Gravimetric Dry Feeders

Gravimetric dry feeders precisely meter and convey dry chemical powders and granules for water treatment applications by continuously weighing material as it's dispensed from a hopper onto a belt or screw conveyor. The system uses load cells to monitor actual material weight loss, automatically adjusting feed rates to maintain target dosages regardless of material density variations or bridging issues. These units typically achieve ±2% accuracy across feed rates from 5 to 2,000 pounds per hour, making them ideal for lime, sodium bicarbonate, and polymer feeding in 0.5-50 MGD municipal plants. The primary trade-off is significantly higher capital cost compared to volumetric feeders, though operational reliability and accuracy often justify the investment for critical chemical applications.

• Lime/Soda Ash for pH Adjustment - Gravimetric feeders meter calcium hydroxide or sodium carbonate into water treatment clarifiers or post-filtration contact chambers. Selected for precise dosing accuracy (±1-2%) needed to maintain pH within 7.2-8.5 range. Connects upstream to chemical storage silos, downstream to static mixers or flash mixers ahead of clarification.

• Polymer Powder Feed - Dry anionic/cationic polymers fed to makeup tanks for dewatering operations. Gravimetric control prevents overdosing that causes gel formation. Typical rates: 0.1-5 lb/hr for 1-10 MGD plants. Integrates with polymer makeup systems and centrifuge/belt press operations.

• Activated Carbon Addition - Powdered activated carbon (PAC) fed to raw water intake or clarifier influent for taste/odor control. Gravimetric accuracy essential for cost control given carbon prices ($0.75-1.50/lb). Feed rates typically 5-50 lb/hr depending on seasonal taste/odor episodes.

• Fluoride Compound Dosing - Sodium fluoride or fluorosilicic acid powder fed to finished water systems. Gravimetric precision required to maintain 0.7-1.0 mg/L fluoride levels per EPA recommendations while avoiding overdosing violations.

Daily Operations - Operators monitor feed rates on HMI displays, checking actual vs. setpoint values within ±2% tolerance. Verify hopper levels and refill scheduling. Adjust feed rates based on process demand signals from pH meters or residual analyzers. Record chemical consumption totals for inventory management and regulatory reporting.

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Maintenance - Weekly calibration verification using test weights (typically 50-100 lb standards). Monthly cleaning of hoppers and conveyors, requiring confined space entry procedures and dust masks for chemical exposure. Load cell recalibration every 6 months by certified technicians. Screw conveyor bearing lubrication quarterly. Expect 10-15 year service life for mechanical components.

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Troubleshooting - Feed rate drift indicates load cell fouling or calibration issues. Hopper bridging causes erratic feeding - check agitation systems and moisture intrusion.

• Loss-in-Weight Scale System - Load cells (typically 3-4 per hopper) continuously weigh chemical hopper and contents. Stainless steel construction for corrosive chemicals. Capacity ranges: 50-2000 lb hoppers for municipal applications. Selection based on chemical bulk density and maximum feed rates.

• Variable Speed Screw Conveyor - Stainless steel or polymer-coated auger meters chemical from hopper. Screw pitch and diameter sized for specific chemical flow characteristics. VFD-controlled motors (0.5-2 HP typical) provide precise speed control for accurate feed rates.

• Hopper and Agitation System - Conical or mass-flow hoppers prevent bridging of cohesive chemicals. Pneumatic vibrators or mechanical agitators maintain material flow. Hopper sizing: 8-24 hour chemical storage capacity based on delivery schedules and plant size.

• Control and Instrumentation - PLC-based controllers integrate with plant SCADA systems. Provide feed rate adjustment, totalizer functions, and alarm outputs. Include load cell signal conditioning and motor speed control interfaces.

• Feed Rate Capacity: 5-5,000 lbs/hr depending on chemical and plant size. Polymer feeders typically 5-200 lbs/hr for 0.5-10 MGD plants; lime feeders 100-5,000 lbs/hr for larger facilities. Size for 150% of maximum anticipated demand.

• Accuracy: ±1-2% of set point for gravimetric systems. Loss-in-weight designs achieve ±0.5% accuracy but require more maintenance than belt-scale configurations.

• Hopper Capacity: 8-72 hour storage typical. Calculate based on delivery frequency, weekend operations, and emergency reserves. Standard sizes: 500, 1,000, 2,000, 5,000, 10,000 lb capacity.

• Power Requirements: 1-15 HP depending on feeder size and conveying distance. Include variable frequency drives for turndown ratios of 20:1 to 50:1.

• Environmental Conditions: -20°F to 120°F operating range. IP65 enclosures minimum for outdoor installations. Consider heated hoppers for lime in cold climates.

• Material Compatibility: 316 stainless steel contact surfaces for most chemicals. Hastelloy C-276 for ferric chloride applications. HDPE-lined hoppers acceptable for polymer service.

• Dust Collection: 99.9% efficiency baghouse filters sized for 6-8 air changes per minute during filling operations.

• What feed rate range and turndown ratio is required? Municipal plants need 20:1 minimum turndown for seasonal variations and process upsets. Undersized feeders cannot handle peak demands during wet weather events or process optimization. Oversized units lose accuracy at low feed rates. Analyze 5-year flow projections and chemical dose optimization studies.

• Loss-in-weight versus belt-scale configuration? Loss-in-weight provides superior accuracy (±0.5% vs ±2%) but costs 40-60% more and requires skilled maintenance. Belt-scale systems offer better reliability for 24/7 unmanned operation. Choose loss-in-weight for critical applications like polymer feed where overdosing causes operational problems.

• What level of automation and remote monitoring is needed? Basic systems provide local indication only. Advanced units integrate with plant SCADA, provide predictive maintenance alerts, and enable remote rate changes. Staffing levels and operational philosophy determine requirements. Unmanned facilities require comprehensive remote monitoring.

• How will material handling and dust control be addressed? Pneumatic conveying systems cost $15,000-50,000 additional but eliminate manual handling. Bag dump stations with integral dust collection prevent worker exposure. Consider truck unloading systems for plants using >50 tons/month.

• Division 40-48: 40 31 33 - Dry Chemical Feed Equipment

• Primary specification section covering gravimetric feeders, hoppers, and associated controls

• May reference Division 23 (HVAC) for dust collection systems and Division 26 (Electrical) for motor controls and instrumentation integration

• Material/Equipment Verification: Verify load cell accuracy certification (±0.25% typical), Confirm chemical compatibility ratings for wetted parts, Review calibration procedures and required test weights

• Installation Requirements: Isolated concrete pad to minimize vibration interference, Adequate headroom for hopper filling and maintenance access, 480V/3-phase power with isolated ground for load cells

• Field Challenges: Load cell sensitivity to temperature variations and electrical interference, Dust control during chemical loading operations, Proper belt tracking and tensioning during startup

• Coordination Issues: 12-16 week lead times for custom configurations, Early coordination with chemical storage suppliers for hopper interfaces

• Schenck Process - AccuRate dry material feeders, widely used in 1-50 MGD plants for lime and carbon feeding applications

• Acrison - Model 105 volumetric-gravimetric feeders popular for polymer and PAC applications in municipal facilities

• K-Tron - Smart Force Transducer (SFT) gravimetric feeders, common in larger plants (10+ MGD) for precise chemical dosing

• Hapman - Helix flexible screw feeders with gravimetric controls for smaller municipal applications

• Volumetric Belt Feeders - 30-40% lower cost, acceptable for non-critical applications like lime feeding where ±5% accuracy suffices.

• Loss-in-Weight Hoppers - Better for sticky materials like polymer, 20% higher cost but superior handling characteristics.

• Liquid Feed Systems - Often preferred for polymers despite higher chemical costs; eliminates dust issues and provides more precise control for smaller plants (<5 MGD).

Establish calibration schedules with chemical suppliers - many will provide certified test weights and perform quarterly accuracy checks as part of service contracts. For polymer applications, specify anti-bridging devices upfront rather than retrofitting later. Consider redundant feeders for critical applications; the 15-20% cost premium is justified compared to emergency chemical truck deliveries during failures.