Microwave measuring device for defining the load of a...

Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters

Reexamination Certificate

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C324S637000, C324S639000

Reexamination Certificate

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06771080

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention, in general, relates to a microwave measuring device for defining the load of a two-phase flow comprising a gaseous carrier medium with small and minute solid particles and/or liquid particles as well as for defining the gas in a fluid flow. More particularly, the invention relates to defining the load of a gaseous flow of solids particles in a large-volume pneumatic solids transport system of the kind used, for instance, in pulverized coal furnaces of coal-fired power plants.
2. The State of the Art
It is known not only to define the particle load of two-phase flows comprising a gaseous carrier medium but also the proportion of gas contained in fluid flows, by means of microwaves. In a great many of the known systems of this kind, microwaves of a certain frequency are coupled into a portion of a feed duct prepared as a measuring conduit, and at the end of the conduit any changes in amplitude and phase of the microwave is registered. A preferred method of operation involves a waveguide fundamental wave to avoid undue complications and interferences. The physical background of the measuring principle resides in the fact that a change in the load of the carrier gas with solids and/or liquids or a change of the proportion of gas in a fluid flow leads to a change in the complex dielectric constant in the feed duct and that microwaves suffer an attenuation and phase shift as a function of this dielectric constant. German laid-open patent specification 44 26 280 A1 describes a method of determining the load of a gaseous flow with solids particles and, more particularly, for controlling the firing with pulverized coal of a boiler in a coal-fired power plant, by determining the solids content of the gaseous flow as a function of the attenuation of electromagnetic waves along a measuring conduit carrying the gaseous flow. As described in German patent 33 17 215 C1, the quantity of the particle load of exhaust gas is similarly derive from the attenuation of microwaves during their passage through the particle-bearing exhaust gas. A method is described in WO 91/05243 in which the content of oil or water in an oil-water mixture as well as the velocity thereof are determined by evaluating the attenuation and phase shift of microwaves along a measuring conduit. The methods described above are subject to significant problems because of the effect of interferences resulting from reflected microwaves or because of geometric changes in the feed duct system. With small loads in particular the microwave attenuation is so little that microwaves, like in a waveguide, are propagating over long distances and are reflected and/or diffracted at narrows, branches, curves or ends. The result is heterodyning of waves propagating to and fro and, hence, measuring results difficult to evaluate or significant distortions in the evaluation.
To avoid the effect of such disturbances, systems have been developed which utilize geometrically defined microwave resonators. A device of this kind has been described in European published patent specification 0,669,522 A2 in which a powder-mass flow in a powder-gas-mixture is measured while it is being fed through a feed duct. In this system, the microwave resonator is either mounted on the exterior of the feed duct or it circumscribes the feed duct in the manner of a cavity resonator. With the cavity resonator mounted exteriorly of the feed duct the measurement is taken of only a portion of the flow of the powder-gas-mixture. When, as is often the case in large volume feed ducts, different particle loads of the powder-gas-mixture occur over the cross-section of the feed duct, or when, sometimes, ropes of increased particle concentration have to be taken into consideration, the results of measurements derived from a cavity resonator mounted on the exterior of the feed duct may suffer from significant errors. Such errors do not occur with a cavity resonator circumscribing a feed duct. However, such resonators entail significant structural complexities and in large-volume feed ducts they cannot usually be realized for reasons of lacking space. Hence, such an arrangement is limited in its application to areas in which relatively small feed cross-sections occur such, as, for instance, in power coating apparatus.
As a further development of the system described in European published patent specification, German patent 196 50 112 C1 describes a microwave resonator which is characterized by relatively low structural complexity. In principle, the resonator consists of a cylindrical coil surrounding a non-conductive portion and shielded to the exterior by electrically conductive cylinder (helix resonator). The coil is caused to resonate by a high frequency alternating voltage (in the microwave range). By evaluating, as is well known, the shift in resonant frequency current, it is possible to define the powder mass flow. This arrangement, too, can either not be used in connection with large volume feed ducts which usually consist of an electrically conductive material (metal), or would entail significant structural complexity as a portion of the feed duct would have to be made of a non-conductive material. At the relevant structural sizes and resultant low resonant frequency the helix resonator consisting of the coil and the electrically conductive shield would be quite voluminous.
For defining the proportions of gas-oil-water-mixtures in transport pipe lines, U.S. Pat. No. 5,351,521 A describes a system for measuring large changes of the complex dielectric constant. This is accomplish by a consecutive arrangement, within the transport pipe line, of stepped continuous pipe sections of reducing diameter. Measuring electrodes are disposed within the pipe sections for measuring and evaluating, in accordance with the reducing diameter of the pipe sections, different threshold frequencies and, hence, different frequency ranges of the input microwave. The pipe sections are supported by electrically conductive rods which extend in radial patterns between the outer wall of each pipe section to the inner wall of the transport pipe line. The described arrangement of the electrically conductive rods prevent the input microwaves from passing through the space between the inner wall of the transport pipe line and the outer wall of the pipe sections. The arrangement is characterized by a relatively large measuring range. The attainable measuring accuracy is insufficient, however, for the preferred application of defining the load in gaseous flow of solids particles in large volume pneumatic solids transport systems of the kind used, for instance, in pulverized coal fired systems of coal fired power plants. Moreover, the necessary installations of the pipe sections into transport pipe lines are complex and have a significant effect upon the flow conditions within the transport pipe line.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a microwave measuring device for defining the load of a two phase flow of a gaseous carrier medium and small and minute solids and/or liquid particles.
It is a further object to provide a microwave measuring device for defining the gas contained in a fluid flow.
Moreover, it is an object of the invention to provide a microwave measuring device of the kind referred to which its high measuring accuracy notwithstanding may be cost-efficiently fabricated and which may be utilized in a broad field of applications.
A further object of the invention resides in the provision of a microwave measuring system of the kind referred to which is particularly useful in large volume feed ducts, even at low loads or small load differences.
It is yet another object of the invention, the provide a microwave measuring system of relatively simple construction and which may easily be integrated into existing feed ducts of feed duct systems.
It is another object of the invention to provide a microwave measuring system which in performing measurements always includes the entire cross-section of the fl

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