Scroll conveyor

The scroll conveyor is a critical internal component of a decanter centrifuge, consisting of a helical screw that rotates at a differential speed relative to the centrifuge bowl. Its primary function is to continuously convey the separated solid phase (sediment) along the conical section of the bowl towards the discharge ports, enabling continuous operation of the centrifuge. The conveyor is typically manufactured from stainless steel (e.g., 316L or 304) or hard-surfaced alloy steel with tungsten carbide coating, depending on the abrasiveness of the solids and the corrosiveness of the process liquid. Key dimensional parameters include screw diameter (200–800 mm), screw pitch (100–400 mm), and conveying capacity (5–50 m³/h), which are selected based on the required throughput and particle size distribution. The screw speed typically ranges from 10 to 60 rpm, with higher speeds reducing residence time. Operating pressure is specified as 1.0–1.6 MPa, and operating temperature ranges from -20°C to 120°C; above 120°C, special seals may be required. Material grade options are SS304 or 316L per ASTM A240, with 316L recommended for corrosive environments. Surface roughness can be specified as Ra 0.8–1.6 μm for food or pharmaceutical applications. The weight of the conveyor varies from 150 to 800 kg depending on size and material. These values are reference ranges and must be verified for the specific model and application with the legal manufacturer or supplier. The scroll conveyor operates on the principle of differential speed: it rotates slightly slower than the bowl, creating a relative motion that pushes settled solids along the bowl wall. The helical flights provide mechanical force to transport solids against centrifugal force and friction. Proper selection of screw pitch and speed is critical for efficient conveying and compression. Maintenance signals include increased vibration, reduced conveying capacity, or visible wear on the flights. Failure boundaries include excessive wear to reduced efficiency or breakage. Always consult the manufacturer for application-specific data and verification.

The scroll conveyor rotates at a slightly different speed (typically 5-50 RPM slower) than the centrifuge bowl. This differential speed creates a relative motion that 'scrolls' or pushes the settled solids along the inner wall of the bowl. The helical flights of the conveyor provide the mechanical force to transport the solids against centrifugal force and friction. The differential speed is a key parameter that determines the conveying rate and the residence time of solids in the bowl. A higher differential speed increases the conveying rate but may reduce the compression of the solids, while a lower speed allows more time for liquid drainage but may reduce throughput. The screw pitch and diameter also influence the conveying efficiency and the maximum particle size that can be handled. The conveyor is driven by a separate gearbox or motor, and its speed is controlled relative to the bowl speed to achieve the desired performance.