High-Efficiency Electrostatic Precipitators
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Indoor air pollution is a source of risk for diseases, such as stroke, ischemic heart disease, respiratory infections, and lung cancer. According to the World Health Organization, nearly 4.3 million people die each year due to exposure to household air pollution. Residential, commercial, and medical facilities use air filters to remove particles from the air stream to maintain indoor air quality. Electrostatic precipitators (ESPs), a common type of air filter, use the corona effect to charge particles, and collect the charged particles using a designed electrostatic field. Most of the components of ESPs are placed along the path of airflow so that there are very few obstacles that obstruct the airflow, making the pressure drop across ESPs low and allowing ESPs to operate in an energy efficient fashion. However, certain collected particles re-enter the environment due to external disturbances, such as strong airflow, vibrations, and spark discharge (sparkover). This act of particle re-entrainment lowers collection efficiency. Commercial ESPs attempt to mitigate particle re-entrainment by adding a fibrous post-filter. However, the implementation of a fibrous filter impedes airflow, increasing the pressure drop across the ESP. Thus, one of the main design considerations for an ESP is determining the engineering trade-off, where the ESP is capable of capturing the required percentage of particles in the air, yet the pressure drop is not high enough to become economically prohibitive. This dissertation presents two particle-trapping mechanisms that are added to the collecting electrodes of two-stage ESPs to suppress particle re-entrainment without introducing additional pressure drop. The main idea behind particle-trapping mechanisms is to store the collected particles in secured spaces, where disturbances are less likely to cause particle re-entrainment. One promising particle-trapping mechanism involves covering the collecting electrodes with porous foam, allowing particles to attach to the surface inside the pores of the foam, instead of the flat surface of the bare collecting electrode. Another particle-trapping mechanism involves covering the collecting electrodes with perforated guidance plates. Gaps are intentionally left between the guidance plate and the collecting electrode to allow particles to enter through the holes, and remain inside the gaps. Particles collected by either particle-trapping mechanism have a lower chance of returning to the air stream, because there are fewer disturbances inside the pores or gaps than on the flat surfaces of the bare collecting electrodes. The inclusion of either mechanism improves collection efficiency, and therefore, a post-filter is no longer required. Foam-covered ESPs have been tested to have up to 99\% collection efficiency. The energy analysis showed that when the fiber-based filters in Loew Hall at the University of Washington were replaced with foam-covered electrostatic precipitators, the operating cost per airflow reduced by 26.5\%. Guidance-plate-covered ESPs have been proven to have up to 22\% higher collection efficiency than conventional ESPs that have no particle-trapping mechanism. This dissertation presents and discusses both foam-covered ESPs and guidance-plate-covered ESPs from various perspectives, attempting to move two-stage ESPs a step closer for non-industrial applications.
- Mechanical engineering