Home

Personnel

Publications

Newsletters

Glossary

Bibliography

Calendar

Services

Products

Instructions,
Descriptions,
Manuals

Links

The objective of this study was to develop and test a method to neutralize concentrated charged aerosols. Electrically charged aerosol particles are generally undesirable for use in inhalation chambers because they reduce inhalability. Several methods have been developed to neutralize highly charged particles. Most common is neutralization by mixing the charged particles with bipolar ions produced by corona discharge or radioactive decay of isotopes such as ²¹⁰Polonium, an alpha emitter. Both ion concentration and the time during which ions are mixed are factors in the rate of neutralization. The authors describe a method to neutralize a concentrated stream of large highly charged particles by means of a new ion generator. In addition an isokinetic sampler, developed for the measurement of ion generation, is described.

Aluminum oxide particles were used for inhalability studies in a wind tunnel. The wind tunnel is described elsewhere (Hinds and Kuo, 1995). The Al₂O₃ was a large (30 um with aerodynamic diameter of 52 um) and highly charged particle (240,000 excess charges per particle). This corresponds to a neutralizing current requirement of 0.11 uA or a negative ion concentration in the aerosol stream of 2.5 x 10¹⁴/ m² (2.5 x 10⁸ cm³). Because Al₂O₃ has high electrical resistance (10¹⁴ohm-m) it would take hours for a 30 um particle to loose charge on a grounded surface. The aerosol consisted of Al₂O₃ optical powders made by 3 NBS dust feeders each producing 110 mg/s; the flow rate was 9.6 m³/h (160 L/m), with mass concentration of 43 g/m³. The particles produced were adequately charged to affect their inhalability. For this reason, neutralization was necessary. It was considered impractical to neutralize or discharge with bipolar ions because the radioactive source required would be impractically large.

To evaluate neutralization methods and devices, a real time aerosol monitor was first developed to monitor the charge of the particles in the wind tunnel (see Figure 1). In brief, this used a Faraday cup sampler arranged for isokinetic sampling. Particles were collected on glass fiber filters. The filters were then assessed for charge with a Keithly Model 6512 meter which measures current in pA or accumulated charge in pC. A 47 mm stainless steel filter holder was used as the Faraday cup. This cup was Teflon coated for insulation and placed inside a 90 mm diameter stainless steel enclosure with a 21 mm diameter outlet. Isokinetic sampling was ensured by the sharp edges of the inlet to the sampler.

The charge neutralization device developed was an asymmetrical 5 electrode flow through ion generator (see Figure 2) with a Richmond Model AB-250 power supply. The 5 electrodes were encased within PVC pipe (21 and 32 mm) and connected by high voltage connectors and high voltage wire to the power supply. The 2 mm diameter central electrode was positive and the 4 peripheral electodes of 0.5 mm tungsten wire were negative. The aerosol flowed through the generator at its axis. There were more negative charges than positive produced at the edge of the aerosol stream. Thus, a negative corona effect was created at the 4 peripheral electrodes where there was contact with the charged particles. The ion generator was powered and regulated by an adjustable (0-8.5 kV) power supply.

The evaluation of the efficacy of the charge neutralizer showed the ion generator reduced charges in the test aerosol to 1% of the original charge or abour <2,400 charges per 30 um particle. Although higher than Boltzmann's equilibrium, this was low enough to allow accurate measurement of inhalability. The final charge condition for 30 nm particles was less than 1% of the original charge. The particle charge was maintained with 2% of the original charge for a one hour period due to the stability of the ion generator. Workplace aerosols have comparable charge to those produced in this experiment by this ion generator.

Reference: Hinds, WC and NJ Kennedy (2000): An Ion Generator for Neutralizing Concentrated Aerosols. Aerosol Science and Technology 32:214-220.

See also: Hinds, WC and T.-L. Kuo (1995): A Low-Velocity Wind Tunnel to Evaluate Inhalability and Sampler Performance for Large Dust Particles.  Appl. Occup. Environ. Hyg. 10:549-556.