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

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Potential savings - Typical Mine Fan

Actual efficiency of 55% is much lower than the original,

selected efficiency of 80%

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Potential savings - Typical Mine Fan

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Original dutyAir power = 540kWAbsorbed power = 620kWIn-efficiency lost = 80kW

Potential savings - Typical Mine Fan

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Original dutyAir power = 540kWAbsorbed power = 620kWIn-efficiency lost = 80kW

Actual dutyAir power = 360kWAbsorbed power = 660kWIn-efficiency lost = 300kW

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)

Potential savings - Typical Mine Fan

Theory – Basic Fan laws

1. Volume flow

2. Pressure

3. Power

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)

Theory – Fan Efficiency

Low efficiency, due to losses

Best efficiency

Low efficiency, due to losses

Energy Savings Options

1. Inlet guide vanes (IGV’s)2. Speed control3. Impeller replacement

(1) Inlet Guide Vane (3) Impeller Replacement(2) Speed Control

Energy Savings Options- Inlet Guide Vanes

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Energy Savings Options - Inlet Guide Vanes

60° rpm

90°

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Duty 2

• Power consumption is related to volume flow• Volume flow reduction of 10% - 15% is power savings of 22% -

25%

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Energy Savings Options - Inlet Guide Vanes

60° rpm

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By altering the IGV opening from 90° to 60° a saving of ±150kW can be achieved.

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

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Energy Savings Options - Speed control

600rpm745rpm

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𝑄2

𝑄1

∝(𝑛2𝑛1 )𝑃2

𝑃1

∝(𝑛2𝑛1 )2

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Energy Savings Options - Speed control

600rpm745rpm

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By adjusting the fan speed from 745 rpm to 600 rpm a saving of ±300kW can be achieved.

𝑄2

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∝(𝑛2𝑛1 )𝑃2

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∝(𝑛2𝑛1 )2

Original inefficient paddle wheel impeller

Replaced high efficiency backward inclined impeller

Photo courtesy of FläktWoods Fans Hermit Crab concept

Impeller Replacement – Example

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 exit angle

Impeller width

• Impeller diameter• Impeller width• Impeller exit angle• Impeller blade profile• Number of blades

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Impeller Replacement – Impeller diameter

16% Change in impeller diameter,

results in 58% decrease in

absorbed power

Original Duty Impeller diameter 2610mm

New DutyImpeller 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.

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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

1. Specify required static pressure and volume flow2. Start with a slightly decreased impeller diameter3. Increase impeller width by 100 mm to get a flatter head curve4. Decrease number of blades to get a reduced pressure curve5. Increase exit blade angle to get a reduced pressure curve6. Calculate efficiency using numerical methods (target eff. = 82%)7. Repeat steps 2 till 6 until desired curve is achieved

Finite Element Structural and fatigue analysis essential

Pres

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Flow

745 rpm

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Existing fan IGVsOld impeller/design resistanceOld impeller efficiencyNew resistanceNew impellerNew impeller efficiency

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745 rpm

Impeller Replacement – New fan curve

Original design duty @ 82%

NEW fan curve

Actual operating duty @ 65%

OLD fan curve

NEW fan duty @ 82%

Original mine system resistance

NEW mine system resistance

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