Abstract:
A method for controlling at least one fan or pump of a system having at least one variable frequency speed drive. The method comprises inputting into a controller a plurality of design conditions and VFD operating variables. The controller determines a plurality of measured conditions based on the design conditions and operating variables including a measured head value, efficiency value, and flow rate value. The controller activates or deactivates the at least one fan or pump based on a comparison of a pump or fan performance curve working point and an efficiency value and a comparison of a ratio of the measured head value over a square of the measured flow rate to a ratio of a design head value over a square of a design flow rate, and modulates the speed of at least one fan and pump based on a comparison of the measured and design flow rates.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
       [0003]    Not Applicable 
       TECHNICAL FIELD 
       [0004]    The disclosed embodiments generally relate to any process that uses fans and pumps to transport air, gas, water, and/or liquid, and more particularly to the fans of air handling units, the pumps of hot and chilled water systems, condensing water systems, water treatment systems, and city water distribution systems. 
       DESCRIPTION OF THE RELATED ART 
       [0005]    Many types of buildings require the use of air handling unit (AHUs) systems to supply air at specific temperatures to indoor spaces. Buildings also use chilled water systems to condition rooms at a set temperature but use water as part of the cooling process. Over the years, a variety of configurations and methods for controlling AHUs and chilled and hot water pump systems have been proposed. 
         [0006]      FIG. 1  is an example of one such prior art fan speed control system for controlling single duct variable air volume air handling units. Outdoor air enters into outdoor air damper  134  of prior art air handling unit  100  and mixes with the return air from return air damper  132 . It is then drawn as supply air through AHU  100  by supply air fan  114 . The temperature at which the outdoor air enters AHU  100  is measured by outdoor air temperature sensor  128 . Supply air fan  114  draws supply air through heating coil  108  and cooling coil  110  where it is heated or cooled at the desired temperature so that it can be distributed to end users. Fan  114  also draws air through a device that measures the air flow rate such as flow station  126  shown in  FIG. 1 . Eventually, the air makes its way through the ductwork to the zone dampers. Zone Damper  122  is configured in the ductwork and controls the amount of air delivered to the end user (in each zone/building/area). At least one temperature sensor (such as temperature sensor  124 ) is typically installed in the zone served by AHU  100  to measure the ambient temperature of said zone. 
         [0007]    Controller  118  receives signals from outdoor air temperature sensor  128 , static pressure sensor  130 , and flow station  126 . It then uses that information to control air handling unit  100 . In VAV AHUs (variable air volume air handling units), the set point of the supply air temperature is maintained at 55° F. (the temperature is adjustable, and not limited to that stated herein) using the dampers, heating coils, and cooling coils. Static pressure sensor  130  is installed upstream of the zone dampers and measures the static pressure downstream of the supply air duct. Supply air fan  114  is driven by VFD  112 . The supply fan speed is modulated to maintain the static pressure at a constant set point value. VFD  120  controls the speed of return air fan  116  to ensure that the building has a slightly positive pressure. 
         [0008]    Although prior art air handling units have been controlled to cool spaces in building interiors, they are not very energy efficient. In particular, prior art air handling units like the one described are controlled to maintain the static pressure set point at a constant value. As a result, the system over-pressurizes the terminal box dampers. In some cases, the static pressure setpoint is reset based on the outdoor air temperature. Since the outdoor air temperature is not representative of all the factors that could influence the static pressure, this reset often ends quite conservatively or leads to occupant comfort related complaints. Notably, in such systems the static pressure sensor is located downstream of the ductwork above the ceiling inside of the building. This thus makes it difficult to find issues and perform maintenance procedures. Another problem is that under partial load conditions, the static pressure is very high. A high static pressure can lead to over-pressurization and cause the terminal box to malfunction. Excessive air leakage in the ductwork and terminal box dampers may also waste energy (by 20%) and increase fan energy consumption by half. In some prior art systems, the fan speed is controlled through pre-selection of the terminal box damper position. However, the problem with this method is that some zones may not attain the same comfort standard as others and it can&#39;t be ensured that the preselected zone is is a critical zone. 
         [0009]      FIG. 2  is a schematic diagram of a typical chilled water pump system in the prior art. Chilled water pump system  200  as shown in  FIG. 2  is comprised of chillers  202  and  204 . The chillers are configured to produce chilled water that is circulated by pumps  206  and  208  throughout system  200 . VFDs  210  and  212  are configured in connection with the pumps and function to modulate the pump speed at partial load conditions. The components of system  200  are controlled by controller  216 . Controller  216  receives signals from outdoor air temperature sensor  218 , loop differential pressure sensor  220 , and flow meter  214 . The collected signal information is used by Controller  216  to control the pump speed at partial load conditions. Outdoor air temperature sensor  218  is often mounted outside the building to measure the outdoor temperature. Flow station  214  monitors the water flow rate of system  200 . Valves  226  and  228  open and close to cool down the air passing through cooling coils  222  and  224 . 
         [0010]    As shown in the Fig., a plurality of sensors and a flow station is included in the configuration of the chilled water pump system. The pump speed is controlled to maintain the set point of the loop differential pressure. Such prior art pump systems activate and deactivate based on the distribution of water in the pump system. If the loop differential pressure is lower than the setpoint when the pumps are running at full speed, for example, controller  216  activates one or more pumps to provide more water to system  200 . When the pump speed is low, the one or more pumps are deactivated. The set point is reset based on the outdoor air temperature or determined according to the prior experiences of the user. It should be noted that since the outdoor air temperature is not the only factor that influences the set point and setting the set point based on knowledge gained through prior experience is not a method that is entirely reliable, the reset in prior art systems tends to be very conservative. The result of this is that the chilled water system consumes more energy. 
         [0011]    Additionally, in prior art pump systems such as the one described, the pump working points are often pushed to the position of lowest efficiency as a result of improper pump staging. Thus, even if the design pump efficiency is 75%, in actuality it operates at a low efficiency of 40%. Under partial load conditions, excessive pressure head is often exerted on the cooling coil control valves as well. The pumps consume an excessive amount of energy as a result of the excessive differential pressure set points. As a result, the control valve either gets stuck open or closed (and wastes energy). Otherwise, the control valve must be manually adjusted into position (resulting in extra labor). Prior art systems also tend to have excessively high differential pressure set points that lead to significant pump energy consumption. Finally, prior art systems use differential pressure sensors. These sensors require a lot of maintenance in order to function properly. 
         [0012]    In order to solve the issues present in the prior art as well as to increase the energy efficiency of air handling unit and chilled water systems, a novel control system and method is proposed. This control system controls the fan speed of air handling units and the pump speed and staging in chilled water pump systems, thus eliminating the need for the installation of sensors and other system components that are used to help perform these tasks in prior art units. 
         [0013]    Accordingly, it is one aspect of an embodiment to improve the energy efficiency of and reduce the costs associated with air handling units and chilled water pump systems. This is accomplished through the addition of a control system that reduces the number of pressure sensors, outdoor air temperatures sensors, flow stations, and static pressure sensors needed. 
       DRAWINGS REFERENCE NUMERALS 
     Prior Art 
       [0000]    
       
           100  Prior Art Air Handling Unit 
           108  Heating Coil 
           110  Cooling Coil 
           112  &amp;  120  Variable Frequency Drives (VFDs) 
           114  Supply Air Fan 
           116  Return Air Fan 
           118  Air Handling Unit Controller 
           122  Damper (end user) 
           124  Temperature Sensor 
           126  Flow Station 
           128  Outdoor Air Temperature Sensor 
           130  Static Pressure Sensor 
           132  Return Air Damper 
           134  Outdoor Air Damper 
           200  Prior Art Chilled Water Pump System 
           202  &amp;  204  Chillers 
           206  &amp;  208  Pumps 
           210  &amp;  212  Variable Frequency Drives (VFDs) 
           214  Flow Station 
           216  System Controller 
           218  Outdoor Air Temperature Sensor 
           220  Differential Pressure Sensor 
           222  &amp;  224  Cooling Coils 
           226  &amp;  228  Valves 
           300  Sensorless Fan and Pump Control Device in a VAV Air Handling Unit 
           302  Sensorless Fan and Pump Control Device 
           304  &amp;  316  VFDs 
           306  Supply Fan 
           308  Cooling Coil 
           310  Heating Coil 
           312  Return Fan 
           314  &amp;  318  Dampers 
           320  Temperature Sensor 
           400  Sensorless Fan and Pump Control Device in a Chilled Water Pump System 
           408  &amp;  426  VFDs 
           404  &amp;  406  Chillers 
           410  &amp;  412  Chilled Water Pumps 
           414 ,  416 ,  418  Valves 
           420 ,  422 ,  424  Cooling Coils 
           500  Control Logic of the Sensorless Fan and Pump Control Device 
           502  Input Module 
           504  AHU Power Module 
           506  Air Flow, Head, and Fan Efficiency Module 
           508  AHU Load/Unload Module 
           510  Fan and Design Efficiency Comparison Step 
           512  Fan Ratio Comparison Step 
           514  Pump Power Module 
           516  Water Flow, Head, and Pump Efficiency Module 
           517  Chiller Number Calculation Step 
           518  Pump Load/Unload Module 
           520  Fan Activation Step 
           522  Fan De-activation Step 
           524  Pump and Design Efficiency Comparison Step 
           526  Pump Ratio Comparison Step 
           528  Pump Activation Step 
           530  Pump De-activation Step 
           532  Fan Speed Control Module 
           534  Pump Speed Control Module 
           536  Fan Airflow and High Load Airflow Rate Comparison Step 
           538  h/Q 2 =hd/Qd 2  Fan Speed Modulation Step 
           540  Fan Airflow and Low Load Airflow Rate Comparison Step 
           542  Low Load Airflow Rate Fan Speed Modulation Step 
         
           544 
         
       
     
         [0000]    
       
         
           
             
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       Fan Speed Modulation Step 
       [0000]    
       
           546  Pump Water flow and High Load Water flow Rate Comparison Step 
           548  h/Q 2 =hd/Qd 2  Pump Speed Modulation Step 
           550  Pump Airflow and Low Load Water flow Rate Comparison Step 
           552  Low Load Water flow Rate Pump Speed Modulation Step 
         
           554 
         
       
     
         [0000]    
       
         
           
             
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       Pump Speed Modulation Step 
       [0000]    
       
           600  Flow chart showing how control device  302  may be connected and operable to control multiple VFDs. 
       
     
       SUMMARY 
       [0083]    In one embodiment, a control system for controlling at least one fan or pump and at least one VFD is provided. The control system comprises an input module configured to input a plurality of operating conditions from said vfd and predetermined variables for said system comprising a performance curve, a design flow rate, a design low load flow rate, a design high load flow rate, a VFD current value, a VFD power value, a VFD torque value, and a VFD speed value. The control system also comprises a power module configured to calculate for a measured power value based on the VFD power value, as well as a head, flow rate, and efficiency module configured to calculate for a head value based on the measured power value and the performance curve. It is also configured to calculate for a measured flow rate value based on the VFD current value, VFD power value, VFD torque value, the performance curve, as well as an efficiency value based on the measured flow rate and measured head value. 
         [0084]    The control system further comprises a load/unload module configured to stage and modulate a speed of said at least one pump or fan. The control system comprises an identifying step for identifying a working point on the performance curve. It also comprises an activation step for activating the at least one fan or pump when the efficiency value is less than the working point by a predetermined amount and a ratio of the measured head value over a square of the measured flow rate is lower than a ratio of the design head value over a square of the design flow rate. The control system further comprises a deactivation step for deactivating the at least one fan or pump when the efficiency value is less than the working point by a predetermined amount and a ratio of the measured head value over a square of the measured flow rate is greater than a ratio of the design head value over a square of the design flow rate. 
         [0085]    Finally, the control system comprises a speed modulation step for controlling a speed of the at least one fan or pump when the measured flow rate is greater than the design high load flow rate so that a ratio of the measured head value over a square of the measured flow rate is equal to a ratio of the design head value over a square of the design flow rate. The control system includes a speed modulation step for controlling a speed of the at least one fan or pump to maintain the low load flow rate when the measured flow rate is less than the low load flow rate. The control system also comprises a speed modulation step for controlling a speed of the at least one fan or pump when the measured flow rate is less than the design high load rate and greater than the design low load rate so that the ratio of the measured head value over a square of the measured flow rate is equal to one plus said design high load flow rate minus said measured flow rate over said design high load flow rate multiplied by a distribution factor and further multiplied by said design head over said design flow rate squared. For clarity, this is shown in equation form as: 
         [0000]    
       
         
           
             [ 
             
               
                 h 
                 
                   Q 
                   
                     2 
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         [0086]    In another embodiment, a method of controlling at least one fan or pump to optimize the transport of liquids and/or gases through a system having at least one VFD is proposed. The method comprises interfacing a control device with the system. It further comprises inputting a plurality of system operating conditions comprising a VFD power value, a VFD current value, VFD torque value, and VFD speed value from the variable speed drive into said control device. It also comprises inputting a plurality of operating conditions such as a performance curve, a design flow rate, a design high load flow rate and design low load flow rate into the control device. The method further comprises calculating, by the controller, for a measured power value based on theVFD power value. The method also comprises determining a measured flow rate based on the performance curve, the VFD current value, VFD power value, and VFD torque value. 
         [0087]    The method further comprises determining a measured head value based on the measured power value and the performance curve, and determining a design point efficiency based on the measured flow rate and measured head value. The method further comprises identifying a working point efficiency on the performance curve, and activating the at least one fan or pump when the design point efficiency is less than the working point efficiency by a predetermined amount, and a ratio of the measured head value over a square of the measured flow rate is lower than a ratio of the design head value over a square of the design flow rate. 
         [0088]    The method further comprises inactivating at least one fan or pump when the design point efficiency is less than the working point efficiency within a predetermined range and a ratio of the measured head value over a square of the measured flow rate is greater than a ratio of the design head value over a square of the design flow rate. The method comprises modulating the fan and pump to maintain the low load flow rate when the measured flow rate is lower than the design low load flow rate. It further comprises modulating a speed of the at least one fan or pump so that a ratio of the measured head value over a square of the measured flow rate is equal to a ratio of the design head value over a square of the design flow rate when the measured flow rate is greater than the design high load flow rate. 
         [0089]    Finally, the method comprises modulating a speed of the at least one fan or pump when the measured flow rate is less than the design high load rate and greater than the design low load rate, so that a ratio of the measured head value over a square of the measured flow is equal to one plus the design high load flow minus the measured flow rate over the design high load flow multiplied by a distribution factor and further multiplied by the design head over the design flow rate squared. For clarity, this is shown in equation form as: 
         [0000]    
       
         
           
             
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         [0090]    In some embodiments the system is an air handling unit while in other embodiments the system is a chilled water pump system having at least one chiller. In embodiments in which the system is a chilled water pump system having at least one chiller, the controller calculates for the design water flow rate and measured head of the at least one chiller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0091]    It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the following Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Advantages, features and characteristics of the present disclosure, as well as methods, operation and functions of related elements of structure, and the combination of parts and economies of manufacture, will become apparent upon consideration of the following description and claims with reference to the accompanying drawings, all of which form a part of the specification, wherein like reference numerals designate corresponding parts in the various figures, and wherein: 
           [0092]      FIG. 1  is a schematic diagram embodying the principles of an air handling unit system in the prior art. 
           [0093]      FIG. 2  is a schematic diagram embodying the principles of a chilled water pump system in the prior art. 
           [0094]      FIG. 3  is a schematic diagram embodying the principles of a sensorless fan and pump speed control device implemented in an air handling unit. 
           [0095]      FIG. 4  is a schematic diagram embodying the principles of said sensorless fan and pump speed control device implemented in a chilled water pump system. 
           [0096]      FIG. 5  is a block diagram showing the the control logic of said sensorless fan and pump speed control device. 
           [0097]      FIG. 6  is a block diagram showing the direction of communication between control device  302  and the variable frequency drive(s) of the system in which it is implemented. 
       
    
    
     DETAILED DESCRIPTION 
       [0098]    Before the present methods, systems, and materials are described, it is to be understood that this disclosure is not limited to the particular methodologies, systems and materials described, as these may vary. It is also to be understood that the terminology used in the description is for the purpose of describing the particular versions or embodiments only, and is not intended to limit the scope. 
         [0099]    It must also be noted that as used herein and in the appended claims, the singular forms “a”, “an,” and “the” include plural references unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Although any methods, materials, and devices similar or equivalent to those described herein can be used in the practice or testing of embodiments, the preferred methods, materials, and devices are now described. All publications mentioned herein are incorporated by reference. Nothing herein is to be construed as an admission that the embodiments described herein are not entitled to antedate such disclosure by virtue of prior invention. 
         [0100]    In accordance with one embodiment, a sensorless fan and pump speed control air handling unit system is illustrated in  FIG. 3 . In the embodiment shown, control device  302  is implemented in air handling unit system  300 . As shown in the Figure, existing air handling unit system  300  is comprised of supply air fan  306 , cooling coil  308 , heating coil  310 , return air fan  312 , temperature sensor  320 , and return air dampers and end user dampers  314  &amp;  318 . Fans  306  and  312  are connected in communication with VFDs  304  and  316 . Based on commands from controller  302 , VFDs  304  and  316  are able to control the speed of fans  306  and  312 , respectively.  FIG. 6  (flow chart  600 ) shows how control device  302  may be connected and operable to control multiple VFDs through use of the Modbus communication protocol or other communication channel. In the Fig., the fans are configured in parallel and are powered by multiple VFDs. In other embodiments, however, device  302  can also be configured in communication with a single VFD. 
         [0101]    In accordance with another embodiment, a sensorless fan and pump speed control system  400  is illustrated in  FIG. 4 . In the embodiment shown, control device  302  is implemented in chilled water pump system  400 . System  400  is comprised of VFDs  408  &amp;  426 , chillers  404  &amp;  406 , supply water pumps  410  and  412 , valves  414 ,  416 , and  418  and cooling coils  420 ,  422 , and  424 . 
         [0102]    Chillers  404  and  406  produce chilled water that is circulated throughout system  400  as well as to valves  414 ,  416 , and  418  and cooling coils  420 ,  422 , and  424  by pumps  410  and  412 . When water passes through cooling coils  420 ,  422 , and  424 , it is warmed up by the air. The water is then once again redistributed through the pump system in a cyclical manner and is cooled down by chillers  404  and  406 . VFDs  408  &amp;  426  control the speed of pumps  410  and  412  to maintain the differential pressure across cooling coils  420 ,  422 , and  424 . Control device  302  is configured in communication with VFDs  408  and  426 . Said control device controls the manner in which pumps  410  and  412  are staged. It is also configured to control the speed of said pumps. As illustrated in  FIG. 4 , the pumps are configured in parallel. 
         [0103]    As shown in  FIG. 3 , control device  302  can be installed in both systems  300  and  400 . Before control device  302  can be used, the user must first configure control device  302  with the specific data for the system in which it will be implemented. For example, when the system is implemented in air handling units, the user pre-programs into device  302  data comprising but not limited to the fan performance curve and high and low load airflow rates. When the system is implemented in chilled water pump systems, data pre-programmed into device  302  may include but not be limited to the chilled water pump performance curves and the high and low load water flow rates. Thus, the control method of device  302  differs depending upon the system of implementation. In  FIG. 3 , Device  302  is shown to be installed in communication with VFD  304  of system  300 . 
         [0104]      FIG. 5  is a block diagram showing the control logic of control device  302 . Control device  302  may include a plurality of modules. The modules have different functions depending on whether Device  302  is implemented in a chilled water pump system or in an air handling unit. As shown in  FIG. 5 , Device  302  includes an input module  502  that is configured to receive a plurality of digital or analog signals dictating the operating conditions of system  300  or  400  (depending upon in which system it is implemented) from the one or more VFDs. The analog or digital signals may be delivered to control device  302  wirelessly or via wire connection. In a method of the embodiment, the collected operating conditions may include data detailing the current, power, torque, and speed values from the VFD(s) as well as set information that is pre-programmed into Device  302  by the user to include but not be limited to the fan and pump performance curves and the high and low load flow rates. Based on information on the VFD power values, AHU Power Module  504  calculates for the fan power values by removing the VFD loss and motor loss from the VFD power values. If device  302  is implemented in a chilled water pump system, Pump Power Module  514  similarly calculates for the pump power values by removing the VFD loss and motor loss from the VFD power values. 
         [0105]    Using the pump performance curve and the current, power, and torque values collected from the VFD(s), Air Flow, Fan Head, and Fan Efficiency Module  506  calculates for an airflow rate when used in system  300 . Device  302  calculates for a water flow rate for a chilled water pump system such as system  400  in Water Flow, Pump Head, and Pump Efficiency Module  516  using the same method as in Module  506 . Modules  516  and  506  also calculate for the pump head and pump efficiency (in chilled water pump systems) and the fan head and fan efficiency (in air handling units) values, respectively. The fan or pump power values (calculated in modules  504  or  514 , respectively) as well as the fan or pump performance curve (fan curve if device  302  is implemented in system  300  and pump curve if implemented in system  400 ) are used by Modules  506  and  516  to calculate for the fan head and fan efficiency or the pump head and pump efficiency, respectively. 
         [0106]    Using the fan head and air flow rates calculated in Module  506  or the pump head and water flow rates calculated in Module  516 , AHU Load/Unload Module  508  or Pump System Load/Unload Module  518  identifies the equivalent working points on the fan or pump design curves, respectively. The pump design curve is used if device  302  is implemented in a pump system or the fan design curve is employed if device  302  is implemented in an air handling unit. As shown in steps  510  and  512  of  FIG. 5 , if the fan efficiency is less than the design efficiency by a predetermined value (about 5% for example, but not limited to this percentage), then AHU Load/Unload Module  508  activates the fans by comparing the ratio of the measured fan head over the square of the measured fan airflow rate to the ratio of the design fan head over a square of the design fan airflow rate. Module  508  activates a fan if the ratio of the measured fan head over the square of the measured fan airflow rate is lower than the ratio of the design fan head over the square of the design fan airflow rate (see step  520  in  FIG. 5 ). Module  508  inactivates a fan if the working point is on the left of the design point meaning that the ratio of the measured fan head over a square of the measured fan airflow rate is higher than the ratio of the design fan head over a square of the design fan airflow rate (see step  522  in  FIG. 5 ). It should be noted though that AHU Load/Unload Module  508  is only needed in configurations in which the air handling unit is comprised of multiple fans in parallel. 
         [0107]    The control logic of Module  518  follows the same control logic as described for Module  508  [see the prior paragraph] expect that it activates and deactivates pumps rather than fans. Thus, as can be seen in Steps  524 ,  526  and  528  of  FIG. 5 , if the pump efficiency is less than the design pump efficiency by a predetermined value (about 5% for example, but not limited to this percentage), then Pump Load/Unload Module  518  activates the pumps of the chilled water pump system by comparing the ratio of the measured pump head over a square of the measured pump flow rate to the ratio of the design pump head over a square of the design pump flow rate. Module  518  inactivates a pump if the working point is on the left of the design point meaning that the ratio of the measured pump head over a square of the measured pump airflow rate is higher than the ratio of the design pump head over a square of the design pump airflow rate (see steps  526  and  530  in  FIG. 5 ). 
         [0108]    When device  302  is implemented in an air handling unit such as that shown in system  300 , Fan Speed Control Module  532  controls the speed of the fan based on a comparison of the measured airflow rate and design airflow rates. As shown in steps  536  and  538  of  FIG. 5 , if the measured rate of airflow is greater than the high load airflow rate (adjustable rate of 80% of the design airflow), Module  532  modulates the fan speed so that the ratio of the fan head over the square of the fan airflow rate equals the ratio of the design fan head over the square of the design fan airflow rate (or the in-situ measured or adjusted value). If the measured rate of airflow is less than the high load but greater than the low load or heating flow rate (50% of the design airflow rate (this rate is adjustable)), Module  532  modulates the speed of the fan so that the ratio of the fan head over the square of the fan airflow is a function of the following equation: 
         [0000]    
       
         
           
             
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       Where: 
       [0000]    
       
         h is the fan head as measured by control device  302   
         Q is the airflow rate as measured by control device  302   
         Qh is the high load flow, or about 80% of the design airflow (this percentage is adjustable) 
         Hd is the design fan head 
         Qd is the design airflow rate 
         α-flow is the distribution factor [2] (the number is adjustable)
 
Module  532  modulates the speed of the fan so that the airflow rate is at a low load flow rate (or at 50% of the design airflow rate, though this is adjustable) if the airflow rate is lower than the low load airflow rate (as shown in steps  540 , 542 , and  544  of  FIG. 5 ).
 
       
     
         [0115]    When device  302  is implemented in a pump system such as system  400  as shown in  FIG. 4  of the drawings, Pump Speed Control Module  534  controls the speed of the pumps. The control method is similar to the method used for controlling the speed of the fans when device  302  is implemented in an air handling unit. Thus, as shown in steps  546  and  548  of  FIG. 5 , Module  534  compares the measured and design water flow rates. If the measured water flow rate is higher than the high load water flow rate (adjustable rate of 80% of the design water flow rate), Speed Control Module  534  modulates the pump speed so that the ratio of the pump head over the square of the pump flow rate is equal to the ratio of the design pump head over the square of the design pump water flow rate (or the in-situ measured or adjusted value). However, if the measured water flow rate is lower than the high load flow rate but higher than the low load flow rate (or 50% of the design heating flow rate (this rate is adjustable)), Module  534  modulates the speed of the pump so that the ratio of the pump head over the square of the pump water flow rate is a function of the equation shown previously. If the water flow rate is lower than the low load water flow rate (or 50% of the design water flow rate), the pump speed is modulated to maintain the low load water flow rate as shown in steps  550 ,  552 , and  554  of  FIG. 5 . 
         [0116]    While the method in which device  302  controls the pumps of chilled water pump systems and the fans of air handling units is similar, device  302  makes an additional calculation before solving for the pump speed ratio when implemented in chilled water pump systems like that shown in  FIG. 3  (system  300 ). This is because the number of chillers in operation in a chilled water pump system significantly affects the design head and flow calculations. As such, in order to modulate the pump speed to maintain the ratio of the pump head and the square of the water flow rate as a function of the flow ratio according to the stated equation, it is first necessary to calculate for the design water flow rate and design pump head for each system configuration. For example, chilled water pump system  400  is comprised of two chillers (chillers  404  and  406 ). When both chillers are in operation they process water at a rate of 1000 GPM (gallons per minute). If the design pump head is 100 feet (ft). (60 ft. for the water distribution system and 40 ft. for the chiller and associated pipe), the pump head and water flow design ratio is 100 ft/1000/1000 or 0.0001 when chillers  404  and  406  are in operation. However, if one of the said chillers is not in operation (and the isolation valve closed to prevent the flow of chilled water through the chiller), the design head is instead 55 ft (40 ft. for the chiller and 15 ft. for the distribution loop or 0.025 multiplied by 60). In this scenario, the design water flow rate is 500 GPM. Thus, the ratio of the design pump head and the square of the design water flow rate becomes 0.00022 (55/500/500). It can therefore be seen that the addition of a chiller to the configuration of a chilled water pump system more than doubles the ratio. This additional chiller calculation for the pump system is shown in step  517  in  FIG. 5 . 
         [0117]    Various features and advantages of the invention are set forth in the following claims.