GB2117518A

GB2117518A - Method and device for detecting electrical charge of particles in a gas stream

Info

Publication number: GB2117518A
Application number: GB08209651A
Authority: GB
Inventors: Gyorgy Bucsky; Gyula Florian; Zoltan Kiss; Henrik Muller; Gyula Selyem; Lajos Tatrai
Original assignee: Magyar Tudomanyos Akademia
Current assignee: Magyar Tudomanyos Akademia
Priority date: 1982-04-01
Filing date: 1982-04-01
Publication date: 1983-10-12
Also published as: GB2117518B

Abstract

A device for and a method of detecting and, if necessary, for measuring electrical charge of particles in a gas stream, provide the possibility of measuring the concentration and stream parameters. The method consists in leading back to the stream system the charge led away for measuring. Thus the equilibrium of the space- charge in the environment of the detecting electrode is stabilized and accuracy of measurement is improved. Electrical charge can be detected in an enclosed system in which material such as granular or pulverulent materials are transported by a gas stream. The charge is generated either naturally (by collision or friction) or artificially. The device comprises a first, detecting, electrode and a second, return, electrode, both having the same area measured in directions perpendicular to the stream. The second electrode may be placed downstream of the first, or they may share a common plane perpendicular to the stream.

Description

SPECIFICATION Method of and device for detecting electrical charge of particles in a gas stream The invention relates to a method of and a device for detecting and, if necessary, for measuring electrical charge of particles in a gas stream. The method and the device according to the present invention may be advantageously used for detecting electrical charge of different materials transported by a gas stream.

When materials such as, for example, pulverized or granulated materials, are so transported an electrical charge may be generated either by a natural electrical generating process while the kinetic energy of the particle movement decreases as a result of collisions or friction, or by an artificial process.

The particles of the material being transported by a gas stream in a closed system, (e.g. in pneumatic transport equipments) will be electrostatically charged under the influence of collisions taking place while the stream is moving or as a result of friction. The material properties of the particles transported by the gas stream, of the gas, and of the wall of the system, determine in what measure the statistical average quantity charge of the particles remains proportional to the relative quantity of the material transported in the system by gas as well to the flow velocity of gas. Sometimes it is necessary for various reasons to transport particles which have been electrically charged artificially and for this purpose a gas stream is used.

In both cases the electrical charge being transported by the particles may be divided by an electrode isolated electrically from the wall, if the particles strike the electrode. The charge given by a particle or by the particles being transported in a continuous gas stream may be led away from the electrode arranged in a tube of continuous particle stream. The electrode consists of electrically conductive material arid the charge density on it i.e. the electrical current can be detected. The possibility thus exists to provide indirect measurement of the relative charge of particles or of powder. By ensuring homogenous concentration of the particles (of powder) it is possible also to measure the flow velocity.

The most interesting solution of this kind is described in Hungarian patent no. 167598. In a device according to this patent the electrode is in form of a probe that reaches into the closed space of the system to different depths and is intended to divide the charge as well to conduct a quantity of charge from the closed system.

!n this prior art solution the charge divided and led out by a simple electrode is transmitted to earth potential after using it for measurement. In that time the charge conducted from the system results in upsetting the charge-equilibrium of the system in its parts adjacent to the electrode in that the spacecharge values before and after the electrode (in the direction of the gas flow) are different. The inhomogeneity of the field intensity causes electrical induction the value of which is not proportional to the changes of the particle concentration, to the flow velocity as well to the flow picture. In consequence of this process the electrical current flowing over the electrode, i.e. the measuring signal, is disturbed and the measured values have lower reliability than expected.

An object of the invention is to create conditions for limiting electric induction on the electrode and thus to increase accuracy of measurements.

According to the invention there is a method of detecting electrical charge of particles in a gas stream, comprising the steps of dividing electrical charges in a closed stream system and leading a part of the charges away for measuring, wherein the charge led away is led back to the system.

This method results in the possibility of keeping charge equilibrium in the stream system also in time of the measurements, as well in increasing accuracy of measurements.

Using the proposed method makes it possible to ensure that at the moment of measurement the space-charge density adjacent to the electrode for detecting electrical charge is stabilized and so the measuring current has a stable value. This stable value is proportional to the particle concentration or to the flow velocity of gas. The proportionality results in increasing accuracy of measurement. A further advantage of the proposed method is that in parts of the system surrounding the electrode for detecting electrical charge, and the current generated in this way is missing in disturbing effects of the material stream in the surroundings of the electrode.

The invention further provides a device for detecting electrical charge of particles in a gas stream, comprising a first electrode for dividing and detecting electrical charges and comprising electrically conductive material, the first electrode being electrically isolated from a wall surrounding the stream and reaching into the stream, and a second electrode for leading back to stream the electrical charge led away therefrom by the first electrode, wherein the surface of the first electrode lying perpendicularly to the direction of the stream is equal to the surface of the second electrode in the same direction.

The device according to the invention comprises the two kinds of electrodes advantageously in a common plane perpendicular to the direction of the stream. It is also advantageous for the second electrode to lie behind the detecting fidrst electrode in direction of the stream.

In the device according to the invention a plurality of first and second electrodes may be advantageously used, the combined surface of which is equal in direction of the stream. The number of first electrodes and the number of second electrodes can be equal or different.

The invention is further described in more detail below in connection with an example.

In a pipeline for transporting a powder mixture a detecting electrode is arranged which extends to the inner part of the pipeline. This electrode is connected to a measuring resistor, and results in dividing electric charge of the powder mixture. The electric charge arises, for example, as a result of collision between the particles of powder. This process results in decreasing the kinetic energy of the mixture and converting this energy into electrical energy. So viewed in the direction of the stream a second electrode lies behind or beside the detecting first electrode. The surface of the second electrode lying perpendicular to the direction of the stream is equal with the similar surface of the detecting first electrode. The second electrode is connected to the measuring resistor at a point on it with required potential.The measuring resistor is connected from this point to the second electrode and also to earth.

Through the above mentioned point a quantity of charge can be led back to the second electrode which is equal to quantity led away from the system by the detecting electrode. In this way a charge quantity will be led back to the system which is necessary to restore the charge equilibrium. The charge led back results in space-charge equilibrium in the region of the detecting electrode and the whole measuring system appears to the outside to be free of electrical charge.

It may be suitable, e.g. in case of a pipeline of large dimensions, to use a plurality of detecting and second electrodes. In these systems the surfaces of the two kinds of the electrodes perpendicular to the direction of the stream should be equal. The second electrodes should be placed beside or behind the corresponding detecting electrodes.

The advantages of the method and device according to the invention can be summarised as follows: (a) The system generates a measuring signal of stable value.

(b) The measurement is characterised by high accuracy.

(c) Because the complete measuring system seems to outside to be free of electrical charge, the invention may be used for detecting electrical charge even in the case of explosive powder mixtures.

Claims

1. A method of detecting electrical charge of particles in a gas stream, comprising the steps of dividing electrical charges in a closed stream system and leading a part of the charges away for measuring, wherein the charge led away is led back to the system.

2. A device for detecting electrical charge of particles in a gas stream, comprising a first electrode for dividing and detecting electrical charges and comprising electrically conductive material, the first electrode being electrically isolated from a wall surrounding the stream and reaching into the stream, and a second electrode for leading back to stream the electrical charge led away therefrom by the first electrode, wherein the surface of the first electrode lying perpendicular to the direction of the stream is equal to the surface of the second electrode in the same direction.

3. A device according to claim 2, wherein the first and second electrodes are arranged in the same plane defined perpendicular to the direction of the gas stream.

4. A device according to claim 2, wherein the second electrode lies behind the first electrode in the direction of the gas stream.

5. A device according to any one of claims 2 to 4, wherein a plurality of first and second electrodes is arranged, and the combined surface of the first electrodes perpendicular to the direction of the gas stream is equal to the combined surface of the second electrodes perpendicular to the direction of the gas stream.

6. A device according to claim 5, wherein the number of first electrodes is equal to number of second electrodes.

7. A device according to claim 5, wherein the number of first electrodes is different from the number of second electrodes.