Patent Publication Number: US-7584021-B2

Title: Energy efficient CRAC unit operation using heat transfer levels

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a Continuation Application of copending application Ser. No. 11/594,721, filed on Nov. 8, 2006, which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   A data center may be defined as a location, for instance, a room, that houses computer systems arranged in a number of racks. A standard rack, for example, an electronics cabinet, is defined as an Electronics Industry Association (EIA) enclosure, 78 in. (2 meters) high, 24 in. (0.61 meter) wide and 30 in. (0.76 meter) deep. These racks are configured to house a number of computer systems, about forty (40) systems, with future configurations of racks being designed to accommodate 200 or more systems. The computer systems typically include a number of printed circuit boards (PCBs), mass storage devices, power supplies, processors, micro-controllers, and semi-conductor devices, that dissipate relatively significant amounts of heat during their operation. For example, a typical computer system comprising multiple microprocessors dissipates approximately 250 W of power. Thus, a rack containing forty (40) computer systems of this type dissipates approximately 10 KW of power. 
   The power required to transfer the heat dissipated by the components in the racks to the cool air contained in the data center is generally equal to about 10 percent of the power needed to operate the components. However, the power required to remove the heat dissipated by a plurality of racks in a data center is generally equal to about 50 percent of the power needed to operate the components in the racks. The disparity in the amount of power required to dissipate the various heat loads between racks and data centers stems from, for example, the additional thermodynamic work needed in the data center to cool the air. In one respect, racks are typically cooled with fans that operate to move cooling air across the heat dissipating components; whereas, data centers often implement reverse power cycles to cool heated return air. The additional work required to achieve the temperature reduction, in addition to the work associated with moving the cooling fluid in the data center and the condenser, often add up to the 50 percent power requirement. As such, the cooling of data centers presents problems in addition to those faced with the cooling of the racks. 
   Conventional data centers are typically cooled by operation of one or more computer room air conditioning (CRAC) units. For example, compressors of CRAC units typically consume a minimum of about thirty (30) percent of the required operating energy to sufficiently cool the data centers. The other components, for example, condensers and air movers (fans), typically consume an additional twenty (20) percent of the required total operating energy. As an example, a high density data center with 100 racks, each rack having a maximum power dissipation of 10 KW, generally requires 1 MW of cooling capacity. CRAC units with a capacity of 1 MW of heat removal generally requires a minimum of 300 KW input compressor power in addition to the power needed to drive the air moving devices, for instance, fans and blowers. Conventional data center CRAC units do not vary their cooling fluid output based on the distributed needs of the data center. Instead, these CRAC units generally operate at or near a maximum compressor power level even when the heat load is reduced inside the data center. 
   The substantially continuous operation of the CRAC units is generally designed to operate according to a worst-case scenario. For example, CRAC units are typically designed around the maximum capacity and redundancies are utilized so that the data center may remain on-line on a substantially continual basis. However, the computer systems in the data center may only utilize around 30-50% of the maximum cooling capacity. In this respect, conventional cooling systems often attempt to cool components that may not be operating at a level which may cause their temperatures to exceed a predetermined temperature range. Consequently, many conventional cooling systems often incur greater amounts of operating expenses than may be necessary to sufficiently cool the heat generating components contained in the racks of data centers. 
   Other types of conventional CRAC units are configured to vary the temperature of the cooling fluid as well as the volume flow rate of the cooling fluid supplied into the data center. These types of CRAC units often include cooling systems configured to vary the temperature of the received cooling fluid prior to delivery into the data center. The cooling systems include variable capacity compressors and chilled water systems. In addition, these CRAC units also include blowers with variable frequency drives configured to vary the volume flow rate of the cooling fluid delivered into the data center. 
   The temperatures to which the cooling systems cool the cooling fluid received from the data center are often based upon the detected temperature of the cooling fluid returned into the CRAC units. In addition, the speeds of the blowers are often correlated to the operations of the cooling systems. In this respect, as the cooling systems are operated to reduce the temperature of the cooling fluid, the blowers are also operated to increase the volume flow rate of the cooled cooling fluid. Operating the cooling systems in this manner is inefficient as both the reduction in cooling fluid temperature and increase in the cooling fluid volume flow rate are typically unnecessary to maintain the components in the data center within predetermined temperature ranges. 
   SUMMARY OF THE INVENTION 
   A method for controlling one or more computer room air conditioning (CRAC) units for energy efficient operation is disclosed. In the method, the temperature of the air returned (Trat) into the one or more CRAC units and the temperature of the air supplied (Tsat) by the one or more CRAC units is detected. The caloric heat transfer level (Q) is calculated based upon the Trat and the Tsat and it is determined whether the Q is within a predetermined setpoint caloric heat transfer range. In addition, at least one operation of the one or more CRAC units is reduced in response to the Q being within the predetermined setpoint caloric heat transfer range to thereby increase the efficiencies of the one or more CRAC units. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will become apparent to those skilled in the art from the following description with reference to the figures, in which: 
       FIG. 1A  shows a simplified plan view of a data center, according to an embodiment of the invention; 
       FIG. 1B  shows a cross-sectional side view taken along lines IIA-IIA of  FIG. 1A , according to an embodiment; 
       FIG. 1C  shows a cross-sectional side view taken along lines IIB-IIB of  FIG. 1A , according to another embodiment; 
       FIG. 1D  shows a cross-sectional side view taken along lines IIB-IIB of  FIG. 1A , according to a further embodiment; 
       FIGS. 2A-2C  are respective block diagrams of CRAC control systems operable to control CRAC units according to various embodiments; 
       FIG. 3  illustrates a graph of various cooling system operating levels and the costs associated with their operations, according to an embodiment; 
       FIGS. 4A and 4B  illustrate flow diagrams of operational modes of methods for CRAC unit control based upon setpoint temperatures and setpoint caloric heat transfer determinations, respectively, according to various embodiments; and 
       FIG. 5  illustrates a computer system which may be used as a platform for various operations described in the present disclosure, according to an embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   For simplicity and illustrative purposes, the present invention is described by referring mainly to an exemplary embodiment thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one of ordinary skill in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention. 
   Throughout the present disclosure, reference is made to “cooling fluid” and “heated cooling fluid”. For purposes of simplicity, “cooling fluid” may generally be defined as air that has been cooled by a cooling device, e.g., an air conditioning unit. In addition, “heated cooling fluid” may generally be defined as cooling fluid that has been heated. It should be readily apparent, however, that the terms “cooling fluid” are not intended to denote air that only contains cooled fluid and that “heated cooling fluid” only contains cooling fluid that has been heated. Instead, embodiments of the invention may operate with air that contains a mixture of heated cooling fluid and cooling fluid. In addition, cooling fluid and heated cooling fluid may denote gases other than air, e.g., refrigerant and other types of gases known to those of ordinary skill in the art that may be used to cool electronic components. 
   According to an example, computer room air conditioning (CRAC) units include systems to enable energy efficient cooling and supply of cooling fluid to a data center. In addition, the systems of the CRAC units are operated in manners to generally optimize the costs associated with cooling components contained in the data center. The CRAC units may thus comprise variably controllable systems designed and operated to cool the components under substantially optimized cost structures. 
   In one example, the variably controllable systems include chilled fluid systems having a two-way or a three-way valve for variably controlling the flow of chilled fluid, for instance, water, refrigerant, or other coolant, etc., through a cooling coil. In another example, the variably controllable systems include variable capacity compressors designed to variably control cooling of a refrigerant configured to absorb heat from the cooling fluid received from the data center. In either of the examples above, the variably controllable systems include blowers with variable frequency drives configured to control the outputs of the cooling fluid cooled through heat transfer with the fluid contained in the cooling coil. 
   The variably controllable systems may be operated in manners to generally optimize their energy utilization while maintaining thermal management requirements of the components in the data centers. In one respect, the variably controllable systems may be operated in substantially independent manners to enable the substantial optimization of energy utilization. For instance, the variably controllable systems may be operated to decrease output of cooled cooling fluid in response to a decrease in the temperature of the cooled cooling fluid. In addition, the variably controllable systems may be operated to increase output of cooled cooling fluid in response to an increase in the temperature of the cooled cooling fluid. As the energy requirements of the variably controllable systems may be minimized through these operations, the costs associated with maintaining the components within the bounds of thermal management concerns may also be substantially minimized. 
   With reference first to  FIG. 1A , there is shown a simplified plan view of a data center  100 , according to an embodiment of the invention. The terms “data center” are generally meant to denote a room or other space and are not meant to limit the invention to any specific type of room where data is communicated or processed, nor should it be construed that use of the terms “data center” limits the invention in any respect other than its definition hereinabove. 
   As shown in the  FIG. 1A , the data center  100  includes a plurality of racks  102 , for instance, electronics cabinets, generally positioned in substantially parallel rows. The racks  102  each house one or more components (not shown). These components may include, for instance, computers, servers, monitors, hard drives, disk drives, etc., designed to perform various operations. Some operations of the components may include, for instance, computing, switching, routing, displaying, etc. These components may comprise subsystems (not shown), for example, processors, micro-controllers, high-speed video cards, memories, semi-conductor devices, and the like to perform these functions. In the performance of these electronic functions, the components, and therefore the subsystems, generally dissipate relatively large amounts of heat. Because the racks  102  have been known to include upwards of forty (40) or more subsystems, they may dissipate substantially large amounts of heat. Cooling fluid is therefore supplied to generally flow around and through the components to absorb the dissipated heat through convection, to maintain the subsystems and the components generally within predetermined operating temperature ranges. 
   The cooling fluid is illustrated as being supplied through vent tiles  104  in the floor  106  of the data center  100 . As will be seen in  FIGS. 1B-1D , the floor  106  is a raised floor with a space therebelow. The space generally enables power lines, communication lines, and other wires (not shown), to be located below the floor  106  such that the wires and communication lines are substantially positioned away from an upper surface of the floor  106 . The space may also function as a plenum for delivery of cooling fluid from computer room air conditioner (CRAC) units  108  and  110  to the racks  102 . The vent tiles  104  are illustrated as being positioned between pairs of adjacent rows of racks  102 . 
   Air or other cooling fluid is received by the CRAC units  108  and  110 , cooled through heat transfer within the CRAC units  108  and  110  and supplied into the space below the floor  106 . 
   The cooled cooling fluid is supplied from the space below the floor  106 , through the vent tiles  104  and through the racks  102  to cool the components housed in the racks  102 . The CRAC units  108  and  110  may control various characteristics of the cooling fluid supplied to the racks  102 . For instance, the CRAC units  108  and  110  may contain variably controllable systems (not shown) configured to vary the temperature of the cooling fluid supplied to the racks  102 . In addition, the CRAC units  108  and  110  may contain systems configured to vary the volume flow rate of the cooling fluid supplied to the racks  102 . Various types of systems arranged in various configurations may be employed to control the temperature and volume flow rate of the cooling fluid. Examples of suitable components and configurations are illustrated in  FIGS. 1B-1D , which are described in greater detail hereinbelow. 
   The aisles  116  between the racks  102  having vent tiles  104  located therebetween may be considered as cool aisles  116 . These aisles  116  are considered “cool aisles” because they are configured to receive cooling fluid from the vent tiles  104 . In addition, the racks  102  are positioned to receive cooling fluid from the cool aisles  116 . The aisles  118  between the racks  102  which do not have vent tiles  104  may be considered as hot aisles  118 . These aisles are considered “hot aisles” because they are positioned to receive cooling fluid heated by the components in the racks  102 . 
   Also illustrated in  FIG. 1A  is a computing device  112 . The computing device  112  may comprise a computer system, a controller, microprocessor, etc., configured to control operations of the CRAC units  108  and  110 . More particularly, the computing device  112  may be configured to receive input from sensors (not shown) and to vary operations of the various variable controllable systems contained in the CRAC units  108  and  110 . The computing device  112  may also be configured to receive input from a user, for instance, data center personnel, an administrator, a manager, etc. The input received from a user may comprise various set points by which the computing device  112  may determine how and when to manipulate the operations of the variable controllable systems. The computing device  112  may, in one instance, compare the conditions, for example, temperature, humidity, pressure, etc., detected by the sensors with predetermined set points for those conditions and control the variably controllable systems in response to differences between the set points and the detected conditions. 
   The computing device  112  is illustrated as communicating with the CRAC units  108  and  110  via wired communication lines  114 . However, it should be understood that communications between the CRAC units  108  and  110  and the computing device  112  may be effectuated through a wireless protocol, such as IEEE 802.11b, 802.11g, wireless serial connection, Bluetooth, etc., or combinations thereof, without departing from a scope of the invention. In addition, although a single computing device  112  is illustrated as controlling both CRAC units  108  and  110 , each of the CRAC units  108  and  110  may include their own computing device  112 . Moreover, the computing device  112  may comprise controllers that are integrally formed or otherwise form part of each of the CRAC units  108  and  110 . Thus, although the data center  100  has been illustrated as containing a certain configuration, it should readily be understood that various other configurations are possible for the data center  100  without departing from a scope of the invention. 
   The data center  100  depicted in  FIG. 1A  represents a generalized illustration and other components may be added or existing components may be removed or modified without departing from the scope of the invention. For example, the data center  100  may include any number of racks and various other apparatuses known to be housed in data centers. Thus, although the data center  100  is illustrated as containing four rows of racks  102 , it should be understood that the data center  100  may include any number of racks, e.g., 100 racks, without departing from the scope of the invention. The depiction of four rows of racks  102  is thus for illustrative and simplicity of description purposes only and is not intended to limit the invention in any respect. In addition, the data center  100  may include any number of CRAC units  108  and  110 , each having a number of different types cooling systems. 
   The data center  100  may also include a lowered ceiling (not shown) configured with returns for receiving heated cooling fluid from within the data center  100 . The lowered ceiling may also include or form a plenum for directing the heated cooling fluid to the CRAC units  108  and  110 . An example of a data center  100  having a lowered ceiling may be found in co-pending and commonly assigned U.S. patent application Ser. No. 10/262,879, filed on Apr. 17, 2002, the disclosure of which is hereby incorporated by reference in its entirety. 
   In  FIGS. 1B-1D , there are shown simplified partial sections of the data center  100  with three examples of the CRAC units  108 ,  110 , and  110 ′.  FIGS. 1B-1D  represent generalized illustrations and other components may be added or existing components may be removed or modified without departing from the scope of the invention. In addition, for instance, although the CRAC units  108  and  110  are illustrated as having different configurations from each other, the CRAC units  108  and  110  employed in the data center  100  illustrated in  FIG. 1A  may have the same type of configuration without departing from a scope of the invention. 
   With particular reference first to  FIG. 1B , there is shown a cross-sectional side view taken along lines IIA-IIA of  FIG. 1A . As shown, the CRAC unit  108  comprises a vapor-compression type air conditioning unit. More particularly, the CRAC unit  108  includes a blower  120  or a fan for delivering air or other cooling fluid into a space  122 . The space  122  may be created beneath the raised floor  106  and may include or otherwise function as a plenum. The blower  120  may also operate to draw heated cooling fluid from the data center  100  by generally forcing airflow through the CRAC unit  108 . In this regard, the CRAC unit  108  may include one or more openings to receive the heated cooling fluid from the data center  100 . A variable frequency drive (VFD)  124  is shown as being positioned adjacent to the blower  120 . The VFD  124  generally operates to control the blower  120  to vary the volume flow rate of cooling fluid flow into and out of the CRAC unit  108 . 
   The VFD  124  may comprise any reasonably suitable VFD that is commercially available from any number of manufacturers. The VFD  124  generally operates to variably control the speed of an alternating current (AC) induction motor. More particularly, the VFD  124  may operate to convert power from fixed voltages/fixed frequencies to variable voltages/variable frequencies. By controlling the voltage/frequency levels of the blower  120 , the volume flow rate of the cooling fluid supplied by the CRAC unit  108  may also be varied. 
   Although the VFD  124  is illustrated as being positioned adjacent to the blower  120 , the VFD  124  may be positioned at any reasonably suitable location with respect to the blower  120  without departing from a scope of the invention. The VFD  120  may be positioned, for instance, outside of the CRAC unit  108  or various other locations with respect to the CRAC unit  108 . 
   In operation, the heated cooling fluid (shown as the arrow  126 ) enters into the CRAC unit  108  and is cooled by operation of a cooling coil  128   a , a compressor  130 , a condenser  132 , and an expansion valve  134 , which may operate under a vapor-compression cycle. By way of example, a refrigerant, for instance, R-134a, etc., may be contained in a refrigerant line  136 , which generally forms a loop between the various components of the cooling system containing the CRAC unit  108 . More particularly, the refrigerant is supplied into the cooling coil  128   a  where it absorbs heat through convection from the cooling fluid received from the data center  100 . The cooled cooling fluid then flows out of the CRAC unit  108  and into the space  122  as indicated by the arrow  142 . 
   The heated refrigerant flows into the compressor  130 , which compresses or pressurizes the refrigerant. The compressor  130  may comprise a variable capacity compressor or it may comprise a constant capacity compressor having a hot gas bypass (not shown). In any regard, the pressurized refrigerant then flows into the condenser  132  where some of the heat in the refrigerant is dissipated into the air around the data center  100 . Although not shown, the condenser  132  may include a fan to generally enhance heat dissipation of the refrigerant. The refrigerant then flows through the expansion valve  134  and back through the cooling coil  128   a . This process may be substantially continuously repeated as needed to cool the cooling fluid drawn into the CRAC unit  108 . In terms of cooling system efficiency, it is generally desirable that the heated cooling fluid supplied into the CRAC unit  108  is composed of the relatively warmest portion of air in the room  100 . 
   The cooling system illustrated in  FIG. 1B  has been described in a relatively simplified manner. Therefore, it should be understood that the cooling system incorporating the CRAC unit  108  may include additional components without departing from a scope of the invention. For instance, a three-way valve may be included to allow some of the refrigerant to bypass the compressor  130  and return into the cooling coil  128   a . The three-way valve may be used, for instance, to divert some of the refrigerant exiting the cooling coil  128   a  back into the refrigerant line  136  for re-entry into the cooling coil  128  to generally ensure that the refrigerant is almost entirely in gaseous form prior to entering the compressor  130 . 
   As described hereinabove, the computing device  112  may be configured to control various operations of the CRAC unit  108 . For instance, the computing device  112  may be configured to control the operations of the compressor  130  to thereby control the temperature and flow of the refrigerant flowing through the cooling coil  128   a . The computing device  112  may also be configured to control the VFD  124 . More particularly, the computing device  112  may control the motor speed of a blower  122  to thereby control the volume flow rate of the cooled cooling fluid supplied by the CRAC unit  108 . By controlling the temperature of the refrigerant and the airflow rate through the CRAC unit  108 , the computing device  112  is generally capable of controlling the level of heat transfer between the heated cooling fluid and the refrigerant to thereby control the temperature of the cooling fluid supplied into the data center  100 . 
   According to an example, the computing device  112  is configured to substantially independently control the compressor  130  and the VFD  124 . The computing device  112  may be configured to determine manners in which to control the compressor  130  and the VFD  124  based upon, for instance, environmental condition measurements obtained by sensors  138  and  140 . As shown in  FIG. 1B , the sensor  138  is positioned at an inlet of the CRAC unit  108  and is thus configured to measure one or more conditions of the cooling fluid returning to the CRAC unit  108 . 
   In addition, the sensor  140  is positioned at an outlet of the CRAC unit  108  and is thus configured to measure one or more conditions of the cooling fluid supplied by the CRAC unit  108 . Alternatively, the sensor  140  may be positioned at an inlet of a rack  102  or near a vent tile  104 , provided that the rack  102  or the vent tile  104  is located within a relatively close proximity to the exhaust of CRAC unit  108 . More particularly, the sensor  140  may be positioned at a location substantially downstream of the CRAC unit  108  where the temperature of the cooling fluid supplied by the CRAC unit  108  does not vary beyond a certain level from the time the cooling fluid exits the CRAC unit  108 . In one respect, the computing device  112  may be configured to control the compressor  130  and the VFD  124  to substantially minimize energy usage by the CRAC unit  110  as will described in greater detail hereinbelow. 
   With reference now to  FIG. 1C , there is shown a cross-sectional side view taken along lines IIB-IIB of  FIG. 1A . As shown, the CRAC unit  110  comprises a chiller type air conditioning unit. More particularly, the CRAC unit  110  includes a blower  120  or a fan for delivering air or other cooling fluid into a space  122 . As described hereinabove with respect to  FIG. 1B , the space  122  may be created beneath the raised floor  106  and may include or otherwise function as a plenum. The blower  120  may also operate to draw heated cooling fluid from the data center  100  by generally forcing airflow through the CRAC unit  110 . In this regard, the CRAC unit  110  may include one or more openings to receive the heated cooling fluid from the data center  100 . A variable frequency drive (VFD)  124  is shown as being positioned adjacent to the blower  120 . The VFD  124  generally operates to control the blower  120  to vary the volume flow rate of cooling fluid flow into and out of the CRAC unit  110  as described hereinabove. 
   The arrow  126  indicates the heated cooling fluid received by the CRAC unit  110 . The heated cooling fluid flows past a cooling coil  128   b  and exchanges heat with a coolant contained in the cooling coil  128   b . The coolant may comprise water or other fluid capable of being heated and cooled in a repeated manner. The speed at which the heated cooling fluid flows past the cooling coil  128   b  and the temperature of the coolant contained in the cooling coil  128   b  generally affect the temperature of the cooling fluid. Thus, for instance, as the temperature of the coolant decreases with the blower  120  operating at a constant level, so too does the temperature of the cooling fluid. The cooled cooling fluid then flows out of the CRAC unit  110  and into the space  122  as indicated by the arrow  142 . 
   The temperature of the coolant contained in the cooling coil  128   b  may be controlled through operation of a cooling system comprising the CRAC unit  110 . In operation, the coolant receives heat from the cooling fluid received into the CRAC unit  110 . The heat transfer from the cooling fluid into the coolant in the cooling coil  128   b  may be effectuated through convection. The heated coolant then flows out of the cooling coil  128   b  and into a first coolant line  144   a . The heated coolant flows through the first coolant line  144   a  and into a heat exchanger  146  which may also include a coil  148 . The heated coolant is cooled through heat transfer with a refrigeration circuit  150 , which includes an evaporator  152 , a compressor  154 , a condenser  156  and an expansion valve  158 . The refrigeration circuit  150  may operate under a vapor-compression cycle generally known to those of ordinary skill in the art. 
   The cooled coolant returns toward the cooling coil  128   b  through a second coolant line  144   b . A three-way valve  160  is provided generally upstream from the cooling coil  128   b  along the second coolant line  144   b . The three-way valve  160  generally operates to control the amount of cooled coolant supplied into the cooling coil  128   b . The three-way valve  160  may control the cooled coolant delivery into the cooling coil  128   b  by diverting some or all of the cooled coolant back into the first coolant line  144   a  through a third coolant line  144   c , thereby bypassing the cooling coil  128   b . The three-way valve  160  may thus substantially control the temperature of the coolant delivered into the cooling coil  128   b  by controlling the amount of cooled coolant delivered into the cooling coil  128   b . In one respect, therefore, the three-way valve  160  may also control the temperature of the cooling fluid supplied into the space  122 . 
   A pump  162  is illustrated as being located along the first coolant line  144   a . The pump  162 , however, may be positioned along the second coolant line  144   b  without departing from a scope of the invention. The pump  162  generally operates to pressurize the coolant contained in the coolant lines  144   a - 144   c , such that the coolant may flow along the circuit created by the coolant lines  144   a - 144   c . The pump  162  may be controlled in addition to or in place of the three-way valve  160  to enable reduced energy usage. In one regard, because the pump  162  may be operated to vary the flow rate of the coolant in the coolant lines  144   a - 144   c , the pump  162  operations may be reduced, for instance, commensurate with increases in the cooling fluid temperature. In addition, a valve configured to enable a substantially constant and predictable coolant flow in the coolant lines  144   a  and  144   b  may be positioned upstream of the pump  162 . The valve may include a spring-loaded valve configured to deliver constant flow for certain pressure ranges. A suitable valve may be available from GRISWOLD CONTROLS of Irvine, Calif. 
   In operation, the temperature of the coolant contained in the coolant lines  144   a - 144   c  generally dictates the amount of energy consumed in operating the CRAC unit  110 . More particularly, the refrigeration circuit  150  generally requires less energy when the temperature of the coolant entering into the heat exchanger  146  is lower. In contrast, the refrigeration circuit  150  generally consumes greater amounts of energy when the temperature of the coolant entering into the heat exchanger  146  is higher. In addition, the desired temperature of the coolant supplied from the heat exchanger  146  also generally dictates the amount of energy consumed by the refrigeration circuit  150 . That is, the more work required by the refrigeration circuit  150  in reducing the temperature of the coolant, the greater the energy consumption. 
   In one example, the refrigeration circuit  150  is operated to cool the coolant to substantially the highest temperature where the three-way valve  160  may remain in a generally fully open position to thus cause substantially all of the coolant to flow into the cooling coil  128   b . In this regard, the energy consumed by the refrigeration circuit  150  may be substantially minimized as relatively no coolant is diverted away from the cooling coil  128   b . Moreover, energy consumption of the refrigeration circuit  150  may be lower because the temperature of the refrigerant contained in the refrigeration circuit  150  may be higher and because coolant at higher temperatures generally gains less energy from its surroundings. When multiple CRAC units  110  are employed to cool the components in a data center  100 , at least one of the CRAC units  110  may be operated in this manner to thereby reduce energy usage of the at least one of the CRAC units  110 . 
   The computing device  112  is configured to substantially independently control the three-way valve  160  and the VFD  124  to thereby control the temperature of the cooling fluid and the volume flow rate of the supplied cooling fluid. The computing device  112  may be configured to determine manners in which to control the three-way valve  160  and the VFD  124  based upon, for instance, environmental condition measurements obtained by the sensors  138  and  140 . In one respect, the computing device  112  may be configured to control the three-way valve  160  and the VFD  124  to substantially minimize energy usage by the CRAC unit  110  as will described in greater detail hereinbelow. 
   Although reference is made in  FIGS. 1B and 1C  to the use of a blower  120  to draw heated cooling fluid from the data center  100 , it should be understood that any other reasonably suitable manner of cooling fluid removal from the data center  100  may be implemented without departing from the scope of the invention. By way of example, a separate fan or blower (not shown) may be employed to draw heated cooling fluid from the data center  100 . In addition, the CRAC units  108  and  110  may include a humidifier and/or a dehumidifier as is known to those of ordinary skill in the art. 
   In addition, one or more isolation valves (not shown) may be placed at various locations along the coolant lines  144   a - 144   c  to thereby enable, for instance, preventative maintenance. 
     FIG. 1D  depicts a cross-sectional side view taken along lines IIB-IIB of  FIG. 1A , according to another example. In  FIG. 1D , there is shown a CRAC unit  110 ′. The CRAC unit  110 ′ includes all of the components illustrated in  FIG. 1C  and thus specific reference to those components are not reiterated. Instead, only those elements depicted in  FIG. 1D , which differ from the components depicted in  FIG. 1C  are discussed hereinbelow. 
   The major difference between the CRAC unit  110  and the CRAC unit  110 ′ is that the CRAC unit  110 ′ includes a two-way valve  164  in place of the three-way valve  160 . In addition, the CRAC unit  110 ′ does not include the third coolant line  144   c  illustrated in  FIG. 1C . The CRAC unit  110 ′ also includes a mass flow sensor  166  positioned along the first coolant line  144   a . The mass flow sensor  166  is configured to detect the mass flow rate of the fluid flowing through the first coolant line  144   a . The mass flow sensor  166  may be required in the CRAC unit  110 ′ since the two-way valve  164  does not enable constant coolant flow through the coolant lines  144   a  and  144   b , as is the case with the three-way valve  160  of the CRAC unit  110 . In addition, with use of the two-way valve  164 , the valve orifice opening in the two-way valve  164  may require calibration. 
   The pump  162  may be controlled in addition to or in place of the two-way valve  164  to enable reduced energy usage. In one regard, because the pump  162  may be operated to vary the flow rate of the coolant in the coolant lines  144   a - 144   c , the pump  162  operations may be reduced, for instance, commensurate with increases in the cooling fluid temperature. 
   In addition; the temperature of the coolant supplied from the heat exchanger  146  also generally dictates the amount of energy consumed by the refrigeration circuit  150 . That is, the more work required by the refrigeration circuit  150  in reducing the temperature of the coolant, the greater the energy consumption. In one example, the refrigeration circuit  150  is operated to cool the coolant to substantially the highest temperature where the two-way valve  164  may remain in a generally fully open position to thus cause substantially all of the coolant to flow into the cooling coil  128   b . The energy consumed by the refrigeration circuit  150  may be lower because the temperature of the refrigerant contained in the refrigeration circuit  150  may be higher and because coolant at higher temperatures generally gains less energy from its surroundings. When multiple CRAC units  110  are employed to cool the components in a data center  100 , at least one of the CRAC units  110  may be operated in this manner to thereby reduce energy usage of the at least one of the CRAC units  110 . 
     FIGS. 2A-2C  are respective block diagrams  200 ,  250 , and  250 ′ of CRAC control systems  202 ,  252 , and  252 ′ operable to control the CRAC units  108 ,  110  and  110 ′. The following descriptions of the block diagrams  200 ,  250 ,  250 ′ are some manners of a variety of different manners in which such CRAC control systems  202 ,  252 ,  252 ′ may be configured. In addition, it should be understood that the block diagrams  200 ,  250 ,  250 ′ may include additional components and that some of the components described herein may be removed and/or modified without departing from the scope of the invention. 
   With reference first to  FIG. 2A , the CRAC control system  202  includes a controller  204  for controlling operations of the CRAC control system  202 . The controller  204  may comprise the computing device  112  and thus may also comprise a microprocessor, a micro-controller, an application specific integrated circuit (ASIC), and the like. The controller  204  is generally configured to receive temperature measurements from an inlet temperature sensor  138 , an outlet temperature sensor  140  and an optional power meter  206 . 
   As described hereinabove, the inlet temperature sensor  138  generally operates to detect the temperature of the heated cooling fluid received by the CRAC unit  108 . In addition, the outlet temperature sensor  140  is configured to detect the temperature of the cooled cooling fluid supplied by the CRAC unit  108 . In a general sense, the controller  204  may determine manners in which to control the CRAC unit  108  based substantially upon the temperatures detected by the temperature sensors  138  and  140 . 
   Communications between the sensors  138  and  140  and the controller  204  may be effectuated through, for instance, an Ethernet-type connection or through a wired protocol, such as IEEE 802.3, etc., or wireless protocols, such as IEEE 802.11b, 802.11g, wireless serial connection, Bluetooth, etc., or combinations thereof. 
   The temperature information received from the temperature sensors  138  and  140  may be stored in a memory  208 . In addition, various control schemes for operating the CRAC unit  108  may be stored in the memory  208 . In this regard, the memory  208  may comprise a traditional memory device, such as, volatile or non-volatile memory, such as DRAM, EEPROM, flash memory, combinations thereof, and the like. The controller  204  may thus access information stored in the memory  208  to determine the manners in which the CRAC unit  108  may be operated. 
   The optional power meter  206  may detect the power consumption of the CRAC unit  108  and thus may be positioned or otherwise configured to measure the power consumption of the CRAC unit  108 . The power meter  206  may comprise any reasonably suitable, and commercially available power meter capable of measuring the CRAC unit  108  power consumption. The controller  204  may receive the detected power consumption and may also store this information in the memory  208 . The power meter  206  is considered as being optional because the controller  204  may be configured to calculate the CRAC unit  108  power consumption based upon operations of the various components, for instance, compressor  130 , blower  120 , etc. As an example, the controller  204  may be configured to determine the power consumption of the compressor  130  based upon its current operating load. A correlation between the power consumption levels and the operating loads of the compressor  130  may be employed to make this determination. 
   With reference now to  FIG. 2B , the CRAC control system  252  includes similar components to those described hereinabove with respect to the CRAC control system  202 . Therefore, only those components that differ from the elements described hereinabove with respect to the CRAC control system  202  will be described. More particularly, the CRAC control system  252  includes the CRAC unit  110  instead of the CRAC unit  108 . In this regard, the CRAC control system  252  is configured to control the three-way valve  160  to vary the temperature of the cooling fluid supplied to the data center  100 . 
   As an example, the controller  204  may operate to control the three-way valve  160  and the blower  120  in manners to substantially minimize the power consumption of the CRAC unit  110  while maintaining the temperature of the cooling fluid supplied by the CRAC unit  110  within the threshold setpoint temperature range. The controller  204  may thus determine various operating conditions for the three-way valve  160  and the blower  120  to substantially minimize the power consumptions associated with their operations. Although reference is made throughout the present disclosure to the control of the blower  120 , the controller  204  may control the VFD  124  to thereby control the blower  120  speed. 
   As depicted in  FIG. 2C , the CRAC control system  252 ′ includes similar components to those described hereinabove with respect to the CRAC control systems  202  and  252 . Therefore, only those components that differ from the elements described hereinabove with respect to those CRAC control systems  202  and  252  will be described. As shown, the CRAC control system  252 ′ includes the CRAC unit  110 ′ instead of the CRAC units  108  and  110 . In this regard, the CRAC control system  252 ′ is configured to control the two-way valve  164  to vary the temperature of a coolant and therefore vary the temperature of the cooling fluid supplied by the CRAC unit  110 ′. 
   In addition, the controller  204  may control the blower  120  to control the volume flow rate of the cooling fluid supplied by the CRAC unit  110 ′. In this regard, the controller  204  may control the temperature and the volume flow rate of the cooling fluid supplied by the CRAC unit  110 ′ in manners to substantially minimize the power consumption of the CRAC unit  110 ′ while maintaining the temperature of the cooling fluid supplied by the CRAC unit  110  within the threshold setpoint temperature range. The controller  204  may thus determine various operating conditions for the two-way valve  164  and the blower  120  to substantially minimize the power consumptions associated with their operations. 
   In each of the CRAC control systems  202 ,  252 ,  252 ′, the controller  204  may be configured to receive input from a user, for instance, a technician, an administrator, etc. As described in greater detail hereinbelow, the controller  204  may include one or more input devices, for instance, keyboard, mouse, disk drives, etc., for receiving input from the user. The input may, for instance, be in the form of predetermined operating set points for the CRAC units  108 ,  110 ,  110 ′. By way of example, a user may input a setpoint temperature (Tset) range into the controller  204 . The setpoint temperature (Tset) range may be based upon desired heat removal characteristics in the data center  100 . In one respect, the setpoint temperature (Tset) range may comprise temperatures that ensure safe operating conditions for the components housed in the data center  100 . The safe operating conditions for the components may be based upon the specifications provided by the component manufacturers. Alternatively, the safe operating conditions may be determined through testing of the components or through historical data. For instance, the components may be operated at various temperatures to determine at which temperatures the performance characteristics of the components being to decline or when the components begin to fail. 
   A maximum setpoint temperature (Tset,max) of the setpoint temperature (Tset) range may constitute an upper limit of safe operating conditions for the components. In other words, if the heated cooling fluid returning to the CRAC units  108 ,  110 ,  110 ′ is above the maximum setpoint temperature (Tset,max), it may be determined that the temperature of the components may be beyond the safe operating conditions. As another example, a minimum setpoint temperature (Tmin,set) of the setpoint temperature (Tset) range may constitute a lower limit indicating a temperature at which operations of the CRAC units  108 ,  110 ,  110 ′ may be ceased. In addition, the controller  204  may store the inputted threshold setpoint temperature (Tset) range in the memory  208 . 
   In addition, the controller  204  may utilize the information received from one or both of the sensors  138 ,  140 , the power meter  206 , and the user received input, to determine manners in which to operate the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  of the CRAC unit  108 . In one example, the controller  204  may operate the compressor  130 , three-way valve  160 , or the two-way valve  164 , and the blower  120  to substantially minimize power consumption of the respective CRAC units  108 ,  110 ,  110 ′ while maintaining the temperature of heated cooling fluid returned to the CRAC units  108 ,  110 ,  110 ′ within the setpoint temperature (Tset) range. Thus, for instance, the controller  204  may manipulate the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  operations to various levels so long as the temperature of the heated cooling fluid returned to the CRAC units  108 ,  110 ,  110 ′ remains within the setpoint temperature (Tset) range. 
   As another example, the controller  204  may determine manners in which to operate the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  based upon the loading of the CRAC unit  108 . In this instance, the controller  204  may be configured to calculate the caloric heat transfer from the heated cooling fluid to the refrigerant of the CRAC units  108 ,  110 ,  110 ′. The caloric heat transfer (Q) may be calculated from the following equation:
 
 Q=mC   p ( T   out   −T   in ),  Equation (1)
 
where m is the mass flow rate of the cooling fluid, C p  is the heat capacity of the cooling fluid, T out  is the temperature of the cooled cooling fluid supplied and T in  is the temperature of the heated cooling fluid received by the CRAC units  108 ,  110 ,  110 ′.
 
   According to this example, a setpoint caloric heat transfer (Qset) range may be used in place of the setpoint temperature (Tset) range. Thus, for instance, the controller  204  may be configured to substantially minimize the power consumptions of the CRAC units  108 ,  110 ,  110 ′ by varying operations of the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  so long as the caloric heat transfer (Q) is within the setpoint caloric heat transfer (Qset) range. In one respect, the caloric heat transfer (Qset) range may comprise heat transfer rates that ensure safe operating conditions for the components housed in the data center  100 . The safe operating conditions for the components may be based upon the specifications provided by the component manufacturers. Alternatively, the safe operating conditions may be determined through testing of the components or through historical data. For instance, the components may be operated at various temperatures to determine at which temperatures the performance characteristics of the components being to decline or when the components begin to fail. 
   In similar fashion to those manners described hereinabove, if the calculated caloric heat transfer (Q) is above a maximum setpoint caloric heat transfer level (Qset,max), the components in the data center  100  may be insufficiently cooled. In addition, if the calculated caloric heat transfer (Q) is below a minimum setpoint caloric heat transfer level (Qset,min), operations of the CRAC units  108 ,  110 ,  110 ′ may be ceased as the CRAC unit  108  may be drawing power unnecessarily. 
   In operation, the controllers  204  of the CRAC control systems  202 ,  252 ,  252 ′ may determine the compressor  130 , three-way valve  160 , or the two-way valve  164 , and the blower  120  operations to substantially minimize CRAC unit  108 ,  110 ,  110 ′ power consumptions when the temperatures of the heated cooling fluid returned to the CRAC units  108 ,  110 ,  110 ′ are within the setpoint temperature (Tset) range. In addition, operations of these systems may be varied when the caloric heat transfer is within the setpoint caloric heat transfer (Qset) range. More particularly, the controllers  204  may determine which combinations of compressor  130 , three-way valve  160 , or two-way valve  164 , and blower  120  operations substantially minimize CRAC unit  108 ,  110 ,  110 ′ power consumption levels when the temperatures of the cooling fluid received from the components are within an acceptable range. 
   Thus, for instance, if the temperatures of the cooling fluid supplied from the components are acceptable, the controllers  204  may select operating levels of the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  that substantially minimize the costs associated with their operations. These operating levels and costs may be considered in terms of the graph  300  illustrated in  FIG. 3 . In the graph  300 , there is illustrated two x-axes  302  and  304  and a y-axis  306 . The first x-axis  302  denotes the speed of the blower  120  and the second x-axis  304  denotes the temperature of the cooling fluid (Tcf,out) supplied by the respective CRAC units  108 ,  110 ,  110 ′. The y-axis  306  denotes the energy consumptions and thus the costs associated with various cooling fluid temperatures (i.e., compressor  130 , three-way valve  160 , or two-way valve  164 , operations) and blower  120  speeds. 
   The power consumption levels or costs associated with operating the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  at various levels may be based upon manufacturer provided specifications. In addition, or alternatively, the power consumption levels or costs may be determined through testing. In terms of testing, for instance, the power meter  206  may be used to measure the power draws of the compressor  130 , the refrigeration circuit  150  (under various three-way valve  160  and the two-way valve  164  settings), and the blower  120  at different levels of operation. The correlations between the power consumption levels or costs and the operating levels of the compressor  130 , the refrigeration circuit  150 , and the blower  120  may be stored in the memory  208 . This information may be stored in the form of, for instance, a look-up table, or through other searchable means. 
   As shown in the graph  300 , the energy consumption level of the compressor  130  (or a refrigeration circuit  150  in the case of the CRAC units  110 ,  110 ′, decreases as the temperature of the cooling fluid (Tcf,out) supplied by the CRAC unit  108  decreases at constant CRAC unit loading. In addition, the energy consumption level of the blower  120  increases as the speed of the blower  120  increases. Thus, the controllers  204  of the CRAC units  108 ,  110 ,  110 ′ may be configured to vary the operations of the compressor  130 , the three-way valve  160 , or the two-way valve  164 , and the blower  120  such that they consume the least amount of power while maintaining the temperatures of the cooling fluid returned into the CRAC units  108 ,  110 ,  110 ′ within the setpoint temperature ranges. 
     FIGS. 4A and 4B  illustrate flow diagrams of operational modes  400  and  450  of methods for CRAC unit control based upon setpoint temperatures and setpoint caloric heat transfer determinations, respectively. It is to be understood that the following descriptions of the operational modes  400  and  450  are two manners of a variety of different manners in which CRAC unit control may be effectuated. It should also be apparent to those of ordinary skill in the art that the operational modes  400  and  450  represent generalized illustrations and that other steps may be added or existing steps may be removed, modified or rearranged without departing from a scope of the invention. 
   The descriptions of the operational modes  400  and  450  are made with reference to the block diagrams  200 ,  250 ,  250 ′ illustrated in  FIGS. 2A-2C , respectively, and thus makes reference to the elements cited therein. It should, however, be understood that the operational modes  400  and  450  are not limited to the elements set forth in the block diagrams  200 ,  250 ,  250 ′. Instead, it should be understood that the operational modes  400  and  450  may be practiced by CRAC unit control systems having a different configuration than those set forth in the block diagrams  200 ,  250 ,  250 ′. 
   The operational modes  400  and  450  may be initiated or started at steps  402  and  452 , respectively, for instance, by activating one or more CRAC units  108 ,  110 ,  110 ′, activating one or more components in the data center  100 , etc. In addition, or alternatively, the operational mode  400  may be manually initiated, after a predetermined period of time, etc. It is to be understood that either or both of the operational modes  400  and  450  may be performed depending upon the configuration of the CRAC units  108 ,  110 ,  110 ′. For instance, those CRAC units  108 ,  110 ,  110 ′ configured to operate based upon setpoint temperatures, may perform the operational mode  400 , whereas those CRAC units  108 ,  110 ,  110 ′ configured to operate based upon setpoint caloric heats may perform the operation mode  450 . Additionally, performance of either operational mode  400  and  450  may be user-specified. 
   With reference first to the operational mode  400  of  FIG. 4A , the controllers  204  of one or more of the CRAC control systems  200 ,  250 ,  250 ′ may receive a setpoint temperature (Tset) range as indicated at step  404 . The setpoint temperature (Tset) range may be supplied by the CRAC manufacturers or they may user-specified and inputted into the computing device  112  through any known input means. Step  404 , however, may be omitted for situations in which the controllers  204 , for instance, have previously received the setpoint temperature (Tset) range. 
   At step  406 , one or more of the sensors  138  may detect the temperatures of the return air (Trat). A comparison of the detected return air temperatures (Trat) and the setpoint temperature (Tset) range may be made at step  408 . More particularly, at step  408 , it may be determined whether the temperatures of the heated cooling fluid returning into the CRAC units  108 ,  110 ,  110 ′ are within the setpoint temperature (Tset) range. For those CRAC units  108 ,  110 ,  110 ′ having detected return air temperatures (Trat) outside of the setpoint temperature (Tset) range, the controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may determine whether the detected return air temperatures (Trat) are below minimum setpoint temperature levels (Tset,min), at step  410 . The minimum setpoint temperature levels (Tset,min) for the CRAC units  108 ,  110 ,  110 ′ may be the same for each of the CRAC units  108 ,  110 ,  110 ′ or they may vary for each of the CRAC units  108 ,  110 ,  110 ′. In this regard, for instance, each of the CRAC units  108 ,  110 ,  110 ′ may be operated in substantially independent manners. 
   At step  410 , for those CRAC units  108 ,  110 ,  110 ′ having detected return air temperatures (Trat) are not below the minimum set point temperature levels (Tset,min), the detected return air temperatures (Trat) are considered as being above maximum setpoint temperature levels (Tset,max), since they are outside of the setpoint temperature (Tset) ranges. The controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may therefore decrease the temperature and/or increase the volume flow rate of cooling fluid supplied to the data center  100 , as indicated at step  412 . The decreased temperature and/or the increased cooling fluid volume flow rate may be required to bring the detected return air temperatures (Trat) within the maximum setpoint temperature levels (Tset,max). 
   Additionally, at step  412 , the controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may decrease the temperature of the refrigerant/coolant and/or increase the volume flow rate of cooling fluid supplied based upon the costs associated with each action. For instance, if the costs associated with decreasing the temperature of the refrigerant/coolant is relatively less than increasing the volume flow rate, the controllers  204  may cause the refrigerant/coolant temperature to be decreased while maintaining the volume flow rate level. As another example, if the controllers  204  determine that a combination of actions are associated with the lowest costs, the controllers  204  may find substantially optimum combinations of actions to achieve the desired results at the lowest costs. 
   As another example, at step  412 , the controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may decrease the temperature of the refrigerant/coolant and/or increase the volume flow rate of cooling fluid supplied based upon the known effectiveness of each action. Thus, for instance, the controllers  204  may have access to historical data indicating the effects of the various actions taken by the CRAC units  108 ,  110 ,  110 ′. By way of example, if it is determined that reducing the refrigerant/coolant temperature to a certain level requires X amount of energy and increasing the volume flow rate to another certain level requires the same amount of energy, and increasing the volume flow rate is more effective, the controllers  204  may decide to increase the volume flow rate as this action is more efficient. 
   At step  410 , for those CRAC units  108 ,  110 ,  110 ′ having detected return air temperatures (Trat) that are below the minimum set point temperature level (Tset,min), those CRAC units  108 ,  110 ,  110 ′ may enter a sleep mode as indicated at step  414 . The sleep mode may include a powered down mode in which the CRAC units  108 ,  110 ,  110 ′ draw reduced amounts of power as compared to when the CRAC units  108 ,  110 ,  110 ′ are fully operational. The reduced amounts of power may comprise power states that are somewhere between the fully operational mode and a completely shut down mode. In addition, the sleep mode may constitute a power saving mode in which the CRAC units  108 ,  110 ,  110 ′ may be reactivated or otherwise brought back to fully operational status in a relatively short period of time. The reduced power state of the CRAC units  108 ,  110 ,  110 ′ may vary for differing types of CRAC units. 
   In any regard, the sleep mode may include a mode in which the power supply to the temperature sensor  138  positioned to detect the temperature of the cooling fluid around the inlet of the CRAC unit  108 ,  110 ,  110 ′, remains active. In addition, the sleep mode may also include the supply of a small amount of power to enable the blower to substantially continuously cause a relatively small amount of cooling fluid flow through the CRAC units  108 ,  110 ,  110 ′. In this regard, the temperatures of the cooling fluid supplied into the CRAC units  108 ,  110 ,  110 ′ may be substantially continuously monitored when the CRAC units  108 ,  110 ,  110 ′ are in the sleep mode. 
   The CRAC units  108 ,  110 ,  110 ′ may exit from the sleep mode, for instance, when the detected return air temperature (Trat) exceeds the maximum setpoint temperature (Tset,max), as indicated at step  412 . Alternatively, the CRAC units  108 ,  110 ,  110 ′ may be configured to exit from the sleep mode when the return air temperature (Trat) exceeds another predefined temperature, which may be defined according to, for instance, operating requirements of the components to which the CRAC units  108 ,  110 ,  110 ′ delivers cooling fluid. As another alternative, the CRAC units  108 ,  110 ,  110 ′ may exit the sleep mode after a predetermined period of time, manually revived, in response to receipt of a setpoint temperature range, etc. 
   In another example, a plurality of CRAC units  108 ,  110 ,  110 ′ may be networked or otherwise configured to communicate with one another. For instance, the same controller  204  may control the plurality of CRAC units  108 ,  110 ,  110 ′. In any regard, the controllers  204  of the CRAC units  108 ,  110 ,  110 ′ may be configured to communicate their statuses to the other CRAC units  108 ,  110 ,  110 ′. The statuses of the CRAC units  108 ,  110 ,  110 ′ may be used by the controllers  204  to determine the provisioning levels of the CRAC units  108 ,  110 ,  110 ′. By way of example, if one of the CRAC units  108 ,  110 ,  110 ′ is in the sleep mode and the provisioning level of a neighboring CRAC unit  108 ,  110 ,  110 ′ is too high, for instance, the return air temperature (Trat) into the neighboring CRAC unit  108 ,  110 ,  110 ′ is above a predefined level, the CRAC unit  108 ,  110 ,  110 ′ may be brought out of the sleep mode. In this instance, the return air temperature (Trat) may not need to be measured during the sleep mode thereby enabling that CRAC unit  108 ,  110 ,  110 ′ to draw less power when in the sleep mode. 
   With reference back to step  408 , for those CRAC units  108 ,  110 ,  110 ′ having return air temperatures (Trat) within the setpoint temperature (Tset) range, the controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may determine the power consumption of the respective cooling systems at step  416 . The cooling systems may comprise, for instance, the compressor  130  in  FIG. 1B  or the refrigeration circuits  150  in  FIGS. 1C and 1D . The controllers  204  may also determine the power consumptions of the blowers  120  at step  418 . 
   The power meter  206  may be employed to determine the power consumptions of the cooling system components. Alternatively, the power consumption may be calculated based upon operations of the various components, for instance, the compressor  130 , the blower  120 , etc. As an example, the controllers  204  may be configured to determine the power consumption of the compressor  130  based upon its current operating load. A correlation between the power consumption levels and the operating loads of the compressor  130  may be employed to make this determination. 
   At step  420 , the power consumptions of the cooling systems and the blower  120  may be correlated to a cost function. For instance, the costs associated with the power consumed by the cooling systems and the blower  120  may be determined. In addition, the power consumed by the cooling systems and the blower  120  may be utilized in determining the operations of the cooling systems and the blower  120 . The power consumptions of the cooling systems may include a determination of the conditions external to the condenser  132  or refrigeration circuit  150 . That is, for instance, the costs incurred by the cooling systems may vary according to the external conditions. For instance, if ambient conditions are relatively hot and/or humid, greater amounts of energy may be expended by the cooling systems to enable sufficient heat transfer between the refrigerant and/or coolant to thereby maintain the refrigerant and/or coolant at desired temperatures. 
   At step  422 , the controllers  204  may determine whether the costs may be reduced. The controllers  204  may ascertain whether costs may be reduced through, for instance, a determination of the output requirements of the CRAC units  108 ,  110 ,  110 ′ to substantially maintain the cooling fluid temperature and delivery to the components in the data center within the setpoint temperature ranges. 
   If the costs cannot be reduced, that is, the controllers  204  determine that the CRAC units  108 ,  110 ,  110 ′ are operating at or near optimum energy levels, the controllers  204  may not vary the cooling system operations and the operational mode  400  may be continued, for instance, at step  406 . However, if the controllers  204  determine that costs may be reduced, the controllers  204  may determine a scheme to enable the costs associated with operating the cooling systems to be reduced at step  424 . The controllers  204  may determine how costs may be reduced based upon the costs associated with increasing refrigerant/coolant temperature and/or decreasing the volume flow rate of the cooling fluid supplied. For instance, if the cost savings associated with increasing the temperature of the refrigerant/coolant is relatively higher than the cost savings associated with decreasing the volume flow rate, the controllers  204  may cause the refrigerant/coolant temperature to be increased while maintaining the volume flow rate level. In the alternative, if the cost savings associated with decreasing the volume flow rate is relatively higher than the cost savings associated with increasing the refrigerant/coolant temperature, the controllers  204  may cause the volume flow rate to be decreased while maintaining the refrigerant/coolant temperature level. As another example, if the controllers  204  determine that a combination of actions produces the greatest cost savings, the controllers  204  may find substantially optimum combinations of actions to achieve the desired results at the greatest cost savings. 
   At step  426 , the controllers  204  may implement the reduced cost scheme determined at step  424 . The reduced cost scheme may be implemented according to an iterative process or it may implemented according to historical data. If an iterative process is implemented, the controllers  204  may cause either or both of the cooling fluid temperature to the increased and the volume flow rate to the decreased incrementally until the CRAC units  108 ,  110 ,  110 ′ are operating at or near optimal levels. If historical data is relied upon, the controllers  204  may know how to manipulate the CRAC units  108 ,  110 ,  110 ′ based upon previously performed manipulations to reach the substantially optimal performance levels. 
   In addition, the operational mode  400  may be continued to thereby enable substantially continuous monitoring and control over the CRAC units  108 ,  110 ,  110 ′. In one respect, operations of the CRAC units  108 ,  110 ,  110 ′ may be substantially continuously altered to enable energy and cost savings. 
   With reference now to the operational mode  450  of  FIG. 4B , the controllers  204  of one or more of the CRAC control systems  200 ,  250 ,  250 ′ may receive a setpoint caloric heat transfer (Qset) range at step  454 . The caloric heat transfer (Q) of a CRAC unit  108 ,  110 ,  110 ′ may be used to determine the workload on the CRAC unit  108 ,  110 ,  110 ′ and may be determined through Equation (1) recited hereinabove. In this regard, and as described in greater detail hereinbelow, if the caloric heat transfer (Q) of a CRAC unit  108 ,  110 ,  110 ′ is within a predefined range, operations of the compressor  130 , the three-way valve  160 , or the two-way valve  164  may be varied to substantially minimize their energy consumptions. It should be appreciated that step  454  may be omitted for situations in which the controllers  204 , for instance, have previously received the setpoint caloric heat transfer (Qset) range. 
   At step  456 , one or more of the sensors  138  may detect the temperatures of the return air (Trat) and one or more of the sensors  140  may detect the temperatures of the supply air (Tsat) at step  458 . At step  460 , the controllers  204  may calculate the caloric heat transfer rates (Q). In addition, the controllers  204  may determine whether the calculated caloric heat transfer rates (Q) are within the setpoint caloric heat transfer (Qset) range at step  462 . 
   For those CRAC units  108 ,  110 ,  110 ′ having calculated caloric heat transfer rates (Q) that are within the Qset range, steps  416 - 426  set forth in the box A of  FIG. 4A  may be performed at step  464 . However, for those CRAC units  108 ,  110 ,  110 ′ having calculated caloric heat transfer rates (Q) that are outside of the Qset range, the controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may determine whether the calculated caloric heat transfer rates (Q) are below minimum setpoint caloric heat transfer levels (Qset,min) at step  466 . The Qset,min for the CRAC units  108 ,  110 ,  110 ′ may be the same for each of the CRAC units  108 ,  110 ,  110 ′ or they may vary for each of the CRAC units  108 ,  110 ,  110 ′. In this regard, for instance, each of the CRAC units  108 ,  110 ,  110 ′ may be operated in substantially independent manners. 
   At step  466 , for those CRAC units  108 ,  110 ,  110 ′ having calculated caloric heat transfer rates (Q) are not below the minimum set point caloric heat transfer levels (Qset,min), the calculated caloric heat transfer rates (Q) are considered as being above maximum setpoint caloric heat transfer levels (Qset,max), since they are outside of the setpoint caloric heat transfer (Qset) ranges. The controllers  204  of those CRAC units  108 ,  110 ,  110 ′ may determine whether the flow rates (FR) of the cooling fluid supplied by those CRAC units  108 ,  110 ,  110 ′ are below a flow rate set point (FRset). The flow rate (FR) of the cooling fluid supplied by the CRAC units  108 ,  110 ,  110 ′ may be detected through use of, for instance, an anemometer. In addition, or alternatively, the flow rate (FR) may be determined based upon the speed of the VFD. In any regard, the flow rate set point (FRset) may be based upon, for instance, historical data that indicates, for instance, a flow rate of cooling fluid supplied by the CRAC units  108 ,  110 ,  110 ′ are optimal for a given CRAC unit  108 ,  110 ,  110 ′. The optimum flow rates may be based, for instance, on the configuration and airflow patterns of the areas in which the CRAC units  108 ,  110 ,  110 ′ are configured to deliver the cooling fluid. In this regard, the flow rate setpoints may vary for each of the CRAC units  108 ,  110 ,  110 ′ and may also vary as airflow patterns change. 
   If it is determined at step  468  that the flow rate (FR) exceeds the flow rate setpoint (FRset), the flow rate may not be varied. If, however, it is determined that the flow rate (FR) does not exceed the flow rate setpoint (FRset), the volume flow rate of the CRAC unit  108 ,  110 ,  110 ′ may be increased as indicated at step  470 . The level of increase in the volume flow rate may be based upon various factors. For instance, the level of increase may be based upon a set percentage of increase and may be based upon an iterative process where the level of increase is performed during each cycle until the flow rate (FR) equals or exceeds the flow rate setpoint (FRset). As another example, the level of increase may be based upon historical data that indicates the level of temperature change in the areas affected by the CRAC units  108 ,  110 ,  110 ′ in response to various VFD speeds. 
   Also, at step  466 , for those CRAC units  108 ,  110 ,  110 ′ having caloric heat transfer rates (Q) that are below the minimum set point caloric heat transfer level (Qset,min), those CRAC units  108 ,  110 ,  110 ′ may enter a sleep mode as indicated at step  414 . The sleep mode may include a powered down mode in which the CRAC units  108 ,  110 ,  110 ′ draw reduced amounts of power as compared to when the CRAC units  108 ,  110 ,  110 ′ are fully operational. The reduced amount of power may comprise a power state that is somewhere between the fully operational mode and a completely shut down mode. In addition, the sleep mode may constitute a power saving mode in which the CRAC units  108 ,  110 ,  110 ′ may be reactivated or otherwise brought back to fully operational status in a relatively short period of time. The reduced power state of the CRAC unit  108 ,  110 ,  110 ′ may vary for differing types of CRAC units. 
   In any regard, the sleep mode may include a mode in which the power supply to the temperature sensor  138  positioned to detect the temperature of the cooling fluid around the inlet of the CRAC unit  108 ,  110 ,  110 ′, remains active. In addition, the sleep mode may also include the supply of a small amount of power to enable the blower to substantially continuously cause a relatively small amount of cooling fluid flow through the CRAC unit  108 ,  110 ,  110 ′. In this regard, the temperature of the cooling fluid supplied into the CRAC unit  108 ,  110 ,  110 ′ may be substantially continuously monitored when the CRAC unit  108 ,  110 ,  110 ′ is in the sleep mode. 
   The CRAC units  108 ,  110 ,  110 ′ may exit from the sleep mode, for instance, when the calculated caloric heat transfer rates (Q) exceed the maximum setpoint caloric heat transfer level (Qset,max), as indicated at step  412 . Alternatively, the CRAC units  108 ,  110 ,  110 ′ may be configured to exit from the sleep mode when the return air temperature (Trat) exceeds another predefined temperature, which may be defined according to, for instance, operating requirements of the components to which the CRAC units  108 ,  110 ,  110 ′ delivers cooling fluid. As another alternative, the CRAC units  108 ,  110 ,  110 ′ may exit the sleep mode after a predetermined period of time, manually revived, in response to receipt of a setpoint temperature range, etc. 
   In another example, a plurality of CRAC units  108 ,  110 ,  110 ′ may be networked or otherwise configured to communicate with one another. For instance, the same controller  204  may control the plurality of CRAC units  108 ,  110 ,  110 ′. In any regard, the controllers  204  of the CRAC units  108 ,  110 ,  110 ′ may be configured to communicate their statuses to the other CRAC units  108 ,  110 ,  110 ′. The statuses of the CRAC units  108 ,  110 ,  110 ′ may be used by the controllers  204  to determine the provisioning levels of the CRAC units  108 ,  110 ,  110 ′. By way of example, if one of the CRAC units  108 ,  110 ,  110 ′ is in the sleep mode and the provisioning level of a neighboring CRAC unit  108 ,  110 ,  110 ′ is too high, for instance, the return air temperature (Trat) into the neighboring CRAC unit  108 ,  110 ,  110 ′ is above a predefined level, the CRAC unit  108 ,  110 ,  110 ′ may be brought out of the sleep mode. In this instance, the return air temperature (Trat) may not need to be measured during the sleep mode thereby enabling that CRAC unit  108 ,  110 ,  110 ′ to draw less power when in the sleep mode. 
   Through operation of the operational modes  400  and  450 , the energy consumption levels of the CRAC units  108 ,  110 ,  110 ′ and therefore the costs associated with their operations may substantially be minimized. In one regard, the CRAC units  108 ,  110 ,  110 ′ may be operated substantially independently from one another in manners to generally enable their energy efficient operations. 
   The operations set forth in the operational modes  400  and  450  may be contained as utilities, programs, or subprograms, in any desired computer accessible medium. In addition, the operational modes  400  and  450  may be embodied by computer programs, which can exist in a variety of forms both active and inactive. For example, it can exist as software program(s) comprised of program instructions in source code, object code, executable code or other formats. Any of the above can be embodied on a computer readable medium, which include storage devices and signals, in compressed or uncompressed form. 
   Exemplary computer readable storage devices include conventional computer system RAM, ROM, EPROM, EEPROM, and magnetic or optical disks or tapes. Exemplary computer readable signals, whether modulated using a carrier or not, are signals that a computer system hosting or running the computer program can be configured to access, including signals downloaded through the Internet or other networks. Concrete examples of the foregoing include distribution of the programs on a CD ROM or via Internet download. In a sense, the Internet itself, as an abstract entity, is a computer readable medium. The same is true of computer networks in general. It is therefore to be understood that any electronic device capable of executing the above-described functions may perform those functions enumerated above. 
     FIG. 5  illustrates an exemplary computer system  500 , according to an embodiment of the invention. The computer system  500  may include, for example, the controllers  204  and/or the computing device  112 . In this respect, the computer system  500  may be used as a platform for executing one or more of the functions described hereinabove with respect to the various components of the CRAC control systems  202 ,  252 ,  252 ′. 
   The computer system  500  includes one or more controllers, such as a processor  502 . The processor  502  may be used to execute some or all of the steps described in the operational modes  400  and  450 . Commands and data from the processor  502  are communicated over a communication bus  504 . The computer system  500  also includes a main memory  506 , such as a random access memory (RAM), where the program code for, for instance, the controllers  204  and/or the controller of the computing device  112 , may be executed during runtime, and a secondary memory  508 . The secondary memory  508  includes, for example, one or more hard disk drives  510  and/or a removable storage drive  512 , representing a floppy diskette drive, a magnetic tape drive, a compact disk drive, etc., where a copy of the program code for the provisioning system may be stored. 
   The removable storage drive  510  reads from and/or writes to a removable storage unit  514  in a well-known manner. User input and output devices may include a keyboard  516 , a mouse  518 , and a display  520 . A display adaptor  522  may interface with the communication bus  504  and the display  520  and may receive display data from the processor  502  and convert the display data into display commands for the display  520 . In addition, the processor  502  may communicate over a network, e.g., the Internet, LAN, etc., through a network adaptor  524 . 
   It will be apparent to one of ordinary skill in the art that other known electronic components may be added or substituted in the computer system  500 . In addition, the computer system  500  may include a system board or blade used in a rack in a data center, a conventional “white box” server or computing device, etc. Also, one or more of the components in  FIG. 5  may be optional (e.g., user input devices, secondary memory, etc.). 
   What has been described and illustrated herein is a preferred embodiment of the invention along with some of its variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.