Air filtration systems are used universally in commercial, industrial, medical, and manufacturing facilities. Energy consumed by these systems depends mostly on the length of air ducts, density of mesh filters inserted in the airflow, and speed of operation of fans used to propel air through the ducts. The denser the filter mesh, the greater the pressure drop across the filter. In order to overcome a higher pressure drop, fans need to operate at higher speeds, therefore consuming larger amounts of energy. Thus, one of the main elements of an air filtration system is finding an engineering trade-off, where the filter mesh is dense enough to capture the required percentage of particles in the air and yet not so dense as to become economically prohibitive.
Evaluating the performance of an air filter by looking only at the filter's collection efficiency or pressure drop is misleading. The collection efficiency is not always in a linear relationship with the pressure drop, especially in the case of non-industrial applications. The key energy performance (KEP) involves both collection efficiency and pressure drop, and is a more impartial method of evaluating air filters. Lowering pressure drop and increasing collection efficiency is crucial to improving the KEPs of air filters. This paper analyzes and compares the KEPs of fiber-based filters and electrostatic precipitators (ESPs).
The particle-removing mechanism of fiber-based filters is a passive process in which particles in the air stream are removed when they attach to the fibers. The presence of the mesh in the path of the airflow results in a large pressure drop. In contrast, electrostatic precipitators have significantly lower pressure drop than fiber-based filters, because the plate electrodes of electrostatic precipitators are arranged along the direction of the airflow. In two-stage ESPs, the subject of this study, the particles are charged by gaining additional ions generated from the ionization processes near the corona electrode. The charged particles move along the electric field that exists between the repelling and collecting electrodes, and settle on the collecting electrodes. In summary, fewer obstacles obstruct the airflow in electrostatic precipitators than in fiber-based filters, which, in principle, allows for a more energy-efficient operation.
However, state-of-the-art ESPs, generally speaking, are less efficient in removing particles from the air stream than their mesh-based counterparts. Many commercial filters have an ESP stage, augmented by a pre-filter and/or a post-filter, exactly for the reasons that the ESP stage does not remove particles across the desired range of particle sizes. Nevertheless, the implementation of pre- and/or post-filters impedes airflow and increases the pressure drop of the system. Our research group previously presented two novel particle-trapping mechanisms that promoted the collection efficiencies of ESPs despite the absence of fibrous filters [1], [2], [3]. For a foam-covered ESP (FC-ESP), the collecting electrode is covered by porous foam. Particles attach to the surface inside the pores of the foam instead of the flat surface of the bare collecting electrode. For a guidance-plate-covered ESP (GPC-ESP), the collecting electrode is covered by a guidance plate that has patterned holes on it. Gaps are intentionally left between the guidance plate and the collecting electrode to allow particles to enter through the holes and stay inside the gaps. The particles collected by such particle-trapping mechanisms have a lower chance of returning to the air stream because there are fewer disturbances inside the pores or gaps than there are on the flat surfaces of the bare collecting electrodes. FC-ESPs are able to collect ultra-fine particles and lower the chance of sparkover between the electrodes. GPC-ESPs, on the other hand, have non-consumable parts on the collecting electrodes and are able to accommodate larger amounts of particles.
At the beginning of this paper, an introduction to fiber-based filters and electrostatic precipitators, including two newly developed ESPs with particle-trapping mechanisms, is presented. After that, this paper compares the KEPs of fiber-based filters and foam-covered electrostatic precipitators. A parametric study on how operating conditions affect the KEPs of guidance-plate-covered ESPs is then brought out. At the end of this paper, a real-life example of how replacing fiber-based filters with electrostatic precipitators in a filtration system affects energy costs is demonstrated.