Belaid Tabti

Corona-Charging and Charge-Decay Characteristics of Nonwoven Filter Media

The surface-potential-decay measurement techniques are commonly employed for the adjustment of Xerox photography processes, development of electrets, and assessment of polyethylene films for cable insulation. In all these situations, it is important to evaluate the surface charge density and compare it to the limit imposed by the dielectric rigidity of atmospheric air. The aim of the experiments reported in this paper is to enhance the efficiency of the corona charging of nonwoven filter media for heat, ventilation, and air-conditioning applications. Samples of nonwoven polypropylene fibrous media were exposed to positive corona discharges from a wire electrode, as the efficiency of the filter can be enhanced when the insulating fibers are electrically charged. The observed surface potential levels and the potential decay characteristics indicate that the charge of the filter is limited by the local discharges that occur inside the fibrous dielectric. The performance of the filter can be further improved by using a triode-type electrode system that assures a uniform distribution of the electric charge at the surface of the filtering media.

Corona Charging and Charge Decay Characteristics of Non-Woven Filter Media

The surface-potential-decay measurement techniques are commonly employed for the adjustment of Xerox photography processes, development of electrets, and assessment of polyethylene films for cable insulation. In all these situations, it is important to evaluate the surface charge density and compare it to the limit imposed by the dielectric rigidity of atmospheric air. The aim of the experiments reported in this paper is to enhance the efficiency of the corona charging of nonwoven filter media for heat, ventilation, and air-conditioning applications. Samples of nonwoven polypropylene fibrous media were exposed to positive corona discharges from a wire electrode, as the efficiency of the filter can be enhanced when the insulating fibers are electrically charged. The observed surface potential levels and the potential decay characteristics indicate that the charge of the filter is limited by the local discharges that occur inside the fibrous dielectric. The performance of the filter can be further improved by using a triode-type electrode system that assures a uniform distribution of the electric charge at the surface of the filtering media.

Accelerated Discharge of Corona-Charged Nonwoven Fabrics

Fibrous polymers with extremely low electrical conductivity such as polypropylene (PP), polycarbonate, polyurethane, and polyethylene are commonly employed as air-filter materials. The charge accumulated on such materials due to tribocharging effects inherent to the manufacturing process might be harmful either to the operator or to the electronic equipment of the production line. Whenever charge buildup cannot be avoided, it is important to have an effective method available in order to rapidly discharge the materials. The present work aims to evaluate the efficiency of active neutralization of charged nonwoven fabrics. The experiments have been carried out on 0.4-mm-thick PP and polyester fibrous media, with the average diameter of the two types of fibers being 28 and 19 , respectively. The nonwoven fabrics were charged by exposing them to a negative corona discharge generated by a wire-grid-plate electrode system. The samples, laid on the surface of the grounded electrode or suspended at a small distance (4.6 mm) above it, were then subjected to the action of the bipolar ions generated by a commercial neutralizer (model 6430, Ion Systems Inc., Berkeley, CA). The monitored variable was the surface potential detected by the probe of an electrostatic voltmeter. The controlled variables were the potential of the grid electrode, the neutralization time, and the distance between the neutralizer and the media. The results of the experiments enabled a crude evaluation of each factor effect. Research should continue, using the experimental design methodology, in order to establish the optimum conditions for charge neutralization.

Factors That Influence the Decay Rate of the Potential at the Surface of Nonwoven Fabrics After Negative Corona Discharge Deposition

Surface potential decay (SPD) measurements are considered as the most appropriate technique for the investigation of the corona charging of dielectric surfaces. The aim of the experiments reported in the present paper was to point out the peculiarities of SPD in the case of nonhomogeneous dielectrics, such as the nonwoven fabrics employed for heat, ventilation, and air conditioning. This paper reports experimental data collected on two types of polypropylene (PP) media, characterized by different fiber diameters. The experiments were performed on 60 mm × 50 mm samples of nonweaved sheets of PP (sheet thickness: 400 μm; average fiber diameter: 20 μm), in ambient air (temperature: 18°C to 22°C; relative humidity: 25% to 60%). The samples were charged for 10 s by exposing them to the negative corona discharge, using a triode-type electrode arrangement, energized from a dc high-voltage supply. Their surface potential was then measured with an electrostatic voltmeter. The measured data indicate that the charge of the filter is limited by the local discharges that occur inside the fibrous dielectric.

Surface potential decay characterization of non-woven electret filter media

Electric charging of the non-woven electret filter media leads to increased particle collection efficiency. Estimation of the charge state of the media can be done by measuring its surface potential. The aim of this paper is to evaluate the effects of the factors that influence the surface potential decay characteristics of these media: the grid potential of the triode-type corona electrode system employed for the electric charging, the active carbon, the ambient relative humidity and the temperature at which the media are thermally pre-conditioned. The experiments were carried out on two types of samples, one of which includes an active carbon layer between two 400 μm thick layers of non - woven polypropylene fibers, the diameter of each fiber being approximately 20 μm. The results show that the surface potential values increase with the increasing of the electric potential of the grid. Thus, if the grid potential Vg increases from 3 to 10 kV, the surface potential value increases over 3 times. A 20% increase of the surface potential values is also recorded in the case of thermally conditioned samples. The surface potential decreases by roughly 10% in the case of the samples with carbon layer, and by about 20%, and in the case of relative humidity increase from 21% to 80%.

Distribution of Electric Potential at the Surface of Corona-Charged Polypropylene Nonwoven Fabrics After Neutralization

Accumulation of electric charges on insulating surfaces is frequently at the origin of severe electrostatic hazards. The best solution to this problem is to neutralize these charges by the use of ac corona discharges. The aim of this work is to evaluate the efficiency of this process in the specific case of fibrous electrets, by measuring the repartition of the electric potential at the surface of nonwoven polypropylene fabrics before and after neutralization. The samples were charged for 10 s,in ambient air, using a triode-type corona electrode system of positive polarity. In all the experiments, the neutralization was performed 180 s after the charging process. In some of them, the neutralization electrode (tungsten wire with a diameter of 0.3 mm) was fixed at a given distance (50 mm) above the samples and energized from an ac voltage amplifier (model 30/20A, Trek, Inc., Medina, NY, USA). In other experiments, the samples moved at constant speed (3 cm/s) in the ac corona discharge zone generated by the neutralization electrode. The experimental results show that the efficiency of neutralization (expressed as the relative reduction of the average potential measured at the surface of the sample) depends on the amplitude and the frequency of the sinusoidal high voltage, as well as on the charging time and the neutralization duration. Better effects were recorded in the case of the samples that moved through the corona discharge zone.

Surface Potential Versus Electric Field Measurements Used to Characterize the Charging State of Nonwoven Fabrics

Surface potential and electric field measurement techniques are widely used for the investigation of the corona charging of dielectric surfaces in a wide range of industry applications. The aim of this paper is to estimate which of these techniques are the most appropriate for characterizing the charging state of nonwoven fibrous dielectrics and point out the “noise factors” that might distort the results of the measurements. The experiments were performed on samples of nonwoven polypropylene sheets, in contact with or at a well-defined distance from a grounded plane electrode. The effect of the variability of the position of the probes with respect to the samples was also investigated. Several recommendations have been formulated regarding the use of these techniques for monitoring the charging state of nonwoven fabrics in industry applications.

Implementation and interpretation of surface potential decay measurements on corona-charged non-woven fabrics

The aim of this paper is to discuss the peculiarities of the surface potential decay (SPD) curves obtained for certain non-woven media. The experiments were performed on samples of non-woven poly-propylene (PP) sheets, which are typically employed in the construction of air filters for heat, ventilation and air conditioning. The samples were in contact with a grounded plane, in order to: (1) ensure better charging and measurement reproducibility; (2) simulate the worst situation of practical interest. They were charged using either a high-voltage wire-type dual electrode or a triode-type electrode arrangement. The aspect of the SPD curves depends on the electrode configuration. When the electric field is strong enough, it can activate charge injection at the insulator-metal interface and extrinsic conduction.

Sinusoidal, Triangular, or Square Alternating Voltages Neutralization of Electrostatic Charges on the Surface of Polypropylene Nonwoven Fabric

The most widely used techniques for neutralizing the static charge at the surface of insulating bodies make use of the bipolar ions generated in an ac corona discharge. The aim of the present work is to evaluate the efficiency of the neutralization achieved with a corona electrode energized from a generator of sinusoidal, triangular and square ac voltages of various amplitudes and frequencies. The experiments were performed on 120 mm × 90 mm samples that were cut from the same nonwoven polypropylene (PP) fabric. The samples were charged for 10 s, in ambient air, using a triode-type corona electrode system of positive polarity. The neutralization was performed 300 s after the charging process, with the samples moving at constant speed (3 cm/s) in the ac corona discharge zone. The electric potential distribution at the surface of the samples was measured with an electrostatic voltmeter (model 3450, Trek Inc., Medina, NY). The efficiency of the neutralization was evaluated by calculating the inverse ratio between the maximum absolute values of the potential before and just after the neutralization. The amount and the polarity of the residual charge after neutralization depend on the frequency, amplitude, and type of ac voltage. The results show that an excellent neutralization can be obtained for any of the three wave shapes of ac voltage, at industrial frequency (i.e., 50 Hz) by varying the amplitude of the offset.

AC corona neutralization of positively and negatively charged polypropylene non-woven fabrics

The insulating materials can acquire high levels of electric charges, which are susceptible to generate electrostatic discharges. The present article aims at evaluating the efficiency of AC corona as neutralization mechanism for positively- and negativelycharged polypropylene non-woven fabrics. The samples were charged for 10 s, in ambient air, using a triode-type corona electrode system of positive polarity in the first set and negative polarity in the second set of experiments. In all experiments, the neutralization was performed 180 s after the charging process, with the samples moving at constant speed (3 cm/s) in the AC corona discharge zone generated by a dual-type corona electrode. The electric potential distribution at the surface of the samples was measured both before and after the neutralization. The experimental results show that the efficiency of neutralization depends on the amplitude and the frequency of the sinusoidal high-voltage that energizes the corona electrode. The negative charge proves to be more difficult to neutralize than the positive one.