Electret Fibers Research Articles

High-speed cinematographic recording and numerical simulation of particles depositing on electret fibres

A method is described for visualization and recording, as well as for numerical simulation of particle motion during the deposition onto single electrically charged filter fibres (electret fibres). For this a microscope and a high-speed cine camera are used to record the trajectories of particles in the process of flowing around, and being deposited on, a fibre (two-dimensionally in a plane perpendicular to the fibre axis). The particle trajectories are then modeled, based on the experimental data and suitable mathematical models for the fluid flow and the electric fields of the fibre charges. As the fibre charge density, in general, is not known, it is used as a fitting parameter in the model calculation. Using the example of split-fibres with a rectangular cross-section and a bipolar electric charge it is shown that, using models known from the literature, very good agreement can be achieved between experiment and simulation. With this electret fibre type, the trajectory simulations, assuming an ideal bipolar charge configuration, gave effective charge densities between roughly 100 and 500 μAs m −2, corresponding to total surface charge densities between 230 and 1150 μAs m −2.

Efficiency of Fibrous Filters with Rectangular Fibers

A theoretical study of filtration characteristics of rectangular electret fibers arranged in a staggered array perpendicular to the flow direction was conducted assuming the Brownian diffusion and interception as the dominant particle capture mechanisms. The single-electret fiber efficiency due to pure diffusion was found to be proportional to -2/3, 0.278, and 1/2 powers of the Peclet number (based on the fiber hydraulic diameter), filter solid-volume fraction, and fiber aspect ratio, respectively. For pure interception, the single fiber efficiency was determined to be proportional to 1.55 and 0.54 powers of the interception parameter and filter solid-volume fraction, respectively. This is Particle Technology Laboratory Publication No. 741.

Electret Fiber

Electret Fiber

Effect of Charging State of Particles on Electret Filtration

Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 μm in diameter and at different charging states. A single fiber collection efficiency ηED was found to be expressible as a function of dimensionless parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K In as, here, A, B, C, and D are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 μm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 μm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.

COLLECTION MECHANISMS OF ELECTRET FILTER

Abstract Collection efficiency of a single electret fiber, which carries permanent positive and negative charges, was studied. Theoretically, it was obtained by solving the equation of particle motion taking account of the induced and Coulombic forces simultaneously. When either induced or Coulombic forces dominates particle collection, the collection efficiency was found to be proportional to 2/5 power of induced force parameter KIn, and 3/4 power of Coulombic force parameter KC, respectively. However, when both forces are effective simultaneously, the efficiency was not expressed by the simple superposition of both effects because of negative interaction between both forces. Experimentally, collection efficiency of a single electret fiber was measured by using monodisperse sodium chloride particles ranging from 0.01 to 0.4 μm in diameter for filtration velocities from 5 to 200 cm/s, under different charging state of particles, i.e., uncharged, singly or doubly charged and charged in Boltzmann equilibriu...

Deep-Bed Electret Filters: The Determination of Single Fiber Charge and Collection Efficiency

Deep-bed electret filters are used to separate small solid or liquid particles from gas flows. Compared with conventional deep-bed filters of similiar pressure loss, they operate with substantially higher collection efficiencies. During the course of filtration, however, the electrical charges of the deposited particles neutralize the electret fibers, temporarily decreasing the collection efficiency. In this paper, experimentally determined fractional separation functions of different types of electret filters are presented for the particle size range from 10 nm to 10 , um at a filter face velocity of 10 cm/s. The initial efficiencies are compared to that of a conventional (glass fiber) filter and also to those of the electret filters in discharged state. The results of long-term filtration experiments show that a complex time-dependent behavior exists for different filter materials. Furthermore, two experimental methods are presented and discussed which determine the charge characteristics of single electret fibers.

An Air Filter Made from an Electret and a Conductor

Electrically charged filter fibers, and, in particular, electret fibers, can be used in air filters. Planar electrets have a high internal and a low external electric field; but only external fields contribute to filtration, and so if electrets are used in filters, the fundamental problem is to increase the external field. In a conventional electret filter this problem is solved simply by reducing the width of the electret sheet by making it into fibers, but an alternative solution is to place conductors close to the surface of the electret. A filter with a low resistance to airflow can be produced by this means, but the level of charge that can be sustained on the electret is limited by dielectric breakdown in the air between dielectric and conductor. The total charge on such a filter has been measured microscopically by X-irradiation, and macroscopically with a probe; the two methods give similar results. The electric field inside the filter has been calculated by a self-consistent method. Specimen results for filtration efficiency are given.

Electrets as a cascade impactor substrate

A theoretical investigation has shown that electret fiber substrates can be used to reduce particle bounce and blow-off by increasing the force of attraction of particles to the substrate. Uncharged particles are attracted to the fibers by induced dipole and van der Waals forces while charged particles experience an additional Coulombic force. The magnitudes of these forces are dependent on the particle size, electret surface charge density, the dimensions of the electret fibers, and the particle's net charge. These attractive forces dominate over different particle size ranges. In a strong electric field gradient, such as exists with an electret fiber, the induced dipole force is predicted to be the strongest force on large particles ( d p > 1–2 μm) and van der Waals attraction will be the dominant force on small particles ( d p < 1 μm).

Filtration of Cotton Dust by Lint Filter Media

The performance of the media used in cotton-mill lint filters has been examined. Efficiencies were found to be only of the order of fifty percent for the test aerosols used, and this mainly reflects the high proportion of fibrous material, or lint, which is stopped at the surface of the filter. The finer dust component of the aerosol is captured with an efficiency of only about ten percent. Experimental evidence supports the view that such low efliciencies are partly due to particle bouncing, associated with the high Reynold's number at which the medium operates. Reynold's numbers are lower for the accumulated lint layer, which is composed of finer fibers than the medium, or when face velocity is reduced. Both these effects can be obtained without reducing flow through the equipment by using pleated media, which give higher efficiencies. Further increase in efficiency may be obtained by applying an external electric field near the medium, or by using a medium made of electret fibers.

Electret filters for high-efficiency air cleaning

The main feature of electret filters is their ability to remove particulate matter from polluted air by strong electrostatic forces generated by the electret fibres from which they are made. Tests against various model aerosols and industrial air have shown that electret filters capture fine dust particles very effectively, while offering a minimum of flow resistance. This combination of filter characteristics makes them suitable for a number of applications, of which a few are discussed. The filters are not significantly affected by high temperatures and high humidities.

The use of polymers for electrets

Investigation of the charge decay of commercial polymer foils has revealed their potential usefulness in electret devices. It has been confirmed that Teflon—FEP and Teflon—TFE are the best materials for making stable homo-electrets. However, the charge retention of the recently developed polymer Teflon—PFA, which has better mechanical properties, also appears to be promising. Methods are described by which the thermal stability and mechanical properties of fluorocarbon electrets can be improved. These methods are based on thermal aging and on lamination, and they are essential for better durability in long-term applications. Several practical electret devices are reviewed. Special attention is paid to the use of electret fibres in high-efficiency filters for submicron particles in air.