A screening deck is running at design feed rate but effectiveness has fallen steadily over a shift.
What is the most likely cause?
Answer: A
Effectiveness declining at constant feed points to the apertures gradually plugging with near-size or damp material. Ball trays or brushes beneath the deck, which knock the plugged apertures clear, are the standard cure.
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An aspirator is set to 10 m/s to be sure of removing all chaff from grain whose terminal velocity is 9 m/s.
What will happen?
Answer: B
Correct answer: Sound grain will be carried away too
The air velocity must lie between the terminal velocities of what is to be removed and what is to be kept. At 10 m/s, above the grain’s own terminal velocity of 9 m/s, the grain is lifted along with the chaff and leaves with it. The right setting is comfortably below 9 m/s and above the terminal velocity of the chaff.
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A fruit packhouse finds that some fruit graded into the top size class is unexpectedly light.
Which change addresses the problem?
Answer: C
A hollow or under-filled fruit occupies the same volume as a sound one, so a diameter sizer cannot tell them apart. Only a weighing mechanism catches the difference, which is why electronic weigh cups have displaced simple sizers on high-value fruit.
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The first and coarsest screen in a cleaner, which removes oversize trash, is the _____ screen.
Answer: D
Correct answer: scalping
Scalping is the first and coarsest stage, removing straw, string, stems and other oversize trash before anything finer is attempted. It comes first for a practical reason: that material would blind the finer decks downstream, so removing it protects everything that follows and is why the order of a cleaning line is not arbitrary.
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Apertures becoming plugged by near-size or sticky particles is known as _____.
Answer: A
Correct answer: blinding
Blinding is the plugging of apertures, usually by near-size particles that wedge in them or by damp or sticky material that adheres. Both capacity and effectiveness fall as the open area is lost. It is countered by ball trays or brushes beneath the deck, by rapping, and by choosing an aperture shape less prone to holding the particular material.
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A separator that removes stones and other dense foreign matter from grain is a _____.
Answer: B
Correct answer: destoner
A destoner separates on density, using an inclined vibrating deck with air rising through it: the fluidised bed lets the dense stones sink to the deck and climb it while the lighter grain floats and flows down. It is fitted in every mill line because a grain-sized stone will pass every screen and every aspirator and then damage the rolls.
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A separator that picks short seeds out of a bed using pockets in a rotating drum is an _____ cylinder.
Answer: C
Correct answer: indented
An indented cylinder is a rotating drum whose inner surface carries pockets of a chosen depth. As the drum turns, short particles seat fully in the pockets and are carried up to drop into a trough, while long ones project and fall back into the bed. Changing the indent size changes the length at which the separation occurs.
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The device that recovers entrained dust and chaff from an aspiration air stream is a _____.
Answer: D
Correct answer: cyclone
The aspiration air leaves carrying the dust and chaff it has removed, and a cyclone recovers them by spinning the stream so that the particles are thrown against the wall and fall out of the bottom while the cleaned air leaves through the top. It is placed ahead of the fan so that abrasive material never passes through the impeller.
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Screen performance at four feed rates on a fixed deck area.
| Feed (t/h) | Bed depth (mm) | Undersize recovery | Effectiveness |
|---|---|---|---|
| 3.0 | 12 | 0.96 | 0.93 |
| 4.5 | 18 | 0.93 | 0.89 |
| 6.0 | 26 | 0.86 | 0.81 |
| 7.5 | 38 | 0.72 | 0.66 |
What does the table demonstrate?
Answer: A
Correct answer: Effectiveness falls as the bed deepens
Read the two middle columns together: as the feed rises from 3.0 to 7.5 t/h the bed deepens from 12 to 38 mm and effectiveness falls from 0.93 to 0.66. Undersize buried in a deep bed never reaches the deck to be presented to an aperture. This is the capacity against effectiveness trade that governs every screening decision.
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Terminal velocities of the components of a harvested wheat sample.
| Component | Terminal velocity (m/s) |
|---|---|
| Dust and fine chaff | 1.2 |
| Straw fragments | 3.5 |
| Shrivelled grain | 6.5 |
| Sound grain | 9.5 |
Which air velocity removes dust, straw and shrivelled grain while retaining sound grain?
Answer: B
Correct answer: 8.0 m/s
The velocity must lie above the terminal velocity of everything to be removed and below that of what is to be kept. Shrivelled grain is the heaviest of the unwanted fractions at 6.5 m/s and sound grain the lightest of the wanted at 9.5, so anything between those figures serves; 8.0 m/s sits comfortably in the middle. At 10.0 the sound grain would go too, and at 5.0 the shrivelled grain would stay.
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