Abstract:
An improved apparatus and method for pulse-jet cleaning of filter bags in a baghouse using pulsed, high-pressure/low-volume, intermediate-pressure/intermediate-volume, or low-pressure/high-volume pulsed air flow. Rotation of one pipe relative to another pipe about a shared longitudinal axis causes apertures in the pipes to align intermittently. When the apertures are aligned, pressurized air is fed, through a pulse valve, into the inner tube of the two and flows out of the nested pulse pipe arrangement in a short, energetic pulse. The pulse is directed down into a filter bag arranged below the pulse pipe to pulse-clean the filter bag.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to pulse-jet cleaning of sleeve-type or tubular filter bags. Such filter bags are employed in baghouses that are part of industrial gas cleaners used, for example, to clean gaseous process streams. More particularly, the invention relates to an apparatus and method for improving the efficiency of pulse-jet cleaning. 
     BACKGROUND OF THE INVENTION 
     An industrial flue gas cleaner of the sort in which the invention may be used is illustrated in FIG.  1 . Dirty flue gas enters the installation through inlet manifold  10 . The dirty gas is admitted into the various compartments  12  of the installation and flows upward through an array of sleeve-type or tubular filter bags  14 , which are supported on the outside surfaces of cylindrical support cages  16 . (See FIG. 2A.) The filter bags remove dust, soot, and other particulate matter from the gas as it passes through the filters. The clean gas then passes into and exits the installation via outlet manifold  18 . Flow into and out of the individual baghouses is controlled by appropriate means such as inlet poppet dampers and outlet poppet dampers, as indicated in FIG.  1 . 
     As further illustrated in FIGS. 1,  2 A, and  2 B, the filter bags are supported at their upper, open ends  20  by a tubesheet  22 , which spans the entire cross section of the baghouse  12 . The tubesheet  20  functions like a gasket, forming a seal around the upper ends of the filter bags and along the perimeter of the baghouse such that the baghouse is separated into distinct, upper and lower portions. Depending on the specific method of cleaning, the filter bags are arranged in either a rectangular or a circular array. 
     Common industry practice is to clean rectangular arrays of bags with compressed gas typically ranging in pressure from about 40 psig to about 120 psig (more or less depending on details of the specific design). A series of pulse pipes  24  extend across the baghouse, with one pulse pipe extending across each row of filter bags in the array. Each pulse pipe  24  has a series of orifices  26  extending along the bottom portion thereof, with one orifice positioned over each of the dust bags. 
     When compressed gas is used for cleaning, it is referred to as either “high-pressure/low-volume” or “intermediate-pressure/intermediate-volume” cleaning, depending on the characteristic pressure. High-pressure systems generally operate at a pulse pressure on the order of 80 psig to 120 psig; intermediate-pressure systems generally operate at a pulse pressure on the order of 40 psig to 60 psig. 
     Circular arrays of bags, on the other hand, are cleaned by gas that is pressurized with a blower to pressures typically on the order of 10 psig to 20 psig (again, more or less depending on the specific design). Because lower pressures and larger volumes of gas are used in this form of cleaning, it is referred to as “low-pressure/high-volume” cleaning. 
     As shown in FIG. 2A, for all but low-pressure/high-volume cleaning, during normal filtering operation, gas with entrained particulate matter enters the baghouse  12  through inlet  30  at the lower end of the baghouse. The gas flows through the filter bags  14  (which are supported on the exterior surfaces of the cages  16 ) from the outside in, as indicated by the schematic cross-section of the filter bag at the top of FIG.  2 A. Dust, soot, ash, and other particulate matter or debris accumulates on the outside surfaces of the filter bags, and the now-clean gas exits the baghouse through the clean gas exhaust  32  at the upper portion of the baghouse. 
     When debris accumulates to the point that pressure drop across the bags exceeds a preset limit, i.e., where flow through the baghouse is restricted (or in many instances on a regular, timed basis), the filter bags are cleaned of debris using the pulse pipes  24 . Each of the pulse pipes is supplied with pressurized gas by pressure header  34 . At the appropriate time, a valve  25  is actuated and pressurized gas flows into the pulse pipe. An energetic pulse of pressurized gas flows out of the pulse pipe through each of the orifices  26  and down into the interior of each of the sleeve-type filter bags in the row, as illustrated schematically by the cross-section of the filter bag at the top of FIG.  2 B. The filer bag rapidly expands to its full circumference and then stops expanding suddenly. This rapid expansion and deceleration causes the “cake” of debris which has accumulated on the filter bag to fracture and be dislodged from the filter bag. The dislodged dust cake then falls into hopper  36  at the bottom of the baghouse, where it is collected and removed by an ash removal system (not shown). (The flow of dirty gas into the compartment may be suspended during cleaning of the filter bags such that the dislodged dust and other debris settles into the hopper, rather than being blown up toward the tops of the filter bags.) 
     Various experiments which have been conducted by, for example, Southern Research Institute, the assignee of this application, have shown that low-pressure/high-volume pulse-jet cleaning is generally superior to high-pressure/low-volume and intermediate-pressure/intermediate-volume pulse-jet cleaning. In low-pressure/high-volume pulse-jet cleaning, a blower is used to supply only moderately compressed air for the cleaning, in contrast to a high-pressure or intermediate-pressure header as shown in FIGS. 1,  2 A, and  2 B. Because a blower is required to supply the relatively large volume of air utilized in this form of cleaning, it generally has been conceded by those skilled in the art that multiple blowers would be required in order to apply this type of cleaning to filter bags arranged in the more conventional square or rectangular array, as they are arranged in high-pressure/low-volume and intermediate-pressure/intermediate-volume pulse-jet cleaning systems. 
     Providing multiple blowers, however, is not economical. Accordingly, low-pressure/high-volume pulse-jet cleaning has only been able to be realized on a commercial, practical scale by arranging the filter bags in concentric circles and supplying the pulses of air to the filter bags by means of a rotating arm. The arm rotates about an axis that is centered in the middle of the concentric circles of filter bags and is supplied with air through a central conduit, as shown, for example, in U.S. Pat. No. 4,157,899. Air is discharged into the filter bags through a series of outlets in the bottom of the rotating arm. This arrangement is not ideal, however. In particular, it is not possible to clean every bag directly below the arm during any one pulse of air because of the manner in which the bags are geometrically distributed beneath the arm. Advocates of this arrangement point out that with multiple passes of the arm, and with pulse timing adjusted so that pulses are not directed at the same point on each rotation, statistically and over some period of time almost every bag will be pulsed. Still, however, many bags are not directly pulsed—i.e., a pulse of air is not directed down through the center of the bag—and the overall efficiency of cleaning therefore is significantly less than what it could be and what would be desired. 
     SUMMARY OF THE INVENTION 
     The present invention improves the efficiency of pulse-jet cleaning in general. Perhaps most advantageously, it eliminates the requirement of circular symmetry and rotating arms for low-pressure/high-volume pulse-jet cleaning, thereby making the superior cleaning performance of low-pressure/high-volume pulse-jet cleaning available for use in more conventional baghouse arrangements in which the filter bags are arranged in rectangular arrays. Pre-existing high-pressure/low-volume and intermediate-pressure/intermediate-volume systems could also be retrofitted to take advantage of the invention, with a concomitant reduction in the compressed air volume required for cleaning. 
     The invention accomplishes this by means of a novel pulse pipe in which only a few of the holes or orifices are open at any given time to permit cleaning airflow to only a subset of the filter bags in a given row at any given time. The pulse pipe remains in position over the filter bags, so every pulse is directed straight down the center of each bag—the most effective location for the pulse. Moreover, with only a few of the holes open for each pulse event, it becomes possible to use a relatively small, inexpensive blower to supply air to each individual pulse pipe; alternatively, a larger blower can be used to supply air to several pipes at a time using a header, valves, or suitable manifold arrangement. 
     In one aspect, the invention features a pulse pipe for use in pulse-jet cleaning of filter bags in a baghouse. The novel pulse pipe includes a cylindrical, hollow inner tube and a cylindrical, hollow outer tube, with the inner tube being arranged coaxially within the outer tube. The inner tube and the outer tube are configured for relative rotation therebetween about a common longitudinal axis, and the inner tube and the outer tube each have a series of longitudinally spaced apertures formed therein. The apertures in one of the tubes—either the inner tube or the outer tube—are longitudinally aligned with each other along the pulse pipe; and the apertures in the other tube are longitudinally aligned with the apertures in the first tube, but are not all longitudinally aligned with each other. As a result, different subsets of the apertures in the second tube are located at different circumferential positions on the second tube. Accordingly, as the second tube rotates relative to the first tube, different subsets of the apertures in the second tube intermittently come into alignment with the various apertures in the first tube and allow gas supplied to the interior of the inner tube to pass out of the pulse pipe. 
     In various embodiments of the invention, the pulse pipe may include a source of gas, and the source of gas preferably has pulse valving which regulates the supply of gas into the interior of the inner tube. Preferably, the pulse pipe also includes means for determining the relative angular position between the inner and outer tubes, and the pulse valving is regulated such that it opens to permit gas to flow into the interior of the inner tube only when apertures in the tubes are aligned. 
     In another aspect, the invention features a filter bag baghouse arrangement, including a chamber with a lower, inlet portion and an upper, outlet portion with a rectangular array of sleeve-type or tubular filter bags disposed therein. The filter bags are arranged in rows and columns and have closed lower ends disposed toward the inlet portion of the chamber and open upper ends disposed toward the outlet portion of the chamber. A plurality of pulse pipes as described above are disposed over the open upper ends of the filter bags, with each of the plurality of pulse pipes aligned over the filter bags in one of the rows or columns of the array. Rotation of one of the tubes relative to the other tube, as described above, causes different subsets of the apertures in the tubes to come into alignment intermittently such that gas supplied to the interior of the inner tube passes out of the pulse pipe and into a corresponding subset of the filter bags disposed below the pulse pipe in pulsed fashion. 
     In another aspect, the invention features a method of pulse-jet cleaning sleeve-type or tubular filter bags disposed in a rectangular array in a filter bag baghouse, which array constitutes rows and columns of filter bags. The inventive method entails intermittently injecting a low-pressure/high-volume flow of gas into varying subsets of the filter bags in each row or column in the array, with the subsets each being less than all of the filter bags in each row or column, respectively. 
     In preferred embodiments of the inventive method, a pulse pipe as described above is provided over each of the rows or columns in the array; gas is caused to flow into the interior of the inner tube; and gas is then injected intermittently into the varying subsets of the filter bags in each row or column by causing one of the inner and outer tubes to rotate relative to the other of the inner and outer tubes. Preferably, the pulse pipes include means for determining the relative angular position between the inner and outer tubes, and the flow of gas into the inner tube is controlled such that gas flows into the inner tube only when the tubes are oriented with the apertures therein aligned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail in connection with the drawings, in which: 
     FIG. 1 is a schematic, perspective view of a rectangular-array baghouse installation as is known in the art. 
     FIGS. 2A and 2B are schematic, side elevation views illustrating the operational, filtering mode and the pulse-jet, filter bag cleaning mode, respectively, of one of the compartments shown in FIG.  1 . 
     FIG. 3 is a side view, partially in section, of one embodiment of a low-pressure/high-volume pulse pipe according to the invention. 
     FIGS. 4-7 are cross-section views taken along the lines  4 — 4 ,  5 — 5 ,  6 — 6 , and  7 — 7  in FIG. 3, respectively. 
     FIG. 8 is a side view, partially in section, of an alternate embodiment of a low-pressure/high-volume pulse pipe according to the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A first embodiment of a low-pressure/high-volume pulse-jet cleaning pulse pipe  100  is shown in FIG.  3 . The pulse pipe  100  is constructed with a stationary inner tube  102  and a rotating outer tube  104  which nests coaxially over the stationary inner tube  102 . 
     The outer tube  104  is supported by the walls  106  of the baghouse. More particularly, a cylindrical stud  108  extends from the closed end  110  of the outer tube, and the stud is rotationally supported by a bearing  112  fixed within an aperture in the baghouse wall  106 . A pair of collars  114 —one on either side of the bearing  112 —are attached to the stud to keep the outer tube  104  properly positioned longitudinally and are intended to provide a gas-tight seal. The opposite, open end  120  of the outer tube is rotationally supported by a bearing  122  fixed in an aperture in the opposite baghouse wall, also with an appropriate gas-tight seal. 
     The inner tube  102  fits concentrically within the outer tube  104  and extends down the entire length of the outer tube. The inner tube is closed at its downstream end  126  and is supplied with low-pressure air at its upstream end  128  via input conduit  130 . The input conduit is supplied with pressurized air from a dedicated blower (not shown) or may be connected to a manifold (not shown) which receives pressurized air from a relatively larger blower. Flow of air to the input conduit is regulated by a pulse valve (not shown). The inner tube  102  may be joined in communication with the input conduit  130  in any convenient, appropriate manner, such as by an elbow joint. The input conduit  130 , which is rigidly secured either to its dedicated blower or to a manifold, or to the baghouse wall  106  (not shown), rigidly supports the inner tube  102  centrally within the outer tube  104 . The inner tube  102  and outer tube  104  are sized such that there is a minimal gap or clearance  134  between them that is on the order of {fraction (1/16)} to ⅛ of an inch wide, although slightly more or less clearance is permissible. 
     The inner tube has a series of holes or apertures  140  extending all the way through its wall, evenly spaced along the bottom of it. The holes  140 , which are longitudinally aligned with each other, are each positioned over one of the filter bags  14  suspended from the tube sheet  22 . 
     The outer tube  104  also has a series of holes or apertures  142  extending all the way through it. Longitudinally, the holes  142  in the outer tube  104  are evenly spaced, with the same longitudinal spacing as the holes  140  in the inner tube, i.e., such that they are longitudinally aligned with the filter bags  14 . 
     Unlike the holes  140  in the inner tube, however, the holes  142  in the outer tube vary in their circumferential location. In the embodiment shown in FIG. 3, for example, each successive pair of holes  142  (as one proceeds down the length of the pulse pipe assembly) is offset ninety degrees circumferentially from the preceding pair of holes, as illustrated in FIGS. 4-7. Accordingly, as the outer tube  104  rotates coaxially around the inner tube  102 , successive subsets of the holes  142  in the outer tube will line up with corresponding holes in the inner tube. Where holes  142  in the outer tube align with holes  140  in the inner tube, a pulse of air will be able to flow out of the pulse pipe assembly and down into the filter bags to pulse-clean the filter bags below the aligned holes. 
     The pulse pipe assembly also includes appropriate means  150  to rotate the outer tube. The means  150  could be, for example, a chain drive, a worm gear, a rack-and-pinion gear, or any other convenient means of rotating the outer tube longitudinally and concentrically around the inner tube. Alternatively, a direct drive motor (not shown) could be attached, e.g., to the cylindrical stud  108  from the outside of the baghouse to cause the outer tube to rotate. If so desired, rotation of the outer tubes of all the pulse pipes in each baghouse could be slaved together so as to rotate in coordinated fashion. 
     When it has been determined that a compartment of bags needs to be cleaned, the outer tube is made to rotate around the inner tube. When openings in the inner and outer tube coincide, the filter bags are pulse-cleaned in sequential fashion (sequentially in pairs in the embodiment shown in FIG.  3 ), with just a subset of the bags in each row being pulsed with cleaning air at any given time. Following current industry practice, the pulse valve referenced above (not shown) is used to admit the low-pressure air from a receiver tank (not shown) to the pulse pipe for cleaning. In the case of this and all embodiments of the invention, however, the pulse valve is opened and cleaning occurs only when holes in the inner tube  140  line up with holes in the outer tube  142 . 
     So that it can be determined when holes in the inner tube  140  and outer tube  142  are aligned such that the pulse valve should be opened, the apparatus also includes rotational position sensing means  152  for monitoring the angular position of the outer tube. The position sensing means could be configured using a photodiode, a photocell, a hall effect sensor, a magnetic switch, a continuous potentiometer (linked, for example, by gearing or direct contact with the drive means  150 ), or any other suitable position sensing means. 
     An alternative embodiment of a pulse pipe  200  according to the invention is shown in FIG.  8 . The primary difference between the embodiment shown in FIG.  8  and the embodiment  100  shown in FIG. 3 is that, in the embodiment  200  shown in FIG. 8, the outer tube  204  remains stationary and the inner tube  202  rotates within it, around their common longitudinal axis. 
     The closed end  210  of the outer tube  204  is supported by an aperture or indentation  207  in one wall  206  of the baghouse, with an appropriate seal therebetween if required. The outer tube  204  can also be held stationary by any other appropriate means of support. Near the opposite, open end  211  of the outer tube  204 , an annular boss  213  is affixed to the wall  206  of the baghouse, surrounding aperture  215  in the wall  206  and supporting a sealing bearing  217  within recessed shoulder  219 . The sealing bearing  217  substantially seals the open end  211  of the outer tube  204  while, at the same time, permitting the inner tube  202  to rotate therein. 
     The outer tube  204  has a cylindrical stud  208  extending inwardly from the closed end  210 . A bearing  209  attached to the outer surface of the closed end  226  of the inner tube mates with the end of the cylindrical stud  208  and supports the end  226  of the inner tube for rotation, within the outer tube, about the common longitudinal axis of the inner and outer tubes. The opposite, open end  228  of the inner tube is supported for rotation relative to the stationary outer tube by the sealing bearing  217 . 
     An air inlet tube  230  extends into the open end  228  of the inner tube  202  and is stationary relative to the baghouse, e.g., by virtue of being attached to a common pressure header (not shown) or a dedicated blower which, itself, may be attached to the wall of the baghouse. (As in the case of the previous embodiment  100 , the flow of air into the air inlet tube is regulated by a pulse valve, not shown.) A bearing  232  positioned between the inner tube  202  and the outlet end  233  of the air inlet tube  230  allows the inner tube to rotate relative to the end of the air inlet tube. 
     Similar to the embodiment shown in FIG. 3, the embodiment shown in FIG. 8 includes means  250 , attached to the open end  228  of the inner tube, for rotating the inner tube. Like the means  150  for rotating the outer tube  104  in the preceding embodiment, the means  250  for rotating the inner tube  202  can be a driven gear, a chain drive, a worm gear, a rack-and-pinion gear, or any other suitable means for causing the inner tube to rotate. Similarly, the embodiment of the invention shown in FIG. 8 includes rotational position sensing means  252 , which are analogous to the rotational position sensing means  152  shown in FIG.  3  and which are used to control opening of the pulse valve such that air flows into the pulse pipe only when holes in the inner and outer tubes are aligned. 
     Because the inner tube rotates in the embodiment shown in FIG. 8, the holes  240  extending through it vary in their circumferential position, from one end of the inner tube to the other, while the holes  242  extending through the outer tube are all aligned along the bottom of it. As in the embodiment shown in FIG. 3, all holes  240  and  242  are longitudinally evenly spaced, aligned over the individual filter bags in a given row of filter bags (not shown). 
     Operation of the embodiment shown in FIG. 8 is otherwise essentially the same as operation of the embodiment shown in FIG.  3 . In particular, as the inner tube rotates relative to the outer tube and about the two tubes&#39; common longitudinal axis, the holes  240  in the inner tube will line up with the holes  242  in the outer tube in sequential fashion, with only a subset thereof—two in the exemplary embodiment shown in FIG.  8 —lining up with holes in the outer tube at any given moment (every ninety degrees). 
     Although the embodiments shown in FIGS. 3 and 8 are similar in that, in both cases, the holes in the inner and outer tubes will line up in pairs of adjacent holes, and such alignment will only occur with every ninety degrees of rotation of whichever pipe is being rotated (with one pair of holes being aligned for each of the four angular positions of the rotating tube in which alignment occurs), other configurations certainly are possible. For example, the pulse pipes could be configured such that either less holes (i.e., one) or more holes (but not all) align simultaneously. 
     Additionally, the “grouping” of the holes that align at any given time could be changed; in other words, it is not necessary for the holes to align in adjacent pairs. Still further, each filter bag could be pulsed more frequently (for a given rotational speed of the tube being rotated) by providing more holes around the circumference of the rotating tube. Other modifications will occur to those having skill in the art and are deemed to be within the scope of the following claims.