This invention relates generally to systems for controlling the voltage of electrostatic filters, and more particularly, to a system wherein the filter voltage is decreased by a predetermined amount after a voltage breakdown, the filter voltage being increased in accordance with a predetermined voltage-time characteristic until a further voltage breakdown occurs.
Several known systems for controlling the voltage of an electrofilter are described in Siemens-Zeitschrift, 1971, No. 9, pages 567-572. It is known from the prior art that the effectiveness of an electrofilter in removing particulate matter from a flowing gas increases approximately with the square of the applied DC filter voltage. Accordingly, it is desirable to maintain as a high a DC filter voltage as possible. The filter voltage, however, is limited to a maximum value which corresponds with the dielectric strength of the gas which is being purified. Since the precise composition of the gas and the cleanliness of the electrofilter are continually varying, the breakdown voltage of the electrofilter must be determined empircally. Accordingly, only by causing voltage breakdowns will one know the magnitude of the voltage at which they will occur for a particular system at a particular time. In order to achieve such a sampling of the breakdown voltage limit of the electrofilter, without unduly inhibiting the gas purification function of the electrofilter, the transmission of electrical energy to the filter is discontinued immediately after a voltage breakdown. After a short pause to allow deionization, the filter voltage is quickly raised in accordance with a predetermined voltage-time function to a new level which is lower than the most recently sampled breakdown voltage by a small amount. From this value, the voltage is raised further accordingly to a somewhat slower voltage-time function until a new voltage breakdown occurs. The foregoing sequence is repeated after each such voltage breakdown. As is evident from the foregoing, the frequency of voltage breakdowns depends upon the difference in voltage between the most recent voltage breakdown and the new magnitude to which the voltage is quickly raised, and the voltage gradient of the slower voltage-time function which controls the rate at which the voltage is raised to sample a further voltage breakdown.
In addition to the foregoing, consideration should also be given to the magnitude of the current which flows through the filter because, in many cases, the electrical resistance of the dust in the gas will permit the current rating of the filter to be exceeded before a voltage breakdown occurs. Accordingly, the automatic control system must limit the filter current, the maximum current limit being advantageously adjustable in accordance with operating conditions.
In known control systems for electrical filters, the magnitudes of the breakdown voltage and the maximum current are initially established when the filter is placed in operation, and are not subsequently varied. However, since the operating parameters of a filter must vary in accordance with the overall conditions at an installation in which the electrofilter is operated, it is evident that fixed, predetermined values of the voltage and current values will not lead to the optimization of filter operation. For example, under some circumstances, excessive power is conducted to the electrofilter for a given dust loading of the purified gas.
It is, therefore an object of this invention to provide an electrofilter control system wherein filter operation is optimized in response to changing conditions in an overall installation.