Aerosols electrostatic effects

D3. Dawkins, G. S., Electrostatic effects in the deposition of aerosols on cylindrical shapes, Univ. of Illinois Eng. Exptl. Sta. Tech. Rept. 15, AEC Rept. COO-1017, Contr. AT(ll-l)-276 (1958). 

These compressed air nebulizers produce polydisperse aerosols. After the aerosol is produced, the size distribution may change due to evaporation of liquid from the droplets. In addition, the particles may be electrically charged due to an ion imbalance in the droplets as they form if such charges become further concentrated due to evaporation, the particle may break up into smaller particles. Thus electrical neutralization of the aerosol, for example, by exposure to a radioactive source, is usually necessary to prevent electrostatic effects from dominating the particle motion, coagulation, and other behavior. 

Aerosol particles, because of their small size, present a large amount of surface for chemical reactions such as burning, adsorption, absorption, or other chemical reactions or for such physical properties as wettability or electrostatic effects. The amount of area per gram of material increases as the particle size decreases, and for a given average size, increasing polydispersity decreases the surface area per gram. As particle size becomes very small, the boundary conditions between the particle and the air around it become confused, but also become more important. 

The electrostatic effects on particle deposition illustrated by computational simulation may have implications on charged aerosol delivery. However, more clinical research is needed since these models have not been validated hitherto by in vivo data using charged aerosols. 

Figure 5 The effect of spacer volume on dose delivery. The aerosol from a budesonide pMDI was analyzed by the Anderson impactor (U.S. throat) 2 s after actuation (28 L/min) through Nebuhaler and Babyhaler, both of which had been primed to prevent electrostatic effects. Both spacers are of similar length but have different diameters (80 versus 50 mm). The aerosol was airborne for a longer time in the spacer with the larger diameter correspondingly, the total dose was larger, mainly due to an increased coarse particle dose.

This survey does not consider the interactions of aerosol particles once they are in "contact." For this reason, the electrostatic effects of mobile charge species in uncharged particles are not treated. However, they are of central importance in considerations of the chemical and structural evolution of the contacting particles whenever they contribute to double layer formation. In that case. 

Industrial painters may suffer adverse health effects from over exposure to paint by skin contact or accidental ingestion, from excessive inhalation of paint aerosol, solvent vapour, or of dust in the case of electrostatically-applied powder coatings (e.g. polyesters containing triglycidyl isocyanurate), or from exposure to thermal degradation products from heated paint or plastic coatings (Table 5.48). 

The electrostatic force is directly proportional to the net charge of an aerosol particle. Therefore, effective charging of the particles is of great importance. Airborne particles are normally charged either due to their birth processes or due to charge transfer from gas ions to particles. The natural charging of particles is normally so weak that it has no practical importance for electrostatic air cleaning. 

Zhibin, Z. and Guoquan, Z. Investigations of the Collection Efficiency of an Electrostatic Precipitator with Turbulent Effects. Aerosol Science and Technology. 20 (1994) 2, pp. 169-176. 

Under normal household conditions the effect of electrostatic precipitators would be less dramatic than shown here, because these measurements were carried out in a closed room. With normal movement between rooms, the aerosol concentration would not be reduced by as much and the increase in unattached fraction of 218Po would be less. 

Electrostatic precipitators were effective in reducing the radon decay-product concentration below the reference level, but analysis of the room aerosol indicated that the reduction was largely offset by an increase in lung dose per unit exposure. 

Effects of carrier gas flow rate, dilution flow rate, and the combustion boat temperature were studied by sampling the aerosol stream with an electrical aerosol analyzer to obtain the particle size distribution. Filter samples were taken for chemical analysis to determine mass concentrations. Aerosol samples were also collected in an electrostatic sampler for electron microscopic examination. 

Quenching in product oil or in an immiscible hydrocarbon solvent is widely practiced. Orthodox aerosol capture devices such as demisters and other commonly used impingement devices are not very effective and electrostatic precipitation is currently the preferred method. 

Electrostatic precipitation is also very efficient for retention of very fine particles, as long as these are not highly electrically conductive. The structural features of these units dictate that they are only cost effective for particulate and aerosol emission control for low pressure drop clean-up of very large volumes of gas hence their extensive use for the treatment of combustion gases of fossil-fueled power stations. Next generation developments have been reviewed [27]. 

Aerosol can be separated by size using devices such as cyclones, classical impactors, virtual impactors, and filters before detection. These same components can also be used for aerosol concentration and collection as described below. Other methods involving electrostatic and ultrasonic effects are being investigated for aerosol concentration and separation, but these methods are less developed and are not available for near-term deployment. 

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