The effect of electrostatic forces on filtration efficiency of granular filters Raheleh Givehchi a , Qinghai Li b , Zhongchao Tan a, ⁎ a Department of Mechanical & Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada b Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, PR China abstract article info Article history: Received 23 November 2014 Received in revised form 8 January 2015 Accepted 31 January 2015 Available online 7 March 2015 Keywords: Granular filtration Airborne NaCl nanoparticles Electrostatic force Residence time The effect of electrostatic forces on the granular filtration of nanoaerosol NaCl particles in the range of 10 nm to 100 nm was investigated experimentally in this study. The test granular filters were made of 2 mm homogeneous glass beads at three media thicknesses (25, 76 and 127 mm), and they were tested at three air flow rates (27, 45, and 65 lpm). The filtration efficiencies were measured for neutralized and charged NaCl nanoparticles. The corresponding difference was considered as the filtration efficiency attributed to the electrostatic attraction between the charged NaCl particles and the glass granules. Results showed that the electrostatics played a great role in nanoaerosol filtration, which is different from conventional filtration theories. Its contribution to filtration efficiency increased with the size of the nanoparticles to a level of 30% or so. Results also showed a positive correlation between the separation efficiency due to electrostatic forces and the residence time of the air flow. The correlation is relatively strong (between 0.6 and 0.9) for particles in the range of 20–100 nm. However, it is weak, although positive, for sub-20 nm particles. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Clean air is a vital resource for human life. However, population growth, enhanced human activities, and the rapid expansion in indus- trial production have led to unprecedented demand on clean air all over the world. The report of the World Health Organization (2013) showed that over seven million premature deaths each year are attrib- uted to air pollution. Among the air pollutants, nanosized aerosol (nanoaerosol) particles suspended in the air are proven to cause adverse impact on human health. In addition, they also negatively impact on global climate change and extreme weather by interacting with the solar radiation. It is important to capture the nanoaerosol particles, at their sources and from the ambient air. Among all of the technologies for removing nanoaerosol particles from their carrier gases, air filtration is the simplest and most widely used method. A great amount of works have been conducted with fibrous filters, membrane filters and fabric filters [1–3]. However, much less attention has been paid to granular air filtration. Limited information has shown that granular air filtration has high removal efficiency for a wide range of particle size [4–8]. Granular filters may also be the only and promising option for air filtration at high temperature and high pressure [8–13]. The mechanisms of granular air filtration are similar to those of fi- brous air filtration, except that the particles are deposited on surfaces of the granules. Among all the particle transport mechanisms, it has been widely accepted that diffusion is the dominant mechanism for removal of nanoparticles (1–100 nm in diameter), and the electrostatic effect is often ignored in conventional air filtration models that were de- veloped based on single fiber theory [14,15], it is not certain for granular filtration. Aerosol particles and granules often carry electrostatic charges which may influence particle transport, and the consequent re- moval efficiency. The electrostatic forces between particles and granules may include image forces, dielectrophoresis due to collector charge, columbic force, space charge effect, columbic force due to external field, and dielectrophoresis due to external electric field. However, the columbic force due to particle charge and an external electrostatic field are the most dominated forces [16]. Several researchers have employed external electric field in granular filtration to enhance the particle removal efficiency [17]. The external electric field causes the granular beads to polarize. In this case, either neutral or charged particles are attracted to the polarized granules, lead- ing to increased filtration efficiency. Particles travel a shorter distance in granular filters than in a conventional electrostatic precipitator to reach the collector surface. And, the collection surface area per volume of granules is larger than that of an electrostatic precipitator; therefore, the electrostatic attraction effect is likely to increase the collision chance of particles and consequently filtration efficiency [18]. Although a number of studies have been conducted by considering external electric field to enhance electrostatic forces between microsized particles and granular beads, there is very limited informa- tion about the effect of electrostatic forces on granular filtration of nanoparticles. In this work, experiments were carried out to investigate the effect of electrostatic forces on nanoaerosol filtration. The granular filters tested were made of uniform 2 mm glass beads at three media thicknesses of 25, 76 and 127 mm, and they were tested at three flow Powder Technology 277 (2015) 135–140 ⁎ Corresponding author. Tel.: +1 519 888 4567ext38718. E-mail address: tanz@uwaterloo.ca (Z. Tan). http://dx.doi.org/10.1016/j.powtec.2015.01.074 0032-5910/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec