Removal of micron-size droplets from an air stream by means of electric fields

In many tank retrieval and waste treatment operations conducted for the U.S. Department of Energy, small droplets of aqueous solutions containing radioactive materials are formed in air streams. These droplets need to be separated before the air is released to the environment. The use of an electric field to separate water droplets from an air stream has been investigated. A test chamber of 10 × 10 cm cross-section and approximately 94 cm long mounted vertically was set up without packing, with two parallel-plate steel electrodes facing each other at a distance of 8 cm. An air stream containing water droplets formed by ultrasonic humidifiers was forced through the chamber. A laser-light-scattering particle-sizing system was used to measure the droplet size distribution. The droplet removal efficiency increased approximately linearly with electric field strength over the range investigated except when the field was raised to 3.0 to 3.50 kV/cm, suggesting a saturation effect. The analysis showed that the removal efficiency is a function of the initial droplet concentration. For instance, for a droplet concentration of 24.2 g/m ³, the maximum removal efficiency was approximately 85%, while that for a concentration of 8.3 g/m³ was 65% under the same experimental conditions. Droplet size measurements revealed that the average size of droplets did not change significantly with voltage; however, the number distribution of drops did change. Appreciable changes in the number of droplets were observed for droplet sizes in a typical range of 4 to 10 μm. The results of this research may be useful in developing effective applications of electric fields for the elimination of mist from air or gaseous streams.