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Elimination of dust particle sedimentation in industry environment
Journal of Electrostatics 71 (2013) 208e213
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Elimination of dust particle sedimentation in industry environment Karol Marton, Jozef Balogh, Jaroslav D zmura, Jaroslav Petrás* Technical University of Kosice, Faculty of Electrical Engineering and Informatics, Department of Electrical Power Engineering, Mäsiarska 74, 040 01 Kosice, Slovak Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2012 Received in revised form 18 December 2012 Accepted 20 December 2012 Available online 9 January 2013
This paper deals with dust particle elimination on industry plastic foils in industry environment and with technical air cleaning for this environment. We describe electro-physical processes that are in progress during electric charge neutralization. These charges arise during foil manufacturing process. We describe a method for electric parameter measurement for such foils, surface charge measurement on foils and methods of their neutralization. We describe an unconventional type of contamination separator for various technical areas. The application of negative or positive unipolar discharge is used in area of air cleaning in medical and other environments. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Neutralization Air cleaning Surface charge High voltage
1. Introduction During technological manipulation of foil which is made of polymer organic material (e.g. polypropylene, polyvinyl fluoride), creation of electrical charges was observed on surface of these foils. This phenomenon proved to be a consequence of so called “harmful” tribo-effect which is caused by electro-kinetic phenomenon occurring when two or more material surfaces touch with same or similar physicalechemical properties. It is possible that created charges, after reaching their saturation, evoke a status that interferes with technological process and invades its operation in the case of high technological hygiene and low relative humidity with insufficient technological device earth connection. Force effects of electric field between equivalently charged foil surfaces (by same charge), force effects of devices involved in manufacturing process or high potential difference between devices involved in technological process and personnel belong to this category. In such cases electric flash-arc occurs that invokes undesirable and often malicious physiological influences. The voltage generated by such processes can reach up to 8e16 kV. In cases when personnel is not actively earthed the probability of such electric discharge between the personnel and semi-product in technological process increases (material with dielectric properties).
* Corresponding author. E-mail addresses: [email protected] (K. Marton), [email protected] (J. Balogh), [email protected] (J. D zmura), [email protected] (J. Petrás). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2012.12.026
From product quality’s point of view there is much higher probability for danger because even with very high care for clean environment it is not possible to eliminate the sedimentation of dust particles with macroscopic size on foils’ surface. These particles are electro-statically coupled to the surface by strong coulomb forces. This effect is especially undesirable during heat processing of foil at temperatures over 135 C or even 160 C, so that the dust particles with fiber, sphere or other shapes get moulded into polymer foil’s surface and they considerably deteriorate the foil’s quality in this way. We have to notice that the abovementioned planar polymer foils are not the only sources of electric charges. Another source of such charges is the quick transmission motion (circle cross-section with diameter approx. 20 mm) during transporter steel roller motion. As the earth connection of the belt is imperfect, there is a high value of accumulated charge on this “infinite” loop. This generated charge finds its way through conductive rollers to critical parts made of organic polymer foils. It is possible to avoid these unwanted effects by electrostatic charge neutralization on foil’s surface.
2. Generation of electric charges on polymer foils and moving parts Theoretical analysis of electric (and also electrostatic) charges on dielectric or insulation foil’s surface can be made in detail on the basis of physical process analysis by band models of materials in touch. Another idea for explanation of these effects leads to process
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
analysis when electric bilayer is created or when it splits. We can start from these ideas: a) splitting of two opposite oriented charge layers. This leads to charge generation on every material (foil) with both polarities. b) giving over the charges from one foil to another by touch. The basic foil loses its original charge. c) receiving of charges by foil surface, e.g. by injection from external source. The abovementioned three processes can invoke various effects (separation of touching surfaces, fracturing of material, electrostatic distribution of charges, photoionization, high voltage discharges, cumulating of charges, tribo-effect e electric-kinetics related to friction, bumps, pressures etc.) which are closely related to the structure of used material and to its electro-physical and also chemical properties. The existence of free or coupled electric charges in foils (non-polar or polar materials) plays one of the most important roles together with related polarization effects. Outgoing from technological processes during foil manufacturing we have to point out the fact that the generation and accumulation of charge occurs not only in one layered foil systems but also often with more layered foil systems. From electro-physical point of view it behaves as layered dielectrics of capacitor with capacities connected in series. If we choose one element from this complex dielectrics (one layer), we can depict a model of it and we can also write an electro-physical interpretation of this model. We have to take into account the energy balance which causes that as a consequence of tribo-effect (friction, movement) the charges depositing or been created on foil’s surface element are in balance with current i0 which flows out from this surface. This can be expressed by the equation:
dQ ¼ i0 ¼ iR þ iC dt
(1)
and schematically depicted on Fig. 1: The solution of electric circuit on Fig. 1 leads to differential equation:
dU U C$ þ ¼ i0 dt R
(2)
By calculation we get the U voltage characteristics in dependence of the t time:
t UðtÞ ¼ i0 $R$ 1 eR$C
(3)
This equation shows the Umax voltage saturation depending on time (Fig. 2) on capacitor model with capacity C and resistor R connected in parallel:
Umax ¼ i0 $R
(4)
Maximal value of charge on ReC elements will be:
Qmax ¼ i0 $R$C
(5)
when taking into account the basic equation:
Qmax ¼ C$Umax If we substitute into equation (5) real values of width h and area S of foil, we get:
h S Qmax ¼ i0 $r$ $ε0 $εr $ ¼ i0 $r$ε0 $εr S h
(6)
We have proved that the value of charge on foil directly depends on basic material properties of foil or planar insulator. This is represented by material’s specific resistivity r and by relative permittivity εr. The abovementioned calculation ignores climatic conditions of environment [5]. 3. Measurement of polymer foil’s dielectric properties Outgoing from theoretical basics which describe electrical charge generation as described in previous chapter we focused on experimental analysis of dielectric properties in applied material technologies. We measured these materials with various widths:
semitransparent polyester h ¼ 0.105 mm polyvinyl fluoride h ¼ 0.035 mm red polymer rubber h ¼ 1.84 mm red polymer rubber h ¼ 3.37 mm grey rough rubber h ¼ 2.13 mm
The knowledge of complex properties required measurements of these parameters:
Fig. 1. Elementary current draining away charge from dielectric foil to earth [5].
209
relative permittivity dielectric loss factor insulating or specific resistivity electrical strength
Fig. 2. Voltage U(t) depending on time.
210
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
Fig. 3. Material characteristics e semitransparent polyester.
Because foil or planar polymer materials were applied in praxis at low frequencies, all measurements were executed at network frequency 50 Hz. Samples were measured at thermo-electrical strain in measurement system of TETTEX-Instruments. Samples were stressed mechanically by constant weight during measurement. As measurements were made with thin foils, measured samples had “electrodes” laid in the form of paste. Semiconductor layer was created which did not reacted with base material in physical or chemical way. We avoided thus the creation of air “pillows” between extremely smooth metallic electrodes of tri-electrode system and uncovered foil. Otherwise in such air pillows electrical discharges at higher field gradient arise and disturb the measurement. Despite these arrangements undesirable case occurred which influenced dielectric loss factor. At electrical strength measurements it is advisable to immerse samples into insulation liquid with non-polar characteristics, eliminating thus surface discharges. The importance of these measurements reposes on the fact that they showed qualitative differences of used materials and their categorization into weak polarized and polarized materials. This fact is very important in terms of their charging capability. Another important aspect of these measurements was the description of obvious difference between measured materials regarding to the compactness of material. While first material samples (PP and PVF) represent compact materials (from polymer chain structure’s point of view) without air cavities and defects, materials based on rubber (red/grey rubber) have spongy structure that depends strongly on temperature, pressure and naturally on applied voltage. This depicts unstable electro-physical properties as well as different C(T)
characteristics, considerable fluctuation of tg d during temperature or electric stress (Figs. 3 and 4). In general, increase of capacity value is expected with increasing temperature which can be explained by activation of weak polar molecules in material. In porous materials with spongy structure mechanical barriers are cleared with increased temperature, material is compressed and densified by constant weight of electrode. Our hypothesis about spongy material structure and about its influence on electrical properties supports also the fact that the value of dielectric loss factor changes in non-standard way at temperatures higher than 60e70 C. At these temperatures the defective places in material filled with air or gas mixture are eliminated. A combination of insulationegas invokes conditions for ionization in gaseous environment. A simple calculation proves that in our conditions the examined insulation materials achieve ionization status at about 107 V/m. This unwanted effect can be observed also at higher voltage levels (above 0.8 kV) in case of compact materials as a consequence of surface discharge activation on electrode’s edges. It is noticeable that for all three rubber materials the dielectric power loss factor attenuates at voltage level about 500 V. We assume that the pressure of gas filling in gas closures increases the value of ignition voltage according to Paschen’s law. Electrical strength of materials was examined by standard method according to STN IEC 243-1 standard. Achieved results fulfilled the expectations and showed us that compact polymer foils have high electric strength and it is 5e13 times higher than the electric strength of rubber materials. Reduced electric strength is caused by material structure. This fact is not an obstacle for using them as separator by thermal manipulation of polypropylene foils (Table 1).
Fig. 4. Material characteristics e red polymer rubber.
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
211
Table 1 Table of electric strengths. Sample
h [mm]
Up [kV]
Ep [kV/mm]
Semitransparent polyester Polyvinyl fluoride Red polymer rubber Red polymer rubber Grey rough rubber
0.105 0.035 1.84 3.37 2.13
9.0 3.2 34.0 47.0 15.5
85.71 90.00 18.48 13.95 7.28 Fig. 6. Capacitive separation of corona electrodes in neutralizer [5].
4. Neutralization of surface charges
5. Air cleaning
Failure free technological process of polymer foil manipulation and manipulation of supporting materials can be achieved by surface electric charge elimination. These charges are generated on foil insulator surfaces usually by tribo-effect. There is a plenty of methods that ensure electrically charged foil neutralization. We can divide them in basic types as depicted on Fig. 5. Considering the possibility of positive and negative electric charge generation on polymer foil surface, neutralizers supplied by AC 50 Hz high voltage are used. It is a corona-ionizing type of charge eliminator which belongs to active neutralizers. We examined a neutralizer of TECNEC type with supply voltage 7.3 kV supplied from built-in high voltage transformer in compact epoxide insulation. The construction of power source was adapted in order to ensure high electric strength by epoxide resin and in order to seal hermetically the system for even difficult climatic conditions. The neutralizer manufacturer applied spark free active type of neutralizer because of work safety purposes. It has capacitive separation of high voltage circuit from high voltage coronizing spike-shaped electrode (Fig. 6). The quality of surface charge neutralization ensures a monitoring device which consists of feedback connection between electric charge probe and a high voltage supply unit. The simplest method of monitoring is depicted on Fig. 7. Measuring probe is placed in defined distance from neutralizing electrode. Signal from probe is lead to electronic regulation of power source voltage, ensuring thus the effectiveness of charge elimination. Considerably higher effectiveness can be achieved by element alignment as depicted on Fig. 8. Preliminary to neutralizer lies one measuring probe which measures the level of area charge. The output voltage of high voltage source is regulated according to this output voltage. The monitoring chain ends up with check probe with regulation unit connected to high voltage power source.
Electrostatic air filtration by negative or positive unipolar discharge can be used in these areas: air cleaners for medical purposes, air cleaners for environment with micro-electronic device manipulation, and air cleaners for other environments. The research has proved that the electrons carrying negative charge and negative ions have stimulating effect on human’s body. By acute breathe passage diseases as well as in cases of cardiovascular diseases and for right brain operation of patients in hospitals it is necessary to ensure clean air with appropriate parameters. One of the ways to achieve this is microscopic dust particle separation (particle diameter smaller than 1 mm) including macroscopic (stamen or other shaped) particles. The contribution of this air cleaning method is also in air enhancement by electric charges while their concentration can be regulated by high voltage power source and by electrode system. In process of micro-electronic device production and manipulation (with very large scale of integration) extremely high hygiene criteria are required including environment purity. These requirements can be fulfilled by high voltage air filtration built-in into air-conditioning system. In this case the overproduction of
Fig. 7. Neutralizer control by one control unit.
Fig. 5. Basic neutralizer types [5].
212
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213 Table 2 Average values of efficiency for each cascade section in tube-type precipitator supplied by alternating voltage. Efficiency [%]
Powder with re ¼ 3 U m Powder with re ¼ 4 kU m Powder with re ¼ 4.8 MU m
1st section
2nd section
3rd section
4th section
83.16 96.82 93.45
70.72 81.72 93.56
73.94 83.7 97.82
62.02 72.12 95.14
Fig. 8. Neutralizer control by control and verification unit.
Fig. 9. Air cleaner construction: DC voltage (a) and AC voltage (b) [1].
electric charges with specific polarity is not important and unwanted as it is advisable to maintain a balance of charges in environment [3]. There are some other environments that require high purity of air, e.g. abovementioned factory environments which produce cover foils for devices. In the case of dusty environment the final product would be devaluated by dust particles moulded into foil. We designed and experimentally tested a channel type air cleaner supplied by DC and AC voltage with frequency 50 Hz. Due to higher effectiveness we used two and four level filter. The principle is depicted on Fig.9 [2].
In the case of DC filter polluted air enters the pipe with quadratic cross-section, in which ionizing (corona) electrodes are placed and connected to DC voltage. In defined distances grid collectors are placed. They can be removable or in the form of infinite band which is depolluted outside of the pipe. After their cleaning they return back to the pipe channel. Depending on air purity requirement 2-4-6 level filters can be constructed. Filter for AC voltage supply uses the principle of barrier in high voltage AC field. Dust particles are accumulated on removable dielectric cartridges which are removed after 8 h time operation, cleaned and returned back into the chamber. Corona electrodes have linear construction with spiral placed and perpendicularly placed electrodes. Barrier material is made of glass, PVC or other plastic material in the form of plane-parallel plates [4]. For experiments a cascade coaxial precipitator that consisted from four sections was created (Fig. 10). The PVC tube as collecting electrode is the basis of precipitator section. Thin aluminium electrode is attached on the outer side of the tube and it creates the ground electrode. Inside of tube there is a fixed thin cooper wire serving as corona electrode. Sections of cascade can contain different number of corona electrodes and they can be placed in different distances from collecting electrode. The aim of corona electrode is to create strong inhomogeneous electric field. This field can be created by electrodes with small radius of bend. If corona electrode is positive new electrons are created. If corona electrode is negative the situation is analogical with the difference that avalanches emitted from negative point are moved to more homogeneous field, as a consequence of their mobility and ability to ionize decreases. In electrical precipitator supplied by alternating voltage with metallic corona electrode insulating barrier as a collecting electrode is used. For maximal efficiency it is necessary to choose insulating materials with great value of resistance and permittivity. The choice of materials depends on another non electrical quantity e.g. temperature of flying gas, sufficient mechanical strength. Insulating barrier must not change its mechanical and dielectric properties under different temperature conditions. In order to find the efficiency of precipitator model three types of dust with different values of resistance were used. The model of
Fig. 10. Section configuration of cascade tubular precipitator supplied by alternating high voltage [4].
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
precipitator with length of 30 cm has efficiency in the range from 60% to 97% for each of dusts (Table 2). According to measured values it is possible to say that the precipitation of solid admixture from flying air at alternating voltage is comparable with precipitation at direct voltage and under some specific conditions achieves better values of efficiency.
6. Conclusion This paper showed the undesirable effect of electrostatic charge generated by tribo-effect. It causes sedimentation of dust particles on foils causing thus problems. Elimination of such effects can be achieved by neutralization. We tested for this purpose a neutralizer of static charges based on the principle of AC voltage neutralizer. For environment air purity improvement dust particles have to be eliminated. By appropriate filtration of air we can achieve high air purity. For air filtration various electro-static filters can be used. This paper also refers to possibility of application alternating voltage for precipitating macroscopic particles in unconventional electrode system: corona electrode e dielectric barrier as collector e grounded electrode. These systems use physical phenomena from high voltage field with barrier theory. Based on numerous measurements and experiments this coaxial flying dust precipitator supplied by alternating high voltage with linear coronary electrode proves to be one of the possible options for precipitation of fly-ash with great conductivity. Advantage of this system opposite the system supplied by direct voltage is the economic aspect of
213
regulation in precipitation process. This fact can be an important step to general utilization of this technology in practice. Acknowledgement The authors wish to acknowledge Scientific Grant Agency of The Ministry of Slovak Republic and Slovak Academy of Science for funding of experimental works in the frame of VEGA No. 1/0487/12 grant Research of Degradation Influences of Electrical and Thermal Fields on Electro-Physical Structure of High Voltage Insulation Materials.
We support research activities in
Slovakia. Project is co-financed from EU funds. This paper was developed within the Project “Centrum excelentnosti integrovaného výskumu a vyu zitia progresívnych materiálov a technológií v oblasti automobilovej elektroniky”, ITMS 26220120055. References [1] K. Marton, Stúdium unipolárneho korónového výboja a jeho vyu zitie v praxi, Doktorská dizerta cná práca, Kosice, 1989. [2] K. Marton, Kaskádny odlucovac prachu na striedavé napätie 50 Hz, Prihláska vynálezu PV 7263/89. [3] K. Marton, Vysokonapätový cisti c vzduchu, Prihláska vynálezu PV 7262/89. [4] J. D zmura, Nabíjatelnost a transport castíc makroskopickej velkosti v silne nehomogénnom elektrickom poli, Dizerta cná práca, Kosice, 2002. } szaki Könyv[5] T. Horváth, I. Berta, J. Pohl, Az elektrosztatikus feltöltödések, Mu kiadó, Budapest, 1984, ISBN 96310 5033 5.
Contents lists available at SciVerse ScienceDirect
Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat
Elimination of dust particle sedimentation in industry environment Karol Marton, Jozef Balogh, Jaroslav D zmura, Jaroslav Petrás* Technical University of Kosice, Faculty of Electrical Engineering and Informatics, Department of Electrical Power Engineering, Mäsiarska 74, 040 01 Kosice, Slovak Republic
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 August 2012 Received in revised form 18 December 2012 Accepted 20 December 2012 Available online 9 January 2013
This paper deals with dust particle elimination on industry plastic foils in industry environment and with technical air cleaning for this environment. We describe electro-physical processes that are in progress during electric charge neutralization. These charges arise during foil manufacturing process. We describe a method for electric parameter measurement for such foils, surface charge measurement on foils and methods of their neutralization. We describe an unconventional type of contamination separator for various technical areas. The application of negative or positive unipolar discharge is used in area of air cleaning in medical and other environments. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Neutralization Air cleaning Surface charge High voltage
1. Introduction During technological manipulation of foil which is made of polymer organic material (e.g. polypropylene, polyvinyl fluoride), creation of electrical charges was observed on surface of these foils. This phenomenon proved to be a consequence of so called “harmful” tribo-effect which is caused by electro-kinetic phenomenon occurring when two or more material surfaces touch with same or similar physicalechemical properties. It is possible that created charges, after reaching their saturation, evoke a status that interferes with technological process and invades its operation in the case of high technological hygiene and low relative humidity with insufficient technological device earth connection. Force effects of electric field between equivalently charged foil surfaces (by same charge), force effects of devices involved in manufacturing process or high potential difference between devices involved in technological process and personnel belong to this category. In such cases electric flash-arc occurs that invokes undesirable and often malicious physiological influences. The voltage generated by such processes can reach up to 8e16 kV. In cases when personnel is not actively earthed the probability of such electric discharge between the personnel and semi-product in technological process increases (material with dielectric properties).
* Corresponding author. E-mail addresses: [email protected] (K. Marton), [email protected] (J. Balogh), [email protected] (J. D zmura), [email protected] (J. Petrás). 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2012.12.026
From product quality’s point of view there is much higher probability for danger because even with very high care for clean environment it is not possible to eliminate the sedimentation of dust particles with macroscopic size on foils’ surface. These particles are electro-statically coupled to the surface by strong coulomb forces. This effect is especially undesirable during heat processing of foil at temperatures over 135 C or even 160 C, so that the dust particles with fiber, sphere or other shapes get moulded into polymer foil’s surface and they considerably deteriorate the foil’s quality in this way. We have to notice that the abovementioned planar polymer foils are not the only sources of electric charges. Another source of such charges is the quick transmission motion (circle cross-section with diameter approx. 20 mm) during transporter steel roller motion. As the earth connection of the belt is imperfect, there is a high value of accumulated charge on this “infinite” loop. This generated charge finds its way through conductive rollers to critical parts made of organic polymer foils. It is possible to avoid these unwanted effects by electrostatic charge neutralization on foil’s surface.
2. Generation of electric charges on polymer foils and moving parts Theoretical analysis of electric (and also electrostatic) charges on dielectric or insulation foil’s surface can be made in detail on the basis of physical process analysis by band models of materials in touch. Another idea for explanation of these effects leads to process
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
analysis when electric bilayer is created or when it splits. We can start from these ideas: a) splitting of two opposite oriented charge layers. This leads to charge generation on every material (foil) with both polarities. b) giving over the charges from one foil to another by touch. The basic foil loses its original charge. c) receiving of charges by foil surface, e.g. by injection from external source. The abovementioned three processes can invoke various effects (separation of touching surfaces, fracturing of material, electrostatic distribution of charges, photoionization, high voltage discharges, cumulating of charges, tribo-effect e electric-kinetics related to friction, bumps, pressures etc.) which are closely related to the structure of used material and to its electro-physical and also chemical properties. The existence of free or coupled electric charges in foils (non-polar or polar materials) plays one of the most important roles together with related polarization effects. Outgoing from technological processes during foil manufacturing we have to point out the fact that the generation and accumulation of charge occurs not only in one layered foil systems but also often with more layered foil systems. From electro-physical point of view it behaves as layered dielectrics of capacitor with capacities connected in series. If we choose one element from this complex dielectrics (one layer), we can depict a model of it and we can also write an electro-physical interpretation of this model. We have to take into account the energy balance which causes that as a consequence of tribo-effect (friction, movement) the charges depositing or been created on foil’s surface element are in balance with current i0 which flows out from this surface. This can be expressed by the equation:
dQ ¼ i0 ¼ iR þ iC dt
(1)
and schematically depicted on Fig. 1: The solution of electric circuit on Fig. 1 leads to differential equation:
dU U C$ þ ¼ i0 dt R
(2)
By calculation we get the U voltage characteristics in dependence of the t time:
t UðtÞ ¼ i0 $R$ 1 eR$C
(3)
This equation shows the Umax voltage saturation depending on time (Fig. 2) on capacitor model with capacity C and resistor R connected in parallel:
Umax ¼ i0 $R
(4)
Maximal value of charge on ReC elements will be:
Qmax ¼ i0 $R$C
(5)
when taking into account the basic equation:
Qmax ¼ C$Umax If we substitute into equation (5) real values of width h and area S of foil, we get:
h S Qmax ¼ i0 $r$ $ε0 $εr $ ¼ i0 $r$ε0 $εr S h
(6)
We have proved that the value of charge on foil directly depends on basic material properties of foil or planar insulator. This is represented by material’s specific resistivity r and by relative permittivity εr. The abovementioned calculation ignores climatic conditions of environment [5]. 3. Measurement of polymer foil’s dielectric properties Outgoing from theoretical basics which describe electrical charge generation as described in previous chapter we focused on experimental analysis of dielectric properties in applied material technologies. We measured these materials with various widths:
semitransparent polyester h ¼ 0.105 mm polyvinyl fluoride h ¼ 0.035 mm red polymer rubber h ¼ 1.84 mm red polymer rubber h ¼ 3.37 mm grey rough rubber h ¼ 2.13 mm
The knowledge of complex properties required measurements of these parameters:
Fig. 1. Elementary current draining away charge from dielectric foil to earth [5].
209
relative permittivity dielectric loss factor insulating or specific resistivity electrical strength
Fig. 2. Voltage U(t) depending on time.
210
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
Fig. 3. Material characteristics e semitransparent polyester.
Because foil or planar polymer materials were applied in praxis at low frequencies, all measurements were executed at network frequency 50 Hz. Samples were measured at thermo-electrical strain in measurement system of TETTEX-Instruments. Samples were stressed mechanically by constant weight during measurement. As measurements were made with thin foils, measured samples had “electrodes” laid in the form of paste. Semiconductor layer was created which did not reacted with base material in physical or chemical way. We avoided thus the creation of air “pillows” between extremely smooth metallic electrodes of tri-electrode system and uncovered foil. Otherwise in such air pillows electrical discharges at higher field gradient arise and disturb the measurement. Despite these arrangements undesirable case occurred which influenced dielectric loss factor. At electrical strength measurements it is advisable to immerse samples into insulation liquid with non-polar characteristics, eliminating thus surface discharges. The importance of these measurements reposes on the fact that they showed qualitative differences of used materials and their categorization into weak polarized and polarized materials. This fact is very important in terms of their charging capability. Another important aspect of these measurements was the description of obvious difference between measured materials regarding to the compactness of material. While first material samples (PP and PVF) represent compact materials (from polymer chain structure’s point of view) without air cavities and defects, materials based on rubber (red/grey rubber) have spongy structure that depends strongly on temperature, pressure and naturally on applied voltage. This depicts unstable electro-physical properties as well as different C(T)
characteristics, considerable fluctuation of tg d during temperature or electric stress (Figs. 3 and 4). In general, increase of capacity value is expected with increasing temperature which can be explained by activation of weak polar molecules in material. In porous materials with spongy structure mechanical barriers are cleared with increased temperature, material is compressed and densified by constant weight of electrode. Our hypothesis about spongy material structure and about its influence on electrical properties supports also the fact that the value of dielectric loss factor changes in non-standard way at temperatures higher than 60e70 C. At these temperatures the defective places in material filled with air or gas mixture are eliminated. A combination of insulationegas invokes conditions for ionization in gaseous environment. A simple calculation proves that in our conditions the examined insulation materials achieve ionization status at about 107 V/m. This unwanted effect can be observed also at higher voltage levels (above 0.8 kV) in case of compact materials as a consequence of surface discharge activation on electrode’s edges. It is noticeable that for all three rubber materials the dielectric power loss factor attenuates at voltage level about 500 V. We assume that the pressure of gas filling in gas closures increases the value of ignition voltage according to Paschen’s law. Electrical strength of materials was examined by standard method according to STN IEC 243-1 standard. Achieved results fulfilled the expectations and showed us that compact polymer foils have high electric strength and it is 5e13 times higher than the electric strength of rubber materials. Reduced electric strength is caused by material structure. This fact is not an obstacle for using them as separator by thermal manipulation of polypropylene foils (Table 1).
Fig. 4. Material characteristics e red polymer rubber.
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
211
Table 1 Table of electric strengths. Sample
h [mm]
Up [kV]
Ep [kV/mm]
Semitransparent polyester Polyvinyl fluoride Red polymer rubber Red polymer rubber Grey rough rubber
0.105 0.035 1.84 3.37 2.13
9.0 3.2 34.0 47.0 15.5
85.71 90.00 18.48 13.95 7.28 Fig. 6. Capacitive separation of corona electrodes in neutralizer [5].
4. Neutralization of surface charges
5. Air cleaning
Failure free technological process of polymer foil manipulation and manipulation of supporting materials can be achieved by surface electric charge elimination. These charges are generated on foil insulator surfaces usually by tribo-effect. There is a plenty of methods that ensure electrically charged foil neutralization. We can divide them in basic types as depicted on Fig. 5. Considering the possibility of positive and negative electric charge generation on polymer foil surface, neutralizers supplied by AC 50 Hz high voltage are used. It is a corona-ionizing type of charge eliminator which belongs to active neutralizers. We examined a neutralizer of TECNEC type with supply voltage 7.3 kV supplied from built-in high voltage transformer in compact epoxide insulation. The construction of power source was adapted in order to ensure high electric strength by epoxide resin and in order to seal hermetically the system for even difficult climatic conditions. The neutralizer manufacturer applied spark free active type of neutralizer because of work safety purposes. It has capacitive separation of high voltage circuit from high voltage coronizing spike-shaped electrode (Fig. 6). The quality of surface charge neutralization ensures a monitoring device which consists of feedback connection between electric charge probe and a high voltage supply unit. The simplest method of monitoring is depicted on Fig. 7. Measuring probe is placed in defined distance from neutralizing electrode. Signal from probe is lead to electronic regulation of power source voltage, ensuring thus the effectiveness of charge elimination. Considerably higher effectiveness can be achieved by element alignment as depicted on Fig. 8. Preliminary to neutralizer lies one measuring probe which measures the level of area charge. The output voltage of high voltage source is regulated according to this output voltage. The monitoring chain ends up with check probe with regulation unit connected to high voltage power source.
Electrostatic air filtration by negative or positive unipolar discharge can be used in these areas: air cleaners for medical purposes, air cleaners for environment with micro-electronic device manipulation, and air cleaners for other environments. The research has proved that the electrons carrying negative charge and negative ions have stimulating effect on human’s body. By acute breathe passage diseases as well as in cases of cardiovascular diseases and for right brain operation of patients in hospitals it is necessary to ensure clean air with appropriate parameters. One of the ways to achieve this is microscopic dust particle separation (particle diameter smaller than 1 mm) including macroscopic (stamen or other shaped) particles. The contribution of this air cleaning method is also in air enhancement by electric charges while their concentration can be regulated by high voltage power source and by electrode system. In process of micro-electronic device production and manipulation (with very large scale of integration) extremely high hygiene criteria are required including environment purity. These requirements can be fulfilled by high voltage air filtration built-in into air-conditioning system. In this case the overproduction of
Fig. 7. Neutralizer control by one control unit.
Fig. 5. Basic neutralizer types [5].
212
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213 Table 2 Average values of efficiency for each cascade section in tube-type precipitator supplied by alternating voltage. Efficiency [%]
Powder with re ¼ 3 U m Powder with re ¼ 4 kU m Powder with re ¼ 4.8 MU m
1st section
2nd section
3rd section
4th section
83.16 96.82 93.45
70.72 81.72 93.56
73.94 83.7 97.82
62.02 72.12 95.14
Fig. 8. Neutralizer control by control and verification unit.
Fig. 9. Air cleaner construction: DC voltage (a) and AC voltage (b) [1].
electric charges with specific polarity is not important and unwanted as it is advisable to maintain a balance of charges in environment [3]. There are some other environments that require high purity of air, e.g. abovementioned factory environments which produce cover foils for devices. In the case of dusty environment the final product would be devaluated by dust particles moulded into foil. We designed and experimentally tested a channel type air cleaner supplied by DC and AC voltage with frequency 50 Hz. Due to higher effectiveness we used two and four level filter. The principle is depicted on Fig.9 [2].
In the case of DC filter polluted air enters the pipe with quadratic cross-section, in which ionizing (corona) electrodes are placed and connected to DC voltage. In defined distances grid collectors are placed. They can be removable or in the form of infinite band which is depolluted outside of the pipe. After their cleaning they return back to the pipe channel. Depending on air purity requirement 2-4-6 level filters can be constructed. Filter for AC voltage supply uses the principle of barrier in high voltage AC field. Dust particles are accumulated on removable dielectric cartridges which are removed after 8 h time operation, cleaned and returned back into the chamber. Corona electrodes have linear construction with spiral placed and perpendicularly placed electrodes. Barrier material is made of glass, PVC or other plastic material in the form of plane-parallel plates [4]. For experiments a cascade coaxial precipitator that consisted from four sections was created (Fig. 10). The PVC tube as collecting electrode is the basis of precipitator section. Thin aluminium electrode is attached on the outer side of the tube and it creates the ground electrode. Inside of tube there is a fixed thin cooper wire serving as corona electrode. Sections of cascade can contain different number of corona electrodes and they can be placed in different distances from collecting electrode. The aim of corona electrode is to create strong inhomogeneous electric field. This field can be created by electrodes with small radius of bend. If corona electrode is positive new electrons are created. If corona electrode is negative the situation is analogical with the difference that avalanches emitted from negative point are moved to more homogeneous field, as a consequence of their mobility and ability to ionize decreases. In electrical precipitator supplied by alternating voltage with metallic corona electrode insulating barrier as a collecting electrode is used. For maximal efficiency it is necessary to choose insulating materials with great value of resistance and permittivity. The choice of materials depends on another non electrical quantity e.g. temperature of flying gas, sufficient mechanical strength. Insulating barrier must not change its mechanical and dielectric properties under different temperature conditions. In order to find the efficiency of precipitator model three types of dust with different values of resistance were used. The model of
Fig. 10. Section configuration of cascade tubular precipitator supplied by alternating high voltage [4].
K. Marton et al. / Journal of Electrostatics 71 (2013) 208e213
precipitator with length of 30 cm has efficiency in the range from 60% to 97% for each of dusts (Table 2). According to measured values it is possible to say that the precipitation of solid admixture from flying air at alternating voltage is comparable with precipitation at direct voltage and under some specific conditions achieves better values of efficiency.
6. Conclusion This paper showed the undesirable effect of electrostatic charge generated by tribo-effect. It causes sedimentation of dust particles on foils causing thus problems. Elimination of such effects can be achieved by neutralization. We tested for this purpose a neutralizer of static charges based on the principle of AC voltage neutralizer. For environment air purity improvement dust particles have to be eliminated. By appropriate filtration of air we can achieve high air purity. For air filtration various electro-static filters can be used. This paper also refers to possibility of application alternating voltage for precipitating macroscopic particles in unconventional electrode system: corona electrode e dielectric barrier as collector e grounded electrode. These systems use physical phenomena from high voltage field with barrier theory. Based on numerous measurements and experiments this coaxial flying dust precipitator supplied by alternating high voltage with linear coronary electrode proves to be one of the possible options for precipitation of fly-ash with great conductivity. Advantage of this system opposite the system supplied by direct voltage is the economic aspect of
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regulation in precipitation process. This fact can be an important step to general utilization of this technology in practice. Acknowledgement The authors wish to acknowledge Scientific Grant Agency of The Ministry of Slovak Republic and Slovak Academy of Science for funding of experimental works in the frame of VEGA No. 1/0487/12 grant Research of Degradation Influences of Electrical and Thermal Fields on Electro-Physical Structure of High Voltage Insulation Materials.
We support research activities in
Slovakia. Project is co-financed from EU funds. This paper was developed within the Project “Centrum excelentnosti integrovaného výskumu a vyu zitia progresívnych materiálov a technológií v oblasti automobilovej elektroniky”, ITMS 26220120055. References [1] K. Marton, Stúdium unipolárneho korónového výboja a jeho vyu zitie v praxi, Doktorská dizerta cná práca, Kosice, 1989. [2] K. Marton, Kaskádny odlucovac prachu na striedavé napätie 50 Hz, Prihláska vynálezu PV 7263/89. [3] K. Marton, Vysokonapätový cisti c vzduchu, Prihláska vynálezu PV 7262/89. [4] J. D zmura, Nabíjatelnost a transport castíc makroskopickej velkosti v silne nehomogénnom elektrickom poli, Dizerta cná práca, Kosice, 2002. } szaki Könyv[5] T. Horváth, I. Berta, J. Pohl, Az elektrosztatikus feltöltödések, Mu kiadó, Budapest, 1984, ISBN 96310 5033 5.