Material Handling in Agro Industries Questions and Answers

Answer: C

Correct answer: 0.44 m\(^3\)/s

Multiply the duct area by the required velocity:

\[ A = \frac{\pi d^2}{4} = \frac{\pi (0.20)^2}{4} = 0.0314\ \mathrm{m^2} \]

\[ Q = 0.0314 \times 14 = 0.44\ \mathrm{m^3/s} \]

The saltation velocity is the floor below which particles drop out of suspension and settle along the pipe, eventually blocking it. Design velocity is set with a margin above it, since a system that saltates on start-up or on a surge in feed rate may never recover.

Answer: D

Area goes as the square of diameter, so doubling gives four times the area. Enlarging the outlet is the most reliable cure for bridging, and it is why arching criteria are written in terms of a critical outlet dimension.

Answer: A

Correct answer: 4.4 kW

Compute the theoretical lifting power, then divide by the efficiency:

\[ P_{th} = \frac{30\,000 \times 9.81 \times 24}{3600} = 1962\ \mathrm{W} \]

\[ P_{motor} = \frac{1.96}{0.45} = 4.4\ \mathrm{kW} \]

The efficiency covers bucket digging resistance, belt and bearing friction, and drive losses, and 45 per cent is typical for an elevator. Selecting on the theoretical figure alone would leave the machine unable to start under load, let alone run.

Answer: B

Correct answer: 0.0338 m\(^2\)

Apply the stated fraction to the square of the belt width, in metres:

\[ A = 0.08 \times (0.650)^2 = 0.08 \times 0.4225 = 0.0338\ \mathrm{m^2} \]

Rules of this form are used for a first estimate because the load area scales with the square of the width for a given trough geometry and surcharge angle. The coefficient itself depends on the idler troughing angle and on the material's surcharge angle, so it varies with the installation.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. A plain belt is limited to about 20 degrees because above that the material rolls back down the belt surface faster than the belt carries it up.

The limit follows from the material's angle of repose, so it is a property of what is being carried rather than of the machine. Vertical lift therefore requires a different machine - a bucket elevator - or a belt with cleats or sidewalls that physically retain the material.

Answer: D

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect, and statement II is exactly why. In a screw conveyor the rotating flight drags the material against the trough wall along the whole length of the run.

That sliding contact abrades the product and consumes power, so a screw damages grain more than a belt, on which the material rides without relative motion. The screw's advantages lie elsewhere - compactness, full enclosure, discharge at any point and low capital cost - which is why it is chosen for short runs despite the damage.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Janssen's analysis shows that as depth increases, an increasing share of the stored material's weight is carried by friction against the silo walls rather than by the floor.

The result is that floor pressure approaches an asymptote instead of rising linearly with depth, as it would in a liquid. This is why a deep silo's floor need not be designed for the full weight of its contents - and why the wall must be designed for the substantial vertical load it does carry.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both statements are correct, the second being the definition from which the first follows. If all the material moves whenever any is withdrawn, then by definition nothing is left standing - there are no stagnant regions.

The practical benefits follow directly: first-in first-out discharge so nothing sits and spoils, no ratholing, and a large measure of remixing that undoes segregation during filling. The price is steep, smooth walls, making a mass flow hopper taller and more expensive than a funnel flow one.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. A cohesive material forms an arch by bridging across the outlet, and the shorter the span the easier that arch is to sustain - the same reason a small arch in masonry carries load more readily than a large one.

Enlarging the outlet beyond the material's critical arching dimension therefore prevents the arch forming at all, which is why it is the most reliable cure. Vibrators, air cannons and rodding break arches after they form and are palliatives by comparison.

Answer: D

Correct answer: Statement I is incorrect but Statement II is correct

Statement I is incorrect. Dilute phase pneumatic conveying uses considerably more power per tonne than a belt - often several times as much.

Statement II is correct and is the reason. Keeping the particles fully suspended requires a fast air stream, typically 15 to 30 m/s, and the whole of that air must be moved and compressed for every tonne of product carried. The system is chosen for flexibility of routing and full enclosure, not for efficiency.