Screening efficiency

What efficiency means here

Screening efficiency describes how much of the material that could pass the openings actually does so before the product leaves the deck. Material that should have passed but stayed on top is carried over into the oversize. Efficiency is not a fixed property of a machine; it changes with the product, the feed rate and the settings.

Factors

Opening size and shape

Pieces close to the opening size pass slowly: they need several contacts with the deck before they find an opening. The further the product size is from the opening, the faster the separation. Panels are therefore made for the product group rather than chosen from a standard list.

Layer thickness

Undersize can only pass when it reaches the deck surface. A thick layer keeps fine pieces away from the openings; a thin layer exposes them. For a given deck, higher throughput means a thicker layer and lower efficiency. A feeder ahead of the screen keeps the layer even.

Deck angle

A steeper deck moves the product faster, so each piece has less time on the deck. A flatter deck gives more time but can let the product bed down. The angle is set with the stroke so that the product keeps moving and keeps turning over.

Stroke and frequency

Stroke and frequency set how far the product is thrown each cycle and how often. More agitation turns the product over and brings fines to the deck; too much throws the product off the deck surface, which reduces contact. The combination is chosen for the product and left fixed in operation.

Product condition

Wet, sticky or oily product blinds the openings and clings together. Dried fruit varies by season and by lot. Where the product is wet, a dewatering stage comes first; where it is sticky, the opening shape and the panel surface are chosen to limit blinding.

Panel condition

Worn or stretched openings change the separation size; blinded openings reduce the open area. Regular inspection and cleaning of the panels keep the result stable. See maintenance.

Double deck

On a double-deck screen the top deck removes coarse material and protects the finer lower deck. The lower deck then works on a narrower size range, which helps its efficiency. The trade-off is a taller machine and two sets of panels to maintain.

Efficiency and capacity

For a given screen, capacity and efficiency pull in opposite directions: more throughput thickens the layer and lowers efficiency. Selection therefore starts from the required throughput and the acceptable carry-over, and sizes the deck area from there. See sizing and selection.