Abstract:
A valve and a method of using the valve to clean a filter unit are provided. As such, the valve is a small vessel penetration diameter cleaning valve that requires no or less installation welding and a reduced pressure vessel wall thickness while not jeopardizing cleaning valve reliability or efficiency. The valve also includes a dampening mechanism to decrease mechanical stresses, decrease operating noise and increase operational life expectancy of the valve.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    In general, the present disclosure relates to a cleaning valve useful for cleaning at least a portion of a filter unit, such as filter bags, arranged in a filter installation useful for filtering polluted gas passed therethrough. More specifically, the present disclosure relates to a relatively small vessel penetration diameter cleaning valve that requires less installation welding and a reduced pressure vessel wall thickness while not jeopardizing cleaning valve reliability or efficiency. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    Commercial “bag house” type filter installations typically consist of a plurality of parallel filter units, each containing a plurality of parallel rows of vertically arranged filter elements in the form of filter bags. Each such filter bag has a top end opening. A gas polluted with particulates is channeled through the filter bags to filter and collect particulates entrained in the gas. Hence, upon filtering and collecting the particulates entrained in the gas, a “cleaned gas” is produced. More specifically, cleaned gas is produced by channeling a polluted gas into a filter installation for passage through one or more filter units for gas flow from an exterior surface of a plurality of filter bags through to an interior area within the filter bags via a flow path through the sides of the filter bags. As the polluted gas passes from the exterior surface of the filter bags through to the interior area within the filter bags, particulate pollutants entrained in the gas are filtered and collected forming dust cakes on the exterior surfaces of the filter bags. Hence, gas in the interior area of the filter bags is the so produced cleaned gas. Cleaned gas exits the interior areas of the filter bags via a top end opening in each such filter bag. Cleaned gas flows from the top end openings through an outlet duct common to the filter units. During operation of the filter installation, a negative pressure is typically generated by a fan arranged downstream of the filter installation to cause gas flow through the filter units and filter bags. 
         [0003]    As noted above, dust and particulates entrained in the polluted gas are filtered by and collected on the exterior surfaces of the filter bags, thus forming dust cakes thereon. Cleaning of the filter bags to remove the dust cakes is necessary for effective and efficient equipment performance. Cleaning of the filter bags is accomplished using a pressure medium in the form of compressed air pulses injected into the filter bags in a direction opposite to that of gas filtering. Rows of filter bags are cleaned successively using cleaning units arranged for each such given row. A cleaning unit cleans a row of filter bags by generating a compressed air pulse delivered substantially simultaneously to each filter bag in the given row. More specifically, each cleaning unit comprises a nozzle pipe arranged above and extending the length of the associated row of filter bags for cleaning. Each nozzle pipe typically has a plurality of vertically downwardly projecting pipe sockets connected thereto. Each pipe socket is positioned straight above a filter bag top end opening within the associated row. The function of these pipe sockets is to direct via nozzles compressed air pulses into the respective filter bag top end openings. The pipe sockets usually have a diameter of about 1.5 to 2 times greater than the diameter of the nozzle associated therewith. The nozzles associated therewith consist of circular holes of varying diameter formed in the nozzle pipe. The varying diameter of the circular holes along the nozzle pipe is determined empirically based on the total number of pipe sockets/nozzles, requiring a uniform distribution of compressed air pulsed therethrough. As such, circular holes arranged in the nozzle pipe are smaller or larger in diameter depending on the circular holes&#39; distance from the nozzle pipe inlet. By so varying the diameter of the circular holes, a uniform distribution of compressed air pulsed therethrough is achieved. 
         [0004]    In the cleaning of filter bags using a pulse of compressed air, a valve is temporarily opened to establish fluid flow between a compressed air tank or pressure vessel and the nozzle pipe. Upon fluid flow between the compressed air tank or pressure vessel and the nozzle, compressed air is pulsed through the nozzle pipe and its associated pipe sockets and nozzles. As such, a compressed air pulse is supplied to each of the filter bags in the associated row of filter bags. Compressed air pulses supplied to the filter bags dislodge dust and particulates that collect and cake in and on the walls of the filter bags. Dust cakes formed on the filter bags are thereby loosened by the flow of compressed air from the interior areas of the filter bags, through the filter bag side walls, to an area in the filter unit exterior thereto. The resultant loosened dust cakes fall off the exterior of the filter bags for hopper collection. 
         [0005]    In operating a cleaning unit, it is essential that the above-described pulse valve delivers a cleaning pulse of compressed air at a relatively high pressure with a relatively low consumption of compressed air. Pulse valves function by a cavity behind a plunger or membrane emptying through either a solenoid valve or a pilot valve, whereby the plunger or membrane is displaced by the differential pressure between the pressure vessel or air tank pressure on one side of the plunger or membrane and the cavity pressure on the other side of the plunger or membrane. The plunger or membrane undergoes considerable acceleration and achieves considerable velocity upon displacement as a result of this pressure differential. Eventually the plunger or membrane impacts an end position with very high momentum. The plunger or membrane impacting the end position with very high momentum creates a significantly loud noise upon impact. Likewise, when the plunger or membrane impacts the end position, such impact creates relatively high mechanical stresses. Mechanical stresses on the valve shorten the operational life expectancy of the valve and add to the system&#39;s operation costs when performance is hampered and/or replacement is necessary. Installation and replacement of valves on compressed air tanks or pressure vessels typically requires bolting of the valve to a flange welded to the compressed air tank or pressure vessel. Welding of the flanges to the compressed air tank or pressure vessel is relatively costly due to the relatively large amount of welding required therefor. Bolting valves to tanks or vessels requires a relatively large valve outer diameter to accommodate the bolts. As such, these relatively large diameter valves must sometimes be staggered in their placement in order to accommodate their larger size. Such is especially true for smaller valve pitches, i.e., smaller distances between valves. Staggered placement of valves is also relatively costly. Hence, to increase system performance and decrease system operational costs, a valve with decreased mechanical stresses, decreased installation and replacement cost, and increased operational life expectancy is desired. 
       SUMMARY OF THE DISCLOSURE 
       [0006]    In view of the above, disclosed herein is a pulse valve with a relatively small vessel penetration diameter. Also disclosed herein is a method of using the subject pulse valve with relatively small vessel penetration diameter for pulsed compressed air cleaning of a plurality of filter elements, such as filter bags. The subject valve requires decreased or no installation welding thereby reducing costs associated therewith. Also, the subject valve has a relatively small valve housing diameter since the valve housing diameter does not have to accommodate for bolting thereof, thus reducing valve spacing requirements and costs associated therewith. The relatively small vessel penetration diameter of the subject valve is important in that it reduces tank or vessel thickness requirements thereby also reducing costs associated with using thicker walled compressed air tanks or pressure vessels. Larger sized plunger extension tabs of the subject valve also provides for high performance filter cleaning. As such, the subject pulse valve with a relatively small vessel penetration diameter is useful for efficient pulsed compressed air cleaning of at least a portion of a filter unit. 
         [0007]    The subject pulse valve with relatively small vessel penetration diameter comprises a housing with a plunger slideably positioned therein. The housing is arranged in an opening of a pressure vessel, or compressed air tank, containing compressed air having a pressure of about 10 pounds per square inch (psi) to about 145 psi, or about 60 psi. The housing is air tightly fixed in an opening of the pressure vessel by engagement of a base thereof within a free end of a nozzle pipe arranged within the pressure vessel. As such, the base is sized with a relatively small vessel penetration diameter suitable for engagement thereof within a free end of the nozzle pipe. Once engaged within the free end of the nozzle pipe, one or more openings formed within the housing allow for a flow of compressed air to pass from the pressure vessel through the housing and into the fluidly connected nozzle pipe. Compressed air flowing through the nozzle pipe effectively cleans at least a portion of a filter unit when the plunger is in a first “open” position. 
         [0008]    A fluid supply is fluidly connected to or on the housing to supply a fluid, such as air, to an interior vault or cavity within the housing. Filling of the interior vault with air causes a downward movement of the plunger within the housing to a second “closed” position. When in the second closed position, the plunger blocks the one or more openings in the housing and thus blocks the flow of compressed air from the compressed air tank or pressure vessel to the nozzle pipe. Compressed air in the pressure vessel or compressed air tank has a pressure of about 10 psi to about 145 psi, or about 60 psi. When the plunger is in the first open position, the fluid pressure within the interior vault or cavity of the housing is significantly less than that of the compressed air within the pressure vessel. When the plunger is in the second closed position, the fluid pressure within the interior vault or cavity of the housing is equal or greater than that of the compressed air within the pressure vessel and significantly greater than the pressure in the nozzle pipe. 
         [0009]    A dampening mechanism, such as one or more cushions, is arranged inside the housing to reduce or dampen the impact between the housing and the plunger upon movement of the plunger into the first open position. Cushioning the impact between the housing and the plunger reduces mechanical stresses caused by such impacts and improves valve reliability even with higher pressure vessel or tank pressure. The internal dampening mechanism likewise reduces the impact noise of the plunger upon displacement or movement of the plunger into the first open position. In addition to the dampening mechanism, fluid within the interior vault or cavity of the housing provides a cushioning effect that reduces impact and impact noise of the plunger upon movement thereof into the first open position. 
         [0010]    A method of using the subject pulse valve device for cleaning at least a portion of a filter unit comprises reducing fluid pressure within an interior vault or cavity of a valve housing to cause pressure differential displacement or movement of a plunger into a first open position allowing for a flow of compressed air from a pressure vessel or compressed air tank through the valve housing to a nozzle pipe in fluid connection with a filter unit, thereby cleaning the filter unit with a pulse of compressed air. Increasing fluid pressure within an interior vault or cavity of the valve housing likewise causes pressure differential displacement or movement of the plunger into a second closed position blocking flow of compressed air from the pressure vessel or compressed air tank and hence from the nozzle pipe, until a further pulse cleaning of the filter unit is indicated by buildup of a filter dust cake. 
         [0011]    For purposes of this method, the compressed air in the pressure vessel has a pressure of about 10 psi to about 145 psi, or about 60 psi. When the plunger is in the first open position, the fluid pressure within the interior vault of the housing is significantly less than that of the compressed air within the pressure vessel or compressed air tank. When the plunger is in the second closed position, the fluid pressure within the interior vault of the housing is equal or greater than that of the compressed air within the pressure vessel and significantly greater than that of the nozzle pipe. Pressure within the housing is controlled using a solenoid valve or the like for purposes of the subject method to control fluid flow into the interior vault of the housing to thus control the pressure differential between that of the interior vault and that of the pressure vessel or compressed air tank. 
         [0012]    The subject method further comprises providing a dampening mechanism within the housing to reduce or cushion the impact between the housing and the plunger upon movement of the plunger into the first open position. Providing a dampening mechanism as herein described also reduces the impact noise of the plunger with the housing upon movement of the plunger into the first open position. Fluid within the interior vault of the housing likewise provides a cushioning effect to reduce impact and impact noise of the plunger upon movement of the plunger into the first open position. 
         [0013]    The subject pulse valve device and method for using the same to clean at least a portion of a filter unit, such as filter bags, arranged in a filter installation to filter polluted gas passed therethrough includes among other features noted, a relatively small vessel penetration diameter, and no weld or low weld installation, to reduce costs while not jeopardizing reliability or efficiency. 
         [0014]    In summary, a valve is provided useful for pulsed compressed air cleaning of a filter unit. The valve comprises a housing with a plunger slideably positioned therein arranged in an opening of a pressure vessel containing compressed air and removably threadedly fixed to a nozzle pipe for a fluid connection thereto, one or more openings in the housing for the flow of compressed air from the pressure vessel through the housing to the nozzle pipe useful for cleaning at least a portion of a filter unit when the plunger is in an open position, and a fluid supply on the housing for control of a fluid to a vault within the housing to cause movement of the plunger to a closed position for blocking the one or more openings to block the flow of compressed air from the pressure vessel to the nozzle pipe. The compressed air in the pressure vessel has a pressure of about 10 psi to about 145 psi. When the plunger is in the closed position, the fluid pressure within the vault of the housing is equal or greater than that of the compressed air within the pressure vessel and significantly greater than that of the nozzle pipe. When the plunger is in the open position, the fluid pressure within the vault of the housing is less than that of the compressed air within the pressure vessel. Also, the housing further comprises a dampening mechanism to reduce impact between the housing and the plunger upon movement of the plunger into the open position. The dampening mechanism also reduces impact noise of the plunger upon movement of the plunger into the open position. Further, a valve is either provided on the housing or connected to the housing through tubing. As such, a solenoid valve is provided on the housing to control fluid flow to and from the vault of the housing. 
         [0015]    In summary, a method is provided for using a valve for cleaning at least a portion of a filter unit. The method comprises increasing fluid pressure within a vault of a valve housing causing pressure movement of a plunger into a closed position blocking flow of compressed air from a pressure vessel through to a nozzle pipe, and decreasing fluid pressure within a vault of a valve housing causing pressure movement of the plunger into an opened position allowing flow of compressed air from a pressure vessel through to a nozzle pipe in fluid connection with a filter unit thereby pulse cleaning the filter unit. The compressed air in the pressure vessel has a pressure of about 10 psi to about 145 psi. When the plunger is in the closed position, the fluid pressure within the vault of the housing is equal to or greater than that of the compressed air within the pressure vessel, and significantly greater than that of the nozzle pipe. When the plunger is in the open position, the fluid pressure within the vault of the housing is less than that of the compressed air within the pressure vessel. A dampening mechanism is also provided to reduce impact between the housing and the plunger upon movement of the plunger into the open position. The dampening mechanism is also used to reduce impact noise of the plunger upon movement of the plunger into the open position. A valve is provided on the housing for controlling movement of the plunger. As such, a solenoid valve is provided on the housing or connected to the housing through tubing to control fluid flow to and from the vault to control movement of the plunger. By controlling movement of the plunger, the solenoid valve on the housing also controls flow of compressed air to the nozzle pipe and cleaning of the filter unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will now be described in more detail with reference to the accompanying drawings. 
           [0017]      FIG. 1  is a schematic side cross-sectional view of a pulse valve device according to the present invention with a plunger in a first “open” position. 
           [0018]      FIG. 2  is a schematic side cross-sectional view of a pulse valve device according to the present invention with a plunger in a second “closed” position. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    A pulse valve  10  with a relatively small vessel penetration diameter D for arrangement in tank or pressure vessel  24  opening  46  and suitably sized for installation within a nozzle pipe  28  arranged in tank or pressure vessel  24  is useful for effective compressed air pulsed cleaning of a plurality of filter elements, such as filter bags, in a filter unit  27 . Pulse valve  10  requires decreased installation or replacement welding reducing costs associated therewith, decreased vessel penetration diameter D reducing valve spacing requirements on pressure vessel  24  and reducing pressure vessel  24  thickness requirements thereby reducing costs associated therewith, and relatively large extended tabs  54   a  on plunger  18  enabling a relatively small vessel penetration diameter D, as is described herein. As illustrated in  FIG. 1 , the subject pulse valve  10  comprises a valve housing  12 . Valve housing  12  is manufactured of a sturdy natural, e.g., iron, aluminum, or other metal, or synthetic, e.g., plastic, resin or other polymer, material suitably rigid and durable for robust industrial uses and forces. Housing  12  is formed with sides  32 . Sides  32  include one or more openings  32   a  therethrough. Sides  32  also include an exterior surface  36  and an interior surface  38 . Top  34  of housing  12  includes an exterior surface  40  and an interior surface  42 . Extending from interior surface  42  of top  34  is a cylinder member  34   a . Cylinder member  34   a  is formed with an attached end  34   b  opposite a free end  34   c . Extending from interior surface  42  between side  32  interior surface  38  and cylinder member  34   a  are one or more dampening mechanisms or cushions  30 . Cushions  30  may be manufactured from natural or synthetic rubber, polyurethane, silicone or a like flexible material capable of providing cushioning effects upon repeated impact between solid surfaces. Valve housing  12  also includes a base  12   a  defining an opening  13 . Base  12   a  is tubular and sized to threadedly engage interior surface  28   a  of nozzle pipe  28 . As such, threads  29  are provided on each the exterior surface  36  of housing  12  near base  12   a  and the interior surface  28   a  of nozzle pipe  28  near free end  28   b  for a removably interlocking connection therebetween. On interior surface  28   a  of nozzle pipe  28  abutting exterior surface  36  of housing  12 , is a channel  15  with a pliable material  17 , such as an O-ring, therein to ensure an air tight seal between housing  12  and nozzle pipe  28 . In removably engaging housing  12  and nozzle pipe  28 , valve  10  is positioned within an opening  46  of pressure vessel  24  with base lip  44  of valve housing  12  abutting exterior surface  48  of pressure vessel  24 . In base lip  44  of valve housing  12  is a channel  50  with a pliant sealing member  52  therein creating an airtight seal between exterior surface  48  of pressure vessel  24  and base lip  44  of valve housing  12 . Valve  10  is sized so that the distance between opposed outer edges  44   a  of base lips  44  is reduced in size as compared to a like valve requiring bolting, to allow for linear valve placement rather than requiring staggered valve placement for filter cleaning. Within interior area  22  of housing  12  extending from interior surface  38  of walls  32  are valve seats  39 . Valve seats  39  abut a portion of base  60  when plunger  18  is in a second closed position as will be described in more detail below. 
         [0020]    Slideably positioned within interior area  22  of housing  12  is a plunger  18 . Plunger  18  is likewise manufactured of a sturdy natural, e.g., iron, aluminum, or other metal, or synthetic, e.g., plastic, resin or other polymer, material suitably rigid and durable for robust industrial uses and forces. Extended tabs  54   a  of sides  54  of plunger  18  contact interior surfaces  38  of sides  32  of valve housing  12  for an air tight seal therebetween. With an air tight seal between extended tabs  54   a  and interior surfaces  38 , one or more air holes  55  are provided through sides  54  of plunger  18 . In addition to or as an alternative to air holes  55  through sides  54  of plunger  18 , air may be allowed to leak between extended tabs  54   a  and surfaces  38 , as explained in more detail below. Extending between sides  54  of plunger  18  opposite free ends  56  of extended tabs  54   a  is a base  60  with a center plug portion  62  extending upwardly therefrom cylinder portion  66   a  into interior vault  22   a  of valve housing  12 . Plug portion  62  is manufactured to extend from plunger  18  interior surface  66  on cylinder portion  66   a  away from opposed exterior surface  64  of base  60 . Interior surface  66  of plunger  18  is in fluid communication with interior area  68  of plunger  18 . Between interior surface  66  of plunger  18  and interior surface  42  of top  34  of valve housing  12  is interior area  68 . The area A 1  of interior area  68  varies as plunger  18  moves or slides within valve housing  12 . The area A 1  of interior area  68  is minimized when base  60  of plunger  18  moves toward top  34  of valve housing  12  for contact of free ends  56  with pliable dampening mechanisms  30  at interior surface  42  of top  34  of valve housing  12 . Air in interior area  68  flows from decreasing area A 1  of interior area  68  through air holes  55  and/or leaks around extended tabs  54   a  into increasing area A 2  of interior area  22 . In this first “opened” position of  FIG. 1 , free ends  56  of sides  54  contact pliable dampening mechanisms  30  and plug portion  62  slides within interior vault  22   a  of valve housing  12  to abut sealing seat  20  of valve housing  12  causing fluid “F” to flow from interior vault  22   a  through solenoid valve  14  integrally formed with, securely affixed to, or connected through tubing (not shown) to valve housing  12 . In this first “open” position, interior vault  22   a  is an area A 3  of low pressure causing plug portion  62  to slide into contact with sealing seat  20 . 
         [0021]    The area A 2  of interior area  22  is maximized when pressure within internal vault  22   a  is decreased causing a flow of fluid F from internal vault  22   a  outwardly through passage  14   a  and solenoid valve  14 . As such, plug portion  62  of plunger  18  moves inwardly into interior vault  22   a  and into contact with sealing seat  20  in a first open position which allows a higher pressure flow of compressed air CA to flow from compressed air tank  24  through one or more openings  32   a  of housing  12  and into fluidly connected nozzle pipe  28 . Now, referring to  FIG. 2 , the area A 2  of interior area  22  is minimized when pressure within area A 3  of internal vault  22   a  is increased causing a flow of fluid F from a fluid source  14   b  through solenoid valve  14  and passage  14   a  into internal vault  22   a . As such, plug portion  62  of plunger  18  moves outwardly from sealing seat  20  into interior vault  22   a  into a second closed position which blocks the now lower pressure compressed air CA from flowing from the compressed air tank or pressure vessel  24  through one or more openings  32   a  of housing  12  and into fluidly connected nozzle pipe  28 . As such, air in decreasing area A 2  of interior area  22  flows through air holes  55  and/or leaks around extended tabs  54   a  into increasing area A 1  of interior area  68 . In summary, upon outward movement of plug portion  62  away from sealing seat  20  in interior vault  22   a , compressed air CA from pressure vessel  24  is blocked by plunger  18  from flowing through one or more openings  32   a  in sides  32  of valve housing  12  into area A 2  of internal area  22 . Fluidly connected to internal area  22  is housing base  12   a  opening  13 . Base  12   a  of housing  12  is fluidly connected to nozzle pipe  28 . As such, in this second “closed” position, plunger  18  is positioned within internal area  22  of valve housing  12  to block compressed air CA flow through one or more openings  32   a  and into fluidly connected nozzle pipe  28 . Likewise, plug portion  62  moves a distance from sealing seat  20  of interior vault  22   a  upon fluid F flow from solenoid valve  14  into interior vault  22   a  via passage  14   a . In this second closed position, the compressed air CA pressure inside interior area  26  of pressure vessel  24  and inside interior area  22  of valve housing  12  are less than that of the fluid F pressure inside interior vault  22   a.    
         [0022]    A method of using the subject pulse valve  10  for cleaning at least a portion of a filter unit  27  comprises decreasing fluid F pressure within interior vault  22   a  of a valve housing  12  to cause pressure movement of a plunger  18  into a first “open” position allowing flow of compressed air CA from a pressure vessel or compressed air tank  24  through valve housing  12  and into a nozzle pipe  28  in fluid connection therewith. This pulse of compressed air CA into nozzle pipe  28  cleans the filter unit  27  from dust cake build up for hopper collection. Increasing fluid F pressure within interior vault  22   a  of the valve housing  12  likewise causes pressure movement of the plunger  18  into a second “closed” position blocking flow of compressed air CA from the pressure vessel or compressed air tank  24  to the fluidly connected nozzle pipe  28  until the next filter unit  27  cleaning. 
         [0023]    For purposes of this method, the compressed air CA in the pressure vessel  24  has a pressure of about 10 psi to about 145 psi, or about 60 psi. When the plunger  18  is in the first open position, the fluid F pressure within the interior vault  22   a  of the valve housing  12  is significantly less than that of the compressed air CA within the pressure vessel or compressed air tank  24 . When the plunger  18  is in the second closed position, the fluid F pressure within the interior vault  22   a  of the valve housing  12  is equal to greater than that of the compressed air CA within interior  26  of pressure vessel  24 , and significantly greater than that of the nozzle pipe  28 . Pressure within the valve housing  12  interior vault  22   a  is controlled using a solenoid valve  14  or the like for purposes of the subject method to control fluid F flow into and out of the interior vault  22   a  of the valve housing  12  and to thus control the movement of plunger  18  and compressed air CA flow to nozzle pipe  28 . 
         [0024]    The subject method further comprises providing a dampening mechanism  30   a  comprising one or more cushions  30 , within the valve housing  12  to reduce or cushion the impact between the valve housing  12  interior surface  42  and the plunger  18  free ends  56  upon movement of the plunger  18  into the first open position. Providing cushions  30  as herein described also reduces the impact noise of the plunger  18  with the valve housing  12  upon movement of the plunger  18  into the first open position. 
         [0025]    Additionally, dampening mechanism  30   a  enables the use of an increased tank pressure for increased filter area cleaning per cleaning valve without jeopardizing cleaning valve reliability. Without dampening mechanism  30   a , increased tank pressure jeopardizes cleaning valve reliability due to damage or wear caused by increased mechanical stresses from the resultant higher velocity impact of the plunger  18  with the valve housing  12 . Dampening mechanism  30   a  cushions the impact of plunger  18  within valve housing  12  thus lessening mechanical stresses of such impacts and reducing damage or wear to the cleaning valve  10 . Hence, with dampening mechanism  30   a , cleaning valve  10  reliability is not jeopardized with increased tank or pressure vessel  24  compressed air CA pressure. 
         [0026]    While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.