Material Handling in Agro Industries Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. Dust extraction at transfer points addresses the dust explosion hazard by removing the fuel; relief panels address structural damage once an explosion has begun; and enclosed conveyors address product contamination.

The fourth is wrong. A larger hopper outlet addresses bridging - it exceeds the material's critical arching dimension so an arch cannot form. It has nothing to do with dust explosions, though a hopper that flows freely does avoid the rodding and vibrating that can themselves raise dust.

Answer: D

Correct answer: Only three

Three pairs are right. The boot is where the buckets pick up material at the bottom of the elevator; the head is where discharge takes place; and centrifugal discharge requires a relatively high belt speed to throw the material clear.

The fourth is wrong. Continuous buckets, set close together so material slides out over the back of the bucket ahead, are used at low speed - precisely where gentle handling of a fragile product matters more than throughput.

Answer: A

Correct answer: Pneumatic conveyor

A pneumatic conveyor uses air as the transport medium, entraining the particles in a moving stream rather than pushing or carrying them with a mechanical element.

The other three are mechanical: a screw pushes material with a rotating flight, a drag conveyor pulls it along a trough with flights on a chain, and a bucket elevator carries it in buckets on a belt or chain. The distinction matters for power - moving air for every tonne of product is expensive - and for the routing flexibility a pipe allows.

Answer: B

Correct answer: Wall pressure by Janssen

Janssen's analysis of wall pressure is a structural matter: it determines the loads the silo wall and floor must carry, and hence the thickness and reinforcement required.

The other three are flow problems, concerning whether and how the material comes out. Bridging is an arch across the outlet, ratholing is a channel emptying through static material, and funnel flow is the pattern in which only a central core moves. Structure and flow are the two independent halves of silo design.

Answer: C

Correct answer: Adding a further transfer point

Each transfer point is an impact, and each impact breaks a further small percentage of kernels. Adding one therefore increases damage, and because the losses compound through a plant the effect on the final proportion of brokens is more than marginal.

The other three all reduce damage: shorter drops mean lower impact velocities, lower conveyor speeds mean gentler handling, and a belt is markedly kinder than a screw, which drags the product against the trough along the whole run.

Answer: D

Correct answer: High material moisture content

High moisture is not an element of the explosion pentagon - indeed it works against an explosion, since damp material generates less airborne dust and damp dust is harder to disperse and to ignite.

The five elements are combustible dust, dispersion of that dust as a cloud, oxygen, confinement and an ignition source. All five must coincide, which is why control attacks several at once: extraction and housekeeping remove the fuel, and hot-bearing monitoring removes the ignition source.

Answer: A

Correct answer: Conveyor length

Capacity is the mass crossing a point per unit time, so it depends on how much material the belt carries per metre - set by width and bulk density - and how fast that passes, set by speed. Length does not appear: a 10 m belt and a 100 m belt of the same width and speed deliver the same tonnes per hour. What length does affect is the power, since friction accumulates along the run.

Answer: B

Correct answer: Cumulative damage from many transfers

Each transfer adds only a fraction of a per cent of damage, so no single machine looks culpable, but the fractions add along the line and a plant with six or eight transfers can lose a substantial part of its germination. The remedy is architectural rather than mechanical - fewer transfers, lower drops, distribution by gravity after a single lift.

Answer: C

Correct answer: Enlarge the outlet or steepen the walls

Damp maize is cohesive, so it can form a self-supporting arch across the outlet. Every arching criterion makes the outlet size the governing variable: enlarge it beyond the critical dimension and no arch can support itself. Steepening and smoothing the walls promotes mass flow and helps too. Reducing the outlet does the opposite.

Answer: D

Correct answer: A bucket elevator at low speed

A 22 m vertical lift in a confined footprint leaves little choice: a plain belt cannot exceed about twenty degrees, a vertical screw is harsh and power-hungry, and a dilute phase pneumatic line damages grain at every bend. A bucket elevator lifts vertically at high capacity, and running it slowly reduces the impact at loading and discharge.