Cleaning and Grading Equipment Questions and Answers

Answer: C

Correct answer: Comparing reflected light with a reference

An optical sorter illuminates each particle as it falls past a detector and compares the reflected light with a reference standard set for acceptable material. Where the signal departs from the reference beyond a set tolerance - a discoloured, mouldy, immature or foreign particle - a jet of compressed air deflects it from the stream.

Modern machines extend the principle beyond visible colour to infrared and to shape analysis, allowing defects invisible to the eye, such as internal fungal damage, to be detected as well.

Answer: D

Correct answer: How completely each fraction is recovered

Screen effectiveness measures how well the separation was actually made: what proportion of the undersize in the feed reported to the underflow, and what proportion of the oversize reported to the overflow. The overall figure is the product of the two.

It is a quality measure and quite distinct from capacity, which is throughput per unit area. The two trade against each other - pushing more material over a deck deepens the bed and lowers effectiveness - so a screen must be specified for both together.

Answer: A

Correct answer: Apertures becoming plugged by particles

Blinding is the plugging of screen apertures by particles that lodge in them and neither pass through nor fall back. Near-size particles are the usual culprits, and damp or sticky material makes it far worse.

The effective open area falls, so both capacity and effectiveness deteriorate - and the deterioration is progressive, since each blocked aperture increases the load on the rest. The remedies are mechanical: ball or brush cleaners running beneath the deck, tapping devices, and screens tensioned so they flex as the deck oscillates.

Answer: B

Correct answer: Reduces effectiveness, the bed being too deep

Beyond a screen's design capacity the bed on the deck deepens, and undersize particles lower in the bed cannot reach the screen surface during their travel along it. They leave with the oversize, so the recovery of undersize falls.

The trade is between capacity and effectiveness: a screen can always be pushed harder, but the separation degrades. Where both are needed, the answer is more screen area or an additional deck, not a higher feed rate - and the loss shows up as good material rejected, which is expensive.

Answer: C

Correct answer: Round perforations

A round hole passes a particle only if both its width and its thickness fit, and since width is the larger of the two it governs - so round perforations sort on width.

A slotted aperture is long in one direction, so a particle can turn edge-on and pass provided its thickness fits, whatever its width or length; slots therefore sort on thickness. Choosing the aperture shape is thus choosing which dimension the separation works on, which is why cleaners carry both types.

Answer: D

Correct answer: Material lighter in air than sound grain

Aspiration separates on terminal velocity - the air speed at which a particle just stays suspended. The air stream is set above the terminal velocity of chaff, dust, empty husks and shrivelled grain but below that of sound grain, so the light material is entrained and carried away while the grain falls through.

It removes precisely the fraction the screens cannot: material that matches the grain in size but not in weight. The entrained material is then recovered in a cyclone before the air is discharged.

Answer: A

Centrifugal action throws the entrained particles to the wall where they spiral down to the discharge, allowing the cleaned air to leave through the top. Without it the chaff would simply be blown into the working area.

Answer: B

Cleaning removes what should not be there; grading sorts what remains into saleable categories. The two operations normally run in that sequence on the same line, with cleaning always coming first and grading afterwards.

Answer: C

Correct answer: Diameter

A roller or diverging belt grader sorts fruit by diameter. The gap between the diverging rollers or belts widens along the machine, so each fruit is carried along until the gap exceeds its diameter and it falls through into the collecting channel for that size class.

The method is simple, gentle and continuous, which suits fruit. Its limitation is that it cannot detect a hollow or under-filled fruit, which has the right diameter but the wrong mass - the reason weight grading is used alongside it for some crops.

Answer: D

A hollow or under-filled fruit occupies the same volume as a sound one but weighs less, and only a weighing mechanism catches the difference. Electronic weigh cups have made the practice commonplace on high-value produce.