MATERIALS HANDLING
CONTROLLING CARRYBACK
R. Todd Swinderman explains why dabbling in dribble chutes can pay dividends
Air cannons prevent clogging and help direct the fl ow of material A
s belt conveyor cargo travels over the idlers on the carry run, the weight of the cargo and the
undulating surface causes dust and fi nes to adhere to the belt and lodge into cracks and gouges. Upon discharge, material that does not fall with the main cargo stream and remains clinging to the belt’s return is known as “carryback.” If it is not cleaned from the belt by a primary and secondary cleaner, the carryback falls from the belt and accumulates in piles under the belt, building up as spillage or becoming airborne dust. Studies have shown that mitigating carryback can reduce fugitive spillage and dust by up to 90% while increasing idler and belt life by 25-30%.
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www.engineerlive.com Material that is cleared by a primary
or secondary cleaner is collected on a slope in the rear of the head chute called a “dribble chute.” However, cleaning these fi nes increases the volume of material passing through the head chute and can build up directly under the head pulley. To control buildup and chute clogging, a vibrating dribble chute and mounted air cannons were developed to reduce downtime from clogging and cleaning and increase production.
A CLEAN BELT IS AN EFFICIENT BELT If a belt has no cleaners, operators should expect to add labour to their budgets to clean spillage and replace fouled components from excessive dust and carryback. When spillage is
left to build up, the belt rides on the dried pile of abrasive material which prematurely erodes the belt covers and damages the unprotected return resulting in a lower belt life. Further down the return path, carryback can foul rollers and spills into walkways requiring regular maintenance to retain eff iciency and adequate workplace safety standards. Without adequate cleaning, a belt
in a heavy-duty dry bulk handling operation can carryback more than 0.5 tons (0.45 tonnes) of material per hour. The addition of just a primary cleaner will still experience carryback, leaving only an average of 0.02 lbs/ft2
(100 g/m2
Adding multiple cleaners can get the carryback level down to 0.01- 0.004 lbs/ft2
(50 to 20 g/m2 ) on the
) on the belt.
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