Airflow visualization in clean environments is a practice prescribed by ISO 14644-3 among the control methods for cleanrooms.
Methods for airflow visualization
ISO 14644 provides that, to visualise airflow, several methods can be used, leading to both qualitative and quantitative assessments.
Among the methods described are:
- tracer threads made of wool or similar, suitably positioned on support grids or on the surfaces under investigation;
- seeding the airflow with tracer particles based on water, glycol or alcohol, of suitable size to accurately follow the flow to be analysed;
- measurement of local velocities using anemometers or velocimeters.
Visualization using smoke generators
The most suitable and immediate method for visualizing airflow in large volumes is the use of tracers in the form of a visible aerosol dispersed in the air through generators.
A key requirement for the success of these methods is that the generated aerosol consists of droplets with a diameter small enough to faithfully follow the airflow and to minimise any gravitational or inertial effects that would distort the result of the visualization.
Particles that are too large and heavy settle quickly and do not follow the airflow along curved trajectories.
The smoke suitable for visualization must not contaminate the environment and must not disperse too easily, so as not to limit the time and distance over which the flow can be observed.
Application of airflow visualization to studies on contaminant droplet dispersion
Airflow visualization is widely used to study how droplets and aerosol particles are transported in indoor environments, especially in the presence of air-conditioning or ventilation systems.
By observing the trajectories of a visible tracer, engineers and hygienists can identify possible short-circuits, recirculation zones and preferential paths that may favour the spread of contaminants.
Transport phenomena involving particles smaller than one micron can be studied in advance through visualization with smoke generators. These tools make it possible to optimise the positioning and orientation of air outlet points in order to minimise unwanted transport of contaminants and improve overall air quality.
Table of settling velocities for aerosol droplets
The table below reports indicative settling velocities that highlight how strongly droplet diameter affects dispersion and residence time in airflow.
| Particle diameter (µm) | Settling velocity (m/s) | Settling 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 |
| 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 |
| 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 |
Values valid for spherical particles of standard density at 293 K and 101325 Pa.
Source: “Aerosol Technology”, William C. Hinds, Wiley-Interscience, 1999.
Airflow visualization tools
To put these principles into practice, you can use instruments such as our
Air Flow Indicator / Air Current Tester Kit, as well as other smoke generators and airflow visualization tools for cleanrooms and controlled environments. Proper airflow visualization helps you validate ISO 14644 requirements, optimise diffuser layout and reduce the risk of unwanted contaminant transport.