Drag Chain Conveyors

Drag chain conveyors transport screenings, grit, and dewatered biosolids through enclosed troughs using continuous chains with attached flights or paddles. Steel chains pull material horizontally or at inclines up to 45 degrees, with typical conveying capacities ranging from 5-150 cubic feet per hour for municipal applications. These systems excel in harsh wastewater environments due to their enclosed design and robust construction, but require frequent chain tensioning and sprocket maintenance due to abrasive material handling and corrosive conditions.

• Screenings Removal at Headworks: Drag chain conveyors transport screenings from fine screens (1-6mm) to washers/compactors. Selected for reliable conveyance of fibrous materials that jam screw conveyors. Connects downstream to screenings washers or directly to dumpsters. Typical capacity 2-15 cubic yards/day for 5-25 MGD plants.
• Grit Handling Systems: Conveys washed grit from classifiers to storage hoppers or trucks. Chain flights provide positive material control for heavy, abrasive grit that causes excessive wear in belt conveyors. Handles 50-500 lbs/day grit loads with flights spaced 12-24 inches for 1-20 MGD facilities.
• Biosolids Transfer: Moves dewatered cake (18-25% solids) from belt filter presses to storage bins. Enclosed design prevents odors while chain flights handle sticky cake that bridges in screw conveyors. Typical installations handle 2-20 tons/day cake production.
• RAS/WAS Screenings: Conveys screenings from return activated sludge screens protecting downstream pumps and digesters. Operates intermittently based on screen differential pressure signals.

Daily Operations: Operators monitor chain tension, motor amperage (typically 60-80% of nameplate), and material buildup at discharge. Speed adjustments via VFD optimize throughput while preventing overloading. Visual inspection through observation windows checks for proper flight engagement and material carryover.

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Maintenance: Weekly lubrication of drive components and take-up mechanisms. Monthly chain tension adjustment and flight inspection for wear/damage. Quarterly drive alignment checks. Annual chain replacement typically required. Confined space entry procedures required for trough access. Standard PPE plus fall protection for elevated installations.

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Troubleshooting: Chain stretch causes tracking problems and increased power draw. Worn flights reduce conveyance efficiency, evidenced by material spillback. Drive overloads indicate material jams or seized bearings. Typical service life: chains 12-18 months, flights 18-24 months, drives 10-15 years with proper maintenance.

• Drive Unit: Gear reducers (10:1 to 40:1 ratios) with 1-5 HP motors provide 15-50 fpm chain speeds. TEFC motors with VFDs common for variable loading. Torque limiting devices protect against jams.
• Chain and Flights: Heat-treated steel chains (3/8" to 3/4" pitch) with welded or bolted polymer flights. Flight spacing 6-24 inches depending on material. Stainless steel chains for corrosive environments add 40-60% to costs.
• Trough: Carbon steel with epoxy lining or 304/316 stainless steel construction. Typical widths 6-24 inches, lengths 20-100 feet for municipal applications. Removable covers for maintenance access.
• Take-up Assembly: Adjustable tensioning maintains proper chain tension (1500-3000 lbs typical). Spring-loaded or screw-type adjustment compensates for chain stretch.
• Discharge Chute: Adjustable height and angle for material direction control. Often includes dust suppression connections.

• Chain Speed: 15-40 fpm typical, with 25-30 fpm standard for most municipal screenings applications. Higher speeds (35-40 fpm) for longer conveyors or higher throughput requirements.
• Conveyor Capacity: 2-15 cubic feet per minute for typical municipal screening applications. Size based on peak hydraulic loading and screening removal rates of 0.5-2.0 ft³/MG treated.
• Incline Angle: Maximum 45° for screenings transport, with 30-35° preferred to prevent material rollback. Steeper angles require higher chain speeds or flight modifications.
• Chain Pull Force: 5,000-25,000 lbs depending on conveyor length, load, and incline. Critical for drive sizing and structural support requirements.
• Conveyor Length: 20-150 feet typical, with most municipal applications in 40-80 foot range. Longer conveyors require intermediate support structures and higher drive capacities.
• Trough Width: 12-24 inches standard, sized for 1.5-2x maximum particle size. Wider troughs accommodate higher flow rates but increase chain loads.
• Material Loading Rate: 15-50 lbs/ft³ for typical screenings mixture. Affects chain sizing, drive requirements, and structural loading calculations.
• Drive Power: 3-15 HP typical, with 5-10 HP most common for municipal screening conveyors under 100 feet.

• What is the required conveyor capacity and peak loading rate? Municipal plants typically generate 0.5-2.0 ft³ screenings per MG treated. Undersizing by 50% leads to overflow conditions and emergency shutdowns. Need accurate flow data, screening efficiency, and peak factor (typically 2.5-3.0x average).
• What conveyor length and elevation change are required? Distances over 80 feet require intermediate drives or tensioning systems, adding $15,000-25,000 to project costs. Elevation changes over 25 feet may require multiple-stage systems. Need site survey with accurate elevations and obstacle locations.
• Should the system use enclosed or open trough design? Enclosed systems cost 20-30% more but eliminate odor complaints and meet stricter environmental regulations. Open systems allow easier maintenance access but require odor control provisions. Consider proximity to property lines and local regulations.
• What level of wash system integration is needed? Basic spray wash adds $8,000-12,000 but reduces downstream handling issues. High-pressure wash systems with compaction can reduce disposal volumes by 40-60% but double equipment costs. Evaluate disposal costs versus capital investment.

• MasterFormat 46 23 13 - Screening Equipment
• Primary section covering mechanical screening systems and associated conveyors. May reference 40 06 00 for process piping connections and 46 05 00 for common process equipment requirements.

• Material/Equipment Verification:
• Verify stainless steel grade (316SS minimum for municipal applications)
• Confirm chain pitch matches sprocket specifications
• Check flight attachment method and spacing
• Installation Requirements:
• Level concrete pad within 1/8" over conveyor length
• Adequate headroom for maintenance access
• Proper alignment critical - use laser alignment tools
• Field Challenges:
• Chain stretch during initial operation requires re-tensioning
• Grit accumulation in return section needs cleanout provisions
• Coordination Issues:
• 12-16 week lead times typical for custom municipal units

• KWS Manufacturing - Model TFC (Trough Flight Conveyor) series, widely used in 1-50 MGD plants
• Vulcan Industries - VDC series drag conveyors, popular for biosolids handling
• Chain-Vey - Municipal Series models, common in smaller plants
• Rapat - German-engineered systems gaining traction in larger facilities, known for heavy-duty applications exceeding 20 MGD capacity

• Screw Conveyors - 15-25% lower capital cost, preferred for shorter runs under 40 feet with lower capacities.
• Belt Conveyors - Better for longer distances over 100 feet, roughly equivalent costs but require covered enclosures.
• Pneumatic Conveyors - Higher operating costs but eliminate mechanical wear, suitable when conveying distances exceed 200 feet or multiple elevation changes required.

Establish direct relationships with manufacturer service technicians - they provide invaluable troubleshooting support for chain tensioning and flight replacement. Order spare chains and flights with initial purchase (typically 20-30% cost savings versus later procurement). Consider variable frequency drives even on smaller units - the soft-start capability significantly extends chain life and reduces maintenance intervals from quarterly to semi-annual inspections.