Abstract:
Cooling control methods and systems include measuring a temperature of air provided to one or more nodes by an air-to-liquid heat exchanger; measuring a temperature of at least one component of the one or more nodes and finding a maximum component temperature across all such nodes; comparing the maximum component temperature to a first and second component threshold and comparing the air temperature to a first and second air threshold; and controlling a proportion of coolant flow and a coolant flow rate to the air-to-liquid heat exchanger and the one or more nodes based on the comparisons.

Description:
RELATED APPLICATION INFORMATION  
       [0001]    This application is related to application Ser. No. TBD, Attorney Docket No. YOR920120012US1 (163-476), entitled “PROVISIONING COOLING ELEMENTS FOR CHILLERLESS DATA CENTERS”, filed concurrently herewith and incorporated herein by reference. 
     
    
     GOVERNMENT RIGHTS  
       [0002]    This invention was made with Government support under Contract No.: DE-EE0002894 (Department of Energy). The government has certain rights in this invention. 
     
    
     BACKGROUND  
       [0003]    1. Technical Field 
         [0004]    The present invention relates to data center design and, more particularly to energy-efficient cooling systems in large data centers. 
         [0005]    2. Description of the Related Art 
         [0006]    Data centers are facilities that house numerous computer systems arranged in the form of electronics racks. Typically, a data center houses on the order thousands of electronic racks. Each computer system in a rack may include one or more processors, memory devices, controllers, power converters and manipulators, and other such electronic components. Depending upon the state of operation, a computer system may dissipate on the order of hundreds of Watts to thousands of Watts. Therefore, a significant amount of cooling is used to keep the electronic components within an optimum operating temperature range. Server driven power usage amounts to a significant portion of total US energy consumption. Liquid cooling solutions, which may include transferring 100% of the heat dissipated by the rack(s) to water, eliminating the facility air conditioning units, use of building chilled water to cool the racks, use of energy efficient chillers to provide relatively lower temperature coolants to the rack(s), and many other liquid cooling solutions, have been proposed as a means to reduce data center cooling/total power consumption. However, such solutions are far from optimal in their cooling energy efficiency. 
         [0007]    Furthermore, many cooling systems are at least partially based on air cooling. Cool air is pumped into servers, cools auxiliary components, and exists as warmer air. A heat exchanger cools the air, which re-enters the server as cool air. Although liquid-cooled components can be overcooled, the temperature difference between coolant temperature entering the air heat exchanger and the air temperature leaving the air heat exchanger can become a limiting factor. 
       SUMMARY  
       [0008]    A cooling control method includes measuring a temperature of air provided to one or more nodes by an air-to-liquid heat exchanger; measuring a temperature of at least one component of the one or more nodes and finding a maximum component temperature across all such nodes; comparing the maximum component temperature to a first and second component threshold and comparing the air temperature to a first and second air threshold; and controlling a proportion of coolant flow and a coolant flow rate to the air-to-liquid heat exchanger and the one or more nodes based on said comparisons. 
         [0009]    A further cooling control method includes measuring a temperature of air provided to one or more nodes by an air-to-liquid heat exchanger; measuring a temperature of at least one component of the one or more nodes and finding a maximum component temperature across all such nodes; comparing the maximum component temperature to a first and second component threshold and comparing the air temperature to a first and second air threshold; and controlling a proportion of coolant flow and a coolant flow rate to the air-to-liquid heat exchanger and the one or more nodes based on said comparisons by adjusting one or more valves that control relative flow rate between the air-to-liquid heat exchanger and the one or more nodes. 
         [0010]    A cooling system includes one or more nodes, each node having at least one temperature sensor to monitor a temperature of internal node components; an air-to-liquid heat exchanger configured to accept a liquid coolant input and to provide cooled air to the one or more nodes; a temperature sensor to monitor a temperature of the air provided by the air-to-liquid heat exchanger; a liquid cooling system configured to provide liquid coolant to components of the one or more nodes; a valve configured to control coolant flow to the air-to-liquid heat exchanger and the liquid cooling system based on the temperature of internal node components and the temperature of the air provided by the air-to-liquid heat exchanger; and a pump configured to provide liquid coolant to the liquid cooling system and the air-to-liquid heat exchanger, having a pump strength that is based on the temperature of internal node components and the temperature of the air provided by the air-to-liquid heat exchanger. 
         [0011]    These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0012]    The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
           [0013]      FIG. 1  is a diagram of single-loop and double-loop cooling systems; 
           [0014]      FIG. 2  is a diagram of an exemplary intra-rack cooling system according to the present principles; 
           [0015]      FIG. 3  is a diagram of an intra-server cooling system according to the present principles; 
           [0016]      FIG. 4  is a diagram of an exemplary intra-rack cooling system according to the present principles; 
           [0017]      FIG. 5  is a diagram of an exemplary intra-rack cooling system according to the present principles; 
           [0018]      FIG. 6  is a block/flow diagram of an exemplary method for cooling control according to the present principles; and 
           [0019]      FIG. 7  is a diagram of an exemplary intra-rack cooling system according to the present principles. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    The present principles provide for temperature measurements at various points within a cooling system that combines liquid- and air-based cooling. This temperature information is used to control coolant flow through a rack and through individual servers. By tuning the temperature difference between liquid coolant and air flowing through the servers, cooling efficiency can be maximized. 
         [0021]    Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIG. 1 , an exemplary data center cooling system  100  is shown. The data center includes a number of racks  102 , which circulate coolant. Low-temperature coolant  112  enters the racks  102 , picks up heat, and leaves the racks  102  as high-temperature coolant  114 . Although the present invention is described herein with respect to racks of servers, it is contemplated that any appropriate structure could be employed. In particular, any clustering, grouping, or other organization of computing devices or structures could be cooled using the present principles. 
         [0022]      FIG. 1  shows a system that has both liquid-to-air heat exchangers  104  and liquid-to-liquid heat exchangers (LLHx)  108  In a liquid-to-air cooling arrangement, high-temperature coolant  114  passes directly to an air-side outdoor exchanger  104 , for example a set of cooling fins. Any appropriate type of heat exchange may be used in place of the liquid-to-air exchanger  104 , including dry coolers, a building&#39;s chilled water supply, a cooling tower, a wet cooler, a building&#39;s heating or heat recovery systems, a geothermal loop, or a combination of multiple kinds. In a liquid-to-liquid cooling arrangement, high-temperature coolant  114  passes through a paired cooling coil. A heat exchanger  106  has a separate coolant circulation system that also feeds into the paired cooling coil of LLHx  108 . The coolant from the heat exchanger  106  reduces the temperature of the high-temperature coolant  114  without mixing, before dissipating its heat at heat exchanger  106 . The LLHxes  108  may be optionally turned off by shutting off the flow of coolant through the paired cooling coil. Additionally, multiple LLHxes  108  may be arranged along a single double-loop line, such that external heat dissipation may be controlled by enabling an appropriate number of heat exchangers  108 . 
         [0023]    The rate of heat transfer at the rack(s)  102  is predominantly governed by the liquid coolant flow rate through them. At the outdoor heat exchangers  104  and  106 , the heat transfer rate is governed by the outdoor heat exchanger&#39;s air-side flow rate and the liquid coolant flow rate through the outdoor heat exchanger  104 . The heat transfer rate is a non-linear monotonically increasing function of air-side flow rate and liquid coolant flow rate. For any given heat exchanger design, there is a limit to the air-side flow rate and liquid flow rate. These limits are used to guide the heat exchanger selection so as to meet the maximum cooling requirements (the worst case scenario) by a safe margin. “Worst case scenario” here refers to the highest ambient air temperature and highest heat dissipation expected at the rack(s), and in a more general sense, highest heat dissipation at the data center, occurring simultaneously. The “worst case scenario” should be rare and might not even occur over the entire life cycle of the data center. 
         [0024]    In some more common situations, an electronic rack  102  might be partially filled. Moreover, with data center provisioning (for example, powering off servers whose resources are not being used, etc.) being widely used to reduce the IT power usage, powered-off servers within a rack  102  might also be cooled, even those servers which would not generate heat. These situations may result in more cooling power consumption than is needed for almost the entire life cycle of data center. Hence, liquid cooling distribution hardware and controls based on physical infrastructure and environmental conditions both inside and outside the data center, may be used to properly optimize the cooling power consumption and further reduce the data center energy usage. 
         [0025]    Referring now to  FIG. 2 , a system for managed cooling of servers at an intra-rack level is shown. A plurality of managed servers  204  are shown, each connected to a hardware management console (HMC)  206  by a management network  202 . The HMC  206  controls workload implementation in the servers  204  and may include, e.g., one or more hypervisor nodes. Each managed server  204  has a corresponding cooling unit  208 , and the cooling units  208  are controlled by a cooling component logic controller  212  through a cooling management network  210 . Together, the cooling components and controls form cooling system  214 . The logic controller  212  receives information about outdoor ambient conditions, such as temperature information. Because outdoor temperature is related to cooling efficiency, the logic controller  212  can use that information to control factors such as coolant flow rate. 
         [0026]    The present principles reduce cooling power consumption by providing liquid cooling only to the components that require cooling. For example, if a managed server  204  is in off-state, then this status information can be fed to the cooling logic controller  212 , which would then take steps to close the coolant flow to that server  204  without affecting the coolant flow to any other server. To take another example, if the managed server  204  needs to be powered ON, then this information can also be fed to the cooling logic controller  212  so that cooling to the server  204  can be activated. Cooling can furthermore be tuned to particular levels corresponding to the workload at a server  204 , with higher workloads allocating more cooling. This system applies to the inter-rack level as naturally as to the intra-rack level. 
         [0027]    Referring now to  FIG. 3 , a schematic of an air- and liquid-cooled server  300  is shown. In addition to liquid-cooled components, such as CPU cold plates  302  and memory banks  304 , many components in a server  300  may be air-cooled. For example, hard drives  308  are frequently air-cooled. Additionally, memory banks  304  may be cooled by a combination of liquid- and air-cooling. Cool coolant liquid  312  enters the server  300  from an external cooling system. The coolant  312  enters memory banks  304  and CPU cold plates  302 , being warmed in the process and becoming warm coolant liquid  314  to exit the server  300 . 
         [0028]    An air-to-liquid heat exchanger (ALHx)  310  may be mounted on server  300  or on the side of a rack  102  as a sidecar unit and is attached to the coolant lines  312  and  314 . The ALHx may be connected to the coolant lines in either order, taking either warm coolant  314  or cool coolant  314  as its input, depending on desired air temperature. Air circulates within the server  300  by the fans  306  and is warmed by, e.g., hard drives  308  and memory banks  304 . The air exits the server as warm air and is then passed through the ALHx  310 , which cools the air before recirculating it into server  300 . There may be substantial air temperatures within the server  300 , and so multiple ALHxes  310  may be employed to provide uniform conditions. 
         [0029]    As noted above, the ALHx  310  may be connected to coolant lines  312  and  314  in either order, taking either cool coolant or warm coolant as input. In some situations, memory banks  304  may be liquid cooled as well as air cooled. In this case, part of the heat dissipated by the memory banks  304  goes into the air, while part of the heat goes into the liquid coolant. This fraction of heat is dependent on the air and liquid temperature that the memory banks  304  are exposed to. As such, by having warmer air enter the server  300 , heat going in to the air from the memory banks  304  may be minimized. This increases the efficiency of cooling at the rack level. The ALHx  310  may also be connected to the coolant lines  312  and  314  using valves that allow the coolant flow to be reversed through ALHx  310 , taking warm coolant, cool coolant, or a combination of the two, as input as circumstances demand. The cooling input to the ALHx  310  may be controlled using valves  322 . 
         [0030]    Liquid cooling at the server level may also be tuned. For example, memory banks  304  may be partially populated and individual CPUs  302  may have varying workloads or be shut off entirely. Individual memory slots within banks  304  may be selectively cooled according to whether those slots are in use, and CPU cold plates  302  may be adjusted or shut off using valves  318  according to CPU usage. Cooling for entire memory banks  304  may be shut off using valves  320 . Cooling within the server  300  may further be controlled based on direct measurements of ambient temperature using, e.g., temperature sensor  316 . Temperature sensor may be used to provide direct feedback to, e.g., ALHx  310  as well as to external cooling logic  212 , which may in turn tune cooling settings according to desired conditions. 
         [0031]    Referring now to  FIG. 4 , an embodiment of an intra-rack cooling system is shown. A set of servers  300  are connected in parallel to a coolant inlet plenum  404  and a coolant outlet plenum  406 . Inlet plenum  404  receives cold input coolant  412  from a pump  401  that draws from outside the rack  400 . Outlet plenum  406  collects warm coolant  410  from the servers  300 , which leaves the rack  400  to be cooled as shown above in, e.g.,  FIG. 1 . 
         [0032]    An ALHx side car  402  is connected to the input coolant line  412  and the output coolant line  410  by actively controlled three-way valves  408 . Valves  408  are used to regulate the flow to the inlet plenum  404  and to side car  402 . Because the side car  402  is inside the rack  400 , by regulating the flow of coolant to the side car  402 , the rack ambient air temperature leaving the side car  402  and entering servers  300  can be controlled. This side car  402  could be connected either to the cold coolant line  412 , where coolant would flow through the sidecar  402  before entering the inlet plenum  404 , or to the warm coolant line  410 , where coolant would flow through the sidecar  402  after leaving the outlet plenum  406 . In an alternative embodiment, coolant could be drawn from both coolant lines  410  and  412 . Connecting the side car  402  to the cold coolant line  412  results in cooler air going through the servers  300  and may be useful in situations where air-cooled components, such as hard drives  308 , require additional cooling. Connecting the side car  402  to the warm coolant line  410  results in warmer air going through servers  300 , which may be useful in situations where heat going into the air at the server level should be minimized and heat going into the liquid at the server level should be maximized. 
         [0033]    Referring now to  FIG. 5 , an alternative embodiment of an intra-rack cooling system  500  is shown. A three-way, flow-directing valve  502  is disposed between side car  402  and the inlet plenum  404 , so that liquid can be directed to one or the other and a relative flow rate can be controlled.  FIG. 5  also shows a set of temperature measurements that may be used to monitor and control cooling efficiency. In particular, the temperature of input coolant  412  is measured as T wi , the temperature of the air output by side car  402  is measured as T ai , the temperatures of important components within servers  300  is measured, and the maximum of those temperatures is measured as T C, max , the temperature of the coolant output  410  by outlet plenum  406  is measured as T wo2 , and the coolant output by the sidecar  402  is measured as T wo1 . 
         [0034]    The cooling system  500  may optionally include two two-way valves  504  and  506 . These valves offer more precise control of coolant flow to the inlet plenum  404  and the side car  402 . Although adjustments to the three-way valve  502  and the pump  401  can accommodate any balance of coolant flow, doing so may be accomplished with fewer steps simply by adjusting one of the two-way valves  504  and  506 . 
         [0035]    As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention 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, aspects of the present invention 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. 
         [0036]    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. 
         [0037]    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, electro-magnetic, 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. 
         [0038]    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 present invention 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). 
         [0039]    Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. 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. 
         [0040]    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. 
         [0041]    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 of the present invention. 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. 
         [0042]    Referring now to  FIG. 6 , a block/flow diagram of a method for controlling coolant and air temperature is shown. This can be accomplished by adjusting the position of valve  502 , thereby changing the proportion of coolant flowing through the side car  402  and servers  300 . Block  602  begins by setting the pump to maximum RPM and opening the valve to 50%, thereby sending equal coolant to the side car  402  and inlet plenum  404 . Next, block  604  checks the monitored temperatures, in particular obtaining measurements for T ai , T wo1 , T wo2 , and taking the maximum component temperature T C, max . 
         [0043]    Four set temperatures, T spec1 , T spec2 , T spec3 , and T spec4 , are used for comparison. These values represent, respectively, a high temperature threshold for server components, a high temperature threshold for the temperature of air leaving the side car  402 , a low temperature threshold for server components, and a low temperature threshold for the temperature of air leaving the side car  402 . Block  606  determines whether the maximum component temperature, T C, max , exceeds the high temperature threshold for components and whether the measured air temperature, T ai , exceeds the high temperature threshold for air. If so, it is determined that there is not enough coolant flow to the rack  500 . Block  608  increases the RPM of a pump  401 , thereby increasing the flow to the rack  500 , proportionally increasing flow to the inlet plenum  404  and the side car  402 . Block  608  may also trigger a notification regarding the status of the determined cooling system. This may, for example, include a visual message such an error display on a screen or an indicator light, or an audio message such as a verbal notification or a beep code. After the pump RPM is increased at block  608 , block  630  waits t seconds before returning to block  604 . The amount of time waited can be any appropriate amount of time, and may be made to depend on the variability of cooling needs. For example, if the servers operate under a constant workload, then the wait time may be long. In data centers where workloads vary over time, the wait time may be made shorter, to allow for rapid adjustments to cooling parameters. 
         [0044]    Block  610  checks whether T C, max , falls below the high temperature threshold for components and whether the measured air temperature, T ai , exceeds the high temperature threshold for air. If so, block  612  adjusts valve  502  to increase flow to side car  402 . Alternatively, two-way valve  504  may be opened, allowing additional coolant flow to the side car  402 . The amount of change may be based on the amount of excess in T ai  or may be a fixed percentage. Block  612  may also trigger a notification regarding the status of the determined cooling system, in particular notifying an administrator that there is too little coolant flow to the side car  402 . 
         [0045]    Block  614  checks whether T C, max , exceeds the high temperature threshold for components and whether the measured air temperature, T ai , falls below the high temperature threshold for air. In this case, block  616  may notify a system administrator that there is too little coolant flow to the servers  300 . Block  616  also increases flow to the servers  300  by adjusting valve  502 . Alternatively, two-way valve  506  may be opened, allowing additional coolant flow to the servers  300 . The amount of change may be based on the amount of excess in T C, max , or may be a fixed percentage. 
         [0046]    Block  618  determines whether T C, max , falls below the low temperature threshold for components and whether the measured air temperature, T ai , falls below the low temperature threshold for air. In this case, block  620  may notify a system administrator that there is too much coolant flow to the servers  300 . Block  620  also decreases the RPM of pump  401 , thereby increasing the flow to the rack  500 , proportionally increasing flow to the inlet plenum  404  and the side car  402 . 
         [0047]    Block  622  determines whether T C, max , exceeds the low temperature threshold for components and whether the measured air temperature, T ai , falls below the low temperature threshold for air. In this case, block  624  may notify a system administrator that there is too much coolant flow to the sidecar  402 . Block  624  also adjusts valve  502  to decrease flow to side car  402 . Alternatively, two-way valve  504  may be opened, allowing additional coolant flow to the side car  402 . The amount of change may be based on the amount of shortfall in T ai  or may be a fixed percentage. 
         [0048]    Block  626  determines whether T C, max , falls below the low temperature threshold for components and whether the measured air temperature, T ai , exceeds the low temperature threshold for air. In this case, block  628  may notify a system administrator that there is too much coolant flow to the servers  300 . Block  628  also adjusts valve  502  to decrease flow to servers  300 . Alternatively, two-way valve  506  may be opened, allowing additional coolant flow to the servers  300 . The amount of change may be based on the amount of shortfall in T C, max  or may be a fixed percentage. 
         [0049]    After each adjustment, processing goes to block  630 , waits for t seconds as described above, and returns processing to block  604  to repeat. In this manner, the cooling parameters may be continually adjusted, including the RPM of pump  402  and the state of the three-way valve  502  and two-way valves  504  and  506 . 
         [0050]    In an alternative embodiment, T wi , T wo1 , or T wo2  may be used instead of T C, max . Additionally, T wi  and T wo2 , along with the liquid coolant flow rate through the servers/nodes, can be used to measure the heat load going into the liquid coolant at the server/rack level. Similarly, T wi  and T wo1 , along with the liquid coolant flow rate through the side car air-to-liquid heat exchanger  402  can be used to measure the heat load going into the air. The percentage of heat load going into the liquid at the server/rack level is related to the temperature difference between T ai  and T wi . The greater T ai  is with respect to T wi , the greater the heat load transferred to the liquid will be. Under optimal circumstances, T ai  will be as much larger than T wi  as possible. However, there is an upper as well as lower limit to both Tai and Twi. As such, T ai  and T wi  can be controlled to obtain a desired heat load distribution to liquid coolant and to air. 
         [0051]    Referring now to  FIG. 7 , an alternative embodiment of an intra-rack cooling system  700  is shown. Side car  402  is arranged in serial with the inlet plenum  404 , such that input coolant flow  412  from pump  401  first passes through side car  402  and then enters the inlet plenum  404 . A bypass two-way valve  702  is configured to provide a bypass path for the coolant to skip the side car  402 . The valve  702  may be adjusted in steps, such that some coolant flows through side car  402  and some does not. This valve  702  may be adjusted in a manner similar to that discussed above in  FIG. 6 , where relative flow may be adjusted in the same manner, replacing changes to the three-way valve  502  with opening and closing the two-way valve  702 . 
         [0052]    Having described preferred embodiments of a system and method for coolant and ambient temperature control for chillerless liquid cooled data centers (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.