Screw Conveyors (Shafted, Shaftless)

Screw conveyors transport dewatered sludge, screenings, and grit within municipal treatment plants using rotating helical flights along a trough or tube. Shafted units feature a central shaft supporting the flights, while shaftless designs use a self-supporting spiral for handling stringy materials. These conveyors typically move 5-150 cubic yards per hour of material at inclinations up to 45 degrees, with standard lengths ranging from 10-100 feet. The key trade-off is maintenance complexity versus material handling capability - shafted units require more frequent bearing service but handle abrasive materials better than shaftless designs.

• Biosolids Dewatering Systems: Shaftless conveyors transport dewatered sludge cake (15-25% solids) from belt filter presses or centrifuges to storage hoppers or loading areas. Selected for ability to handle sticky, stringy material without wrapping or plugging. Flow rates typically 2-15 cubic yards/hour for 5-20 MGD plants
• Screenings Handling: Both shafted and shaftless units convey screenings from fine screens (1-6mm) to washing/compacting equipment or dumpsters. Shaftless preferred for longer runs (>20 feet) with debris containing rags and plastics. Typical capacity 0.5-3 cubic feet/minute
• Grit Removal Systems: Shafted conveyors move classified grit from cyclone separators to grit washers or disposal containers. Selected for abrasion resistance and precise metering capability. Standard sizing: 9-12 inch diameter, 10-30 RPM for 1-25 MGD facilities
• Chemical Feed: Shafted conveyors meter dry chemicals (lime, polymer, carbon) from storage silos to dissolving tanks. Chosen for accurate volumetric feeding and dust containment. Variable speed drives provide 0.1-50 cubic feet/hour dosing rates

Daily Operations: Operators monitor amperage draw on drive motors, checking for increases indicating material buildup or bearing wear. Visual inspection through sight ports confirms proper material flow and identifies any wrapping on shafted units. Flow rate adjustments made via VFD speed control, typically maintaining 60-80% of maximum rated capacity for optimal efficiency.

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Maintenance: Weekly bearing lubrication and monthly coupling alignment checks required. Quarterly inspection of flight wear and trough liner condition. Annual gearbox oil changes and drive belt tension adjustment. Confined space entry procedures needed for internal inspection. Standard PPE includes hard hat, safety glasses, and chemical-resistant gloves when handling biosolids conveyors.

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Troubleshooting: Excessive motor current indicates material wrapping (shafted) or bearing failure. Reduced discharge flow suggests worn flights or plugged discharge. Unusual noise signals bearing problems or foreign object interference.

• Screw Flight: Helical steel flights welded to central shaft (shafted) or formed as continuous spiral (shaftless). Carbon steel with optional hard-facing for abrasive service. Pitch typically equals diameter for standard duty; reduced pitch for inclined applications. Diameters range 6-24 inches for municipal applications
• Trough/Housing: U-shaped or tubular steel housing containing the screw assembly. 304/316 stainless steel standard for wastewater contact. Includes inlet/discharge openings, inspection ports, and mounting feet. Length-to-diameter ratios typically 20:1 to 40:1
• Drive System: Gear reducer with AC motor, typically 1-10 HP for municipal sizes. Variable frequency drives common for flow control. Torque ratings 5,000-50,000 in-lbs. Includes overload protection and shaft seals
• Bearings/Supports: Shafted units use pillow block bearings at discharge end; shaftless use intermediate hangers every 10-12 feet. Sealed bearings with grease fittings. Materials selected for chemical compatibility and expected loads

• Capacity Requirements: Volumetric flow rates: 5-500 cubic feet per hour for biosolids; Mass flow rates: 0.5-50 tons per hour (dry basis); Solids concentration: 15-35% for dewatered biosolids, 2-8% for thickened sludge
• Physical Parameters: Screw diameter: 6-36 inches (shafted), 9-24 inches typical for municipal; Conveyor length: 10-200 feet, with 6-foot increments standard; Incline angles: 0-45 degrees maximum, 15-30 degrees typical for biosolids; Trough loading: 15-45% fill factor depending on material characteristics
• Performance Specifications: Torque requirements: 1,000-50,000 in-lbs depending on diameter and material; Speed range: 10-60 RPM for biosolids applications; Power consumption: 2-50 HP motors typical; Wear life: 8,000-12,000 hours minimum for abrasive materials
• Material Considerations: Bulk density: 35-65 lbs/cubic foot for dewatered biosolids; Abrasiveness factor: 2-4 on modified scale; Temperature range: 32-180°F operating conditions

• Shafted vs. Shaftless Configuration Selection: Shaftless required for materials with long stringy debris (>6 inches), high moisture content (>25%), or significant rag content. Shafted conveyors limited to free-flowing materials under 25% moisture. Wrong selection causes frequent plugging, requiring 2-4 hour shutdowns for cleaning and $15,000-30,000 annual maintenance cost increases
• Incline Angle and Capacity Relationship: Capacity reduces 10-15% for every 10-degree increase above horizontal. Materials above 30% moisture content limited to 20-degree maximum incline. Exceeding limits causes material rollback, reducing effective capacity by 25-40% and increasing power consumption 20-30%
• Trough Design and Wear Protection: Abrasive materials (grit, screenings) require AR400 steel or ceramic liners, adding $8,000-15,000 initial cost but extending service life from 2-3 years to 8-10 years. Non-abrasive biosolids use standard carbon steel construction
• Drive System Sizing and Redundancy: VFDs provide 15-25% energy savings and allow capacity adjustment for varying loads. Backup drives or dual-motor systems justify $12,000-20,000 additional cost for critical applications where 4+ hour downtime costs exceed $50,000

• Primary: Division 44 33 00

• Material/Equipment Verification: Verify stainless steel grade (304 vs 316) for corrosive applications; Confirm motor IP ratings and explosion-proof classifications; Check bearing specifications for continuous duty
• Installation Requirements: Requires substantial concrete foundations for larger units; Access needed for periodic shaft removal/maintenance; Electrical coordination for VFD integration
• Field Challenges: Alignment critical during installation; Grouting requirements often underestimated
• Coordination Issues: Interface points with upstream screening equipment; Discharge chute connections to containers/trucks
• Lead times: 12-16 weeks typical for custom municipal units

• KWS Manufacturing - Model SC Series shafted conveyors, widely specified in municipal sludge applications
• Spirac - ShaftPower shaftless conveyors, dominant in screenings/grit handling
• Conveyor Engineering & Manufacturing (CEM) - Heavy-duty municipal models for biosolids
• Martin Sprocket & Gear - Standard shafted units for lime/chemical feed applications

• Belt conveyors - Better for longer distances (>100 feet), lower maintenance, but higher capital cost and space requirements
• Pneumatic conveyors - Preferred for dry materials like lime/carbon, enclosed transport, but higher energy consumption
• Pump systems - For materials that can be slurried, lower maintenance but require dilution water. Screw conveyors typically 30-40% less expensive than belt systems for municipal applications under 50 feet

Size conservatively - municipal applications rarely operate at design capacity continuously. Specify removable covers every 10-12 feet for maintenance access. Establish relationships with local manufacturer reps early; they often provide free troubleshooting support and spare parts guidance. Consider standardizing on one manufacturer across multiple units to simplify maintenance training and parts inventory. VFD installation typically pays for itself within 2-3 years through energy savings.