Abstract:
A regenerative air dryer system including a plurality of air dryer modules, wherein air dryer modules of the plurality of air dryer modules have different air flow rates. A controller selectively operates different combinations of air dryer modules of the plurality of air dryer modules so that a selected combination of air dryer modules has a combined air flow rate based on system air flow requirements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Utility Patent Application is a Continuation application of U.S. Ser. No. 13/735,645, filed Jan. 7, 2013, now issued as U.S. Pat. No. 8,999,045, and which claims benefit to Provisional Application No. 61/583,552, filed Jan. 5, 2012, all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Compressed air systems are used in a wide variety of applications to power a wide variety of devices, such as spray or paint guns in a compressed air painting system, for example. In order to prevent damage to air powered devices, such as through corrosion, for example, or to prevent adversely affecting processes, such as contaminating paint in a spray painting process, for example, the compressed air is dried and other contaminants removed prior to being used. 
     Air drying systems, including regenerative air drying systems, have been developed for such purposes. Regenerative air drying systems pass air through a desiccant material which removes moisture from the compressed air, with the desiccant material being able to be dried, or regenerated, and reused. Such systems typically employ more than one desiccant container so that one container can continue to provide dry air while the desiccant material in the other container is being regenerated. While such systems are effective at drying air, they employ complex systems for switching between the desiccant containers and have complicated piping and valve systems that produce large pressure losses in the system and make system expansion difficult. 
     SUMMARY OF THE INVENTION 
     One embodiment provides a regenerative air dryer which is smaller, has fewer moving parts, has single-point control, has simplified air paths, reduces external piping, and provides decreased air pressure losses relative to conventional air dryers. 
     One embodiment provides a regenerative air dryer system including at least one dryer module including a housing having an inlet air passage, an outlet air passage, and a wash air passage. The air dryer system further includes a desiccant canister mounted to the housing and in communication with the inlet, outlet, and exhaust air passages, and a single controllable valve selectively moveable between a first position and a second position, wherein the single controllable valve, when in the first position, forms a supply air flow path from the inlet air passage, through the desiccant canister in a first direction, and to the outlet air passage, and when in the second position, forms a wash air path from the outlet air passage, through the desiccant canister in a direction opposite the first direction, and to the exhaust air passage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view generally illustrating a modular regenerative air dryer system including a dryer module according to one embodiment. 
         FIG. 2  is a perspective view generally illustrating a dryer module according to one embodiment. 
         FIG. 3  is a perspective view generally illustrating a dryer module according to one embodiment. 
         FIG. 4  is a cross-sectional view of a dryer module and illustrating the operation thereof according to one embodiment. 
         FIG. 5  is a cross-sectional view of a dryer module and illustrating the operation thereof according to one embodiment. 
         FIG. 6  is a perspective view generally illustrating a modular regenerative air dryer system including multiple dryer modules according to one embodiment 
         FIG. 7  is a perspective view generally illustrating a modular regenerative air dryer system including multiple dryer modules according to one embodiment 
         FIG. 8  is a perspective view generally illustrating a modular regenerative air dryer system according to one embodiment. 
         FIG. 9  is block and schematic diagram of a modular regenerative air dryer system according to one embodiment. 
         FIG. 10  is a flow diagram illustrating the operation of a modular regenerative air dryer system according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following Detailed Description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
       FIG. 1  is a perspective view generally illustrating a modular regenerative air dryer system  8 , according to one embodiment of the present application, for drying air for compressed air systems (e.g. an air compressor). Modular regenerative air dryer system  8  includes a dryer module  10 , with dryer module  10  including a housing  12 , a desiccant canister  14 , and a valve cartridge  16  including a threaded portion  17  that serves as a mounting stud for coupling desiccant canister  14  to housing  12 . Although illustrated in  FIG. 1  as including only a single dryer module  10 , as will be described in greater detail below, modular regenerative air dryer system  8  may include two or more dryer modules  10  which are coupled together to provide increased air drying capacity (see  FIGS. 6-9 , for example). 
     According to one embodiment, housing  12  includes a pair of inlet air ports  18  on opposite ends of an inlet air passage  20  extending through housing  12  from one end face  22  to an opposing end face  24  (see  FIG. 2 ), a pair of outlet air ports  26  on opposite ends of an outlet air passage  28  extending through housing  12  from end face  22  to opposing end face  24  (see  FIG. 2 ), an actuating mechanism, such as a piston or spool valve  30 , and an exhaust vent, such as a muffler  32 . According to one embodiment, spool valve  30  is disposed within housing  12  via a side face  34  of and muffler  32  extends from a bottom face  36  of housing  12 . According to one embodiment, housing  12  includes a device outlet air port  35  disposed on side face  34  that is in communication with outlet air passage  28  and which can be used as a connection point for a compressed air device (e.g. a paint gun), wherein device outlet port  35  can be plugged when not being used (as illustrated). 
       FIG. 2  is a perspective view of dryer module  10  illustrating end face  24 , which opposes end face  22 , with inlet and outlet air ports  18  and  26 , and inlet and outlet air passages  20  and  28  extending through housing  12 . As will be described in greater detail below, dryer module  10  receives a compressed “wet” supply air flow  40 , such as from a compressor or compressor system (not shown), via inlet air port  18 , dries the wet air via desiccant canister  14 , and provides a “dry” outgoing air flow  42  via outlet air port  46 . 
     The inlet air port  18  to which a compressor system is connected, and the outlet air port  26  to which a device is connected, such as compressed air storage tank (not shown), is selectable, with the unused port able to be plugged. For example, inlet air port  18  on end face  22  and outlet air port  26  on opposing end face  24  can be selected, with inlet and outlet air ports  18  and  26  on end faces  24  and  22  being plugged, and vice versa. Similarly, inlet and outlet air ports  18  and  26  on a same end face, such as end face  22 , can be selected, with inlet and outlet air ports  18  and  26  on opposing end face  24  being plugged. This flexibility in selection and configuration of inlet and outlet air ports  18  and  28  enables modular regenerative air dryer system  8  to be installed in a wider variety of positions and locations as compared to conventional air drying systems having only one set of inlet and outlet ports which are typically positioned on a front side of the system. 
       FIG. 3  is a perspective view illustrating a top face  15  of housing  12  with desiccant canister  14  being removed, and illustrates generally valve cartridge  16  and a plurality of canister holes  21  arrayed in an arced pattern within and along a perimeter edge of desiccant canister  14 . As will be described in greater detail below, desiccant canister  14  has an air flow path in communication with outlet air passage  28  via valve cartridge  16 , with canister holes  21  being in communication with a chamber internal to housing  12  which, in-turn, is in communication with inlet air passage  20  and a fluid passageway that leads to muffler  32 . 
     As will be described and illustrated in greater detail below (see  FIGS. 4 and 5 ), spool valve  30  extends within a shaft or bore in housing  12  and is moveable between at least a first position and a second position (e.g. a retracted position and an extended position) to control the path and direction of air flow between inlet and outlet air passages  20  and  28 , desiccant canister  14 , and muffler  32 , and thereby control the mode of operation of dryer module  10 . According to one embodiment, the position of spool valve  30  dictates whether dryer module  10  in an air drying mode or a regeneration mode. 
     According to one embodiment, spool valve  30  is placed in a retracted position to operate dryer module  10  in the air drying mode. In air drying mode, wet supply air flow  40  is received (e.g. ambient air being drawn in by an air compressor or compressor system) via inlet port  18  and directed through inlet air passage  20  to desiccant canister  14  via fluid passages leading to canister holes  21 . The wet supply air flow  40  then passes through desiccant canister  14 , which removes moisture from the wet supply air  40 , via desiccant media therein, to provide the resulting dry outgoing air flow  42  which exits canister  14  via a drying orifice in valve cartridge  16  and enters outlet air passage  28  where it is directed via outlet air port  26  to a downstream device, such as storage tank (not shown). 
     According to one embodiment, spool valve  30  is placed in the retracted position to operate dryer module  10  in the regenerative mode, wherein moisture is removed from desiccant media within desiccant canister  14  (i.e. the desiccant media is dried). In regenerative mode, dry air  42  is received via outlet port  28  from a dry air source (for example, a storage tank or another dryer module  10  (not illustrated) which is in a drying mode) and is directed through outlet air passage  28  and into desiccant canister  14  via a regeneration orifice in valve cartridge  16 . The flow of dry through the regeneration orifice, also referred to as “wash” air, passes through desiccant canister  14  in a direction opposite to which the flow of wet supply air  40  passes through desiccant canister  14  when dryer module  10  is operating in the drying mode. The flow of dry wash air removes collected moisture from the desiccant material in desiccant canister  14  to form a flow of wet wash air  44  which is directed out of desiccant canister  14  through canister holes  21  and is ultimately expelled from dryer module  10  via muffler  32 . 
       FIGS. 4 and 5  are cross-sectional views through dryer module  10 , and illustrate in greater detail housing  12 , desiccant canister  14 , valve cartridge  16 , and spool valve  30 , and which respectively illustrate the air drying and regenerative operating modes of dryer module  10 . 
     With reference to  FIG. 4 , inlet and outlet air passages  20  and  28  extend through housing  12  in directions into and out of the page, between inlet and outlet air ports  18  and  26  (see  FIGS. 1 and 2 ). Housing  12  includes a supply air shaft  50  extending between inlet air passage  20  and an air chamber  52 . According to one embodiment, air chamber  52  is semicircular in shape with a plurality of bi-directional air shafts  54  along its circumference (only one shaft  54  is illustrated) which extend through housing  12  and form a corresponding canister hole  21  in top face  15  (see  FIG. 3 ). 
     Spool valve  30  is positioned within a valve shaft  56  extending into housing  12  from side face  34  to supply air shaft  50 . A first wash air shaft  58  extends between air chamber  52  and valve shaft  56 , and a second wash air shaft  60  extends between valve shaft  56  and bottom face  36  of housing  12 , terminating at muffler  32 . According to one embodiment, as illustrated, muffler  32  is threaded into a threaded vent or muffler opening  33  in housing  12 . 
     According to one embodiment, as illustrated generally by  FIG. 4 , spool valve  30  includes a plurality of sealing devices spaced apart along a body  31 . According to one embodiment, as illustrated, body  31  has a stepped, cylindrical shape. According to one embodiment, the plurality of sealing devices includes a first gasket  62 , a second gasket  64 , a third gasket  66 , and a fourth gasket  68 . According to one embodiment, each of the gaskets  62 ,  64 ,  66 , and  68  is an O-ring positioned about the stepped, cylindrical body of spool valve  30 . A spring  69  is disposed about a portion of body  31  of spool valve  30  between second and third gaskets  64  and  66  and biases spool valve  30  to the retracted position (i.e. the position illustrated in  FIG. 4 ). Spool valve  30  is actuated within valve shaft  56  between an extended position and a retracted position by any number of actuating means, including pneumatic and electric means, for example. 
     According to one embodiment, spool valve  30  is retained within valve shaft  56  by a retaining cover  57  which attached to housing  12  via one or more screws  57   a . According to one embodiment, to remove spool valve  30  from valve shaft  56 , retaining cover  57  is first removed, and a screw can be temporarily threaded into a threaded female shaft  29  within spool valve  30  and employed to pull spool valve  30  from valve shaft  56 . 
     According to one embodiment, as illustrated generally by  FIG. 4 , valve cartridge  16  has a body  70  with a threaded portion  71   a  that screws into a corresponding female threaded opening  71   b  that extends through housing  12  from top face  15  to an outlet air shaft  80 . Body  70  further includes threaded portion  17  which extends above top face  15  and serves as a mounting stud to which desiccant canister  15  is threadably mounted. According to one embodiment, body  70  includes a hexagonal flange  77  which limits how far valve cartridge  16  can be screwed into threaded opening  70 , and also serves as a nut to enable a tool, such as a socket, to be used to install or remove valve cartridge  16  from housing  12  (or to remove valve cartridge  16  from desiccant canister  14  should it remain attached thereto upon removal of desiccant canister  14  from housing  12 ) without damaging threaded portions  15  and  71   a.    
     Valve cartridge  16  further includes a plunger or check valve  72  disposed within body  70  having a spring loaded flange portion  74  extending from a base portion  76 , with flange portion  74  including a regeneration orifice  75  extending there through. As will be described in greater detail below, check valve  72  opens and closes a drying orifice  78  in body  70  to control the flow of dry air  42  between desiccant canister  14  and dry outlet air passage  28  via outlet air shaft  80 , with the spring-loaded flange portion  74  being biased so as to close orifice  78  in the absence of a flow of wet supply air  40  into desiccant canister  14   
     According to one embodiment, valve cartridge  16  is modular in design and can be replaced with valve cartridges  16  having different threaded portions  15  accommodate different types and sizes of desiccant canisters  14 . Similarly, different valve cartridges  16  may have differently sized check valves  72 , regeneration orifices  75 , and drying orifices  78  to provide different volumes of air flow. The modular nature of valve cartridge  16  enables one valve cartridges to be quickly replaced to accommodate changing system requirements. 
     According to one embodiment, as illustrated by  FIG. 4 , desiccant canister  14  includes a housing  81  forming an outer air passage  82  about an inner portion  84 . According to one embodiment, inner portion  84  includes beads of a desiccant media forming a molecular sieve bed  86 . According to one embodiment, desiccant canister  14  is removable and threads on to valve cartridge  70 . 
       FIG. 4  illustrates dryer module  10  when operating in the air drying mode, with spool valve  30  being in a first or retracted position so that gaskets  62 ,  64 , and  66  are positioned to prevent the flow of wet supply air  40  from exiting housing  12  via second wash air shaft  60  and muffler  32 . With spool valve  30  in the retracted position, the flow of wet supply air  40  (as indicated by the unbroken directional arrows) travels from inlet air passage  20 , through supply air shaft  50  to air chamber  52 . From air chamber  52 , the wet supply air  40  enters the outer portion of desiccant canister  14  via the plurality of air shafts  54  and the corresponding plurality canister holes  21 . The flow of wet supply air  40  then travels up through outer air passage  82  and enters the molecular sieve bed  86  at the top of the desiccant canister  14 . 
     As the flow wet supply air  40  travels downward through molecular sieve bed  86 , moisture is removed from the flow of wet supply air  40  to form the flow of dry output air  42  (as indicated by the broken directional arrows). The pressure from the flow of dry air  42  forces check valve  72  to the open position, and dry output air  42  flows through orifice  78  and through valve cartridge  16  to dry outlet air passage  80 . The flow of dry output air  42  then flows through outlet air passage  28  to a storage container, such as a pressurized storage tank (not shown), with a portion of the flow of dry output air  42  potentially being employed by a different air dryer module  10  which is in regenerative mode (see  FIG. 5  below) or being employed by a device (e.g. a paint spray gun) via device output port  35  (see  FIG. 1 ). 
     According to one embodiment, housing  12  is machined in two pieces  94  and  96 , which are bolted together with a gasket  98  disposed there between in a gasket channel. 
       FIG. 5  illustrates dryer module  10  when operating in the regenerative mode with spool valve  30  being in a second or extended position such that first gasket  62  is positioned to seal and prevent a flow wet supply air  40  from entering supply air shaft  50  from inlet air passage  20 . Additionally, third gasket  66  is in a position such that it no longer seals first wash air shaft  58  from second wash air shaft  60 , so that first and second wash air shafts  58  and  60  are now in communication with one another via valve shaft  56 . 
     With spool valve  30  in the extended position, there is no flow of wet supply air  40  to desiccant canister  14  so that back-pressure from dry output air  42  (e.g. from a storage tank or another dryer modules  10 ) spring-loaded flange portion  74  keep check valve  72  in the closed position thereby sealing drying orifice  78 . However, regenerative orifice  75  allows a small flow of dry output air  42  through flange portion  74  of check valve  72 , thereby creating a flow of dry wash air  90  to flow through desiccant canister  14  in a direction opposite to the flow of wet supply air  40  through desiccant canister  14  when dryer module  10  is being operating in the air drying mode. 
     In regenerative mode, the flow of dry wash air  90  flows from output air passage  28 , through dry outlet air passage  80 , through regenerative orifice  75  of check valve  72 , and into molecular sieve bed  86  (as indicated by the broken directional arrow). As the flow of dry wash air  90  passes through molecular sieve bed  86 , it absorbs moisture (and potentially other contaminants) collected by the molecular sieve bed  86  and forms a flow of wet wash air  44  which exits molecular sieve bed  86  and enters outer air passage  82 . The flow of wet wash air  44  then travels down outer air passage  82  to air chamber  52  of housing  12  via canister holes  21  and bi-directional air shafts  54 . The wet wash air  44  then continues through first wash air shaft  58 , past third gasket  66 , and through valve shaft  56  about a portion of spool valve  30  where it enters second wash air shaft  58  and is expelled from housing  12  via muffler  32 . 
     By removing the accumulated water and other contaminants from the desiccant material of molecular sieve bed  86  in this fashion, the beads of desiccant material are dried and cleaned and can once again be used to clean wet supply air  40  upon returning spool valve  30  to the retracted position. The above described process is employed each time the beads of descant material of molecular sieve bed  86  are cleaned. 
     While described above in terms of a single dryer module  10  for ease of illustration, modular regenerative air dryer system  8  may include multiple dryer modules  10  which are coupled together to provide increased air drying capacity. For example, modular regenerative air dryer system  8  may include two, three, four, etc. air dryer modules  10  coupled together to form a system. 
       FIG. 6 , for example, is a perspective view generally illustrating an embodiment of modular regenerative air dryer system  8  including two dryer modules  10 . According to one embodiment, as illustrated, side faces  22  and  24  of the two dryer modules  10  are bolted together at each of the four abutting corners of housings  12  using bolts, such as indicated at  100 . When coupled together, the inlet and outlet air passages  20  and  28  on side face  24  of one of the modules  10  align with the inlet and outlet air passages  20  and  28  on side face  22  of the other one of the modules  10  so as to combine to form continuous inlet and outlet air passages  20  and  28  that extend through both of the housings  12 . In one embodiment, seals are positioned about inlet and outlet ports  18  and  26  of the adjacent modules  10  prior to their being bolted together so as to eliminate leaks at the “joints” of the continuous inlet and outlet air passage  20  and  28 . As before, the non-selected input and output ports  18  and  26  of continuous inlet and outlet air passages  20  and  28  are sealed with a plug. 
     Similarly,  FIG. 7  is a perspective view illustrating an embodiment of modular regenerative air dryer system  8  including three dryer modules  10 . Again, the dryer modules  10  are bolted together at each of the four corners of each of the abutting faces of housings  12  using bolts, such as indicated at  100 . When coupled together, the inlet and outlet air passages  20  and  28  of each of the modules  10  align with one another and combine to form continuous inlet and outlet air passages  20  and  28  extending through the three housing  12 . 
     The modular configuration of air dryer modules  10 , including the positioning of pars of inlet and outlet air ports  18  and  26  on opposing faces of housing  12 , enables any number of air dryer modules  10  (e.g. two, three, four, and even more) to be simply and easily coupled together, with no external piping, to form regenerative air dryer system  8  of varying air drying capacity. Such a modular configuration enables air dryer system  8  to be easily adapted (e.g. expanded or downsized) to meet changing compressed air requirements. In contrast to simply bolting a pair of housings  12  together with four bolts, in order to add additional air drying capacity, conventional air drying systems typically require that the existing system be at least partially disassembled and require complicated piping, valves, and connectors to be installed to add system components (e.g. “banjo” fittings, elbows, tees, crosses, etc.), wherein such external piping and components are difficult and time consuming to install and restrict air flow, thereby resulting in increased pressure losses in the system. 
     Regenerative air dryer system  8 , according to the present application, also enables one air dryer module  10  to added (or subtracted) at a time and thereby enables system capacity to be changed in smaller increments relative to conventional systems. Typically, conventional air drying systems require that “pairs” of interconnected desiccant containers (along with the associated piping and control valves) to added at a given time (so that one canister can dry air while the other is being regenerated), thereby forcing system capacity to be expanded (or reduced) in larger increments. As a result, system capacity in such conventional air drying systems may be forced to be greater than what is required due to the restriction of having to expand the system using “pairs” of desiccant containers. 
     According to one embodiment, each dryer module  10  of air dryer system  8  is rated for a 5-horsepower air compressor, such that an air dryer system  8  having two dryer modules  10  is rated for a 10-horsepower air compressor, an air dryer system  8  having three dryer modules  10  is rated for a 15-horsepower air compressor, and so on. As described above, according to one embodiment, dryer module  10  has a valve cartridge  16  with a check valve  72  having an orifice  78  sized and selected to correspond to different horsepower requirement. As such, in addition to being able to add one or more air drying modules  10  in a modular fashion to increase the air drying capacity of a regenerative air drying system  8  as described herein, the air drying capacity of system  8  can also be adjusted in a modular fashion by changing out the modular valve cartridge  16 . 
     According to one embodiment, each dryer module  10  is rated for 40 cfm (cubic feet per minute) of flow, with each dryer module  10  having a flow rate based on specific system requirements, wherein the flow rate can be modified based on the size of orifice openings and on sequencing in the mode of operation between multiple dryer modules  10  of air dryer systems  8  employing multiple air dryer modules. 
     According to one embodiment, when modular regenerative air dryer system  8  is formed using multiple dryer modules  10 , air dryer system  10  is operated so that only one dryer module  10  is in the regenerative mode at a given time. 
       FIG. 8  is a perspective view generally illustrating modular regenerative air dryer system  8  including two dryer modules  10 , and further including a controller  110  (e.g. a PLC controller) for controlling the operation of spool valves  30  of each dryer module  10  to coordinate regeneration of the desiccant media of desiccant canisters  14 . According to one embodiment, as mentioned above, only one dryer module  10  of air dryer system  8  is placed in the regenerative mode at a given time. 
     For example, after both dryer modules  10  are first pressurized and air dryer system  8  is providing a stable flow of dry output air  42 , controller  110  is configured to direct spool valve  30  of a first one of the dryer modules  10  to move to the extended position to begin the regenerative drying process, while spool valve  30  of a second one of dryer modules  10  is maintained in the retracted position so as to continue operating in the air drying mode. Upon completion of the regenerative drying process of the first dryer module  10 , controller  110  is configured to direct the spool valve  30  of the first dryer module  10  to return to the retracted position so as to return the first dryer module  10  to the air drying mode. 
     Thereafter, controller  110  repeats the above described process with the second dryer module  10 . By controlling the regenerative drying processes of the two dryer modules  10  so as to have only one of the two dryer modules  10  operating in the regenerative mode while the other is operating in the air drying mode, modular regenerative air dryer system  8 , according to the present application, eliminates sudden and undesirable air pressure changes caused by simultaneous or overlapping regeneration of desiccant canisters as done in conventional air drying systems. 
     It is noted that above control process can be adapted and applied to a modular regenerative air dryer system  8  formed by any number of coupled dryer modules  10 . Whatever the number of dryer modules  10  (other than one), controller  110  is configured to control the spool valves  30  of each of the dryer modules  10  so that only one of the dryer modules  10  is being operated in the regenerative mode at a given time, with no overlap between the regeneration processes of the air dryer modules  10 . 
     According to one embodiment, the normal operation of a regenerative air dryer system  8  having two dryer modules  10 , for example, includes the first air dryer module  10  being operated in a regenerative mode and the second air dryer module  10  being in an air drying mode. When the desiccant material in canister  14  of the second air module  10  needs to be regenerated (i.e. cleaned), the respective positions of spool valves  30  of the first and second air dryer modules  10  are reversed so that the second air dryer module  10  is placed in a regenerative mode and the first air dryer module  10  is placed in an air drying mode. To avoid a pressure surge or spike that could result from a simultaneous changeover of the respective spool valves  30 , the first air dryer module  10  is switched from the regenerative mode to the air drying mode while the second air dryer module  10  remains in the air drying mode. Only after stable air flow has commenced through the first air dryer module  10  after being placed in the air drying mode is the second air dryer module  10  placed in the regenerative mode. 
     As described above, in operation of regenerative air dryer system  8 , each of the one or more air dryer modules  10  includes only two moveable parts, check valve  72  and spool valve  30 , and only a single control point, that being spool valve  30 . The use of only two moving parts, single-point control via spool valve  30 , modular valve cartridge  70 , and the reduction/elimination of external piping greatly simplifies control, maintenance, and operation of a regenerative air dryer system  8 , according to the present application. In contrast, conventional air drying systems include numerous moving parts and multiple control valves that must be operated in precise sequences in order to provide air drying and regenerative operating modes, and further include complicated piping and valve systems that must be disassembled/added in order to modify the air drying capacity of the system, and which require time consuming and costly maintenance. 
     Additionally, each of the air drying modules  10  of regenerative air drying system  8  is configured with short and direct air flow paths within housing  12  that greatly reduce air pressure losses relative to conventional air drying systems having complex air passages. As mentioned above, whereas conventional air drying systems employ complicated external piping and valve systems, the “piping” of air dryer modules  10 , according to the present application, is completely internal to housings  12  and is configured so as to reduce/minimize path lengths and turns in order to reduce pressure losses (wherein pressure losses increase the amount of energy required to provide desired output pressures and flows). 
     For example, with reference to  FIG. 4 , according to one embodiment, a flow path of supply air through housing  12 , when dryer module  10  is operating in drying mode (excluding desiccant canister  14 ), is through inlet air passage  20 , through supply air shaft  50 , air chamber  52 , and bi-directional air shafts  54  to desiccant canister  14 , and then through valve cartridge  18  to outlet air shaft  80 , and lastly through outlet air passage  28 . According to one embodiment, housing  12  has dimension of approximately 6-inches between opposing side faces (e.g. between side faces  22  and  24 ) and a height between top face  15  and bottom face  36  of 4-inches (6″×6″×4″). According to one embodiment, a flow path of supply air through housing  12 , as described above, has a total length of approximately only 12.5 inches and a combined 450 degrees of turns within housing  12  (excluding desiccant canister  14 ). According to one embodiment, a flow path of supply air through housing  12  does not exceed 14 inches and does not have more than 450 degrees of turns within housing  12 . According to one embodiment, to further reduce pressure losses, supply air shaft  50  is rectangular in shape to increase its surface area and thereby decrease drag on air passing there through, and edges of passages, such as the edges of bi-directional air-shafts  54  and chamber  52 , have edges with radiuses to reduce pressure losses as the air transitions from one passage to another. 
     Additionally, according to one embodiment, housing  12  of dryer module  10  is machined from solid billet aluminum, as opposed to cast aluminum typically employed by cast aluminum. According to one embodiment, the machined billet aluminum is hard coat anodized and Teflon-impregnated to reduce friction of the air passages to further reduce air pressure losses within housing  12  and to minimize corrosion and damage to the air passages from moisture and other contaminants within the supply air as it passes through housing  12 . The use of hard-coat anodized aluminum greatly reduces the occurrence of oxidation on the surfaces of housing  12  which otherwise increases air flow resistance and causes increased wear on any moving parts. The use of hard-coat anodized, Teflon-impregnated, machine billet for housing  12  greatly increases the expected operating life of air dryer modules  10 , with air drying modules  10  having an expected operating life of up to 18 years, as opposed to 1-2 years for conventional air compressor systems employing cast aluminum. 
     Together, the elimination of external piping, the short and simple configuration of internal passages forming a flow path of supply air through housing  12 , and the materials and coatings on the surfaces of housing  12 , work together to greatly reduce pressure losses within air drying system  8  as compared to conventional air drying systems. For example, according to one embodiment, an air drying system  8  employing two air dryer modules  10 , as described herein, was measured to have only 2 psi of pressure drop at full flow (40 cfm) as compared to a conventional system having a substantially equal capacity, and being operated under substantially equal parameters, which had a measured pressure drop of 12 psi. In other words, according to such configuration, air drying system  8  according to the present application had only one-sixth the pressure drop of a conventional regenerative air drying system. 
       FIG. 9  is a block and schematic diagram illustrating generally a compressed air system  120 , including an air compressor  122 , a storage container  124  for storing compressed and dried air, a controller  126  (e.g. a PLC), and a regenerative air dryer system  8 , according to the present application, including four air dryer modules  10 , indicated as dryer modules  10   a - 10   d . As described above, the housings  12  of the four air dryer modules  10   a - 10   d  are coupled together and form common input and output air passages  20  and  28  shared by air dryer modules  10   a - 10   d.    
     In operation, regenerative air dryer system  8  receives a supply of “wet” compressed supply air  40  from air compressor  122  via input air passage  20 . Dryer modules  10   a - 10   d  dry the wet supply air  40  by directing the air through desiccant canisters  14  and provide a flow of dry output air  42  to a storage container  124  (e.g. pressurized tank) via output air passage  28 . 
     According to one embodiment, as described above, after all of the air dryer modules  10   a - 10   d  are pressurized and regenerative air dryer system  8  is providing a stable flow of dry output air  42  to storage container  124 , controller  126  cycles the air dryer modules  10   a - 10   d  through regeneration modes in order to clean or dry the desiccant media within desiccant canisters  14  such that the desiccant canister  14  of only one of the air dryer modules  10   a - 10   d  is being regenerated at a time while the remaining three of the air dryer modules continue to dry the supply air flow  40 . According to one embodiment, controller  30  controls the positions of spool valves  30  of air dryer modules  10   a - 10   d  via control wires  128  to cycle the air dryer modules  10   a - 10   d  between air drying and media regenerating modes. 
       FIG. 10  is flow diagram illustrating one embodiment of a drying process  140  employing a modular regenerative air dryer system  8  according to the present application, such as air dryer system  8  illustrated by  FIG. 9 . Process  140  begins at  142  wherein all of the air dryer modules  10  of air dryer system  8  are pressurized and begin providing a stable flow of dry output air  42 , such as air dryer modules  10   a - 10   d  of  FIG. 8  providing a flow of dry output air  42  to storage container  124 . 
     At  144 , controller  126  directs spool valve  30  of a first air dryer module  10  of the air dryer system  8 , such as air dryer module  10   a , to the extended position to begin the regenerative drying process of the desiccant media within the corresponding desiccant canister  14 , while the spool valves  30  of the remaining air dryer modules  10   b - 10   d  are maintained in the retracted position to so that the remaining air dryer modules  10   b - 10   d  continue operating in the air drying mode. 
     After the regeneration process of air dryer module  10   a  is complete, process  140  proceeds to  146  where it is queried whether the air dryer module  10  which has just been regenerated is a last one of the four air dryer modules  10   a - 10   d  to be regenerated. If the answer to the query at  146  is “no”, process  140  proceeds to  148 , where a next air dryer module  10  of the four air dryer modules  10   a - 10   d , for example, air dryer module  10   b , is regenerated. Process  140  then returns to  144 . 
     If the answer to the query at  146  is “yes”, meaning that each of the for air dryer modules  10   a - 10   d  has been regenerated to complete a regeneration cycle of the air dryer modules  10 , process  140  returns to  144  where the first of the air dryer modules  10   a - 10   d  is again regenerated to start the next regeneration cycle. 
     Again, as described above, by controlling the regenerative drying process so that only one of the dryer modules  10   a - 10   d  is operating in the regenerative mode at a given time, while the remaining modules  10   a - 10   d  continue to operate in the air drying mode, modular regenerative air dryer system  8 , according to the present application, eliminates sudden and undesirable air pressure changes caused by simultaneous or overlapping regeneration of desiccant canisters as done in conventional air drying systems. 
     Although described as regenerating air dryer modules  10  sequentially from air dryer module  10   a  to air dryer module  10   d , it is noted that the air dryer modules  10   a - 10   d  could be regenerated in any selected order. It is also noted that process  140  can include time delays between the drying of one dryer module  10  and a next dryer module  10 . Additionally, the process  140  can also be applied to air dryer systems  8  have more or fewer than four air dryer modules  10 . 
     In summary, by employing only two moveable parts, spool valve  30  and check valve  72 , and having only a single control point via spool valve  30  to control a mode of operation, air dryer module  10  of modular regenerative drying system  8  according to embodiments of the present application, greatly simplifies control, maintenance, and operation relative to conventional air drying systems. Additionally, the small size and modular design of air dryer modules  10  (e.g., selectable inlet and outlet ports  18  and  26  on opposing faces of housing  12 , modular valve cartridge  70 , bolt-together housings  12 , etc.) enable modular air drying system  8  to be easily installed and repaired, and easily adapted to meet changing compressed air requirements in comparison to conventional compressed air systems. Also, the short and simple flow path for supply air through housing  12 , and the hard-coat anodized, and Teflon-impregnated surfaces of housing  12  greatly reduce air pressure losses and greatly increase the operating life (e.g. reduces corrosion) of modular air drying system  8  relative to conventional compressed air systems. Furthermore, staggering the regeneration of desiccant canisters  14  through simple control of spool valves  30  eliminates sudden and undesirable pressure drops during operation of modular air drying system  8  while continuing to provide a stable flow of dry air.