Particle Size & Settling Velocities

Particle Size

All of Concept’s mains powered fog machines have a precision engineered heat exchanger block which is manufactured to tolerances that are measured in microns. This technique delivers a consistent high quality fog and fog particle size that is unmatched by other manufacturers.

Benefits of producing such a consistent particle size are:

  • .The smaller the particle size of the fog, the less fog fluid is required within the atmosphere to achieve a given visibility. This has a significant effect on running costs.
  • .The less fog fluid is in the atmosphere , the safer the fog if measured in terms of relative OEL’s (Occupational Exposures Levels)
  • .The smaller the particle size of the fog, the less it’s mass and therefore the lower its rate at which it settles (the speed at which a particle will fall, due to gravity). A particle of fog that measure 0.2 micron mmd, will settle at a rate of 8mm / hour, which when compared to a 2 micron mmd particle, which falls at 468mm / hour. In essence the smaller the fog particle, the less chance of any deposition occurring .
Most typical fog machines create a fog particle size of 1-5 micron. This may not seem like much but if you take into account that the adjacent picture shows a 0.2 micron mmd Concept particle on the right hand side, the particle on the left is just over 1.5 micron diameter.

Settling Velocities

The below table indicates settling velocities for particles ranging from 0.05—30 micron mmd.

Particle diameter (µm): Setting Velocity (m/s): Setting Velocity (mm/hr)
0.05 3.85E-07 1.39
0.06 4.75E-07 1.71
0.08 6.69E-07 2.41
0.1 8.82E-07 3.18
0.15 1.50E-06 5.40
0.2 2.26E-06 8.14
Typical Concept particle
0.3 4.21E-06 15.16
0.4 6.76E-06 24.34
0.5 9.91E-06 35.68
0.6 1.37E-05 49.32
0.8 2.30E-05 82.80
1 3.48E-05 125.28
1.5 7.47E-05 268.92
2 1.30E-04 468.00
3 2.85E-04 1026.00
4 5.00E-04 1800.00
5 7.76E-04 2793.60
Typical competitor particle size
6 1.11E-03 3996.00
8 1.96E-03 7056.00
10 3.06E-03 11016.00
15 6.84E-03 24624.00
20 1.21E-02 43560.00
30 2.72E-02 97920.00

The terminal settling velocity of an artificial fog particle increases dramatically with particle size. The settling velocity is proportional to the square of the particle diameter, although sub micron particles, such as those produced by Concept’s mains powered fog machines require the use of slip correction factors.

Calculated for standard density spheres at 293 K [20° C] and 101 kPa (1 atm). "Aerosol Technology", William C. Hinds, Wiley-Interscience, 1999