Parallel point to plane electrostatic precipitator particle size sampler

[57] ABSTRACT

An electrostatic precipitator is provided for precipitating particulate matter within a fluid onto a grid. An elongated electrode with a sharpened point is positioned with its long axis in spaced-apart relationship parallel to the grid with the grid and electrode in a chamber of a housing. A high voltage is imposed between the grid and the electrode sufficient to cause a corona discharge from the electrode which will precipitate the particulate matter from the fluid onto the grid.

Claims, 5 Drawing Figures

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H/Gti YOITAGE supp/γ

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PATENTED APR 2 91975

3,879,986

SHEET 1 OF 2

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PATENTEDAPR29I975

3,879,986

SHEET 2 OF 2

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3,879,986

PARALLEL POINT TO PLANE ELECTROSTATIC PRECIPITATOR PARTICLE SIZE SAMPLER

CONTRACTUAL ORIGIN OF THE INVENTION

The invention described herein was made in the course of, or under a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION

Electrostatic precipitation is a method of removing particles from fluids by charging the particles to one polarity and then attracting and collecting the charged particles on an electrode of opposite polarity to the charged particles. One application of this technique is a point to plane precipitator which may be used in conjunction with electron microscope studies of particulate matter. The particulate matter is deposited on a grid electrode which is mountable in an electron microscope for examination of properties such as particle size distribution.

The prior art device, herein referred to as the perpendicular point to plane electrostatic precipitator, requires a high-voltage elongated wire electrode whose end is sharpened to a point. The sharpened end is positioned directly opposite and perpendicular to a grid, which is mountable on an electron microscope. By applying a sufficient voltage between the grid and the pointed end of the electrode a corona discharge from the electrode is produced which will charge particulate matter coming in contact with the corona discharge. These charged particles are then attracted by the grid because of the difference in potential between the particles as charged by the wire electrode and the grid.

In order to hold the electrode perpendicular to the grid and to supply the necessary high voltage to the electrode, the dimensions of the precipitator, particularly parallel to the long axis of the electrode, are relatively large and cumbersome for many applications. As fluid access to the electrode and grid is perpendicular to the long axis of the electrode, application of the perpendicular point to plane precipitation geometry is not feasible where the area of access to the fluid is limited. For example, to sample particulate matter in the fluid in a smoke stack, a rather large hole would have to be drilled in the stack wall to allow the electrode and grid to be positioned adjacent to fluid flow in the stack for sampling. Similarly where the desired mounting is within a pipe the perpendicular point to plane orientation is impractical.

Distortion of sampling results may occur in tests such as particle-size sampling due to premature particle separation prior to the precipitation of the particulate matter onto the grid. In the perpendicular point to plane design premature separation may result from the manner in which the fluid to be tested is provided with access to the electrode and grid. It is common practice in the perpendicular point to plane design to provide access with an inlet tube or other device interposed between the fluid to be sampled and the electrode and grid. Due to interaction of the particulate matter with the inlet tube walls, the inlet tube may cause premature particle separation. Even with no separate inlet tube and access of the fluid provided by an opening in the precipitator extending to the electrode and grid, in order to insulate properly the perpendicular electrode from energy losses, where the voltage levels of the electrode may be 7000 volts or more, the length of the opening may be large enough so that the precipitator structure itself causes premature separation.

It is therefore an object of this invention to provide an improved electrostatic precipitator.

Another object of this invention is to provide an electrostatic precipitator having minimal premature particle separation.

SUMMARY OF THE INVENTION

Precipitation of particulate matter in a fluid onto a grid is achieved with a parallel point to plane design. An elongated wire electrode with one end sharpened to a point is positioned in spaced-apart relationship from and parallel to a grid. Both the grid and electrode are supported by a housing and are located within a chamber in the housing. A voltage is applied between the electrode and the grid sufficient to cause a corona discharge from the sharpened end of the electrode. An opening is provided in the mounting member to allow fluid access to the grid and electrode. The distance from the electrode and grid to the beginning of the opening is minimized to retard premature particle separation. The device may be operated in a directed fluid stream by positioning the electrode with its long axis parallel to and its sharpened point directed against the direction of flow, or the device may be operated by positioning the opening in contact with a static fluid supply.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of the precipitator;

FIG. 2 is a sectional view along line 2—2 of FIG. 1; FIG. 3 is a sectional view along line 3—3 of FIG. 1; FIG. 4 is a view of the precipitator incorporated into a conduit system; and

FIG. 5 is a view of the precipitator installed in the wall of an exhaust stack.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, FIG. 2 and FIG. 3, there is shown a parallel point to plane electrostatic precipitator. The main functional elements of the precipitator are supported by housing 10 which includes individual mounting pieces 11 and 12, which are held together by set screw 13. Mounting pieces 11 and 12 should be constructed of an insulating material.

In this embodiment, mounting piece 11 is designed to provide a mounting for the grid assembly. The grid assembly includes pedestal 16 upon which grid 17 is mounted. Grid 17 is constructed of wire mesh formed into a flattened pancake providing a flat conductive surface and may be a standard electron microscope specimen holder suitable for removal and mounting on an electron microscope for examination of the collected specimen. Grid 17 is secured to pedestal 16 by grid cap 18. Best results may be obtained if grid cap 18 is made of a nonconducting material or is coated with a nonconducting material to avoid collection of particles on the cap during precipitation. Pedestal 16 is screw threaded and screwed into threaded grid mounting hole 19 of mounting piece 11. As pedestal 16 is screwed into hole 19, grid 17 and grid cap 18 will eventually protrude into chamber 20. Grid 17 is connected to wire 21 which extends through the body of housing 10.

Mounting piece 12 provides mounting for the wire electrode 24. Electrode 24 is positioned in mounting

3,879,986 hole 25 of mounting piece 12 and maintained there by set screw 26 engaging the insulator 23 of electrode 24. One end of the elongated wire electrode 24 is formed into a sharpened point 27. By means of mounting piece 12, the elongated wire electrode 24 is positioned with its long axis parallel to the diameter of the flattened pancake shape of grid 17 and its sharpened end 27 is generally positioned over grid 17 so that a corona discharge may be developed between end 27 and grid 17.

Electrode 24 and wire 21 are coupled to a highvoltage supply 28. With a sufficient voltage provided between electrode 24 and grid 17, a corona discharge without arcing will occur between sharpened end 27 and grid 17. Varying the separation between sharpened end 27 and grid 17 with screw pedestal 16 will vary the voltage difference necessary to cause a corona discharge so that the greater the separation the greater the voltage required. At a particular separation between grid 17 and sharpened end 27 an increase in voltage above the minimum voltage necessary to cause corona discharge will produce an increase in the associated corona current therebetween. With a 0.039 inch diameter tungsten wire as elongated wire electrode 24 and with end 27 sharpened to a point of 20 micrometers in diameter, corona currents of 5 to 10 microamperes were obtained for voltages of 4,000 to 6,000 volts. At 5,000 volts corona currents between 1 and 10 microamperes were obtained by varying the separation between sharpened end 27 and grid 17 from 0.235 inch to 0.181 inch.

When a fluid is introduced into chamber 20 in the presence of a corona discharge from sharpened end 27, particulate matter in the fluid will become charged to the polarity of the corona by interaction with the corona discharge and be attracted to grid 17 because of the difference in polarity between grid 17 and the charged particles. Chamber 20 is a hollowed-out portion of housing 10 encompassing grid 17 and electrode 24 and access for fluid to grid 17 and electrode 24 is provided through opening 29 in housing 10. If separation “A,” which is the distance between sharpened end 27 and the beginning of opening 29 adjacent the fluid, is increased with respect to separation “B,” which is the narrowest cross-sectional dimension of opening 29 over the length of separation “A,” or if separation “B” is decreased with respect to separation “A,” premature particle separation may result due to the interaction of particulate matter within the fluid with the body of housing 10 prior to contact of the particulate matter with the corona. It has been observed that the ratio of separation “A” to separation “B” should not be greater than 2 to 3 to limit the possibility of premature separation caused by housing 10. An example of appropriate dimensions found to limit premature separation is .2 inch for separation “A” and .3 inch for separation “B.”

There are several methods of introducing fluid into chamber 20 and of positioning housing 10 relative to a volume of fluid so that fluid will enter chamber 20. In one embodiment, fluid enters chamber 20 through opening 29 in the direction of arrow 30 with sharpened end 27 directed against the flow so that premature separation which might be caused by the interaction of the body of electrode 24 with fluid particles is limited. The fluid exhausts chamber 20 through passageway 31. Best results are obtained if passageway 31 is positioned below the center of chamber 20 and below grid 17 so that the fluid flow will be brought closer to grid 17, im proving the efficiency of particle deposition on grid 17.

When the fluid is flowing through a pipe system, as shown in FIG. 4, housing 10 may be incorporated within the pipe system between pipes 32 and 33 and fastened by set screws (not shown), with the direction of fluid flow as indicated by arrow 36 toward sharpened end 27 and continuing through passageway 31. Note that the limited diameter of the parallel point to plane design perpendicular to the direction of flow works a great saving of space, not requiring as much diametric space as would be the case with a perpendicular point to plane design and that the limited separation A will not unduly obstruct the flow to cause premature separation upstream from grid 17. A pump 37 may be provided to draw fluid through housing 10.

In other applications it may be desirable to expose the precipitator directly to the fluid. For example, in sampling particles in the air in a room the precipitator, as shown in FIG. 1, can be placed on a table and the fluid drawn through chamber 30 by a pump 40 coupled to housing 10 by pipe 41 and set screw 42. Here again the narrow separation “A” limits premature separation which might be caused by an inlet tube and allows accurate size sampling. This method of sampling is also well adapted to exhaust stack sampling, as shown in FIG. 5. All that is required is that a hole conforming to the size of housing 10 be drilled in the wall 45 of a stack and the precipitator is then inserted into the hold with opening 29 beginning at the interior side 46 of wall 45 and passageway 31 exhausting at exterior side 47 of wall 45. This type of in situ stack sampling yields a truer picture of particle characteristics in a stack than top of the stack samplers. This would be difficult using the prior art perpendicular point to plane precipitator because of the size of the hole required for insertion.