Patent Publication Number: US-2018031292-A1

Title: Condenser Pressure Control System and Method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     Not Applicable 
     TECHNICAL FIELD 
     The disclosed embodiments generally relate to a system and method for controlling condenser fans, and more particularly for controlling condenser fans in HVAC (heating, ventilation, and air conditioning) units as well as commercial and industrial refrigeration system applications. 
     DESCRIPTION OF THE RELATED ART 
     Condensers are devices that condense a substance from a gaseous state to a liquid state. Air-cooled condensers enable this phase change using ambient air, while water cooled condensers use water as the cooling means. They are a common component of air conditioning and refrigeration systems. A condenser control system may include at least one VFD (variable frequency drive) for modulating the speed of the condenser fan (in air-cooled condensers) or a control valve for controlling the water flow rate (in water-cooled condensers). Condenser control systems generally comprise at least one temperature or pressure sensor that measures the temperature or pressure, respectively, of a refrigerant at the liquid outlet pipe of the condenser. In typical control systems, temperature sensors are located in the middle, outlet, or inlet pipes of the condenser, while pressure sensors are typically located just in the outlet pipe of the condenser. When the ambient air temperature (in the case of air-cooled condensers) or water temperature (in the case of water-cooled condensers) is high, the pressure in the condenser is determined by the design of the condenser. The condenser pressure can also be determined by taking the temperature difference between the refrigerant and the outside air while the fan (in the case of air-cooled condensers) is operating at full speed or the control valve (in the case of water cooled condensers) is in a fully open position. 
     When the ambient air temperature (in the case of air-cooled condensers) or water temperature (in the case of water-cooled condensers) is low, the pressure in the condenser must be high enough so that the rated refrigerant can flow through the expansion valve. This is the defining principle for determining the minimum condensing pressure, also called the head pressure. The minimum head pressure can be found by summing together the total pressure in the evaporator with the pressure drop across the expansion valve (under the design conditions) and the pressure drop in the pipe located between the condenser &amp; evaporator. There are a variety of ways in which the controller of the condenser can maintain the minimum head pressure. It may turn the fan on and off, modulate the fan speed, and/or (in applications having water-cooled condensers) open/close a two position valve. 
     Since the head pressure setpoint significantly influences the compressor power, scholars in the prior art have developed a method called the floating head pressure control method to save energy. The optimal head pressure set point in the floating pressure control method is determined based on the ambient air temperature and rated refrigerant flow rate. The optimal head pressure is set at or higher than the minimum head pressure. As an example, when a refrigerant such as monochlorodifluoromethane (R-22) is used in a system employing the floating head pressure method, the suction pressure is typically set at 70 Psig. At the design flow rate, the design pressure drop for the expansion valve is supposed to be about 100 Psi, while the design liquid pressure drop is about 5 Psi. The minimum head pressure set point should thus be set to 175 Psig. Under real system conditions, the minimum head pressure set point is often set from 195 Psig to as high as 250 Psig. The corresponding condensing temperature becomes about 100° F. and 118° F., respectively. However, the majority of the time the refrigerant flow rate is less than the rated flow rate under partial load conditions. As a result of the high condensing pressure, when the flow rate of the refrigerant is less than the design rate, the expansion valve is configured to partially close. This in turn results in the excessive consumption of compressor power. 
     There are a variety of methods proposed in the prior art to control the operation of the condenser fan in air-cooled condensers or the cooling water valve in water-cooled condensers. It is common to use the sensed temperature and pressure values to control the fan or cooling water valve under low ambient temperatures. In a prior art temperature based fan control method, for example, a temperature sensor is installed in the condenser fan control system. Using the readings from this sensor, the controller of the condenser fan control system can be programmed to compare the measured condensing temperature with the set point temperature. Based on the comparison, in some applications the controller is programmed to command the fan on or off or the control valve open or closed to maintain the condensing temperature to within specific high/low temperature limits (for the R-22 refrigerant, for example, this is 100° F. to 118° F.). In other applications, the controller is programmed to modulate the fan speed (for air-cooled condensers) or control the valve position (for water-cooled condensers) to maintain the condensing temperature at a single, pre-set setpoint value (For R-22, this is 100° F.). 
     The controller activates or deactivates the fan and/or opens or closes the control valve to maintain the pressure of the refrigerant to within the specified high and low limits of the setpoint (195 Psig to 250 Psig for R-22).  FIG. 1  is an illustration of a prior art condenser fan control system. In the figure ( FIG. 1 , System  100 ), condenser  106 , pressure sensor  112 , expansion valve  114 , evaporator  116 , compressors  118  and  119 , condenser fan  120 , controller  122 , thermal bulb  124 , and variable frequency drive (VFD)  140  are connected via conduit in a typical refrigeration system. Any type of compressor may be employed (including constant speed and multiple speed compressors). Condenser fan  120  (in condenser  106 ) of the refrigeration system is connected in communication with controller  122 . Controller  122  is also configured in connection with pressure sensor  112 . In the prior art method, controller  122  collects pressure values from pressure sensor  112  and modulates the speed of condenser fan  120  based on those values. Variable frequency drive  140  is modulated to maintain the condensing pressure at a single setpoint. 
     Thermal bulb  124  is also configured within system  100  at the tailpipe of evaporator  116  so that it is in connection with and can be used to control the opening and closing of expansion valve  114 . This temperature sensing bulb controls the flow of refrigerant through the refrigeration system. The bulb is filled with a gas that is the same as the gas in the refrigeration system. As the temperature in the bulb increases, the gas in the bulb expands and puts pressure on expansion valve  114 . The pressure causes expansion valve  114  to open, thereby increasing the flow of refrigerant through the system. When the temperature in the suction line of the refrigeration system decreases, the pressure of the gas in thermal bulb  124  also decreases causing expansion valve  114  to close. Closure of valve  114  results in a restriction in the flow of refrigerant. 
     While this prior art system is functional, there are certainly problems with controlling the condenser fan to maintain the condensing temperature or pressure at a single setpoint. For one, the amount of refrigerant that flows through the expansion valve and liquid pipe line may be much smaller than the design value when refrigeration systems use or are equipped with the following:
         1. They employ a VFD (variable frequency drive) to modulate the speed of the compressor   2. They have a small number of operating compressors in a system equipped with multiple compressors   3. They have multiple stage reciprocating compressors   4. They employ a slide valve to modulate the capacity   5. They use a high temperature liquid or hot gas for reheat purposes   6. They have a hot gas by-pass (whereby hot gas is bypassed through the high pressure valve and returned to the inlet of the compressor).       

     Under all of the previously stated conditions, the flow rate of the refrigerant that flows through the expansion valve may be as low as 30% of the design flow rate. Table 1 below lists the pressure loss across the expansion valve and pipelines as well as the evaporator pressure of the R-22 refrigerant for different flow rates. Based on the given information, it is possible to determine the optimal head pressure at each flow rate. When the flow rate is reduced from 100% to 50%, for example, the minimum head pressure set point is reduced from 178 Psig to 96 Psig. At the same time, the liquid temperature set point is reduced from 87° F. to 55° F. The compressor head lift can be reduced by as much as 75%. In real system applications in which the R-22 refrigerant is employed, the minimum floating head pressure can be as high as 190 Psig. The fixed head pressure set point is often set as high as 250 Psig. Therefore, optimizing the head pressure can significantly reduce the power of the compressor. It is important to point out that the condensing temperature (also called the optimal head pressure) varies according to the refrigerant flow rate and is thus not a fixed value. An additional problem is that under the design head pressure the expansion valve can malfunction when the refrigerant flow rate is low. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Variance in the pressure values of a refrigeration system using 
               
               
                 the R-22 refrigerant under differing refrigerant flow rates. 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Flow 
                 50% 
                 60% 
                 70% 
                 80% 
                 90% 
                 100% 
                 120% 
                 140% 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Expansion valve loss 
                 25 
                 36 
                 49 
                 64 
                 81 
                 100 
                 144 
                 196 
               
               
                 Pipe loss 
                 2.5 
                 3.6 
                 4.9 
                 6.4 
                 8.1 
                 10 
                 14.4 
                 19.6 
               
               
                 Evaporator pressure 
                 83 
                 83 
                 83 
                 83 
                 83 
                 83 
                 83 
                 83 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Optimal 
                 Psia 
                 111 
                 123 
                 137 
                 153 
                 172 
                 193 
                 241 
                 299 
               
               
                 Head 
                 Psig 
                 96 
                 108 
                 122 
                 139 
                 157 
                 178 
                 227 
                 284 
               
               
                 pressure 
                 Bar 
                 6.7 
                 7.6 
                 8.6 
                 9.8 
                 11.1 
                 12.6 
                 16.0 
                 20.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Liquid Temp 
                 55.0 
                 60.0 
                 65.0 
                 72.0 
                 79.0 
                 87.0 
                 105.0 
                 125.0 
               
               
                 Lift reduction 
                 75% 
                 64% 
                 51% 
                 36% 
                 19% 
                 0% 
                 −44% 
                 −96% 
               
               
                   
               
            
           
         
       
     
     In addition, while thermal bulb  124  is able to control the opening and closing of expansion valve  114 , by itself it is not able to maintain the position of expansion valve  114  at an optimal fully open position for long periods of time. Therefore, in order to overcome the problems posed in the prior art, a new system is proposed herein. The following describe key advantages of embodiments of this new system: In the embodiments, the expansion valve is connected in direct communication with a controller. This controller is configured with a control algorithm that ensures that the expansion valve is constantly maintained in an optimal open position and can thus optimally circulate refrigerant throughout the refrigerant system. Prior art systems, such as the one shown in  FIG. 1 , tend to rely solely on the thermal bulb to control the position of the expansion valve and are thus not able to optimally regulate the flow of refrigerant. In addition, another key advantage of the disclosure is that in at least some of the disclosed embodiments, the proposed method saves more energy than the prior art methods since it is able to modulate the condensing temperature or pressure at multiple setpoints rather than at a single setpoint. Other related advantages of the proposed embodiments disclosed herein are summarized in the following: 
     It is therefore an advantage of an embodiment to reduce the excessive consumption of compressor power under partial load conditions and to extend the lifetime of the compressor. 
     It is another advantage of an embodiment to reduce the power of the condenser fan and condenser pressure under partial load conditions to extend the lifetime of the condenser. 
     It is yet another advantage of an embodiment to improve the energy efficiency in terms of the coefficient of performance (COP). 
     It is a further advantage of an embodiment to reduce excessive pressure on the expansion valve. The reduction results in reduced O&amp;M costs, and ensures that the HVAC system is operating smoothly. 
     SUMMARY OF THE INVENTION 
     The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to an embodiment of the present invention and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. The embodiments described in this disclosure offer some key advantages of the invention over the prior art as summarized below: 
     In one embodiment, a method of controlling a condenser fan to maintain a valve position of an expansion valve of a heating, ventilating, and air conditioning (HVAC) system at a valve position setpoint is proposed. The method involves providing a controller in communication with the expansion valve of the HVAC system. The controller is operable to receive a signal indicating the valve position. It also involves providing a refrigerant flow rate sensing device in communication with the HVAC system and controller operable to measure a refrigerant flow rate of the HVAC system. The method further entails configuring the controller with a critical refrigerant flow rate value and the valve position setpoint. The controller compares the refrigerant flow rate with the critical refrigerant flow rate value and modulates the condenser fan to maintain the valve position of the expansion valve at the valve position setpoint when the refrigerant flow rate is higher than the critical refrigerant flow rate. 
     In another embodiment, a method of controlling a condenser fan of a heating, ventilating, and air-conditioning (HVAC) system to maintain a liquid condensing measurement of a refrigerant at a plurality of condensing measurement setpoints is proposed. The HVAC system comprises at least one evaporator, expansion valve, condenser, and compressor configured in a refrigerant circuit. The method involves providing a condensing measurement device in connection with the condenser and controller and is configured to provide a condensing measurement. The method further entails providing a refrigerant flow sensing device in communication with the controller and HVAC system that is operable to measure a refrigerant flow rate value. The method further comprises determining, by the controller, a system load ratio (ω) for the HVAC system. It involves programming the controller with a plurality of variables for the HVAC system comprising a design flow rate value of said refrigerant, the system load ratio value (ω), a critical flow rate value of the refrigerant, a subcooling temperature of the refrigerant, a saturated pressure measurement corresponding to the evaporative temperature (P evaporator ) of the evaporator; and a sum of a pressure loss value from said expansion valve and a pressure loss value in a liquid line of the refrigerant circuit at said design flow rate value (ΔP). The method further involves determining, by said controller, a condensing pressure setpoint (P set ) of the condenser based on said plurality of variables, wherein P set =P evaporator +ω 2 ΔP, and modulating, by the controller, the speed of the condenser fan to maintain the condensing measurement value at the plurality of condensing measurement setpoints when the flow rate value of the refrigerant is higher than the critical flow rate value. 
     Some embodiments of the method comprise providing a pressure sensor in communication with the controller and operable to measure and send to the controller a pressure measurement from the liquid line of the refrigerant circuit. The controller modulates the speed of the condenser fan to maintain the pressure measurement at a plurality of condensing pressure setpoints when the refrigerant flow rate value is higher than the critical flow rate value. The condensing measurement setpoint is the condensing pressure setpoint (P set ). Other embodiments of the method comprise providing a temperature sensor in communication with and operable to measure and send a temperature measurement to the controller. The controller modulates the speed of the condenser fan to maintain the temperature measurement at a plurality of condensing temperature setpoints when the refrigerant flow rate value is higher than the critical flow rate value. The plurality of condensing temperature setpoints is defined as the saturated temperature value of the refrigerant at the condensing pressure setpoint(P set ). 
     In some embodiments of the method the sensing device is a flow meter operable to measure a refrigerant flow rate, and the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises dividing the refrigerant flow rate value over the design refrigerant flow rate value. 
     In other embodiments of the method, the sensing device is a compressor status device operable to measure a refrigerant flow rate as well as to collect and transmit to the controller a signal indicating when the compressor of the HVAC system is in an active state of operation. When this method is used, the HVAC system must have at least two compressors connected in communication with the controller and the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises transmitting, by the compressor status device, the compressor status signal for the compressors to the controller. The controller divides a sum of the compressor status signal for the compressors in operation over a sum of the compressors in the HVAC system. 
     In still yet other embodiments of the method, the sensing device is a compressor speed device operable to measure a refrigerant flow rate as well as to collect and transmit to the controller a signal indicating a speed of the compressor. In the method, the step in which the controller determines the system load ratio (ω) for the HVAC system further comprises providing the controller with a compressor design speed and transmitting, by the compressor status device, the signal indicating the speed of the compressor to the controller. The step further entails dividing, by the controller, the speed of the compressor over the compressor design speed. 
     In another embodiment, a method of controlling a condenser fan of a condenser of a heating, ventilating, and air-conditioning (HVAC) system to maintain a condensing measurement value at a plurality of condensing measurement set points based on an ambient air temperature measurement is proposed. The HVAC system has at least one evaporator and expansion valve configured in a refrigerant circuit. The method involves providing a controller in communication with the HVAC system and providing an ambient air temperature sensor in communication with and operable to measure and send to the controller an ambient air temperature value (T amb ). It also comprises providing a condensing measurement device in communication with the controller and operable to measure and send to the controller a condensing measurement value. The method involves determining, by the controller, a condensing temperature value (T cond ). It also entails programming the controller with a ratio of a speed of the condenser fan over a design speed of the condenser fan (ω) a design condenser split temperature value of the HVAC unit (ΔT), an on/off heat transfer coefficient ratio for the condenser fan (α), a suction pressure value (P suc ) from the evaporator, a subcooling temperature of the refrigerant, and a sum of a pressure loss value across a liquid line and a suction line of the refrigerant circuit and a pressure loss at the expansion valve of the HVAC unit under a design flow rate (ΔP d ). Using those variables, the controller is configured to determining a cooling load ratio (β) for the condenser, wherein: 
     
       
         
           
             β 
             = 
             
               { 
               
                 
                   
                     
                       
                         
                           
                             T 
                             cond 
                           
                           - 
                           
                             T 
                             amb 
                           
                         
                         
                           Δ 
                            
                           
                               
                           
                            
                           T 
                         
                       
                        
                       
                         ω 
                         0.76 
                       
                     
                   
                   
                     
                       ω 
                       &gt; 
                       0.1 
                     
                   
                 
                 
                   
                     
                       
                         
                           
                             T 
                             cond 
                           
                           - 
                           
                             T 
                             amb 
                           
                         
                         
                           Δ 
                            
                           
                               
                           
                            
                           T 
                         
                       
                        
                       α 
                     
                   
                   
                     
                       ω 
                       ≤ 
                       0.1 
                     
                   
                 
               
             
           
         
       
     
     The controller determines a condensing liquid pressure set point (P cond.set ) of the refrigerant based on the cooling load ratio (β), wherein: 
         P   cond.set   =P   suc +β 2   ΔP   d  
 
     The method further entails determining, by the controller, a plurality of condensing measurement setpoints based on a condensing measurement value and the condensing liquid pressure set point (P cond.set ). It also involves modulating, by the controller, the speed of the condenser fan to maintain the condensing measurement at the plurality of condensing measurement setpoints when the condensing temperature value is higher than the ambient temperature value plus at or around 5° F. 
     In some embodiments of the method, the step of determining, by the controller, the condensing temperature value (T cond ), further comprises providing a temperature sensor in communication with and operable to measure and send to the controller the condensing temperature value (T cond ) of the refrigerant. Other embodiments of the method comprise providing a pressure sensor in communication with the condenser and controller and operable to measure and send a condensing pressure measurement value (P cond ) to the controller. The method further entails finding a saturated temperature of the refrigerant at the condensing pressure value (P cond ). 
     In some embodiments of the method, a temperature sensor is provided in communication with and operable to measure and send to the controller a liquid temperature condensing measurement value (T cond ). The condensing measurement setpoint is determined as a corresponding saturated refrigerant temperature at the condensing pressure setpoint (P cond.set ). The controller modulates the speed of the condenser fan to maintain the temperature measurement value (T cond ) at the plurality of condensing temperature measurement setpoints when the temperature measurement value (T cond ) is higher than the ambient air temperature value plus at or around 5° F. (this temperature value is not limited to 5° F. and is thus adjustable). 
     In other embodiments of the method, a pressure sensor is provided in communication with and operable to measure and send to the controller a liquid pressure condensing measurement value (P cond ). The condensing measurement setpoint is determined as the condensing pressure setpoint (P cond.set ). The controller is configured to modulate the speed of the condenser fan to maintain the pressure measurement value (P cond ) at condensing measurement setpoint (P cond.set ) when the pressure measurement value (P cond ) is higher than said saturated pressure under the ambient temperature value plus at or around 5° F. (this temperature value is not limited to 5° F. and is thus adjustable). 
     Finally, in certain embodiments in which the pressure sensor is implemented, the method further comprises inactivating, by the controller, the condenser fan when the condensing pressure value (P cond ) is less than the corresponding saturated pressure value at the ambient air temperature value plus at or around 5° F. In certain other embodiments in which the pressure sensor is implemented, the method further comprises activating, by the controller, the condenser fan when the condensing pressure value (P cond ) is higher than said corresponding saturated pressure value at the ambient temperature value plus at or around 5° F. In certain embodiments in which the temperature sensor is implemented, the method further comprises activating, by the controller, the condenser fan when the condensing temperature value (T cond ) is higher than the ambient air temperature value plus at or around 5° F. In certain other embodiments in which the temperature sensor is implemented, the method further comprises inactivating, by the controller, the condenser fan when the condensing temperature value (T cond ) is less than the ambient air temperature value plus at or around 5° F. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of a prior art system. 
         FIG. 2  schematically illustrates a system embodying the principles of the disclosure in the air-cooled expansion valve based condenser fan system configuration. 
         FIG. 3  is a schematic diagram illustrating an alternative embodiment of  FIG. 2  of the disclosure in the air-cooled expansion valve based condenser fan system configuration. 
         FIG. 4  is a schematic diagram illustrating an additional alternative embodiment of  FIGS. 2 and 3  in the air-cooled expansion valve based condenser fan system configuration. 
         FIG. 5  is a schematic diagram illustrating a system embodying the principles of the disclosure in the condenser based fan system configuration. 
         FIG. 6  is a schematic diagram illustrating an alternate embodiment of  FIG. 5  of the disclosure in the condenser based fan system configuration. 
         FIG. 7  is a schematic diagram illustrating a system embodying the principles of the disclosure in the compressor based fan system configuration. 
         FIG. 8  is a schematic diagram illustrating an alternative embodiment of  FIG. 7  of the disclosure in the compressor based fan system configuration. 
       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. 
       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. 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. 
     
    
    
     DRAWINGS REFERENCE NUMERALS 
     Prior Art 
     
         
           100  Prior Art Condenser Fan System 
           106  Prior Art Condenser 
           112  Prior Art Pressure Sensor 
           114  Prior Art Expansion Valve 
           116  Prior Art Evaporator 
           118  Prior Art Compressor I 
           119  Prior Art Compressor II 
           120  Prior Art Condenser Fan 
           122  Prior Art Controller 
           124  Thermal Bulb 
           140  Prior Art Variable Frequency Drive (VFD) 
         *********************************** 
           200  Air-cooled Expansion Valve based Condenser Fan System Configuration I 
           300  Air-cooled Expansion Valve based Condenser Fan System Configuration II 
           400  Air-cooled Expansion Valve based Condenser Fan System Configuration III 
           500  Condenser based Condenser Fan System Configuration I 
           600  Condenser based Condenser Fan System Configuration II 
           700  Compressor based Condenser Fan System Configuration I 
           800  Compressor based Condenser Fan System Configuration II 
           206  Condenser 
           212  How Meter I 
           214  Expansion Valve 
           216  Evaporator 
           218  Compressor I 
           219  Compressor II 
           220  Condenser Fan 
           222  Controller 
           224  Thermal Bulb 
           312  Flow Meter II 
           314  Pressure Sensor I 
           324  Pressure Sensor II 
           428  Temperature Sensor II 
           430  Temperature Sensor III 
           532  Temperature Sensor IV 
           534  Pressure Sensor III 
           632  Temperature Sensor V 
           634  Temperature Sensor VI 
           708  Compressor Status and Speed Device I 
           738  Pressure Sensor IV 
           808  Compressor Status and Speed Device II 
           838  Temperature Sensor VI 
       
    
     DETAILED DESCRIPTION 
     Seven possible embodiments of the condenser fan control system are provided herein. All of the possible configurations shown in the described embodiments use the same existing refrigeration system. The existing components that are the same are thus labeled identically in all of the figures. Components that are not labeled identically in the figures are components that are different in each embodiment (such as the added temperature or pressure sensors). The existing refrigeration system can be any already installed refrigeration system. In the embodiments illustrated in the figures, air-cooled condensers with condenser fans are employed. It should be noted however that in other embodiments not illustrated herein, the heating, ventilating, and air-conditioning (HVAC) system may instead be equipped with water-cooled condenser(s) instead of air-cooled condensers. In such embodiments, control valves that open and close to control a water flow rate are used. 
     The existing refrigeration system shown in  FIGS. 2-4  is comprised of compressor I  218  and II  219 , condenser  206 , flow meter  212 , expansion valve  214 , condenser fan  220 , thermal bulb  224 , and VFD II  240  connected in series in a typical refrigerant loop. The existing refrigeration system shown in  FIGS. 5-6  is comprised of evaporator  216 , compressor I  218  and II  219 , condenser  206 , expansion valve  214 , condenser fan  220 , thermal bulb  224 , and VFD II  240  connected in series in a typical refrigerant loop. The existing refrigeration system shown in  FIGS. 7-8  is comprised of condenser  206 , expansion valve  214 , evaporator  216 , compressor I  218  and II  219 , condenser fan  220 , thermal bulb  224 , and VFD II  240  connected in series in a typical refrigerant loop. In addition,  FIGS. 7 &amp; 8  also include compressor speed and status device I  708  (for  FIG. 7 ), or II  808  (for  FIG. 8 ) which can be implemented in or be an already existing component of the refrigerant loop. It should also be noted that the refrigeration system shown in all of the figures is representative of an existing refrigeration system and is not limited to the stated components and could include more or fewer components than those mentioned herein. For example, in embodiments not illustrated, the existing refrigeration system may have only one compressor, or more than two. In all of the embodiments and figures described, the compressor(s) employed can be of any type. Thus, for example, in some embodiments the compressors may be single stage constant speed compressors, while in other embodiments they may be multiple speed/multiple stage compressors or variable capacity/variable speed compressors. In all of the embodiments disclosed herein, as in the prior art, the thermal bulb is configured in connection with and functions to open and close the expansion valve to control the amount of refrigerant that flows into the evaporator. 
     The purpose of the refrigeration system is to remove heat from an area that is low temperature into an area that is high temperature and thus cool a space. As is typical in refrigerant systems such as the one shown in the figures, cold refrigerant flows through evaporator  216  where it cools a space by absorbing heat. This heat is then transferred to condenser  206  via compressors I  218  and II  219 . Compressors I  218  and II  219  have a suction line that pulls the low pressure and low temperature refrigerant from evaporator  216 , and includes a discharge line to compress the refrigerant into a high temperature and high pressure vapor which is then pushed into condenser  206 . In condenser  206 , the heat is removed and the vapor cools into a liquid. Condenser fan  220  is located in the middle of condenser  206  and blows outdoor air across the coils of condenser  206  to cool and condense the refrigerant. 
     It should be noted that while the refrigeration system shown in the Figs. herein has multiple compressors ( 218  and  219 ), in other embodiments not illustrated the refrigeration system may only have one compressor. Expansion valve  214  is installed in the liquid line between condenser  206  and evaporator  216  and functions to reduce the high pressure, high temperature refrigerant from the condenser line into a low pressure and low temperature refrigerant that flows into evaporator  216 . The refrigerant then proceeds along the flow line through evaporator  216  where the liquid is heated and becomes a vapor. Variable frequency drive (VFD)  240  is configured to drive condenser fan  220 . The VFD is an optional component, however. In some embodiments, VFD  240  is not included. In other embodiments, such as those shown in  FIGS. 7 and 8  of the drawings illustrated herein, a compressor status and speed device is configured within the refrigerant loop that functions to send to controller  222  a signal relaying the speed of the compressor(s) or the compressor status (indicating the number of compressors in the refrigeration system that are currently in operation). 
     In the first three embodiments shown in  FIGS. 2-4 , flow meter  212  is also included as part of the existing refrigeration system (systems  200 - 400 ). Flow meter  212  may be a component of the already existing refrigeration system or can be installed in the existing refrigeration system along with controller  222  and the pressure and/or temperature sensors. Flow meter  212  is configured with controller  222  and measures the flow rate of the refrigerant flowing through the existing HVAC system. It should be noted that other components can be used in place of flow meter  212 . For example, in some embodiments a VFD can be implemented instead of flow meter  212  to measure and send to the controller the refrigerant flow rates of the refrigeration system. 
     The first three embodiments of the system (shown in the Figs. by systems  200 ,  300 , and  400  in  FIGS. 2-4 ) are called the expansion valve based configurations because the expansion valve of the HVAC system is either directly or indirectly controlled at a maximum open position. Keeping the expansion valve at its maximum open position functions to both increase the efficiency and reduce the power of the compressor. System configurations  200 ,  300 , and  400  are thus most suitable for use in heating, ventilation, and air-conditioning (HVAC) refrigeration systems such as that illustrated in the figures as well as in rooftop units, split units, or CRAC (computer room air conditioning) units. 
     In the first embodiment of the disclosure, herein referred to as expansion valve based configuration I  200  and shown in  FIG. 2  of the drawings, the condenser fan is controlled to directly maintain the position of expansion valve  214  at a position close to the maximum. This position is measured or given by controller  222 . In the method of the embodiment, controller  222  is configured within the existing refrigeration system in connection with expansion valve  214  and is configured to collect signal information on the valve position of the expansion valve. In the figure shown, the existing refrigeration system comprises flow meter  212 , evaporator  216 , condenser  206 , condenser fan  220 , compressors I  218  and II  219 , expansion valve  214 , thermal bulb  224 , and variable frequency drive (VFD) II  240  (this VFD is used to control condenser fan  220 ). 
     Controller  222  is implemented so that it is configured in communication with flow meter  212 , VFD II  240  and compressors I  218  and II  219  of the existing refrigeration system. Controller  222  is pre-programmed with a critical refrigerant flow rate parameter. It is also configured with an expansion valve setpoint position. The valve is at a nearly fully open position at this setpoint (generally 95% open, but this rate is adjustable). Controller  222  compares the measured flow rate (sensed by flow meter  212 ) with the pre-programmed critical flow rate parameter. This comparison determines the on/off status of condenser fan  220 . If the refrigerant flow rate is less than the critical flow rate, controller  222  is configured to command condenser  220  off. If the refrigerant flow rate is greater than the critical flow rate, controller  222  is configured to activate condenser fan  220  and then modulate the speed of the fan so that the position of expansion valve  214  is maintained at the pre-programmed setpoint position. 
     In the second and third configurations shown in the embodiments illustrated in  FIGS. 3 and 4  (systems  300  and  400 ), the position of expansion valve  214  is indirectly maintained at the maximum open valve position. In the method for the configuration shown in  FIG. 3 , controller controls the speed of condenser fan  220  to maintain a condensing pressure measurement at a plurality of setpoint values based on a comparison of the system refrigerant flow rate with a critical flow rate. In the method for the configuration shown in  FIG. 4 , the controller controls the speed of condenser fan  220  to maintain a condensing temperature measurement at a plurality of setpoint values based on a comparison of the refrigerant flow rate with the critical flow rate. 
     In the second embodiment, herein referred to as expansion valve based configuration II  300  and shown in  FIG. 3  of the figures, controller  222  measures the pressure across expansion valve  214  of the refrigeration system. As in the first configuration (system  200 ), controller  222  is configured in communication with an existing refrigeration system comprised of compressors  218  and  219 , condenser fan  220  (in condenser  206 ), evaporator  216 , expansion valve  214 , thermal bulb  224  and VFD II  240 . Flow meter  312  is also configured in the HVAC system in connection with controller  222 . In some embodiments the flow meter can be installed into the existing refrigeration system while in others it may be a part of the already existing refrigeration system. 
     Controller  222  is configured to receive a signal indicating the valve position of expansion valve  214  of the existing system. At least one pressure sensor is also implemented in the existing system at the inlet of the expansion valve to sense the pressure in the liquid line. In  FIG. 2 , pressures sensors  314  and  324  are provided at the inlet and outlet of expansion valve  214 . They are configured to measure the pressure across expansion valve  214  and to send the measured pressure value (P cond ) to controller  222 . Flow meter  312  is configured in the outlet pipeline of condenser  206 , however it is not limited to this location. Flow meter  312  is configured to measure the flow rate of the refrigerant in the liquid pipeline and to send this information to controller  222 . 
     Once implemented in the refrigeration system, controller  222  is programmed with measurements obtained from the system including a critical flow rate value, a saturated pressure at the evaporative temperature (P evaporator ) from evaporator  216 , and a sum of the pressure loss from expansion valve  214  and the pressure loss from the liquid line of the HVAC system at the critical flow rate (ΔP). Using the measured flow rate obtained from flow meter  312  and the pre-programmed critical flow rate value, controller  222  is able to calculate for a refrigerant flow ratio (ω). This ratio (ω) is obtained by dividing the refrigerant flow rate over the design flow rate. 
     Based on the information, controller  222  can then calculate for the liquid condensing pressure setpoint of condenser  206 , defined herein as (P set ), using the following equation: 
         P   set   =P   evaporator +ω 2   ΔP  
 
     Wherein: 
     P set  represents the optimal condensing pressure set point (also called the optimal head pressure set point) of condenser  206 .
 
P evaporator  is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed).
 
ω is the relative flow rate of the refrigerant. It is a ratio of the refrigerant flow rate over the design flow rate.
 
ΔP is the sum of the pressure loss of expansion valve  214  and the pressure loss in the liquid line under the design flow rate conditions.
 
     Based on the calculated condensing pressure setpoint (P set ), controller  222  controls expansion valve  214  so that it is maintained at the maximum open position and thus ensures the lowest head pressure. Controller  222  then compares the refrigerant flow rate with the critical flow rate (the critical value is generally 20%, but this value depends on the system). If the refrigerant flow rate is less than the critical flow rate minus a control band, controller  222  keeps condenser fan  220  off. 
     If the refrigerant flow rate value is higher than the critical flow rate value, controller  222  is programmed to activate condenser fan  220 . When condenser fan  220  is activated, controller  222  modulates the speed of fan  220  to maintain the condensing pressure (P cond ) at the condensing pressure setpoint (P set ). The particular method in which controller  222  maintains the setpoint depends on the configuration. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller  222  activates or inactivates condenser fan  220  to maintain the setpoint. In embodiments in which water-cooled condensers are employed, controller  222  can open/close a cooling liquid control valve to maintain the calculated set point. In embodiments in which constant speed fans and/or two position control valves are employed, controller  222  can control the fans and/or the position of the valve so that the set point value is maintained. Finally, in embodiments in which at least one VFD is employed to control the speed of condenser fan  220 , controller  222  can maintain the setpoint by controlling the speed of the condenser fan at the desired rate. 
     A third embodiment of the invention, herein referred to as expansion valve based configuration III  400  and shown in  FIG. 4  of the figures, is similar to expansion valve based configuration II  300  shown in  FIG. 3  of the figures. As in the embodiment illustrated in  FIG. 3 , controller  222  is configured into an existing refrigeration system comprised of compressors I  218  and II  219 , condenser fan  220  (in condenser  206 ), evaporator  216 , expansion valve  214 , flow meter  212 , thermal bulb  224 , and VFD  240 . The difference between the embodiment illustrated in  FIG. 3  and that shown in  FIG. 4  is that in the third embodiment (configuration III  400 ), temperature sensor II  428  and III  430  are configured in communication with controller  222  rather than pressure sensor I  314  and II  324 . While there are two temperature sensors illustrated in the Figs., in other embodiments only one temperature sensor is required (in order to collect a condensing temperature value (T cond )). In  FIG. 4 , the two temperature sensors are installed within the refrigeration system at the inlet and outlet of expansion valve  214 . Temperature sensors  428  and  430  measure the temperature at the inlet and outlet of expansion valve  214  and send the collected temperature information to controller  222 . 
     Like in the art prior configuration, once it is implemented in the refrigeration system, controller  222  is programmed with measurements obtained from the system including a critical flow rate value, a saturated pressure corresponding to the evaporative temperature (P evaporator ) from evaporator  216 , and a sum of a pressure loss from expansion valve  214  and a pressure loss from the liquid line at the critical flow rate (ΔP). Using the measured flow rate obtained from flow meter  212 , and the pre-programmed critical flow rate value, controller  222  calculates for a refrigerant flow ratio (ω) by dividing the refrigerant flow rate over the design flow rate. 
     Based on the information, controller  222  can then calculate for the condensing liquid pressure setpoint of condenser  206 , defined herein as P (set), using the following equation: 
         P   set   =P   evaporator +ω 2   ΔP  
 
     Wherein: 
     P set  represents the condensing liquid pressure set point of condenser  206 .
 
P evaporator  is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed).
 
ω is representative of the relative flow rate of the refrigerant. It is a ratio of the refrigerant flow rate over the design flow rate.
 
ΔP is the sum of the pressure loss of expansion valve  214  and the pressure loss in the liquid line under the design flow rate conditions.
 
In configuration III  400 , an additional step can be taken to find the condensing liquid temperature setpoint (T set ). The condensing liquid temperature setpoint (T set ) is the saturated temperature of the refrigerant at the condensing liquid pressure set point (P set ), the condensing subcooled liquid temperature setpoint can be found by subtracting the refrigerant sub-cooled temperature from the condensing liquid temperature set point (T set ). In applications like configuration III  400 , the subcooled temperature correction is not necessary if the temperature sensor measures the refrigerant temperature inside the condenser.
 
     Based on the calculated liquid condensing temperature setpoint (T set ), controller  222  indirectly controls expansion valve  214  so that it is maintained at the maximum open position and can thus ensure that the lowest head pressure is obtained. Controller  222  then compares the refrigerant flow rate with the critical flow rate (the critical value is generally 20%, but this value depends on the system in which controller  222  is employed). If the refrigerant flow rate is less than the critical flow rate minus a control band, controller  222  keeps condenser fan  220  off. If the refrigerant flow rate is higher than the critical flow rate, controller  222  is programmed to activate condenser fan  220 . When condenser fan  220  is activated, controller  222  modulates the speed of the fan to maintain the liquid condensing temperature (T cond ) at the liquid condensing temperature setpoint (T set ). The particular method in which controller  222  maintains the liquid condensing temperature setpoint (T set ) depends on the configuration. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller  222  activates or inactivates condenser fan  220  to maintain the setpoint value. In embodiments in which water-cooled condensers are employed, controller  222  can open/close the cooling liquid control valve to maintain the calculated set point value. In embodiments in which constant speed fans and/or two position control valves are employed, controller  222  can control the fans and/or the position of the valve so that the set point value is maintained. In embodiments in which at least one variable speed drive is employed to control the speed of condenser fan  220 , controller  222  can maintain the setpoint by controlling the speed of condenser fan  220  at the desired rate. 
     The systems shown in  FIGS. 5 and 6  (systems  500  and  600 ) are referred to herein as the Condenser Based Condenser Fan System Configurations. The following summarizes these embodiments. In a fourth embodiment of the invention (shown in  FIG. 5  and system  500 ), controller  222  is configured to determine the pressure setpoint of condenser  206  based on measurements of the condensing pressure and ambient air temperature. These measurements are used to find the load ratio (β) and condensing pressure setpoint. The pressure setpoint is maintained by modulating the fan speed and/or the condenser fan on or off based on a comparison of the measured condensing pressure value and the condensing pressure setpoint. Condenser fan  220  is programmed to inactivate if the pressure of condenser  206  is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this amount is adjustable so can be at or around this value). In a fifth embodiment of the invention (as shown in  FIG. 6  by system  600 ), controller  222  is configured to determine the temperature setpoint of the condenser based on variables pre-programmed into controller  222  as well as the condenser and ambient temperature measurements. These measurements are used to find the load ratio and condensing liquid temperature setpoint. The temperature setpoint is maintained by modulating the condenser fan on/off, and/or by modulating the fan speed based on a comparison of the measured condensing temperature value and the condensing temperature setpoint. In this embodiment, the condensing temperature setpoint is the temperature value that corresponds to the pressure at the condensing pressure setpoint. Condenser fan  220  is programmed to inactivate if the pressure of condenser  206  is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this amount is adjustable). The configurations shown in the fourth ( FIG. 5 ) and fifth ( FIG. 6 ) embodiments (systems  500  and  600 ) are thus particularly suitable for implementation in split HVAC units, computer room air conditioning (CRAC) units, and industrial and commercial refrigeration systems. 
     The following describes the fourth embodiment in more detail. In the fourth embodiment, illustrated in  FIG. 5 , Controller  222 , ambient air temperature sensor IV  532 , and pressure sensor IV  534  are implemented in the existing refrigeration system. The existing refrigeration system in this embodiment comprises condenser  206 , expansion valve  214 , evaporator  216 , compressor I  218  and II  219 , condenser fan  220 , thermal bulb  224 , and VFD II  240 . Also, as in the other embodiments, this refrigeration system is an example of a refrigeration system in which the invention could be implemented and so is not limited to the parts described herein and may thus include fewer or additional components. 
     Once implemented in the refrigeration system, controller  222  is programmed with the system variables obtained from the refrigeration system. These variables are a condenser fan speed ratio (ω), a ratio of a speed of said condenser fan over a design speed of said condenser fan; a design condenser split temperature value from said refrigeration system (ΔT), an on/off heat transfer coefficient ratio of said condenser fan (li, a suction pressure value (P suc ) from the evaporator, and a sum of the pressure loss value across the liquid pipeline, suction pipeline, and the expansion valve of the refrigeration system under the design flow rate (ΔP d ). 
     Pressure sensor III  534  is configured in the liquid line of the existing refrigeration system in the pipe at the outlet of condenser  206  and measures the pressure of the refrigerant in the liquid line after it leaves the condenser. Pressure sensor III  534  is configured in communication with and sends the collected pressure measurements to controller  222 . Temperature sensor IV  532  measures the temperature of the ambient air and is configured in communication with and operable to send to controller  222  the collected ambient air temperature measurements. 
     The following details the method for finding the pressure setpoint of the system in condenser based fan system configuration I  500  ( FIG. 5  of the drawings). In the method, the system cooling load ratio (β) of condenser  206  is first calculated by controller  222  based on the collected temperature and pressure measurements. Then, the load ratio is employed in an algorithm to determine the liquid pressure setpoint of the refrigerant in condensor  206  (P cond,set ). The measured variables are the speed of condenser fan  220 , the ambient air temperature (obtained from temperature sensor IV  532 ), and the condensing pressure (P cond ) as measured by pressure sensor III  534 . 
     In the method for the embodiment, controller  222  receives ambient temperature measurements from temperature sensor IV  532  and measurements of the refrigerant pressure from pressure sensor III  534 . In some embodiments (not illustrated in the figures), controller  222  may also obtain the temperature of the refrigerant inside condenser  206  from a temperature sensor located in the condenser. Using the following variables such as the preprogrammed fan speed ratio (ω), and heat transfer coefficient ratio (α), controller  222  calculates for the condenser load ratio (β): 
     
       
         
           
             β 
             = 
             
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                             T 
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                       0.1 
                     
                   
                 
                 
                   
                     
                       
                         
                           
                             T 
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                             T 
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                           Δ 
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     Wherein: 
     β—condenser load ratio
 
ΔT—design condenser split temperature (from the existing rooftop unit)
 
ω—condenser fan speed ratio (ratio of the fan speed over the design speed)
 
α—the fan on/off heat transfer coefficient ratio
 
T cond —condensing temperature. (This variable is measured directly by temperature sensor IV  634  in condenser based configuration II  600  ( FIG. 6 ). It can be determined by finding the saturated refrigerant temperature at the measured head pressure for condenser based configuration I  500  ( FIG. 5 )).
 
     In the embodiment shown in configuration  1   500 , once the condenser load ratio (β) is found, it can then be used to find the condensing pressure setpoint of the refrigerant in the condenser (P cond,set ). Controller  222  uses the following algorithm to calculate for the condensing pressure set point: 
         P   cond,set   =P   suc +β 2   ΔP   d  
 
     Wherein: 
     P cond,set —The liquid pressure set point of the refrigerant in the condenser
 
P suc —The suction pressure. This is defined as the saturation pressure that corresponds to the evaporating temperature.
 
ΔP d —The sum of the pressure loss across the expansion valve, liquid pipe line, and suction pipe line under the design refrigerant flow rate.
 
β—The condenser load ratio
 
     If the condensing pressure measurement is less than the corresponding saturated pressure at the ambient temperature plus 5° F. (this temperature value is adjustable), controller  222  is programmed to disable the operation of condenser fan  220 . If the condensing pressure measurement (P cond ) is higher than the corresponding saturated pressure value at said ambient temperature value plus 5° F. (this temperature value is adjustable and not limited to 5° F.), controller  222  is programmed to activate the condenser fan. Controller  222  is configured to modulate the speed of condenser fan  220  to maintain the condensing pressure measurement (P cond ) at the condensing pressure setpoint (P cond,set ) when the condensing pressure measurement is greater than the corresponding saturated pressure value at said ambient temperature value plus 5° F. (adjustable). 
     In the embodiment shown in  FIG. 6  and described by system  600 , ambient air temperature measurements, condensing temperature measurements, as well as the variables already pre-programmed in Controller  222  (such as the fan speed ratio and heat transfer coefficient ratio), are used to find a condensing pressure setpoint (P cond,set ) and a condensing temperature setpoint (T cond,set ) which can be used to maintain a condensing temperature value at a plurality of condensing temperature set points. Controller  222  can solve for the load ratio (β) as shown above in the description for system  500 ). This load ratio (β) is used to determine the condensing pressure setpoint (P cond,set ) using the same method and equation shown in the description for the fourth embodiment (system  400 ). Once the condenser load ratio (β) is calculated, controller  222  is programmed to calculate for the condensing pressure setpoint (P cond,set ) as described in the embodiment for system  500 . In the embodiment for system  600 , however, the condensing temperature set point must be found once the condensing pressure setpoint is known. Controller  222  is programmed to find the temperature setpoint of the liquid refrigerant (T cond,set ) as the saturated temperature under the condensing pressure setpoint (P cond,set ). 
     If the condensing temperature measurement (T cond ) is less than the ambient temperature plus 5° F. (this temperature value is adjustable), controller  222  is programmed to disable the operation of condenser fan  220 . If the condensing temperature measurement (T cond ) is higher than the ambient temperature value plus 5° F. (this temperature value is adjustable), controller  222  is programmed to activate the condenser fan. Controller  222  is configured to modulate the speed of condenser fan  220  to maintain the condensing temperature measurement (T cond ) at the condensing temperature setpoint (T cond set ) when the condensing temperature measurement is higher than the ambient temperature value plus 5° F. (this temperature value is adjustable). 
     Thus, in both systems  500  and  600  ( FIGS. 5 and 6 ), controller  222  is configured to modulate condenser fan  220  and/or (in the case of a water-cooled condenser) modulate a control valve to maintain the set points. In the fourth embodiment (system  500 ), this setpoint is the condensing pressure setpoint. In the fifth embodiment (system  600 ), this setpoint is the condensing temperature setpoint. 
     In a sixth and seventh embodiment of the disclosure, shown in  FIGS. 7 and 8  of the drawings, controller  222  is also configured to control the condenser fan of the HVAC system to maintain the condensing temperature or condensing pressure at a plurality of condensing liquid temperature setpoint (T set ) or condensing liquid pressure setpoints (P set ). The difference is that in this configuration the speed of compressors I  118  and II  119  is used to determine the set points used to control the condenser fan (in systems with air-cooled condensers) or the position of the condenser control valve (in systems equipped with water-cooled condensers not illustrated in the Figs.). As such, the sixth and seventh embodiments are referred to herein as the Compressor Based Condenser Fan System Configurations. The method for these embodiments is similar to the method in the second and third embodiments. The main difference is that in these embodiments the speed of the compressors or the compressor status (the number of compressors in operation) is used to determine the refrigerant flow rate instead of a flow meter. In the Figs., the compressor speed/status is measured by a compressor speed and status device. Data collected from the device is used to determine the liquid condensing pressure setpoint (P set ). 
     In the embodiment illustrated in  FIGS. 7 and 8  (systems  700  and  800 ), controller  222  is implemented in the existing refrigeration system comprising condenser  206 , expansion valve  214 , evaporator  216 , compressors I  218  and II  219 , condenser fan  220 , thermal bulb  224 , VFD II  240 . If not already a part of the existing refrigeration system, compressor status &amp; speed device I  708  (when using the configuration illustrated in  FIG. 7 ) or II  808  (when using the configuration illustrated in  FIG. 8 ) is installed to measure the speed of compressors I  218  and II  219 . There are no differences between compressor status &amp; speed device I  708  and II  808 . The difference in the numbering of the Figs. is merely for illustrative purposes to show that a compressor status and speed device is intended to be included in the embodiments shown in  FIGS. 7 and 8  but not in the other described embodiments. Compressor status &amp; speed device I  708  and II  808  are configured to receive a signal of the compressor speed or status and send to controller  222  a compressor speed ratio (ω) that controller  222  uses to calculate for the liquid condensing pressure set point (P set ). The compressor speed ratio (ω) for compressor based condenser valve configurations I and II  700  and  800  is defined as the ratio of the speed of the compressor over a design speed of the compressor. As an alternative to the compressor speed ratio (ω), in other embodiments of compressor based condenser valve configurations I and II  700  and  800 , the controller can instead be configured to find a compressor status ratio where (ω) represents the ratio of the number of compressors in the HVAC system that are in active operation over the total number of compressors (in the HVAC system). 
     Controller  222  is also programmed with a plurality of variables for the HVAC system in which it is installed. These variables comprise a design refrigerant flow rate, the compressor speed ratio (ω) from compressor status &amp; speed devices I  708  and II  808  when using the configuration illustrated in  FIG. 8 ), a measurement of the saturated pressure corresponding to the evaporative temperature (P evaporator ) from evaporator  216 , and a sum of a pressure loss from expansion valve  214  and the liquid and suction lines of the HVAC system under the design flow rate conditions (ΔP). Controller  222  then calculates for a condensing pressure set point (P set ) based on the selected ratio (ω), (either the compressor speed or compressor status), a measurement of the saturated pressure corresponding to the evaporative temperature (P evaporator ) from the evaporator of the HVAC system, and a sum of a pressure loss from the expansion valve and liquid and suction lines of the HVAC system under the design flow rate conditions (ΔP). Controller  222  calculates for the condensing pressure set point (P set ) using the following equation: 
         P   set   =P   evaporator +ω 2   ΔP  
 
     Wherein: 
     P set  represents the liquid pressure set point, or the optimal pressure set point of condenser  206 .
 
P evaporator  is the saturated pressure corresponding to the evaporative temperature (for most HVAC applications this temperature is 40° F., however this is adjustable depending on the type of system employed).
 
ω is representative of the compressor speed ratio. In some embodiments (like those shown in  FIGS. 7 and 8 , it can be defined as the relative speed of the compressor/the design speed of the compressor. In other embodiments it is defined as the compressor operating status ratio (the number of compressors in operation over the total number of compressors in the HVAC system). In some embodiments, like those shown in  FIGS. 2 &amp; 3 , it is representative of the refrigerant flow rate over the design flow rate.
 
ΔP is the sum of the pressure loss of expansion valve  214  and the liquid line under the design flow rate conditions.
 
     The controller is configured to activate condenser fan  220  when the flow rate is higher than the critical flow rate. In the same way, condenser fan  220  is deactivated when the refrigerant flow rate is lower than the critical refrigerant flow rate minus a control band. The critical flow rate value is generally 20%, but this value depends on the system in which controller  222  is employed). 
     Compressor based configurations I &amp; II  700  &amp;  800  are very similar but use different sensors. This difference results in differences in the method of controlling condenser fan  220  to maintain the liquid condensing pressure setpoint (P set ). In condenser based compressor fan speed configuration I (system  700 ), pressure sensor VI  738  is configured at the outlet and measures the pressure of the refrigerant exiting condenser  206 . The pressure sensor then sends the collected pressure measurement (P cond ) to controller  222 . When the refrigerant flow rate in the HVAC system is higher than the critical flow rate, Controller  222  is configured to modulate the speed of the condenser fan so that the condensing pressure measurement (P cond ) is maintained at the condensing liquid pressure setpoint (P set ), which can be a plurality of values. 
     In compressor based fan speed configuration II (system  800  as shown in  FIG. 8 ), temperature sensor VI  838  is configured in the outlet pipes of condenser  206  of the existing refrigeration system. The temperature sensor measures the temperature of the refrigerant exiting condenser  206  and sends the collected temperature measurement (T cond ) to controller  222 . Controller  222  then uses the calculated liquid condensing pressure setpoint (P set ), to calculate for a liquid condensing temperature setpoint (T set ). The (T set ) value is found as the saturated temperature of the refrigerant under the condensing pressure setpoint (P set ). If the temperature sensor does not directly measure the saturated temperature inside of the condenser, the condensing temperature setpoint needs to be corrected. This correction is achieved by subtracting the condensing temperature setpoint minus a subcooling temperature. When the refrigerant flow rate is higher than the critical refrigerant flow rate, controller  222  is configured to modulate the speed of condenser fan  220  so that the condensing temperature measurement (T cond ) is maintained at the condensing liquid temperature setpoint (T set ), which can be a plurality of values. 
     In summary, the particular method in which controller  222  maintains the liquid pressure setpoint (P set ) (in configurations like the one shown in  FIG. 7 ) or the liquid condensing temperature setpoint (T set ) (in configurations like the one shown in  FIG. 8 ) depends on the configuration of the HVAC (heating, ventilating, and air-conditioning) system in which controller  222  is implemented. In embodiments in which air-cooled condensers are employed, such as that shown in the illustrations, controller  222  activates or inactivates condenser fan  220  to maintain the setpoint value. In embodiments in which water-cooled condensers are employed, controller  222  can open/close the cooling liquid control valve to maintain the calculated set point value. In embodiments in which constant speed fans and/or two position control valves are employed, controller  222  can control the fans and/or the position of the valve so that the set point values are maintained. In embodiments in which at least one variable speed drive is employed to control the speed of condenser fan  220  (as shown in  FIGS. 7 and 8 ), controller  222  can maintain the setpoint by controlling the speed of condenser fan  220 . 
     The above-described features and advantages of the present disclosure thus improve upon aspects of those systems and methods in the prior art.