Abstract:
An apparatus for comminuting material includes two disks arranged coaxially to one another inside a housing that encloses a comminuting room. At least part of the opposing surfaces of the disks are provided with interacting comminuting tools thus forming a comminuting zone, whereby at least one of the disks rotates around a mutual axis to generate a relative movement of the disks. At the same time, the material, which is a mixture of gaseous and solid materials is axially fed by one of the disks into the comminuting room and is radially conveyed to the comminuting zone. Thereby, cooling gas is additionally channeled into the comminuting room. The comminuting room is partitioned into one chamber through which the mixture of gaseous and solid materials flows, and at least one additional chamber dedicated to the cooling gas.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on German Patent Application No. DE 102004050002, which was filed in Germany on Oct. 14, 2004, and which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus for comminuting material with a separate air supply.  
         [0004]     2. Description of the Background Art  
         [0005]     Devices of this class are characterized by an air-ventilated mode of operation. Air, together with a mixture of gaseous and solid materials, is thereby axially channeled into a comminuting room, and after radial rerouting is conveyed by centrifugal forces to an annular comminuting zone, where it is comminuted between the comminuting tools to a desired size. After exiting the comminuting zone, the suitably milled material gathers in a ring channel, which is located between the housing and the comminuting tools, where it is tangentially discharged by the air stream via the material discharge. Apart from the centrifugal forces, the driving force for the transport of the material through the comminuting apparatus is essentially the air flow, the sweeping force of which affects the material.  
         [0006]     When the material is comminuted in the comminuting zone, a considerable part of the energy required for the comminuting is converted into heat. This is caused by friction and impact forces the material is subjected to during comminuting, which primarily affect the comminuting tools. The heating up of the material resulting therefrom carries the risk on the one hand, particularly with regard to heat-sensitive materials and/or fine and finest-milled materials, of the material to be irreversibly ruined, and on the other hand, of the comminuting device to suffer damage due to thermal stress.  
         [0007]     Conventionally, the cooling of devices of this class is done via the air portion in the mixture of gaseous and solid materials that passes through the milling gap. A heat transfer from the comminuting tools to the air thereby takes place, whereby the desired cooling effect is achieved. Thus, devices of this class are characterized in that during the comminuting operation, the air flowing through the device has a transport function as well as a cooling function.  
         [0008]     Furthermore, it is known to channel additional air into the comminuting room. The additional air volume is able to remove heat, thus increasing the cooling effect. Again, the heated air is discharged together with the suitably milled material.  
         [0009]     The disadvantage of conventional comminuting devices is the dual function of the mixture of gaseous and solid materials, which on the one hand has the task of transporting the material, and on the other hand has the task of cooling. Under certain circumstances, for example, in the case of fine and finest milling, the air portion in the mixture must be increased beyond the volume needed for transport for reasons of cooling. As a consequence, large volumes must be filtered to separate the milled material from the mixture of gaseous and solid materials exiting the device. From a structural-technical point of view, this requires large filter surfaces and large conduit cross sections, which, apart from high investment and operation costs, also has the additional consequence of increased spatial requirements.  
         [0010]     This disadvantage is also a characteristic of devices of this class, where additional cool air is channeled in because upstream to the comminuting zone, the additional cool air merges with the mixture of gaseous and solid materials.  
         [0011]     Furthermore, only as an exception does the dual function of the mixture of gaseous and solid materials during the comminuting process lead to an optimal utilization of the comminuting device. In most cases, either the conveying potential of the air portion in the mixture of gaseous and solid materials is exhausted while there are still cooling reserves, or the cooling potential of the air portion is exhausted, although reserves in the conveying capacity would still be available. This leads to a diminished efficiency of conventional devices.  
       SUMMARY OF THE INVENTION  
       [0012]     It is therefore an object of the present invention to improve comminuting devices of this class economically and functionally.  
         [0013]     The invention recognizes the previously described correlations and based thereon, to provide a spatial separation of cooling and material transport utilizing a gas, primarily air.  
         [0014]     The separation of the cooling gas stream from the mixture of gaseous and solid materials makes it possible to calculate the gas portion in the mixture of gaseous and solid materials solely in view of the desired conveying power. The result is a reduction of the gas volume in the mixture of gaseous and solid materials to a minimum. Because the cooling gas stream does not contain any solid materials, and only the volume-reduced mixture of gaseous and solid materials has to be run through filters, this measure has the advantage that smaller filter surfaces and conduit cross sections are sufficient to separate the milled material, resulting in lower investment as well as operational costs.  
         [0015]     Simultaneously, the required amount of cool air can be channeled into the comminuting room, independently from the necessary conveying power and merely dependent on the prevailing temperature and the kind of material. This independent and thus varying control of the mixture of gaseous and solid materials and the cooled gas allows a maximal adaptation of the device of the present invention to outer parameters. This makes is possible to further minimize the operational costs and to achieve a more efficient operation.  
         [0016]     An additional benefit of the separate conduit of the cooling gas is that the cool air stream is not hindered by the solid materials in the mixture of gaseous and solid materials. Thus, the present invention provides for an even and improved cooling effect on the comminuting tools.  
         [0017]     According to a further embodiment of the present invention, a partitioning of the comminuting room by a wall arranged in a plane that is radial to the axis of rotation is provided. The beneficial feature is the forming of two ringwheel-shaped chambers that primarily extend in a direction that is parallel to their flow-through direction.  
         [0018]     Beneficially, the wall is partially formed by the disk that is provided with comminuting tools, adjacent to which, in a radial direction, is a ring wheel. Thus, the device of the present invention is reduced to a minimum of components. Because the wheel is also a part of the chamber for the cooling gas stream, an optimal cooling effect can be achieved in this way.  
         [0019]     Due to the staggered arrangement in a peripheral direction of the two outlets for the mixture of gaseous and solid materials and the cooling gas in an embodiment of the present invention, an equalization of the two parallel line systems is possible with the benefit of better utilization of the available space.  
         [0020]     In a particularly preferred embodiment of the present invention, a stationary disk is formed by the front and rear walls of the housing. In this way, a compact construction of a device of the present invention is attained.  
         [0021]     Beneficially, the stationary disk is formed by the intake side of the housing wall because this results in an extremely simple axial feeding of the material into the comminuting zone.  
         [0022]     In a further embodiment of the invention, two rotating disk forming three separate chambers are provided. This allows an application of the invention in comminuting devices with differently rotating disks resulting in the desired effect that the comminuting tools are subjected to even attrition, and thus to even wear and tear.  
         [0023]     In further development of such devices, the two outlets for the cooling gas can stream-upwardly and can be combined to eliminate the need for dual conduits.  
         [0024]     To better utilize the cooling potential of the cooling gas, the cooling gas stream can be systematically channeled along the temperature-affected components by adding suitable fittings to the chamber. By arranging the fittings on a level with the comminuting tools, the cool air is channeled past the area with the highest heat development so that a maximum heat transfer takes place.  
         [0025]     Further, the side of the disks facing the cool air chamber can be provided with ribs in order to enlarge the surface for the cooling gas, thus increasing the heat transfer.  
         [0026]     By orienting the ribs radially, a flow-through of the chamber is achieved that results in a greater cooling effect. Additionally, with the disk rotating, the radially oriented ribs add a motion impulse to the cooling gas brushing by, thus further advancing the flow of cool air. The arrangement of the ribs in the area of the fittings thereby causes an interaction of these components and thus an improved cooling as well as conveying effect.  
         [0027]     It is further preferred to arrange a temperature sensor in the comminuting room to emit, for example, an infrared beam, which, either continuously or at preset time intervals, registers the temperature in the comminuting zone. By evaluating the data, a temperature-depending control of the cooling gas stream is possible. In a further development of this idea, an automatic control, preferably by a microprocessor-powered control, is provided, which controls both the mixture of gaseous and solid materials and the volume of the cooling gas. In this way, an automated operation of the device of the present invention with continuous optimization is possible.  
         [0028]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. For example, pin mills, refiners and the like are also within the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0030]      FIG. 1  is a longitudinal section of a device according to an embodiment of the present invention along the line I-I illustrated in  FIG. 2 ;  
         [0031]      FIG. 2  is a front view of the device illustrated in  FIG. 1 ;  
         [0032]      FIGS. 3 and 4  are additional partial cross sections according to further embodiments of the invention; and  
         [0033]      FIG. 5  is a front view of an additional embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0034]      FIGS. 1 and 2  show a first embodiment of the present invention, that is, a disc mill. To begin with, a machine substructure  1  is shown, which is illustrated in  FIG. 2  only, the feet  2  of which rest on the ground floor. The upper part of the machine substructure  1  forms a platform, on which the comminuting apparatus of the present invention is mounted.  
         [0035]     The comminuting apparatus includes a drum-shaped housing  3  that encircles a rotational axis  8  such that a comminuting room  4  is formed. On its front side  5 , the housing  3  has a central circular opening  6 , which can be closed and bolted shut with a housing door  7  that is pivotable around a vertical axis.  
         [0036]     The housing door  7  also has a central circular feeder opening  9 , to the outside of which a chute  10  is connected via a round arc  11 . The inside of the housing door  7  expands conically starting from the rim of the feeder opening  9 . The wider inner cross section of the feeder opening  9  resulting therefrom is surrounded by comminuting tools  12 , which are arranged around the axis  8  in a circular shape, thus forming a milling ring. The comminuting tools are therefore fixedly mounted to the inside of the housing door  7 , which in this way assumes the function of the first milling disk.  
         [0037]     In the area of the rotational axis  8 , a rear wall  15  also has a circular opening  16 . Adjacent to the outside of the rear wall  15  is a box-shaped reinforcement  18 , which encloses a cavity  17  that is parallel to the comminuting room  4 . The box-shaped reinforcement  18  also has a circular opening  19  in the area of the axis of rotation  8 . A further opening  36  is provided in the bottom area of the box-shaped reinforcement  18 , through which the cavity  17  can be supplied with cool air  35 . Coaxially to the axis of rotation  8 , an arrangement of horizontal shaft bearings  20  is provided, which is fixedly connected to the box-shaped reinforcement  18  and extends into the opening  19 .  
         [0038]     In the arrangement of shaft bearings  20 , a freely rotatable drive shaft  22  is mounted within the bearing group  21 , the front end of which extends through the opening  16  in the rear wall  15  of the housing  3  into the comminuting room  4 . Attached to its exterior end is a multiple-groove pulley  23 , which is connected by straps to the drive motor  24 , which is only illustrated in  FIG. 2 . The straps extend thereby inside a protective sheathing  25 .  
         [0039]     On the opposite end of the drive shaft  22  extending into the comminuting room  4 , a circular hub plate  26  is mounted. Thus, the hub plate  26  is arranged plane-parallel and at a distance to the housing door  7 . The hub plate  26  is also provided with comminuting tools  27  forming a milling ring, which are positioned opposite the comminuting tools  12  at a narrow axial distance, thus forming a milling gap, both interacting to comminute the material.  
         [0040]     At the level of the comminuting tools  27 , radially oriented ribs  28  are evenly distributed around the periphery of the side of the hub plate  26  that faces the rear wall  15 . The ribs can be 5-25 mm high and can be spaced at mutual peripheral intervals of 20-100 mm. Due to their rigid attachment to the hub plate  26 , the ribs  28  and the hub plate  26  together execute a rotational motion around the axis  8 .  
         [0041]     On the rear wall  15  of the housing  3 , axially across from the ribs  28 , air-conducting elements  37  are attached, which narrow the flow-through cross section in this area. In this way, cool air  35  is systematically directed to the components that show the highest heat development. Furthermore, the rotating ribs  28  interact with the air-conducting elements  37  such that the cool air stream also has a conveying effect.  
         [0042]     Adjacent to the peripheral side of the hub plate  26 , located in a radial plane, is a ring wheel  29 . Across its outer periphery, the ring wheel  29  is rigidly connected to the housing  3 , whereas its inner periphery forms a gliding connection to the hub plate  26 . In this way, the ringwheel-shaped comminuting room  4  is separated into two chambers  30  and  31 , which are also ringwheel-shaped. The partition wall formed by the hub plate  26  and the ring wheel  29  extends in a radial plane.  
         [0043]     As can be particularly seen in  FIG. 1 , this partition also extends into the area of the material discharge  14 , which is connected to a subsequent line system. A first line  32  is thereby connected to the chamber  30  to extract the milled material, and a second line  33  is connected to the chamber  31  to extract the cool air  35 .  
         [0044]     In addition, the comminuting device of the present invention is provided with a temperature sensor  38 . The temperature sensor  38  is attached to the periphery of the housing  3  ( FIG. 2 ), for example, and preferably includes an infrared sensor, which records the temperature in the comminuting zone, either continuously or at preset time intervals. The measured temperature can be directly displayed on a screen, or else can be transmitted to an automatic control.  
         [0045]     During operation, the device of the present invention works as follows:  
         [0046]     As indicated by the arrow  34 , the material comprised of a mixture of gaseous and solid materials is fed axially into the comminuting room  4 , via the chute  10  and the round arc  11 , where it first encounters the top side of the hub plate  26 . There it is rerouted into a radial direction and is drawn into the milling gap between the comminuting tools  12  and  27  by centrifugal forces. After exiting the milling gap, the milled material, together with the air portion of the mixture of gaseous and solid materials  34 , passes on to the chamber  30  of the comminuting room  4 , and is then conveyed via the first line  32  to a filter device (not shown), where a separation of the solid phase from the gaseous phase takes place. The mixture of gaseous and solid materials  34  is thereby characterized by its mixing ratio, whereby the gaseous portion is calculated such that it is able to transport the desired quantity of material to and through the device of the present invention. Although the gaseous portion of the mixture of gaseous and solid materials  34  has also a cooling effect in the comminuting zone, this does not have to be the deciding factor when determining the gaseous portion.  
         [0047]     To cool down the comminuting zone, additional cool air, as indicated by arrows  35 , is channeled into the comminuting device. The cool air  35  can be extracted from the ambient air, or can be derived from an air-conditioning system, and is channeled via the opening  36  into the cavity  17  of the box-shaped reinforcement  18 . From there, the cool air  35  is channeled via the circular gap between the hub plate  26  and the opening  16  to the chamber  31  of the comminuting room  4 . There, the cool air  35  is radially rerouted, and by utilizing the air-conducting elements  37 , is directed to the ribs  28 . When flowing through the ribs  28 , a heat transfer from the ribs  28  to the cool air  35  occurs, resulting in a cooling effect at the same time. Subsequently, the cool air  35  exits the chamber  31  through the second line  33 . Since the cool air  35  does not mix with the material, that is, with the milled material, there is no need for the cool air  35  to be run through filter devices to filter out solid materials.  
         [0048]     During the comminuting process, the temperature in the comminuting zone is monitored with the temperature sensor  38 . If a value is reached that may damage the material or the comminuting device, the volume of cool air  35  that is fed into the device is increased and/or the quantity of material that is fed into the device is reduced in order to attain the desired temperature in the comminuting zone. In this way, a device of the present invention can always be operated with an optimal mixing ratio of material to cool air at a predefined temperature. By using automatic controls, a fully automated operation can be realized.  
         [0049]      FIG. 3  illustrates a further embodiment of the present invention. The illustration is thereby limited to areas essential of the invention since the remaining structure is identical to the device described in  FIGS. 1 and 2  so that the same applies. The layout corresponds with  FIG. 1 .  
         [0050]      FIG. 3  also shows a drum-shaped housing  41  surrounding an axis of rotation  40 , which encloses a comminuting room  42 . The front side of the housing is formed by a pivotable housing door  43 , which in the area of the axis  40  is provided with a concentric, circular feeder opening  44 . Furthermore, additional openings  45  are provided in the housing door  43 , which are arranged in a circle around the feeder opening  44 .  
         [0051]     In the area of the axis  40 , the rear wall  46  of the housing  41  has a shaft exit for a drive shaft  47  (only partially illustrated), which extends into the comminuting room  42 . At this end of the drive shaft  47 , there is a milling disk  48  located in a radial plane.  
         [0052]     In the outer peripheral area of the side of the milling disk  48  that faces the rear wall  46 , a milling ring  49  is attached. Opposite thereto, at an axial distance, thus forming a milling gap, an additional milling ring  50  is located, which is fixedly connected to the rear wall  46  of the housing  41 . In this embodiment, the rear wall  46  functions like a stationary milling disk.  
         [0053]     On the side of the milling disk  48  that faces the housing door  43 , radially oriented ribs  51  are positioned at a level with the milling ring  49 , which are even distributed around the periphery of the milling disk  48  and rigidly attached thereto.  
         [0054]     In the area between the drive shaft  47  and the milling ring  49 , the milling disk  48  has openings  52 , which connect the front side of the milling disk  48  to its rear side. Furthermore, an annular guiding plate  53  can be seen on the front side of the milling disk  48 , which is fixedly attached to the milling disk  48  and glidingly connected to the feeder opening  44  in the housing door  43 .  
         [0055]     The comminuting room  42  of the present invention is divided into a chamber  54  and a chamber  55 . Once again, the milling disk  48  and the ring plate  39  that connects to the milling disk  48  in a radial direction, serve as a partition wall. On its outer periphery, the ring wheel  39  is connected to the housing  41 , and with its inner periphery, slidingly connects to the milling disk  48 . The chamber-like partitioning of the comminuting device continues into the material discharge, where a first line  56  connects to the chamber  54  and a second line  57  to the chamber  55 .  
         [0056]     During operation, a mixture of gaseous and solid materials  58  is fed through the feeder opening  44  along the guiding plate  53  and through the openings  52  into the chamber  55 , where is passes through the milling gap due to centrifugal forces while being milled. The sufficiently milled material, together with the air, is channeled to line  57 , which leads to a filter device (not shown) for filtering out the solid particles.  
         [0057]     Through the openings  45  in the housing door  43 , the cool air  59  is channeled to the chamber  54 , where it brushes radially along the ribs  51 , whereby once again a cooling down of the comminuting zone takes place. The cool air  59  is discharged from the comminuting device via the line  56  and can be released directly into the ambient air without prior filtering, for example.  
         [0058]      FIG. 4  illustrates an embodiment of the idea of this invention with a disc mill having two counter-rotating milling disks, whereby once again only the parts that are essential to the invention are shown. The remaining components that are not illustrated are almost identical to the device illustrated in  FIGS. 1 and 2  so that reference is made to that part of the description.  
         [0059]     The device illustrated in  FIG. 4  has a drum-shaped housing  62  that surrounds an axis of rotation  60  and encloses a comminuting room  61 . The front side of the housing  62  is formed by a pivotable housing door  63  that allows access to the housing interior.  
         [0060]     In the area of the rotational axis  60 , the housing door  63  has a central opening  64 , through which material is fed into the device. The opening  64  is surrounded by additional openings  65 , which are positioned on a circular periphery. The inside of the housing  63  has a circular connecting piece  66  that is concentric towards the axis of rotation  60 .  
         [0061]     In the area of the axis of rotation  60 , the rear side  67  of the housing  62  has an opening  68  for receiving the drive shafts for the milling apparatus. Grouped in a circle around the opening  68  are yet again additional openings  69 .  
         [0062]     Extending in the area of the axis of rotation  60  is a first drive shaft  70  designed as a hollow shaft, the end of which extends into the comminuting room  61 . The first drive shaft  70  is mounted, freely rotatable, inside a horizontal bearing arrangement. The horizontal bearing arrangement is not illustrated in  FIG. 4  but is essentially identical to the one described in  FIG. 1 .  
         [0063]     The end of the first drive shaft  70  supports a first milling disk  71 , which is oriented in a radial plane to the axis of rotation  60 . The milling disk  71  is thereby positioned at an axial distance to the rear wall  67  as well as to the housing door  63 . On the side facing the housing door  63 , the outer peripheral area of the first milling disk  71  is provided with a first milling ring  72 . On the opposite side of the milling disk  71 , in the outer circumferential area, first radial ribs  73  are evenly distributed around the periphery.  
         [0064]     Inside the first drive shaft  70 , a second, freely rotatable drive shaft  74  is arranged, the end of which extends beyond the end of the first drive shaft  70  into the comminuting room  61 . This end supports a plane-parallel second milling disk  75 , the outer peripheral area of which is provided with a second milling ring  76 . The second milling ring  76  is thereby located axially opposite the first milling ring  72 , thus forming a radial milling gap. On the opposite side of the second milling disk  75 , second radial ribs  77  are evenly distributed around the periphery.  
         [0065]     In addition, there is a plurality of openings  78  in the area between the second milling ring  76  and the drive shaft  74 , which allow the passing-through of material from the front side to the rear side of the second milling disk  75 . In the area of the openings  78 , the second milling disk has an annular shoulder  79 , which forms a sliding connection to the circular connecting piece  66 .  
         [0066]     On its peripheral side, the first milling disk  71  is surrounded by a first ring wheel  80 , which is arranged in a radial plane. With its outer periphery, the ring wheel  80  is fixedly connected to the housing  62 , whereas the inner periphery is glidingly connected to the first milling disk  71 . In this way, a first disk-shaped chamber  81  is formed in the comminuting room  61 .  
         [0067]     On its peripheral side, the second milling disk  75  is surrounded by a second plane-parallel ring wheel  82 , which with its outer periphery is also fixedly connected to the housing  62 , whereas with its inner periphery, it is glidingly connected to the second milling disk  75 . In this way, a second chamber  83  and a third chamber  84  are formed in the comminuting room  62 . Upstream, the first chamber  81  and the second chamber  83  are merged in a common line, which is not illustrated in  FIG. 4 .  
         [0068]     During operation, the device illustrated in  FIG. 4  works as follows:  
         [0069]     With the milling disks  71  and  75  counter-rotating, or rotating unidirectional with rotational speed difference, the material as indicated by arrows  85  is axially channeled through the openings  64  and  78  to the area between the millings disks  71  and  75 . After encountering the milling disk  71 , the mixture of gaseous and solid materials is radially rerouted and is drawn by centrifugal forces into the milling gap formed by the two milling disks  72  and  76 . After comminuting, the sufficiently milled material is channeled into the annular chamber  84 , where it gathers to be tangentially conveyed by the air stream via the material discharge to a filter device (not illustrated).  
         [0070]     In order to prevent an overheating of the comminuting tools and the material, a first cool air stream indicated by the arrows  86  is channeled through the openings  69  in the rear side  67  of the housing  62  into the first chamber  81 . In this way, a flow is generated in the first chamber  81  along the first milling disk  71 , and particularly along the first radial ribs  73 . Thereby, a heat transfer and thus a cooling of the comminuting tools takes place before the cool air  86  is tangentially discharged from the housing  62 .  
         [0071]     Likewise, a second cool air stream indicated by the arrows  87  is channeled from the front of the device through the openings  65  in the housing door  63  into the second chamber  83 . The air flow thereby generated along the second milling disk  75 , and particularly the second cooling ribs  77 , allows a heat transfer and thus a cooling of the comminuting tools. The cool air stream  87  is also tangentially discharged from the housing  62 .  
         [0072]     The air  86  and  87  used for cooling can be directly taken from the ambient air, or else can be obtained via lines (not shown) from an air-conditioning system.  
         [0073]     The best-possible symmetrical feeding of the device of the present invention with material and cool air allows a uniform temperature distribution in the comminuting zone and thus the best-possible utilization of the comminuting potential of a device of the present invention.  
         [0074]      FIG. 5  shows a device of the present invention, which is almost identical to the one illustrated in  FIG. 1  so that by using identical reference numerals, reference is made to the corresponding part of the description of  FIG. 1 . The only difference is in the construction of the material discharge.  
         [0075]      FIG. 5  illustrates a material discharge  88  that is split in two, comprised of a first discharge piece  89 , which leads vertically upwards, and a second discharge piece  90 , which terminates at an offset in a peripheral direction from the housing  3 . In the chamber  30  illustrated in  FIG. 1 , the first discharge piece  89  is designated for the milled material, whereas the second discharge piece  90  is designated for the discharge of the cool air  35  from the chamber  31 . The offset arrangement of the two discharge pieces  89  and  90  allows a better utilization of the available space with better accessibility.  
         [0076]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.