Measuring and testing – Gas analysis – Solid content of gas
Reexamination Certificate
1995-11-22
2001-02-27
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Solid content of gas
C324S454000
Reexamination Certificate
active
06192740
ABSTRACT:
This invention relates to a method and apparatus for monitoring particles.
The invention particularly relates to an arrangement in which a probe projecting into a flow of particles is charged triboelectrically by flowing particles colliding with the probe.
WO 86/02454 describes an apparatus for monitoring particles in a gas flow through a conduit. A metal probe is installed in a flow of gas containing solid particles and the probe is coupled to an electric circuit containing processing means. The probe is charged triboelectrically by the particles colliding with the probe and the resulting current in the circuit is processed to give an output that gives a measure of the flow rate of the particles.
U.S. Pat. No. 5,054,325 shows an apparatus for measurement of fluid flows with suspended solid particles, using a triboelectric probe embedded in the wall of the conduit through which the fluid flows, where the fluid flow is a liquid or gas.
U.S. Pat. No. 5,591,895 the content of which is incorporated herein by reference also describes an arrangement for monitoring particles in a gas flow. An electrically conducting rod is mounted in a stack and is coupled to a processing circuit. The rod is charged triboelectrically by the particles in the gas flow and the signal generated in the circuit is evaluated to give an output giving an indication of the particle flow. The rod is insulated at the point where it is mounted in the stack wall to prevent currents being transmitted to and from the stack wall, but of course the insulating material does not extend over the whole outer surface of the conducting rod.
In the arrangements described above, a probe extends into the fluid flow and obstructs the flow of the particles. It has been found that if the probe is mounted in a duct wall or the like, particles can build up in the region between the electrically conducting probe and the duct wall and, especially in damp conditions, form an electrically conducting path between the probe and the duct wall.
As a result, the charge transferred to the probe by the particles in the flow can pass through the built-up material and through the duct wall to earth. Thus the output of the processing circuit may not give an accurate measure of the particle flow.
In addition, small currents that exist in the duct wall can be transmitted via the probe into the processing circuit connected to the probe. The magnitude of the currents generated by the triboelectrical charging of the probe in the circuit may be of the same order as those that exist in the duct wall and so the output of the circuit may not give an accurate measure of the particle flow.
A further problem is that the metal probe and the duct wall may begin to operate as a battery, introducing more undesirable currents into the processing circuit, again giving an inaccurate measure of the particle flow.
It is an object of the invention to provide an improved method and apparatus for detecting particles in a flow that avoids or mitigates the above problems and gives a reliable indication of the particle flow.
Accordingly, the present invention provides a method of detecting particles in a flow in which a probe is positioned so that it projects into the flow of particles and is charged triboelectrically by the particles in the flow and a signal from the probe is evaluated to provide an indication of the particle flow, characterised in that the part of the probe that projects into the particle flow comprises an electrically conducting core covered with an insulating layer which insulates the core from the particle flow.
Advantageously the A.C. component of the signal is evaluated to provide an indication of the particle flow. Although the A.c. component of the signal generated in the circuit by triboelectrical charging of the probe is small when compared with the D.C. component, it has been found that the A.C. component of the signal gives a more accurate reflection of the particle flow than the absolute value of the signal. It is believed that factors such as humidity, electrical charges already on the particles and a build-up of particles on the probe all affect the absolute value of the current without affecting the alternating component of the current as much. We have found furthermore that the combination of providing an insulated probe and evaluating the A.C. component is especially advantageous because the use of A.C. is especially suited to the case where the probe is insulated.
Preferably the alternating component of the signal from the probe is filtered to limit the frequency to below about 5 Hz. The frequency may be limited to below 2 Hz, preferably about 1.5 Hz. By eliminating higher frequencies the risk of spurious signals derived from mechanical vibration of the probe is substantially reduced since the resonant frequency of such vibration is likely to be substantially higher than 5 Hz.
Preferably the alternating component of the signal from the probe is filtered to limit the frequency of the signal to above about 0.1 Hz, preferably about 0.15 Hz. By eliminating lower frequencies the risk of spurious signals derived from transient temperature-generated voltages is substantially reduced.
Preferably the alternating component of the signal from the probe is amplified in a plurality of stages. In that case low frequencies, which may be those below 0.15 Hz, are preferably attenuated at the first stage of amplification.
The particles may be suspended in a fluid flow. The fluid flow may be either a gas or a non-electrically conducting liquid and the particles may be either liquid or solid particles.
The flow may be a gas flow through a stack with suspended particles that are emitted through the stack.
The flow may be through a duct having a probe mounted in the duct.
The present invention also provides an apparatus for detecting particles in a flow comprising a probe to be positioned so that it projects into the flow to be charged triboelectrically by the particles in the flow, and an electric circuit coupled to the probe having evaluating means for monitoring a signal from the probe and for providing an output in dependence on the signal generated by the triboelectric charging of the probe, characterised in that the part of the probe to project into the particle flow comprises an electrically conducting core covered with an insulating layer which insulates the core from the particle flow.
Advantageously, the electric circuit comprises evaluating means for monitoring the A.C. component of the signal from the probe. The probe may be in the form of a rod. The rod probe may be of circular cross-section.
It is, of course, entirely unconventional to use an insulated probe to monitor triboelectrical charging but we have found the use of such a probe to be surprisingly effective in the present invention, especially in terms of overcoming the problems referred to above.
In the prior art particle monitoring arrangements, a conducting rod probe is mounted in a duct wall with the electrically conducting surface of the probe exposed to the gas flow and the probe is coupled to a processing circuit. It is believed that a current is generated by the rod probe in the following ways:
(1) When a particle collides with the probe there is a “rubbing” of the particle against the probe leading to direct triboelectric charging.
(2) Particles in the flow may become charged by collisions with other particles. When a charged particle collides with the conducting probe, the particle gives up some or all of its charge to the probe. The particle may be charged positively or negatively and the current generated will vary accordingly.
(3) A charged particle in the flow passing the probe may, even though it does not touch the probe, induce a charge in the probe which causes a current to flow.
In the case of the present invention it is believed that currents are usually generated as a result of all three of the effects described above although precisely what happens is not fully understood. It is believed that the probe and evaluating means of the present invention may be likened in
Rigby Michael
Thomas Victor Frederick
PCME Limited
Pitney Hardin Kipp & Szuch LLP
Politzer Jay L.
Williams Hezron
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