Duane H. Pontius

Experimental investigations of fine particle charging by unipolar ions — A review

Abstract Many theories have been offered in attempts to describe accurately the rate of charge accumulation of fine particles in a unipolar ion field. There has been little experimental data, however, for comparison with these new theories. In this study, the existing experimental data are reviewed and compiled, and additional particle charging experiments done to extend the quantity of data. Before this study was performed the majority of the experimental data which was available for comparison with particle charging theories was contained in a paper by G. Hewitt, published in 1957. Hewitts experiments were carefully done, but the range of experimental variables was limited and his work was never independently verified. The experiments reported here followed Hewitts procedures closely and were designed to extend the quantity of experimental data. There is some overlap between this and Hewitts data, and in these instances, the agreement is good. Experimental results are presented in this paper for the charging rate of particles of 0.109–7 μm dia. at various values of electric field strength, ion density and charging time. A limited amount of data is also given which shows the important effects on the charging rate of variations in the particle dielectric constant and the polarity of the corona ion source.

Laboratory analysis of back-corona discharge

Abstract An experimental research program was designed and performed to characterize back-corona generation and behaviour in a range of environments and geometries common to electrostatic precipitators (ESPs). A wire-parallel plate device was utilized to monitor the intensity and distribution of back-corona discharges. Back-corona was generated by subjecting a deposited layer of fly ash to corona current. Visual observations of back-corona emissions were made with a sensitive video camera. These observations provided new insight into the characteristics of back-corona phenomena. Conditions that were varied in the experiments included ash resistivity, corona electrode design, corona polarity, and ash-layer thickness and surface condition. The effect of pulsed energization on back-corona was also studied. Analyses of visual observations and measured distributions of corona- and back-corona current allowed an assessment of the effects of back-corona emissions on ESP-operating parameters.

Laboratory measurements of the spatial distribution of gas velocities through seasoned baghouse fabrics throughout a simulated filtration cycle

The spatial distribution of the gas velocity across seasoned filter bag samples was measured with high resolution throughout a simulated filtration cycle. The samples were obtained from utility baghouses employing shake/deflate and reverse-gas cleaning. The tests simulated a filtration cycle with a constant air to cloth ratio of 2 acfm/ft/sup 2/ for a two-hour period with a gas having a mass loading of 3 gr/ft/sup 3/. The spatial distribution of velocities was determined at selected times throughout the filtration cycle by measuring the amount of a fine fluorescent powder that was intermittently introduced as a tracer into the test aerosol. Results of these tests showed that the initial flow of gas was almost entirely through fractures in the dust cake resulting from the cleaning operation. As the filtration cycle progressed, the flow became more uniformly distributed over the filter surface.

Performance of Large-Diameter Wires as Discharge Electrodes in Electrostatic Precipitators

Conventional practice in the design of corona discharge electrodes for electrostatic precipitators tends toward those having a relatively low corona Inception voltage. But the theory indicates that maximal electric field strength is beneficial to both charging and collection of particles. Only those particles smaller than about 0.2 μm diameter are more readily charged by increasing the ion current density instead of the electric field strength. Tests carried out on high-resistivity fly ash In a large pilot system (30,000 acfm) comparing performance of 3/8-in. diameter wires with more nearly conventional 1/8-in. wires resulted in substantially superior performance for the larger electrodes

An electrostatic precipitator backup filter for sampling systems

Abstract An alternative technique for filtering the aerosol effluent from particle sampling systems has been developed. A two-stage electrostatic precipitation system was designed to collect with high efficiency the paniculate material in a gas stream. The principal advantage of an electrostatic precipitator over conventional glass fiber backup filters is the ability to sample for long times without encountering filter pluggage or an excessive pressure differential across the system. Tests of a prototype electrostatic precipitator backup indicate collection efficiency for submicrometer diameter particles of 97 to greater than 99%.

A Novel Device for Charging High Resistivity Dust

The aerosol precharger described in the preceding paragraphs can provide a charging environment with relatively high unipolar ion density for particulate matter having high electrical resistivity. The result is a much higher level of charge on the particles than can be achieved by a conventional electrostatic precipitator which is limited by back corona to a low current density. Used in conjunction with a downstream collector operated at a low current density and high electric field strength, the precharger may be of significant value in the collection of high resistivity particulate materials.

Patch testing of ceramic barrier filters

The objectives of this work are to construct, install and operate a patch testing unit on a hot gas stream at a coal-fired fluidized-bed boiler. Long-term ``patch tests`` will be conducted on ceramic disks of the same materials used in the fabrication of ceramic candles and ceramic crossflow filters. The primary issues to be addressed in these tests are the long-term physical, thermal and chemical stability of the ceramic materials; long-term pressure drop and filtration characteristics of the ceramic filters; potential for irreversible clogging of filter elements; and long-term performance and reliability of auxiliary hardware, such as the tube require about 3 to 4 months of nearly continuous operation. Progress is discussed.

Method for computing flow distribution and pressure drop in multichambered baghouses

A new method has been developed for calculating the complex, time-dependent gas flow rates and pressure drops in baghouses. In addition to accounting for the flow resistance of the fabric and dust cake, this approach also includes the effects of connective ducting, dampers, and valves on the dynamics of the gas flow. The computational method is embodied in a computer program which is designed for simple and convenient operation. In this paper the method is outlined, and the program is described. Results are given from examples that were chosen to illustrate some of the principal features of the model.

Progress in Fabric Filter Technology for Utility Applications

The use of fabric filtration for controlling particulate emissions from power generating plants has proven effective, both technically and economically. In most installations, the collection efficiency of these systems ismuch better than is required by law. Recent developments have centered on improvements in methods for removing collected ash (dustcake) from filter bags, applications in integrated emissions control systems, and studies leading toward a better understanding of the fundamental processes involved.

Characterization of sonic devices used for cleaning fabric filters

The values of the fundamental frequency and integrated output power of horns, or other sonic sources, do not provide adequate information to evaluate their potential effectiveness in baghouse applications, because these data give no indication as to the distribution of the sonic power as a function of frequency. The fundamental frequency denotes only the lowest possible mode of vibration. It is physically possible for a system to emit no sound at all at its fundamental frequency while producing intense sounds at higher frequencies. However, the use of a power-weighted mean frequency, and to a lesser extent, a geometric standard deviation provides a quantitative approach to the evaluation of horns for applications in cleaning fabric filters.