Screw conveyors: which statement about operating speed and material suitability reflects standard practice?

Introduction / Context:
Screw conveyors are ubiquitous for short, controlled transfers of powders, granules, and pastes. Their enclosed design limits dusting and allows metering, heating/cooling, and mixing along the trough.

Concept / Approach:
Standard screws do not run at “very high” rpm; excessive speed causes undue wear and product degradation. With correct flight design, clearance, and low speed, screws can move sticky or pasty materials that would bridge on belts. Conversely, highly abrasive solids cause significant screw and trough wear; while abrasion-resistant liners and hardfacing help, screws are not the first choice for severe abrasion.

Step-by-Step Solution:
Assess speed claim: typical screws operate at modest rpm to prevent surging and wear → (a) is incorrect.Assess sticky handling: with proper design, screws are used for sticky materials → (b) is correct.Assess abrasion: screws suffer wear with highly abrasive solids → (c) is incorrect as stated.

Why Other Options Are Wrong:
(a) Overstates speed; (c) ignores wear realities; (d) aggregates incorrect claims.

Final Answer:
They are suitable for sticky or pasty materials (with proper design).