Material Handling in Agro Industries Questions and Answers

Answer: A

It carries large tonnages over long distances with very low power per tonne-kilometre and treats the product gently. Its weakness is inclination, since above about twenty degrees the material simply rolls back down the belt.

Answer: B

Correct answer: 20 degrees

About 20 degrees is the practical limit for a plain belt handling grain. Beyond it the material rolls back down the belt surface faster than the belt carries it up, so capacity collapses and spillage begins.

The angle of repose of the material is what sets the limit, and grain's is around 25 to 30 degrees. Steeper inclines require a different belt - cleated, ribbed or sidewall - or a different machine altogether, which is why a bucket elevator is used for vertical lifts.

Answer: C

Correct answer: Rotating a helical flight in a trough

A screw conveyor consists of a helical flight rotating within a fixed trough or tube. The flight pushes the material along as it turns, so the machine is compact, fully enclosed and can discharge at any point along its length through a gate in the trough.

The enclosure suits dusty and hazardous materials. The cost is that the material is dragged against the trough wall, so power per tonne is high and product damage significant - which is why screws are used for short runs rather than long ones.

Answer: D

Correct answer: Consumes more power per tonne and damages the product more

A screw drags the material along the trough wall, so friction acts on the product over the whole length of the run. That costs power and abrades the grain, giving both a higher energy consumption per tonne and more breakage than a belt, on which the material rides without relative motion.

The compensations are compactness, complete enclosure, the ability to discharge at any point and a much lower capital cost - all of which make the screw the right choice for short runs, and the wrong one for long ones.

Answer: A

Correct answer: Lift material vertically at high capacity

A bucket elevator is the standard machine for vertical lift. Buckets fixed to a belt or chain scoop material from the boot at the bottom, carry it up the casing and discharge it at the head, achieving high capacity in a small footprint.

Nothing else does the job as economically: a belt cannot exceed about 20 degrees, and a pneumatic system uses far more power per tonne. It is a vertical machine only, and it neither dries nor sorts the material passing through it.

Answer: B

Material fed into it is scooped up by the passing buckets, and a poorly designed boot is the commonest cause of spillage and of buckets running only part full. Its condition largely determines the elevator's real capacity.

Answer: C

Correct answer: A relatively high belt speed

Centrifugal discharge relies on the material being thrown clear of the bucket as it rounds the head pulley, which requires enough speed for the centrifugal effect to overcome gravity at the top of the arc.

The buckets are spaced apart so the discharging material has a clear path. The alternative, continuous discharge, runs at low speed with buckets close together, the material sliding out over the back of the bucket in front - gentler on fragile products, and used where breakage matters more than throughput.

Answer: D

Air velocity must exceed the material's saltation velocity, and routing is easy since the pipe can turn in any direction. The costs are high power per tonne, appreciable product attrition and the need for a cyclone or filter at the discharge.

Answer: A

Correct answer: High air velocity with particles fully suspended

Dilute phase conveying carries the particles fully suspended in a fast air stream, typically 15 to 30 m/s, at a low ratio of solids to air. The velocity must stay above the saltation velocity, below which particles drop out and settle along the bottom of the pipe.

It is simple and flexible in routing, but it is energy-intensive and abrasive to both product and pipework. Dense phase conveying is the contrast: low velocity, high solids loading, material moving as plugs, gentler but requiring higher pressure.

Answer: B

Correct answer: Hot or sticky material over short runs

A vibratory conveyor moves material by imparting a series of small throws, so the product travels in a succession of short hops with little sliding contact. There are no moving parts touching the material along the run, and the trough can be made of anything and left open or enclosed.

That makes it well suited to hot material that would damage a belt, and to sticky material that would build up on a screw. Its capacity and run length are limited, so it is used over short distances.