Ask20

Sorry I cannot help but am on the move at present.

I am confident George Baker will be able to assist he is a very generous moderator.

Good luck

Mike. ■

Ask20 & George

I may be able to throw a little more light on this problem unit having recently inspected it....

It's purpose is to screen metal fragments like small nuts and pipe wear slivers out of hydrated lime being blown into a day storage bin.

The hydrated lime is 95% passing 150micron and can surge up to 40t/h

The current wire mesh screen aperture is approx 8mm but smaller would be preferred if blinding can be eliminated. It was originally 3.83mm 5 mesh (1.25mm wire dia). The mesh is bolted in with very little camber.

The screen angle is not known but "looks" OK.

It was designed locally in NZ some years ago and has been inefficent ever since installation relying on air fluidising by lean phase conveying to pass the powdered lime which should be delivered to it as a dense phase slug and screened by vibration.

It has a two vee belt drive but frequently loses belts.

The lime supplier says the product density is 910kg/m3 but they have mixed it with quicklime the true density of hydrated lime is around 500kg/m3 but their is some conveying air fluidisation so it could even be lower than 400kg/m3.

The screen has an out of balance shaft with small counterweights providing the throw.

Hope that helps.....Mike. ■

HELLO Ask20 & Mike: Thanks Mike........now we are back on the right thread here.........

Thank you both for the detailed "Field information" this type of reality truly dictates the answer. We always ask our sales guys in the field to go DIG the info out ..........so we can truly be helpful.

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The purpose of the unit is the screen ....actually not screen but, SCALP a small percentage of contaminants ....metal, bolts, fragments out of a product........namely hydrated lime.

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Beautiful information MIKE..........the key phrase you stated was SURGING.........UP to 40 TPH...........yikes........problem.... of course at this point we have very deep BED DEPTH or mat of material going over the deck.........and the TINY LITTLE stroke.......in the circle is not anywheres adequate to toss the material up in the air and separate the COARSE .....pieces of metal from the small particles of LIME........hence, we do not separate at all and we carry over the contaminants and product which should pass due to BED DEPTH PROBLEMS. this is clearly addressed in VSMA after the size of the screen is picked as a double check.... to assure efficiency is in fact attained.

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95% passing 150 micron is VERY TOUGH SCREENING.......the smaller the size......the tougher the task to achieve efficiency

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BLINDING is stated clearly as a PROBLEM in the field. This of course equals NON availability of calculated screen area to allow the fines to pass. IF i blind the screencloth completely ....NIL PASSES........EFFICIENCY equals zero.

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Material is very fine and will build up on edges of square wirecloth openings and over due course blind over completely. If fully blinded over.......metal and fines will all go over the single deck which AINT desirable of course.

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CAMBER is very slight......if wirecloth.....it should be very tightly stretched over the rails with a fairly good ARCH OR CAMBER which ensures the back of the wirecloth is being BROKEN or tightly stretched over the camber to ensure TIGHT WIRECLOTH TENSION. if cloth is not very tight.....we BLIND.

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This situtation sounds ripe for a ball tray deck to keep the blinded deck opened up. A heated wirecloth deck would also be a possible solution if moisture content is present. A product with moisture over 6% will corrode the wire, which pits and under a magnifying glass looks like crates which grab the material, allow it to build up and BLIND the aperture or opening.... STAINLESS steel should be used here as standard for this small wire diameter and for this fine of screening.

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SCREEN ANGLE: was not stated......honestly we want to be a lot steeper than 20 degrees to just quickly PASS the minus 150 micron and quickly scoot off the metal contaminants. What is the smallest big piece of metal???

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What is the feed - sieve analysis of material going onto screen? If it is already sized and just safety screen for metal.......we can open up the deck .........possibly.? If just screening for METAL...use cross belt magnet......or head pulley magnet to pull it out?

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SCREEN SIZE; very small.......footage.....like 1.83 ft x 5.3 ft or total screen area of 9.699 FEET .... is not much to handle surges up to 40 TPH.......we need to size for the max .......not the average TPH.

Lets think of this......max surge 40TPH ....screening over say nominal 10 sq ft of wirecloth.....we are asking the screen to pass nominal 4 TONS PER HOUR PER SQ FOOT......not possible at all. Even at 7 TPH........ we are not big enough to do the job by VSMA rules or practical in the field calucation.

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SPEED & SLOPE: We need to go quite FAST on the rpm here to screen the lime........and slope has to be in excess of 25-30 degrees TO keep foot travel up and bed depth or thickness of mat on the screen down to manageable depths by the rules of VSMA. VSMA says max bd at discharge end of any screen length should not exceed 4 times opening size on the actual deck being tested.

Sorry for the long winded....but we need to think of all variables to attain success.

GEORGE BAKER........ ■

George & Co

You asked....

"SCREEN ANGLE: was not stated......honestly we want to be a lot steeper than 20 degrees to just quickly PASS the minus 150 micron and quickly scoot off the metal contaminants. What is the smallest big piece of metal??? "

I have found the screen angle on the old drawing I have to be 14 degrees.

We should probably assume we want to scalp bits of metal of 4.0 mm and larger from the minus 150um hydrated lime powder, remembering the original mesh was 3.8mm aperture and is currently 8.0 mm aperture.

One other thing we haven't mentioned that may make it a little clearer for George is that the screen cloth covers the whole area of the single deck and there is just a ledge on the outlet end to catch the pieces of metal for occasional manual cleaning and so when it blinds during a surge it actually pressurises above the deck and backpressures into the pneumatic blow line feeding it.

Mike. ■

Hi Mike & Co:

14 degrees is a scalper screen type of install angle

We need steeper for the tough fine screening. Yes......we just want the metal to get the heck off....as quick as possible.

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Okay, one a single deck as you are using.........with about 8mm opg in wirecloth........and surging up to 40tph on this small deck we are wrong in quite a few areas. Not enough footage (screen is too small) bed depth too deep too often.

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So........if I can get the angle up........thin out the bed depth from feed end to discharge end quicker, I may be able to screen. But, the screen is undersize....in its present config.

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Concept 2: go to double deck config. say........4 mm over 8 mm and this would help load relieve the screening deck to allow the bed depth to thin DOWN to an acceptable thickness to allow separation to take place.

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BALL TRAY DECK: This device would be a very good mechanical means of ACTIVATING the underside of the BLINDED wires to help keep them opened up and NON BLINDED.

Are we havin' fun yet............Keep Smiling.......George ■