GB2125314A - Method of and apparatus for cleaning fabrics - Google Patents

Abstract

For cleaning an article formed of non-rigid fabric such as a tubular filter (5) low frequency air pulses are directed at the article in the form of a concentrated beam to cause the material to vibrate while the article is rotated to expose different portions to direct impingement. The frequency is varied in the range 10 to 100 Hz while observing the article through window 24, and the frequency is held constant for a period at those values where the article resonates. The frequency may return to resonant values more than once in a cleaning operation. <IMAGE>

Description

SPECIFICATION Method of and apparatus for cleaning fabrics In the cleaning of dry powdered contaminants from relatively delicate fabrics or fabric-like limp materials such as are used in dust filters it has been common to use a relatively high pressure continuous air jet. In the case of large, expensive filter installations such as are used in fossil fueled boiler plants to remove dust from the stack gases, and in some other industrial applications, current practice requires an operator to use an air hose for many hours in order to clean the filters. Severe damage to the filtering material is apt to be caused by this currently used cleaning method, and frequently no more than a single cleaning is possible before it is necessary to discard the filter.

The overall objective of the present invention is to provide an improved method and apparatus whereby such difficult cleaning operations can be performed with very little or no damage to the material being cleaned, and in a much quicker and more economical manner.

A related object is to provide improved means for dislodging dry contaminants from limp fabrics wherein air pulses of sonic and subsonic frequencies may be applied to the material at low pressures but with sufficient intensity and at such frequencies as to cause the material to flutter.

According to the invention there is provided a method of cleaning limp porous material which comprises subjecting it to air pulses of different frequencies lying within a range between ten and one hundred hertz.

Another aspect of the invention provides apparatus for cleaning drum-shaped filters formed of limp fabric or the like comprising a housing having a side wall, means for rotatably supporting such a filter in the housing at a position opposite such side wall, and a sound generator capable of generating pulses in a continuously variable range lying between ten and one hundred hertz and having a discharge portion of limited cross sectional area directed inwardly from the side wall toward the position at which the filter is supported, whereby the operation of the sound generator in use of the apparatus while the filter is rotated will subject different portions of the filter to pulses directed toward the same.

The pulse frequency may be matched to one or more natural frequencies of oscillation of the material, or harmonics of such frequencies.

Preferably the maximum pressure of the pulses is concentrated in the area or a portion of the area of the article being cleaned, and the article or material being cleaned is moved angularly with relation to the pulse path to permit the pulse portions of maximum pressure to strike different portions of the article, and the frequency being varied to cause fluttering or vibrations at different resonances and/or harmonics of resonances.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of an assembly embodying and adapted to be used in accordance with the present invention; Figure 2 is a sectional elevational view taken substantially on the line Il-Il of Figure 1 and looking in the direction of the arrows, the horn being partially broken away; Figure 3 is an axial diametric sectional elevational view of the impulse generator on a larger scale, showing a portion of the motor, taken as indicated by the line and arrows Ill-Ill in Figure 1; and Figure 4 is a developed projection of the obturating wall of the rotor of the pulser.

The fabrics which are used in stack gas dust filters become tightly impregnated with dry particulates which, as indicated above, are often so difficult to remove by air hosing that frequently removal of the dust by this customary method cannot be achieved satisfactorily without destroying the filtering material. The present disclosure, therefore, illustrates a preferred application of the invention to this difficult cleaning operation, i.e., the cleaning of a typical baghouse stack gas dust filter. It will be recognized, however, that the invention is readily applicable to to the dislodgement of dry particulates from any relatively limp fabric or fabric-like material.

Figures 1-4 show a preferred apparatus which has been used in carrying out the invention. The article to be cleaned is illustrated as a generally cylindrical dust filter, F, of the type used in the filtering of stack gases. The filtering element comprises a fabric sleeve which is fitted on a supporting metallic reticulated frame. The frame is not shown. As indicated, the particulars of the article to be cleaned do not in themselves form a part of the invention, although such filters, large numbers of which are used in a typical powerplant baghouse array, tend to become tightly packed with dust, and are difficult and sometimes impossible to clean without damage by conventional methods.Typically the frame has parts by which the filter can conveniently be removably hung, as shown, from a work support 10 carried by a rotatable shaft 12 journaled in the top wall 14 of a sheet metal housing generally designated 15. A hand wheel 18 fast on the shaft above the top wall 14 permits the support 10 and the workload carried thereby to be rotated during the cleaning operation. Obviously a motor drive could if desired by used to rotate the support 10 and workload. The housing forms a dust-tight enclosure, except for its hopper bottom 20, and has an access door 22 to permit inserting and removing the filter or article to be cleaned. A transparent observation window 24 is provided in the door 22, as shown, or another convenient part of the housing.

An air pulse generator and pulse conveyor conduit assembly is provided, consisting of a pulse generator generally designated 25, and a horn, generally designated 26. The horn is shown as of folded form, in the interest of compactness, having its open mouth tightly coupled to an opening 28 in a mid portion of one of the side walls of the housing in a position to project pulses directly at the filter or other workload. The mouth of the horn is approximately 12" in diameter, and the horn is preferably of a substantially exponential form, and is so arranged as to project a beam-like succession of pulses at the material to be cleaned, the path defined by the beam being smaller in cross-section than the area of the impacted material.

The pulse generator 25 consists of a cylindrical housing 30 having an axial shaft 32 journaled therein drivable by an electric motor 33. An air inlet 34 in one wall of the housing 30 is adapted to be connected to a source (not shown) of air under pressure. The air supply in the unit that has been constructed furnishes air at approximately 300 cfm during operation of the system. An outlet 35 in another wall of housing 30 is coupled to the horn 26. A rotor 36 fast on shaft 32 has a cylindrical obturating wall 41, the axial length of which varies substantially sinusoidally to define two lobes 38, 39 which, as the rotor turns, alternately open and close outlet 35. Wall 41 accurately conforms to and is rotatable in close proximity to the cylindrical inner wall of the housing 30.In the embodiment illustrated, in which a two lobed rotor is used, contoured as shown in Figure 4, the motor is a conventional single phase induction electric motor having nominal speed of 1800 rpm, so that the two-lobed rotor, under normal full actual operating speeds, creates a pulse frequency of between 55 and 60 hertz. A conventional manually adjustable, continuously variable speed controller, diagrammatically indicated at 40, is incorporated in the electric circuit to the motor 33 to enable driving the rotor at any lower speed, and varying the speed at will.

The material being cleaned is rotated by means of hand wheel 18 while being subjected to air pulses which are of an intensity insufficient to cause substantial oscillation of the material except at resonant frequencies. In using the described apparatus it has been found that by adjusting the speed of the rotor upwardly and downwardly, a plurality of resonant frequencies are found at which the workload material vibrates or flutters.

These sympathetic vibrations are not of such severity or amplitude as to damage the limp material of filtering units of the character indicated, but at each such resonance, embedded dust is effectively shaken loose. Although the air flowing into the housing is insufficient to clean the material in the absence of such sympathetic or resonant vibration, the air assists in causing the dislodged dust to flow downwardly and out of the hopper for disposal.

If sympathetic vibration occurs at approximately 20, 40 and 60 hertz, for example, and the unit is adjusted to each of these resonant frequencies in succession, it is desirable to pause at and maintain each such frequency for a period of time, until it appears that no more dust is being dislodged. After all dust possible has been removed at each such frequency in succession, it is preferred to readjust the frequency to, or to a frequency close to, one or more of the previously used resonant frequencies. This "return" can be done more than once, and causes additional dust to be dislodged.The reason for this phenomenon is not fully understood, but it is presumed that the material that is being cleaned vibrates at frequencies which are both fundamental and harmonic, and that the dust removal which occurs at each pause, which for convenience is referred to as being at a resonant or fundamental frequency, causes a change in the mass and stiffness of the material, and thereby a change in the actual resonant or fundamental and harmonic frequencies. The change in the patterns of fundamental and harmonic vibrations may explain the fact that it is possible to repeatedly return to frequencies close to frequencies previously used, and thereby dislodge additional material that could not previously be dislodged.

It should be recognized that dust varies widely in its physical characteristics, such as particle size, shape, lubricity, static charge, etc., and that fabric and fabric-like materials which are to be cleaned also vary widely. Thus although the embodiment of the invention which has thus far been constructed and tested operated with high efficiency when used in the manner described to clean a supply of fouled baghouse filters which has been furnished by a power station which was faced with a severe filter cleaning problem, it is believed that the most effective cleaning pulse frequencies may vary substantially, and for example may lie in any range between 10 and 100 hertz, under differing conditions as to fouling materials and work-load materials. Similarly, while an air supply at 300 cfm (8500 litres per min), delivered through a horn having a discharge mouth 1 2" (30.5 cms) in diameter, was found to be highly effective in cleaning the baghouse filters, the optimum air flow may also vary with different materials and conditions. An air flow control in the form of a hand valve 44 is preferably provided, to permit varying the air flow and thereby the intensity of the pulses.

Claims (25)

1. Apparatus for cleaning drum-shaped filters formed of limp fabric or the like comprising a housing having a side wall, means for rotatably supporting such a filter in the housing at a position opposite such side wall, and a sound generator capable of generating pulses in a continuously variable range lying between ten and one hundred hertz and having a discharge portion of limited cross sectional area directed inwardly from the side wall toward the position at which the filter is supported, whereby the operation of the sound generator in use of the apparatus while the filter is rotated will subject different portions of the filter to pulses directed toward the same.

2. The method of cleaning limp porous material which comprises subjecting it to air pulses of different frequencies lying within a range between ten and one hundred hertz.

3. The method of cleaning limp porous material having portions which are oscillatable at a plurality of different resonant and/or harmonic frequencies lying between ten and one hundred hertz, which comprises subjecting the material to air pulses of one or more of such frequencies.

4. A method as defined in Claim 3 wherein the frequency is maintained substantially constant for a period of time at each of a plurality of different frequencies at which resonance or a harmonic of a resonance is indicated by vibration of the material.

5. A method according to Claim 2 wherein the frequency of the pulses is varied while observing the reaction of the material, and the frequency is maintained substantially constant for a period of time at each of a plurality of different frequencies at which vibration of the material appears to indicate a resonant or harmonic condition.

6. A method in accordance with any one of Claims 2, 3, 4 or 5 wherein the pulses are projected toward the material in a concentrated beam-like path having a cross sectional area which is less than the area of the material, and moving the material at an angle to the direction of projection of such path to expose different portions of the material thereto.

7. Apparatus as defined in Claim 1 including a sound generator comprising a body having an air inlet, an air outlet, an interrupter operative to periodically and sequentially block and unblock passage of air from said outlet, and a pulse conveyor conduit having an inlet connected to said air outlet, said pulse conveyor conduit having an outlet which is connected to an opening in said side wall and which defines said discharge portion.

8. In combination with apparatus as defined in Claim 7, variable speed motor means for actuating said interrupter, and control means for selectively varying the speed of said motor means and interrupter.

9. The method of cleaning limp fabric-like material which is mounted on a rigid frame without removing the material from the frame which comprises subjecting the material to air pulses of different frequencies lying within a range between ten and one hundred hertz to cause the material to vibrate.

10. A method as defined in Claim 9 wherein the intensity of the pulses is sufficient to cause the material to vibrate at fundamental and/or harmonic frequencies, but is not of sufficient intensity to cause substantial vibration at other frequencies.

11. A method as defined in Claim 10 wherein the frequency is maintained substantially constant for a period of time at a plurality of frequencies at which resonance or a harmonic of a resonant frequency is indicated by vibration of the material.

12. In combination with apparatus as set forth in either of Claims 7 or 8, means for supplying air under pressure to said air inlet, and control means for varying such pressure.

13. A method as defined in any one of Claims 2, 3, 4, 5, 9, 10 or 11 including the further step of varying the air pressure of said pulses.

14. Apparatus as set forth in either of Claims 7 or 8 wherein said pulse conveyor conduit is a horn of generally exponential shape.

15. A method as defined in Claims 4, 5, 9 or 10 wherein the frequency is adjusted upwardly and downwardly between said frequencies at which resonance or a harmonic is indicated, and the adjustment is halted to maintain the frequency constant a second time at one of said frequencies after the material has been subjected to pulses of constant frequency at another of said frequencies.

16. A method as defined in Claim 7 wherein the pulses are directed toward the material in a beamlike path of limited cross section.

17. A method as defined in any one of Claims 2, 4, 5, 9, 10 or 11 wherein the pulse intensity varies substantially sinusoidally.

18. Apparatus as defined in any of Claims 7, 8 or 14 including means for supplying air under constant pressure to said air inlet, and supporting means for removably holding an article to be cleaned in a position to be impacted by said pulses.

19. In combination with means as defined in Claim 18, means for moving an article on said supporting means to positions such that different portions of the article are aligned with said air outlet.

20. Means as defined in Claim 19 wherein said supporting means is rotatable to move said article around a generally vertical axis in said housing.

21. A method as defined in Claim 1 5 wherein the frequency is adjusted upwardly and downwardly more than once and the adjustment is halted to maintain the frequency constant more than once at a plurality of said frequencies.

22. A method as defined in either Claim 15 or Claim 16 wherein said frequencies are approximately 20, 40 and 60 hertz.

23. A method according to any one of Claims 5, 11 or 21 wherein the air pressure and intensity of the pulses are too low to cause substantial movement or distortion of the material at frequencies other than said frequencies.

24. A method of cleaning fabrics substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

25. Apparatus for cleaning fabrics constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.