Patent Publication Number: US-2022213896-A1

Title: Step seal for refrigerant compressors

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/133,471, filed Jan. 4, 2021, and also claims the benefit of U.S. Provisional Application No. 63/224,479, filed Jul. 22, 2021. The entirety of the &#39;471 and &#39;479 applications are herein incorporated by reference. 
    
    
     BACKGROUND 
     Refrigerant compressors are used to circulate refrigerant in a chiller via a refrigerant loop. Refrigerant loops are known to include a condenser, an expansion device, and an evaporator. The compressor compresses the fluid, which then travels to a condenser, which in turn cools and condenses the fluid. The refrigerant then goes to an expansion device, which decreases the pressure of the fluid, and to the evaporator, where the fluid is vaporized, completing a refrigeration cycle. 
     Many refrigerant compressors are centrifugal compressors and have an electric motor that drives at least one impeller to compress refrigerant. The fluid is then directed downstream for use in the chiller system. Known refrigerant compressors have seals. 
     SUMMARY 
     In some aspects, the techniques described herein relate to a refrigerant compressor, including: a stator; a rotor configured to rotate with respect to the stator; and at least one step seal between the rotor and the stator, wherein the step seal includes a first tooth and a second tooth extending from the rotor toward the stator, wherein a downstream surface of the first tooth and an upstream surface of the second tooth are arranged at an angle relative to one another, wherein the angle is less than 90°. 
     In some aspects, the techniques described herein relate to a refrigerant compressor as recited claim  1 , wherein the first tooth and the second tooth include a pair of teeth, and the step seal includes a plurality of pairs of teeth, and wherein each pair of teeth is provided in a stepped arrangement. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the first and second teeth are formed in the rotor and an axial tooth is formed in the stator, and wherein the axial tooth extends in a substantially axial direction radially outward of the first and second teeth. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the first tooth has a first point and the second tooth has a second point, and wherein the first and second points are arranged at a common radial position. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the downstream surface and the upstream surface meet at a curved surface to form a curved cavity. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein a radially inner cavity wall is arranged downstream of the second tooth to form a second cavity downstream of the curved cavity. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the second cavity is a curved cavity. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the second cavity is a square cavity. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the stator has an abradable portion, and wherein the first and second teeth extend toward the abradable portion. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the first and second teeth are configured to contact the abradable portion and carve tracks into the abradable portion over time. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein the refrigerant compressor is used in a heating, ventilation, and air conditioning (HVAC) chiller system. 
     In some aspects, the techniques described herein relate to a refrigerant compressor, wherein a stator cavity is arranged in the stator, the stator cavity is arranged axially between the first tooth and the second tooth. 
     In some aspects, the techniques described herein relate to a refrigeration system, including: a condenser; an evaporator; an expansion device; and a compressor, wherein the compressor includes a stator, a rotor configured to rotate with respect to the stator, and at least one step seal between the rotor and the stator, wherein the step seal includes a first tooth and a second tooth extending from the rotor toward the stator, wherein a downstream surface of the first tooth and an upstream surface of the second tooth are arranged at an angle relative to one another, wherein the angle is less than 90°. 
     In some aspects, the techniques described herein relate to a refrigeration system as recited claim  13 , wherein the first tooth and the second tooth include a pair of teeth, and the step seal includes a plurality of pairs of teeth, each pair of teeth provided in a stepped arrangement. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein the first tooth has a first point and the second tooth has a second point, and wherein the first and second points are arranged at a common radial position. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein the downstream surface and the upstream surface meet at a curved surface to form a curved cavity. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein a radially inner cavity wall is arranged downstream of the second tooth to form a second cavity downstream of the curved cavity. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein the second cavity is a curved cavity. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein the second cavity is a square cavity. 
     In some aspects, the techniques described herein relate to a refrigeration system, wherein the stator has an abradable portion, wherein the first and second teeth extend toward the abradable portion, and wherein the first and second teeth are configured to contact the abradable portion and carve tracks into the abradable portion over time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a refrigerant loop. 
         FIG. 2  shows a schematic view of a refrigerant compressor. 
         FIG. 3  shows a schematic view of an example step seal arrangement. 
         FIG. 4A  shows an example step seal arrangement. 
         FIG. 4B  shows the example step seal arrangement of  FIG. 4A . 
         FIG. 5A  shows another example step seal arrangement. 
         FIG. 5B  shows the example step seal arrangement of  FIG. 5A . 
         FIG. 6A  shows another example step seal arrangement. 
         FIG. 6B  shows the example step seal arrangement of  FIG. 6A . 
         FIG. 7  shows a schematic view of another example step seal arrangement. 
         FIG. 8A  shows an example step seal arrangement. 
         FIG. 8B  shows the example step seal arrangement of  FIG. 8A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a refrigerant system, which includes a compressor  10 , a condenser  11 , an evaporator  13 , and an expansion device  15  arranged in a main refrigerant loop, or circuit,  17 . This refrigerant system may be used in a chiller, for example. In that example, a cooling tower may be in fluid communication with the condenser  11 . While a particular example of the refrigerant system is shown, this application extends to other refrigerant system configurations, including configurations that do not include a chiller. For instance, the main refrigerant loop  17  can include an economizer downstream of the condenser  11  and upstream of the expansion device  15 . The refrigerant system may be part of a heating, ventilation, and air conditioning (HVAC) chiller system, for example. 
       FIG. 2  illustrates a portion of the compressor  10  from  FIG. 1  in more detail, which in this example is a refrigerant compressor  10  (“compressor  10 ”). The compressor  10  includes a housing  12 , which encloses an electric motor  14 . The housing  12  may comprise one or more pieces. The electric motor  14  rotationally drives at least one impeller about an axis A to compress refrigerant. Example refrigerants include chemical refrigerants, such as R-134a and the like. The motor  14  may be driven by a variable frequency drive. The compressor  10  includes a first impeller  16  and a second impeller  18 , each of which is connected to the motor  14  via a shaft  19 . In the illustrated example, the impellers  16 ,  18  are centrifugal impellers. While two impellers are illustrated, this disclosure extends to compressors having one or more impellers. In some embodiments, the compressor  10  may have two axial compression stages, or may have a mixed stage (i.e., a stage with a radial and an axial component) and an axial compression stage. 
     The housing  12  establishes a main refrigerant flow path F. In particular, the housing  12  establishes an outer boundary for the main refrigerant flow path F. A first, or main, flow of refrigerant is configured to flow along the main refrigerant flow path F between a compressor inlet  20  and a compressor outlet  22 . In the illustrated example, there are no inlet guide vanes disposed at the compressor inlet  20 . The lack of inlet guide vanes reduces the number of mechanical parts in the compressor  10 . In other examples, inlet guide vanes may be arranged near the inlet  20 . 
     From left to right in  FIG. 2 , the main refrigerant flow path F begins at the compressor inlet  20 , where refrigerant is drawn toward the first impeller  16 . The first impeller  16  is provided in the main refrigerant flow path F, and is arranged upstream of the second impeller  18  relative to the main refrigerant flow path F. The first impeller  16  includes an inlet  161  arranged axially, generally parallel to the axis A, and an outlet  160  arranged radially, generally perpendicular to the axis A. 
     Immediately downstream of the outlet  160 , in this example, is a first vaned diffuser  24 . The main refrigerant flow path F extends through the diffuser  24  in a direction generally radially away from the axis A. Next, the main refrigerant flow path F turns 180 degrees in a cross-over bend  25 , and flows radially inward through a return channel  27  toward the second impeller  18 . Like the first impeller  16 , the second impeller  18  includes an axially oriented inlet  181  and a radially oriented outlet  180 . 
     The compressor  10  has a plurality of seals  30 A- 30 F. The seals  30 A- 30 F prevent the main refrigerant from escaping the flow path F. The seal  30 A is located between an outer diameter of the first impeller  16  and the housing  12 , near the inlet  161 . The seal  30 B is located between the shaft  19  and the housing  12  between the first and second impellers  16 ,  18 . The seal  30 C is located between an outer diameter of the second impeller  18  and the housing  12 , near the inlet  181 . The seal  30 D is located at an inner diameter of the second impeller  18  and the motor  14 . At least one of the seals  30 A- 30 D is a step seal. In one particular embodiment, all of seals  30 A- 30 D are step seals. 
     Step seals are used in turbomachinery to restrict or prevent the flow of fluids, such as liquid or gas, between adjacent internal compartments with different pressures. A step seal prevents fluid flow from travelling from a higher pressure location to a lower pressure location. Step seals may generally include a plurality of fins or teeth that define a plurality of cavities. The cavities entrap working fluid between a moving component and a stationary component. The trapped fluid thus creates a barrier that isolates a high pressure region within the machine from a region of lower pressure. In one example, the stationary and moving components are a stator and a rotor, such as an impeller. In another example, the stationary component may be provided by an insert within the compressor housing. 
       FIG. 3  schematically shows an example stepped seal  30 , which is representative of any one of the seals  30 A- 30 D. A seal flow path  32  is formed between a stator  38  and a rotor  40 . In the illustrated embodiment, a plurality of teeth  42  extend generally radially outward (i.e., in a direction normal to axis A) from the rotor  40  in a direction towards the stator  38  to define a plurality of cavities  48 ,  50  along the flow path  32  between the teeth  42 . In another embodiment, a plurality of teeth  42  extend outward from the stator  38 . 
     The teeth  42  on the rotor  40  are arranged in a stepped arrangement, meaning some are arranged at a different radial position than others. In particular, in  FIG. 3 , the teeth  42  are spaced-apart radially by steps  44 A. In the illustrated example, the steps  44 A are formed such that the teeth are arranged in pairs  60 . Each pair  60  has a first tooth  42 A and a second tooth  42 B. The teeth  42 A,  42 B in each pair  60  are at the same position in a radial direction relative to an axis of rotation of the rotor  40  (i.e., the axis A). The steps  44 A and teeth  42  are cut out of the rotor  40  using known manufacturing techniques, in an example Similar steps  44 B are also cut into the stator  38  to align with the rotor  40 . The steps  44 B may be formed from an insert within the housing, for example. The insert may be metallic. In the illustrated example, there are seven of each of the steps  44 A,  44 B. In other examples, there may be at least five of each of the steps  44 A,  44 B. In a further example, there may be ten or fewer of each step  44 A,  44 B. The teeth  42  and the steps  44 A,  44 B introduce reverse flow, which stalls refrigerant flow (such as trapping flow in the cavities  48 ,  50 ), and helps decrease total leakage. 
       FIG. 4A  illustrates further details of the step seal  30 . The pairs  60  of teeth  42 A,  42 B form two differently shaped cavities  48 ,  50 . The cavities  50  have a squared shape, while the cavities  48  have a rounded shape. The first tooth  42 A has an upstream surface  62  and a downstream surface  64 . The second tooth  42 B has an upstream surface  66  and a downstream surface  68 . In this example, the upstream surface  62  of the first tooth  42 A and the downstream surface  68  of a second tooth  42 B form the walls of each cavity  50 . The cavity  50  has a radially inner cavity wall  51 . The cavity wall  51  is substantially parallel to the axis A. The upstream surface  62  and downstream surface  68  each extend radially outward from the cavity wall  51  at approximately a right angle. In other words, the cavity  50  has a square-shaped bottom. 
     The downstream surface  64  of the tooth  42 A and upstream surface  66  of the tooth  42 B are angled at an angle with respect to the radial direction. The surfaces  64 ,  66  are joined at a curved inner wall  47  to form the cavity  48 . The cavity  48  is a curved cavity, while the cavity  50  is a square cavity. The surfaces  64 ,  66  are arranged at an angle θ with respect to one another. The angle θ is less than 90°. In a further example, the angle θ is between 45° and 90°. 
     The upstream surface  62  and downstream surface  64  of the first tooth  42 A meet at a point  72 A. The upstream surface  66  and the downstream surface  68  of the second tooth  42 B meet at a point  72 B. The points  72 A,  72 B are the radially outermost portion of the rotor  40  in each step. In this example, the points  72 A,  72 B within each pair  60  extend to a same position in the radial direction. A radial clearance  80  is defined between the points  72 A,  72 B and the stator  38 . In an example, the radial clearance  80  is at least 0.15 mm. An axial clearance  82  is defined between the downstream surface  68  of the second tooth  42 B and the upstream surface  62  of an adjacent pair of teeth  60 . In an example, the axial clearance is at least 0.7 mm. An axially extending tooth  70  extends in a substantially axial direction from the stator  38 . The axial tooth  70  extends into the flowpath from the step  44 B. The axial tooth  70  may have an inner surface  74  and an outer surface  76 . In one example, the inner surface  74  is substantially parallel to the axis of rotation A. The inner and outer surfaces  74 ,  76  are arranged at an angle iv with respect to one another. The angle iv is less than 60°, for example. The inner and outer surfaces  74 ,  76  extend in an upstream direction and meet at a point  78 . In one example, the point  78  is aligned with the point  72 A in the axial direction. 
       FIG. 4B  illustrates the fluid flow through the step seal  30 . As shown, the radial teeth  42  and axial teeth  70  create eddies in the fluid, trapping fluid in the cavities  48 ,  50 . Although an example seal  30  is shown, the particular shape and size may be tailored to a particular compressor size, speed, and refrigerant. The cavities  48 ,  50  provide recirculation zones within the refrigerant leakage path to help decrease leakage. The three sharp teeth  42 A,  42 B,  70  create vortices  90 ,  92 ,  94  in the flow field. This tooth arrangement provides three recirculating regions  91 ,  93 ,  95  per step to help passively control the flow, decreasing total leakage and increasing efficiency. 
       FIG. 5A  illustrates another example step seal  130 . To the extent not otherwise described or shown, the step seal  130  corresponds to the step seal  30  of  FIGS. 3, 4A, and 4B , with like parts having reference numerals preappended with a “1.” In this example, the two teeth  142 A,  142 B in each pair  160  form two angled cavities  148 ,  150 . The upstream surface  162  of the first tooth  142 A extends from the radially inner cavity wall  151  at an angle α that is less than 90°. The downstream surface  168  of the second tooth  142 B extends from the cavity wall  151  at substantially a right angle, or perpendicular to the axis A. Thus, the cavity  150  formed between adjacent pairs of teeth  60  has an angled shape. The downstream surface  164  of the first tooth  142 A and the upstream surface  166  of the second tooth  142 B meet at a point defining an angle θ. The angle θ may be less than 90°, for example. 
     The upstream surface  162  and downstream surface  164  of the first tooth  142 A meet at a point  172 A. The upstream surface  166  and the downstream surface  168  of the second tooth  142 B meet at a point  172 B. The points  172 A,  172 B within each pair  160  extend to a same position in the radial direction. In other words, the points  172 A,  172 B touch the locus of the radial clearance between the rotor  140  and the stator  138 . 
     An axially extending tooth  170  extends in a substantially axial direction from the stator  138 . The axial tooth  170  extends into the flowpath from the step  144 B. The inner surface  74  of the axial tooth  170  is substantially parallel to the axis of rotation A. The inner and outer surfaces  174 ,  176  of the axial tooth  170  are arranged at an angle ψ with respect to one another. The angle ψ is less than 60°, for example. The inner and outer surfaces  174 ,  176  extend in an upstream direction and meet at a point  178 . In one example, the point  178  extends upstream of the first tooth  142 A. In another example, the point  178  is substantially aligned with the first tooth  142 A in the axial direction. 
       FIG. 5B  illustrates the fluid flow through the step seal  130 . This tooth arrangement provides three recirculation zones  191 ,  193 ,  195  per step. The three sharp teeth  142 A,  142 B,  170  create vortices  190 ,  192 ,  194  in the flow field. In some examples, this arrangement passively controls the flow to decrease total leakage, and may reduce leakage by 40-80% compared to known conventional seals. 
       FIG. 6A  illustrates another example step seal  230 . To the extent not otherwise described or shown, the step seal  230  corresponds to the step seal  30  of  FIGS. 3, 4A, and 4B , with like parts having reference numerals preappended with a “2.” In this example, the step seal  230  reduces leakage flow by using a stepped arrangement and using abradable materials in the step seal. The performance of the seal  230  depends on the stepped design and the radial clearance at the tips of the teeth  242 . In some known step seals, it can be difficult to control the amount of radial clearance, because thermal gradients, centrifugal and gas pressure forces, and shaft flexing, among other things may cause deflections between the components. The stator  238  includes an abradable portion  246  that is formed from an abradable material. In some examples, the abradable portion  246  is an insert arranged within a compressor housing. The abradable portion  246  helps to minimize clearance between the tips of the teeth  242  and the stator  238 . The abradable portion  246  starts off at a very close clearance to the rotor  240  and teeth  242  and gradually wears away over time as the teeth  242  come into contact with the abradable portion  246 . 
     As shown in  FIG. 6A , the abradable portion  246  wears away as the compressor  10  runs over time. A track  249  is carved into the abradable portion  246  radially outward of each of the teeth  242 . Each of the teeth  242  has a flat tip  272  that forms the track  249 . As the rotor  240  rotates, the teeth  242  contact the abradable portion  246  and wear off some of the abradable portion  246  in the tracks  249  where the teeth  242  contacted the abradable portion  246 . The tracks  249  provide a very small gap between the teeth  242  and the stator  238 . In other words, once the tracks  249  are formed by the teeth  242 , a portion  253  of the abradable material extends radially inward of the tracks  249  between tracks  249 . The tracks  249  may be wider in the axial direction than the tips  272  of the teeth  242  due to axial movement of the impeller  16 ,  18 . 
     In this arrangement, the upstream surface  262  of the first tooth  242 A and the downstream surface  268  of the second tooth  242 B are substantially perpendicular to the radially inner cavity wall  251 . The surfaces  262 ,  268  meet the cavity wall  251  at a rounded edge to form a curved cavity  250 . The downstream surface  264  of the first tooth  242 A meets the upstream surface  266  of the second tooth  242 B at a curved surface  247  to form a second curved cavity  248 . The surfaces  264 ,  266  are arranged at an angle θ to one another. The angle θ may be less than 90° in one example. In a further example, the angle θ is between 45° and 90°. The particular tooth arrangement may be selected based on the particular compressor size and speed, for example. 
     The abradable portion  246  is formed from an abradable material. Example abradable materials may include polytetrafluoroethylene (“PTFE”), polyamide, and other low strength alloys. The rotor  240  and teeth  242  are generally formed from a hard material that can wear away the abradable portion  246 , such as an aluminum alloy, stainless steel, carbon steel, nickel alloy (such as Inconel), etc. The abradable portion  246  and the tracks  249  formed over time permit a minimal gap size, which makes it more difficult for the flow to continue, and thus improves the sealing capability of the seal  230 . 
     The use of abradable materials may result in debris as the abradable portion is worn down. Although the abraded amount may be small, the system may include high precision parts. For example, bearings, sensors, and power electronics within the system cannot have intrusion of debris. In some examples, a debris trap may be arranged downstream of the teeth  242  to capture any debris from the abradable portion  246  as it is worn away. The debris trap may be arranged on a discharge path to redirect the debris away from any sensitive components downstream of the seal  230 . 
       FIG. 6B  illustrates the fluid flow through the step seal  230 . The abradable material may provide a significantly smaller radial clearance than an arrangement with a hard material, which may further reduce leakage. The combination of two teeth  242 A,  242 B followed by a rounded rectangular cavity  250  provides a particular flow pattern having two vortices  290 ,  294  and two recirculation regions  291 ,  295 . This recirculation arrangement may decrease total leakage flow. In some examples, this arrangement passively controls the flow to decrease total leakage, and may reduce leakage by 40-80% compared to known conventional seals. 
       FIG. 7  illustrates another example step seal  330 . To the extent not otherwise described or shown, the step seal  330  corresponds to the step seal  30  of  FIGS. 3, 4A, and 4B , with like parts having reference numerals preappended with a “3.” In this example, the teeth  342  on the rotor  340  are spaced-apart radially by steps  344 A. In the illustrated example, the steps  344 A are formed such that the teeth are arranged in pairs  360 . Each pair  360  has a first tooth  342 A and a second tooth  342 B. The teeth  342 A,  342 B in each pair  360  are at the same position in a radial direction relative to an axis of rotation of the rotor  340  (i.e., the axis A). Cavities  350  are formed between each pair of teeth  360 , and cavities  348  are formed between the teeth  342 A,  342 B within each pair  360 . 
     The steps  344 A, teeth  342 , and cavities  350  are cut out of the rotor  40  using known manufacturing techniques. Similar steps  344 B are also cut into the stator  338  to align with the rotor  340 . In this example, cavities  371  are formed in the stator  338  between the steps  344 B. In some examples, the cavity  371  forms a tooth  359  in the stator  338  that extends towards the rotor  340 . The tooth  359  may be have a substantially similar size and shape as the tooth  342 A. The teeth  342 ,  359  and the steps  344 A,  344 B introduce reverse flow, which stalls refrigerant flow (such as trapping flow in the cavities  348 ,  350 ,  371 ), and helps decrease total leakage. 
       FIG. 8A  illustrates the example step seal arrangement of  FIG. 7 . In this example, the two teeth  342 A,  342 B in each pair  360  form two cavities  348 ,  350 . The cavity  350  is formed between an upstream surface  362  of the first tooth and a downstream surface  368  of the second tooth  342 B. In this example, the cavity wall  351  is rounded at a radially innermost portion. The cavity wall  351  may have a semi-circle or radius, for example. A wall  355  opposite the rounded end of the cavity wall  351  may be substantially parallel to the axis A, for example. The cavity  350  has a rectangular shape with a rounded end, for example. The surfaces  362 ,  368  may extend substantially perpendicular relative to the axis A. The tooth  359  may meet the wall  355  at a fillet  357 , in some examples. 
     The cavity  348  formed between the teeth  342 A,  342 B within a pair of teeth  360  has an angled shape. The downstream surface  364  of the first tooth  342 A and the upstream surface  366  of the second tooth  342 B are angled relative to the axis A. The downstream surface  364  of the first tooth  342 A and the upstream surface  366  of the second tooth  342 B are arranged at an angle θ relative to one another. The angle θ may be about 90°, for example. In other examples, the angle θ may be less than 90°. A cavity  371  is formed in the stator  338  opposite the cavity  348 . The cavity  371  may have a similar shape as the cavity  348  and be substantially aligned with the cavity  348 . The cavity  371  is a reflection of the cavity  348  about a plane parallel to the axial direction and arranged radially between the cavities  348 ,  371 , in one example. 
     The upstream surface  362  and downstream surface  364  of the first tooth  342 A meet at an end surface  372 A. The upstream surface  366  and the downstream surface  368  of the second tooth  342 B meet at an end surface  372 B. The end surfaces  372 A,  372 B within each pair  360  extend to a same position in the radial direction. The geometry of the flow path changes the speed and trajectory of fluid flow, which may decrease leaks through the seal. 
       FIG. 8B  illustrates the fluid flow through the step seal  330 . As fluid flows from left to right in this example, the tooth and cavity arrangement provides three recirculation zones  391 ,  393 ,  395  per step. The arrangement of teeth  342  and cavities  348 ,  350 ,  371  create vortices  390 ,  392 ,  394  in the flow field. In some examples, this arrangement passively controls the flow to decrease total leakage. 
     Any of the above described step seals  30 ,  130 ,  230 ,  330  may be used in any of the seal locations  30 A- 30 D. In some examples, different types of step seals  30 ,  130 ,  230 ,  330  may be used in different seal locations  30 A- 30 D within the same compressor  10 . 
     Although the different examples have the specific components shown in the illustrations, embodiments of this disclosure are not limited to those particular combinations. It is possible to use some of the components or features from one of the examples in combination with features or components from another one of the examples. In addition, the various figures accompanying this disclosure are not necessarily to scale, and some features may be exaggerated or minimized to show certain details of a particular component or arrangement. 
     One of ordinary skill in this art would understand that the above-described embodiments are exemplary and non-limiting. That is, modifications of this disclosure would come within the scope of the claims. Accordingly, the following claims should be studied to determine their true scope and content.