Patent Abstract:
Cross flow trays are provided with a support system. The support system interconnects the upper cross flow tray to a downcomer of the lower cross flow tray to provide support for the upper tray. In some aspects, the support system may also interconnect the downcomer of the lower cross flow tray to an upper tray of an underlying pair, providing additional support for the assembly. Such tray assemblies may facilitate easier installation without impeding the performance of the column.

Full Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to cross flow trays used in mass transfer columns in which mass transfer and/or heat exchange processes occur and, more particularly, to apparatus and methods for supporting such cross flow trays. 
         [0002]    Cross flow trays are used within mass transfer columns to facilitate interaction between fluid streams flowing in countercurrent relationship within the column. The term mass transfer column as used herein is not intended to be limited to columns in which mass transfer is the primary objective of the processing of the fluid streams within the column, but is also intended to encompass columns in which heat transfer rather than mass transfer is the primary objective of the processing. The fluid streams are typically an ascending vapor stream and a descending liquid stream, in which case the cross flow trays are commonly referred to as vapor-liquid cross flow trays. In some applications, both fluid streams are liquid streams and the cross flow trays are commonly referred to as liquid-liquid cross flow trays. In still other applications, the ascending fluid stream is a gas stream and the descending fluid steam is a liquid stream, in which case the cross flow trays are referred to as gas-liquid cross flow trays. 
         [0003]    The cross flow trays are positioned within the column in vertically spaced-apart relationship with each of the tray decks extending horizontally to fill the entire internal cross-section of the column. Each of the cross flow trays has a planar tray deck on and above which interaction between the ascending fluid stream and the descending fluid stream occurs, a plurality of apertures to allow upward passage of the ascending fluid stream through the tray deck and into the descending fluid stream to create a froth or mixture in which the desired mass transfer and/or heat exchange occurs, and at least one downcomer that directs the descending fluid stream from the associated tray deck to a tray deck on an underlying cross flow tray. The portion of the tray deck that receives the descending fluid stream from the downcomer of an overlying cross flow tray typically comprises an inlet panel that is either imperforate or contains bubble promoters or other structures that allow upward passage of the ascending fluid stream but impede the descending fluid stream from weeping through the inlet panel. 
         [0004]    Cross flow trays having a single side downcomer located at one end of the tray deck are known as single-pass trays. In other applications, typically those involving higher descending liquid flow rates, multiple downcomers may be used on some or all of the cross flow trays. For example, in two-pass configurations, two side downcomers are positioned at opposite ends of one cross flow tray and a single center downcomer is positioned in the center of the adjacent cross flow trays. In four-pass configurations, one cross flow tray has two side downcomers and a center downcomer and the adjacent contact trays have two off-center downcomers. 
         [0005]    The tray decks of cross flow trays are typically secured by clamps to support rings welded to the interior surface of the column shell. The downcomer walls are also normally bolted at their opposite ends to bolting bars that are welded to the interior surface of the column shell. In some applications, such as in larger diameter columns and in columns in which vibratory forces are a concern, it is known to add further support to portions of the tray deck by using a strut that extends upwardly from major beams, lattice trusses or a system of hangers to connect the tray deck of a cross-flow tray to the downcomer walls of a similar tray located directly above, or below. When hangers are utilized, the downcomer walls act as beams to carry a portion of the load of the coupled tray, thereby reducing sagging and bracing against uplift of the tray deck. These hangers and other structures, however, add complexity to the design and increase the cost of fabrication and installation of the cross flow tray. 
         [0006]    In other applications, the inlet panel on the tray deck is formed as a structural beam to provide added support to the tray deck. The inlet panel must then be interconnected to the adjacent portions of the tray deck using fasteners of various types, thereby adding to the complexity in the design and the installation of the tray deck. A need has thus arisen for a method of supporting and bracing the tray deck while reducing the disadvantages resulting from the conventional methods of providing additional support in larger diameter columns and in columns in which vibratory forces are present. 
       SUMMARY OF THE INVENTION 
       [0007]    In one aspect, the present invention is directed to a tray assembly for use in a mass transfer column. The tray assembly comprises a first upper cross flow tray comprising a first generally planar upper tray deck and at least one downcomer for removing liquid from the first upper deck and a first lower cross flow tray vertically spaced from the first upper tray. The first lower tray comprises a first generally planar lower tray deck and an inlet area positioned proximate the downcomer of the first upper tray for receiving liquid from the first upper tray. The first lower tray further comprises at least one centrally located downcomer positioned at or near the center of the lower deck for removing liquid therefrom and the centrally located downcomer comprises a pair of spaced-apart parallel walls extending in a direction generally perpendicular to the direction of liquid flow across the first lower deck. The tray assembly also comprise a support system at least partially disposed between the first upper and lower trays. The support system comprises a plurality of horizontally-spaced braces coupled to and extending between the walls of the centrally located downcomer, a support beam coupled to a lower surface of the upper deck, and a plurality of elongated strut members coupled to and extending between the support beam and each of the braces. 
         [0008]    In another aspect, the invention is directed to a mass transfer column comprising an outer column shell defining an open internal volume and at least one cross-flow tray assembly as described above positioned in the open internal volume of the shell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a side elevation view of a mass transfer column in which mass and/or heat transfer are intended to occur and in which a portion of the column shell is broken away to show the upper and lower cross flow trays of the present invention; 
           [0010]      FIG. 2  is an enlarged, fragmentary, top perspective view of the mass transfer column shown in  FIG. 1  with portions of the column shell broken away to show the upper and lower cross flow trays; 
           [0011]      FIG. 3  is an fragmentary, bottom perspective view of a pair of the upper and lower cross flow trays shown in  FIG. 2 , but taken from a bottom perspective; 
           [0012]      FIG. 4  is an enlarged, fragmentary, side elevation view of the upper and lower cross flow trays shown in  FIG. 2 ; 
           [0013]      FIG. 5  is an enlarged, fragmentary, top plan view showing a portion of a support system for the upper cross flow tray taken in horizontal section; 
           [0014]      FIG. 6  is a top perspective view of the support system and a center downcomer; 
           [0015]      FIG. 7  is a bottom perspective view of the support system and the center downcomer; 
           [0016]      FIG. 8  is a fragmentary, side perspective view of a mass transfer column with portions of the column shell broken away to illustrate a second embodiment of the lower cross flow trays of the present invention; 
           [0017]      FIG. 9  is a top perspective view of another embodiment of a support system used with the center downcomer of the second embodiment of the lower cross flow trays; 
           [0018]      FIG. 10  is a fragmentary, side perspective view of a mass transfer column with portions of the column shell broken away to illustrate a third embodiment of the lower cross flow trays of the present invention; 
           [0019]      FIG. 11  is a top perspective view of another embodiment of a support system used with the center downcomer of the third embodiment of the lower cross flow trays of the present invention; 
           [0020]      FIG. 12  is a fragmentary, side perspective view of a mass transfer column with portions of the column shell broken away to illustrate a fourth embodiment of the lower cross flow trays and a second embodiment of the upper cross flow trays of the present invention; 
           [0021]      FIG. 13  is a top perspective view of another embodiment of a support system used with the center downcomer of the fourth embodiment of the lower cross flow trays of the present invention; 
           [0022]      FIG. 14  is a fragmentary, side perspective view of a mass transfer column with portions of the column shell broken away to illustrate a fifth embodiment of the lower cross flow trays of the present invention; and 
           [0023]      FIG. 15  is a fragmentary, side perspective view of a mass transfer column with portions of the column shell broken away to illustrate a sixth embodiment of the lower cross flow trays of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Turning now to the drawings in greater detail and initially to  FIG. 1 , a mass transfer column suitable for use in processes in which mass transfer and/or heat exchange is intended to occur between countercurrent-flowing fluid streams is represented generally by the numeral  10 . Mass transfer column  10  includes an upright, external shell  12  that is generally cylindrical in configuration, although other orientations, such as horizontal, and configurations, including polygonal, are possible and are within the scope of the present invention. Shell  12  is of any suitable diameter and height and is constructed from one or more rigid materials that are desirably inert to, or are otherwise compatible with the fluids and conditions present during operation of the mass transfer column  10 . 
         [0025]    Mass transfer column  10  is of a type used for processing fluid streams, typically liquid and vapor streams, to obtain fractionation products and/or to otherwise cause mass transfer and/or heat exchange between the fluid streams. For example, mass transfer column  10  can be one in which crude atmospheric, lube vacuum, crude vacuum, fluid or thermal cracking fractionating, coker or visbreaker fractionating, coke scrubbing, reactor off-gas scrubbing, gas quenching, edible oil deodorization, pollution control scrubbing, and other processes occur. 
         [0026]    The shell  12  of the mass transfer column  10  defines an open internal region  14  in which the desired mass transfer and/or heat exchange between the fluid streams occurs. Normally, the fluid streams comprise one or more ascending vapor streams and one or more descending liquid streams. Alternatively, the fluid streams may comprise both ascending and descending liquid streams or an ascending gas stream and a descending liquid stream. 
         [0027]    The fluid streams are directed to the mass transfer column  10  through any number of feed lines  16  positioned at appropriate locations along the height of the mass transfer column  10 . One or more vapor streams can also be generated within the mass transfer column  10  rather than being introduced into the mass transfer column  10  through the feed lines  16 . The mass transfer column  10  will also typically include an overhead line  18  for removing a vapor product or byproduct and a bottom stream takeoff line  20  for removing a liquid product or byproduct from the mass transfer column  10 . Other column components that are typically present, such as reflux stream lines, reboilers, condensers, vapor horns, and the like, are not illustrated in the drawings because they are conventional in nature and an illustration of these components is not believed to be necessary for an understanding of the present invention. 
         [0028]    Turning additionally to  FIGS. 2 and 3 , any desired number of pairs of an upper cross flow tray  22  and a lower cross flow tray  24  constructed according to the present invention are positioned within the open internal region  14  of the mass transfer column  10  in vertically-spaced apart relationship with respect to each other. Each of the upper cross flow trays  22  comprises a generally planar tray deck  26  and two side downcomers  28  and  30  that are positioned at opposite ends of the tray deck  26 . The tray deck  26  is formed from individual panels  32  that are joined together using any of various conventional methods. The panels  32  extend longitudinally in the direction from one end to the other end of the tray deck  26 . In one embodiment, most or all of the panels  32  extend continuously between the ends of the tray deck  26 . In another embodiment, most or all of the panels  32  extend from the ends of the tray deck  26  to the centerline of the tray deck  26  where they are joined together in a conventional fashion. Some or all of panels  32  include stiffening flanges  34  that extend perpendicularly downward from the panels  32 , typically along one of the longitudinal edges of each of the panels  32 . 
         [0029]    Most of the tray deck  26  includes apertures  36  to allow an ascending vapor, gas or liquid stream to pass through the tray deck  26  for interaction with a liquid stream traveling along an upper surface of the tray deck  26 . The apertures  36  can be in the form of simple sieve holes or directional louvers or they may include structures such as fixed or movable valves. The portion of the tray deck  26  containing the apertures  36  is known as the active area of the cross flow tray  24 . A center portion of the tray deck  26  is imperforate and functions as an inlet region  38  for receiving liquid flowing downwardly from an overlying liquid distributor (not shown) or a center downcomer  40  of an overlying one of the lower cross flow trays  24 . The inlet region  38  may include bubble promoters or other structures to allow the ascending fluid stream to pass upwardly through the center portion of the tray deck  26 . 
         [0030]    The lower cross flow tray  24  likewise comprises a tray deck  42  formed from interconnected individual panels  44  and containing apertures  36  of the type described above. The panels  44  contain downwardly extending stiffening flanges  46  along their longitudinal edges. Both end portions of the tray deck  26  of the lower cross flow tray  24  are normally imperforate and function as inlet regions  48  and  50  to receive the liquid flowing downwardly through the side downcomers  28  and  30  from the upper cross flow tray  22 . The inlet regions  48  and  50  may include bubble promoters or other structures to allow the ascending fluid stream to pass upwardly through the end portions of the tray deck  42 . 
         [0031]    The lower cross flow tray  24  includes the center downcomer  40  that receives the liquid flowing towards the center of the tray deck  42  from the end portions and delivers it to the inlet panel  38  of the upper cross flow tray  22  in the next underlying pair of upper and lower cross flow trays  22  and  24 . 
         [0032]    The center downcomer  40  in each cross flow tray  24  comprises a pair of spaced apart, parallel walls  52  that extend in a chordal fashion across the open internal region  14  within the mass transfer column  10 . Each wall  52  comprises an upper wall segment  54  and a lower wall segment  56  that may be vertically adjusted in relation to each other during installation of the upper and lower cross flow trays  22  and  24 . Opposite ends of each wall  52  are bolted to bolting bars (not shown) that are welded to the internal surface of the shell  12 . 
         [0033]    A plurality of horizontally-spaced-apart upper braces  58  extend perpendicularly between and are bolted to the upper wall segments  54  of the center downcomer  40  to stiffen and maintain the desired spacing between the walls  52  of the center downcomer  40 . A plurality of similar, horizontally-spaced-apart, lower braces  60  extend perpendicularly between and are bolted to the lower wall segments  56 . The lower braces  60  also serve to stiffen and maintain the desired spacing between the walls  52 . The lower braces  60  extend below a lower edge of the lower wall segments  56  and rest on and are bolted to the tray deck  26  of an underlying upper cross flow tray  22 . In this manner, the lower braces  60  interconnect the walls  52  of the center downcomer  40  to the tray deck  26  of the underlying upper cross flow tray  22  to provide support to the center downcomer  40  and to maintain the desired vertical clearance between the lower edge of the lower wall segments  56  and the tray deck  26  of the underlying upper cross flow tray  22 . 
         [0034]    Turning additionally to  FIGS. 4-6 , in accordance with the present invention, a support system  62  is provided to interconnect and support the center portion of the tray deck  26  of one or more of the upper cross flow trays  22  with the center downcomer  40  of the paired lower cross flow tray  24 . The support system  62  comprises the upper braces  58 , elongated struts  64  that are bolted or otherwise secured to the upper braces  58  and extend upwardly therefrom, and a horizontally-extending beam  66  that is bolted or otherwise secured to an upper end of each of the struts  64 . The struts  64  are formed from angled metal stock or other suitable materials having the necessary strength, rigidity, and compatibility with the conditions within the mass transfer column  10 . The number of struts  64  and the horizontal spacing between adjacent struts  64  can be varied and are selected based on the loads and vibratory forces expected in the intended application. 
         [0035]    The beam  66  is likewise formed from metal or other suitable materials and, in one embodiment, comprises a vertical segment  68 , an upper flange  70  that extends perpendicularly from an upper edge of the vertical segment  68  in one direction, and a lower flange  72  that extends perpendicularly from a lower edge of the vertical segment  68  in an opposite direction. Cutouts  74  ( FIG. 6 ) are provided at spaced apart locations in the upper flange  70  and in part of the vertical segment  68  to accommodate the stiffening flanges  34  of the individual panels  32  in the tray deck  26  of the upper cross flow tray  22 . Alternatively, the cutouts  74  may be omitted by removing the stiffening flanges  34  in the portions of the tray deck  26  that overlap the beam  66 . The panels  32  of the tray deck  26  rest on the beam  66  and are bolted or otherwise secured the upper flange  70  of the beam  66 . Shims may be placed between the lower end of one or more of the struts  64  and a flange on the upper brace  58  to which the strut  64  is mounted to level the tray deck  26  during installation. 
         [0036]    Anti-jump baffles  76  extend between and are bolted or otherwise mounted to the struts  64 . The anti-jump baffles  76  function to impede liquid and dense froth from jumping over rather than entering the center downcomer  40 . The anti-jump baffles  76  are positioned so that their lower edges are roughly at the same horizontal elevation as an upper edge of weirs  78  positioned on the tray deck  42  at the inlet to the center downcomer  40 . The anti-jump baffles  76  have a height sufficient to block the liquid and dense froth from jumping over the center downcomer  40 , while at the same time allowing an upper edge of each anti-jump baffle  76  to be spaced a sufficient distance below the overlying upper cross flow tray  22  to permit vapor to readily flow above the anti-jump baffles  76 . 
         [0037]    Each anti-jump baffle  76  comprises a vertically-extending plate  80 , a strengthening flange  82  at an upper and/or lower edge of the plate  80 , and, as best shown in  FIG. 5 , a dog-legged mounting flange  84  positioned along each of the side edges of the plate  80 . The mounting flanges  84  overlap and are bolted or otherwise secured to the struts  64 . The dog-legged construction of the mounting flanges  84  allow the plates  80  and the struts  64  to be positioned along the longitudinal centerline of the center downcomer  40 . Alternatively, the struts  64  can be offset slightly from the centerline so that the plates  80  can be positioned along the centerline without requiring the mounting flanges  84  to have a dog-legged construction. 
         [0038]    It can be seen that the support system  62  interconnects the upper and lower cross flow trays  22  and  24  and functions to support the center portion of the upper cross flow tray  22  using the center downcomer  40  of the lower cross flow tray  24 . The support system  62  eliminates the need for the tray deck  26  of the upper cross flow tray  26  to have a separate inlet panel that functions as a support beam and must be interconnected with the panels  32 . Using the support system  62  to provide support at the center of the tray deck  26  allows some or all of the panels  32  to span from end to end of the tray deck  26  as a single piece, thereby greatly simplifying the design and installation of the tray deck  26 . Alternatively, some or all of the panels  32  may extend from the end to the center of the tray deck  26  where they may abut or overlap panels  32  extending to the center from the other end of the tray  26 . The abutting or overlapping panels  32  may then be bolted or otherwise secured to the beam  66  that underlies the panels  32 . 
         [0039]    The upper and lower cross flow trays  22  and  24  that are interconnected by the support system  62  are also interconnected to the underlying pair of upper and lower cross flow trays  22  and  24  by the lower braces  60  that join the center downcomer  40  to the tray deck  26  and/or the beam  66  of the support system  62  associated with the underlying pair of upper and lower cross flow trays  22  and  24 . By interconnecting and supporting the upper and lower cross flow trays  22  and  24  in this manner, the upper and lower cross flow trays  22  and  24  are better able to resist the loading and vibratory forces exerted during operation of the column  10 . The tray decks  26  and  42  are thus more likely to remain level and stationary during such operation. 
         [0040]    The support system  62  can be used or readily adapted for use with center downcomers of various constructions. For example, in  FIGS. 8 and 9 , the support system  62  is used with a lower cross flow tray  124  that has a center downcomer  140  with inclined downcomer walls  152  rather than the vertical downcomer walls  52  associated with the center downcomer  40  of the lower cross flow tray  24  illustrated in  FIGS. 1-4  and  6 - 8 . In all other respects, the lower cross flow tray  124  of  FIGS. 8 and 9  may have the same construction as the previously described lower cross flow trays  24  and the same reference numerals preceded by the numeral “1” are used to reference the like components. 
         [0041]    Turning now to  FIGS. 10 and 11 , upper and lower cross flow trays  222  and  224  and a support system  262  are shown and differ in the following respects from upper and lower cross flow trays  22  and  24  and support system  62  shown in  FIGS. 1-4  and  6 - 8 . All of the panels  226  except for the outer panels  226  of the upper cross flow tray  222  extend from the side downcomers  228  and  230  to the centerline of the tray deck  226  where they abut and are bolted to the beam  266  of the support system  262 . Because the panels  226  abut at the centerline, the stiffening flanges  234  can be removed in the area of the beam  266  so that the beam  266  does not need to include cutouts for the stiffening flanges  234 . The center downcomer  240  includes additional rectangular-framed braces  286  are of a sufficient height to be bolted to both the upper and lower wall segments  254  and  256  of the downcomer walls  252 . These additional braces  286  are spaced apart at predetermined intervals from each other and from the upper braces  258  and  260 . In all other respects, the upper and lower cross flow trays  222  and  224  and support system  262  are generally the same as the upper and lower cross flow trays  22  and  24  and support system  62 . For ease of reference, the same reference numerals preceded by the numeral “2” have been used in  FIGS. 10 and 11  to refer to like components shown in  FIGS. 1-4  and  6 - 8 . 
         [0042]    Turning now to  FIGS. 12 and 13 , another embodiment of a center downcomer with which the support system  62  may be used is designated by the numeral  340 . Center downcomer  340  has inclined downcomer walls  352  that terminate at a floor plate  388  containing louvers  390  ( FIG. 12 ) or simple apertures  392  ( FIG. 13 ). The lower braces  360 , rather than extending between the downcomer walls  352 , underlie the floor plate  388  and support the downcomer walls  352  on the inlet region  238  of the tray deck  226  of the underlying lower cross flow tray  224 . In all other respects, the upper cross flow tray  322  may be of the same construction as the upper cross flow tray  222 . The same reference numerals preceded by the numeral “3” rather than “2” have been used to refer to like components shown in  FIGS. 10 and 11 . 
         [0043]    Turning now to  FIG. 14 , adjacent pairs of upper and lower cross flow trays  422  and  424  are shown, along with corresponding first and second support systems  462  and  562 . The upper and lower trays  422  and  424  and the support systems  462  and  562  differ in the following respects from the upper and lower cross flow trays  22  and  24  and the support system  62  shown in  FIGS. 1-4  and  6 - 8 . The horizontally-extending beams  466  and  566  of the support systems  462  and  562  are also vertically elongated. The beam  566  of the support system  562  extends to the same vertical elevation as, or to a vertical elevation just below, the upper edges of the walls  452  of the central downcomer  440  of the lower tray  424  of the second pair of trays. As also shown in  FIG. 14 , the elongated beam  566  further includes a plurality of laterally-spaced cut-outs  567 , positioned at a desired interval along the length of the beam  566 . Although shown as being polygonal in cross-section, the cut-outs  567  may be of any size and/or shape, and the number and spacing of such cut-outs can be determined based on the specific application. 
         [0044]    Additionally, the beams  466  and  566  each include a knife edge  469  located at or near the terminal edge of the support beam  466  or  566  for helping to control foam and/or heavy froth within each of the central downcomers  440 . Neither of the support systems  462  or  562  includes a set of lower braces  60  coupled to an underlying upper tray. Accordingly, in the embodiment shown in  FIG. 14 , the lower tray  424  of each pair is not physically coupled to the upper tray  422  of the underlying pair of trays. In all other respects, both sets of the upper and lower cross flow trays  422  and  424  and the support systems  462  and  562  shown in  FIG. 14  are generally the same as the upper and lower cross flow trays  22  and  24  and the support system  62 . For ease of reference, the same reference numerals preceded by the numeral “4” have been used in  FIG. 14  to refer to like components shown in  FIGS. 1-4  and  6 - 8 . 
         [0045]    Turning now to  FIG. 15 , adjacent pairs of upper and lower cross flow trays  622  and  624  and first and second support systems  662  and  762  are shown and differ in the following respects from the adjacent pairs of upper and lower cross flow trays  422  and  424  and the support systems  462  and  562  as shown in  FIG. 14 . As shown in  FIG. 15 , the first support system  652  is at least partially disposed between the upper set of upper and lower cross flow trays  622  and  624  and includes one or more pairs of anchor tabs positioned along the direction of extension of the downcomer  640 . Each pair of anchor tabs includes an upper anchor tab  651  and a lower anchor tab  653  arranged in a staggered configuration with respect to each other and the direction of extension of the central downcomer  640 . The upper anchor tab  651  of each pair is coupled to a lower surface of the upper tray deck  626 , while the lower anchor tab  653  is coupled to the upper surface of the upper tray deck  626  of the underlying upper tray  622  of the adjacent pair. 
         [0046]    A transverse support beam  677  extends generally diagonally between the upper  651  and lower  653  anchor tabs of each pair. Because of the staggered configuration of the anchor tabs  651  and  653 , the adjacent transverse beams  677  extend in generally opposite diagonal directions. The elongated strut members  654  of the support system  652  are also coupled to the upper anchor tabs  653  of each pair and are generally disposed between pairs of oppositely-extending transverse support beams  677 . The second support system  762  shown in  FIG. 14  is nearly the same as the support system  62  shown in  FIGS. 2 and 3 , except that the support system  762  does not include anti-jump baffles. In all other respects, the upper and the lower cross flow trays  622  and  624 , as well as the first and second support systems  652  and  752 , may be of the same construction as the similar components described with respect to  FIGS. 1-4  and  6 - 8 . The same reference numerals preceded by the numeral “6” rather than “1” have been used to refer to like components shown in  FIGS. 1-4  and  6 - 8 . 
         [0047]    While the invention has been described with respect to a support system that connects to a center downcomer of a lower cross flow tray, it is to be understood that the invention also may include using such a support system with an intermediate, rather than a center, downcomer in a multi-pass cross flow tray. 
         [0048]    From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objectives hereinabove set forth together with other advantages that are inherent to the structure. 
         [0049]    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 invention. 
         [0050]    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 and not in a limiting sense.

Technology Classification (CPC): 1