Estimating cyclone efficiency with the Lapple method
Cyclone performance boils down to one number, the cut size d50. That is the particle size collected exactly half the time; larger particles are captured more reliably and smaller ones slip through. This tool solves the widely used Lapple expression for d50, then applies the fractional efficiency curve η = 1 ÷ (1 + (d50 ÷ d)²) to estimate capture at the size you care about.
The formula is d50 = √(9 × gas viscosity × inlet width ÷ (2π × effective turns × inlet velocity × particle density)). Air viscosity is taken as 1.81×10⁻⁵ Pa·s at 68 °F, 2.18×10⁻⁵ at 212 °F, 2.58×10⁻⁵ at 392 °F and 2.95×10⁻⁵ at 572 °F. An 8 in inlet at 50 ft/s with 5 effective turns and 125 lb/ft³ dust gives a d50 near 5.88 micron and about 74 % capture on 10 micron particles.
| Particle size ÷ d50 | Fractional efficiency |
|---|---|
| 0.5× | 20 % |
| 1× | 50 % |
| 2× | 80 % |
| 3× | 90 % |
| 5× | 96 % |
Treat this as a screening estimate only. Lapple ignores re-entrainment, wall buildup and agglomeration at high dust loading, so measured performance differs. Raising inlet velocity shrinks d50 but pressure drop climbs with the square of velocity and re-entrainment grows, which can undo the gain. Base equipment selection on vendor performance curves and stack test data, and confirm permit compliance with your air permitting authority.
Frequently Asked Questions
It comes from barrel and cone length divided by inlet height, but conventional high efficiency cyclones land between 5 and 10. Without drawings, enter 5 for a conservative estimate.
Rarely. Capture below about 5 micron is poor, so cyclones normally serve as pre-cleaners ahead of a baghouse or scrubber. Judge final outlet loading with the downstream device included.