The article’s assumed fan, motor and VFD efficiencies give about 6.90 kW electrical input for 4.00 kW air power. The diagram uses rounded values; direct drive has no separate transmission loss in this example.
1. State airflow and pressure on a consistent basis
Start with actual volumetric flow at the fan inlet, gas density and the required pressure rise. Specify whether the pressure is static or total, and use the corresponding fan efficiency. Mixing total pressure with a static efficiency produces an inconsistent estimate.
The required duty includes the relevant hood, duct, equipment and discharge losses. Check clean and loaded filters and the intended combinations of operating branches. The actual operating point is set by the intersection of the fan curve and system resistance curve; a required flow entered in a spreadsheet does not force a real fan to deliver it.
2. Follow power through the equipment
The motor nameplate rating is generally a rated mechanical output capability, not a measurement of electrical input during operation. Similarly, a calculated shaft requirement is not automatically the correct motor size: the selection must cover the allowed operating range, starting conditions, ambient conditions and manufacturer requirements.
3. Worked example: a steady duty
Assume 7,200 actual m³/h, a total pressure rise of 2,000 Pa, total fan efficiency of 0.65, direct drive with no separate transmission loss, motor efficiency of 0.92 and VFD efficiency of 0.97. These are illustrative assumptions.
| Quantity | Calculation | Result |
|---|---|---|
| Flow | 7,200 / 3,600 | 2.00 m³/s |
| Air power | 2.00 × 2,000 | 4.00 kW |
| Shaft power | 4.00 / 0.65 | 6.15 kW |
| Electrical input | 6.15 / (0.92 × 0.97) | 6.90 kW |
| Annual energy at 4,000 h | 6.90 × 4,000 | About 27,600 kWh |
The annual figure assumes the same duty for every operating hour. For variable production, calculate energy as Σ(Pᵢ × tᵢ) across representative load periods. Include auxiliary equipment separately when estimating the whole system’s consumption.
4. Use affinity laws within their assumptions
For the same fan geometry, comparable operating points and constant gas density, the affinity laws give Q proportional to rotational speed, pressure rise proportional to speed squared and shaft power approximately proportional to speed cubed when efficiency remains similar.
At 80% speed, the idealized ratios are 0.80 for flow, 0.64 for pressure and 0.512 for shaft power. This does not guarantee a 48.8% reduction in measured electrical consumption. Fan efficiency, motor and drive losses, control settings and the system curve can all change.
A loaded filter, a controlled constant-pressure system or a process with a fixed pressure requirement may not follow a simple resistance curve through the origin. In dust extraction, reducing speed can also leave hoods short of capture flow or ducts below an adequate transport velocity. Verify both duties before using speed reduction as an energy measure.
5. Compare offers at the same duty
- Ask for the fan curve, efficiency definition, gas density and test conditions.
- Check the operating envelope, including filter loading and branch combinations.
- Review stable operation, maximum absorbed power, noise and installation effects with the supplier.
- Use measured electrical input and flow to establish a commissioning baseline.
- Reduce avoidable system resistance while retaining required source capture and dust transport.
The duct pressure-loss guide explains how to build the resistance estimate. The dedusting system guide places fan selection in the wider process.
Sources and scope
The US Department of Energy fan-system resources and DOE Fan System Cheat Sheet provide system-efficiency guidance. Equations and numerical examples here are preliminary estimates, not manufacturer selection data. Obtain curves and component efficiencies for the intended operating conditions.