Patent Publication Number: US-2009217679-A1

Title: Refrigeration cooling system control

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is related to co-pending U.S. patent application Ser. No. 11/086,527 filed on Mar. 22, 2005 by Sridharan Raghavachari and entitled MULTIPLE COMPRESSOR CONTROL SYSTEM, the full disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Cooling systems are used in a variety of applications such as refrigeration systems and air-conditioning systems. Many cooling systems are energy inefficient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematically straight and of a refrigeration cooling system and control system according to an example embodiment. 
         FIG. 2  is a block diagram schematically illustrating control logic for the control system of  FIG. 1  according to an example embodiment. 
         FIGS. 3-10  are graphs comparing performance of a refrigeration cooling system not under control of the control system of  FIG. 1  with the performance of the refrigeration cooling system under the control of the control system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXAMPLE EMBODIMENTS 
       FIG. 1  schematically illustrates controlled cooling apparatus  20  according to one example embodiment. Apparatus  20  includes refrigeration cooling system  22  and control system  24 . As will be described hereafter, control system  24  controls various components of refrigeration cooling system  22  to enhance energy efficiency while satisfying cooling objectives for system  22 . 
     Refrigeration Cooling system  22  comprises an arrangement of compressors, condensers, evaporators, and pumps, etc configured to withdraw heat directly or indirectly from a cooled environment and to transmit the withdrawn heat to a remote environment and or atmosphere outside. In the example illustrated, refrigeration cooling system  22  comprises a two-stage cooling system including circulation system  28 , holding tank  30 , intermediate temperature evaporators  32 , intermediate stage gas suction tank  34 , low temperature in evaporators  38 , low stage gas suction tank  40 , low stage compressors  42 , high stage compressors  44  and condenser/s  46 . Circulation system  28  delivers or directs refrigerant between holding tank  30 , intermediate temperature evaporators  32 , intermediate stage gas suction tank  34 , low temperature evaporators  38 , low stage gas suction tank  40 , low stage compressors  42 , high stage compressors  44  and condenser  46 . Circulation system  28  includes piping system  50 , expansion valves  52 ,  53  and level maintenance valve  54 . Piping system  50  comprises headers, and piping, plenums and the like configured to direct the flow of refrigerant, whether in gaseous or liquid form. Piping system  50 , along with the other components of refrigeration cooling system  22 , form a closed circuit refrigerant cooling system in which refrigerant is contained as it is repeatedly compressed, condensed and expanded or evaporated to transfer or conduct heat from one or more cooling areas (in communication with evaporators  32 ,  38 ), where heat is absorbed, to condensers  46 , where heat is discharged. 
     Expansion valve  52  (schematically illustrated) comprises one or more expansion valves along conduit  50  between holding tank  30  and intermediate temperature evaporators  32 . Expansion valve  52 , when actuated or opened, permits liquid refrigerant to expand and flow across intermediate temperature evaporators  32 . Likewise, expansion valve  53  (schematically illustrated) comprises one or more expansion valves along conduit  50  between holding tank  30  and low temperature evaporators  38  and/or between intermediate stage gas suction tank  34  and low temperature evaporators  38 . Expansion valve  53 , when actuated or opened, permits liquid refrigerant to expand and flow across low temperature evaporators  38 . 
     Holding tank  30  comprises one or more tanks configured to store and contain liquid refrigerant. Holding tank  30  is supplied with liquid refrigerant after the refrigerant gas has been compressed and condensed. One example of a refrigerant includes ammonia gas. In other embodiments, other refrigerants may be utilized. 
     Intermediate temperature evaporators  32  comprise one or more coils, conduits or other structures configured to contain and direct the flow of liquid and refrigerant while facilitating the absorption of heat from the processes to be cooled ing or from the surrounding volume of in such a room to be cooled. Intermediate temperature evaporators  32  receive expanded refrigerant after it is passed across expansion valve  52 . In one embodiment, air from the room or other region to be cooled may be directed across the evaporators  32  using a fan. In other embodiments, evaporators  32  may be provided as part of other cooling arrangements. 
     Intermediate stage gas suction tank  34  comprises a tank or other container configured to collect and store and contain refrigerant from evaporators  32 . Most of such refrigerant collected from evaporators  32  may be in gaseous form. Such gaseous refrigerant is contained in tank  34  until taken up by compressors  44 . In the example illustrated, tank  34  also receives the gas refrigerant from the low stage gas compressors  42 . Tank  34  further contains and supplies liquid refrigerant to low temperature evaporators  38 . As noted above, level maintenance valve  54  maintains a predetermined level or amount of liquid refrigerant within tank  34  for supply to low temperature evaporators  38 . 
     Low temperature evaporators  38  comprise one or more coils, conduits or other structures configured to contain and direct the of refrigerant while facilitating the absorption of heat from the processes to be cooled or from the surrounding volume in such a room to be cooled by the ing. Low temperature evaporators  38  receive expanded refrigerant after it is passed across expansion valve  53 . In one embodiment, air from the room or other region to be cooled may be directed across the evaporators  38  using a fan. In other embodiments, evaporators  38  may be provided as part of other cooling arrangements. 
     Low stage gas suction tank  40  comprises a tank or other container configured to collect and to act as a buffer tank to dynamically store and contain refrigerant from evaporators  38  until such evaporated refrigerant is taken up by low stage compressors  42 . In the example illustrated, tank  40  includes a suction mechanism for drawing evaporated refrigerant from evaporators  38  and directing the refrigerant to compressors  42 . 
     Low stage compressors  42  comprise one or more compressors configured to receive gaseous refrigerant and to compress the gaseous refrigerant to higher pressure. Compressed refrigerant is discharged from low stage compressors to intermediate gas suction tank  34 . In one embodiment, low stage compressors  42  may comprise reciprocating, rotary screw, centrifugal, scroll or vane type compressors. Each compressor is specified load capacity and a specified maximum discharge pressure. The discharge pressures of compressors  42  are adjustable within some range up to the specified maximum discharge pressure. In another embodiment, one or more of the compressors  42  have a fixed discharge pressure. In one embodiment, compresses  42  have controllable slide valves for adjusting an inlet volume of such compressors. Prime movers for such compressors  42  may be driven by electricity, fossil or other fuels, or steam, for example. Compressors  42  may comprise any combination of types, makes or models of compressors. 
     High stage compressors  44  are similar to low stage compressors  42  but are configured to compress gaseous refrigerant to a greater pressure level. High stage compressors  44  gaseous refrigerant from intermediate stage gas suction tank  34  and discharge compressed gaseous refrigerant to condenser/s  46 . Like compressors  42 , compressors  44  may comprise reciprocating, rotary screw, centrifugal, scroll or vane type compressors each compressor is specified by load (TR or Volume rate) capacity and a specified maximum discharge pressure. The discharge pressures of compressors  44  are adjustable within some range up to the specified maximum discharge pressure. In another embodiment, one or more of the compressors  44  have a fixed discharge pressure. Prime movers for such compressors  44  may be driven by electricity, fossil or other fuels, or steam, for example. Compressors  44  may comprise any combination of types, makes or models of compressors. 
     Condensers  46  comprise one more devices configured to receive compressed refrigerant gas and to extract heat from such refrigerant. In one embodiment, condenser  46  comprises one or more in parallel condenser coils through which the compressed refrigerant flows and from which heat is extracted. In one embodiment, condenser  46  may extract heat using one or more fans. In one embodiment, condenser  46  may comprise an evaporative condenser in which water showered upon the coils, wherein the water vaporizes and mixes with the ambient air. In this case, the latent heat of vaporization of the water is supplied by the hot refrigerant inside the condenser tubes. Air force on the outside of the evaporative condensers carries evaporated water vapor from the condenser surface to the ambient air. In another embodiment, condenser  46  may comprise a direct heat transfer condenser. In one embodiment, heat extraction may be performed by directing water across such coils, wherein the water is heated while extracting heat from the gas refrigerant surrounding the outside of the tubes. For example, in one embodiment, condenser  46  may include one or more water cooling towers. In other embodiments, other mechanism for devices may be utilized to extract heat from the refrigerant (cool and condense the compressed refrigerant). The condensed refrigerant is directed to the holding tank  30  via conduit  50 , ready to absorb heat once expanded across one or more of expansion valve  52 ,  53  and directed across evaporators  32 ,  38 . 
     Control system  24  comprises a system or arrangement of sensors and one or more controllers that are configured to monitor cooling demands and various parameters of refrigerant cooling system  22  and the environment of cooling system  22 . In particular, control system  24  is configured to receive and store various analog (pressures, temperatures, flows etc. and digital signals (compressor on/off etc.) and manually in put data (such as compressor parameters, temperature set points etc. Control system  24  is programmed to compute dynamically the total enthalpy of circulating liquid refrigerant of the cooling system and a rate of change of the enthalpy of the evaporated refrigerant gas contained in cooling system  22 . Based upon such values, control system  24  adjusts the operating parameters of cooling system  22  to reliably satisfy cooling demands while enhancing energy efficiency. In one embodiment, cooling system  24  controls the loading and unloading of compressors  42  and  44  to satisfy cooling demands while enhancing energy efficiency. In other embodiments, cooling systems  24  may control and adjust other operating parameters of cooling system  22  as well. 
     Control system  24  generally includes pressure transmitters  60 ,  62  and  63 , temperature transmitters  64 ,  66 ,  68 ,  70 ,  72 ,  74  and, flow transmitters  78 ,  80 ,  82  and  84 , wet bulb temperature transmitter  88 , dry bulb temperature transmitter  90 , variable frequency drive  92  and controller  94 . Pressure transmitters  60 ,  62  and  63  comprise devices configured to sense pressure of refrigerant. Transmitter  60  is retrofitted on the low stage gas suction tank  40  and senses and detects the pressure of gaseous refrigerant in tank  40 . Transmitter  62  is retrofitted on the intermediate stage gas suction tank  34  and senses the pressure of gaseous refrigerant in tank  34 . Pressure transmitter  3  is retrofitted or otherwise connected to the inlet side of holding tank  30  and is configured to sense or detect the pressure of condensation of holding tank  30 . 
     Temperature transmitters  64 ,  66 ,  68 ,  70 ,  72 ,  74  and comprise devices configured to sense and transmit temperatures of refrigerant. Transmitter  64  is retrofitted on a liquid outlet line of holding tank  30  and senses the temperature of the liquid refrigerant discharged from holding tank  30 . Transmitter  66  is retrofitted at an upstream side of expansion valve  53  and senses &amp; transmits the temperature of liquid refrigerant from holding tank  30  and from tank  34  prior to the liquid refrigerant passing through expansion valve  53 . Transmitter  68  is retrofitted on low stage gas suction tank  40  and senses the temperature of gaseous refrigerant in tank  40 . Transmitter  70  is retrofitted on intermediate stage gas suction tank  34 . Transmitter  72  is retrofitted to the water line/s to condenser/ 46  and senses the temperature of the inlet water being supplied to condenser/s  46 . Transmitter  74  is retrofitted to an outlet water line of condenser  46  and senses the temperature of the return or remaining water that has passed through condenser  46 . Transmitter  76  is retrofitted to holding tank  30  and senses the condensing temperature of the refrigerant in condenser/s  46  as well as the holding temperature of the refrigerant in tank  30 . 
     Flow transmitters  78 ,  80 ,  82  and  84  comprise the sensors configured to detect and transmit the volume/mass flow of the refrigerant liquid and or gas. Flow transmitter  78  is retrofitted or otherwise connected to the refrigerant liquid outlet line of holding tank  30  so as to detect and transmit the total flow of liquid refrigerant from holding tank  30 . Flow transmitter  80  is retrofitted or otherwise connected to an upstream or inlet side of expansion valve  53  so as to detect t and transmit the flow of liquid refrigerant through expansion valve  53  prior to expansion of such liquid refrigerant. Flow transmitter  82  is retrofitted and or connected to the water inlet line of condenser  46  and is configured to sense and transmit the flow of water to condenser  46 . Flow transmitter  84  is retrofitted or otherwise connected to the water outlet line of condenser  46  and is configured to sense and transmit the flow of water from condenser  46 . 
     Wet bulb temperature transmitter  88  comprises a sensor configured to sense and transmit a wet bulb temperature of ambient air proximate condenser  46 . Dry bulb temperature transmitter  90  comprises a sensor configured to measure and transmit a dry bulb temperature of ambient air proximate condenser  46 . Transmitters  88  and  90  enable controller  94  to adjust operation of cooling system  22  based upon the ambient conditions such as the temperature, humidity, etc of the air which may affect the ability of heat to be extracted from liquid refrigerant passing through condenser  46 . 
     Variable frequency drive  92  comprises a device associated with controller  94  that is configured to receive signals or data from the sensors or transmitters to a control system  24  and, based upon optimization algorithms and analysis performed by one or both of drive  92  or controller  94 , is further configured to transmit control signals that would selectively increase or decrease the volume of the refrigerant gas being compressed prior to condensation and accordingly load and or unload a selected one of compressors  42 ,  44  operating at a partial load (a trim compressor) at a variable frequency. In other embodiments, drive  92  may be incorporated into or as part of controller  94 . In still other embodiments, where the one or more trim compressors are variably controlled by adjusting controllable slide valves, drive  92  may be omitted. 
     Controller  94  comprises a processing unit configured to receive input or data from transmitters  64 - 90  as well as inputs from the human operators, and to generate control signals based upon such data directing the operation of compressors  42 ,  44  and condenser  46 . For purposes of this application, the term “processing unit” shall mean a presently developed or future developed processing unit that executes sequences of instructions. Execution of the sequences of instructions causes the processing unit to perform steps such as generating control signals. The instructions may be loaded in a random access memory (RAM) for execution by the processing unit from a read only memory (ROM), a mass storage device, or some other persistent storage. In other embodiments, hard wired circuitry may be used in place of or in combination with software instructions to implement the functions described. For example, controller  94  may be embodied as part of one or more application-specific integrated circuits (ASICs). Unless otherwise specifically noted, the controller is not limited to any specific combination of hardware circuitry and software, nor to any particular source for the instructions executed by the processing unit. 
     As shown by  FIG. 1 , in one embodiment, controller  94  may comprise a computer having a monitor  96 , a hard drive  97  and user input  98 . Monitor  96  provides one mechanism by which data or information may be communicated to a person. Hard drive  97  includes processing circuitry, memory and ports for portable memory reading and writing (disk drive, USB port, memory card reader and the like). User input  98  comprises a keyboard, mouse, microphone and associated speech recognition software, stylus, touch screen or other device configured to facilitate entry of information to controller  94 . In another embodiment, controller  94  may have other configurations or may be connected to a remote user interface such as via a network or the Internet. 
       FIG. 2  is a block diagram illustrating one example of control logic according to an example embodiment. As shown in block  300  of  FIG. 2 , drive  92  and controller  94  receives various analog inputs including low stage gas temperature from transmitter  68 , low stage gas pressure from transmitter  60 , intermediate or high stage gas temperature from transmitter  70 , intermediate or high stage gas pressure from transmitter  62 , refrigerant flow from flow transmitter  78 , refrigerant temperature from temperature transmitter  64 , refrigerant flow from transmitter  80 , refrigerant temperature from transmitter  66 , holding tank pressure and temperature from transmitter  63  and  85 , respectively, condenser water/air outflow from transmitters  84 , condenser water outlet temperature from transmitter  74 , condenser water/air inlet flow from transmitter  82 , condenser water/air inlet temperature from transmitter  72 , a wet bulb temperature from transmitter  88  and ambient dry bulk temperature from transmitter  90 . In addition, controller  94  may also receive inputs regarding the level of liquid refrigerant in holding tank  30  and tank  32 . 
     As shown in  FIG. 2 , block  301 , controller  94  additionally receives various operator inputs. For example, controller  94  may receive compressor information such as kW, ton refrigeration (TR) rating, service factor, start delay, rest delay and stop delay information for each compressor. Controller  94  may also receive information regarding the volumes in which gaseous and liquid refrigerant is contained. For example, controller  94  may receive information regarding the volume of various sections of segments of conduit  50  as well as the various tanks  30 ,  34  and  40  of cooling system  22 . Controller  94  may also receive information regarding the type or refrigerant used in various operational parameters such as a system set temperature and pressure for each stage. Operator input additionally includes minimum and/or maximum levels of liquid refrigerant in the various liquid holding tanks  30 ,  34 , internal size and geometry of the holding tanks, overriding set points and limits of the variable frequency drives  92 . Additional analog or operator input values may also be provided to controller  94  in other embodiments. 
       FIG. 2 , blocks  302 - 307  are performed by controller  94  for each stage of the cooling system. In the example illustrated, block  302 - 307  are performed by controller  94  (utilizing drive  92 ) for each of the low-temperature stage (area being cooled by low-temperature evaporators  38 ) and the intermediate temperature stage (the area being cooled by intermediate temperature in evaporators  32 ). As shown by block  302 , for each stage, controller  94  dynamically determines the instant thermal content or load (enthalpy), a dynamic rate of change of thermal load (rate of change of enthalpy), a response time and the immediate future thermal load (enthalpy). To determine the immediate future load or enthalpy for the low temperature stage, controller  94  utilizes the determined current enthalpy and the rate of change of enthalpy. To determine the response time (the time at which additional gaseous refrigerant must be compressed and condensed to refrigerant in order to meet the cooling demands at the particular stage or cooled area or the time at which the amount of gaseous refrigerant being compressed and condensed may be reduced while still satisfying the cooling demands at the particular stage or cooled area), controller  94  utilizes the current enthalpy for the particular stage, the immediate future enthalpy for the particular stage and the response times of the various available compressors for the particular stage. 
     As shown by block  303 , based upon the determined the instant thermal content or load (enthalpy), a dynamic rate of change of thermal load (rate of change of enthalpy), a response time and the immediate future thermal load (enthalpy), controller  94  selects a combination of compressors for the particular stage that together, have a total capacity, that will closely approximate, but generally not exceed, the immediate future thermal load. Such compressors (base compressors) are operated at full load. Controller  94  will also select one of the remaining compressors for the particular stage as a partially loaded or trim compressor. Only one compressor serves as a partially loaded compressor for each stage at any moment in time. The partial loading of the selected compressor may be enabled either by drive  92  or compressor&#39;s own volumetric control or a combination of both. 
     As indicated by blocks  304  and  305  in  FIG. 2 , once the full load compressors and the trim compressor are selected for each stage, controller  94  will generate control signals initiating the loading of such compressors based upon the determined response time which is in turn based upon the rate of change of the immediate future cooling load and the lead time of each of the selected full load and trim compressors. For example, if each of the selected full load and trim compressors must be started and loaded in one minute in order to match the thermal load requirements or demands for a particular stage given the determined immediate future thermal load, controller  94  will generate control signals initiating the loading to the selected full load and trim compressors at the appropriate time such that each compressor is loaded at approximately the one minute mark. For example, if one compressor has a response time of 20 seconds, controller  94  will initiate loading of the compressor in 40 seconds. If another of the selected compressors has a response time of 25 seconds, controller  94  will initiate loading of this compressor in 35 seconds. This process of selecting particular combinations of full load or base compressors and partial load or trim compressors for each stage is dynamically performed and repeated over time depending upon changes in the cooling load demands for the different areas being cooled by the different cooling stages. 
     As shown by blocks  304 ,  306  and  307  in  FIG. 2 , with respect to the selected partial load or trim compressor for each stage, controller  94  will vary the inlet volume of the trim compressor to satisfy the remaining cooling load that is not satisfied by the selected full load compressors. As shown by block  306 , in one embodiment, controller  94  generates control signals directing drive  92  to vary the frequency of the trim compressor. As indicated by block  307 , controller  94  may also, or alternatively, generate control signals to control the slide valve of the selected trim compressor to vary its discharge pressure. 
     As shown by block  303 , controller  94  may further adjust the operational parameters of condenser  46  which may permit controller  94  to further adjust the operation of the compressors to enhance energy efficiency. Likewise, controller  94  may adjust the inlet volume or discharge pressure of one or more the compressors to adjust to the condensing pressure in condenser  46 , which again is determined dynamically from the measured ambient wet bulb and dry bulb temperatures through transmitters  88  &amp;  90 . In addition, controller  94 , in some embodiments, may adjust the operational parameters of condenser  46 , such as by adjusting the number of fans or fan speed of condenser  46  which may allow controller  94  to also adjust the particular discharge pressure or inlet volume of one or more of the selected base &amp; trim compressors. By increasing the ability of condenser  46  to extract heat, such as by increasing the number of fans or increasing their speed, the discharge pressure of all the selected compressors may be lowered when the ambient conditions permit so while still satisfying the cooling load demands. In one embodiment, controller  94  controls the variable parameters of condenser  46  as well as the inlet volume or discharge pressure of one or more of the selected trim compressors for enhanced energy efficiency. In particular, based upon a known energy consumption of such fans and the known or determined differences in the amount of energy consumed by the compressor to operate at a different discharge pressures or set pressures, controller  94  may optimize the parameters of each. In other words, controller  94  may select a particular combination of condenser fans at selected speeds and may select a discharge pressure appointed for the compressor to optimize or at least enhance energy efficiency. 
     In addition to adjusting the inlet volume and or discharge pressure of one or more selected compressors based upon the controllable variables or parameters of condenser  46 , controller  94  may also adjust the inlet volume or discharge pressure of the one or more (transient only) selected trim compressor based upon environmental conditions which also impact the ability of condenser  46  to extract heat and condense the gaseous refrigerant. For example, in situations where cooling system  22  is in a location having a seasonal climate, the ability of condenser  46  to extract heat from the refrigerant may greatly vary depending upon ambient outside temperature and humidity. Based upon the detected outside temperature and humidity from transmitters  88 ,  90 , controller  94  adjusts the inlet volume or discharge pressure of the one or more selected trim compressors for enhanced energy efficiency. For example, in response to a more humid and/or warmer condensing environment, controller  94  may increase the discharge pressure of the selected compressors for a given cooling load. Alternatively, in response to a more dry and/or cooler condensing environment, controller  94  may lower the discharge pressure of one of more selected compressors for the same given heat load. 
     In the particular example illustrated, cooling system  22  includes two stages: a low temperature evaporator stage and an intermediate temperature evaporator stage. For the low temperature evaporator stage, controller  94  determines the instant thermal content or load (enthalpy), a dynamic rate of change of thermal load (rate of change of enthalpy), a response time and the immediate future thermal load (enthalpy) for the low stage. The enthalpy of the refrigerant gas is determined using the temperature and pressure of the refrigerant gas from transmitters  60  and  68  in conjunction with the input or determined volume containing the gas. In the example illustrated, gas refrigerant is contained in tank  40 , portions of conduit  50  from tank  40  to compressors  42 . 
     The enthalpy of the liquid refrigerant is determined using the flow lbs/min, and temperature of refrigerant (from flow transmitters  78   80  and temperature transmitters  64 ,  66 . The total enthalpy is the sum of the enthalpy of the gas refrigerant and the liquid refrigerant. In some embodiments, the total enthalpy may be estimated using just the enthalpy of the liquid refrigerant since the enthalpy of the gas refrigerant may comprise a small percentage of the total enthalpy. 
     To determine the enthalpy for the low temperature stage, controller  94  utilizes data from transmitters  66 ,  80 ,  60  and  68 . To determine the rate of change of enthalpy for the low temperature stage, controller  94  utilizes data from transmitters  60  and  68 . 
     To determine the enthalpy for the intermediate temperature stage, controller  94  utilizes data from transmitters  64 ,  78 ,  62 ,  70  as well as the determined volume of refrigerant gas in tank  34  (based upon a sensed level of liquid refrigerant and tank  34  and the known volume of tank  34  and open piping or conduit extending from tank  34 ). The enthalpy of the refrigerant gas is determined using the temperature and pressure of the refrigerant gas from transmitters  62  and  70  in conjunction with the input or determined volume containing the gas, portions of conduit from compressors  42  to tank  34 , portions of conduit  50  from compressors  44  to condenser  46  and portions of tank  34  not occupied by liquid refrigerant. Since the volume of liquid refrigerant in tank  34  is measured and transmitted to controller  94 , controller  94  may determine the instant volume of gas in tank  34 . To determine the rate of change of enthalpy for the intermediate temperature stage, controller  94  utilizes data from transmitters  60  and  68  as well as the determined volume of refrigerant gas in tank  34  based upon a sensed level of liquid refrigerant and tank  34  and the known volume of tank  34  and open piping or conduit extending from tank  34 . To determine the immediate future load or enthalpy for the intermediate temperature stage, controller  94  utilizes the determined current enthalpy and the rate of change of enthalpy. To determine a response time (the time at which the inlet gas volume to the running compressors is to be increased or decreased while still meeting the cooling demands at the low temperature stage or cooled area), controller  94  utilizes the current enthalpy for the intermediate temperature stage, the immediate future enthalpy the intermediate temperature stage, the capacities of the compressors  44  and the response times of the various available compressors  44 . 
     In one embodiment, controller  94  validates the determined heat load or enthalpy against the amount of heat being extracted by condenser  46 . The amount of heat extracted by condensers  46  may be determined from the information from transmitters  72  and  82  and transmitters  74 ,  84 . The amount of the extracted may approximate the enthalpy. In other embodiments, this validation may be omitted. 
     In the example illustrated, controller  34  is configured to operate in either a set pressure mode or a floating pressure mode, as selected by an operator. In the set pressure mode, a minimum pressure is maintained in tank  34  to facilitate defrosting or other requirements. In the floating pressure mode, controller adjustably controls the pressure in tank  34  for energy savings. For example, it has been found that energy savings is achievable by maintaining the pressure with tank in proportion to the condensing pressure and the pressure of low stage gas suction tank  40 . In one embodiment, the pressure in tank  40  is maintained so as to be equal to the square root of the product of the condensing pressure and the low stage gas suction tank pressure. Since the condensing pressure and the low stage gas suction tank pressure may vary, so will the controlled pressure of tank  34 . 
     In the particular example illustrated, refrigeration cooling system  22  includes two stages: a low temperature evaporator stage and an intermediate temperature evaporator stage. For the low temperature evaporator stage, controller  94  determines the instant thermal content or load (enthalpy), a dynamic rate of change of thermal load (rate of change of enthalpy), a response time and the immediate future thermal load (enthalpy) for the low stage. The enthalpy of the refrigerant gas is determined using the temperature and pressure of the refrigerant gas from transmitters  60  and  68  in conjunction with the input or determined volume containing the gas. In the example illustrated, gas refrigerant is contained in tank  40 , portions of conduit  50  from tank  40  to compressors  42 . 
     The enthalpy of the liquid refrigerant is determined using the flow (lbs/min) and temperature of refrigerant (from flow transmitters  78  and  80  and temperature transmitters  64 , and  66 . The total enthalpy is the sum of the enthalpy of the gas refrigerant and the liquid refrigerant. In some embodiments, the total enthalpy may be estimated using just the enthalpy of the liquid refrigerant since the enthalpy of the gas refrigerant may comprise a small percentage of the total enthalpy. 
     To determine the enthalpy for the low temperature stage, controller  94  utilizes data from transmitters  66 ,  80 ,  60  and  68 . To determine the rate of change of enthalpy for the low temperature stage, controller  94  utilizes data from transmitters  60  and  68 . 
     To determine the enthalpy for the intermediate temperature stage, controller  94  utilizes data from transmitters  64 ,  78 ,  62 ,  70  as well as the determined volume of refrigerant gas in tank  34  (based upon a sensed level of liquid refrigerant and tank  34  and the known volume of tank  34  and open piping or conduit extending from tank  34 ). The enthalpy of the refrigerant gas is determined using the temperature and pressure of the refrigerant gas from transmitters  62  and  70  in conjunction with the input or determined volume containing the gas, portions of conduit from compressors  42  to tank  34 , portions of conduit  50  from compressors  44  to condenser  46  and portions of tank  34  not occupied by liquid refrigerant. Since the volume of liquid refrigerant in tank  34  is measured and transmitted to controller  94 , controller  94  may determine the instant volume of gas in tank  34 . To determine the rate of change of enthalpy for the intermediate temperature stage, controller  94  utilizes data from transmitters  60  and  68  as well as the determined volume of refrigerant gas in tank  34  based upon a sensed level of liquid refrigerant and tank  34  and the known volume of tank  34  and open piping or conduit extending from tank  34 . To determine the immediate future load or enthalpy for the intermediate temperature stage, controller  94  utilizes the determined current enthalpy and the rate of change of enthalpy. To determine a response time (the time at which the inlet gas volume to the running compressors is to be increased or decreased while still the meeting the cooling demands at the low temperature stage or cooled area), controller  94  utilizes the current enthalpy for the intermediate temperature stage, the immediate future enthalpy the intermediate temperature stage, the capacities of the compressors  44  and the response times of the various available compressors  44 . 
     In one embodiment, controller  94  validates the determined heat load or enthalpy against the amount of heat being extracted by condenser  46 . The amount of heat extracted by condensers  46  may be determined from the information from transmitters  72  and  82  and transmitters  74 ,  84 . The amount of the extracted may approximate the enthalpy. In other embodiments, this validation may be omitted. 
     Overall, controller  94  performs one or more of the following functions. First, controller  94  selects optimal combinations of base, full load compressors and a single trim compressor at each stage and also determines an optimal start time for loading of each of the selected compressors based upon a predicted or forecasted future cooling load which is determined based upon an existing enthalpy for the particular stage and the rate of change of enthalpy for the particular stage. 
     Second, controller  94  adjusts operational parameters of condenser  46  based upon existing ambient conditions (temperature and humidity) in combination with a predicted or forecasted future cooling load which is determined based upon an existing enthalpy for the particular stage and the rate of change of enthalpy to conserve energy. 
     Third, controller  94  controls the condensing rate such as by controlling the number of condensers online or such as by controlling fan speed of the condensers so as to maintain minimum pressure requirements for defrosting or for circulation of refrigerant. For example, controller  94  may decrease the condensing rate (lower fan speed or reduce the number of condensers online) to ensure that the minimum pressure of gaseous refrigerant is maintained. 
     Fourth, controller  94  further adjusts or controls interstage pressure of refrigerant within tank  34 . Such adjustment is based upon the condensing pressure at condenser  46  and the low stage pressure at tank  40 . In particular, the adjustment is based upon the square root of the product of the condensing pressure at condenser  46  and the low stage pressure at tank  40 . 
     The following is an example comparing performance of refrigeration cooling system  22  riot under control of control system  24  with the performance of refrigeration cooling system  22  under the control of control system  24 . In the particular example described, refrigeration cooling system  22  is in the meat processing &amp; packing industry facility. The particular facility requires Minus 40 F (−40 F) for the process area. It requires Plus 17 F (17 F) for the packing and ware house area. 
     1. Cooling System  22  not Under Control of Control System  24   
     
         
         
           
             1.1. LOW STAGE COMPRESSORS: 
             Table 1.1 lists the compressors included in the low stage compressor group  42 : 
           
         
       
    
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2.1 
               
               
                   
                   
               
               
                   
                   
                 HP 
                 FULL LOAD 
                 TR 
               
               
                   
                 COMPRESSOR # 
                 RATING 
                 KW 
                 RATING 
               
               
                   
                   
               
             
            
               
                   
                 C1 
                 300 
                 270 
                 200 
               
               
                   
                 C2 
                 350 
                 315 
                 240 
               
               
                   
                 C3 
                 450 
                 405 
                 310 
               
               
                   
                 C4 
                 250 
                 225 
                 175 
               
               
                   
                 C5 
                 150 
                 135 
                 110 
               
               
                   
                   
               
            
           
         
       
         
         
           
             
               
                 1.1.1. Low stage process requires a temperature of minus 45 (−45 F) degree Fahrenheit, corresponding to a saturation pressure (of Ammonia refrigerant) of 8.92 PSIA. The compressors are set to maintain a suction pressure of 8.0 PSIA (corresponding to a saturation temperature of minus (−) 48.5 F, in the low stage suction tank  40 .  FIG. 3  illustrates the actual pressure reading in the tank  40  over a period of fifteen days. 
                 1.1.2. Compressors are controlled by stand alone individual controller of each compressor&#39;s “start/load/mod u late/stop” controller. 
                 1.1.3. All low stage compressors under group  42  are controlled through one or more of the following methods:
               1.1.3.1. Mechanical loading and unloading of the individual compressors based on the suction pressure or process temperature   1.1.3.2. Modulating controls of the individual compressors using variable volume control by inlet throttling and or inlet port restrictions also based on suction pressure   
             
                 1.1.4. One or more compressors may start and load when the pressure goes above the set pressure. Similarly one or more compressors may start modulating the inlet volume/s by opening the slide valve. As a result almost all the compressors are operating at various fractions of the full load capacities resulting in more energy consumption. 
               
             
             1.2. HIGH STAGE COMPRESSORS: 
             Table 1.2 lists the compressors included in the high stage compressor group  44 : 
           
         
       
    
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1.2 
               
               
                   
                   
               
               
                   
                   
                 HP 
                 FULL LOAD 
                 TR 
               
               
                   
                 COMPRESSOR # 
                 RATING 
                 KW 
                 RATING 
               
               
                   
                   
               
             
            
               
                   
                 C6 
                 600 
                 540 
                 550 
               
               
                   
                 C7 
                 700 
                 630 
                 630 
               
               
                   
                 C8 
                 700 
                 630 
                 650 
               
               
                   
                 C9 
                 600 
                 540 
                 570 
               
               
                   
                  C10 
                 450 
                 405 
                 480 
               
               
                   
                   
               
            
           
         
       
         
         
           
             
               
                 1.2.1. High stage process requires a temperature of 17 degree Fahrenheit (F), corresponding to a saturation pressure (of Ammonia refrigerant) of 45 PSIA (˜30 PSIG). The compressors are set to maintain a suction pressure of 30 PSIG (corresponding to a saturation temperature of 17 F), in the high stage suction tank  34 .  FIG. 4  illustrates the actual pressure reading in the tank  34  over a period of fifteen days. 
                 1.2.2. Compressors are controlled by stand alone individual controller of each compressor&#39;s “start/load/mod u late/stop” controller. 
                 1.2.3. All high stage compressors under group  44  are controlled through one or more of the following methods:
               1.2.3.1. Mechanical loading and unloading of the individual compressors based on the suction pressure or process temperature   1.2.3.2. Modulating controls of the individual compressors using variable volume control by inlet throttling and or inlet port restrictions also based on suction pressure   
             
                 1.2.4. One or more compressors may start and load when the pressure goes above the set pressure. Similarly one or more compressors may start modulating the inlet volume/s by opening the slide valve/s when the pressure goes below the set point. As a result almost all the compressors are operating at various fractions of the full load capacities resulting in more energy consumption. 
               
             
             1.3. CONDENSERS 
             The compressed gas from the high stage compressors are condensed in the six evaporative condensers  46 .
           1.3.1. An evaporative condenser is a heat exchanger in which water is showered on the outside of the tube coil and the compressed refrigerant gas circulates through the inside of the coil tubes. The hot compressed gas supplies the latent heat of vaporization for the showered water. The water vaporizes and mixes with the ambient air. The refrigerant gas gets condensed and collects in the holding tank  30 . Air is forced on the outside of the evaporative condensers by the condenser fans to carry the moisture vapor from the condenser surfaces to the ambient air.   1.3.2. CONDENSER FANS:   Table 1.3 lists the condenser fan motors:   
         
           
         
       
    
     
       
         
           
               
               
               
             
               
                 TABLE 1.3 
               
               
                   
               
               
                 CONDENSER # 
                 FAN HP 
                 FULL LOAD KW 
               
               
                   
               
             
            
               
                 CON 1 
                 60 
                 54 
               
               
                 CON 2 
                 50 
                 45 
               
               
                 CON 3 
                 50 
                 45 
               
               
                 CON 4 
                 40 
                 36 
               
               
                 CON 5 
                 60 
                 54 
               
               
                 CON 6 
                 50 
                 45 
               
               
                   
               
            
           
         
       
         
         
           
             
               
                 1.3.3. Condensing pressure varies with the condensing temperature. Condensing temperature is influenced by the ambient wet &amp; dry bulb temperatures, indicators of the saturation level of the humidity in the air. The lower the ambient temperature, the higher the rate of evaporation of the water and the condensation of the refrigerant. In the example under chapter 2, condensing temperature (and pressure) is controlled by adding or removing the number of condensers on line.  FIG. 5  illustrates the actual condenser pressure reading over a period of fifteen days. 
               
             
             1.4. All the controls described above are designed for proper functioning for maintaining the process temperatures; they do not necessarily include energy performance optimization 
           
         
       
    
     2. Energy Analysis of Example not Under Control of Control System  24   
     
         
         
           
             2.1. Energy, Ton Refrigeration (TR) and Pressure Data 
             Table 2.1 lists the measured operational data as weekly averages for both stages of compressors as well as the condensers. The data includes average kWs of motors measured; pressures at the various stages including condensers&#39;, and TR arrived from published charts. 
           
         
       
    
     
       
         
           
               
             
               
                 TABLE 2.1 
               
               
                   
               
             
            
               
                 ENERGY ANALYSIS - PRIOR ART 
               
               
                   
               
               
                 LOW STAGE 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 FULL 
                   
                 ACTUAL 
                 % 
                 % 
                   
                   
               
               
                   
                 LOAD 
                 TR 
                 LOAD 
                 ELECTRIC 
                 TR 
                 ACTUAL 
               
               
                 COMP. # 
                 kW 
                 RATING 
                 kW 
                 LOAD 
                 LOAD 
                 TR 
                 kWhrs/year 
               
               
                   
               
               
                 C1 
                 270 
                 200 
                 230 
                 85% 
                 70% 
                 140 
                 2,014,800 
               
               
                 C2 
                 315 
                 240 
                 220 
                 70% 
                 28% 
                 67 
                 1,927,200 
               
               
                 C3 
                 405 
                 310 
                 340 
                 84% 
                 67% 
                 208 
                 2,978,400 
               
               
                 C4 
                 225 
                 175 
                 170 
                 76% 
                 44% 
                 77 
                 1,489,200 
               
               
                 C5 
                 135 
                 110 
                 100 
                 74% 
                 42% 
                 46 
                 876,000 
               
               
                 Total 
                 1,350 
                 1,035 
                 1,060 
                   
                   
                 538 
                 9,285,600 
               
               
                   
               
            
           
           
               
               
            
               
                 Rated TR/kW efficiency 
                 0.7667 
               
               
                 Actual TR/kW efficiency 
                 0.5076 
               
               
                 Efficiency reduction 
                 34% 
               
            
           
           
               
            
               
                 HIGH STAGE 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 FULL 
                   
                 ACTUAL 
                 % 
                 % 
                   
                   
               
               
                   
                 LOAD 
                 TR 
                 LOAD 
                 ELECTRIC 
                 TR 
                 ACTUAL 
               
               
                 COMP. # 
                 kW 
                 RATING 
                 kW 
                 LOAD 
                 LOAD 
                 TR 
                 kWhrs/year 
               
               
                   
               
               
                 C6 
                 540 
                 550 
                 350 
                 65% 
                 51% 
                 281 
                 3,066,000 
               
               
                 C7 
                 630 
                 630 
                 350 
                 56% 
                 40% 
                 252 
                 3,066,000 
               
               
                 C8 
                 630 
                 650 
                 400 
                 63% 
                 49% 
                 319 
                 3,504,000 
               
               
                 C9 
                 540 
                 570 
                 300 
                 56% 
                 40% 
                 228 
                 2,628,000 
               
               
                 C10 
                 405 
                 480 
                 200 
                 49% 
                  0% 
                 — 
                 1,752,000 
               
               
                 Total 
                 2,745 
                 2,880 
                 1,600 
                   
                   
                 1,079 
                 14,016,000 
               
               
                   
               
            
           
           
               
               
            
               
                 Rated TR/kW efficiency 
                 1.0492 
               
               
                 Actual TR/kW efficiency 
                 0.6744 
               
               
                 Efficiency reduction 
                 36% 
               
            
           
           
               
            
               
                 COMBINED TOTAL 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 TOTAL DESIGN TR RATING 
                 3,915 
               
               
                   
                 TOTAL DESIGN KW RATING 
                 4,095 
               
               
                   
                 TOTAL ACTUAL TR 
                 1,617 
               
               
                   
                 TOTAL ACTUAL KW 
                 2,660 
               
               
                   
                 TR RATIO - ACTUAL/DESIGN 
                 41% 
               
               
                   
                 KW RATIO - ACTUAL/DESIGN 
                 65% 
               
               
                   
                   
               
            
           
           
               
            
               
                 CONDENSER FANS 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 FULL 
                 ACTUAL 
                 % 
                   
               
               
                   
                   
                 LOAD 
                 LOAD 
                 ELECTRIC 
               
               
                   
                   
                 kW 
                 kW 
                 LOAD 
                 kWhrs/year 
               
               
                   
                   
               
               
                   
                 CON # 1 
                 54 
                 54 
                 100% 
                 473,040 
               
               
                   
                 CON # 2 
                 45 
                 45 
                 100% 
                 394,200 
               
               
                   
                 CON # 3 
                 45 
                 45 
                 100% 
                 394,200 
               
               
                   
                 CON # 4 
                 36 
                 0 
                  0% 
                 — 
               
               
                   
                 CON # 5 
                 54 
                 54 
                 100% 
                 473,040 
               
               
                   
                 CON # 6 
                 45 
                 0 
                  0% 
                 — 
               
               
                   
                 Total 
                 279 
                 198 
                   
                 1,734,480 
               
            
           
           
               
               
               
            
               
                   
                 TOTAL TONNAGE HOUR OF REFRIGERATION 
                 14,165,796 
               
               
                   
                 TOTAL ENERGY CONSUMPTION 
                 25,036,080 
               
               
                   
                   
               
            
           
         
       
     
     3. Cooling System  22  Under Control of Control System  24 : 
       FIG. 1  is a schematic representation of the two-stage industrial refrigeration system in the same meat processing and packing facility as described in  FIG. 1  &amp; chapter 2 above but retrofitted with the instruments and control system  24 .
         3.1. The controller  24  receives the following analog inputs from the various equipment and surrounding ambience of the refrigeration system:
           3.1.1. Low stage gas temperature from low stage gas suction tank  40 , through transmitter  68 .   3.1.2. Low stage gas pressure from low stage gas suction tank  40 , through transmitter  60 .   3.1.3. High stage gas temperature from high stage gas suction tank  34 , through transmitter  70 .   3.1.4. High stage gas pressure from High stage gas suction tank  34 , through transmitter  62 .   3.1.5. Refrigerant flow, from the holding tank  30 , through transmitter  78 .   3.1.6. Refrigerant temperature from the holding tank  30 , through transmitter  64 .   3.1.7. Refrigerant flow from the suction tank  34 , through transmitter  80 .   3.1.8. Refrigerant temperature from the suction tank  34 , through transmitter  66 .   3.1.9. Temperature of condensation from the holding tank  30 , through transmitter  76 .   3.1.10. Pressure of condensation from the holding tank  30 , through transmitter  63 .   3.1.11. Condenser water outlet flow from the outlet water line  75 , through transmitter  84 .   3.1.12. Condenser water outlet temperature from the outlet water line  75  through transmitter  74 .   3.1.13. Condenser water/air inlet flow from the inlet or suction water or air line  73  through transmitter  82 .   3.1.14. Condenser water/air inlet temperature from the inlet water line  73 , through transmitter  72 .   3.1.15. Ambient vet bulb temperature from the ambience through transmitter  88 .   3.1.16. Ambient dry bulb temperature from the ambience through transmitter  90 .   
           3.2. The controller receives the following data inputs from the operator:
           3.2.1. Compressor list including compressor kW, TR rating, service factor, start delay, rest delay, stop delay etc   3.2.2. Volume of each system in which the respective refrigerant (both gas and liquid) is contained.   3.2.3. The type of refrigerant used   3.2.4. Various operational parameters such as system set temperature, pressure, etc., for each stage.   3.2.5. Set levels of the liquid in various refrigerant liquid holding tank   3.2.6. Internal size and geometry of the holding tank   3.2.7. Over riding set points   3.2.8. Critical limit of the Variable Frequency drive/s   3.2.9. Any other inputs not covered above but required by the design   
           3.3. The controller sends out the following digital &amp; analog output signals:
           3.3.1. Start/stop/load/unload/modulate signals to the compressor motors   3.3.2. Frequency variation signal to the frequency drive for the compressors   3.3.3. Set points of pressures to the high stage suction tank and discharge of high stage compressors   3.3.4. Frequency variation signal to the frequency drive for the fans   3.3.5. Any other output not covered above but required by the design   
               
     4. Control Strategy of Control System  24   
     Almost all of the industrial and or commercial refrigeration and air conditioning systems are controlled for maintaining one or more of the following physical conditions:
         4.1. Control Parameter/s
           4.1.1. Comfort Temperature—Building Air conditioning   4.1.2. Statutory Temperature Levels—Cold storages and ware houses   4.1.3. Process Temperature—Food Processing   4.1.4 Surrounding Humidity Level—Food processing and Textile mills, printing industry etc   4.1.5. Cooling Rate required for the process—Food industry   4.1.6. Chilled water or glycol temperature—All industrial facilities which require indirect cooling for processes; e.g. plastic molding, forming, extrusion industry; hydraulic presses etc.   
               

     The control parameters described above are all based on temperature bands. For e.g. if the temperature goes up beyond the temperature band the control if any will start compressing more refrigerant gas, condense and circulate for evaporation to reduce the temperature. Similarly, when the temperature falls below the band, it will reduce the amount of gas compressed, condensed, and circulated for evaporation. 
     The refrigerant liquid and vapor will be at equilibrium at the saturation temperature. There is only one saturation temperature corresponding to a particular pressure. Therefore if you control the pressure you can control the temperature. Therefore, most users of refrigeration systems, in a bigger scale, control the pressure to control the temperatures. 
     The trending (ups and downs) of temperature does not follow a predictable pattern in a continuous process industry especially when the process conditions vary dramatically. The unpredictability is even more severe in a refrigeration system which is influenced by ambient temperature and relative humidity.  FIGS. 3 ,  4  and  5  illustrate this phenomenon very clearly. See  FIG. 6  also: 
     Therefore, maximum number of compressing, condensing and circulation equipment is run to satisfy the temperature set points all the time irrespective of the actual refrigeration thermal load. For e.g. in the system described in Table 2.1, compressors of total capacity of 3,915 Tons are run to a refrigeration thermal load of 1,617 Tons. The capacity utilization is only 41%. However the electric power consumption is 2,660 kW OR 65% of the running compressors&#39; full load motor power of 4,095 kW. There is an efficiency reduction of 36% because of the partial loading. 
     The present invention relates to the control of refrigeration fluids during the stages of compression, condensation, distribution to optimize energy efficiency performance of the compressors, cooling fans, distribution pumps etc. of the refrigerant fluids and the carrier of cooling or heating energy like water or air, pumping or blowing systems for the cooling mediums of the refrigerants, and all the above energy performance obtainable without affecting the associated process integrity. 
     The optimum energy efficiency of these stages is achieved simply by including the thermal load and the ambient conditions as additional control parameters to the process temperatures.
         4.2. Control Logic:   The following steps are included in the algorithm of controller  94 .
           4.2.1. Refrigerant vapor pressure and temperatures are dynamically measured at least in one holding tank of each stage (1 st , stage suction, 2 nd , stage suction &amp; condenser etc.).   4.2.2. Total Refrigerant flow to the system from the holding tank  30  is measured.   4.2.3. Total Refrigerant flow to the low stage system from the holding tank  34  is measured.   4.2.4. Total Water consumption by the condensers is measured   4.2.5. The ambient wet bulb and dry bulb temperatures are measured   4.2.6. Full Load “Tonnage Hour” (TR) capacity of each refrigeration compressor in the system is listed in a table; the TR may be either measured or chosen from the manufacturers published data   4.2.7. Operating Power (kW) of each individual compressor is continuously measured   4.2.8. From chapters 4.2.1 through 4.2.7 the following calculations and validations are conducted
               4.2.8.1. Total instant heat loads are computed from the measured flow, temperature and pressures of the refrigerant   4.2.8.2. The computed heat load is validated by the heat load absorbed by the cooling water and/or the cooling air flow.   
               4.2.9. Chapters 4.2.8.1 and 4.2.8.2 can be interchanged depending on the in situ conditions.   4.2.10. From the pressure and temperature changes, the rate of change of mass and enthalpies are computed.   
           4.3. From chapter 4.2.8 actual instant refrigeration demand is computed   4.4. From chapter 4.2.10 rate of change refrigeration demand is determined   4.5. From chapters 4.3 &amp; 4.4 the total refrigeration demand in the immediate future is determined   4.6. The refrigeration demand determined by Item 4.5 will be mapped with the Capacity Tables 5.1 &amp; 5.2 in chapter five to select the optimum number of compressors to be fully loaded and the one compressor to be partially loaded or trimming in each stage.   4.7. The compressors selected for full load in Item 4.5 will have the inlet ports completely open. For e.g. if the inlet port is controlled by slide valve, the slide valve will be in a 100% closed position allowing the inlet port area to be 100% open to the suction reservoir.   4.8. The compressor selected for trim or partial load in chapter 4.6 will be controlled by either partial opening and closing the inlet ports by available means or by an external variable electrical frequency mechanism that will increase or decrease speed of the motor shaft of the selected trim compressor.   4.9. Chapters 4.7 &amp; 4.8 enable to select the optimum number compressors to be in operation to the current and instantly changing refrigeration load
 
To summarize, steps 4.1 through 4.9, the controller dynamically determines the following:
   The instant thermal load   The dynamic rate of change of thermal load   The response time   The immediate future thermal load   Selection of the compressors to be fully loaded in each stage   Selection of the trim compressor for each stage   Time available to add or remove compressor   Condenser fan speed   The number of condensers effectively transferring the heat to the atmosphere   4.10. The other compressor operating parameters are the suction and discharge pressures.
           4.10.1. The suction pressure in each stage is influenced by the process temperature requirements   4.10.2. The intermediate stage suction pressure may be optimized as a function of the condensing pressure and lowest suction pressure of the system.   4.10.3. The intermediate stage pressure can be configured as a choice by the user between item 4.10.1 and 4.10.2   4.10.4. The condensing pressure is influenced by the ambient wet bulb temperatures; for a constant condensing surface area, the condensing pressure will fall as the ambient wet bulb temperature falls; therefore the condensing pressure can be set as dynamic set point which will be determined by the control program as a function of the ambient wet bulb temperature and an allowable tolerance in temperature.   4.10.5. Some processes require minimum level of pressures for the liquid refrigerant holding tank for effective pumping or for defrosting purposes.   4.10.6. The condensing pressure can be maintained at a minimum level within a set band of pressures by reducing condensing surface area and or by shutting of the condenser fans in case of item 4.10.5.   4.10.7. Controller  24  described above provides a chance to the operator to select the minimum condensing pressure for optimum energy efficiency and at the same time, satisfying process condition described in item 4.10.5.   
           4.11. Chapters 4.9 &amp; 4.10 will enable optimizing the refrigeration compressors&#39; operation.   4.12. The volume of air to be forced by the evaporative condenser fan is also a function of the heat load to be removed.   4.13. Chapter 4 5 will determine the speeds of the fans to be operated with installed variable frequency mechanism       

     5. Control Algorithm 
       FIG. 2  is block diagram of the control logic of controller  94 .
         5.1. Analog inputs ( FIG. 2  # 300 ) are fed in to the controller. They include but not limited to the following:
           5.1.1. Low stage gas; temperature from low stage gas suction   5.1.2. Low stage gas; pressure from low stage gas suction tank   5.1.3. High stage gas temperature from high stage gas suction tank   5.1.4. High stage gas pressure from High stage gas suction tank   5.1.5. Refrigerant flow from the holding receiver   5.1.6. Refrigerant temperature from the holding receiver.   5.1.7. Refrigerant flow from the high stage suction tank   5.1.8. Refrigerant temperature from the high stage suction tank   5.1.9. Temperature of condensation from the holding receiver.   5.1.10. Pressure of condensation from the holding tank receiver.   5.1.11. Condenser water outlet flow from the outlet water line   5.1.12. Condenser water outlet temperature from the outlet water line   5.1.13. Condenser water/air inlet flow from the inlet water/suction line   5.1.14. Condenser water/air inlet temperature from the outlet line   5.1.15. Ambient wet bulb temperature   5.1.16. Ambient dry bulb temperature   
           5.2. The operator enters all the operating data ( FIG. 2  # 301 ). The data includes but is not limited to the following:
           5.2.1. Compressor list including compressor kW, TR rating, service factor, start delay, rest delay, stop delay etc   5.2.2. Volume of each system in which the respective refrigerant (both gas and liquid) is contained.   5.2.3. The type of refrigerant used   5.2.4. Various operational parameters such as system set temperature, pressure, etc., for each stage.   5.2.5. Set levels of the liquid in various refrigerant liquid holding tank   5.2.6. Internal size and geometry of the holding tank   5.2.7. Over riding set: points   5.2.8. Critical limit of the Variable Frequency drive/s   
           5.3. Dynamic Load Balancing   Controller computes the dynamic operational parameters ( FIG. 2  # 302 ). They include but not limited to the following:
           5.3.1. Thermal load on the condenser—From mass flow difference of air/water and temperature difference between inlet and outlet   5.3.2. Thermal load clue to heat of compression   5.3.3. Thermal load of refrigeration—Thermal load of condenser minus heat of compression   5.3.4. Determine enthalpies of liquid and gas at various stages—From formula or Look up table for the analog input of pressure and temperature in each stage   5.3.5. Validate Thermal load—From refrigerant flow measurements * enthalpies and steps 5.3.1 through 5.3.3.   5.3.6. Volume of gas—Total Volume minus the liquid volume   5.3.7. Density of gas—From formula or Look up table for the analog input of pressure and temperature in each stage   5.3.8. Calculate instant mass of gas in each stage—from formula “Mass in lbs=d*V” Where d=density in lbs/cubic feet, of the gas at the measured temperature &amp; pressure and V=Total volume in cubic feet occupied by the evaporated gas.   5.3.9. The rate of change of mass/second equals the change of refrigerant flow in lbs/second   5.3.10. Available Response time—From gas volume and rate of change of gas mass   5.3.11. Practical Response time From 5.3.10 and compressor operational parameters   5.3.12. Total refrigerant flow—Instant flow plus refrigerant flow during the response time   5.3.13. The refrigeration load on the compressors of both stages—Total refrigerant (lbs/min) recirculated as measured by flow transmitter  78  multiplied by (*) enthalpy (btu/lb) of the refrigerant at the instant temperature as transmitted measured by temperature transmitter  64  from the look up table or by calculation.   5.3.14. The refrigeration load on the compressors ( 42 ) of the low stage—Total refrigerant (lbs/min) flowing to the expansion valve  53  as measured by flow transmitter  80  multiplied by (*) enthalpy (btu/lb) of the refrigerant at the instant temperature as transmitted measured by temperature transmitter  66  from the look up table or by calculation.   5.3.15. The refrigeration load on the compressors ( 44 ) of the high stage equals the enthalpy as computed in chapter 5.3.13 minus the enthalpy as computed in chapter 5.3.14.   
           5.4. Selection Of Compressors &amp; Condenser Fan Speeds   The controller decides the actions. They include but not limited to the following:
           5.4.1. Identifies and selects the number of compressors for full loads ( FIG. 2  # 303 )—From the operator data ( FIG. 2  # 301 ) and chapter 5.3.14 &amp; 5.3.15. For e.g., in the facility under  FIG. 1  and chapter 4.0 above, the refrigeration thermal loads are 538 and 1,079 Tons in the low and high stage respectively. The nearest full load capacity to thermal load is of compressor C 1  &amp; C 3  in the low stage and of compressor C 8  in the high stage respectively as evident from the compressor tables below;   
               
     
       
         
           
               
             
               
                 TABLE 5.1 
               
             
            
               
                   
               
               
                 LOW STAGE 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 HP 
                 FULL LOAD 
                 TR 
               
               
                   
                 COMPRESSOR # 
                 RATING 
                 KW 
                 RATING 
               
               
                   
                   
               
               
                   
                 
                   
                 
                 
                   
                 
                 
                   
                 
                 
                   
                 
               
               
                   
                 C2 
                 350 
                 315 
                 240 
               
               
                   
                 
                   
                 
                 
                   
                 
                 
                   
                 
                 
                   
                 
               
               
                   
                 C4 
                 250 
                 225 
                 175 
               
               
                   
                 C5 
                 150 
                 135 
                 110 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5.2 
               
             
            
               
                   
               
               
                 HIGH STAGE 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 HP 
                 FULL LOAD 
                 TR 
               
               
                   
                 COMPRESSOR # 
                 RATING 
                 KW 
                 RATING 
               
               
                   
                   
               
               
                   
                 C6 
                 600 
                 540 
                 550 
               
               
                   
                 C7 
                 700 
                 630 
                 630 
               
               
                   
                 
                   
                 
                 
                   
                 
                 
                   
                 
                 
                   
                 
               
               
                   
                 C9 
                 600 
                 540 
                 570 
               
               
                   
                  C10 
                 450 
                 405 
                 480 
               
               
                   
                   
               
            
           
         
       
         
         
           
             
               
                 5.4.2. Controller  94  computes the balance thermal capacity required by the process as 18 Tons in the Low stage and 429 Tons in the high stage; accordingly it selects the trim compressors ( FIG. 2  # 303 ) C 5  in the low stage, and C 10  in the high stage because they have the nearest higher capacity to the short fall to meet the demand in the low and high stages respectively. 
                 5.4.3. Computes the most efficient way ( FIG. 2  # 304 ) of operating the trim compressors; either by mechanically controlling the inlet volume ( FIG. 2  # 307 ) or by varying the speed of the motor shaft through the Variable frequency drive ( FIG. 2  # 306 ). 
                 5.4.4. CONDENSER FANS&#39; SPEED:
               5.4.4.1. Condenser fans force the air to the outside of the condenser coils to carry the condenser thermal load to the atmosphere and improve the heat transfer efficiency. Since the amount of air to be circulated depends on the thermal load, the controller per the present invention Varies the speeds of the fans uniformly (through a common variable frequency drive for all the fans) to match with the thermal load. In the process it also checks the critical speed of the fans. If computed speed is less than the critical speed of the fans, the controller reduces the number of condensers on line to obtain the best energy efficiency of operation.   See Table 5.3:   
             
               
             
           
         
       
    
     
       
         
           
               
             
               
                 TABLE 5.3 
               
             
            
               
                   
               
               
                 CONDENSER FANS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 FULL 
                 ACTUAL 
                 % 
                   
               
               
                   
                 LOAD 
                 LOAD 
                 ELECTRIC 
               
               
                   
                 kW 
                 kW 
                 LOAD 
                 kWhrs/year 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 CON # 1 
                 54 
                 27.648 
                 80% 
                 242,196 
               
               
                 CON # 2 
                 45 
                 23.04 
                 80% 
                 201,830 
               
               
                 CON # 3 
                 45 
                 23.04 
                 80% 
                 201,830 
               
               
                 CON # 4 
                 36 
                 0 
                  0% 
                 — 
               
               
                 CON # 5 
                 54 
                 0 
                  0% 
                 — 
               
               
                 CON # 6 
                 45 
                 0 
                  0% 
                 — 
               
               
                 Total 
                   
                 73.728 
                   
                 645,857 
               
               
                   
               
            
           
         
       
     
     6. Energy Analysis Of System  22  Under Control System  24   
     
         
         
           
             Table 6.1 summarizes the energy analysis of the example facility in chapter 2 and  FIG. 1 , after retrofitted with control system  24  and according to  FIG. 1  and described in chapters 4 and 5. 
           
         
       
    
     
       
         
           
               
             
               
                 TABLE 6.1 
               
               
                   
               
             
            
               
                 ENERGY ANALYSIS - CURRENT INVENTION 
               
               
                   
               
               
                 LOW STAGE 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 FULL 
                   
                 ACTUAL 
                 % 
                 % 
                   
                   
               
               
                   
                 LOAD 
                 TR 
                 LOAD 
                 ELECTRIC 
                 TR 
                 ACTUAL 
               
               
                 COMP. # 
                 kW 
                 RATING 
                 kW 
                 LOAD 
                 LOAD 
                 TR 
                 kWhrs/year 
               
               
                   
               
               
                 C1 
                 270 
                 200 
                 270 
                 100% 
                 100% 
                 200 
                 2,365,200 
               
               
                 C2 
                 315 
                 240 
                 0 
                  0% 
                  0% 
                 — 
                 — 
               
               
                 C3 
                 405 
                 310 
                 405 
                 100% 
                 100% 
                 310 
                 3,547,800 
               
               
                 C4 
                 225 
                 175 
                 40 
                  18% 
                  16% 
                 28 
                 351,651 
               
               
                 C5 
                 135 
                 110 
                 0 
                   
                   
                   
                 — 
               
               
                 Total 
                 1,350 
                 1,035 
                 715 
                   
                   
                 538 
                 6,264,651 
               
               
                   
               
            
           
           
               
               
            
               
                 Rated TR/kW efficiency 
                 0.7667 
               
               
                 Actual TR/kW efficiency 
                 0.7524 
               
               
                 Efficiency reduction 
                 2% 
               
            
           
           
               
            
               
                 HIGH STAGE 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 FULL 
                   
                 ACTUAL 
                 % 
                 % 
                   
                   
               
               
                   
                 LOAD 
                 TR 
                 LOAD 
                 ELECTRIC 
                 TR 
                 ACTUAL 
               
               
                 COMP. # 
                 kW 
                 RATING 
                 kW 
                 LOAD 
                 LOAD 
                 TR 
                 kWhrs/year 
               
               
                   
               
               
                 C6 
                 540 
                 550 
                   
                   
                   
                   
                 — 
               
               
                 C7 
                 630 
                 630 
                   
                   
                   
                   
                 — 
               
               
                 C8 
                 630 
                 650 
                 630 
                 100% 
                 100% 
                 650 
                 5,518,800 
               
               
                 C9 
                 540 
                 570 
                   
                   
                   
                   
                 — 
               
               
                 C10 
                 405 
                 480 
                 419 
                  99% 
                  89% 
                 429 
                 3,669,961 
               
               
                 Total 
                 2,745 
                 2,880 
                 1,049 
                   
                   
                 1,079 
                 9,188,761 
               
               
                   
               
            
           
           
               
               
            
               
                 Rated TR/kW efficiency 
                 1.0492 
               
               
                 Actual TR/kW efficiency 
                 1.0287 
               
               
                 Efficiency reduction 
                 2% 
               
            
           
           
               
            
               
                 COMBINED TOTAL 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 TOTAL DESIGN TR RATING 
                 3,915 
               
               
                   
                 TOTAL DESIGN KW RATING 
                 4,095 
               
               
                   
                 TOTAL ACTUAL TR 
                 1,617 
               
               
                   
                 TOTAL ACTUAL KW 
                 1,764 
               
               
                   
                 TR RATIO - ACTUAL/DESIGN 
                 41% 
               
               
                   
                 KW RATIO - ACTUAL/DESIGN 
                 43% 
               
               
                   
                   
               
            
           
           
               
            
               
                 CONDENSER FANS 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 FULL 
                 ACTUAL 
                 % 
                   
               
               
                   
                   
                 LOAD 
                 LOAD 
                 ELECTRIC 
               
               
                   
                   
                 kW 
                 kW 
                 LOAD 
                 kWhrs/year 
               
               
                   
                   
               
               
                   
                 CON # 1 
                 54 
                 27.648 
                 80% 
                 242,196 
               
               
                   
                 CON # 2 
                 45 
                 23.04 
                 80% 
                 201,830 
               
               
                   
                 CON # 3 
                 45 
                 23.04 
                 80% 
                 201,830 
               
               
                   
                 CON # 4 
                 36 
                 0 
                  0% 
                 — 
               
               
                   
                 CON # 5 
                 54 
                 0 
                  0% 
                 — 
               
               
                   
                 CON # 6 
                 45 
                 0 
                  0% 
                 — 
               
               
                   
                 Total 
                   
                 73.728 
                   
                 645,857 
               
            
           
           
               
               
               
            
               
                   
                 TOTAL TONNAGE HOUR OF REFRIGERATION 
                 14,165,796 
               
               
                   
                 TOTAL ENERGY CONSUMPTION 
                 16,099,270 
               
               
                   
                   
               
            
           
         
       
         
         
           
             6.1. The energy saving obtainable by optimization of the supply and demand of the “REFRIGERATION LOAD” with the retrofit of the controller and accessories as described by the Current invention is summarized as below: 
           
         
       
    
     Summary of Savings 
       
     
       
         
           
               
             
               
                   
               
             
            
               
                 PRIOR ART 
               
            
           
           
               
               
               
            
               
                 TOTAL TONNAGE HOUR OF 
                 14,165,796 
                 TONS/YEAR 
               
               
                 REFRIGERATION 
               
               
                 TOTAL ENERGY CONSUMPTION 
                 25,036,080 
                 KWHRS/YEAR 
               
            
           
           
               
            
               
                 CURRENT INVENTION 
               
            
           
           
               
               
               
            
               
                 TOTAL TONNAGE HOUR OF 
                 14,165,796 
                 TONS/YEAR 
               
               
                 REFRIGERATION 
               
               
                 TOTAL ENERGY CONSUMPTION 
                 16,099,270 
                 KWHRS/YEAR 
               
               
                 ENERGY SAVINGS 
                  8,936,810 
                 KWHRS/YEAR 
               
               
                 PERCENTAGE OF SAVING 
                 36% 
               
               
                   
               
            
           
         
       
     
     7. Optimization of System Parameters: 
     
         
         
           
             The controller and equipment per the current invention is capable of producing more energy saving in addition to the energy saving obtainable in chapter 7.1, by optimizing the system operational parameters to match with the need and talking advantage of the natural atmospheric conditions. 
             7.1. LOW STAGE SUCTION PRESSURE:
           7.1.1. Low stage process requires a temperature of minus forty five (−45 F) degree Fahrenheit, corresponding to a saturation pressure (of Ammonia refrigerant) of 8.92 PSIA. The compressors are set to maintain a suction pressure of 8.0 PSIA (corresponding to a saturation temperature of −48.5 F), in the low stage suction tank  40 .  FIG. 3  shows the actual pressure reading in the tank  40  over a period of fifteen days.   7.1.2. The controller per the current invention is capable of controlling within a tighter band of suction pressure without compromising the required temperature of minus (−) 45 degrees F. see  FIG. 7 . This is achieved solely due to the pro-active ability of the controller to accurately predict the thermal load changes and thereby the temperature changes. The resultant energy savings in this example can be as high as two percentage points (2%) of the power for the corresponding compressors.   
         
             7.2 HIGH STAGE COMPRESSORS:
           High stage process requires a temperature of 17 degree Fahrenheit (F), corresponding to a saturation pressure (of Ammonia refrigerant) of 45 PSIA (˜30 PSIG).   7.2.1. As described in Chapter 2, and  FIG. 1 , the high stage compressors are set to maintain a suction pressure of 30 PSIG (corresponding to a saturation temperature of 17 F), in the high stage suction tank for all seasons and conditions through out the year. It does not take advantage of ambient conditions to maximize the energy efficiency of the compressors.   7.2.2. The controller per the present invention described in Chapter 4 and  FIG. 2 , is designed and programmed to change the inter stage suction pressure (which is also the low stage compressors&#39; discharge pressure) for optimizing the energy efficiency of the refrigeration compressors. In other words the inter stage pressure is not fixed set point as in the prior art. The optimum inter stage pressure (as far as the energy efficiency is concerned) is obtained by the following formula:   
         
           
         
       
    
         P 2=Square Root of  P 1(Low stage suction pressure)* P 3 (Condensing Pressure), 
       where, 
       P1=Low stage suction pressure in PSIA, P2=Inter stage pressure in PSIA, and P3=condensing pressure in PSIA.             7.2.3. The inter stage pressure is made dynamic because the condensing pressure is made dynamic as described in chapter 7.3 following this chapter.   7.2.4.  FIG. 8  shows the dynamic inter stage pressure as calculated by the controller as against the variations of the fixed suction pressure set by the controller of the prior art.   7.2.5. The resultant energy savings in this example can be as high as two percentage points (2%) per one PSI reduction in the inter stage pressure. The energy savings can be as high as 18% in this example.   7.2.6. The controller is also configured to provide the operator with the chance to select the floating dynamic set pressure calculated in chapter 7.2.2 or a mandatory set pressure required by the intermediate stage cooling loads.       7.3. CONDENSING PRESSURE:
           The condensation temperature depends on the ambient temperature and humidity in an evaporative condenser. The lower the wet bulb temperature, the lower would be the condensing temperature. When the condensing temperature is lower the condensing pressure also can be lower. If the condensing pressure is lower, the high stage compressors need to do less amount of compression and therefore less energy consumption.   The controller per the current invention capitalizes on the above natural phenomenon and can dynamically set the condensing pressure dependent on the ambient conditions.   7.3.1. AVERAGE AMBIENT CONDITIONS:
                 FIG. 9  depicts the monthly average ambient temperatures measured for the facility per the prior art described in the chapter 2. The chart also includes the constant condensing temperature and pressure as set by the controller in the prior art.   It also shows the condensing temperature and the corresponding condensing pressure as set by the controller  94 .   
               7.3.2. POTENTIAL SAVING:   
               
       FIG. 10  depicts the potential saving effect by varying the condensing pressure as per the ambient temperature as shown in  FIG. 9 . The savings can be as high as twenty percentage points of the energy consumption of the prior art. 
     The ambient wet and dry bulb temperatures will be measured constantly. From the temperatures and using psychometric charts and formulas the condensing pressure will be computed by the controller  94  as described in chapter 4-Control Strategy of Control System  24  and chapter 5-CONTROL ALGORITHM and  FIG. 2 . 
     Although the present disclosure has been described with reference to example embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the claimed subject matter. For example, although different example embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described example embodiments or in other alternative embodiments. Because the technology of the present disclosure is relatively complex, not all changes in the technology are foreseeable. The present disclosure described with reference to the example embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements.