The spiral classifier is a common screening and dressing equipment for mechanical classification by means of different precipitation speeds of solid particles in a liquid. The classifier mainly has four types of sorghum single helix and double helix, sunken single helix and double helix. Among them, the high-grade classifier is suitable for coarse-grain classification, and the maximum overflow particle size is generally 0.4-0.15mm; the submerged classifier is suitable for fine-grain classification, and the maximum overflow size is generally below 0.2mm.