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A new apparatus for ignition tests with brush discharges
Journal ot
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 383-388
A new apparatus for ignition tests with brush discharges K. Schwenzfeuer and M.Glor Ciba-Geigy Ltd., CH-4002 Basle, Switzerland
Abstract To study the ignition behaviour of brush discharges a new experimental approach was chosen. The main features of the new experimental set up are the following: Brush discharges are generated by the pneumatic transport of polymeric granules through insulating pipes. The spherical electrode is placed close to the charged surface on the outside of the pneumatic transport tube and the occurrence &brush discharges is measured with a computer controlled RC-circuit. In the present work the pneumatic transport system is tested related to the quantity of produced charge.
1. Introduction The values of the minimum ignition energy of dusts, as determined with spark discharges from purely capacitive circuits became lower and lower during the last decades reaching in some cases the range of gases and vapours [1]. The equivalent energy of brush discharges for the ignition of flammable gases has been determined in earlier work, for example by Gibson and Gior [2, 3], as 1 to 4 nO. The minimum ignition energy of some dusts is thus in the energy range of brush discharges, as Glor already pointed out earlier [4]. This fact led to the set up of a test equipment [5]. Brush discharges were generated by rubbing a rotating polyethylene disc with a cat fur and by approaching an electrode to the charged disc. Ignition tests with the brush discharges between this electrode and the rotating polyethylene disc have not been successful. Therefore ignition tests with so called "deviated" brush discharges in a spatially separated ignition tube have been performed. They were partially successful [6]. These test results led to an extensive debate about the incendiary of brush discharges and about the question whether there is any differerlce in the temporal and spatial distribution of energy between the generated brush discharge and the following discharge in the ignition tube. And if there is any difference, whether than the equivalent energy would still be the same.
Since it is very difficult to answer the last question on a theoretical basis even if it would be possible to measure the temporal and spatial energy distribution it was decided to choose a new experimental approach. Brush discharges are generated by the pneumatic transport of polymeric granules through insulating pipes. The most important improvement compared to the old set up is the feature that the brush discharges and the ignition tests will no longer be disturbed by the high turbulence due to the fast rotating disc. 0304-3886/97/$17.00 © Elsevier Science B.V. All rights reserved. S0304-3886(97)00075-2
PII
384
K. Schwenzfeuer, M. Glor /Journal of Electrostatics 40&41 (1997) 383-388
2. Measurement principle and experimental arrangement To generate the brush discharges a charge distribution on an insulating surface is necessary. The insulating surface was charged by the impact of polymeric granules transported in a pneumatic transport system. The pneumatic transport system was set up as a closed circuit of insulating pipes. Highly insulating polymeric granules were blown with a fan through these pipes. The schematic experimental arrangement is shown in figure 1. The experimental set-up was placed in a climate chamber with 20 % relative humidity.
ignition collision chamber spherical / c h a m b e r , ~ ~ electrode 1---1 -/1 ~"~R \ l ^ l @1 filling
compressed air m::zh~ n °°mR
Figure 1. Schematic experimental arrangement of the pneumatic transport system. 1) powder to be ignited 2) powder for charge build up
charged surface
The details of the pneumatic transport system are the following: The fan produces a volume stream of 21 m3/min. The pipes are made by PVC and have a diameter of 100 ram. The insulating surface which must be charged was originally made by polyethylene with a thickness of 0.6 mm. This surface is placed on one side of a rectangular box in such a way, that the stream of granules hits the surface. From the beginning of the construction it was planned, that this surface will be removable. This allows to change the material of the surface to be charged, The dimensions of the box are height and width 250 mm and depth 200 mm. Adjacent to the chargeable surface a second box is placed with nearly the same dimensions. This box is made by transparent plastic and contains a spherical electrode. Between the electrode and the charged surface the brush discharges should occur. A mushroom-shaped nozzle is placed on the bottom of the transparent box. It allows to generate a nearly homogeneous dust cloud with the help of a pulsed low pressure air stream. The transparent box is used as the ignition chamber of the experimental set-up. The dimensions over all are approximately 1200 mm in height and 1000 mm in width and 700 mm in depth. The amount of polymeric granules inside the pneumatic transport system varies between 100 g and 1000 g The density of the granules is about 518 g/l. The technique to measure the charge transfer during a brush discharge is still the same as reported in previous papers [5, 6]. An RC-circuit is installed on the spherical electrode and the voltage of the charged capacitor is measured with an oscilloscope. With a capacitance of 25 nF and a resistance of 10 k~, the RC-circuit has a time constant of 0.25 ms.
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
385
3. Measurement of the charge build-up The first experiment was designed to measure the charge transfer signals from the spherical electrode. Though the registration set-up was the same as with the rotating disc [5, 6] and three different materials (polyethylene, polycarbonate, polypropylene) for the chargeable surface have been used, no strong brush discharges could be produced. Therefore it was decided to study more in detail the charge build-up on the interior surface hit by the polymeric granules. For this purpose a metallic plate was mounted on top of the insulating surface inside the pneumatic system and connected to a capacitor of 10 nF and to a static voltmeter. With this system the amount of charge generated by the impact of the granules could be measured. Figure 2 shows a typical curve of charge increase with time. In addition to a continuous pneumatic transport the system was also used with a pulsed pneumatic transport by switching the fan on and off. In Figure 3 a typical curve for a pulsed pneumatic transport is shown. Figure 2. Charge accumulated on the metallic sheet as a function of time during the continuous pneumatic transport. The total amount of the polymeric granules circulated was about 500 g.
charge [pC]
4 3 2 1 0 -1 0
i
i
i
J
20
i
i
40
i
60
i
i
80
i
i
100
120
time [s]
Figure 3. Charge accumulated on the metallic sheet as a function of time during the pulsed pneumatic transport. The total amount of the polymeric granules circulated was about 500 g.
2.4 2.0 1.6 1.2
0.8 0.4 0.0 ,I
0
i
20
i
t
40
i
i
60
i
i
80
i
i
100
i
120
time Is]*
These measurements have been performed with different mass flow rates in the pneumatic transport system. Depending on the amount of granules the transport can be classified into two different types. With a low amount of polymeric granules (between 100g and approximately 600 g) the granules were well dispersed in the pneumatic transport system.
386
K. Schwenzfeuer, M. Glor /Journal of Electrostatics 40&41 (1997) 383-388
Above this value a certain part of the polymeric granules was slowly moving on the bottom of the pipe. Above 1200 g the granules were no longer dispersed in the system. Depending to these different types of transport the charging rate varied with the amount of polymeric granules inside the pneumatic transport system as shown in figure 4 and 5 for a continuous and a pulsed pneumatic transport.
Figure 4. Charging rate of the metallic plate as a function of the amount of the polymeric granules in the pneumatic flow during continuous pneumatic transport.
80 70 60 e
50 40
e-
30 20 10 0 0
200
400
600
800
1000
t200
mass [g]
Figure 5. Peak charging rate of the metallic plate as a function of the amount of the polymeric granules in the pneumatic flow during a pulsed pneumatic transport (duration of the peak current is about 150ms, see fig. 3)
2200 2000 1800 1600 1400 1200 1000 800 600 400 0
200
400
600
800
1000
1200
mass [g]
4. Discharges across a spark gap
The experiments with the charge build-up on the metal plate were performed at low potential of the metal plate (additional capacitor of high capacitance). In order to test whether charge build-up continues at high potentials the metallic plate without the additional capacitor
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
387
was connected to a spark gap with a width of 5 mm and the charge transfer was observed with the same measuring technique as described for the spherical electrode [5, 6]. The quantity of polymeric granules was about 500 g. In figure 6 the frequency distribution of the measured charge transfer is shown.
Figure 6. Measurement of the charge transfer. 109 Measurements correspond to 100% in each case. The mass of the polymeric granules was about 500 g.
50
4o
~,
3o
•
2o
U-
lO
80
100 120 140 160 180 200 220 240 260 280 charge [nC]
5. D i s c u s s i o n of results and conclusions
This experiment was build in order to get brush discharges from an insulating surface in such a way, that the discharges can be compared with the results in earlier work [5, 6]. The pneumatic transport system was chosen, because it is a more realistic system compared with a rotating polyethylene disk used in the earlier work [5, 6]. The brush discharges should be used for ignition tests of dusts but in the current state of the experiment the quantity of charge build-up on the insulating surface is too low. If the insulating surface is replaced by a metallic surface the following results are obtained: - T h e highest charging rate during the continuous pneumatic transport occurs with the polymeric granules fully dispersed in the pneumatic transport system. If the amount of polymeric granules does not allow this transport type the charging rate decreases. - The charging current amounts to 45 nA and would be able to produce a brush discharge of 300 nC every 7 seconds. - Charge build-up on the metallic plate continues at least up to a potential of about 6 kV (break down voltage of the spark gap). The poor charge build-up in case of the insulating surface needs further investigations. Two aspects may strongly influence the observed results: The combination of materials tested so far may not result into optimal tribocharging, or the formation of a double layer of charges across the insulating plate could have prevented the build-up of a strong external field. Independent on the scientific background, the results at least indicate that the reproducible generation of high energy brush discharges is not easy under realistic conditions prevailing in industrial practice.
388
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
References 1.
2. 3. 4.
5. 6.
R.Siwek, Ch.Cesan6 Process Safety Progress, 14 (1995) 107-119 N.Gibson, F.C.Lloyd, British Journal of Appl. Phys., 16 (1965) 1619 M.Glor, Journal of Electrostatics, 10 (1981) 327-332 M.Glor, Journal of Electrostatics, 16 (1984) 175-191 K.Schwenzfeuer, M.Glor, Journal &Electrostatics, 30 (1993) 115-122 K.Schwenzfeuer, M.Glor, Inst. Phys. Conf. Ser., No 143 (1995) 125-128
ELECTROSTATICS ELSEVIER
Journal of Electrostatics 40&41 (1997) 383-388
A new apparatus for ignition tests with brush discharges K. Schwenzfeuer and M.Glor Ciba-Geigy Ltd., CH-4002 Basle, Switzerland
Abstract To study the ignition behaviour of brush discharges a new experimental approach was chosen. The main features of the new experimental set up are the following: Brush discharges are generated by the pneumatic transport of polymeric granules through insulating pipes. The spherical electrode is placed close to the charged surface on the outside of the pneumatic transport tube and the occurrence &brush discharges is measured with a computer controlled RC-circuit. In the present work the pneumatic transport system is tested related to the quantity of produced charge.
1. Introduction The values of the minimum ignition energy of dusts, as determined with spark discharges from purely capacitive circuits became lower and lower during the last decades reaching in some cases the range of gases and vapours [1]. The equivalent energy of brush discharges for the ignition of flammable gases has been determined in earlier work, for example by Gibson and Gior [2, 3], as 1 to 4 nO. The minimum ignition energy of some dusts is thus in the energy range of brush discharges, as Glor already pointed out earlier [4]. This fact led to the set up of a test equipment [5]. Brush discharges were generated by rubbing a rotating polyethylene disc with a cat fur and by approaching an electrode to the charged disc. Ignition tests with the brush discharges between this electrode and the rotating polyethylene disc have not been successful. Therefore ignition tests with so called "deviated" brush discharges in a spatially separated ignition tube have been performed. They were partially successful [6]. These test results led to an extensive debate about the incendiary of brush discharges and about the question whether there is any differerlce in the temporal and spatial distribution of energy between the generated brush discharge and the following discharge in the ignition tube. And if there is any difference, whether than the equivalent energy would still be the same.
Since it is very difficult to answer the last question on a theoretical basis even if it would be possible to measure the temporal and spatial energy distribution it was decided to choose a new experimental approach. Brush discharges are generated by the pneumatic transport of polymeric granules through insulating pipes. The most important improvement compared to the old set up is the feature that the brush discharges and the ignition tests will no longer be disturbed by the high turbulence due to the fast rotating disc. 0304-3886/97/$17.00 © Elsevier Science B.V. All rights reserved. S0304-3886(97)00075-2
PII
384
K. Schwenzfeuer, M. Glor /Journal of Electrostatics 40&41 (1997) 383-388
2. Measurement principle and experimental arrangement To generate the brush discharges a charge distribution on an insulating surface is necessary. The insulating surface was charged by the impact of polymeric granules transported in a pneumatic transport system. The pneumatic transport system was set up as a closed circuit of insulating pipes. Highly insulating polymeric granules were blown with a fan through these pipes. The schematic experimental arrangement is shown in figure 1. The experimental set-up was placed in a climate chamber with 20 % relative humidity.
ignition collision chamber spherical / c h a m b e r , ~ ~ electrode 1---1 -/1 ~"~R \ l ^ l @1 filling
compressed air m::zh~ n °°mR
Figure 1. Schematic experimental arrangement of the pneumatic transport system. 1) powder to be ignited 2) powder for charge build up
charged surface
The details of the pneumatic transport system are the following: The fan produces a volume stream of 21 m3/min. The pipes are made by PVC and have a diameter of 100 ram. The insulating surface which must be charged was originally made by polyethylene with a thickness of 0.6 mm. This surface is placed on one side of a rectangular box in such a way, that the stream of granules hits the surface. From the beginning of the construction it was planned, that this surface will be removable. This allows to change the material of the surface to be charged, The dimensions of the box are height and width 250 mm and depth 200 mm. Adjacent to the chargeable surface a second box is placed with nearly the same dimensions. This box is made by transparent plastic and contains a spherical electrode. Between the electrode and the charged surface the brush discharges should occur. A mushroom-shaped nozzle is placed on the bottom of the transparent box. It allows to generate a nearly homogeneous dust cloud with the help of a pulsed low pressure air stream. The transparent box is used as the ignition chamber of the experimental set-up. The dimensions over all are approximately 1200 mm in height and 1000 mm in width and 700 mm in depth. The amount of polymeric granules inside the pneumatic transport system varies between 100 g and 1000 g The density of the granules is about 518 g/l. The technique to measure the charge transfer during a brush discharge is still the same as reported in previous papers [5, 6]. An RC-circuit is installed on the spherical electrode and the voltage of the charged capacitor is measured with an oscilloscope. With a capacitance of 25 nF and a resistance of 10 k~, the RC-circuit has a time constant of 0.25 ms.
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
385
3. Measurement of the charge build-up The first experiment was designed to measure the charge transfer signals from the spherical electrode. Though the registration set-up was the same as with the rotating disc [5, 6] and three different materials (polyethylene, polycarbonate, polypropylene) for the chargeable surface have been used, no strong brush discharges could be produced. Therefore it was decided to study more in detail the charge build-up on the interior surface hit by the polymeric granules. For this purpose a metallic plate was mounted on top of the insulating surface inside the pneumatic system and connected to a capacitor of 10 nF and to a static voltmeter. With this system the amount of charge generated by the impact of the granules could be measured. Figure 2 shows a typical curve of charge increase with time. In addition to a continuous pneumatic transport the system was also used with a pulsed pneumatic transport by switching the fan on and off. In Figure 3 a typical curve for a pulsed pneumatic transport is shown. Figure 2. Charge accumulated on the metallic sheet as a function of time during the continuous pneumatic transport. The total amount of the polymeric granules circulated was about 500 g.
charge [pC]
4 3 2 1 0 -1 0
i
i
i
J
20
i
i
40
i
60
i
i
80
i
i
100
120
time [s]
Figure 3. Charge accumulated on the metallic sheet as a function of time during the pulsed pneumatic transport. The total amount of the polymeric granules circulated was about 500 g.
2.4 2.0 1.6 1.2
0.8 0.4 0.0 ,I
0
i
20
i
t
40
i
i
60
i
i
80
i
i
100
i
120
time Is]*
These measurements have been performed with different mass flow rates in the pneumatic transport system. Depending on the amount of granules the transport can be classified into two different types. With a low amount of polymeric granules (between 100g and approximately 600 g) the granules were well dispersed in the pneumatic transport system.
386
K. Schwenzfeuer, M. Glor /Journal of Electrostatics 40&41 (1997) 383-388
Above this value a certain part of the polymeric granules was slowly moving on the bottom of the pipe. Above 1200 g the granules were no longer dispersed in the system. Depending to these different types of transport the charging rate varied with the amount of polymeric granules inside the pneumatic transport system as shown in figure 4 and 5 for a continuous and a pulsed pneumatic transport.
Figure 4. Charging rate of the metallic plate as a function of the amount of the polymeric granules in the pneumatic flow during continuous pneumatic transport.
80 70 60 e
50 40
e-
30 20 10 0 0
200
400
600
800
1000
t200
mass [g]
Figure 5. Peak charging rate of the metallic plate as a function of the amount of the polymeric granules in the pneumatic flow during a pulsed pneumatic transport (duration of the peak current is about 150ms, see fig. 3)
2200 2000 1800 1600 1400 1200 1000 800 600 400 0
200
400
600
800
1000
1200
mass [g]
4. Discharges across a spark gap
The experiments with the charge build-up on the metal plate were performed at low potential of the metal plate (additional capacitor of high capacitance). In order to test whether charge build-up continues at high potentials the metallic plate without the additional capacitor
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
387
was connected to a spark gap with a width of 5 mm and the charge transfer was observed with the same measuring technique as described for the spherical electrode [5, 6]. The quantity of polymeric granules was about 500 g. In figure 6 the frequency distribution of the measured charge transfer is shown.
Figure 6. Measurement of the charge transfer. 109 Measurements correspond to 100% in each case. The mass of the polymeric granules was about 500 g.
50
4o
~,
3o
•
2o
U-
lO
80
100 120 140 160 180 200 220 240 260 280 charge [nC]
5. D i s c u s s i o n of results and conclusions
This experiment was build in order to get brush discharges from an insulating surface in such a way, that the discharges can be compared with the results in earlier work [5, 6]. The pneumatic transport system was chosen, because it is a more realistic system compared with a rotating polyethylene disk used in the earlier work [5, 6]. The brush discharges should be used for ignition tests of dusts but in the current state of the experiment the quantity of charge build-up on the insulating surface is too low. If the insulating surface is replaced by a metallic surface the following results are obtained: - T h e highest charging rate during the continuous pneumatic transport occurs with the polymeric granules fully dispersed in the pneumatic transport system. If the amount of polymeric granules does not allow this transport type the charging rate decreases. - The charging current amounts to 45 nA and would be able to produce a brush discharge of 300 nC every 7 seconds. - Charge build-up on the metallic plate continues at least up to a potential of about 6 kV (break down voltage of the spark gap). The poor charge build-up in case of the insulating surface needs further investigations. Two aspects may strongly influence the observed results: The combination of materials tested so far may not result into optimal tribocharging, or the formation of a double layer of charges across the insulating plate could have prevented the build-up of a strong external field. Independent on the scientific background, the results at least indicate that the reproducible generation of high energy brush discharges is not easy under realistic conditions prevailing in industrial practice.
388
K. Schwenzfeuer, M. Glor/Journal of Electrostatics 40&41 (1997) 383-388
References 1.
2. 3. 4.
5. 6.
R.Siwek, Ch.Cesan6 Process Safety Progress, 14 (1995) 107-119 N.Gibson, F.C.Lloyd, British Journal of Appl. Phys., 16 (1965) 1619 M.Glor, Journal of Electrostatics, 10 (1981) 327-332 M.Glor, Journal of Electrostatics, 16 (1984) 175-191 K.Schwenzfeuer, M.Glor, Journal &Electrostatics, 30 (1993) 115-122 K.Schwenzfeuer, M.Glor, Inst. Phys. Conf. Ser., No 143 (1995) 125-128