Abstract:
A gas compression system is operable to deliver a flow of compressed gas to a point of use. The gas compression system includes a compressor that is operable to produce a flow of compressed gas at a first pressure, a motor operable to drive the compressor at a compressor speed, and a gas treatment member positioned to receive the flow of compressed gas from the compressor and deliver the flow of compressed gas to the point of use at a second pressure. A controller is operable to vary the compressor speed in response to a difference between the first pressure and the second pressure.

Description:
RELATED APPLICATION 
       [0001]    The present application claims the benefit of co-pending provisional patent application Ser. No. 60/764,243, filed Feb. 1, 2006, the subject matter of which is hereby fully incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to gas compressor systems. More particularly, the invention relates to a control system for a gas compressor system. 
         [0003]    Compressed gas, and particularly air, can be used in many applications such as for process, shop air, or other applications. Compressor systems generally include several components disposed within one or more housings. Exemplary components of these systems include a motor and drive assembly, a compressor module, and a separator system (moisture and/or oil). The motor may drive the compressor module through a belt and pulley system that transfers power from the motor to the compressor module, other to other transmission systems, or may directly drive the compressor module. 
         [0004]    In many compressed air systems, the air carries an amount of moisture and, in some constructions, lubricants that are trapped in the air as part of the compression process. Many compressor systems include an air dryer that operates to remove a significant portion of the moisture from the air. The air dryer dries the compressed air with a drying process that may include cooling, absorption, adsorption, and the like. During the air drying process, there is generally a pressure drop within the flow of compressed air. Thus, the air dryer reduces the overall efficiency of the compressor system. 
       SUMMARY 
       [0005]    A controller is used to increase an efficiency of an airflow system by monitoring pressure values at several points of the system. Increased efficiency reduces energy costs and can also reduce maintenance necessity. Particularly, the controller can dynamically measure and monitor a pressure differential between a plurality of pressure points. For example, the controller can monitor a discharge (after-cooler) pressure value at or near a compressor discharge. Similarly, the controller can also monitor a system discharge pressure at or near a dryer, or at or near a point-of-use. The controller then uses the pressure differential between the discharge (after-cooler) pressure value and the system discharge pressure to dynamically change or adjust a speed of the compressor to maintain a desired quantity of air flow at a desired pressure at the point-of-use, while reducing or minimizing an energy or power consumption demanded by a motor and the controller. 
         [0006]    In one form, the invention provides a gas compression system that is operable to deliver a flow of compressed gas to a point of use. The gas compression system includes a compressor that is operable to produce a flow of compressed gas at a first pressure, a motor operable to drive the compressor at a compressor speed, and a gas treatment member positioned to receive the flow of compressed gas from the compressor and deliver the flow of compressed gas to the point of use at a second pressure. A controller is operable to vary the compressor speed in response to a difference between the first pressure and the second pressure. 
         [0007]    In another form, the invention provides a method of controlling airflow generated by a compressor. The method includes the steps of measuring a first pressure value associated with the compressor, measuring a second pressure value indicative of a rate of use of air at a point of use, and determining a pressure differential between the first pressure value and the second pressure value. The method also includes adjusting a motor speed based at least partially on the pressure differential. 
         [0008]    In yet another form, the invention provides a gas compression system that is operable to deliver a flow of compressed gas to a point of use. The gas compression system includes a compressor that is operable to produce a flow of compressed gas at a first pressure and a variable speed motor that is operable to drive the compressor at a compressor speed between a low speed and a high speed. A dryer is positioned to receive the flow of compressed air from the compressor and deliver a flow of dried compressed air to the point of use at a second pressure. A first pressure sensor is positioned to measure the first pressure and generate a first signal indicative of the first pressure, and a second pressure sensor is positioned to measure the second pressure and generate a second signal indicative of the second pressure. A controller is operable to receive the first signal and the second signal and to calculate a third signal indicative of the pressure difference between the first pressure and the second pressure. The controller is also operable to vary the compressor speed in response to the third signal. 
         [0009]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an airflow system embodying the present invention; and 
           [0011]      FIG. 2  is a flow chart of processing carried out in some embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
         [0013]      FIG. 1  shows an airflow system  100  in a schematic format. The airflow system  100  includes a compressor module  104  that is controlled by a controller  108 . The compressor module  104  includes an engine or a motor  112  and a compressor  116 . The compressor includes a housing that contains one or more moving elements that operate to compress fluid within the compressor  116 . In one construction, a rotary screw compressor is employed with other constructions employing other types of compressors (e.g., centrifugal, reciprocating, gear, gerotor, and the like). 
         [0014]    The motor  112  includes an output shaft  118  that couples to the compressor  116  to drive the moving element or elements and generate compressed air. In one construction, the motor  112  operates at a fixed speed. In more preferred constructions, a variable speed drive, such as a variable frequency drive  120  controls the motor  112  and operates the motor  112  and the compressor  116  within a desired speed range. By varying the speed of the compressor  116 , the drive  120  is able to vary the quantity and the pressure of the compressed air discharged at a compressor discharge  122 . 
         [0015]    A compressor discharge pressure sensor  128  is disposed near the compressor discharge  122  or in piping  124  downstream of the discharge  122  to measure a discharge pressure of the compressed air. The pressure sensor  128  can continuously measure the discharge pressure or can periodically sample the discharge pressure. The pressure sensor  128  generates an electrical signal indicative of the measured discharge pressure and transmits that signal to the controller  108  for use. 
         [0016]    The airflow system  100  also includes an air dryer  132  that receives the compressed air from the compressor module  104  through the piping  124 . The air dryer  132  dries the compressed air to reduce the dew point of the air and inhibit condensation. For example, in one construction, the dryer  132  first cools the air flow to produce condensation within the flow of compressed air. The flow is then directed to a moisture separator that removes the condensed water. The stream of compressed air is then heated well above its dew point, thus making the condensation of water during use of the air less likely. In other constructions, a desiccant drying system is employed. In the desiccant drying system, the stream of compressed air is passed through a water-absorbing substance that removes moisture from the flow of compressed air. In addition, the air dryer  132  can include a cycling or a non-cycling dryer  132 , depending on applications at hand. As one of ordinary skill in the art will realize, the use of any air-drying system produces a pressure drop in the flow of compressed air. In addition, the greater the quantity of flow in any given system, the greater the pressure drop. 
         [0017]    The drying module  132  discharges the dry air through piping  136  to a point-of-use  138 . An isolation valve  139  may be fitted at the point-of-use  138  if desired. A dryer discharge pressure sensor  140  is positioned near a dryer discharge  142  or in the piping  136  downstream of the dryer  132  to measure a dryer discharge pressure. The dryer discharge pressure sensor  140  can continuously measure the dryer discharge pressure or can periodically sample the dryer discharge pressure. The dryer discharge pressure sensor  140  generates an electrical signal indicative of the measured dryer discharge pressure and transmits that signal to the controller  108  for use. 
         [0018]    The controller  108  receives the discharge pressure sensor signal and the dryer discharge pressure sensor signal and uses these two signals along with a reference to generate a control signal. More specifically, the controller  108  uses the two sensor signals to calculate a pressure difference or a pressure differential. This calculated value is then compared to the reference to generate the control signal. The control signal is then transmitted to the variable frequency drive  120  to control the speed of the motor  112  and the compressor  116 . 
         [0019]      FIG. 2  is a flow chart  200  that further illustrates processes that occur in some embodiments of the invention including processes that may be carried out by software, firmware, or hardware. As noted, the compressor discharge pressure sensor  128  dynamically monitors or senses the pressure generated by the compressor module  104  at the compressor module discharge  122  at block  204 . The dryer discharge pressure sensor  140  also dynamically monitors or measures the pressure associated with the drying module  132  at the drying module discharge  142  at block  208 . 
         [0020]    At block  212 , the controller  108  dynamically determines a difference between the pressure values measured by the compressor discharge pressure sensor  128  and the dryer discharge pressure sensor  140 . At block  214 , the controller  108  compares the calculated pressure differential to the reference  148  such as a desired pressure differential. The controller  108  then dynamically changes or adjusts a control signal based on this comparison, and transmits this control signal to the variable frequency drive  120 . The variable frequency drive  120  adjusts the driving frequency of the motor  112  at block  216  based on the pressure differential. At block  220 , the controller  108  transmits the driving frequency signal to the compressor module  104  to drive the motor  112 , which in turn drives the compressor  116 . In some constructions, the controller  108  also drives the drying module  132  based on the pressure differential calculated at block  224 . In this way, the operating speeds of the motor  112  and the speed of the compressor  116  within the compressor module  104  are maintained at a speed that produces the needed quantity of air at the needed pressure, thereby reducing the power consumption of the motor  112  and/or the variable frequency drive  120 . 
         [0021]    During operation of the airflow system  100 , the motor  112  operates to rotate the compressor  116 . The compressor  116  outputs a quantity of compressed air at a pressure in response to rotation of the motor  112 . The compressor discharge sensor  128  measures the pressure of the compressed air at the compressor discharge  122  and transmits a signal indicative of this value to the controller  108 . The compressed air then flows through the air dryer  132  where some of the moisture contained within the compressed air is removed. As the compressed air flows through the air dryer  132 , a pressure drop occurs. The magnitude of the pressure drop is largely a function of the mass flow or flow velocity through the air dryer  132 , with higher mass flows or flow velocities producing larger pressure drops. After passing through the air dryer  132 , the dryer discharge pressure sensor  140  measures the pressure of the compressed air and transmits a signal indicative of this measured value to the controller  108 . The compressed air then flows to the point-of-use  138 , such as a manifold or a distribution center. Meanwhile, the controller  108  calculates the pressure difference between the measured pressure values and compares this difference to the reference to generate the control signal. 
         [0022]    During periods when no air is being used, the flow of air at the point-of-use  138  drops to zero or very near zero. As the flow velocity through the air dryer  132  drops, the pressure drop becomes smaller and the difference between the compressor discharge pressure and the air dryer discharge pressure approaches zero. As the pressure difference drops, the control signal generated by the controller  108  continues to indicate to the variable frequency drive  120  to slow the motor operation to reduce the flow from the compressor  116 . In some constructions, a threshold pressure difference is reached and the controller  108  sends a signal to the variable frequency drive  120  that stops operation of the compressor  116 . 
         [0023]    During periods of high air usage, air is quickly drawn from the system at the point-of-use  138 . As air is removed, the flow velocity through the air dryer  132  increases to replace the used air, thereby producing a greater pressure drop between the compressor discharge  122  and the air dryer discharge  142 . As the pressure difference increases, the control signal generated by the controller  108  acts to increase the speed of the motor  112 . The increased speed produces a corresponding increase in speed of the compressor  116 , which produces an increase in the amount of air output by the compressor  116 . The increased air flow passes through the air dryer  132  and aids in maintaining the pressure at the air dryer discharge  142 . In this way, the control system is able to use the pressure differential to control the speed and the output of the compressor  116 . 
         [0024]    It should be noted that the present control system can be used to control a multi-compressor system as well as the single compressor system described above. In the multi-compressor system, the controller may control several variable frequency drives in parallel, may control one variable frequency drive and one or more fixed speed compressors, may control several variable frequency drives in series, or may control several fixed speed drives in series. 
         [0025]    Furthermore, while the foregoing description and illustrations include an air dryer  132 , other constructions may employ other auxiliary equipment in addition to, or in place of the air dryer  132 . For example, another construction employs an oil separator in addition to an air dryer  132 . Still other constructions may include only the oil separator and may omit the air dryer  132 . As such, the invention should not be limited to air dryers alone. 
         [0026]    Although the invention has been described in detail with reference to certain described constructions, variations and modifications exist within the scope and spirit of the invention.