Presented by: John-John Fourie Power Savings by Impeller Replacements for Main Fan Stations in the SA Gold Mining Industry Presented by: John-John Fourie

Agenda Introduction Potential savings - Typical mine fan Theory, Fan laws and Fan curves Energy Saving Options Drop-in Impeller Replacement Conclusion

Introduction Typically 250kW to 2.2MW per single fan can be consumed Potential financial savings significant Intervention methods include centrifugal fan impellers modifications

Potential savings - Typical Mine Fan Original duty Actual duty

Potential savings - Typical Mine Fan Actual efficiency of 55% is much lower than the original, selected efficiency of 80% Original duty Actual duty

Potential savings - Typical Mine Fan Original duty Actual duty

Potential savings - Typical Mine Fan Original duty Air power = 540kW Absorbed power = 620kW In-efficiency lost = 80kW Original duty Actual duty

Potential savings - Typical Mine Fan Original duty Air power = 540kW Absorbed power = 620kW In-efficiency lost = 80kW Actual duty Air power = 360kW Absorbed power = 660kW In-efficiency lost = 300kW Original duty Actual duty

Potential savings - Typical Mine Fan Efficiency achieved > 80% (from 60%) Power Saved approximately 200 kW (per fan) Cost Saving R963 600 per annum (Based on 0.55c/kWh)

Theory – Basic Fan laws 1. Volume flow 𝑄 2 𝑄 1 = 𝑛 2 𝑛 1 x 𝐷 2 𝐷 1 3 Q – Volumetric flow rate (m3/s) P – Fan static pressure (Pa) kW – Absorbed power (kW) n – Fan operating speed (rpm) D – Impeller diameter (m) ρ – Air density (kg/m3) 2. Pressure 𝑃 2 𝑃 1 = 𝑛 2 𝑛 1 2 x 𝐷 2 𝐷 1 2 x ρ 2 ρ 1 1 3. Power 𝑘𝑊 2 𝑘𝑊 1 = 𝑛 2 𝑛 1 3 x 𝐷 2 𝐷 1 5 x ρ 2 ρ 1 1

Theory – Fan Efficiency Low efficiency, due to losses Best efficiency Low efficiency, due to losses

Energy Savings Options Inlet guide vanes (IGV’s) Speed control Impeller replacement (1) Inlet Guide Vane (2) Speed Control (3) Impeller Replacement

Energy Savings Options- Inlet Guide Vanes

Energy Savings Options - Inlet Guide Vanes Power consumption is related to volume flow Volume flow reduction of 10% - 15% is power savings of 22% - 25% Duty 1 Duty 2 90° 60° rpm

Energy Savings Options - Inlet Guide Vanes By altering the IGV opening from 90° to 60° a saving of ±150kW can be achieved. Duty 1 Duty 2 90° 60° rpm

Energy Savings Options - Speed control Fixed speed reduction gearbox Variable frequency drives (VFD’s)

Energy Savings Options - Speed control 𝑄 2 𝑄 1 ∝ 𝑛 2 𝑛 1 𝑃 2 𝑃 1 ∝ 𝑛 2 𝑛 1 2 Duty 1 Duty 2 745rpm 600rpm

Energy Savings Options - Speed control 𝑄 2 𝑄 1 ∝ 𝑛 2 𝑛 1 𝑃 2 𝑃 1 ∝ 𝑛 2 𝑛 1 2 Duty 1 By adjusting the fan speed from 745 rpm to 600 rpm a saving of ±300kW can be achieved. Duty 2 745rpm 600rpm

Impeller Replacement – Example Original inefficient paddle wheel impeller Replaced high efficiency backward inclined impeller Photo courtesy of FläktWoods Fans Hermit Crab concept

Impeller Replacement – Design constraints No change to fan casing and civils Existing impeller should be stored Re-evaluate fan shaft and bearing selection Vibration signature of the fan

Impeller Replacement – Design Parameters Impeller diameter Impeller width Impeller exit angle Impeller blade profile Number of blades Impeller width Impeller exit angle Impeller diameter

Impeller Replacement – Impeller diameter 𝑄 2 𝑄 1 ∝ 𝐷 2 𝐷 1 3 𝑃 2 𝑃 1 ∝ 𝐷 2 𝐷 1 2 Original Duty Impeller diameter 2610mm 16% Change in impeller diameter, results in 58% decrease in absorbed power New Duty Impeller diameter 2200mm

Impeller Replacement - Impeller width Distance between the backplate and the shroud Width is directly proportional to the volumetric flow Width defines the volume capabilities If altered check inlet cone arrangement Increase in power due to increased impeller width. New Duty Original Duty

Impeller Replacement - Impeller exit angle A larger blade exit angle reduces the exit shock losses Backward curved impellers are typically more efficient than radial type impellers Changing the exit angle also impacts the static pressure

Impeller Replacement - Impeller blade profile Blade aerodynamic profile: Reducing drag and increasing lift Reduced power at the same pressure and volume flow

Impeller Replacement - Number of blades Increasing the number of blades increases the fan pressure Increasing the blade numbers can result in secondary flow losses and increased tip blockage

Impeller Replacement – Method of Analysis Specify required static pressure and volume flow Start with a slightly decreased impeller diameter Increase impeller width by 100 mm to get a flatter head curve Decrease number of blades to get a reduced pressure curve Increase exit blade angle to get a reduced pressure curve Calculate efficiency using numerical methods (target eff. = 82%) Repeat steps 2 till 6 until desired curve is achieved Finite Element Structural and fatigue analysis essential

Impeller Replacement – New fan curve Original mine system resistance OLD fan curve Original design duty @ 82% Pressure NEW mine system resistance NEW fan duty @ 82% Actual operating duty @ 65% NEW fan curve Flow

Impeller Replacement – Measured fan efficiency Performance data: Volume flows 85 m3/s - 460 m3/s Static pressures 1.6 kPa - 6.2 kPa. Old Fan efficiency 50% - 60% New Fan Efficiency 75% - 80%. Possible Power Savings ~ 1 MWE Possible Cost Savings R5.6M pa (Megaflex)

Conclusion – Drop-in Impellers Drop-in impeller replacements are feasible on surface fans that are running at poor efficiency Payback period < 2 years Saving R 5.6M per year – 1MWe (60% - 80%) No significant technical or production risk