US2985802A - Magnetic impulse rapper - Google Patents

Description

2 Sheets-Sheet 1 @TTG m. .f d mm, L mm E 10 T@ y W m 7 5 Mm 7 y wim mm J f//v// /f/f L May 23, 1961 Filed March 28, 1958 May 23, 1961 J. W` DRENNING 2,985,802

MAGNETIC IMPULSE RAPPER Filed March 28, 1958 2 Sheets-Sheet 2 United States Patent O MAGNETIC IMPULSE RAPPER John `W. Drenning, Baltimore,

Md., assignor to Koppers Company,

This invention relates to electrostatic precipitators having impulse rappers for the electrode assemblies of the precipitators, and more particularly to a magnetic rapper for such precipitators.

In electrostatic precipitators electrodes are suspended in a precipitator shell and exposed to a prevailing particleladen gas stream which liows through the shell under pressure. By means of suitable electrical connections some of these electrodes are positively charged and some are negatively charged so as to create an electrical field which ionizes the gas ilowing through the shell. By means of this electrical field charges are imparted to the particles and the charged particles are attracted to the electrodes having an opposite charge so that the particles cling thereto with great tenacity. Primarily these particles are attracted to the electrodes which are positively charged but some particles also are attracted to the negatively charged electrodes. The gas, free of particles, then flows out of the precipitator. In time, the particles tend to accumulate on the suspended electrodes to such an extent as to reduce greatly the eiciency of the precipitator and to weigh heavily the electrodes and their supporting structures. Periodically, the electrodes are given a series of sharp blows to jar or vibrate loose the accumulated material and loosened accumulated material falls of its own weight downwardly to a hopper at the bottom of the precipitator shell.

Conventionally mechanical and pneumatic impulse rappers have been used to deliver these blows both to the positively and negatively charged electrodes, but an electromagnetic impulse rapper has been found to be more advantageous for this purpose because the magnitude of the blow imparted to the electrode by the electromagnetic rapper can be so easily adjusted to the proper magnitude for cleaning the electrodes. The heretofore known electromagnetic rappers, however, have been undependable or have required costly and frequent maintenance, such as by the frequent addition of 4lubrication thereto or the elimination of dirt therefrom. Also the presence of moisture in the rapper environment as well as the dust particles has caused frequent breakdown in the coils of the electromagnetic units known heretofore.

An object of this invention, therefore, is to provide an electromagnetic rapper which requires infrequent or no lubrication.

A further object is to provide an electromagnetic rapper which is sealed from the environment of dirt and dust.

A still further object is to provide a magnetic impulse rapper having a moisture resistant casing for an electromagnetic coil therein.

A still further object is to provide a magnetic impulse rapper which is operable at elevated temperatures.

This invention contemplates an impulse rapper having a separator and a base adapted to be struck by a hammer to supply vibrations to the electrodes of an electrostatic precipitator, an encapsulated electromagnetic actuator for striking the hammer against the separator, a resilient device for biasing the hammer away from the separator,

2,985,802 Patented May 23, 1961 and a nonmagnetic bearing for guiding the hammer, the hammer including an arrangement Kfor communicating air from one end of the hammer to the other, and a further arrangement for reducing the adverse effects of the electromagnetic actuator on the resilient device.

The above and further objects and novel features of the invention will appear more fully when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

Fig. l is an elevational and partial cross-sectional view of the novel rapper of this invention.

Fig. 2 is an isometric view of the electromagnetic actuator of Fig. l.

Fig. 3 is a diagrammatic view of the hammer of Fig. 1.

Fig. 4 is a partial cross-section of an electrostatic precipitator with the novel rapper of this invention mounted on a supporting structure for the electrodes.

Electrostatic precipitators are frequently subjected to extremly adverse operating conditions. For example, the outside of the shell of an electrostatic precipitator is subjected to air that contains dirt and moisture and that may have a temperature as high as 200 F. and at these temperatures the lubrication conventionally used in magnetic impulse rappers tends to become very thin or otherwise ineffective.

An electrostatic precipitator is shown in Fig. 4 as having an electrode 11 conventionally in the form of a rod to which are attached wires or plates (not shown), and attached by means of suitable nuts 13 to a plate 15 so as to be suspended in precipitator shell 17. The charged particles from a particle-laden gas stream, which is passed through the shell 17, are deposited on electrode 1'1. Plate 15 is supported by a conventional insulator 19 which is attached to the outside of the precipitator shell 17 by suitable means well known in the art. To the top of the insulator 19 is attached a plate 21 in a manner such that the electrode 11 is suspended through a hole 23 in plate 15, a hole 25 in plate 21 and a hole 27 in the precipitator shell 17 which has a diameter substantially corresponding to the inside diameter of the insulator 19. Interposed between plates 15 and 21 are a plurality of resilient springs 29 which are arranged so that the electrode 11 is resiliently supported and electrically insulated from the shell 17.

Housing 31 provides a closed chamber 33 around the top of the resiliently mounted electrode 11 so that there is no leakage of gas from the inside of shell 17 to the atmosphere. This housing 31 also protects the electrode 11 from weather. Housing 31 is mounted in a conventional fiuid-tight manner on the shell 17 around the top of the insulator 19 and the electrode 11. A projection 35 at one side of housing 31 supports a conventional ceramic insulator 37, and a conventional synthetic rubber gasket 39, such as a neoprene boot, is supported at the top of housing 31. Both members 37 and 39 are attached to the housing 31 in a Huid-tight manner by means well known in the art. A conventional electrical connection 41 connected from plate 21 through insulator 37 to a suitable electrical power source (not shown) supplies the necessary electrical potential to the electrode 11. A suitable insulating rod 43 connected by a conventional metallic coupling 45 to the top of the electrode 11 extends through the gasket 39 in a fluid-tight manner from coupling 45 to rapper 47.

In accordance with this invention, rapper 47 transmits impulses to the electrode 11 through rod 43 and coupling 45. This rapper comprises a hollow base 49 having a raised portion 51 thereon and a separator 53 made of annealled non-magnetic stainless steel mounted on raised portion 51 of base 49, a hollow cap 55 attached to the base 49, a magnetic coil 57 encapsulated in a casing 59 3 made of a moldable material lsuch as a polyester resin, a bearing 61 in contact with the bore 63 of casing 59, a floating piston 65 having a first portion 67, a second extended portion 69 and opening 71 for communicating air secured together by frictional contact but a pin (notv shown) may also be, used through the base 49 and rod 43. Attached to the base 49 by suitable means such as by welding is a collar 79. The cap 55, is substantially cylindrical and has a flange 80 advantageously -weldedv to the cap 55 which is attached to surface 81 of collar 79 by suitable bolts 83 and nuts 85. A gasket 87 between the cap 55 and the collar 79' provides a iluidtight seal. Thus, the inside of cap 55, when the cap 55 is attached to the base 49, is completely closed.

The coil 57 may be conventional. In accordance With this invention coil` 57 is encapsulated with a moldable material such as a polyester resin. The electrical windings of the coil are placed in a metal mold (not shown), a metal plug is inserted in the bore of the coil, the air is withdrawn from the mold, and liquid polyester resin, advantageously Acme 2001 supplied by the Stone and Barron Company of Baltimore, Maryand, is poured into the mold. The maximum operating temperature of Acme 2001 is about 140 F. but the temperature resistance of this resin may be improved to be as high as 475 F. by usingA a filler such as asbestos. Other resins and illers may. also be used in accordance with suitable molding processes well known in the art, provided the other resins have the required molding characteristics, high dielectric strength, dimensional stability, high resistance to moisture, and relatively high temperature resistance, and where a lubricant is used in the rapper 47 for the piston 65, the resin. and ller should also be resistant to such lubricants. Such other resins include phenolic resinoid, phenol-formaldehyde casting resins, polyvinylidene chloride and other materials. This type of molded plastic covered coil has the advantage that dimensions on the surface ofthe coil covering can be held to closer tolerances than was possible with the heretofore known tape covered coils, thereby permitting the resin covered coil to iit easily and snuggly into the cap 55 without elaborate hand adjustments. The molded plastic covered coil has the further advantage of maintaining a high dielectric.

strength for long periods of time despitey the presence of moisture which might en-ter through outlet 89 in the side of cap 55. The molded coil has the still further advantage that the coil `57 and lead wires 91 are resistant to the shock caused by piston 65 hitting the separator 53 during the operation of the rapper 47 and thus the coil windings. and the lead wires are not `damaged by shock as oftenv happened in the coils used heretofore.

In accordance with this invention, the top of the molded casing or cover 9 has a sill 90 through which lead wires 91 to the coil 5,7 are attached. Thus when the coil 57 is inserted in the cap 55 the lead wires 91 are prevented from jamming in the cap 55, jamming of the wires often beingl a problem with the coils known heretofore.

The coil 57 is fitted snugly into the cap 55 so that casing `59` butts against seal 93 at one end and seal 95 at the other end thereby to form a seal between the casing 59 and the cap 55 at one end of thecoil and between the casing 59 and the base 49 at the other end of the coil. Both seals 93 andE 95 `advantageously are made of neoprene but other materials such as polytetrauoroethylene maybe used. The bearing 61 is arranged inside the seals '93, andj95` so thatthe bearing member 61 contacts the borey63 of casing 59 in a snuggly fitting manner and extendsrfrom base; 49 into recess 9.7y in cap,55. Thus the seals 93, and 95, as described above, maintain the inside of bearing 61 free of dirt and dust from the atmosphere and the inside of bearing 61 tight from the escape of lubrication, if present therein.

Bearing `61 is advantageously made of a nonmagnetic material in the form of a hollow cylinder having its inside surface coated with a Teflon coating 101, Teon being the registered trademark of the polytetrauoroethylene compound made by the E. I. du Pont de Nemours & Company, Inc., of Wilmington, Delaware. tageously the nonmagnetic material but other nommagnetic materials such as nonmagnetic stainless steel may be used. This coating 101 substantially reduces friction between the bearing 61 andv the piston 65 as the piston reciprocates in the bearing thus eliminating the need for lubrication in the rapper. This Teflon coating is advantageously at least .005 of an inch thick. To obtain optimum results with coating 101, the inside surface of thebearing 61 should be cleaned initially of any rust,

grease, organic coatings, or dirt, sanded or vapor blasted.

and washed with a volatile solvent. The polytetrauoroa ethylene, as a low-viscosity dispersion in a water medium,

is. applied to the bearing by spraying at room temperature (but it is also possible to dip the bearing repeatedly until the proper coating thickness is achieved), and the coating is dried, preferably at room temperature belowapproxi-A until sintering of the polytetrauoroethylene occurs. It has been found that rap-id cooling of the coating by quenching in cold air or water improves the hardness of the coating and promotes ease of stripping excess material when necessary. Itis desirable to use several coatings, it being preferable. to use the above describeddrying andbaking steps between the application of the polytetrafluoroethylene layers. Each layer should be limited in thickness in order to avoid mud cracking upon dryingv as well as to permit reasonably rapid vaporization of the dispersing agent during the sintering operation.

The Teflon coating 10.1 has a maximum operating temperature range of over 400 F. and since the other parts of the rapper including the casing 59, the seals 93 and 95 have very -high operating temperatures ranges, this no vel rapper can be. used in high temperature locations where rappers known heretofore could not operate satisfactorily.

Inside the bore o'f the bearing 61 is located the floating hammer or piston 65. The first portion 67 of this piston has a diameter corresponding to the insid'ediameter of the coated bearing 61 and the secondportion 69 has a lesser diameter. -T he piston is held at the end 103` of the cap 55 by the spring 73. Both ends of the piston are chamfered and a hole 71 is drilledr through the center of' the piston for the communication of air from one side of the piston to the other when the piston reciprocates. Piston 65 is made of magnetic steel.

Piston 65 is biased away from separator 53Y by spring 73 made of spring steel which is interposed between a conventional solid or split piston ring 107 made of Teflon located on a shoulder 1019 of the piston 65land` a TellenY guide ring 111=1 located on brass spacer ring 75 which is4 supported in bearing 61 and on base 49 around' the s eparator 53. The separator 53 is force tted inside spacer '75 and located on raised portion 51 of base 49. It is important that the separator 53 be made of nonmagnetic material because residual magnetismV exists between the piston 6,5 and the raised portion 51 of basey 49.- This fo'rce may be suiiicient to retain the piston 6,5y in a downward position if the piston is allowed to contact the raised portion `51. Separator 53, therefore, being of nonmagnetic material, creates. a gap between` the piston 65 and the raised portion 51 thus. greatly reduces, the residual force of' attraction and allows the piston to return freely to the top of cap 55'.A This separator S3 is advantageously made of annealed stainless steel to give it toughness,

Brass is advan otherwise the force of the piston 65 might destroy it. Spacer 75 is advantageously brass but other no'nmagnetic materials may be used.

The second extended portion 69 of piston 65 extends downwardly through the coil spring 73 and the spacer 75. It has been `found that without such an extension 69 and spacer 75 the spring 73 may be adversely affected to such an extent that it will fail after a limited number of piston cycles` Surprisingly, therefore, it has been found that by providing this extended po'rtion 69 and by supporting the spring 73 at one end on the spacer 75 adverse effects are not experienced and consequently the spring 73 has long life. The reason for this is not completely understood but it is theorized that if the spring 73 is allowed to span the air gap between the retracted piston, i.e., when the piston is at end 103 of cap 55 and the separator 53, the spring being of a magnetic material, will provide a low reluctance path across the air gap for the field produced by the electrical impulse in coil 57. A still lower reluctance path is formed if adjacent coils of the spring are touching. Since a magnetic field will always take the path of least reluctance, the spring coils are therefore forced to come in contact, each one with its adjacent coil, to form this path. lIt was found by experiment, for example, that magnetic eld alone, in the absence of piston motion, was sufficient to cause coil clash. This coil clash, i.e., the hitting of the coils on each other, flattens the contacting surfaces eventually leading to spring failure. Thus by placing the spring 73 around the piston portion 69, the path of least reluctance is no longer through the spring but rather through the relatively larger mass of magnetic material in the piston portion 69. The magnetic flux is thereby effectively shunted around the spring 73 thus eliminating the forces which cause coil clash. It is theorized that the spacer 75 is effective to reduce the adverse magnetic effects on the spring 73 because the magnetic field is strongest at or near the separator 53 and thus the spacer displaces the spring 73 away from the separator 53 to a position where the magnetic effects on the spring 73 are less adverse.

`Coil 57 is energized through lead wires 91 which may be attached to a suitable power source through a timer and olf-on switch (not shown). With such timers and switches the coil 57 can be energized for short periods of time, and the 'energization may be set at any desirable interval. An interval between each energization found to be desirable is about thirty seconds. The energization of coil 57 irnpels piston 65 against separator 53.

To change the intensity of the rapper impulse under certain conditions present in a precipitator, the voltage maybe increased or decreased by a conventional voltage regulator (not shown) thus resulting in an increased or a decreased rapper intensity.

In operation, the piston 65 is normally held at end 103 of the cap 55 by spring 73, this being the condition when coil `57 is de-energized. Upon the energization of coil 57, the piston 65 is impelled downwardly against separato'r 53 to cause a sharp rapping impulse which is transmitted therefrom through base 49 and rod 43 to the electrode 11 of an electrostatic precipitator. This impulse is substantially constant and repetitive in intensity when the voltage to the coil upon each energization is constant. The use of Teflon ring 107 and coating 101 eliminates the need of lubrication in this novel rapper because of the low coeiiicient of friction of Teon. Furthermore the wear of the piston and Teon parts is negligible so that maintenance is substantially eliminated and the impulse intensity remains constant over long periods of time.

If Teflon coating 101 not be used for bearing member 61 lubrication in the form of natural or synthetic grease or oil or other lubricating substances such a graphite or molydisuliide is required. The novel rapper operates satisfactorily however, even if lubrication is used and the rapper be mo'unted on the precipitator shell 17, as shown in Fig. 3, and requires little or no maintenance because the lubrication is sealed in and dirt sealed out as described above. The maximum operating temperatures under such circumstances are usually limited by the maximum temperature allowable with the lubrication and with Gulf Anti-Friction Grease No. l, for instance, the temperature range is approximately 20 F. to 150 F. A solid Teflon bearing 61 may also be used with goo'd results although it is more expensive than the last embodiment mentioned.

As shown in Fig. l the separator 53 is force fitted into bearing 61 but separator 53 may also be provided which is a part attached by suitable means such as by bolting to the bottom of piston 65.

The ring 107, cut away, may be located between the ends of the first portion 67 of piston 465 in a suitable groove as shown in Fig. 3.

The foregoing has described a novel rapper which provides predetermined rapper impulse intensities, reduced maintenance requirements, infrequent or no lubrication, operation at elevated temperatures, and an interior sealed from the dust and moisture of its environment.

Although the foregoing has illustrated and described the invention in detail it is to be expressly understood that the invention is not limited thereto. Various changes can 'be made in the design and arrangement of the parts without departing from the Spirit and scope of the invention as will now be understood by tho'se skilled in the art.

What is claimed is:

l. A magnetic impulse rapper particularly adapted for electrostatic precipitators exposed to adverse ambient conditions comprising a hammer, a base, electromagnetic actuating means having a hollow inside for producing a magnetic eld for contacting said hammer against said base whereby vibrations of the base are produced, resilient means Ifor biasing said hammer away from said base, a nonmagnetic spacer interposed between said base and said resilient means for reducing adverse magnetic effects on said resilient means whereby said resilient means is operative within said `actuating means substantially without being adversely effected by said actuating means, a nonmagnetic bearing interposed between said electromagnetic means and said hammer for guiding said hammer, and means forming a single closed chamber around said hammer action and bearing so as to provide a rapper which is substantially maintenance free.

2. A magnetic impulse rapper particularly adapted for electrostatic precipitators and exposed to adverse ambient conditions comprising a base for transmitting momentary impulses of high vibrating intensity, electromagnetic actuating means having a hollow core, a sleeve member coated with polytetrafluoroethylene in the core of said electromagnetic actuating means, hammer means reciprocatable in said bearing means, said hammer means being actuated in one direction by `the energization of said electromagnetic actuating means so as to cause a momentary hammer-like blow of high intensity to said base, spring actuating means for biasing said hammer means in the other direction when said electromagnetic actuating means is de-energized and means enclosing said hammer action in a single chamber so as to separate said chamber from said adverse ambient conditions, whereby said hammer is operative without lubrication.

3. A magnetic impulse rapper exposed to adverse ambient conditions comprising a base having an anvil, a substantially closed housing attached to said base in a fluid tight manner having an outlet therein, a hollow electromagnetic coil in said housing having outlet Wires, and producing a magnetic field, a molded casing around said coil which protects said coil from said ambient conditions and so formed that its outside diameter tits snuggly in said housing and so that said outlet wires are connected to said coil through said outlet in said housing without jamming of the wires, means for sealing the hollow of said coil with said base and said housing so as to protect the hollow of said coil against said ambient conditions, a non-magnetic bearing located in said magnetic coil and gest/soa coatedA with polytetrailuoroethylene resin, a reciprocatable piston having an upper and lower portion and being positioned' in said bearing, said piston having means for communicating air from one end of said piston to the other end, said piston also having a supporting means thereon extending around its side portion, a polytetraiuoroethylene ring riding on said supporting means, a spacer ring in said bearing in contact with said base, and a coil spring interposed between said polytetraliuoroethylene ring and said spacer so as to surround said lower portion of said piston, andV to be removed substantially from the strong magnetic tield at said base, whereby energization of said magnetic coil impells said piston in one direction against said anvil and upon de-energization of said magnetic coil said spring impells said piston in the opposite direction thereby to provide a rapper operable substantially without maintenance and without adverse effects from said ambient conditions.

4. An electrical precipitator comprising a precipitator, shell, an electrode assembly resiliently supported on said shell, a rapper housing connected to said electrode assembly and having a hollow portion closed by a base portion, hammer means in said hollow portion, said hammer means having a irst portion of one diameter and a second portion of lesser diameter, a brass bearing coated with polytetraliuoroethylene for said hammer having an inside diameter substantially corresponding to the diameter of said first hammer portion, a spacer ring in said bearing, electromagnetic actuating means surrounding said hammer, said electromagnetic actuating means upon energization, actuating said hammer in one direction so as to cause vibrations in said electrode assembly, and a spring actuating means substantially surrounding said second portion of said-hammer means and interposed between said `first portion of said hammer means and said spacer ring for actuating said' hammer in the opposite direction.

5. A magnetic impulse rapper exposed to adverse ambient conditions comprising a base having an anvil, a housing attached to said base in a fluid tight manner, a

hollow electromagnetic coil enclosed in said housing inl a liuid tight manner having electric wires attached thereto, a polyester resin casing, around said magnetic coil and interposed between said magnetic coil and said housing in a snuggly fit-ting manner and which protects said coil against said ambient conditions, means forming a sill at one end of said resin casing for connecting wires to said magnetic coil through said housing and said resin casing, bearing means enclosed in said resin casing, hammer means having anV upper and lower portion enclosed in said bearing means, said hammer means also having internal means for communicating gas from one end of said hammer means to the other, resilient means in operable association with said hammer means, and spacer means interposed between said resilient means and said base, whereby energization of said magnetic coil irnpells said hammer means in one direction to give a hammer-like blow to said anvil and upon de-energization of said magnetic coil said hammer means is actuated in the other direction so that a rapper is provided which is substantially free from said adverse ambient conditions.

6. A magnetic impulse rapper comprising abase having an anvil, a housing attached to said base in a iiuid tight manner, a hollow cored electromagnetic coil enclosed inV said housing in aiiuid tight manner, a casing for said coil interposed between said coil and said housing in a snugly fitting manner and which protects said coil against ambient conditions, a bearing means enclosed in said casing made with polytetrafluoroethylene, a hammer means enclosed in said bearing means so as to be reciprocatable therein, spacer means located in said bearing means in contact with said base, resilient means interposed between said hammer means and said spacer means, meansr at` tached to said hammer means and extending intoY said, resilient means responsive to electromagnetic forces caused' by said electromagnetic coil whereby energization of said electromagnetic coil impels said hammer. means in one direction to give a hammer-like blowv to saidjanvil and upon` de-energization of said magnetic coil' said hammer means is actuated in theother direction so that a rapper is provided which is operable in said adverse ambient conditions and substantially free of maintenance.

7. A magnetic impulse rapper exposed to adverse ambient conditions comprising a base having an anvil, a housing attached to said base in a fluid tight manner, a hollow cored electromagnetic coil enclosed in said housing in a iiuid tight manner, a casing for said magnetic coil interposed between said coil and said housing in a snugly fitting manner whereby said coil is substantially unaffected by said ambient conditions, a bearing means enclosed in said casing, a hammer means having an upper and lower portion enclosed in said bearing means so as to be reciprocatable therein, resilient means in operable association with said hammer means and a spacer interposed between said' hammer means and said resilient means, whereby energization of said electromagnetic coil impels said hammer means in one direction to give a hammer-like blow to said anvil and upon de-energization of said magnetic coil said hammer means is actuated in the other direction.

8. An electrical precipitator comprising a precipitator shell, an electrode assembly resiliently supported on said shell, a rapper housing connected to said electrode assembly and having a hollow portion closed by an anvil portion, said housing also housing an outlet means, a hollow electromagnetic coil in said hollow portion having outlet wires, a molded polyester resin casing around said coil and so formed that its outside diameter tits snugly in said housing and so that said outlet wires are connected to said coil through said outlet means in said housing without jamming of the wires, means for sealing the hollow bore of said coil with said base andV said housing, a bearing located in said magnetic coil, a reciprocatable piston having an upper and lower portion and being positioned in said bearing, said piston having means for communicating air from one end of said piston to the other end, said piston also having a supporting means thereon extending around its mid-portion, a polytetrafluoroethylene piston ring riding on said supporting means, a spacer ring in said bearing in contact with said base, a coil spring interposed between. said polytetrafluoroethylene ring and said spacer, whereby energization. of said magnetic coil impels said piston in one direction against said anvil of said base to causevibrations in said electrode assembly and: upon de-energization of said magnetic coil said spring impels said piston in the opposite direction.

9. An electrostatic precipitator comprising a precipitator shell, an electrode assembly resiliently supported on said shell, impulse transmitting means connected through said shell to said electrode assembly, sealing means between saidl shell and said impulse transmittingmeans, a rapper housing connected to said impulse transmitting means and having a hollow portion closed by an anvil portion, a hollow bored electromagnetic coil in said rapper housing, a molded resin casing around said coil formedy so that its outside diameter tits snugly in saidV housing, means for sealing the hollow bore of said coil with saidv housing, a nonmagnetic bearing in said magnetic coil and coated'with resin having low coefficient of friction, a, piston reciprocatable in said bearing with an upper and lower portion, said piston having means therethrough coinciding with its longitudinal axis for communicating air from one end of said piston to the other end, said piston also having a supporting means thereon extending around its middle portion, a ring ,of resin having a low coefficient of friction riding on said supporting means, a nonmagnetic spacer ring located in said bearing in contact` with-said anvil portion of said rapper housing, a coil spring interposed between` said polytetrauoroethylene ring and said spacer ring so as to surround said lower portion of said piston whereby energization of said magnetic coil impels said piston in one direction to cause vibrations in said electrode assembly and upon de-energization of said magnetic coil said spring impels said piston in the opposite direction.

10. An impulse rapper not requiring lubrication and particularly adapted for electrical precipitators having adverse ambient conditions comprising a hollow electromagnetic actuator, a piston in said hollow actuator and actuated thereby, a plastic and metal case enclosing said hammer in said actuator and completely separating said hammer from said ambient, said piston having rst portion of one diameter and a second portion of lesser diameter, and a nonmagnetic bearing surface between said actuator and said piston, said bearing surface having thereon a coating of polytetrauoroethylene with a low coecient of friction.

11. An impulse rapper not requiring lubrication and particularly adapted for electrical precipitators comprising a hollow electromagnetic actuator, a piston in said hollow actuator and actuated in one direction thereby, a resilient means for actuating said piston in the opposite direction, means enclosing said hammer and resilient means in said actuator, means for preventing adverse magnetic effects on said resilient means and a nonmagnetic bearing surface between said actuator and said piston, said bearing surface having thereon a coating of polytetrauoroethylene with a low coeficient of friction.

12. An impulse rapper not requiring lubrication and particularly adapted for electrical precipitators comprising a hollow electromagnetic actuator, a piston in said hollow actuator and actuated in one direction thereby, a resilient means for actuating said piston in the opposite direction, means enclosing said hammer and resilient means in said actuator, means for preventing adverse magnetic effects on said resilient means, a nonmagnetic bearing surface between said actuator and said piston, said bearing surface having thereon a coating of polytetrafluoroethylene with a low coeicient of friction, and a means for reducing adverse magnetic effects on said resilient means.

References Cited in the ile of this patent UNITED STATES PATENTS 1,389,625 Churchward Sept. 6, 1921 1,583,583 Dominguez May 4, 1926 1,723,607 Dominguez Aug. 6, 1929 1,837,197 Berman Dec. 22, 1931 1,974,226 Wintermute Sept. 18, 1934 2,268,687 Young Jan. 6, 1942 2,383,411 Obszarny Aug. 21, 1945 2,462,753 Kyle et al. Feb. 22, 1949 2,561,355 Fish July 24, 1951 2,574,762 Schell Nov. 13, 1951 2,777,535 Hull Jan. l5, 1957 2,909,713 Hambor et al Oct. 20, 1959 FOREIGN PATENTS 757,582 Great Britain 1956

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