Charging characteristics for electret filter materials

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Abstract

In order to save energy at the time of dust collection, the authors have tried to provide as much charge as possible to electret filter. Polypropylene (PP) (nonwoven, by melt-blowing) and polytetrafluoroethylene (PTFE) (layering of PTFE fibers in wet system and heat dried) were used as a sample. High voltage DC corona, pulse corona and pulse with DC corona were applied to the sample materials. The discharge characteristics were investigated in order to provide larger charge for the materials. The results indicate that the pulse with somewhat slower rise time with DC superimposition provides a large charge. The maximum charge obtained was approx. 45μC/m2 in the case of PP and approx. 10μC/m2 in the case of PTFE.

Introduction

Electret filter is a good material for environmental protection. If the electret filter is effective, it could save energy for dust collection. It is important to provide more charge to the electret filter material to develop effective electret filter, in view of environmental protection. There are several methods to provide charge to the electret materials [1], [2]. The principal and traditional charging processes are triboelectric charging, corona charging and charging by induction [2]. Since corona discharge produces a lot of ion which will give charge to the materials, the authors tried to improve the corona charging processes. The charging will be also influenced by the electrical property of the materials. High charging characteristics of the electret filter materials mixing with inorganic additive [3] and polymeric additives [4] were reported so far.

As a preliminary research, the authors investigated more effective charging methods for the electret filter materials and effects of additives for the filter materials. Also, electret filters were manufactured experimentally in the laboratory and their dust collection characteristics were investigated. These are reported in this paper.

Section snippets

Experimental

Four types of power supply were applied to provide charge to the electret filter materials. The first was DC power supply with corona needle, grounded electrode (plate) and grid electrode (corona triode system). The second was pulse power supply using rotary spark gap. This produces fast (nanosecond order) rise pulse and the maximum pulse production rate is 200 pulse/s. The third was the pulse power supply coupled with DC power supply. The fourth was boxer charger and this produces about 26 kVpp

Influence of discharge characteristics on the surface charge density

Traditional charging methods, such as contact (inductive) charging, needle-plane corona charging and needle-plane corona triode charging with grid electrode using DC power supply, were investigated. The results show that needle-plane triode corona charging provides the biggest surface charge density among the three systems. The surface charge densities were around 20–50 μC/m2 at applied voltage of 25 kV.

Pulse charging was more effective to charge the sample than traditional charging methods. Fig.

Conclusion

By applying various types of electrical discharges, it was found that pulse with slower pulse rise time with DC superimposition is more effective for providing bigger surface charge density than DC or pulse charging only.

SiO2 and TiO2 additives seemed to improve the surface charge density slightly in the case of PP filter.

The electret filter produced in the authors’ laboratory showed higher collection efficiency, 99.32% at the pressure drop 17.5mmH2O (at air velocity 5.3 cm/s) in the case of

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There are more references available in the full text version of this article.

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