US2864457A - Flashover elimination in precipitator - Google Patents

Description

Patented Dec. 16, 1958 ice ' FLASHOVER ELIMINATION IN PRECIPITATOR Application April 30, 1956, Serial No. 581,495

3 Claims. (Cl. 183-7) This invention relates to electrostatic precipitators and particularly to means for controlling the current supply device for the precipitator to suppress flashovers.

The present invention contemplates the suppression of flashovers that occur between the precipitator electrodes and ground by periodically cutting 01f the electrode charging current for very short time intervals. When applied to precipitators where the gas flows at high velocity, such as at the rate around 40 feet per second, the momentary cutting oil? of the ionizing current serves to minimize flashovers by cutting down the time period during which they might form. These intervals of current interruption are so short that the normal ionizing is sustained in the precipitator by the electrostatic capacity of the electrodes and the current supply cables.

The invention will be best understood upon consideration of the following detailed. description of an illustrative embodiment thereof when read in conjunction with the accompanying drawing in which:

Figure 1 is a schematic view of a gas cleaning system and power supply embodying the present invention.

Figure 2a, 2b, 2c and 2d are graphs illustrating current conditions that obtain in a precipitator in which the control means of the present invention is embodied.

Referring now more particularly to the precipitator shown diagrammatically in the upper part of Figure 1, the reference character indicates a duct delivering gases containing impurities from a furnace or other apparatus and 11 is a discharge duct for carrying away the cleaned gases. Between ducts 10 and 11 a stationary precipitator housing 12 is located. The housing 12 is divided interiorly into a series of sector like compartments of which four designated 13, 14, 15, 16 appear in Figure 1. The collecting surface is provided by a plate structure forming in each compartment a bank comprising a multiplicity of open ended gas channels which may be of hexagonal cross section. Each of the gas channels is traversed longitudinally by a centrally located electrode 18. Current is supplied to each compartment through individual feeders designated 20, 21, 22, 23 in Figure 1, so that the electrodes 18 may be electrically charged.

The housing 12 includes at its bottom a chamber 30 to which the gas inlet 10 leads and communicating through an annular opening with the space in which the banks of electrodes and collecting surfaces respectively are located. Within this chamber there is located a rotatably mounted hopper 32 projecting at its lower end through a suitable sealed opening 33 in the casing structure, being carried by a suitable bearing and having an external discharge outlet 34.

At the upper end of the shell structure a rotatably mounted casing 35 is provided, which comprises a sectorshaped wing providing a. chamber that encloses a cleaning element in the form of a pipe 38 rotatable along with the casing 35 by means of the suitable gearing. Pipe 38 is connected to a source of high pressure fluid, such as hot air. The casing 35 operates to isolate acompartment or bank of electrodes to be cleaned from the compartments through which gas is flowing; in the same manner the hopper effects this isolation at the lower end of the apparatus. It will be evident that as the hopper 32 and casing 35 are rotated electrode banks in different compartments can be successively cleaned without interruption to the gas flow through the apparatus.

Precipitator structor of this type is more fully described in the patent to Per Hilmer Karlsson, No. 2,582,133, dated January 8, 1952.

A single power pack 40 is provided to supply current to all of the electrode banks comprising the precipitator through a line 41 connecting with the individual power control devices for each electrode bank. The power transmitting or control devices include electronic tubes 36 to which a preset negative biasing voltage is applied through a line 42. Line 41 is connected to the filaments 37 of the individual tubes to which the biasing voltage is applied through the secondary winding 43 of the impulse transformer 44 to the control grid 38 of the tube. This bias voltage determines the current carrying capacity of the transmitting tube 36 which thereby serves to regulate the voltage applied through the line 41 and leads 20, 23, etc., to the various electrode banks.

The manner in which transmitter tubes of the type described above serve to regulate the voltage to the precipitator electrodes of the various banks of the precipitator is more completely disclosed in my co-pending application Serial No. 522,479, filed July 18, 1955, now Patent 2,817,412, dated December 24, 1957.

For the purposes of the present invention it may be assumed that the variations of the current biasing voltage directly affects the charging urrent supplied through the lines 20 and 23 to the related banks of electrodes. The present invention contemplates the continual interruption of the supply of current in order to prevent the formation of flashovers and for this purpose an impulse producer is utilized to vary the biasing voltage applied to the currents of the transmitter tubes. A single impulse producer may be utilized for the transmitter tubes of all the electrode banks or alternatively an impulse producer may be provided for each bank of electrodes and its transmitter tube.

The impulse producer shown in the drawing is a special transformer 44 which is connected to a source of alternating current at 60 cycles. The secondary 43 of this transformer 44 is connected by the wires 45 and 46 to the grid 38 of the transmitter tube 36 and to the power supply line 41 that connects with the filaments 37 of the tubes. The impulse transformer applies a voltage to the grid 38 of the tube 36 in the form of a series of short lasting impulses. These pulses as formed by a 60 cycle impulse transformer are preferably about 2 in Width and have an amplitude of a few hundred volts. The impulses occur 60 times per second in one polarity for and last about one millisecond. The application of these negative impulses to the grid of the transmitter tube momentarily cut ofi" the supply of current to the electrodesof the related bank, and during the brief interval any existing flashover current would discharge the capacity of the precipitator and supply cable in an even shorter interval. During these intervals when the transmitter tube 36 discontinues the supply of current from the power lines 42 to the electrodes, the normal ionizing current to the electrodes is nevertheless sustained by the capacity of the electrodes making up the bank and the supply cables leading thereto. In a precipitator system in which the gas flows at 40 feet per second, the gas moves at a rate of about half an inch per millisecond. The chance for the flashover arc to restrike after an interval of that order is small.

The described modulation of the precipitator current supply will not cause the anodes of the transmitter tube to heat up because the status of the tube is varied between full conduction and complete insulation; in the first case the heat developed at the anodes is small, in the second case it is zero.

The graph designated Figure 2a shows the sine wave of the impulse transformer input whereas Figure 2b shows the output of transformer 44. The effect of the application of these impulses to the transmitter tube grid is shown in Figure 2c and the corresponding effect upon the electrode bank current during normal ionization appears in Figure 2d.

Although an impulse transformer has been described as the means for modulating the electrode current it would be possible to use other devices such as an electronic impulse producing means.

What I claim is:

1. ln an electrostatic precipitator having a bank of electrodes and an electrical powersupply for furnishing current to said electrodes; an electronic device comprising a control electrode connected between said power supply and said precipitator electrodes for periodically interrupting the supply of current to the precipitator electrodes of said banks; an impulse producer that converts sine wave excitation into a peaked output wave; a source of alternating current for energizing said impulse producer; and connections between the output side of said impulse producer and said electronic device for modulating the current transmitted through said device to said electrodes from said power supply.

2. In an electrostatic precipitator having a bank of electrodes, an electrical power supply for furnishing current to said electrodes: an electronic device of the transmitter type connected between said power supply and said electrodes for periodically interrupting the supply of current to the electrodes of said banks, said device including a control grid; an impulse producer that converts sine wave excitation into a peaked output wave; a source of alternating current for energizing said impulse producer; and connections between the output side of said impulse producer and the grid of said tube for modulating the current transmitted through said tube to said electrodes from said power supply.

3. In an electrostatic precipitator having a bank of electrodes and an electrical power supply for furnishing current to said electrodes: an electronic device of the transmitter tube type connected between said power supply and said electrodes for periodically interrupting the supply of current to the electrodes of said banks, said tube including a control grid; an impulse producer that converts sine wave excitation into a peaked output wave; a source of alternating current for energizing said impulse producer; and connections between the output side of said impulse producer and the grid of said tube for modulating the current transmitted through said tube to said electrodes from said power supply to effect interruption of the supply current to the electrodes for a period long enough to eliminate flashovers and short enough to sustain normal ionization of the gases by means of the electrostatic capacity of the electrodes and cables connecting with said power source.

References Cited in the file of this patent UNITED STATES PATENTS 1,960,047 Bedford May 22, 1934 2,000,017 Heinrich et a1 May 7, 1935 2,284,101 Robins May 26, 1942

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