Abstract:
A compressor airflow controller is operable to control airflow through a centrifugal compressor. The controller drives the centrifugal compressor and determines if a flow irregularity (e.g., an error in gas flow velocity) has occurred. If a flow irregularity is determined, the controller reduces the irregularity by adjusting compressor operation to thereby change the compressed gas flow.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/764,042, filed Apr. 20, 2010, entitled COMPRESSOR INLET GUIDE VANE CONTROL, which claims the priority benefit of U.S. Provisional Application Ser. No. 61/220,635, filed Jun. 26, 2009, entitled COMPRESSOR DRIVE AND PNEUMATIC CONVEYING COMPRESSOR, all of which are hereby incorporated in their entirety by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates generally to compressor systems, such as those used in pneumatic conveying systems. More specifically, embodiments of the present invention concern a compressor package with a centrifugal compressor and a controller that controls the airflow through the compressor. 
         [0004]    2. Discussion of Prior Art 
         [0005]    Centrifugal compressors with guide vanes are known in the art. For example, compressor systems have been developed with inlet guide vanes and/or diffuser guide vanes. Furthermore, it is also known for a centrifugal compressor to have adjustable guide vanes so as to permit control over airflow to and/or from the compressor. 
         [0006]    Prior art compressor systems suffer from various limitations. For instance, conventional centrifugal compressors are unable to provide compressed airflow at a substantially constant velocity, particularly when the compressor is exposed to widely varying backpressures or other widely varying environmental conditions. This problem is particularly relevant in the application of centrifugal compressors in pneumatic conveying systems. More specifically, centrifugal compressors are generally deficient at overcoming any blockage in the conveying system that develops downstream of the compressor. Furthermore, in the event of such blockage, the compressor is prone to entering a surge condition. 
       SUMMARY 
       [0007]    The following brief summary is provided to indicate the nature of the subject matter disclosed herein. While certain aspects of the present invention are described below, the summary is not intended to limit the scope of the present invention. 
         [0008]    Embodiments of the present invention provide a compressor assembly that does not suffer from the problems and limitations of the prior art compressor systems, such as those set forth above. 
         [0009]    A first aspect of the present invention concerns a method of maintaining a desired gas flow in a pneumatic conveying system. The method broadly includes the steps of driving a centrifugal compressor to direct compressed gas flow into the system; determining if a flow irregularity has occurred, with the flow irregularity determined when a sensed gas flow characteristic is found to be different from a desired value of the gas flow characteristic; and upon such an occurrence, reducing the irregularity by adjusting compressor operation to thereby change the compressed gas flow. 
         [0010]    A second aspect of the present invention concerns a controller to maintain a desired gas flow in a pneumatic conveying system. The controller broadly includes a processing element configured to drive a centrifugal compressor to direct compressed gas flow into the system; determine if a flow irregularity has occurred, with the flow irregularity determined when a sensed gas flow characteristic is found to be different from a desired value of the gas flow characteristic; and upon such occurrence, reduce the irregularity by adjusting compressor operation to thereby change the compressed gas flow. 
         [0011]    A third aspect of the present invention concerns a compressor airflow control assembly operable to control airflow induced by a centrifugal compressor. The control assembly broadly includes an air valve assembly and a controller assembly. The air valve assembly includes a body that presents a valve passage. The air valve assembly further includes an air valve shiftably mounted in the valve passage to control airflow through the passage. The air valve assembly is operable to be mounted relative to the compressor so that the valve passage fluidly communicates with the compressor and shifting of the valve controls the compressor airflow. The controller assembly is operable to sense an airflow pressure and responsively control shifting of the air valve. The controller assembly includes a fluidly driven actuator and a shiftable fluid valve fluidly connected to the actuator to control pressurized fluid flow from a source to the actuator. The actuator is operably coupled to the air valve so that driven movement of the actuator effects shifting of the air valve within the valve passage. The fluid valve is shiftable so as to vary fluid flow to the actuator causing movement thereof and thereby shifting of the air valve. The controller assembly includes an airflow pressure sensor configured to sense the airflow pressure and operably coupled to the fluid valve so that sensing of the airflow pressure is operable to cause shifting of the fluid valve and a corresponding shifting of the air valve. 
         [0012]    Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0013]    Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein: 
           [0014]      FIG. 1  is a fragmentary front left perspective of a compressor package constructed in accordance with a first embodiment of the present invention, and showing a base, motor, centrifugal compressor, intake system, transmission, hydraulic assembly, inlet guide vane assembly, and guide vane controller of the compressor package, with the controller including a bypass valve in fluid communication with the compressor discharge and operable to vent compressor airflow to ambient; 
           [0015]      FIG. 2  is a fragmentary front right perspective of the compressor package shown in  FIG. 1 ; 
           [0016]      FIG. 3  is a fragmentary rear perspective of the compressor package shown in  FIGS. 1 and 2 , with part of the transmission housing being removed to show an internal gear drive of the transmission; 
           [0017]      FIG. 4  is a fragmentary front right perspective of the compressor package shown in  FIGS. 1-3 , showing the compressor, inlet guide vane assembly, and controller; 
           [0018]      FIG. 5  is a front right perspective of the compressor, inlet guide vane assembly, and controller similar to  FIG. 4 , but showing part of the compressor housing and part of the guide vane housing removed to show the guide vanes and transmission that controls positioning of the guide vanes, with the guide vanes in a predetermined preswirl position and the piston in a corresponding central position, and showing part of the controller housing removed to show a sensor assembly, valve, and hydraulic piston of the controller, with the valve being in a closed position; 
           [0019]      FIG. 6  is a right rear perspective of the compressor, inlet guide vane assembly, and controller shown in  FIG. 5 , with the guide vanes in the predetermined preswirl position and the valve in the closed position; 
           [0020]      FIG. 7  is an enlarged front right perspective of the compressor, inlet guide vane assembly, and controller similar to  FIG. 5 ; 
           [0021]      FIG. 8  is a front elevation of the inlet guide vane assembly and controller shown in  FIGS. 1-7 , showing the controller cross-sectioned to depict the valve within the controller housing; 
           [0022]      FIG. 9  is a perspective of the valve shown in  FIGS. 5 ,  7 , and  8 , showing slotted and chamfered ends of the cylindrical valve, with upper and lower slots formed along the slotted end, and showing upper and lower bores extending longitudinally from corresponding upper and lower slots to corresponding ends of the valve; 
           [0023]      FIG. 10  is a cross section of the valve shown in  FIGS. 5 ,  7 ,  8 , and  9 , showing the upper and lower bores extending longitudinally through the valve; 
           [0024]      FIG. 11  is a cross section of the inlet guide vane assembly and controller shown in  FIGS. 1-8 , showing the valve rotated into the closed position, with the closed valve preventing fluid flow between the upper and lower slots and the supply and return bores; 
           [0025]      FIG. 12  is a front right perspective of the compressor, inlet guide vane assembly, and controller similar to  FIG. 5 , but showing the sensor assembly pivoted rearwardly toward the inlet guide vanes in response to a high-velocity intake airflow so that the valve assumes a vane-closing position, with the valve permitting pressurized supply fluid to flow from the supply bore to the right piston chamber so that the piston is shifted into a left endmost position, and with the inlet guide vanes being shifted by the piston into a corresponding maximum preswirl condition to generally reduce the airflow to the compressor; 
           [0026]      FIG. 13  is a cross section of the inlet guide vane assembly and controller similar to  FIG. 11 , but showing the valve pivoted into the vane-closing position, with the upper slot of the valve fluidly communicating with the supply bore and the lower slot of the valve fluidly communicating with the return bore; 
           [0027]      FIG. 14  is a front right perspective of the compressor, inlet guide vane assembly, and controller similar to  FIG. 5 , but showing the sensor assembly pivoted forwardly from the inlet guide vanes in response to a low-velocity intake airflow so that the valve assumes a vane-opening position, with the valve permitting pressurized supply fluid to flow from the supply bore to the left piston chamber so that the piston is shifted into a right endmost position, with the inlet guide vanes being shifted by the piston into a corresponding maximum counterswirl condition to generally increase the airflow velocity and, in instances where the counterswirl condition does not increase the low-velocity airflow to a predetermined velocity, the left piston chamber exceeds a predetermined pressure so that a pressure relief valve allows fluid to flow from the left piston chamber to the bypass valve and thereby open the bypass valve; 
           [0028]      FIG. 15  is a cross section of the inlet guide vane assembly and controller similar to  FIG. 11 , but showing the valve pivoted into the vane-opening position, with the upper slot of the valve fluidly communicating with the return bore and the lower slot of the valve fluidly communicating with the supply bore; 
           [0029]      FIG. 16  is a schematic view of the hydraulic assembly and controller shown in  FIGS. 1-3 , showing a pressure regulator, heat exchanger, pump, and sump of the hydraulic assembly, and showing the valve, actuator, pressure relief valve, and bypass valve of the controller; 
           [0030]      FIG. 17  is a fragmentary cross section of an alternative compressor package constructed in accordance with a second embodiment of the present invention, with the controller including a diaphragm depicted schematically, showing the diaphragm attached to an arm of the sensor assembly and fluidly communicating with the compressor volute to sense compressor discharge pressure; 
           [0031]      FIG. 18  is a graph showing an operational characteristic of the alternative compressor package without the diaphragm and operational characteristics of the alternative compressor package with the diaphragm installed in different configurations; 
           [0032]      FIG. 19  is a schematic view of an alternative compressor package shown as part of a pneumatic conveying system constructed in accordance with a third embodiment of the present invention, with the system also including a media storage tank, main flow line, media valve, and downstream equipment, and with the compressor package including a centrifugal compressor, an inlet guide vane assembly, and a controller with an inlet guide vane actuator and a hydraulic valve; 
           [0033]      FIG. 20  is a fragmentary perspective of the compressor package shown in  FIG. 19 , showing the centrifugal compressor, inlet guide vane assembly, and the actuator, with the actuator including an actuator housing and a double-acting piston, and showing the actuator housing sectioned to depict the piston and various fluid ports that fluidly communicate with opening and closing chambers on both sides of the piston; 
           [0034]      FIG. 21  is a perspective of the hydraulic valve shown in  FIG. 19 , showing a valve housing, valve piston, and a shiftable lever, with the valve housing including a body, a cover, and a lever support; 
           [0035]      FIG. 22  is a fragmentary schematic side elevation of the compressor package shown in  FIG. 19 , showing the hydraulic valve and a sensor assembly, with the sensor assembly including a pitot tube and diaphragm connected to the lever and the pitot tube, and showing another diaphragm connected to the lever and sensing static airflow pressure, with a weight attached to an opposite end of the lever; 
           [0036]      FIG. 23  is a fragmentary schematic cross section of the hydraulic valve shown in  FIGS. 21 and 22 , showing a drain chamber of the housing and the valve piston extending along a bore of the valve housing, with the piston located in a neutral position where the valve restricts hydraulic fluid flow to and from the actuator; 
           [0037]      FIG. 24  is a fragmentary schematic cross section of the hydraulic valve shown in  FIGS. 21-23 , showing the valve piston located in an opening position where hydraulic fluid is supplied to the opening chamber of the actuator and hydraulic fluid is drained from the closing chamber of the actuator; 
           [0038]      FIG. 25  is a fragmentary schematic cross section of the hydraulic valve shown in  FIGS. 21-24 , showing the valve piston located in a closing position where hydraulic fluid is supplied to the closing chamber of the actuator and hydraulic fluid is drained from the opening chamber of the actuator; 
           [0039]      FIG. 26  is a side elevation of the body of the valve housing shown in  FIGS. 21-25 , showing various laterally-extending ports including a relief port, upper and lower pairs of drain ports that extend through the piston bore and to the drain chamber, a supply port, and upper and lower return ports; 
           [0040]      FIG. 27  is a cross section of the valve housing body taken along line  27 - 27  in  FIG. 26 ; 
           [0041]      FIG. 28  is a cross section of the valve housing body taken along line  28 - 28  in  FIG. 26 ; 
           [0042]      FIG. 29  is a cross section of the valve housing body taken along line  29 - 29  in  FIG. 26 ; 
           [0043]      FIG. 30  is a cross section of the valve housing body taken along line  30 - 30  in  FIG. 26 ; 
           [0044]      FIG. 31  is a cross section of the valve housing body taken along line  31 - 31  in  FIG. 26 ; 
           [0045]      FIG. 32  is a cross section of the valve housing body taken along line  32 - 32  in  FIG. 26 ; 
           [0046]      FIG. 33  is a cross section of the valve housing body taken along line  33 - 33  in  FIG. 26 ; 
           [0047]      FIG. 34  is a cross section of the valve housing body taken along line  34 - 34  in  FIG. 26 ; 
           [0048]      FIG. 35  is a flow diagram depicting a method of using the compressor package shown in  FIG. 19 ; 
           [0049]      FIG. 36  is a schematic view of an alternative compressor package shown as part of a pneumatic conveying system constructed in accordance with a fourth embodiment of the present invention, with the system also including a media storage tank, main flow line, media valve, and downstream equipment, and with the compressor package including a centrifugal compressor, an inlet guide vane assembly, and a controller with an inlet guide vane actuator, a PID controller, and a variable frequency drive; 
           [0050]      FIG. 37  is a flow diagram depicting a method of using the compressor package shown in  FIG. 36 ; and 
           [0051]      FIG. 38  is a flow diagram depicting another method of using the compressor package shown in  FIG. 36 . 
       
    
    
       [0052]    The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the preferred embodiment. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    Turning initially to  FIGS. 1 and 2 , a modular compressor package  20  is illustrated for installation as part of a pneumatic conveying system to convey various types of particulate media. However, the compressor package  20  is capable of providing compressed air for other applications without departing from the scope of the present invention. Furthermore, the illustrated compressor package  20  is particularly suitable for use as a retrofit into a pneumatic conveying system in place of a positive-displacement compressor. It has been found that the compressor package  20  is operable to provide compressed air at a substantially uniform velocity, much like a positive-displacement compressor, while providing better overall compressor efficiency. The modular compressor package  20  broadly includes a base  22 , a motor  24 , a centrifugal compressor  26 , an intake system  28 , a transmission  30 , a hydraulic assembly  32 , an inlet guide vane assembly  34 , and a guide vane controller  36 . 
         [0054]    Turning to  FIGS. 1-3 , the base  22  supports the rest of the compressor package  20 . The base  22  includes an elevated frame  38  supported on upright legs  40  and a compressor platform  42  secured adjacent one end of the frame  38 . In the usual manner, the elements of base  22  are preferably formed of steel, although the base  22  could include other materials. While the base  22  has components supported entirely by frame  38 , the base  22  could have an additional frame spaced above or below frame  38  to alternatively support the package components. 
         [0055]    The motor  24  provides power to the compressor package  20 . The motor  24  comprises an electric motor that is drivingly attached to the transmission  30  in the usual manner. Another type of motor, such as an internal combustion engine, could be substituted for the electric motor without departing from the scope of the present invention. 
         [0056]    Turning to  FIGS. 1-4 , the compressor  26  is a conventional centrifugal compressor and is operable to provide compressed air for various applications, such as pneumatic conveying. The compressor  26  includes a housing  44 , an impeller shaft  46  rotatably mounted on the housing  44 , and an impeller  48  mounted on the shaft  46  to rotate with the shaft  46 . The housing  44  presents a chamber that rotatably receives the impeller  48  and presents an inlet  50  and a discharge  52 . In the usual manner, the impeller  48  draws airflow into the housing  44  via the inlet  50  and provides compressed airflow through the discharge  52 . As will be discussed, the inlet guide vane assembly  34  is attached the housing  44  upstream of the inlet  50 . 
         [0057]    The intake system  28  is attached to the compressor  26  and fluidly communicates with the inlet  50 . The intake system  28  includes an air intake plenum  54  and flexible tube  56  that fluidly interconnects the plenum  54  and the inlet guide vane assembly  34 . As will be discussed, a heat exchanger of the hydraulic assembly  32  is mounted to the inlet of plenum  54  to cool hydraulic fluid. 
         [0058]    Turning to  FIG. 3 , the transmission  30  provides power from the motor  24  to the compressor  26  and the hydraulic assembly  32 . The illustrated compressor package  20  is preferably configured so that the transmission  30  is generally external to the compressor  26 . However, the principles of the present invention are applicable where the compressor  26  has an internal transmission. Furthermore, the transmission could be configured to provide self-lubrication. Preferred details of such a compressor and transmission arrangement are disclosed in U.S. Pat. No. 6,439,208, issued Aug. 27, 2002, entitled CENTRIFUGAL SUPERCHARGER HAVING LUBRICATING SLINGER, which is hereby incorporated in its entirety by reference herein. 
         [0059]    The transmission  30  preferably includes a housing  58 , an external belt drive  60 , and an internal gear drive  62 . The belt drive  60  includes a drive sheave (not shown) mounted on the shaft of the motor  24 , a driven sheave  64  mounted on an input shaft  66  of the gear drive  62 , and a toothed belt  68  drivingly entrained on the sheaves so that the motor  24  powers the input shaft  66 . 
         [0060]    The gear drive  62  includes the input shaft  66  and an intermediate shaft  70 . The gear drive  62  further includes a drive gear (not shown) mounted on the input shaft  66 . Gears  72 , 74  are mounted on the intermediate shaft  70 . The gear drive  62  further includes a driven gear  76  mounted to a drive shaft (not shown) of a hydraulic positive-displacement pump of the hydraulic assembly  32 . The gear drive  62  also includes a driven gear  78  mounted on the impeller shaft  46 . 
         [0061]    The transmission  30  provides power to the impeller  48  by transmitting power from the motor  24  to the input shaft  66 . Power is then transmitted to gear  72  and to driven gear  78 , which is drivingly mounted on the impeller shaft  46 . The gears  72 , 78  are preferably sized to provide a step-up arrangement to increase the rotational speed of the compressor  26 , although an alternative drive configuration could be employed to power the impeller  48   
         [0062]    The transmission  30  provides power to the hydraulic pump by transmitting power through the belt drive  60  to the input shaft  66 . Power is then transmitted to gear  72  and intermediate shaft  70 . Power is further transmitted from the gear  74  to the driven gear  76  mounted on the pump shaft. The gears  74 , 76  preferably provide a step-down arrangement to decrease the rotational speed of the hydraulic pump, although an alternative drive could be used to power the pump. 
         [0063]    Turning to  FIGS. 1-3  and  16 , the hydraulic assembly  32  provides lubricant to transmission  30  and also provides hydraulic power to the controller  36 . The hydraulic assembly  32  broadly includes a hydraulic pump  80 , a sump  82 , a heat exchanger  84 , and a pressure regulator  86 , with hydraulic lines  87   a,b,c,d,e,f,g,h  fluidly connecting the components. (see  FIG. 16 ). The pump  80  is preferably a conventional positive-displacement hydraulic pump and includes a pump drive shaft (not shown). The pump  80  is mounted on a sidewall of the housing  58 . The pump drive shaft extends through the sidewall into the transmission chamber and receives the driven gear  76  so that the driven gear  76  rotates with the drive shaft. 
         [0064]    Turning to  FIGS. 5 ,  6 ,  12 , and  14 , the inlet guide vane assembly  34  controls airflow drawn through the inlet  50  of compressor  26 . The inlet guide vane assembly  34  broadly includes a two-piece annular housing  88 , a plurality of vanes  90  rotatably mounted in the housing  88 , and a gear assembly  92 . The housing  88  preferably includes two annular housing sections that are removably attached to one another and cooperatively form an annular chamber that receives the vanes  90  and gear assembly  92 . The housing  88  is removably attached to the housing of compressor  26  adjacent the inlet  50 . However, it is also within the scope of the present invention where the housing  88  is alternatively constructed. Yet further, the assembly  34  could be configured so that housing  88  is eliminated, e.g., where the assembly  34  is integrated into the compressor housing. 
         [0065]    The vanes  90  each preferably include a flat vane body  94  and a shaft  96  attached to the body  94  with fasteners. The principles of the present invention are also applicable where the body  94  takes a different shape, e.g., another shape that provides improved vane performance. 
         [0066]    The vanes  90  are pivotally mounted in the housing  88  and are spaced circumferentially about a passage  98  that extends through the housing and toward the impeller  48 . The inlet guide vane assembly  34  further includes bearings  100  that rotatably receive corresponding shafts  96 . Furthermore, the assembly  34  includes caps  102 , 103  that are attached to and are rotatable with corresponding shafts  96 . The illustrated assembly  34  preferably includes eight vanes  90 , although a larger or smaller number of vanes  90  could be used to provide suitable vane performance. 
         [0067]    The gear assembly  92  drivingly interconnects the vanes  90  with one another so that the vanes  90  are synchronously shiftable. The gear assembly  92  broadly includes a ring gear  104  and pinion gears  106  positioned along the length of the ring gear  104 . The ring gear  104  presents an endless toothed surface  108  that generally faces in an axial direction. The ring gear  104  is rotatably mounted within an annular groove of the housing  88  so that the toothed surface  108  generally faces forwardly. The gear assembly  92  also includes an annular bushing (not shown) and a wave spring (not shown) received between the ring gear  104  and groove. In particular, the bushing is positioned between the ring gear  104  and wave spring, with the wave spring engaging the base of the groove. Thus, the wave spring urges the ring gear  104  into engagement with pinion gears  106 . 
         [0068]    The pinion gears  106  each have a toothed body  110  and a sleeve  112  integrally formed with the body  110 . The sleeve  112  receives a threaded set screw  114 . The teeth of body  110  present a pinion diameter that tapers from adjacent the sleeve  112  to an end of the pinion gear  106  opposite the sleeve  112 . Preferably, the pinion diameter presents a taper angle relative to the rotational axis of the pinion gear  106  that ranges from about five (5) degrees to about six (6) degrees, although the pinion gears  106  could be alternatively configured. 
         [0069]    The pinion gears  106  are mounted on corresponding shafts  96  of the vanes  90 , and each gear  106  is positioned between the respective bearing  100  and cap  102 . The gears  106  are preferably secured by threading set screw  114  into engagement with shaft  96 . Thus, the gears  106  are preferably spaced about the circumference of the ring gear  104 . It has been found that this arrangement of gears  106  and the tapered pinion construction allow the pinion gears  106  to cooperatively maintain the ring gear  104  centered relative to the gears  106 . 
         [0070]    Again, the vanes  90  are spaced about the passage  98 , and the pinion gears  106  each engage the toothed surface  108 . One of the shafts  96  extends radially outwardly from the cap  103  and is attached to a swing arm  116 . Thus, rotation of the swing arm  116  causes rotation of the shaft  96  and pinion gear  106  attached to the swing arm  116 . Because the pinion gears  106  intermesh with the ring gear  104 , this movement causes rotation of the ring gear  104  about the passage  98  and corresponding pivotal movement of the other pinion gears  106 . Thus, the pinion gears  106  and vanes  90  move synchronously with one another. Furthermore, each gear  106  and vane  90  rotates in the same rotational direction as the other gears  106  and vanes  90 . The rotational direction of a vane  90  is determined from a corresponding fixed reference point on the housing  88 . Thus, if one vane  90  is rotated in a clockwise direction, the other vanes  90  also rotate in the clockwise direction at the same time. 
         [0071]    Turning to  FIG. 6 , the vanes  90  can preferably be positioned relative to a passage axis A to provide either preswirl or counterswirl to the incoming airflow, or to impart substantially no swirling motion to the air flow. The preswirl direction coincides with the rotational direction of the impeller  48  and is clockwise when viewing the impeller  48  along the passage axis A. Thus, the counterswirl direction is counterclockwise when viewing the impeller  48  along the passage axis A. When the vanes  90  are in a neutral condition (i.e., the vanes  90  impart no preswirl or counterswirl), the vanes  90  are arranged so that the plane of vane body  90  is aligned with the passage axis A. 
         [0072]    The vanes  90  can be rotated clockwise from the neutral condition into a maximum preswirl condition. In the illustrated maximum preswirl condition, the vanes  90  are preferably rotated to an angle of about eighty (80) degrees clockwise from the axis A and the passage  98  is almost fully occluded. However, as will be discussed, the controller  36  can be adjusted to provide an alternative vane preswirl position associated with maximum preswirl. Similarly the vanes  90  can be rotated counterclockwise from the neutral condition into a maximum counterswirl condition. In the illustrated maximum counterswirl condition, the vanes  90  are preferably rotated to an angle of about fifteen (15) degrees counterclockwise from the axis A. Again, the controller  36  can be adjusted to provide an alternative vane counterswirl position associated with maximum counterswirl. However, for some applications, the controller  36  can also be adjusted so that the vanes  90  are not shiftable into any counterswirl position, i.e., the vanes  90  are always in a preswirl position or the neutral condition. 
         [0073]    The illustrated vanes  90  are preferably arranged upstream of inlet  50  to provide preswirl of incoming airflow. However, the principles of the present invention are applicable where vanes are employed to control the discharge flow of compressor  26 . For instance, the compressor  26  could include vanes mounted into the diffuser of compressor  26 . Alternatively a vane arrangement similar to the illustrated vane assembly  34  could be located downstream of the compressor discharge  52 . For some aspects of the present invention, the compressor package  20  could employ an alternative valve-type mechanism to control compressor airflow. For instance, some types of valves (such as a butterfly valve) could be installed upstream or downstream of the compressor  26  and used to throttle compressor airflow. 
         [0074]    Turning to  FIGS. 4-10 , the illustrated controller  36  is operable to position the vanes  90  of assembly  34  so that the compressor  26  provides a compressed airflow with a substantially constant velocity. It has been found that such operation is desirable in various compressor applications, such as pneumatic conveying. The controller  36  broadly includes a controller housing assembly  118 , a valve assembly  120 , a velocity sensor assembly  122 , a hydraulically-driven actuator  124 , and a bypass valve  125 . 
         [0075]    The housing assembly  118  preferably includes a central housing  126 , end manifolds  128 , 130 , and end caps  132 . The central housing  126  includes a generally rectangular cuboid body that presents an open rear slot  134  spaced between ends of the housing  126  (see  FIGS. 7 and 8 ), end faces  136 , 138  (see  FIG. 8 ), a centrally-located bottom cavity  140  that opens to front and bottom faces of the housing  126 , and transverse valve and piston bores  142 , 144  that extend laterally between the end faces  136 , 138 . The central housing  126  also includes a cover plate  146  that is removably attached to the body with fasteners. The cover plate  146  covers the bottom cavity  140  along the front face (see  FIGS. 1 and 11 ). 
         [0076]    The end manifold  128  comprises a generally rectangular cuboid body and presents end faces  148 , 150  (see  FIG. 8 ). The end face  150  includes a slot  152  spaced from the outer margin of the end face  150  (see  FIG. 7 ). A piston end bore  154  extends from the slot  152  toward the end face  148 , with a threaded hole  156  extending from the end face  148  to the bore  154 . 
         [0077]    The end manifold  128  is removably attached to the central housing  126  with fasteners that extend through the body of manifold  128  and are threaded into the central housing  126 . With the end manifold  128  attached to the central housing  126 , the end face  150  engages the end face  136  of central housing  126  (see  FIG. 8 ). Also, the piston end bore  154  is preferably aligned with the piston bore  144  (see  FIG. 7 ). Furthermore, the recess presented by the slot  152  preferably fluidly communicates with the valve bore  142  (see  FIG. 8 ). 
         [0078]    The end manifold  130  also comprises a generally rectangular cuboid body and presents end faces  158 , 160  (see  FIG. 8 ). A piston end bore  162  extends from end face  158  toward the end face  160 , with a threaded hole  164  extending from the end face  160  to the bore  162 . The end manifold  130  also presents transversely extending supply and return bores  166 , 168  extending from the end face  160  toward the end face  158 . A fore-and-aft bore  170  extends from the front face of end manifold  130  to the return bore  168 . An upright bore  171  extends from the top face of end manifold  130  to a valve end bore  172 . Once the bores  170 , 171  are machined, the open ends of the bores  170 , 171  are closed with plugs (not shown) to prevent fluid leakage from the bores  170 , 171 . 
         [0079]    The end manifold  130  is removably attached to the central housing  126  with fasteners  174  that extend through the body of manifold  130  and are threaded into the central housing  126 . With the end manifold  130  attached to the central housing  126 , the end face  158  engages the end face  138  of central housing  126  (see  FIG. 8 ). Also, the piston end bore  162  is preferably aligned with the piston bore  144  (see  FIG. 7 ). Furthermore, the valve end bore  172  is preferably aligned with the valve bore  142  (see  FIG. 8 ). 
         [0080]    The housing assembly  118  is preferably removably mounted on a platform of the housing  88 , with the housing assembly  118  being located above the passage  98  and located forwardly of the vanes  90 . However, the housing assembly  118  could be alternatively supported relative to the inlet guide vane assembly  34 . 
         [0081]    The central housing and end manifolds  128 , 130  of the housing assembly  118  are preferably formed of aluminum, but could be formed of other materials, such as stainless steel or carbon steel. Yet further, the components of housing assembly  118  could be alternatively configured to provide suitable controller operation. 
         [0082]    The hydraulic assembly  32  and housing assembly  118  are fluidly connected to one another so that pressurized hydraulic fluid can be supplied to the controller  36  and returned from the controller  36 . In particular, line  87   d  runs from the pressure regulator  86  to supply port S so that pressurized hydraulic fluid can flow to supply bore  166 . Line  87   h  runs from return port R to sump  82  so that pressurized hydraulic fluid can be returned from return bore  168 . While the controller  36  is preferably hydraulically powered, for some aspects of the present invention the controller  36  could be pneumatically operated. 
         [0083]    Turning to  FIGS. 7-10 , the valve assembly  120  serves to control and selectively allow hydraulic fluid flow within the controller  36 . The valve assembly  120  includes a rotatable valve  176 , bearings  178 , and seals  180 . The valve  176  preferably comprises a unitary and cylindrical metal body that presents a chamfered end  182  and a slotted end  184  (see  FIGS. 9 and 10 ). The valve  176  presents upper and lower slots  186 , 188  that extend perpendicular to the length of the valve  176  and along the end  184 . An upper longitudinal bore  190  extends from the end  184  to the upper slot  186 . A lower longitudinal bore  192  extends from the end  182  to the lower slot  188 . The valve  176  further presents a centrally spaced fore-and-aft slot  194  and a through hole  196 . 
         [0084]    The valve  176  is rotatably received within the valve bore  142  and valve end bore  172 . In particular, the chamfered end  182  is positioned adjacent the end face  136  and the slotted end  184  is located within the valve end bore  172  so that a gap  198  is defined between the slotted end  184  and the end of the valve end bore  172  (see  FIG. 8 ). 
         [0085]    Bearings  178  are secured in annular slots formed adjacent to and surrounding corresponding ends of the valve bore  142 . The bearings  178  rotatably receive and support the valve  176  within the housing assembly  118  and permit rotation relative thereto. A pair of seals  180  are received in corresponding glands that surround the valve bore  142  and are located between the annular bearing slots. The seals  180  rotatably engage the valve  176  and restrict pressurized fluid from flowing from the recess of slot  152  to the bottom cavity  140  and from the valve end bore  172  to the bottom cavity  140 . Preferably, the seals  180  are formed of PTFE material to provide low friction, although other materials could be used. 
         [0086]    Turning to FIGS.  7  and  11 - 15 , the valve  176  is operable to control hydraulic fluid flow within the controller  36  by selectively allowing hydraulic fluid to flow between piston end bores  154 , 162  and the supply and return bores  166 , 168 . In a closed position, the valve  176  is positioned so that neither of the slots  186 , 188  is in fluid communication with the fore-and-aft bore  170  (see  FIGS. 7 and 11 ). The fore-and-aft bore  170  provides part of the paths for fluid to flow between the bores  166 , 168  and piston end bores  154 , 162 . Thus, the closed valve  176  prevents fluid flow into and out of the controller  36 . 
         [0087]    In a vane-closing position, the valve  176  is positioned so that the upper slot  186  fluidly communicates with the supply bore  166  via a front portion of the fore-and-aft bore  170 , and the upper slot  186  thereby receives pressurized supply fluid (see  FIGS. 12 and 13 ). The piston end bore  162 , upright bore  171 , gap  198 , and upper bore  190  are also in fluid communication with the upper slot  186  and also receive pressurized supply fluid. 
         [0088]    At the same time, in the vane-closing position, the lower slot  188  fluidly communicates with the return bore  168  via a rear portion of the fore-and-aft bore  170 , and the lower slot  188  thereby returns pressurized supply fluid. The piston end bore  154 , slot  152 , and lower bore  192  are also in fluid communication with the lower slot  188  to thereby return pressurized supply fluid in the vane-closing position. 
         [0089]    In a vane-opening position, the valve  176  is positioned so that the lower slot  188  fluidly communicates with the supply bore  166  via the front portion of the fore-and-aft bore  170 , and the lower slot  188  thereby receives pressurized supply fluid (see  FIGS. 14 and 15 ). Again, the piston end bore  154 , slot  152 , and lower bore  192  are in fluid communication with the lower slot  188  and also receive pressurized supply fluid. 
         [0090]    At the same time, in the vane-opening position, the upper slot  186  fluidly communicates with the return bore  168  via the rear portion of the fore-and-aft bore  170 , and the upper slot  186  thereby returns pressurized supply fluid. Again, the piston end bore  162 , upright bore  171 , gap  198 , and upper bore  190  are in fluid communication with the upper slot  186  and also return pressurized supply fluid in the vane-opening position. In this manner, the illustrated vane arrangement simultaneously pressurizes and depressurizes opposite sides of the illustrated hydraulic piston, as will be discussed. 
         [0091]    The illustrated valve assembly  120  preferably comprises a mechanically-driven fluid valve. However, for some aspects of the present invention, the valve assembly  120  could include an alternatively-driven valve arrangement, such as an electronic solenoid valve. 
         [0092]    The velocity sensor assembly  122  is shiftably received in the passage  98  and is operable to rotate the valve  176 . The velocity sensor assembly  122  broadly includes an elongated arm  200 , a plate  202 , a spring  204 , and an adjuster  206 . The arm  200  presents opposite ends, with an upper end being removably attached to the valve  176  within the central slot  194  by a threaded fastener  208 . The plate  202  is generally circular and is removably attached to a lower end of the arm  200  by screws  210  (see  FIG. 5 ). The arm  200  and plate  202  preferably take the illustrated shape, but it is also within the ambit of the present invention where the arm  200  and plate  202  have alternative shapes to provide desired sensor operation. For example, the plate  202  could have a larger or smaller diameter so that the plate  202  applies a correspondingly larger or smaller drag force to the arm  200  when airflow is rushing past the sensor. 
         [0093]    With the arm  200  secured to the valve  176 , the valve  176 , arm  200 , and plate  202  are operable to pivot with one another about a lateral axis. Again, the valve  176  pivots to direct pressurized fluid through the controller  36 . The sensor assembly  122  serves to operate the valve  176 , as will be discussed below. 
         [0094]    Turning to  FIG. 4 , the adjuster  206  provides tension adjustment of the spring  204 . The adjuster  206  includes a slotted body  212  with opposite end slots  214 . The body  212  is slidably received by complemental grooves  216  that form part of the bottom cavity  140  of the central housing  126 . Thus, the body  212  is operable to slide up and down within the cavity  140 . The body  212  is adjustably positioned by a tensioning screw  218  that extends through a hole in the central housing  126  and is rotatably received by the body  212 . 
         [0095]    The spring  204  includes opposite end hooks  220 , with one end hook  220  removably attached to the arm  200  at a location preferably about one-third of the arm length from the upper arm end. The other end hook  220  is removably attached to the body  212  (see  FIG. 7 ). The illustrated adjuster  206  is spaced in front of and above the arm  200  so that the spring  204  urges the arm  200  in a forward direction, i.e., generally opposite the normal airflow direction into the inlet  50 . However, the sensor assembly  122  could have an alternative construction to provide adjustment of the valve  176 . For instance, the assembly  122  could have one or more alternative springs to control positioning of the arm  200 . 
         [0096]    Furthermore, the sensor assembly  122  could include a traditional damping mechanism, such as a fluid-filled damper, connected between the arm  200  and housing  126  to provide additional control of the valve  176 . However, one of ordinary skill will appreciate that suitable damping of the valve  176  and actuator  124  can be provided by various alternative constructions. Furthermore, some damping of the controller  36  is provided inherently by the hydraulic arrangement of the controller  36 . That is, the hydraulic fluid within the controller  36  provides some damping of the actuator  124 , valve  176 , and fluid pressure signals transmitted therebetween. 
         [0097]    In the illustrated embodiment, the spring  204  urges the arm  200  so that the valve  176  assumes the closed position when the airflow is at a predetermined airflow velocity; that is, the arm  200  is preferably held by the spring somewhere between the extreme positions during “ideal” operating conditions. However, the adjuster  206  is operable to change the spring tension (and thereby the length of spring  204 ) to vary the predetermined airflow velocity that corresponds to the valve  176  in the closed position. In particular, as the adjuster  206  is shifted upwardly to provide greater spring tension, the predetermined airflow velocity associated with the closed valve position generally increases. As the adjuster  206  is shifted downwardly to decrease spring tension, the predetermined airflow velocity associated with the closed valve position generally decreases. 
         [0098]    Preferably, the illustrated sensor assembly  122  is primarily responsive to airflow velocity through the passage  98 . However, as will be shown in a subsequent embodiment, the controller can be alternatively configured to include a diaphragm attached to the arm  200 , where the diaphragm senses compressor discharge pressure and applies a force to the arm  200  in response to the pressure. 
         [0099]    The illustrated sensor assembly  122  preferably comprises a mechanical velocity sensor. However, for some aspects of the present invention, an alternative velocity or mass flow sensor, such as a pitot tube or hot wire anemometer, could be used to determine airflow velocity for the compressor package  20 . As discussed above, the valve assembly  120  could include an alternative valve, such as an electronic solenoid valve. For some aspects of the present invention, the compressor package  20  could utilize a solenoid valve in combination with an electronic velocity sensor to alternatively sense airflow and provide fluid control of the inlet guide vanes. 
         [0100]    Turning to  FIG. 7 , the hydraulically-driven actuator  124  serves to drive the swing arm  116 . The actuator  124  preferably includes a piston  222 , a screw  224 , and a bushing  226 . The piston  222  preferably comprises a unitary cylindrical rod that presents opposite ends  228 , a smooth cylindrical surface  230 , and a central flat  232  spaced between the ends  228 . 
         [0101]    The piston  222  is reciprocally received in piston bore  144  and end bores  154 , 162  so as to be slidable between left and right endmost positions corresponding to the vanes  90  in the maximum preswirl condition and the maximum counterswirl condition (see  FIGS. 12 and 14 ). In each endmost position, the piston  222  contacts a corresponding stop  234 , which comprises a threaded screw that is adjustably positionable to change the location of the endmost position. Thus, the left stop  234  can be adjusted to provide an alternative vane preswirl position associated with maximum preswirl. Also, the right stop  234  can be adjusted to provide an alternative vane counterswirl position associated with maximum counterswirl. Again, for some applications, the right stop  234  can also be adjusted so that the vanes  90  are not shiftable into any counterswirl position, i.e., the vanes  90  are always in a preswirl position or the neutral condition. The stops  234  are covered by end caps  132  secured to the respective end manifold with fasteners. The stops  234  can be selectively accessed for adjustment by removing the respective end cap  132 . 
         [0102]    The piston  222  and end bores  154 , 162  cooperatively form corresponding fluid chambers  236 , 238  that receive pressurized fluid. The illustrated piston  222  is preferably a double-acting piston, although the controller  36  could be configured so that the piston  222  is single-acting without departing from the scope of the present invention. Low-friction seals  240  are located at corresponding ends of the central housing  126  and extend around the piston  222  to hold fluid within the chambers  236 , 238 . Preferably, the seals  240  are formed of PTFE material to provide low friction, although other materials could be used. 
         [0103]    The piston  222  is removably attached to the swing arm  116  by inserting the screw  224  and bushing  226  through a slot  242  in the swing arm  116 . Thus, as the piston  222  reciprocates between the endmost positions, the screw  224  and bushing  226  slide along the slot  242 . Thus, the swing arm  116  pivots in a manner corresponding to sliding movement of the piston  222 . 
         [0104]    As discussed above, pivotal movement of the swing arm  116  causes corresponding pivotal movement of the vanes  90 . Thus, sliding movement of the piston  222  results in corresponding pivotal movement of the vanes  90 . With the piston  222  in the left endmost position, the vanes  90  are shifted to the maximum preswirl condition (see  FIG. 12 ). With the piston  222  in the right endmost position, the vanes  90  are shifted to the maximum counterswirl condition (see  FIG. 14 ). 
         [0105]    The actuator  124  uses hydraulic fluid from the hydraulic system to power the vanes  90  depending on the position of valve  176 . As discussed above, the valve  176  has a closed position and opposite open positions (i.e., the vane-opening position and the vane-closing position). 
         [0106]    In the vane-closing position, the valve  176  is positioned so that the upper slot  186  fluidly communicates with the supply bore  166  (see  FIG. 12 ). Consequently, the piston end bore  162  is in fluid communication with the upper slot  186  to receive pressurized supply fluid and fill the fluid chamber  238 . At the same time, the lower slot  188  fluidly communicates with the return bore  168 . Thus, the piston end bore  154  is in fluid communication with the lower slot  188  to return pressurized supply fluid in the vane-opening position and thereby empty the fluid chamber  236 . The simultaneous filling and emptying of chambers  236 , 238  causes the piston  222  to shift to the left, with the vanes  90  shifting to the maximum preswirl condition (see  FIG. 12 ). 
         [0107]    In the vane-opening position, the valve  176  is positioned so that the lower slot  188  fluidly communicates with the supply bore  166 . The piston end bore  154  is in fluid communication with the lower slot  188  to receive pressurized supply fluid and fill the fluid chamber  236 . At the same time, the upper slot  186  fluidly communicates with the return bore  168 . The piston end bore  162  is in fluid communication with the upper slot  186  to return pressurized supply fluid in the vane-closing position and thereby empty the fluid chamber  238 . This simultaneous filling and emptying of chambers  236 , 238  causes the piston  222  to shift to the right, with the vanes  90  shifting to the maximum counterswirl condition (see  FIG. 14 ). 
         [0108]    The inlet guide vane assembly  34  and controller  36  are depicted with the piston  222  and vanes  90  in certain discrete positions. Specifically, the piston  222  is shiftable between endmost positions with the vanes  90  being shiftable between maximum preswirl and maximum counterswirl conditions. However, it will be appreciated that the illustrated piston  222  can position the vanes  90  into nearly an infinite number of intermediate positions between the maximum preswirl and maximum counterswirl conditions. As discussed above, the piston  222  is slidable continuously along a lateral direction between the endmost positions. Also, the piston  222  is slidably attached to the swing arm  116 . Thus, continuous sliding movement of piston  222  between the endmost positions results in corresponding pivotal movement of the swing arm  116 , which causes synchronized continuous pivotal movement of vanes  90  between the maximum preswirl and maximum counterswirl conditions. 
         [0109]    The illustrated actuator  124  preferably receives hydraulic fluid to drive the inlet guide vanes. However, for some aspects of the present invention, the actuator  124  could be alternatively configured to receive hydraulic fluid and power the inlet guide vanes. For instance, an alternative actuator could include a hydraulic motor that receives a continuous hydraulic fluid flow from the hydraulic system. 
         [0110]    The illustrated controller  36  is preferably powered by the hydraulic system so that the hydraulic system amplifies the power input to the sensor assembly (caused by airflow in the passage  98 ) and thereby drives the actuator  124  and guide vanes  90 . Through the direct mechanical connection between the sensor assembly and the valve  176 , the sensor assembly directly effects positioning of the actuator  124  and thereby the vanes  90 . For some aspects of the present invention, the controller  36  could be configured so that actuator  124  is powered by a conventional pneumatic power system, such as a regulated shop-air system. 
         [0111]    The compressor package  20  provides substantially uniform airflow velocity by sensing the airflow and then shifting the inlet guide vanes, if necessary, to maintain a desired airflow velocity. For instance, in the illustrated embodiment, the compressor package  20  could maintain airflow velocity with a preset amount of preswirl (see  FIG. 7 ). 
         [0112]    In some instances, the compressed airflow discharged from the compressor  26  can encounter significant backpressure, such as when the package  20  is used in a pneumatic conveying system and a large volume of particulate matter is injected downstream of the compressor discharge. In such an instance, airflow velocity through the compressor  26  will decrease and the spring  204  will cause the sensor arm  200  to shift forwardly (i.e., away from the impeller) so that the valve  176  shifts into the vane-opening condition, which causes the vanes  90  to open. Thus, the vanes  90  open so that the inlet guide vane assembly  34  provides minimal restriction to intake airflow and thereby permits airflow velocity to increase and return to the desired airflow velocity. 
         [0113]    Preferably, the bypass valve  125  is also operable to provide increased compressor airflow by venting compressed airflow from the compressor discharge to ambient. The illustrated bypass arrangement has been found to be particularly effective for low velocity conditions in pneumatic conveying systems when a large volume of particulate matter causes excessive backpressure in the compressor  26 . The bypass valve  125  is mounted on main compressed air line  244  that runs from the compressor discharge  52 . A bypass line  246  is fluidly connected to the main line  244  and terminates at the bypass valve  125 . The bypass valve  125  receives pressurized hydraulic fluid from the housing assembly  118  via a pressure relief valve  248  and line  250  and returns hydraulic fluid to the sump  82  via line  252  (see  FIG. 16 ). 
         [0114]    When the compressor package  20  encounters a low airflow velocity due to substantial compressor backpressure, e.g., due to clogging of a pneumatic conveying line downstream of the compressor, the controller  36  initially opens the inlet guide vanes to reduce the intake airflow restriction of the inlet guide vane assembly and thereby allow greater airflow into the compressor  26 . Again, the controller  36  opens the vanes  90  in a low velocity condition because the sensor arm  200  pivots forwardly and the valve  176  pivots so that pressurized fluid builds in the chamber  236  to shift the piston  222  to the right (see  FIG. 14 ). In the event that the valve  176  remains in the vane-opening position because of continued low-velocity airflow, fluid pressure will build within chamber  236  up to the pressure of the hydraulic system, which preferably ranges from about fifty (50) psi to about sixty (60) psi. Once the pressure in chamber  236  exceeds the preset pressure value, the pressure relief valve will open and cause bypass valve  125  to open. In this manner, the bypass valve  125  is operable to correct a continued low-velocity airflow condition. 
         [0115]    In other instances, the compressed airflow discharged from the compressor  26  can encounter greatly reduced backpressure. For example, in a pneumatic conveying system, the operator may eliminate particulate matter from the line downstream of the compressor discharge. In this instance, airflow velocity through the compressor  26  will generally increase and the spring  204  will cause the sensor arm  200  to shift rearwardly, i.e., toward the impeller, so that the valve shifts into a vane-closing condition, which causes the vanes  90  to close. Thus, the vanes  90  close so that the inlet guide vane assembly  34  provides a substantial airflow restriction to intake airflow. Consequently, airflow velocity will decrease and return to the desired airflow velocity. 
         [0116]    Turning to  FIGS. 17 and 18 , a second preferred embodiment of an alternative compressor package  300  is depicted. For the sake of brevity, the remaining description will focus primarily on the differences of this embodiment relative to the embodiment described above. The primary difference is the use an airflow sensor with a damping diaphragm. The alternative compressor package  300  includes a compressor  302 , an inlet guide vane assembly  304 , and an alternative sensor assembly  306 . The sensor assembly  306  broadly includes an arm  308 , plate  310 , a spring  312 , an adjuster  314 , and a diaphragm  316 . The diaphragm  316  and spring  312  are both attached to the arm  308  at the same location between ends of the arm  308 . The diaphragm  316  also fluidly communicates with the compressor volute so as to sense a compressor discharge pressure. Consequently, the illustrated sensor assembly  306  controls the inlet guide vanes in response to airflow velocity and compressor discharge pressure. 
         [0117]    Turning to  FIG. 18 , without the diaphragm  316 , the compressor package  300  has an operational characteristic identified by line  318 , i.e., the compressor package  300  would provide a substantially constant airflow velocity over a range of compressor pressures. With the diaphragm  316 , performance of the compressor package  300  can be adjusted to provide alternative operational characteristics identified by lines  320 , 322 . 
         [0118]    The adjustable compressor performance provided by diaphragm  316  has been found to be particularly desirable in a pneumatic conveying application. For example, when little or no particulate media is being conveyed in the system, the compressor will produce a relatively low discharge pressure with relatively high velocity. The diaphragm  316  is configured to push the sensor arm rearwardly to thereby encourage opening of the inlet guide vanes in a low-pressure condition (see  FIG. 17 ). For example, when a heavy media, such as flour, is initially introduced into the conveying system, the flour can readily cause clogging of the media line. Thus, the illustrated diaphragm  316  is preferably connected to the sensor to further increase airflow velocity when the media line has little or no media in it, and the illustrated diaphragm  316  provides a compressor operational characteristic identified by line  322  (see  FIG. 18 ). In this manner, the media line is ready to receive a subsequent slug of media without becoming clogged. 
         [0119]    For other purposes, the diaphragm  316  can be configured so as to push the sensor arm forwardly to encourage at least partial closure of the inlet guide vanes in response to low compressor discharge pressure to thereby reduce airflow velocity. By reducing airflow velocity, the power requirements of the compressor are reduced, thus saving energy. Such a diaphragm configuration provides a compressor operational characteristic identified by line  320  (see  FIG. 18 ). For some types of media, such as plastic granules, it has been found that media can be introduced into the conveying system at relatively low-velocity airflow without clogging the media line. 
       Additional Embodiments 
       [0120]    Turning to  FIGS. 19-35 , an alternative compressor package  400  is depicted as part of a pneumatic conveying system  402 . For brevity, the description of this system  402  will focus primarily on the differences of this embodiment relative to the previously described embodiment. 
         [0121]    Turning to  FIG. 19 , in addition to the compressor package  400 , the illustrated pneumatic conveying system  402  includes a main flow line  404 , media storage tank  406 , media valve  408 , and downstream media equipment  410 , including a separator (not shown). The conveying system  402  is preferably operable to transport various types of particulate media, such as plastics, flours, etc. The tank  406  stores the media and permits media to be discharged into the flow line  404  at media pickup location  407  by opening the media valve  408 . However, as will be appreciated, the system  402  could have various alternative configurations to suitably convey media for different conveying applications without departing from the scope of the present invention. 
         [0122]    Turning to  FIGS. 19-22 , the illustrated compressor package  400  is preferably configured for use with the system  402 . As will be discussed in greater detail, the compressor package  400  preferably provides a substantially constant airflow velocity at the media pickup location  407 , and this velocity is referred to as the pickup velocity. Generally, a desired pickup velocity is selected to be greater than the saltation velocity (i.e., the velocity at which airflow becomes insufficient to maintain solids in suspension and the solids begin to settle in the bottom of the conveying line) for a given media. For example, the desired pickup velocity for flour preferably ranges from about three thousand (3000) feet per minute to about three thousand five hundred (3500) feet per minute. 
         [0123]    Furthermore, the compressor package  400  is preferably operable to overcome media blockage that develops in the flow line  404  downstream of the compressor. As will be explained in greater detail, the compressor package  400  is preferably designed to maintain airflow and pressure in reserve. More particularly, the compressor package  400  includes an inlet guide vane assembly that is preferably operated in a partly closed condition during normal conveying operation so that the compressor can provide additional flow and pressure when necessary, e.g., to overcome downstream blockage. 
         [0124]    As with the previously disclosed embodiments, the compressor package  400  is suitable to be retrofitted into a pneumatic conveying system (e.g., to replace a positive-displacement compressor). However, the compressor package  400  is operable to provide compressed air for other applications without departing from the scope of the present invention. The compressor package  400  broadly includes, among other things, a centrifugal compressor  412 , an inlet guide vane assembly  414 , and an alternative guide vane controller  416 . 
         [0125]    The illustrated compressor  412  is conventional and includes a compressor housing and an impeller. The compressor  412  preferably operates at an impeller rotational speed of 41,200 revolutions per minute and provides a corresponding flow rate of 625 cubic feet per minute. 
         [0126]    The controller  416  is operable to position vanes of the guide vane assembly so that the compressor  412  provides a compressed airflow with a substantially constant velocity. The controller  416  preferably includes a hydraulic actuator  417  with a housing assembly  418  and a hydraulically-driven actuator piston  420 , a valve assembly  422 , a sensor assembly  424 , and a bypass valve  426 . 
         [0127]    Turning to  FIG. 20 , the housing assembly  418  preferably includes a central housing  428 , end manifolds  430 , 432 , and end caps  434 . The central housing  428  includes a generally rectangular cuboid body that presents an open rear slot  438  spaced between ends of the housing  428 , end faces  440 , 442 , a transverse piston bore  444 , and a central fluid bore  446 . 
         [0128]    The end manifold  430  comprises a generally rectangular cuboid body and presents end faces  448 , 450 . The end face  450  includes a slot  452  spaced from the outer margin of the end face  450 . A piston end bore  454  extends from the slot  452  toward the end face  448 . 
         [0129]    The end manifold  430  is removably attached to the central housing  428  with fasteners (not shown) so that the end face  450  engages the end face  440  of the central housing  428 , with the piston bores  444 , 454  being aligned with one another. Also, the recess presented by the slot  452  preferably fluidly communicates with the fluid bore  446 . 
         [0130]    The end manifold  432  also comprises a generally rectangular cuboid body and presents end faces  456 , 458 . A piston end bore  460  extends from end face  456  toward the end face  458 . The end manifold  432  also presents transversely extending bores  462 , 464  extending from the end face  458  toward the end face  456 . A fore-and-aft bore  466  extends from the front face of the end manifold  432  to a port  468  so that the port  468  and bore  462  fluidly communicate with one another. An upright bore  470  extends from the top face of the end manifold  432  to bore  464  and fluidly communicates with bore  464 . Once the bores  466 , 470  are machined, the open ends of bores  466 , 470  are closed with plugs (not shown) to prevent fluid leakage from the bores. 
         [0131]    The end manifold  432  is removably attached to the central housing  428  with fasteners (not shown) so that the end face  456  engages the end face  442  of the central housing  428  and the piston bores  444 , 460  are aligned with one another. Also, the fluid bore  446  fluidly communicates with port  468 . Thus, the piston  420  is slidably received by bores  444 , 454 , 460  so that the housing assembly  418  and piston  420  cooperatively form an opening chamber  471   a  and a closing chamber  471   b . Similar to the previous embodiments, the piston  420  is attached to the inlet guide vane assembly  414  with a swing arm. While the illustrated actuator configuration is preferred, the principles of the present invention are applicable where an alternative actuator is used to move the inlet guide vanes. As will be discussed, the housing assembly  418  and the hydraulic assembly (not shown) are fluidly connected to one another by the valve assembly  422  so that pressurized hydraulic fluid can be supplied to the actuator  417  and returned from the actuator  417 . 
         [0132]    Turning to  FIGS. 21-34 , the valve assembly  422  is operable to control hydraulic fluid flow to and from the actuator  417 . The valve assembly  422  broadly includes a valve housing  472 , a valve piston  474 , and a shiftable lever  476 . The valve housing  472  comprises a generally rectangular cuboid construction and includes a body  478  and a cover  480 . The body  478  is preferably unitary and presents upper and lower ends  482 , 484  and a face  486  that extends between the ends  482 , 484  (see  FIG. 26 ). The body  478  also presents slotted openings  488  and slotted openings  490  that extend along the face  486 , with o-ring glands  492  that surround respective openings  490 . The valve assembly  422  further includes rubber stops (not shown) that are inserted between the body  478  and cover  480  in openings  488  and o-rings (not shown) that are inserted in glands  492 . 
         [0133]    The body  478  presents a piston bore  494  that extends from the upper end  482  toward the lower end  484  and is operable to receive the piston  474 . The body  478  also presents a drain chamber  496  that extends from the lower end  484  toward the upper end  482 . The body  478  further presents lateral drain ports  498   a,b,c,d  that extend from respective slotted openings  488  through the piston bore  494  and to the drain chamber  496  (see  FIGS. 26 ,  28 ,  29 ,  33 , and  34 ). A lateral relief port  500  is positioned adjacent the upper end  482  with a surrounding o-ring gland  501  (see  FIGS. 26 and 27 ). Yet further, the body  478  presents a lateral supply port  502  that extends from the face  486  to the piston bore  494  and is surrounded by an o-ring gland  504  (see  FIGS. 26 and 31 ). The body  478  also presents upper and lower lateral return ports  506 , 508  that extend from the face  489  to the piston bore  494  (see  FIGS. 26 ,  30 , and  32 ). The valve assembly  422  also includes o-rings (not shown) that are inserted in glands  501  and  504 . 
         [0134]    The cover  480  is preferably unitary and presents opposite inner and outer faces. The cover  480  presents several lateral ports including a relief port  510 , a supply port  512 , and upper and lower return ports  514 , 516  (see  FIG. 21 ). The cover  480  is removably attached to the body  478  with multiple threaded fasteners  518 . When attached, the cover  480  is aligned with the body  478  so that the relief ports  500 , 510  are in fluid communication, the supply ports  502 , 512  are in fluid communication, the upper return ports  506 , 514  are in fluid communication, and the lower return ports  508 , 516  are in fluid communication. 
         [0135]    The cover  480  is preferably constructed so that conventional hydraulic lines (not shown) can be removably attached to each of the respective ports. In particular, the supply port  512  is preferably fluidly connected to the hydraulic system (not shown), which supplies pressurized hydraulic fluid to the controller  416 . The upper return port  506  is preferably fluidly connected to the opening chamber  471  a via the port  514 . The lower return port  508  is preferably fluidly connected to the closing chamber  471   b  via the port  516 . 
         [0136]    Turning to  FIGS. 21-25 , the valve piston  474  preferably includes a piston body  520 , a connector  522 , and straps  524 . The piston body  520  presents opposite upper and lower ends and presents a generally outer cylindrical surface that extends between the ends. The piston body  520  also preferably presents endless annular grooves  526   a,b,c,d  spaced between the ends (see  FIGS. 23-25 ). Uppermost groove  526   a  permits removal of fluid from the piston bore  494  through relief ports  500 , 510 . Drain groove  526   b  permits drainage of fluid from the piston bore  494  through one of the drain ports  498   a,b . Diversion grooves  526   c,d  each permit diversion of fluid from the supply port  502  to either a corresponding drain port  498   b,c  or to a corresponding return port  506 , 508 . Adjacent the grooves  526  are piston sections  528   a,b,c,d,e  used to restrict fluid flow. 
         [0137]    The valve piston  474  is slidably received in the piston bore  494  and is operable to slide along the piston axis. The piston  474  and housing  472  preferably do not have an o-ring seal therebetween so that friction is minimized as the piston  474  slides relative to the housing  472 . 
         [0138]    The piston  474  is slidable between opening, closing, and neutral positions to selectively provide pressurized hydraulic flow to the actuator. In the neutral position, the piston section  528   d  covers the supply port  512  and restricts fluid flow through the supply port  512  into the piston bore  494 . Similarly, the sections  528   c,e  cover drain ports  498   b,c  and restrict flow through the return ports  506 , 508 . Thus, in the neutral position, fluid flow into and out of the chambers  471   a,b  is preferably restricted so that the actuator  417  is restricted from moving the guide vanes. 
         [0139]    In the opening position, the piston section  528   d  is located between the supply and lower return ports  502 , 508  to restrict flow therebetween. Thus, fluid is permitted to flow from the lower return port  508  to the drain chamber  496  by passing through the lower diversion groove  526   d.  Also, fluid is permitted to flow from the supply port  502  to the upper return port  506  by passing through the upper diversion groove  526   c.    
         [0140]    The opening position allows fluid flow to cause the actuator to urge the vane assembly  414  open. In particular, the upper return port  506  fluidly communicates with the opening chamber  471  a so that pressurized hydraulic fluid is pumped from the hydraulic system into the opening chamber  471   a.  At the same time, the lower return port  508  fluidly communicates with the closing chamber  471   b  so that hydraulic fluid is drained from the closing chamber  471   b  and returned to the hydraulic system. Consequently, this movement of hydraulic fluid urges the actuator piston  420  into the opening condition (not shown) so that the vanes are shifted open. 
         [0141]    In the closing position, the piston section  528   d  is located between the supply and upper return ports  502 , 506  to restrict flow therebetween. Thus, fluid is permitted to flow from the upper return port  506  to the drain chamber  496  by passing through the upper diversion groove  526   c.  Also, fluid is permitted to flow from the supply port  502  to the lower return port  508  by passing through the lower diversion groove  526   d.    
         [0142]    The closing position allows fluid flow to cause the actuator to urge the vane assembly  414  closed. In particular, the lower return port  508  fluidly communicates with the closing chamber  471   b  so that pressurized hydraulic fluid is pumped from the hydraulic system into the closing chamber  471   b . At the same time, the upper return port  506  fluidly communicates with the opening chamber  471   a  so that hydraulic fluid is drained from the opening chamber  471   a  and returned to the hydraulic system. Consequently, this movement of hydraulic fluid urges the actuator piston  420  into the closing condition (see  FIG. 20 ) so that the vanes are shifted closed. 
         [0143]    Turning to  FIGS. 21 and 22 , the lever  476  is preferably unitary and presents opposite lever ends  476   a,b . The lever  476  is mounted to the housing  472  with a lever support  530 , which includes upright braces  532  and a fastener  534  that extends laterally to interconnect upper ends of the braces  532 . The lever  476  presents a hole spaced between the ends that rotatably receives the lateral fastener  534 . Thus, the supported lever  476  is operable to pivot about the axis of the fastener  534 . 
         [0144]    The threaded connector  522  is attached to the piston body  520  at the upper end thereof. The illustrated connector  522  preferably comprises an eyebolt that presents an eye. The piston  474  and lever  476  are interconnected by straps  524 . Lower ends of straps  524  are attached to the eyebolt with a fastener  538  that is secured through the lower ends of straps  524  and the eye of connector  522 . Upper ends of straps  524  are attached to the lever  476  with a fastener  540  that is secured through the upper ends of straps  524  and the lever  476 . Thus, up and down movement of the piston  474  causes corresponding up and down movement of the lever end  476   a.    
         [0145]    In the illustrated embodiment, a weight  542  is preferably attached to the lever end  476   a  to urge the piston  474  into the piston bore  494  and toward the opening position. However, the principles of the invention are applicable where the valve assembly  422  has an alternative counterbalance mechanism so that the valve assembly  422  is operable to regulate airflow velocity. For example, the lever  476  could be alternatively biased to urge the piston  474  into the opening position, e.g., where a spring is attached to the lever end  476   a  (or at another location along lever  476 ) to urge the lever end  476   a  downwardly. 
         [0146]    Turning to  FIG. 22 , the sensor assembly  424  is preferably configured to sense pressures within the compressor discharge (or volute) and act on the lever  476  of the valve assembly  422  in response to the sensed pressures, although pressure conditions could alternatively be sensed elsewhere in the line (e.g., within the compressor inlet or downstream from the compressor discharge). The sensor assembly  424  preferably includes a conventional pitot tube  544  and a diaphragm  546 . The diaphragm  546  is also conventional and includes a housing  548  and a shiftable diaphragm element  550 . The diaphragm element  550  acts as a piston and is attached to a rod  552 , which is attached to end  476   b.  The diaphragm element  550  and housing  548  cooperatively define chambers  554 , 556  on opposite sides of the diaphragm element  550 . 
         [0147]    The pitot tube  544  preferably includes a high pressure tube  558  with an end that faces into the airflow and is preferably used to measure a high pressure (preferably total pressure) within the compressor discharge. The pitot tube  544  also preferably includes a low pressure tube  560  with an end that faces away from the airflow and is used to measure a low pressure in the compressor discharge. The low pressure tube  560  preferably faces away from the airflow so that the sensed pressure is less than static pressure within the discharge. In this manner, the illustrated pitot tube  544  maximizes the difference between high and low pressures. However, the principles of the present invention are applicable where an alternative pitot tube is used, e.g., where the low pressure tube is designed to sense static pressure. Again, it is also within the scope of the present invention where the sensor assembly  424  is used to sense airflow pressures at another location along the system (e.g., where pressures are sensed at the compressor inlet). The tubes  558 , 560  are connected to the diaphragm  546  so that the high pressure tube  558  is in fluid communication with chamber  554  and the low pressure tube  560  is in fluid communication with chamber  556 . Thus, the illustrated diaphragm  546  is operable to sense a pressure differential, which one of ordinary skill in the art will appreciate is proportional to airflow velocity at the location where pressures are measured. 
         [0148]    The compressor  412 , inlet guide vane assembly  414 , and controller  416  preferably operate so that the compressor  412  provides airflow at a gauge pressure that ranges from about five (5) pounds per square inch (psig) to about eleven (11) psig. More preferably, under normal operating condition, the compressor  412  provides airflow at a pressure of about 8.8 psig. Also, under normal operating condition, each vane of the inlet guide vane assembly  414  is preferably positioned so that the chord of the vane forms a preswirl angle that ranges from about forty (40) degrees to about fifty (50) degrees relative to the passage axis and, more preferably, is about forty-five (45) degrees. 
         [0149]    The controller  416  is operable to increase the throttling of airflow under certain conditions. For instance, when the flow of media into flow line  404  is suddenly decreased or stopped, the controller  416  is operable to sense a corresponding increase in airflow velocity and close the vane assembly  414 . As the sensor assembly  424  senses a relatively large airflow velocity, the diaphragm  546  is urged downwardly and the piston  474  is urged upwardly toward the closing position. Consequently, this shifting of the valve causes the actuator  417  to be shifted toward the closing condition, which shifts the vanes closed to subsequently reduce airflow velocity. For example, the illustrated vane assembly  414  can be shifted to a maximum preswirl condition where the vane is at a preswirl angle of about seventy-nine (79) degrees relative to the passage axis. In this condition, the illustrated compressor provides airflow at a pressure of about 5.3 psig. 
         [0150]    Similarly, the controller  416  is also operable to decrease the throttling of airflow under certain conditions. For example, as discussed above, the compressor package  400  is preferably operable to overcome media blockage that develops in the flow line  404  downstream of the compressor. The controller  416  is operable with the inlet guide vane assembly  414  to maintain airflow and pressure in reserve. The inlet guide vane assembly  414  is preferably operated in a partly closed condition during normal conveying operation so that the compressor can provide additional flow and pressure when necessary, e.g., to overcome downstream blockage. 
         [0151]    As the sensor assembly  424  senses a relatively small airflow velocity, the diaphragm  546  moves upwardly so that the piston  474  moves downwardly toward the opening position. Consequently, this shifting of the valve causes the actuator  417  to be shifted toward the opening condition, which shifts the vanes open to increase airflow velocity. 
         [0152]    For example, the illustrated vane assembly  414  can be shifted to an open condition where the vane is substantially aligned with the passage axis. In the open condition, the illustrated compressor provides airflow at a pressure of about ten (10) psig. In one counterswirl condition, the vanes are positioned with a counterswirl angle of about twenty (20) degrees from the passage axis. In this counterswirl condition, the compressor provides airflow at a pressure of about 10.5 psig. 
         [0153]    The sensor assembly  424  also preferably includes another diaphragm  562 , similar to diaphragm  316 , that fluidly communicates with the compressor volute to sense compressor static discharge pressure. With diaphragm  562 , the performance of compressor package  400  can be adjusted to provide alternative operational characteristics. As with diaphragm  316 , when little or no particulate media is being conveyed in the system  402 , the compressor  412  will produce airflow with relatively low discharge pressure and relatively high velocity. The diaphragm  562  is configured to shift the lever  476  upwardly to encourage opening of the inlet guide vane assembly  414  in a low-pressure condition. Thus, the diaphragm  562  is preferably connected to the lever  476  to further increase airflow velocity when the main flow line  404  has little or no media in it. Thus, the main flow line  404  is prepared to receive a slug of media without becoming clogged. However, the principles of the present invention are applicable where the sensor assembly  424  is devoid of diaphragm  562 . 
         [0154]    Turning to  FIG. 35 , in a method  564  of using the compressor package  400 , the compressor package  400  is started at step  566 . High and low pressures are sensed by the pitot tube  544  of the sensor assembly at step  568 . In response to the sensed pressures, the diaphragm  546  shifts the valve piston  474  at step  570  according to the sensed pressures. The actuator is then controlled at step  572  in response to valve movement to adjust the inlet guide vane assembly  414 . 
         [0155]    Turning to  FIGS. 36-38 , an alternative compressor package  600  is depicted as part of pneumatic conveying system  602 . For brevity, the description of this system  602  will focus primarily on the differences of this embodiment relative to the previous system  402 . 
         [0156]    The illustrated compressor package  600  is preferably configured for use with the system  602 . As with the previously disclosed embodiments, the compressor package  600  is suitable for use as a retrofit into a pneumatic conveying system in place of a positive-displacement compressor. However, the compressor package  600  is operable to provide compressed air for other applications without departing from the scope of the present invention. The compressor package  600  broadly includes, among other things, a centrifugal compressor  604 , an inlet guide vane assembly  606 , a variable frequency drive  608 , and an alternative guide vane controller  610 . 
         [0157]    The variable frequency drive (VFD)  608  is conventional and is operable to drive an A/C motor (not shown) of the compressor package  600 . The motor is drivingly connected to the compressor  604 . The drive  608  is operable to change the rotational speed of the motor and thereby change the rotational speed of the compressor impeller. Rotational speed of the compressor impeller is preferably sensed using a Hall Effect sensor (not shown). 
         [0158]    The illustrated compressor package  600  preferably includes both the inlet guide vane assembly  606  and the drive  608  to control compressor airflow. However, it is also within the ambit of the present invention where the package  600  does not have a variable frequency drive such that airflow control is provided by the inlet guide vane assembly  606 . Similarly, the compressor package  600  could be alternatively constructed without the inlet guide vane assembly  606  so that the drive  608  controls compressor airflow. 
         [0159]    As with the previous embodiments, the controller  610  is operable to position vanes of the guide vane assembly  606  so that the compressor  604  provides a compressed airflow with a substantially constant velocity. Importantly, the illustrated controller  610  is also preferably configured as part of the package  600  so that the package  600  can be used as a convenient retrofit to an existing pneumatic system compressor. However, instead of the mechanical controllers disclosed in the previous compressor package embodiments, the controller  610  provides electronic control of the compressor package. The controller  610  preferably includes, among other things, a guide vane actuator  612  and a programmable logic controller  614 . 
         [0160]    The actuator  612  is preferably a servo motor, which has a closed loop position control. The actuator  612  is drivingly attached to one of the shafts connected to a corresponding guide vane. Thus, movement of the servo causes corresponding movement of the guide vanes. The actuator  612  is operably coupled to the controller  614  so that the controller  614  controls and reads servo position. 
         [0161]    The controller  610  also includes multiple sensors. For instance, the controller  610  includes a hot wire anemometer (not shown), an oil pressure sensor (not shown), a static pressure sensor (not shown), and an ambient temperature sensor (not shown). The anemometer is preferably mounted in the compressor discharge to sense the velocity of compressor discharge airflow. The static pressure sensor is preferably installed in the compressor discharge. However, it is also within the ambit of the present invention where the pressure sensor and/or anemometer are mounted to sense the airflow at another location within the system, such as at the compressor inlet. The oil pressure sensor is installed to sense compressor hydraulic pressure. The ambient temperature sensor is mounted in any suitable location on or adjacent the compressor package to reliably measure ambient temperature. 
         [0162]    The programmable logic controller  614  is preferably programmable as a PID controller. However, the principles of the present invention are equally applicable where other types of controller algorithms are employed to operate the compressor package  600 . 
         [0163]    Turning to  FIG. 37 , in a method  616  of using the compressor package  600 , the compressor package  600  is started at step  618 . At step  620 , the controller  614  is then used to sense mass air flow (MAF), the static compressor discharge pressure (OUTLET_P), the compressor oil pressure (OIL_P), ambient temperature (AMB_TEMP), position of the inlet guide vanes (IGV POSITION), and impeller speed (N_IMP). The controller  614  then determines, at step  622 , if OIL_P is less than ten (10) psi. If so, the controller stops the compressor package at step  624 . If not, the controller  614  proceeds to step  626  to calculate a desired mass air flow rate of the compressor (MAF_TARGET) as a function of OUTLET_P and AMB_TEMP. The MAF TARGET value is also calculated as a function of desired air flow pickup velocity adjacent a media pickup location  628  (see  FIG. 36 ). In calculating the MAF_TARGET value, the pressure at the location  628  is estimated from the OUTLET_P value using conventional pipe flow loss calculations. Also, the temperature adjacent the location  628  is estimated by measuring and recording ambient temperature over time and by predicting air and media temperatures. For the purpose of performing calculations as part of the process of controlling the system  602 , the desired pickup velocity and other flow conditions are preferably determined for a location immediately adjacent and downstream of the media pickup location  628  (e.g., preferably about one foot downstream of the location  628 ). Compressor discharge temperature is calculated as a function of pressure ratio, time, mass of the main line piping system, and cooling through heat transfer of the piping system. Temperature of media is preferably determined as a function of time the media is held in the storage tank, ambient temperature, and media conveying rate. 
         [0164]    It is also within the ambit of the present invention where the media pickup temperature is alternatively determined. For instance, the temperature could be measured directly at a location slightly downstream of the media pickup location  628 . Also, the temperature could be calculated by using a measured temperature of the media prior to media introduction in the main flow line and a measured temperature of airflow prior to the media pickup location  628 . 
         [0165]    At step  630 , the controller  614  determines the error in MAF as the difference between MAF_TARGET and MAF. This error value is used in step  632  to adjust the inlet guide vane position. At step  634 , a desired rotational speed of the impeller is determined as a function of IGV POSITION, OUTLET_P, and AMB_TEMP. At step  636 , the value at which the compressor  604  will enter a surge condition (i.e., the surge flow) is calculated as a function of IGV POSITION, and N_IMP. The controller  614  then determines, at step  638 , if MAF is less than the calculated surge flow. If so, a bypass valve  640  (see  FIG. 36 ) is opened by the controller  614  at step  642  to maintain static pressure in the flow line below a predetermined discharge pressure. If not, the controller  614  proceeds to step  644  where the error in mass air flow is again determined. At step  646 , the calculated mass air flow error is used to adjust the drive frequency provided by the variable frequency drive (VFD), preferably using a PID control algorithm. 
         [0166]    The illustrated method  616  has been found to be particularly effective for controlling the centrifugal compressor package to provide substantially constant airflow velocity. The controller  614  preferably controls the guide vane assembly  606  and variable frequency drive  608  to provide responsive control of airflow velocity while permitting the compressor package to operate at optimum efficiency. The guide vane assembly  606  has been found to provide more responsive change in airflow compared to the drive  608 . Thus, the control algorithm preferably uses a higher gain in connection with the guide vanes compared to the drive  608 . 
         [0167]    Again, the principles of the present invention are applicable where an alternative control algorithm is employed to provide suitable control over system operation. For instance, the controller  614  could be operated using an alternative process where the system  602  does not include an inlet guide vane assembly (or where the inlet guide vane assembly is held in an open condition). It is also within the ambit of the present invention where the controller  614  is operated using a process where the system  602  does not include a variable frequency drive (or where the variable frequency drive is not utilized to provide variable impeller speeds during system operation). Turning to  FIG. 38 , one alternative method  700  includes steps  618 , 620 , 622 , 624 , 626 , 638 , 642 , 644 , 646 . The method also includes step  702 , where a desired rotational speed of the impeller is determined as a function of MAF_TARGET, OUTLET_P, and AMB_TEMP. It is particularly noted that steps  630 , 632 , 636  (as disclosed in the method depicted in  FIG. 37 ) are not included in method  700 . The illustrated method  700  is particularly effective where the compressor package includes a variable frequency drive and a motor with sufficient power to provide adequate variable speed response to control the system. 
         [0168]    The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
         [0169]    The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.