Food industry equipment Unit 10

Text 1

1. Read, translate and retell the text.

Mechanical Apparatuses. Roll mill

Roll mills may contain one, two, or three rolls and can be arranged for

either batch or continuous production. The supply of material is cradled

between the rolls or between the roll and a feed bar, where it is kneaded and

folded. Part of the material is nipped and continuously drawn down between the rolls or between the roll and the feed bar where it is subjected to crushing, shearing, and abrasion. This action calls for the proper formulation as sufficient

liquid must be provided to wet the constantly increasing surface of the dispersed mate­rial. This is necessary both to lubricate the particles as they pass through the mill and to avoid the formation of new agglomerates. Since all the broken or dispersed particles in the thin film must be immediately surrounded with a film of vehicle, the feed to the mill must be uni­form and not contain lumps of dry material. Change-can mixers may be used for preliminary dispersion.

The roll or rolls may be heated or cooled as required. As the action varies with the number of rolls, the different types will be considered separately.

The single-roll mill con­tains a wide-faced grinder bar working against a smooth hardened roll. Practically all the work done in single-roll mills results

from abrasion. The nipping action is much weaker than between two rolls

rotating in the same direction at the point of contact, and there is less

crushing than in a two-roll mill. The larger contact area of the rubbing bar and the roll gives more hydraulic shear than in two- or three-roll mills. As the nip is weak, coarse material collects in the feed between the grinding bar and the roll. This action may change the composition of the product.

Accordingly, single-roll mills are best considered for dispersion rather than grinding. Also, they cannot efficiently handle materials of a wide range of viscosity. Mixes which are too thin are not pulled into the narrow space between the grinder bar and the roll, while pastes which arc too heavy bridge the nip entrance.

Two-roll mills contain two parallel rolls mounted in a heavy frame with

provision for accurately regulating the pressure and distance between the rolls. They are usually mounted in the same hori­zontal plane. As one pass

between the rolls does little blending and only a small amount of work, they are practically always used as batch mixers.

To increase the wiping action, the rolls are usually operated at different speeds. The material passing down between the rolls is returned to the feed

point by the rotation of the rolls. If the rolls are at different tem­peratures the material will usually stick to the hotter roll and return to the feed point as a thick layer.