Abstract:
The present invention provides a variable frequency controlled refrigerant compressor in a dehydrator for compressed air or other cases. In particular, the present invention detects changes in a demand on the pneumatic air supply by monitoring a pressure of a refrigerant system associated with the air supply. Based on the changes in the refrigerant system pressure, a motor speed controller generates and sends a control signal to the variable speed compressor to adjust the speed of the variable speed compressor based on the demand in the air supply.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims is a divisional application of U.S. patent application Ser. No. 09/593,977 filed by James J. Wilson, Donald Neve and William B. Thomas on Jun. 3, 2000 and entitled METHOD AND APPARATUS FOR VARIABLE FREQUENCY CONTROLLED COMPRESSOR AND FAN, 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to compressor controls and more particularly to compressor controls in compressed gas systems having refrigerated dryers.  
       BACKGROUND OF THE INVENTION  
       [0003]     Refrigerant compressors are used in a variety of systems. One type of system that uses refrigerant compressors is a compressed gas system. Compressed gas systems typically provide high volumes of dry, pressurized air or other gases to operate various items or tools (while a multitude of gases can be used, this application typically refers to air as a matter of convenience). Conventional systems dry the air using heat exchangers first to cool the air and lower the dew point of the air, which causes water vapor to condense out of the air, and second to reheat the air and raise the outlet temperature of the air. This system provides a relatively dry air source.  FIG. 1  shows a conventional refrigerated dryer  100  for a compressed gas system. Refrigerated dryer  100  includes both an air heat exchanger circuit  110  and a refrigerant heat exchanger circuit  120 . Air heat exchanger circuit  110  includes an inlet  112 , an air-to-air heat exchanger  114 , a air-to-refrigerant heat exchanger or evaporator  116 , a water separator  120   a  and an air outlet  118 . Refrigerant heat exchanger circuit  120  includes evaporator  116 , a compressor  122 , a condenser  124 , a throttling device  126 , and a hot gas by-pass valve  128 .  
         [0004]     Notice that temperatures used below to describe the operation of dryer  100  are exemplary only. Many different air temperatures and saturation levels are possible. The temperatures and saturation levels of the final operating system depend on a large variety of factors including for example system design specifications and local environmental factors. The factors that determine actual temperatures are beyond the scope of this patent application and, in any event, are well known in the art.  
         [0005]     In operation, dryer  100  receives a high temperature, saturated, pressurized air or gas stream at inlet  112 . For example, the air or gas may be at 100 degrees (all degrees represented are degrees Fahrenheit) with a dew point of 100 degrees (i.e., 100% humidity), although any inlet temperature and dew point is possible. The air or gas stream passes through an inlet side of air-to-air heat exchanger  114 . The air or gas stream cools down to, in this example, 70 degrees with a dew point of 70 degrees (i.e., still 100% humidity). However, because 100 degree air or gas can carry a larger volume of water vapor than 70 degree air or gas, some water vapor condenses. The condensed moisture precipitates out and collects in the separator  120   a . The 70 degree air or gas then travels through the air side of evaporator  116  where the air or gas is further cooled to approximately 35 degrees with a dew point of 35 degrees (i.e., still at 100% humidity). Again, moisture condenses out of the air or gas stream and collects in the separator  120   a . The 35 degree air or gas then travels through the outlet side of air-to-air heat exchanger  114 . This reheats the air or gas stream to approximately 85 degrees with a pressure dew point of 35 degrees. The air or gas stream then exits the dryer  100  at air outlet  118 . Because 85 degree air can hold significantly more moisture vapor than 35 degree air, dryer  100  provides a source of dry, unsaturated, pressurized air or gas at air outlet  118 .  
         [0006]     In refrigerant heat exchanger circuit  120 , refrigerant enters the refrigerant side of evaporator  116  as a cool liquid. While passing through evaporator  116 , the refrigerant heats up and is converted to a gas by the exchange of heat from the relatively hot air side to the relatively cool refrigerant side of evaporator  116 . The low pressure gas travels to compressor  122  where the refrigerant is compressed into a high pressure gas. The refrigerant than passes through air or water cooled condenser  124  where the refrigerant is condensed to a cool, high pressure liquid. The cool, high pressure refrigerant passes through throttling device  126  (typically capillary tubes or the like) to reduce the pressure and boiling point of the refrigerant. The cool, low pressure, liquid refrigerant than enters the evaporator and evaporates as described above.  
         [0007]     When air heat exchanger circuit  110  and refrigerant heat exchanger circuit  120  operate at or near full capacity, hot gas by-pass valve  128  has no particular function. However, as the demand on air heat exchanger circuit  110  decreases, refrigerant heat exchanger circuit  120  has excessive capacity that could cause the liquid condensate in dryer  100  to freeze. Thus, when used in this situation, hot gas by-pass valve  128  functions to prevent the liquid condensate in dryer  100  from freezing. In particular, the hot gas by-pass valve opens feeding hot, high pressure gas around the evaporator (i.e., by-passes) maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing. The particulars regarding the operation of hot gas by-pass valve  128  are well known in the art. Typically, a temperature sensor associated with the hot gas by-pass valve (not specifically shown in  FIG. 1 ) monitors the refrigerant temperature at the outlet of evaporator  116 . When the temperature at the outlet decreases below a predetermined threshold, the hot gas by-pass valve  128  opens feeding hot, high pressure gas around the evaporator maintaining a constant pressure and temperature in the evaporator preventing any condensate from freezing.  
         [0008]     The capacity of compressor  122  depends, in large part, on the maximum required capacity or expected air flow (measured in standard cubic feet per minute) of air heat exchanger circuit  110 . At full capacity (or air flow), compressor  122  operates at 100% capacity and the air temperature and dew point of the air stream is, for example, approximately as described above. The demand on the air system, however, is not always 100% of the designed capacity. Frequently, the demand on air heat exchanger circuit  110  is somewhat below full capacity. With less than 100% demand on air heat exchanger circuit  110 , the refrigerant heat exchanger circuit  120  described above still operates at 100% capacity, thus wasting energy or electric power because compressor  122  does not need to operate at full capacity. Some systems, as described above, compensate using hot gas by-pass valve  128 . Hot gas by-pass solves the problem of providing to much cooling through refrigerant heat exchanger circuit  120 , but does not solve the problem that the compressor is operating at a higher than necessary capacity and consuming a larger amount of electrical power than necessary. Other systems cycle the compressor on and off when the system operates at less than 100% capacity. These systems reduce power consumption somewhat, but cause excessive on and off cycling of compressor  122 , wide fluctuations in the dew point at air outlet  118 , and introduce inefficiencies associated with the heat exchange of mass media. Thus, it would be beneficial to control operation of compressor  122  based on the demand of air heat exchanger circuit  110  to reduce the power consumed by compressor  122  and increase the overall power efficiency of dryer  100 .  
       SUMMARY OF THE INVENTION  
       [0009]     To attain the advantages of and in accordance with the purpose of the present invention, as embodied and broadly described herein, apparatus for controlling the operating speed of a variable speed compressor in a refrigerated air drying system having changing demands on an air supply, include a demand sensor capable of sensing changes in the demand on the air supply and generating a change in demand signal. A motor speed controller receives the generated change in demand signal and generates a motor speed signal. The motor speed controller sends the motor speed signal to a motor of the variable speed compressor to change the speed of the variable speed compressor.  
         [0010]     Other embodiments of the present invention provide methods for controlling the operating speed of a variable speed compressor in a refrigerated air drying system having changing demands on an air supply. These methods include sensing a demand on the air supply. Determining an operating speed for a variable speed compressor based on the sensed air supply demand. Controlling a speed of the variable speed compressor based on the determined operating speed.  
         [0011]     Still other embodiments of the present invention provide computer program products having computer readable code for processing data to control a speed of the variable speed compressor. The computer program product has a demand sensing module configured to sense changes in the demand of the air supply. A generating module is configured to generate a signal indicative of the sensed change in demand. A motor speed controlling module is configured to receive the signal indicative of the sensed change in demand and generate at least one motor speed signal. The motor speed controlling module is adapted to send the motor speed signal to the variable speed compressor.  
         [0012]     The foregoing and other features, utilities and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention, and together with the description, serve to explain the principles thereof. Like items in the drawings are referred to using the same numerical reference.  
         [0014]      FIG. 1  is a system block diagram of a prior art refrigerated air drying system;  
         [0015]      FIG. 2  is a system block diagram of a refrigerated air drying system in accordance with the present invention;  
         [0016]      FIG. 3  is a flow chart describing the motor speed control drive of  FIG. 2  in accordance with the present invention;  
         [0017]      FIG. 4  is a block diagram showing compressors arranged in parallel in accordance with an embodiment of the present invention;  
         [0018]      FIG. 5  is a flow chart describing the operation of the motor speed controller with compressors as arrayed in  FIG. 4 ;  
         [0019]      FIG. 6  is a block diagram showing two variable speed compressors arranged in parallel in accordance with another embodiment of the present invention;  
         [0020]      FIGS. 7A and 7B  are a flow chart describing the operation of the motor speed controller with compressors arrayed as in  FIG. 6 ;  
         [0021]      FIG. 8  is a block diagram showing a compressor with two unload devices in accordance with an embodiment of the present invention; and  
         [0022]      FIG. 9  is a flow chart describing the operation of the motor speed controller with a compressor as shown in  FIG. 8 .  
         [0023]      FIG. 10  is a block diagram showing an alternate embodiment of a refrigerated air drying system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0024]     Some embodiments of the present invention are shown in  FIGS. 2 through 9 .  FIG. 2  shows a refrigerated air dryer  200  in accordance with one possible embodiment of the present invention. Air dryer  200  includes both an air heat exchanger circuit  210  and a refrigerant heat exchanger circuit  220 . Air heat exchanger circuit  210  includes an inlet  212 , an air-to-air heat exchanger  214 , a air-to-refrigerant heat exchanger or evaporator  216 , and an air outlet  218 . Air heat exchanger circuit  210  also has a conventional separator and automatic drain system (not shown) that is known in the art.  
         [0025]     Air heat exchanger circuit  210  operates by receiving an air or gas stream at inlet  212 . The air or gas stream travels through air-to-air heat exchanger  214 . The air or gas stream circulates in piping  214   i  along the inlet side of air-to-air heat exchanges  214  to cool. After cooling, the air or gas stream exists into piping  240 . The air or gas stream travels along piping  240  and enters air side piping  216   a  of evaporator  216 . The air or gas stream is further cooled by evaporator  216 . After this additional cooling, the air or gas stream exists into piping  242 . The air or gas stream travels along piping  242  and enters the reheat side of air-to-air heat exchanger  214 . The air or gas stream circulates in piping  214   o , along the outlet side of air-to-air heat exchanger  214  to reheat. After reheating the air or gas steam exits air heat exchanger circuit  210  as hot, dry air or gas at outlet  218 .  
         [0026]     Refrigerant heat exchanger circuit  220  includes evaporator  216 , a compressor  222 , a condenser  224 , a throttling device  226 , and a hot gas by-pass valve  228 . Refrigerant heat exchanger circuit  220  also has a hot gas by-pass controller  230 , a temperature sensor  232 , a motor speed control  234 , and a pressure sensor  236 . Also, one of ordinary skill in the art would now recognize that the pressure sensors could be replaced with other sensors capable of monitoring system pressure, such as, for example, temperature sensors. Similarly, one of ordinary skill in the art would now recognize that the temperature sensors could be replaced with other sensors, such as, for example, pressure sensors.  
         [0027]     Refrigerant heat exchanger circuit  220  operates by circulating a refrigerant through evaporator  216  along piping  216   r  to cool down the air stream. While circulating through piping  216   r , the refrigerant changes from a liquid to a low temperature vapor and exists evaporator  216  into piping  250 . The pressure sensor  236  is connected to piping  250  to measure the pressure at the inlet to compressor  222 . Compressor  222  receives the low pressure, gas refrigerant traveling in piping  250  and outputs the refrigerant as a high pressure, high temperature gas refrigerant into piping  252 . The refrigerant circulates from piping  252  into condenser piping  224   c  where the refrigerant is condensed to a liquid and cooled. The refrigerant exits condenser  224  as a high pressure liquid into piping  254 . Piping  254  includes throttling device  226 . Piping segments  256  and  258  connect the hot and cool sides of refrigerant heat exchanger circuit  220  through hot gas by-pass valve  228 .  
         [0028]     When dryer  200  is operated at full capacity, compressor  222  operates at its normal operating capacity or frequency similar to the description of dryer  100  above. When air flow through air heat exchanger circuit  210  decreases, however, pressure sensor  236  detects the decrease in demand as a decrease in the system pressure of refrigerant heat exchanger circuit  220  from an expected operating pressure at the inlet of compressor  222 . On sensing the decrease in pressure, sensor  236  generates and sends a decreased pressure signal to motor speed controller  234  through a signal conduit  260 . Motor speed controller  234  registers the decreased pressure signal as a decrease in demand on air heat exchanger circuit  210  and, thereby, sends a signal over signal conduit  262  to compressor  222  that decreases the speed of the compressor motor, i.e. decreases the motor&#39;s operating frequency, which will be described in more detail below. This causes the system pressure of refrigerant heat exchanger circuit  220  at the inlet of compressor  222  to increase back to the expected operating pressure. The decrease in the motor operating frequency of compressor  222  causes a corresponding decrease in energy consumption.  
         [0029]     When demand on air heat exchanger circuit  210  increases, pressure sensor  236  detects the increase in demand as an increase in the system pressure of refrigerant heat exchanger circuit  220  from an expected operating pressure at the inlet to compressor  222 . On sensing the increase in pressure, sensor  236  generates and sends an increased pressure signal to motor speed controller  234  over signal conduit  260 . Motor speed controller  234  registers the increased pressure signal as an increase in demand on air heat exchanger circuit  210  and, thereby, sends a signal over signal conduit  262  to compressor  222  that increases the speed of the compressor motor, i.e., increases the motor&#39;s operating frequency, which will also be described in more detail below. This causes the system pressure of refrigerant heat exchanger circuit  220  at the inlet of compressor  222  to decrease back to the expected operating pressure.  
         [0030]     When demand on air heat exchanger circuit  210  remains constant, pressure sensor  236  can, depending on design choice, send an expected operating pressure signal to motor speed controller  234  or simply not send a signal to motor speed controller  234 . In either case, motor speed controller  234  maintains the operating frequency of compressor  222  to maintain the expected operating pressure of refrigerant heat exchanger circuit  220  at the inlet of compressor  222 .  
         [0031]     In the present example, compressor  222  is sized so that one compressor can satisfy the cooling requirements of dryer  200 . Compressor  222  has a minimum operating frequency. If the motor speed is reduced below that minimum the internal lubrication of the compressor will be insufficient and/or the refrigerant flow rate will not provide adequate oil return. Thus, motor controller  234  can only reduce the operating frequency of compressor  222  to compressor  222  to a predetermined minimum speed. (Note that motor controller  234  could control the speed of compressor  222  over its full range of speeds, i.e., 0 Hz to full frequency, if the minimum speed was not dictated by the compressor.) When compressor  222  operates at its minimum frequency, motor speed controller  234  sends a signal over signal conduit  264  to hot gas by-pass controller  230  to begin hot gas by-pass control of refrigerant heat exchanger circuit  220  to prevent the suction pressure/temperature from falling that, in turn, prevents condensed water vapor from freezing, which will be explained further below.  
         [0032]      FIG. 3  shows a flow chart  300  indicating operation of refrigerant heat exchanger circuit  220 . First, dryer  200  is initialized, step  310 . This can include starting compressor  222  using a “soft-start” mode. A soft start mode is a procedure that brings the motor of compressor  222  up to speed following the motor control curves for the motor of compressor  222 . The motor curves, not shown but generally known in the art, provide ideal voltage supplies to the motor of compressor  222  when the motor is operating at a given frequency. Additionally, these curves supply an optimal rate of change in frequency for a given unit of time. While it is preferred that motor speed controller  234  functions according to the motor control curves it is not necessary.  
         [0033]     Once the system is initialized and compressor  222  soft-started, motor speed controller  234  is placed in an automatic mode, step  320 . In automatic mode, motor speed controller  234  begins monitoring the pressure at the inlet to compressor  222 , step  330 . Next, motor speed controller  234  determines whether the motor speed of compressor  222  is greater than the minimum speed allowed, step  340 . As noted above, the minimum speed of the motor of compressor  222  is based largely on the lubrication ability of the motor and is not a function of motor speed controller  234 .  
         [0034]     If the motor of compressor  222  is operating at greater than the minimum operating speed, motor speed controller  234  next determines whether the pressure at the inlet to compressor  222 , as measured by pressure sensor  236 , is greater than a first pre-established pressure threshold, step  350 . If pressure is greater than the first pre-established pressure threshold, motor speed controller  234  increases the operating speed of the motor, step  360 . Otherwise, motor speed controller  234  determines whether the pressure at the inlet to compressor  222  is less than the first pre-established pressure threshold, step  370 . If pressure is less than the first pre-established pressure threshold, motor speed controller  234  decreases the operating speed of the motor, step  380 . Of course, if the monitored pressure is approximately the same as the first pre-established pressure threshold, motor speed controller  234  simply maintains the operating speed of the compressor. After any required operating speed adjustments, the control loop is returned to step  330 .  
         [0035]     In the preferred embodiment, the above control is referred to as a “pressure mode”  300   p  because motor speed controller  234  uses a pressure signal from pressure sensor  236  to control motor speed. Alternative means of controlling the motor speed are possible. For example, a flow meter in air heat exchanger circuit  210  could be used to measure system demand and control the motor speed of compressor  222 .  
         [0036]     Alternatively, a temperature sensor could be used in place of pressure sensor  236  to measure the system demand. Essentially any conventional demand sensor could be used to control the motor speed.  
         [0037]     If at step  340  motor speed controller  234  had determined that the motor of compressor  222  was already operating at its minimum operating speed, motor speed controller would begin a “hot gas mode”  300   t  of refrigerant heat exchanger circuit  220 . In hot gas mode, motor speed controller  234  maintains the speed of the motor of compressor  222  at the minimum operating speed, step  390 . Pressure sensor  236  continues to monitor the pressure at the inlet to compressor  222 , step  400 . Motor speed controller determines whether the pressure at the inlet of compressor  222  is less than a second pre-established pressure threshold, step  410 . If pressure is less than the second pre-established pressure threshold, hot gas by-pass controller  230  senses the temperature at the outlet of evaporator  216  using sensor  232 , step  420 . Next, hot gas by-pass controller  230  determines whether the temperature at the outlet of evaporator  216  is below a hot gas by-pass pre-established temperature threshold, step  430 . If the temperature is below a hot gas by-pass pre-established temperature threshold, hot gas by-pass controller  230  causes hot gas by-pass valve  228  to cycle and inject hot gas from piping  252  on the outlet side of compressor  222  into piping  250  on the outlet side of evaporator  216 , step  440 . After the hot gas is injected or if pressure was above the hot gas by-pass pre-established pressure threshold, control reverts back to step  400 .  
         [0038]     If at step  410  motor speed controller  234  had determined pressure was not less than the second pre-established pressure threshold, then motor speed control  234  reverts back to pressure control at step  350 , above. In the preferred embodiment, the second pre-established pressure threshold is sufficiently higher than the first pre-established pressure threshold to prevent excessive cycling between hot gas mode  300   t  and pressure mode  300   p . The settings for the first and second pre-established pressure thresholds is, however, largely a matter of design choice. The hot gas by-pass threshold settings are well known in the art.  
         [0039]     The embodiment of the present invention described above shows dryer  200  with one compressor  222  that is sized to accommodate 100% or full demand on air heat exchanger circuit  210 . Under this configuration, the motor speed of compressor  222  could be varied from minimum to full capacity to vary the power consumption of the overall system. Also, as is known in the art, condenser  224  has a fan  224   f  and a fan motor  224   m  associated with it to assist in cooling and condensing the refrigerant. The fan motor  224   m  could be a variable speed motor controlled by motor speed controller  234 . In this case, the fan motor would receive a motor speed control signal over conduit  262  so that the fan motor speed and the speed of the compressor motor would coincide. Thus, if air supply demand on air heat exchanger circuit  210  was 80%, under the above described control scheme, the motor of compressor  222  would be operating at 80% and the fan motor associated with the condenser would be operating at 80%.  
         [0040]     Notice that the fan motor could be controlled by a separate motor speed controller. It is currently preferred to use a separate motor speed controller for fan motor  224   m  to prevent excessive cycling of fan motor  224   m  that could occur if fan motor  224   m  was controlled at the same speed as the motor of the compressor. In one present preferred embodiment, the fan motor is controlled using the same control scheme as outlined in flow chart  300 , but using a separate controller. Using a separate control has the additional advantage that the fan motor can be controlled from 0 Hz to its maximum frequency because the fan motor does not have the same lubrication requirements as the compressor motor. When using a separate motor speed controller to control the operating speed of fan motor  224   m , it is preferable to control the speed based on a demand sensor that measures condensing pressure (a demand sensor that measures condensing pressure is not specifically shown in the drawing, but is generally known in the art) instead of the demand sensor that measures the pressure at the inlet to the compressor.  
         [0041]     More precise control over the power consumption could be obtained by using more compressors or compressors with unloading devices and/or variable speed controlled condenser fan motors. This would be helpful in systems where power consumption is of greater concern, or more precise control over the coolant system is needed. For example,  FIG. 4  shows three compressors  460 ,  470 , and  480  arranged in parallel. In this embodiment, motor speed controller  234  would control compressor  460  in a variable speed mode and control compressors  470  and  480  by simple on/off instructions. Additionally, compressor  460 , being the variably controlled compressor, is preferably capable of twice the capacity of compressors  470  and  480 . In this manner, demand on the air source could be controlled down to about 25% capacity of the air flow. As one of ordinary skill in the art would now recognize, adding more or less compressors allows more or less precise control of the power consumption. While the variably controlled compressor is preferred to be about twice the size of the other compressors, almost any arrangement is possible.  
         [0042]      FIG. 5  is a flow chart  500  representing operation of the present invention with multiple compressors  460 ,  470  and  480 . First, the motor speed controller would be placed in automatic control, step  510 , and the pressure sensor would monitor pressure at the inlet of compressors  520 . Next, the motor controller would determine whether the variable speed compressor motor is operating at a minimum frequency, step  530 . If compressor  460  is operating at a minimum speed, motor speed controller  234  next determines whether two or more compressors are currently operating, i.e., compressor  460  and compressors  470  and/or  480 , step  540 . If only compressor  460  is operating, refrigerant heat exchanger circuit  220  enters hot gas mode control, step  550 . Step  550  is substantially as described in steps  390  to  440  of  FIG. 3 . If motor speed controller  234  determines that one or both of compressors  470  and  480  are operating in addition to variable speed compressor  460 , then motor speed controller turns one of the compressors  470  or  480  off, step  560 , and returns the control to the control loop at step  570 , below.  
         [0043]     If motor speed controller  234  had determined that variable speed compressor  460  was not operating at its minimum, step  530 , motor speed controller  234  would then determine whether pressure at the inlet to compressors  460 ,  470 , and  480  was greater than the first pre-established pressure threshold, step  570 . If pressure is greater than the first pre-established pressure threshold, which indicates an increase in demand on air heat exchanger circuit  210 , then motor speed controller  234  checks whether variable speed compressor  460  is operating at its maximum, step  580 . If variable speed compressor  460  is not operating at its maximum, motor speed controller  234  increases the speed of variable speed compressor  460 , step  590 , and the control loop returns to step  520 . If, however, motor speed controller  234  determines that variable speed controller  460  is operating at its maximum, step  580 , then motor speed controller turns on another compressor, either compressor  470  or  480 , and brings that compressor up to its normal operating speed, step  600 . After turning on the additional compressor, motor speed controller  234  would decrease the speed of variable speed compressor  460 , step  610 , and the control loop would return to step  520 .  
         [0044]     If, at step  570 , motor speed controller  234  had determined that pressure was not greater than the first pre-established pressure threshold, it would determine whether pressure was less than the first pre-established pressure threshold, step  620 . If motor speed controller  234  determines pressure is less than the first pre-established pressure threshold, then it decreases the speed of variable speed compressor  460 , step  630 , and control returns to the control loop at step  520 .  
         [0045]     In this embodiment, if the demand on air heat exchanger circuit  210  is 25% of full capacity, compressor  460  is operating at 50% and both compressors  470  and  480  are off. As demand of air heat exchanger circuit  210  increases, the speed of compressor  460  is increased until demand on air heat exchanger circuit  210  is 50% and compressor  460  is operating at 100% capacity. As demand on air heat exchanger circuit  210  increases past 50%, a second compressor  470  would be turned on to supply 25% of the necessary flow and the speed of compressor  460  would drop down to 50% to supply the other 25%. In other words compressor  460  would be operating at 50% capacity and compressor  470  would be operating at 100% capacity. As demand on air heat exchanger circuit  210  increased from 50% to 75%, the speed of compressor  460  is increased until it is operating at 100% capacity. When demand increases over 75%, compressor  480  is turned on and the speed of compressor  460  is reduced to 50% such that compressor  460  is at 50%, and compressors  470  and  480  are at 100%. Similarly, other combinations of parallel compressors could be used. One example includes a variable speed compressor capable of 40% capacity and three on/off compressors capable of 20% capacity each. Another example includes one variable compressor capable of 70% capacity and two on/off compressors capable of 15% capacity, which is useful when precise control is only necessary at higher capacities. In general, however, any percentage combination is possible. It is beneficial that the variable compressor capacity be larger than the nonvariable speed compressors to avoid gaps in the control.  
         [0046]     In still another embodiment of the present invention, it is possible to control two or more variable speed compressors. For example,  FIG. 6  shows first and second variable speed compressors  660  and  670  arranged in parallel. In this case, motor speed controller  234  could control the speed of both compressors or, in the alternative, a second motor speed controller could be added, not shown. In the preferred embodiment, each compressor is sized to accommodate equal portions of full capacity on refrigerant heat exchanger circuit  220 . Additionally, if only one motor speed controller is used, it is preferable that the compressors be of equal capacity. In this case, compressors  660  and  670  are each capable of approximately one-half of full capacity.  
         [0047]      FIGS. 7A and 7B  show a flow chart  700  indicating operation of the present invention with first and second variable speed compressors  660  and  670 , respectively. As with the previous embodiments, dryer  200  is placed in operation and motor controller  234  is operating in automatic mode, step  710 . In automatic mode, pressure sensor  236  monitors pressure of refrigerant heat exchanger circuit  220  at the inlet of first and second compressors  660  and  670 , step  720 . The motor speed controller next determines whether first and second variable speed compressors are operating, step  730 .  
         [0048]     If first and second variable speed compressors  660  and  670  are not operating, it is further determined whether first variable speed compressor  660  is operating at its minimum speed, step  740 . If first variable speed compressor  660  is operating at the minimum speed, dryer  200  enters hot gas mode as described in Steps  390  to  440  of flow chart  300  of  FIG. 3 , step  750 . Otherwise, it is further determined whether first variable speed compressor  660  is operating at its maximum speed, step  760 . If first variable speed compressor  660  is operating at its maximum speed, second variable speed compressor  670  is turned on, step  770 . If second variable speed compressor  670  is turned on, control moves to step  820 , as will be described below, otherwise the speed of first variable speed compressor  660  is controlled in steps  780 ,  790 ,  800 , and  810 , in a manner substantially identical to steps  350 ,  360 ,  370 , and  380  described in flow chart  300  of  FIG. 3 , above.  
         [0049]     If first and second variable speed compressors are operating, motor speed controller  234  determines whether pressure is greater than a first pre-established pressure threshold, step  820 . If pressure is determined to be greater than the first pre-established pressure threshold, it is further determined whether first variable speed compressor  660  is operating at its maximum operating speed, step  830 . If first variable speed compressor  660  is not operating at its maximum operating speed, the speed of compressor  660  is increased, step  840 , otherwise the speed of compressor  670  is increased, step  850 . The control loop then returns to step  720 .  
         [0050]     If it is determined that pressure is not greater than the first pre-established pressure threshold, it is next determined whether pressure is less than the first pre-established pressure threshold, step  860 . If pressure is less than the first-established pressure threshold, it is next determined whether second variable speed compressor  670  is operating at it minimum speed, step  870 . If compressor  670  is not operating at its minimum speed, then its speed is decreased, step  880 , and the control loop returns to step  720 . If compressor  670  is operating at its minimum speed, then it is determined whether first variable speed compressor  660  is operating at its minimum speed, step  890 . If compressor  660  is not at its minimum speed, then its speed is decreased, step  900 , and the control loop returns to step  720 . If it is determined that first variable speed compressor is also operating at its minimum speed, then second variable speed compressor  670  is turned off, step  910 , and the speed of first variable speed compressor  660  is increased, step  920 , and the control loop returns to step  720 . As before, if pressure is neither greater than nor less than the first pre-established threshold, control simply returns to step  720  without altering the speed or configuration of the compressors.  
         [0051]      FIG. 8  shows a variable speed compressor  1800  with two unload devices  1810  and  1820  arranged in parallel. As one of ordinary skill in the art would now recognize, any number of unload devices could be arranged in parallel. In this embodiment, motor speed controller  234  would control the motor speed of compressor  1800  in variable speed mode and control unload devices  1810  and  1820  by simple on/off instructions. In general terms, compressor  1800  has multiple cylinders. The compressor is controlled using a variable speed motor and, in this example, two unloading devices are controlled by on/off instructions that de-energize and energize the unload devices. When demand on the air supply is low, the variable speed motor controlled compressor is operated and unload devices  1810  and  1820  are energized, which causes the output of the cylinders to be reduced. As the demand ncreases, the unload devices are de-energized as necessary. When all unload devices re de-energized, the compressor supplies its rated output.  
         [0052]     Compressor  1800 , being the variably controlled compressor, supplies 100% of its total capacity when both unload devices are de-energized, 66% of its total capacity with one unload device energized and one unload device de-energized, and 33% of its total capacity with both unload devices energized. In this manner, demand on the air source could be controlled down to approximately 16% capacity of the air flow. As one of ordinary skill in the art would now recognize, altering the number of unload devices allows more or less precise control of the power consumption. While the variably controlled compressor is preferred to have two unloading devices, almost any arrangement is possible.  
         [0053]      FIG. 9  is a flow chart  930  representing operation of the present invention with variable speed compressor  1800  having two unload devices  1810  and  1820 . First, the motor speed controller would be placed in automatic control, step  940 , and the pressure sensor would monitor pressure at the inlet of compressor  1800 , step  950 . Next, the motor controller would determine whether the variable speed compressor motor is operating at greater than a minimum speed, step  960 . If compressor  1800  is operating at a minimum speed, motor speed controller  234  next determines whether two or more unload devices are currently de-energized, i.e., variable speed compressor  1800  and associated unload devices  1810  and  1820  are operating, step  970 . If compressor  1800  is operating with both upload devices energized, hot gas control mode is initiated, step  980 . Step  980  is substantially as described in steps  390  to  450  of  FIG. 3 . If motor speed controller  234  determines that one or both of unload devices  1810  and  1820  are energized in addition to variable speed compressor  1800 , then motor speed controller de-energizes one of the unload devices  1810  or  1820 , step  990 , and returns the control to the control loop at step  1000 , below.  
         [0054]     If motor speed controller  234  had determined the variable speed compressor  1800  was not operating at its minimum, step  960 , motor speed controller  234  would then determine whether pressure at the inlet to compressor  1800  was greater than the first pre-established pressure threshold, step  1000 . If pressure is greater than the first pre-established pressure threshold, which indicates an increase in demand on air heat exchanger circuit  210 , then motor speed controller  234  checks whether variable speed compressor  1800  is operating at its maximum, step  1010 . If variable speed compressor  1800  is not operating at its maximum, motor speed controller  234  increases the speed of variable speed compressor  1800 , step  1020 , and the control loop returns to step  950 . If, however, motor speed controller  234  determines that variable speed compressor  1800  is operating at its maximum, step  1010 , then motor speed controller de-energizes an unload device, either 1810 or 1820, step  1030 . After de-energizing the additional unload device, motor speed controller  234  would decrease the speed of variable speed compressor  1800 , step  1040 , and the control loop would return to step  950 .  
         [0055]     If, at step  1000 , motor speed controller  234  had determined that pressure was not greater than the first pre-established pressure threshold, it would determine whether pressure was less than the first pre-established pressure, step  1050 . If motor speed controller  234  determines pressure is less than the first pre-established pressure threshold, then it decreases the speed of variable speed compressor  1800 , step  1060 , and control returns to the control loop at step  950 .  
         [0056]     In this embodiment, if the demand on air heat exchange circuit  210  is 16% of full capacity, compressor  1800  is operating at 50% and both unload devices  1810  and  1820  are energized. As demand of air heat exchanger circuit  210  increases, the speed of compressor  1800  is increased until demand on air heat exchanger circuit  210  is about 33% and compressor  1800  is operating at maximum speed. As demand on air heat exchanger circuit  210  increases past 33%, an unload device  1810  would be de-energized to supply 33% of the necessary flow and the speed of compressor  1800  would drop down to 50% to supply the other 16%. In other words, compressor  1800  would be operating at 50% capacity and unload device  1810  would be off. As demand on air heat exchanger circuit  210  increased to 66%, the speed of compressor  1800  is increased until it is operating at maximum speed. When demand increases over 66%, unload device  1820  is de-energized and the speed of compressor  1800  is reduced to 50% such that compressor  1800  is at 50%, and the unload devices are de-energized. Many combinations of unload devices and compressors could be used. The above embodiments are only exemplary of the possible combinations. For example, a variable speed compressor could have three unload devices capable of 25% capacity each. Another example includes one variable compressor with five unload devices capable of 15% capacity. In general, however, any percentage combination is possible.  
         [0057]     While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention. Additionally, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.