Abstract:
A baghouse filter having a filter media and an end cap is provided. The filter media has a longitudinal axis, and the end cap has an outer peripheral side wall. The side wall has first and second edges. A first recessed channel extends between the first and second edges of the side wall, and a second recessed channel extends from the first channel. The first and second channels define a connection area at which the filter connects to the tubesheet at an opening in the tubesheet. Also, a method is provided for installing a filter on a tubesheet having first and second sides separating plenums of a baghouse. The method includes providing a filter having structure for attaching to a tubesheet opening from either side, selecting the side to install the filter and installing the filter on the selected side.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/216,717, filed Jul. 7, 2000, and is a continuation of co-pending application Ser. No. 09/707,527, filed Nov. 7, 2000, which issued as Pat. No. 6,440,188, on Aug. 27, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to filters within a baghouse and more particularly to an air filter capable of being mounted from either the top or the bottom of the tubesheet. 
     Filter assemblies are commonly used for removing particulates and aerosols from the air in industrial environments. One type of common filter assembly is an industrial baghouse. Baghouses typically include two large chambers, or plenums, that are divided by a tube sheet. The filter cartridges, preferably containing fabric filtration elements, are secured to the tube sheet at one end to suspend the cartridge in the lower plenum. Fabric filter media having longitudinal pleatings are particularly effective for removing particulates. One such air filter cartridge is disclosed in U.S. Pat. No. 5,632,791 to Reinout G. Oussoren and Jack T. Clements, herein incorporated by reference in its entirety. 
     Air containing unwanted particulates is forced into the first plenum by a forced air means such as a fan. The air passes from the first plenum to the second plenum via the air filter cartridges held by the tube (or pan) sheet. The particulates are retained on the outer surface of the fabric filter element of the cartridge filter, and the clean air is directed into the second plenum and exits the baghouse through an exhaust passageway. 
     To suspend the cartridges from the tube sheet, the cartridges are either loaded from the upper or lower plenum. Specifically, to load the cartridges from the upper plenum or “top load” the cartridges, the cartridge is slidingly received within the appropriate opening and the top cap of the cartridge is frictionally secured to the tubesheet at one of the openings. To load from the lower plenum or “bottom load” the cartridge, the cartridge is raised into proximity with the tubesheet and the top cap of the cartridge is attached to the tubesheet at one of the openings. The decision to top or bottom load the cartridge is often dictated by the accessibility and size of the plenums. If the upper plenum is spacious, a worker may move within the upper plenum to load the cartridges by top loading. Conversely, if the lower plenum is more accessible and provides the necessary space, the cartridges are typically bottom loaded. 
     However, in the past, air filter cartridges are designed for either top or bottom loading, but not both. Accordingly, the operator of the baghouse must have the appropriate type of either bottom load or top load cartridges for the particular baghouse. In industrial environments having both top and bottom load baghouses, an inventory of filter cartridges must be maintained for each baghouse. Accordingly, there is a need for an air filter cartridge capable of top and bottom loading within a baghouse. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an air filter cartridge capable of top and bottom loading within a filter assembly. In accordance with the foregoing and other objects evident from the following description of a preferred embodiment of the invention, a baghouse filter having a filter media and an end cap is provided. The filter media has a longitudinal axis, and the end cap has an outer peripheral side wall. The side wall has first and second edges. A first recessed channel extends between the first and second edges of the side wall, and a second recessed channel extends from the first channel. The first and second channels define a connection area at which the filter connects to the tubesheet at an opening in the tubesheet. 
     In accordance with another aspect of the invention, the filter has a filter media and an end cap. The end cap has first and second connector portions. The first connector portion mates with the tubesheet at a tubesheet opening when the filter is installed from the bottom plenum. The second connector portion mates with the tubesheet at a tubesheet opening when the filter is installed from the bottom plenum. 
     In yet another aspect of the invention, a tubesheet is provided having a planar sheet, a number of openings in the sheet and a number of collars. Each collar is positioned about an opening in the sheet and extends normally from the sheet. Each collar has an inner face directed toward the center of the opening about which the collar is positioned. The collars include a number of elements extending radially from the inner face toward the center of the opening. 
     In still another aspect of the invention, a method is provided for installing a filter on a tubesheet having first and second sides separating plenums of a baghouse. The method includes providing a filter having structure for attaching to a tubesheet opening from either side, selecting the side to install the filter and installing the filter on the selected side. 
     In another aspect of the invention, a tool for securing a filter cartridge to or from a tubesheet is provided. The tool has an elongated shaft, a handle, a disc and a rod. The handle is positioned at a first end of the shaft. The disc is disposed on the other end of the shaft and is aligned normally with respect to the shaft. The rod is coupled with the disc and has opposing ends extending beyond the periphery of the disc. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     The following description of the drawings, in which like reference numerals are employed to indicate like parts in various views: 
     FIG. 1 is a schematic view of the interior of a filter assembly demonstrating top loading of an air filter cartridge; 
     FIG. 2 is a schematic view of the interior of a filter assembly demonstrating bottom loading of an air filter cartridge; 
     FIG. 3 is an exploded perspective view of an adaptor plate and tube sheet according to the present invention; 
     FIG. 4 is a fragmentary, exploded elevational view of an air filter cartridge of the present invention being bottom loaded with the filter assembly; 
     FIG. 5 is a fragmentary, elevational view of the air filter cartridge of FIG. 4 being top loaded within the filter assembly; 
     FIG. 6 is a fragmentary, elevational view of the filter cartridge locked onto the adaptor plate of FIG. 3; 
     FIG. 7 is an enlarged fragmentary sectional view of the air filter assembly taken along line  7 — 7  of FIG. 6; 
     FIG. 8 is a top plan view of the air filter cartridge taken along lines  8 — 8  of FIG. 4; 
     FIG. 9 is a bottom plan view of the air filter cartridge taken along lines  9 — 9  of FIG. 2; 
     FIG. 10 is a perspective view of a tool used in accordance with the present invention; 
     FIG. 11 is an exploded elevational view of the air filter cartridge of FIGS. 5 and 6 and an alternative connection device with parts broken away for ease of illustration; 
     FIG. 12 is an enlarged sectional view of the filter cartridge locked onto the alternative connection device of FIG. 11; 
     FIG. 13 is an enlarged sectional view of the air filter cartridge locked onto a second alternative connection device; 
     FIG. 14 is a fragmentary top plan view of a tube sheet of the air filter assembly having a plurality of notches to engage the air filter cartridge; 
     FIG. 15 is an exploded partial elevational view of an air filter cartridge of a second embodiment of the present invention, wherein the filter cartridge is shown as being bottom loaded on the tube sheet of FIG. 14; 
     FIG. 16 is a fragmentary elevational view demonstrating the air filter cartridge of the second embodiment in the bottom-loaded, locked position; 
     FIG. 17 is a fragmentary side elevational view demonstrating the air filter cartridge of the second embodiment in the top loaded, locked position; 
     FIG. 18 is a plan view taken along lines  18 — 18  of FIG. 16; 
     FIG. 19 illustrates, by plan view, a modification to the second embodiment of the air filter cartridge used in conjunction with the tube sheet of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to FIG. 1, a baghouse, utilizing filter cartridges made in accordance with the present invention is designated generally by the reference numeral  10 . Baghouse  10  is defined by an enclosed housing  12  made from a suitable material such as sheet metal. Gas flows through the baghouse  10  from an inlet  14 . Gas is filtered by a plurality of filter cartridges  16  and exits the baghouse  10  through an outlet  18 . The filter cartridges  16  are secured to a tubesheet  20 , thus defining a lower plenum  22  and an upper plenum  24 . Specifically, each air filter cartridge  16  is secured within the tubesheet  20  at one of a plurality of openings  26  formed within the tube sheet  20 . 
     The housing  12  of the baghouse  10  includes a number of sides  28  and a roof  30 . As shown in FIG. 1, some baghouses have larger or more accessible upper plenums  24  allowing for the filter cartridges  16  to be loaded from the top of the tubesheet  20  and secured within the appropriate opening  26 . For instance, the height of the upper plenum  24  may allow for a worker to move within the plenum  24 , or the roof  30  may be removable to allow access into the baghouse  10  for top loading. As shown in FIG. 2, in some baghouses, the upper plenum  24  does not have a sufficient height to allow for top loading and thus the filter cartridges  16  are loaded from the lower plenum  22 . 
     With reference to FIG. 3, an opening  26  of tubesheet  20  is illustrated. In some prior art connection devices, a number of bolts  32  are placed around the periphery of the circular opening  26 . Typically, the bolts  32  are angularly equidistant from one another. For instance as shown in FIG. 3, each bolt is offset by 90° from the next adjacent bolt. Another conventional design consists of three bolts offset one from another by 120°. In accordance with the principles of the present invention, an adaptor plate  34  is placed over the existing bolts  32 . Annular adaptor plate  34  has a first set of apertures  36  and a second set of apertures  38 . The first set of apertures  36  are spaced 90° one from another to correspond with the bolts  32  on the tubesheet  20 . The second set of apertures  38  are spaced 120° one from another to correspond with a tubesheet having three (rather than four) bolts. Of course, the bolts and apertures may be spaced in an asymmetrical pattern about the end cap. 
     The adaptor plate  34  further comprises a number of L-brackets  40  spaced around the top surface of adaptor plate  34 . In the preferred embodiment, three L-brackets  40  are placed equally around the adaptor plate  34 . Both the adaptor plate  34  and L-brackets  40  are preferably made from a suitable material such as sheet metal. Each L-bracket  40  has a lower portion  42  secured to the adaptor plate  34  and an upstanding portion  44  extending normally from the surface  43  of adaptor plate  34 . Each upstanding portion  44  has an inner face  45  that is flush with the periphery of the inner peripheral edge  47  to define a diameter D 1  of the adaptor plate  34 . A threaded aperture  46  is located within the upstanding portion  44  of each L-bracket  40 . Bolts  48  having generally round heads  49  are threaded within each aperture  46  and extend radially inwardly toward the center of the adaptor plate  34 . With reference to FIGS. 3 and 4, the adaptor plate  34  is placed on the tubesheet  20  so that the bolts  32  are received within the first set of apertures  38 . When the adaptor plate  34  is in contact with tubesheet  20 , nuts  50  are secure on bolts  32  and the adaptor plate  34  is held against the tubesheet  20 . 
     With reference to FIGS. 1 and 2, the filter cartridges  16  include a first (top) end cap  52 , a second (bottom) end cap  54  and a tubular filter media  56  disposed therebetween. As air is cleaned, it passes through the tubular filter media  56  and moves upwardly through a channel defined by the filter media  56  and into the upper plenum  24 . With reference to FIG. 4, the top end cap  52  is formed about the filter media  56  as would be understand by one of ordinary skill in the art. With reference to FIG. 7, the top end cap  52  is typically formed from a potting material  55 . The potting material  55  extends about the upper edge of tubular filter media  56  and a screen member  57  as known to those of ordinary skill in the art. 
     With reference to FIGS. 4 and 8, top end cap  52  is generally annular and has an outer diameter D 2  that is slightly smaller than the diameter of the openings  26  of the tubesheet  20  and the diameter D 1  of the opening defined by adaptor plate  34 . The inner diameter D 3  (FIG. 8) of the top end cap  52  is generally equal in size to the diameter of the air flow channel defined by tubular filter media  56  so that the top end cap  52  does not obstruct the flow therethrough. 
     As best shown in FIG. 4, in the peripheral-sidewall  51  of top end cap  52 , a plurality of connection areas  58  are defined between the upper edge  60  and lower edge  61  of the top end cap  52 . In the preferred embodiment, each of the three connection areas  58  has a longitudinal channel  62  and a locking channel  64 . The longitudinal channel  62  runs parallel to the longitudinal axis of the tubular filter media  56  and has a uniform depth within the top end cap  52 . The locking channel  64  extends generally normal to the longitudinal channel  62  and has the same depth as the longitudinal channel. The sidewalls  65  of the locking channel  64  are tapered inwardly and terminate at the mouth  67  of a locking area  68 . The height H of the locking area is preferably greater than the smallest distance (namely, that distance at mouth  67 ) between the sidewalls of the tapered locking channel  64 . 
     With reference to FIGS. 7 and 8, the top end cap  52  defines an annular ledge  70  which extends inwardly from the sidewall  51  about the interior of the potting material  55  proximate the collar  53 . The annular ledge  70  extends completely about the top end cap  52 , and a groove  72  is disposed completely around the ledge  70 . A pair of opposing notches  76  and  78  are defined by a first pair of upstanding members  80  and a second pair of upstanding members  82 , respectively. The notches  76  and  78  are aligned linearly with one another. 
     As best shown in FIG. 1, the filter cartridge  16  of the present invention is capable of being top loaded from the upper plenum  24  of the baghouse  10 . With specific reference to FIG. 5, the tubular filter media  56  is slidingly received within the opening defined by the inner peripheral edge  47  of the adaptor plate  34  and one of the openings  26  of tubesheet  20 . An annular gasket  79  (or engaging surface) is located around top end cap  52  at the area of the top end cap  52  contacting tubesheet  20 . The bolts  48  of adaptor plate  34  are received within the longitudinal channels  62  of each of the connection areas  58  of top end cap  52 . When the filter cartridge  16  is lowered to the depth at which the bolts  48  are proximate the mouth portions  66  of locking channels  64 , the filter cartridge  16  is rotated in the clockwise direction indicated by the arrow in FIG.  6 . The bolts  48  are received within the locking channels  64  at the locking areas  68  when the rotation is complete. With specific reference to FIG. 7, each bolt  48  contacts the upper surface of its corresponding locking area  68  and the filter is suspended therefrom. 
     Likewise, with reference to FIG. 4, the filter cartridge  16  may be bottom loaded when the adaptor plate  34  is secured to the tubesheet  20  on the lower plenum side of the tubesheet  20 . Specifically, the top end cap  52  is placed in proximity with the adaptor plate  34  and the heads of the bolts  48  are aligned with the longitudinal channels  62  of the connection areas  58 . Finally, the filter cartridge  16  is inserted within the connection areas and rotated so that the bolts  48  are positioned within the locking areas  68 . At this point, an annular gasket  69  contacts the tubesheet  20  to form a seal. 
     As shown in FIG. 10, a tool  84  may be used to facilitate rotation of the filter cartridge  16  to and from the locked position of FIG.  7 . Tool  84  includes a handle  86 , a shaft  88  and a cartridge engagement assembly  90 . The handle  86  and shaft  88  are preferably made from a square tube of metal such as steel and are welded together. The cartridge engagement assembly  90  includes a disc  92  and a rod  94 . The disc  92  is preferably welded to the end of the shaft  88  opposite handle  86 . The rod  94  is welded to either side of the shaft  88  and to the upper surface of disc  92 . The disc  92  is sized to fit onto the ledge  70 . As shown by the phantom lines in FIG. 8, the rod  94  is received within notches  76  and  78  when the disc  92  is properly seated. The user merely grasps the handle  86  and rotates the tool  84  to rotate the cartridge relative to the tubesheet  20 . 
     With reference to FIG. 9, the potting material  96  of the bottom end cap  54  is recessed to present a ledge  95 . A pair of upstanding members  98  and  100 , similar to upstanding members  80  and  82  of the top end cap  52 , are located on the ledge  95  and define notches  104  and  106 , respectively. A circular depression  102  is formed centrally on the bottom end cap  54 . In order to bottom load (or unload) the filter cartridges  16  from the tubesheet  20 , the disc  92  is placed within depression  102  and the rod  94  within the notches  104  and  106  prior to rotation of the filter cartridge  16 . Typically, the space between the bottom end cap  54  and housing  12  is limited and a tool  84  having a relatively short shaft  88  is desired. 
     While one embodiment of the filter cartridge  16  consists of the longitudinal channel  62  and the locking channel  64 , the connection areas may take on any of a number of configurations allowing for top and bottom loading. For instance, a pair of channels may originate from upper edge  60  and lower edge  61  and be formed angularly with respect to one another. The top end cap  52  could be threaded onto the tube sheet at either of the two channels and locked at the intersection of the channels. 
     With reference to FIGS. 11 and 12, an alternative structure for connecting a filter cartridge  16  to a tubesheet  20  is illustrated. A collar  110  extends downwardly from tubesheet  20  into the lower plenum  22  of the baghouse  10 . Each collar  110  is preferably formed from the same material as the tubesheet  20  and is welded thereto. A plurality of bolts  112  corresponding to the number of connection areas  58  of the filter cartridge  16  are threaded within apertures  114  located around the collar  110 . A resilient gasket  116  in the shape of a ring is adhered to the underside of the planar portion of the tubesheet  108  around the opening  26  of the tube sheet. To bottom load the filter cartridge  16 , the top end cap  52  is placed within the collar  110  so that bolts  112  are received within the longitudinal channels  62 . Once the end cap  52  contacts resilient gasket  116 , the filter cartridge  16  is rotated relative to the collar  110  similarly to the rotation with respect to the adaptor plate  34  (described above). The gasket  116  forms a generally air-tight seal between the filter cartridge  16  and tubesheet  20 . 
     With reference to FIG. 13, a collar  120 , which is a variation of collar  110 , is shown. A plurality of bolts  122  corresponding to the number of connection areas  58  that are threaded within apertures  124  are formed within the collar  120 . However, a small annular ledge  126  is located about the terminal edge of collar  120 . A resilient gasket  128  is located on the upper surface of ledge  126 . The tubesheet  20 , having collar  120  thereon, is used for top loading the filter cartridge  16 . Specifically, the bottom end cap  54  and tubular filter media  56  are placed downwardly into the lower plenum  22  through the collar  120 . When the top end cap  52  is in proximity with the bolts  122 , the bolts  122  are aligned relative to the longitudinal channels  62  and the top end cap  52  is slid into a resting position on small ledge  126 . The filter cartridge  16  is rotated so that the bolts  122  are moved to the locked position, the filter cartridge  16  is lifted slightly from small ledge  126 . The resilient gasket  128  forms a generally air-tight seal between the filter cartridge  16  and tubesheet  118 . 
     FIG. 14 illustrates a tubesheet  130  for use in conjunction with an alternative air filter cartridge shown in FIGS. 15-18. Tubesheet  130  includes a plurality of openings  132 . Around the periphery of opening  132 , a plurality of tabs  134  extend inwardly toward the center of each opening. With reference to FIG. 15, the tabs  134  have a beveled edge  136  on a first side of each tab  134 , and a body  138  having a width generally equal to the planar portion of the tubesheet  130 . The tabs  136  are preferably spaced angularly equidistant from one another. Accordingly, the three tabs  134  of the preferred embodiment are offset by 120° from each adjacent tab  134 . Alternatively, the tubesheet  130  may have varying numbers of tabs. 
     With reference to FIGS. 15-18, an alternative filter cartridge is designated generally by the numeral  140 . The filter cartridge  140  is similar to filter cartridge  16  except for the construction of the top end cap  142  formed about a first end of the tubular filter media  144 . With specific reference to FIG. 15, the top end cap  142  includes a first connector portion  146 , a second connector portion  148  and an annular flange  150  disposed therebetween. 
     With reference to FIGS. 15 and 18, the first connector portion  146  has a first outer sidewall portion  152 , a first inner sidewall portion  154  and a top edge  156 . A plurality of recessed channels  158  defining threads are formed within first outer sidewall portion  152  of first connector portion  146 . A plurality of channel openings  160  extends longitudinally from the top edge  156  of the first connector portion  146  and each channel opening  160  is in communication with a corresponding recessed channel  158 . The recessed channels  158  are defined by the upper surface  161  of annular flange  150 , an end wall  162 , and an edge wall  164 . The edge wall  164  has a first portion  166 , extending from end wall  162 , that is parallel with the upper surface  161  of annular flange  150  and a second portion  168  extending between first portion  166  and opening  160  that tapers away from the upper surface  161  of flange  150 . In the preferred embodiment, a ridge  170  is defined at the intersection of first portion  166  and second portion  168 . The depth within the outer sidewall  152  of each recessed channel  158  is slightly greater than the length that each tab  134  extends inwardly from the corresponding opening  132  of the tubesheet  130 . 
     The annular flange  150  has an outer diameter greater than the diameter of opening  132 . A first annular gasket  172  is located on the surface of flange  150  directed toward the first connector portion  146 , and a second annular gasket  174  is located on the surface of flange  150  directed toward the second connector portion  148 . The gaskets  172  and  174  are preferably made from a resilient material that may be adhered or otherwise secured to the annular flange  150 . With specific reference to FIG. 18, the annular flange  150  has an inner diameter and presents a ledge  176  around the opening of top end cap  142 . A groove  177  is formed near a midpoint of ledge  176 . Two pairs of upstanding members  178  and  180  are located on the ledge  176  to define opposing notches  182  and  184 . The ledge  176  and notches  182  and  184  are used in conjunction with the tool  84  (FIG. 10) as described above. 
     With reference back to FIG. 15, the second connector portion  148  is generally similar to the first connector portion  146 . Namely, second connector portion  148  has a second outer sidewall portion  186  and a bottom edge  188 . The inner portion of the second connector portion is formed about the tubular filter media  144  as is known to those of ordinary skill in the art. A plurality of recessed channels  190 , each channel defining a thread are formed within second outer sidewall portion  186  of second connector portion  148 . A plurality of openings  192  extend longitudinally from the bottom edge  188  into communication with each recessed channel  190 . Recessed channels  190  are defined by a lower surface  194  of annular flange  150 , an end wall  196 , and an edge wall  198 . The edge wall  198  has a first portion  200  extending from the end wall  196  and disposed parallel with the lower surface  194 . A second portion  202  extends between first portion  200  and opening  192  and tapers away from flange  150 . In the preferred embodiment, a ridge  204  is defined at the intersection of first portion  200  and second portion  202 . Additionally, the second connector portion  148  includes a tapered skirt  206  extending from the outer sidewall section  186  and around the tubular filter media  144 . The depth of each recessed channel  190  is the same as the depth of the recessed channels  158  of the first connector portion. 
     With reference to FIG. 15, to bottom load the filter cartridge  140  to the tubesheet  130 , the openings  160  are placed in alignment which each of the tabs  134 . When the tabs are placed within openings  160  and are in close proximity with the upper surface  161  of annular flange  150 , the filter cartridge  140  is rotated in the direction indicated by the arrow in FIG. 16 so that the beveled edge  136  moves within the recessed channel  158  between annular flange  150  and the second portion  168  of edge wall  164 . As the filter cartridge  140  is further rotated, each tab  134  slides between the second portion  168  of edge wall  164  and the annular flange  150 . When the rotation is complete, each tab  134  is seated between an end wall  162  and the small ridge  170  at the intersection of first portion  166  and second portion  168  of edge wall  164 . In this position, the first annular gasket  172  is in connection with the bottom of tubesheet  130  to form a seal therebetween. The bottom of filter cartridge  140  may be similar to the bottom end cap of filter cartridge  160  as shown in FIG.  9 . Thus, the tool  84  (FIG. 10) may be used to either secure or remove the filter cartridge  140  from the tubesheet  130  when bottom loading (or unloading). 
     With reference to FIG. 17, the filter cartridge  140  may be top loaded within the baghouse  10  use of second connector portion  148 . Specifically, the tubular filter media  144  of filter cartridge  140  is placed through one of the openings  132  of tubesheet  130 . When the top end cap  142  is in proximity with the tubesheet  130 , the tabs  134  are aligned with the channel openings  192  of second connector portion  148 . The tabs are received within the openings  192  so that the tabs are in communication with the bottom lower surface  194  of annular flange  150 . At this point, the filter cartridge  140  is rotated in the direction indicated by the arrow in FIG. 17 so that the tab is positioned within the portion of the recessed channel  190  between the edge wall  198  and the lower surface  194  of annular flange  150 . As indicated in phantom lines in FIG. 17, when rotation of the filter cartridge  140  is complete, each tab  134  is positioned between the first portion  200  of edge wall  198  and the lower surface  194  of annular flange  150  between the end wall  196  and the ridge  204 . When the filter cartridge  140  is in the top loaded position, the second annular gasket  174  is in communication with the upper surface of tubesheet  130  to form a seal therebetween. To facilitate the unloading and loading of the filter cartridges  140  from the upper plenum  24 , the tool  84  (FIG. 10) may be placed on ledge  176  and the rod  94  placed within notches  182  and  184 . 
     FIG. 19 demonstrates an alternative embodiment of the filter cartridge  140  wherein the channel openings  192  are significantly larger than the channel openings  160  to facilitate the receipt of larger tabs located about the openings  132  of the tubesheet  130 . This is merely illustrative of one of a number of variations of the inventive construction described above. 
     From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 
     Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative of applications of the principles of this invention, and not in a limiting sense.