of split level. It drops material down an additional 6 inches on to the last screen panel. ( I think to offer more access room) I have seen some buildup at this 'transfer point' but it is usually back to a uniform loading by the time the material reaches the discharge end.

    (Yes, this is the deck where I have my 1/2", now 9/16" screens)

    Thanks again.

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