Krebs Coal Spirals are used to make density-based separations on nominal 14M x 100M (1.19mm x 0.149mm) size coal. These devices consist of a 0.96-meter Ø helix with between 2 and 3.25 turns, mounted on a 160-mm Ø center column. To provide greater capacity per volume, either two (duplex) or three (triplex) helices can be mounted on one center column.
Each helix mounted on a center column is typically referred to as a "Start". Each helix contains: feed boxes are used to properly distribute the feed slurry to the helix product-discharge boots are used to collect the products separated by the spiral. Each product discharge boot has two (2) adjustable-position product splitters which direct the "clean-coal" (outer stream), "middlings" (middle-stream), and "refuse" (inner stream) to their respective discharge ports in the boot.
Spirals are further grouped into rectangular-shaped "Packs", consisting of between two and ten duplex or triplex spirals, which are fed from a central radial distributor. For example, a ten-pack of duplex spirals contains 20 starts, a ten-pack of triplex spirals contains 30 starts. Beneath the spiral pack is a discharge piping system which collects the clean-coal, middlings, and refuse from all the product-discharge boots and directs them to a central discharge location.