Patent Publication Number: US-8978401-B2

Title: Data center cooling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 13/551,929 (“IDATA CENTER COOLING METHOD”) filed Jul. 18, 2012. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract No.: DE-EE0002894 (awarded by Department of Energy (DOE)). The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The disclosure relates to the field of computer systems, and, more particularly, to data centers housing computer systems. 
     A data center is a collection of computer systems and associated subsystems housing such. Data centers usually include chillers as part of the environmental control system that regulates the temperature of the computer systems and associated subsystems housed by the data center. 
     SUMMARY 
     In one embodiment, a data center cooling system may include heat transfer equipment to cool a liquid coolant without vapor compression refrigeration, and the liquid coolant is used on a liquid cooled information technology equipment rack housed in the data center. The system may also include a controller-apparatus to regulate the liquid coolant flow to the liquid cooled information technology equipment rack through a range of liquid coolant flow values based upon information technology equipment temperature thresholds. 
     Another system embodiment for a data center cooling system may include heat transfer equipment to cool a liquid coolant without vapor compression refrigeration, and the liquid coolant is used on a liquid cooled information technology equipment rack housed in a data center. The system may also include a controller-apparatus to regulate the liquid coolant flow to the liquid cooled information technology equipment rack through a range of liquid coolant flow values based upon information technology equipment temperature thresholds, and the controller-apparatus comprises a computer processor. The system may further include a cooling system pump and a fan where the controller-apparatus determines the cooling system pump&#39;s rpm and/or the fan&#39;s rpm based upon the data center&#39;s power consumption and/or information technology equipment operating temperature. 
     Another system embodiment for a data center cooling system may include heat transfer equipment to cool a liquid coolant without vapor compression refrigeration, and the liquid coolant is used on a liquid cooled information technology equipment rack housed in a data center. The system may also include a controller-apparatus to regulate the liquid coolant flow to the liquid cooled information technology equipment rack through a range of liquid coolant flow values based upon information technology equipment temperature thresholds, the controller-apparatus comprises a computer processor, and regulates the data center energy usage based upon a user selected upper target temperature and/or a user selected lower target temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a data center cooling system in accordance with various embodiments. 
         FIG. 2  is a flowchart illustrating method aspects according to various embodiments. 
         FIG. 3  is a flowchart illustrating method aspects according to the method of  FIG. 2 . 
         FIG. 4  is a flowchart illustrating method aspects according to the method of  FIG. 3 . 
         FIG. 5  is a flowchart illustrating method aspects according to the method of  FIG. 4 . 
         FIG. 6  is a flowchart illustrating method aspects according to the method of  FIG. 2 . 
         FIG. 7  is a flowchart illustrating method aspects according to the method of  FIG. 6 . 
         FIG. 8  is a flowchart illustrating method aspects according to the method of  FIG. 6 . 
         FIG. 9  is a flowchart illustrating method aspects according to the method of  FIG. 6 . 
         FIG. 10  is a flowchart illustrating method aspects according to the method of  FIG. 6 . 
         FIG. 11  is a flowchart illustrating method aspects according to the method of  FIG. 8 . 
         FIG. 12  is a flowchart illustrating method aspects according to the method of  FIG. 4 . 
         FIG. 13  is a flowchart illustrating method aspects according to the method of  FIG. 12 . 
         FIG. 14  is a flowchart illustrating method aspects according to the method of  FIG. 12 . 
         FIG. 15  is a flowchart illustrating method aspects according to various embodiments. 
         FIG. 16  is a flowchart illustrating method aspects according to various embodiments. 
         FIG. 17  is a block diagram illustrating a system view of the data center cooling system of  FIG. 1 . 
         FIG. 18  is a block diagram illustrating a dual loop view of the data center cooling system of  FIG. 1 . 
         FIG. 19  illustrates an exemplary single loop improved data center cooling system in accordance with various embodiments. 
         FIG. 20  illustrates an exemplary dual loop improved data center cooling system in accordance with various embodiments. 
         FIG. 21  illustrates exemplary controller input parameters from a rack in accordance with various embodiments. 
         FIG. 22  is a flowchart illustrating method aspects according to various embodiments. 
         FIG. 23  illustrates an exemplary rack inlet coolant temperature using servo control aspects according to various embodiments. 
         FIG. 24  is a flowchart illustrating method aspects according to various embodiments. 
         FIG. 25  illustrates exemplary external loop flow rate as a function of Delta T according to various embodiments. 
         FIG. 26  illustrates exemplary external fan revolutions per minute as a function of Delta T according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments are shown. Like numbers refer to like elements throughout. 
     With reference now to  FIGS. 1 ,  17 , and  18 , a cooling system  10  is initially described. In an embodiment, the system  10  includes heat transfer equipment  12  to cool a liquid coolant  14  without vapor compression refrigeration, and the liquid coolant is used to cool a liquid cooled information technology equipment rack  16  housed in the data center  18 . The system may also include a controller-apparatus  20  to regulate the liquid coolant  14  flow to the liquid cooled information technology equipment rack  16  through a range of liquid coolant flow values based upon information technology equipment  22  temperature thresholds. 
     In one embodiment, the controller-apparatus  20  comprises a computer processor  24 . In another embodiment, the range of liquid coolant flow values provides a continuous heat removal runtime means in which there is no off-on cycling of the system  10 . In another embodiment, at least the cooling system pump  26  of the system  10  is not cycled on and off, but rather is continuously powered at varying flow rates dependent on the amount of cooling required. 
     In one embodiment, the system  10  further includes a cooling system pump  26  and a fan  28  where the controller-apparatus  20  determines the cooling system pump&#39;s rpm and the fan&#39;s rpm based upon the data center&#39;s  18  power consumption and/or information technology equipment  22  operating temperatures. For example, the information technology equipment  22  comprises computers and associated subsystems which is housed within a liquid cooled information technology equipment rack  16 . 
     In one embodiment, the controller-apparatus  20  calculates the cooling system pump  26  rpm and the fan  28  rpm to determine the liquid cooled information technology equipment rack&#39;s  16  liquid inlet temperature. In another embodiment, the controller-apparatus  20  ignores the outdoor ambient temperature when calculating the liquid cooled information technology equipment rack&#39;s  16  liquid inlet temperature. 
     In one embodiment, the controller-apparatus  20  regulates the data center&#39;s  18  energy usage based upon a user selected upper target temperature and/or a user selected lower target temperature. In another embodiment, the controller-apparatus  20  sets the energy usage to a reduced energy state while maintaining a liquid coolant  14  operating temperature between the user selected upper target temperature and the user selected lower temperature target. 
     In one embodiment, the controller-apparatus  20  engages additional cooling capacity  30  to limit information technology equipment  22  temperatures to a value at or below the user selected upper target temperature. In another embodiment, the controller-apparatus  20  engages additional cooling capacity  30  while proportionally distributing the additional cooling capacity between a cooling system pump  26  rpm and a fan  28  rpm. 
     In one embodiment, the controller-apparatus  20  reduces cooling capacity to limit information technology equipment  22  temperatures to a value at or above the user selected lower target temperature. In another embodiment, the controller-apparatus  20  powers down information technology equipment  22  when the additional cooling capacity  30  is insufficient. 
     In one embodiment, the heat transfer equipment  12  includes the liquid cooled information technology equipment rack  16 , a side car heat exchanger  32 , an outdoor heat exchanger  34 , and a liquid to liquid heat exchanger  36 , and the controller-apparatus  20  regulates the liquid coolant  14  flow through the liquid cooled information technology equipment rack, the side car heat exchanger, the outdoor heat exchanger, and/or the liquid to liquid heat exchanger by changing the cooling system pump  26  rpm. 
     In one embodiment, the controller-apparatus  20  bypasses the outdoor heat exchanger  34  to reduce cooling capacity to limit information technology equipment  22  temperatures to a value at or above a user selected lower target temperature. In another embodiment, the controller-apparatus  20  includes the outdoor heat exchanger  34  to add cooling capacity to limit information technology equipment  22  temperatures to a value at or below a user selected upper target temperature. 
     Another aspect is a method for data center cooling, which is now described with reference to flowchart  40  of  FIG. 2 . The method begins at Block  42  and may include using liquid coolant cooled without vapor compression refrigeration on a liquid cooled information technology equipment rack at Block  44 . The method may also include regulating liquid coolant flow to the data center through a range of liquid coolant flow values with a controller-apparatus based upon information technology equipment temperature threshold of the data center at Block  46 . The method ends at Block  48 . 
     In another method embodiment, which is now described with reference to flowchart  50  of  FIG. 3 , the method begins at Block  52 . The method may include the steps of  FIG. 2  at Blocks  44  and  46 . The method may also include determining a cooling system pump rpm and/or a fan rpm based upon the data center&#39;s power consumption and/or information technology equipment operating temperature at Block  54 . The method ends at Block  56 . 
     In another method embodiment, which is now described with reference to flowchart  58  of  FIG. 4 , the method begins at Block  60 . The method may include the steps of  FIG. 3  at Blocks  44 ,  46 , and  54 . The method may also include calculating via the controller-apparatus the cooling system pump rpm and/or the fan rpm to determine the liquid cooled information technology equipment rack&#39;s liquid inlet temperature at Block  62 . The method ends at Block  64 . 
     In another method embodiment, which is now described with reference to flowchart  66  of  FIG. 5 , the method begins at Block  68 . The method may include the steps of  FIG. 4  at Blocks  44 ,  46 ,  54 , and  62 . The method may also include ignoring the outdoor ambient temperature when calculating the liquid cooled information technology equipment rack&#39;s liquid inlet temperature at Block  70 . The method ends at Block  72 . 
     In another method embodiment, which is now described with reference to flowchart  74  of  FIG. 6 , the method begins at Block  76 . The method may include the steps of  FIG. 2  at Blocks  44  and  46 . The method may also include regulating the data center energy usage by using the controller-apparatus according to a user selected upper target temperature and/or a user selected lower target temperature at Block  78 . The method ends at Block  80 . 
     In another method embodiment, which is now described with reference to flowchart  82  of  FIG. 7 , the method begins at Block  84 . The method may include the steps of  FIG. 6  at Blocks  44 ,  46 , and  78 . The method may also include setting the cooling energy usage via the controller-apparatus to a reduced energy state while maintaining a liquid coolant operating temperature between the user selected upper target temperature and the user selected lower temperature target at Block  86 . The method ends at Block  88 . 
     In another method embodiment, which is now described with reference to flowchart  90  of  FIG. 8 , the method begins at Block  92 . The method may include the steps of  FIG. 6  at Blocks  44 ,  46 , and  78 . The method may also include engaging additional cooling capacity with the controller-apparatus to limit specific information technology equipment temperatures to a value at or below the user selected upper target temperature at Block  94 . The method ends at Block  96 . 
     In another method embodiment, which is now described with reference to flowchart  98  of  FIG. 9 , the method begins at Block  100 . The method may include the steps of  FIG. 6  at Blocks  44 ,  46 , and  78 . The method may also include engaging additional cooling capacity while proportionally distributing the additional cooling capacity between a pump rpm and a fan rpm at Block  104 . The method ends at Block  104 . 
     In another method embodiment, which is now described with reference to flowchart  106  of  FIG. 10 , the method begins at Block  108 . The method may include the steps of  FIG. 6  at Blocks  44 ,  46 , and  78 . The method may also include reducing cooling capacity with the controller-apparatus to limit specific information technology equipment temperatures to a value at or above the user selected lower target temperature at Block  110 . The method ends at Block  112 . 
     In another method embodiment, which is now described with reference to flowchart  114  of  FIG. 11 , the method begins at Block  116 . The method may include the steps of  FIG. 8  at Blocks  44 ,  46 ,  78 , and  94 . The method may also include powering down specific information technology equipment when the additional cooling capacity is insufficient at Block  118 . The method ends at Block  120 . 
     In another method embodiment, which is now described with reference to flowchart  122  of  FIG. 12 , the method begins at Block  124 . The method may include the steps of  FIG. 4  at Blocks  44 ,  46 ,  54 , and  62 . The method may also include regulating a liquid coolant flow through a liquid cooled information technology equipment rack, a side car heat exchanger, an outdoor heat exchanger, and/or a liquid to liquid heat exchanger by changing the pump rpm at Block  126 . The method ends at Block  128 . 
     In another method embodiment, which is now described with reference to flowchart  130  of  FIG. 13 , the method begins at Block  132 . The method may include the steps of  FIG. 12  at Blocks  44 ,  46 ,  54 ,  62 , and  126 . The method may also include bypassing the outdoor heat exchanger to reduce cooling capacity via the controller-apparatus to limit information technology equipment temperatures to a value at or above a user selected lower target temperature at Block  134 . The method ends at Block  136 . 
     In another method embodiment, which is now described with reference to flowchart  138  of  FIG. 14 , the method begins at Block  140 . The method may include the steps of  FIG. 12  at Blocks  44 ,  46 ,  54 ,  62 , and  126 . The method may also comprise including the outdoor heat exchanger to add cooling capacity via the controller-apparatus to limit information technology equipment temperatures to a value at or below a user selected upper target temperature at Block  142 . The method ends at Block  144 . 
     Another aspect is a method for data center cooling, which is now described with reference to flowchart  146  of  FIG. 15 . The method begins at Block  148  and may include using liquid coolant cooled without vapor compression refrigeration on a liquid cooled information technology equipment rack at Block  150 . The method may also include regulating liquid coolant flow to the data center through a range of liquid coolant flow values with a controller-apparatus based upon information technology equipment temperature threshold of the data center, and the controller-apparatus includes a computer processor at Block  152 . The method may further include determining a cooling system pump rpm and/or a fan rpm based upon the data center&#39;s power consumption and/or information technology equipment operating temperature at Block  154 . The method ends at Block  156 . 
     Another aspect is a method for data center cooling, which is now described with reference to flowchart  158  of  FIG. 16 . The method begins at Block  160  and may include using liquid coolant cooled without vapor compression refrigeration on a liquid cooled information technology equipment rack at Block  162 . The method may also include regulating liquid coolant flow to the data center through a range of liquid coolant flow values with a controller-apparatus based upon information technology equipment temperature threshold of the data center, and the controller-apparatus includes a computer processor at Block  164 . The method may further include regulating the data center energy usage by using the controller-apparatus according to a user selected upper target temperature and/or a user selected lower target temperature at Block  166 . The method ends at Block  168 . 
     In view of the foregoing, the system  10  provides cooling for the data center. For example, system  10  uses a set of temperature-based proportional servo control algorithms, for a fluid, e.g. liquid coolant  14 , cooled chiller-less data center  18 , that is implemented to reduce the data center cooling power consumption while controlling to a specified temperature. The specified temperature could be the liquid coolant  14  temperature entering the liquid cooled information technology equipment rack  16  of servers, e.g. information technology equipment  22 . 
     In an embodiment, system  10  includes a liquid cooled chiller-less data center  18  cooling system that comprises liquid cooled information technology equipment rack  16 , e.g. electronics rack(s), which is liquid cooled, side-car(s) air to liquid heat exchanger(s)  32 , and optional liquid-to-liquid heat exchanger(s)  36  ( FIGS. 19 and 20 ). The heat dissipated by the electronics components, e.g. information technology equipment  22 , within the rack(s)  16  is transferred to the liquid coolant  14  partially by direct thermal conduction using CPU Cold Plates and DIMM spreaders attached to liquid cooled cold rails within the server(s) and partially by air to liquid heat exchange in which air flowing over server components extracts heat from the components which is rejected to the side car(s)  32 . This heat is then transported to the outdoor heat exchanger  34  where it is dissipated to the ambient air environment. 
     The rate of heat transfer at the rack(s)  16  and the side car(s)  32  is governed by the liquid coolant  14  flow rate through them and air flow rate over the server components and side car heat exchanger within the rack  16 . The air flow rate is determined by the rpm of the bank of fans within each server as shown in  FIG. 21C  and/or optionally could be an external fan within the rack  16 . At the outdoor heat exchanger  34 , the heat transfer rate is governed by the outdoor heat exchanger air-side flow rate and the liquid coolant  14  flow rate through the outdoor heat exchanger. The heat transfer rate is a non-linear monotonically increasing function of air-side flow rate and liquid coolant  14  flow rate. 
     In an embodiment, for any given outdoor heat exchanger  34  design there is a limit to the air-side flow rate and liquid coolant  14  flow rate. These limits are used to guide the outdoor heat exchanger  34  selection so as to meet the upper cooling requirements (worst case scenario) by a safe margin. Worst case scenario here refers to highest ambient air temperature and highest heat dissipation at the rack(s)  16 , and in a more general sense, highest heat dissipation at the data center  18 , occurring simultaneously. A worst case scenario which exceeds the cooling capability at the highest heat dissipation may never occur over the life cycle for a system designed with a safety margin. 
     A control algorithm, executing on the controller-apparatus  20 , based on data center  18  heat dissipation and on ambient air temperature, is used to properly improve the cooling power consumption and further reduce the data center energy usage. Also, in certain conditions where the outdoor air temperature is significantly high, it becomes important to maintain the liquid coolant  14  temperature going to the liquid cooled information technology equipment rack  16  below a certain threshold to ensure proper functioning of the IT equipment  22 . 
     System  10  uses a set of temperature-based servo control algorithms, for a liquid cooled chiller-less data center  18  that can be implemented to reduce the data center cooling power consumption while controlling to a specified temperature under varying temperature and workload conditions. In one exemplary embodiment the specified temperature could be the liquid coolant  14  temperature entering the rack  16  of servers  22 . 
     An embodiment is shown in  FIGS. 22 and 23  in which the system operates at a specified lower, e.g. minimum, cooling power setting as long as the temperature being controlled, (T_Measured) is between a Minimum Temperature Target and a Maximum Temperature Target. For example, the rack  16  inlet coolant temperature could be controlled between a Max target and Min target for any given outdoor weather condition and IT Equipment  22  power and/or workload. 
     As shown in flow diagram of  FIG. 22 , the cooling system  10  is started at a specified lower or minimum cooling power setting. This minimum setting need not be the global minimum for the cooling system  10 , but rather a user selectable input. 
     If in element  202  T Measured approaches the T min target the system  10  goes into a winter mode operation and begins to open a recirculation valve to maintain the system above the dew point. If in element  204  the T Measured increases above the T min target, the cooling system  10  begins to close the recirculation valves. 
     In element  206 , T Measured is compared to the T max target and if T_Measured is below the T max target temperature, the cooling system  10  operates at its lower or minimum cooling power setting. 
     If T_Measured is above the T max target, the servo loop  208  is engaged to control the cooling elements to servo T_Measured close to the Target temperature. For example, the external loop pump flow rate and the outdoor heat exchanger  34  fans  28  speed could be changed proportionately to keep T_Measured close to the Target temperature. 
     This approach provides three distinct zones of control: 1) Below T min target—In this zone, the system  10  responds to keep the temperature above the dew point; 2) Above T min target and Below T max target—The system  10  operates in an energy efficient cooling mode to reduce the cooling power; 3) Above T max target—The system  10  servo is initiated to control the cooling elements to maintain a T max target. 
     The system  10  could be implemented in a number of ways. One way is to program the control algorithm onto the programmable logic controller (PLC) unit, e.g. controller-apparatus  20 , controlling the outdoor heat exchanger  34  fans  28  and liquid coolant pumps  26  operation. Another way is to run the control algorithm on a remote computer that takes in the required input information from the data center  18  and outputs an improved solution to the cooling system  10 . 
       FIGS. 19 and 20  represents two liquid cooled chiller-less data center  18  cooling designs and schematic of a typical volume server node.  FIG. 19  represents a single loop configuration that consists of liquid cooled rack(s)  16 , side car(s)  32 , outdoor heat exchanger(s)  34 , electronically controlled by-pass/recirculation valve(s) and controller(s)  20  used to implement the control algorithms. 
     For the single loop configuration of  FIG. 19 , the inputs to the controller  20  could be ambient temperature, humidity, and/or the like, coolant inlet, outlet temperatures, and/or the like, room dew point temperature, temperature of server components, e.g. central processing unit (CPU), dual inline memory modules (DIMMs), hard-drives, and/or the like, coolant flow rate, air flow rate (outside), air flow rate (rack), and/or the like, and server power. The outputs of the controller  20  could be the outdoor Fan  28  RPM, Server Fan(s)  38 , Pump  26  RPM, recirculation valve (percent open), and/or the like. 
       FIG. 20  represents a dual loop configuration which consists of liquid cooled rack(s)  16 , side car(s)  32 , liquid-to-liquid heat exchanger(s)  36 , outdoor heat exchanger(s)  34 , electronically controlled by-pass/recirculation valve(s), and controller(s)  20  used to implement the control algorithms. For the dual loop configuration, the inputs to the controller  20  could be ambient temperature, humidity, and/or the like, indoor loop coolant inlet and outlet temperatures, outdoor loop coolant temperature, room dew point temperature, temperature of server components such as CPU, DIMMs, hard-drives, and/or the like, coolant flow  14  rate, air flow rate (outside), air flow rate (rack), and server  22  power. The outputs of the controller  20  could be the outdoor Fan  28  RPM, Server Fan(s)  38 , outdoor Pump  26  RPM, Indoor Pump  26  RPM, recirculation valve (percent open), and/or the like. 
     The configurations shown in  FIGS. 19 and 20  can be generalized to having up to I number of fans  28 , J number of server fans  38  and K number of liquid coolant pumps  26 . So, in a more general sense, the RPM of I number of outdoor heat exchanger  34  fans and K number of liquid (where I, J, and K are integers) coolant pumps  26  can be regulated individually or simultaneously to reduce/improve the data cooling energy and can subsequently reduce the total data center  18  energy consumption while controlling to a specified temperature. 
       FIG. 21  illustrates a typical volume server rack  16  and a schematic of a partially liquid cooled and partially air cooled volume server node. The inputs from the rack  16  may include air temperature entering each node in each rack, each and every operational CPU&#39;s temperature or platform environment control interface (PECI), each and every DIMM temperature, hard-drives temperatures and temperature of any other key components. The controller  20  may also monitor all the inputs, all the outputs, power consumption by all the various components (data center  18  as well as facility side). 
     The heat dissipated by the electronic rack(s)  16  is transfer to the liquid coolant—partially by direct thermal conduction using CPU Cold Plates and DIMM spreaders attached to liquid cooled cold rails within the server(s) and partially by air to liquid heat exchange in which air flowing over server components extracts heat from the components which is rejected to the side car(s)  32 . In case of a single loop design, the heat is then transported to the outdoor heat exchanger(s)  34  where it is dissipated to the ambient air. 
     In case of a dual loop, the heat is first transferred from the inner coolant loop to the outer coolant loop via liquid-to-liquid heat exchanger(s)  36  and is then transported to the outdoor heat exchanger(s)  34  where it is dissipated to the ambient air. The rate of heat transfer at the rack(s)  16  and the side car(s)  32  is governed by the liquid coolant flow rate through them and air flow rate over the server components and side car heat exchanger within the rack  16 . 
       FIG. 23  shows an expected temperature history when this control is implemented for rack  16  inlet coolant temperature servo control. The upper design specification on the rack  16  inlet coolant temperature could be 40 C. So a target temperature of, say 38 C, can be selected. The lower settings could be specified to be 6 gpm internal loop flow rate, 4 gpm external loop flow rate, 150 rpm outdoor heat exchanger  34  fans speed, server fans speed of 5000 rpm, and recirculation valves fully closed. The maximum settings could be specified to be 6 gpm internal loop flow rate, 10 gpm external loop flow rate, 750 rpm outdoor heat exchanger  34  fans speed, sever fan speed of 15000 rpm, and recirculation valves fully closed. 
     If the T_Measured goes above the Target temperature (38 C), the external loop pump flow rate and the outdoor heat exchanger  34  fans speed could be changed proportionately between the specified minimum and maximum setting to keep T_Measured close to the Target temperature. If the T_Measured is in between the minimum temperature target and the maximum temperature target, T_Measured is allowed to drift and the cooling system operates at the specified minimum setting. 
       FIG. 24  shows a sample servo loop that regulates the external loop flow rate and outdoor heat exchanger  34  fans speed to keep/maintain T_Measured close to the specified target temperature. First, initial values are set: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 SET Initial Values 
               
               
                   
                 Set Minimum Values 
               
               
                   
                 Min Ext Flow = 4 GPM      Min Fan RPM = 150 
               
               
                   
                 Set Maximum Values 
               
               
                   
                 Max Ext Flow = 10 GPM   Max Fan RPM  = 750 
               
               
                   
                 Set Internal Pump Value at Fixed Value 
               
               
                   
                 Internal Pump Flow Rate  = 6 GPM 
               
               
                   
                 Set Target Temperature 
               
               
                   
                 target  =  Input by User (e.g. 38 C) 
               
               
                   
                 Set Gain 
               
               
                   
                 Gain = Input by User 
               
               
                   
                 (Could be 1*13.1/Q; ... Q is IT load in kW) 
               
               
                   
                   
               
            
           
         
       
     
     The Gain here relates the temperature deltas to the IT head load. Next, the servo loop is initiated: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 START of Servo Loop (For rack inlet temperature 
               
               
                   
                 servo to target temperature) 
               
               
                   
                 Measure Rack Inlet Temperature 
               
               
                   
                 T rack inlet = Measured Value 
               
               
                   
                 Calculate an error signal 
               
               
                   
                 T error = T rack inlet − T target 
               
               
                   
                 Calculate Control Temperature 
               
               
                   
                 Tc = T error * Gain 
               
               
                   
                 Divide the Control Temperature proportionately 
               
               
                   
                 between Ext Pump and Fan 
               
               
                   
                 Tp = Tc*Ap/Tpf ... .. (Example, Tp = Tc*5/12.5) 
               
               
                   
                 Tf = Tc*Af/Tpf ...... (Example, Tf = Tc*7.5/12.5) 
               
               
                   
                 where, Delta T = (T rack inlet − T outdoor 
               
               
                   
                 ambient) 
               
               
                   
                 Tpf = Delta T at min − Delta T at max ext pump &amp; 
               
               
                   
                 fan speed settings 
               
               
                   
                 Ap = Delta T at min − Delta T at max ext pump 
               
               
                   
                 settings (that is, max Delta T achievable by changing the 
               
               
                   
                 ext loop flow settings) 
               
               
                   
                 Af = Delta T at min − Delta T at max fan speed 
               
               
                   
                 settings (that is, max Delta T achievable by changing the 
               
               
                   
                 fan speed settings) 
               
               
                   
                   
               
            
           
         
       
     
     This dependence of Tp and Tf on Tc can be altered based on the cooling system components. Fp(T) is external flow required for a given delta T. For example, Fp=37.43*((−1.0916*Tp+9.1526)^−1.01). Ff(T) is external fan rpm required for a given delta T. For e.g., Ff=669.33*((−0.7107*Tf+6.2004)^−0.8197). 
     
       
         
           
               
             
               
                   
               
             
            
               
                      Ext Flow = Fp   (Tp) 
               
               
                      Fan RPM = Ff   (Tf) 
               
               
                      Set Limits on Max and Min RPM and Ext Flow 
               
               
                      If  Ext  Flow &lt; Min Ext Flow   then   Ext Flow 
               
               
                 = Min Ext Flow 
               
               
                      If  Fan RPM  &lt; Min Fan RPM   then  Fan RPM = 
               
               
                 Min Fan RPM 
               
               
                      If  Ext Flow  &gt; Max Ext Flow  then  Ext Flow = 
               
               
                 Max Ext Flow 
               
               
                      If  Fan RPM  &gt; Max Fan RPM  then  Fan RPM = 
               
               
                 Max Fan RPM 
               
               
                      Set New Ext Flow and Fan RPM 
               
               
                      External Pump Flow Rate =  Ext Flow 
               
               
                      Heat Exchanger Fan RPM = Fan RPM 
               
               
                      Back to start of loop 
               
               
                      END SERVO LOOP 
               
               
                   
               
            
           
         
       
     
       FIGS. 25 and 26  illustrate the graph of sample Fp(Tp) and Ff(Tf), respectively, of an example for cooling system  10 . According to these functions, in order to get, for example 3C, from the pump, the pump flow rate should be changed from 4 gpm to ˜6.3 gpm and in order to get 3 C from the external fans, the fans speed should be changed from 150 RPM to ˜210 RPM. 
     However, to get a further 1 C from the pump, the pump flow rate should be changed from ˜6.3 gpm to ˜7.8 gpm and to get a further 1 C from the external fans, the external fans speed should be changed from ˜210 RPM to ˜250 RPM. In general, this control could be extended to I fans and K pumps and the relationship functions such as Fp and Ff can either be generated analytically or numerically. 
     As will be appreciated by one skilled in the art, aspects may be embodied as a system, method, and/or computer program product. Accordingly, embodiments may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, embodiments may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
     A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electromagnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. 
     Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
     Computer program code for carrying out operations for aspects of the embodiments may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     Aspects of the embodiments are described above with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to the embodiments. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the embodiments. The embodiment was chosen and described in order to best explain the principles of the embodiments and the practical application, and to enable others of ordinary skill in the art to understand the various embodiments with various modifications as are suited to the particular use contemplated. 
     While the preferred embodiment has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the embodiments first described.