Patent Publication Number: US-9848516-B2

Title: Liquid-assisted bottom air cooling of electronic racks in data centers

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/936,177, filed Nov. 9, 2015, which claims the benefit of U.S. provisional patent application No. 62/239,227, filed Oct. 8, 2015. The disclosure of the above applications is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the present invention relate generally to data centers. More particularly, embodiments of the invention relate to a liquid-assisted cooling system for cooling electronic racks of IT components in a data center. 
     BACKGROUND 
     Heat removal is a prominent factor in computer system and data center design. The number of information technology (IT) components such as servers deployed within a data center has steadily increased as the server performance has improved, thereby increasing the amount of heat generated during the ordinary operation of the servers. The reliability of servers used within a data center decreases if the environment in which they operate is permitted to increase in temperature over time. A significant portion of the data center&#39;s power is used for cooling electronics at the server level. As the number of servers within a data center increases, a greater portion of the power is commensurately consumed by the data center to cool electronic components within the servers. 
     In conventional data center structures, a computer room air conditioner (CRAC) circulates cold air throughout a data center. Conventionally, the CRAC is a closed-loop system that cools returning air drawn from within the data center and recirculates the cooled air to the servers within the data center. Because air drawn by the CRAC originates within the data center, the air has an increased temperature from cooling the servers in the data center. Certain conventional data centers utilize heat exchangers disposed on the top of electronic racks. However such a configuration may cause maintenance problems such as liquid leakage, which may damage the IT components or elements. Further, such a design is inefficient in a modular data center configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements. 
         FIGS. 1A-1C  are block diagrams illustrating a data center system according to one embodiment of the invention. 
         FIGS. 2A-2C  are block diagrams illustrating a data center system according to another embodiment of the invention. 
         FIGS. 3A-3B  are block diagrams illustrating a data center system according to another embodiment of the invention. 
         FIGS. 4A-4B  are block diagrams illustrating a data center system according to another embodiment of the invention. 
         FIGS. 5A-5B  are block diagrams illustrating a data center system according to another embodiment of the invention. 
         FIG. 6  is a block diagram illustrating a data center system according to another embodiment of the invention. 
         FIGS. 7A and 7B  are block diagrams illustrating an example of a cooling unit according to one embodiment of the invention. 
         FIG. 8  is a block diagram illustrating a data center system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments and aspects of the inventions will be described with reference to details discussed below, and the accompanying drawings will illustrate the various embodiments. The following description and drawings are illustrative of the invention and are not to be construed as limiting the invention. Numerous specific details are described to provide a thorough understanding of various embodiments of the present invention. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present inventions. 
     Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in conjunction with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. 
     According to some embodiments, a cooling mechanism or cooling unit is provided to utilize liquid heat exchangers (as part of cooling units). The cooling units having liquid heat exchangers are placed underneath or below the electronic racks of IT components of a data center system, also referred to as a data center unit (e.g., a modular data center unit). A data center may include an array of one or more data center systems. The liquid exchangers are utilized for the purpose of cooling, reducing power utilization energy, and increasing the reliability of the data center hardware. 
     In one embodiment, liquid (e.g., water or dielectric liquid) pumped from a data center infrastructure chiller, data center condensing units, or any other means of achieving cooling liquid, is utilized for the purpose of cooling data center IT components by passing cooling liquid through heat exchangers located below IT components of the electronic racks in the data centers. The electronic racks are placed within an enclosure or container in a manner, such that rack cold and hot aisles are formed. There are fans located underneath of the racks to push returned air through the heat exchanger. The combination of the electronic racks, liquid heat exchangers located underneath, heat exchanger fans, and the outer enclosure of the electronic racks are considered to be the basic elements of a data center cooling system design. 
     In one embodiment, main elements of a data center cooling system include one or more electronic racks with IT components, cabling, power, and cooling aisles enclosed by an enclosure that act as cold and hot aisles of an electronic rack for the purpose of efficiently cooling the IT components. The cooling is achieved by passing cold liquid through a heat exchanger located underneath of an electronic rack. There are few auxiliary fans located adjacent to or within a proximity of the liquid heat exchanger that act as an assistant for pushing air through the heat exchanger. The electronic rack is located inside of a pod cluster (also referred to as a data center system or data center unit) along with several other electronic racks. Each electronic rack permits a certain amount of cooling air to be pushed into the rack cluster enclosure of the pod or container. 
       FIGS. 1A-1C  are block diagrams illustrating a data center system according to one embodiment of the invention. In this example, a perspective view is shown in  FIG. 1A . A cross-section view or side view of a data center system is shown in  FIG. 1C  while a top view of the data center system is shown in  FIG. 1B . Data center system  100  is also referred to as a data center unit, where a typical data center may include one or more of data center units  100 . 
     Referring to  FIGS. 1A-1C , according to one embodiment, data center system  100  includes a housing structure, such as a room or a container, to house rows of electronic racks of IT components, equipment or instruments  101 - 102 , such as, for example, computer servers that provide data services to a variety of clients. In this embodiment, data center system  100  includes electronic racks arranged in row  101  and row  102 . However, more or fewer rows of electronic racks may be implemented. Row  101  is positioned aligned with a first wall or side of the container while row  102  is positioned aligned with a second wall or side of the container. The first and second walls or sides may be opposite walls or sides, which form a pod aisle  103  between row  101  and row  102  within the container. 
     Each of rows  101 - 102  includes an array of rack slots (e.g., vertical slots) to host electronic racks, such as electronic racks  110 A to  110 N. An electronic rack having a stack of IT components therein can slide in and out of a rack slot. Typically, an electronic rack includes a front penal and a back penal operating as front and back doors, which can be opened by an operator to perform management or maintenance actions. In one embodiment, a front panel of an electronic rack is positioned on a first side or first end away from pod aisle  103 , while a back panel of the electronic rack is positioned on a second side or second end towards pod aisle  103 . Typically, the front penal can be used to access the IT components, such as network connectors of the IT components, while the back penal is used to access one or more cooling fans mounted therein configured to blow cool air into an airspace between the IT components for the purpose of heat exchange. An IT personnel or operator can walk into pod aisle  103  to access the back panels of the electronic racks. 
     In one embodiment, a cooling unit having one or more heat exchangers (not shown) is disposed underneath or below the electronic racks of the IT components to receive first liquid (e.g., cold or cool liquid) from chiller system or chiller unit  120  via first liquid channel  131  (also referred to as a cold/cool liquid channel or supply line). The cooling unit is to exchange heat generated from the IT components using the first liquid, where the exchanged heat transforms the first liquid into second liquid having a higher temperature (e.g., hot or warm liquid). The cooling unit then transmits the second liquid back to chiller system  120  via liquid channel  132  (also referred to as a hot/warm liquid channel or supply line). 
     In one embodiment, each electronic rack, such as electronic racks  110 A- 110 N, includes a housing (also referred to as a rack housing, rack enclosure, or rack container) to house the IT components in a stack. A first rack aisle (not shown) is formed between a first side of the stack of the IT components and a first wall of the housing. A second rack aisle (not shown) is formed between a second side of the stack and a second wall of the housing. The first rack aisle (also referred to as a rack cool aisle or RCA) is adapted to direct cool air received from a cooling unit, which is located underneath the corresponding electronic rack, upwardly to reach the IT components in the stack. The second rack aisle (also referred to as a rack hot aisle or RHA) is adapted to direct hot air to the cooling unit downwardly. The hot air is transformed from the cool air received from the first rack aisle by flowing through an airspace between the IT components in the stack. One of the advantages having a cooling unit at a position lower than the position of the electronic racks is that if the cooling unit fails to operate properly (e.g., liquid leakage), the IT components would not be damaged significantly. 
       FIGS. 2A-2C  are block diagrams illustrating an electronic rack according to one embodiment of the invention. In this example,  FIG. 2A  shows a perspective view of an electronic rack, while  FIGS. 2B-2C  show a side view or cross view of the electronic rack. Referring to  FIG. 2A-2C , electronic rack  110  includes a rack housing having a front panel  201  and a back panel  202 , which may operate as a front door or front window and a back door or back window, respectively. The rack housing is configured to house a stack of IT components  205 A- 205 B. An IT component may operate as a server to provide data services to a variety of clients over a network. For example, an IT component may operate as a Web or cloud server, a storage server, an application server, a backend server, or any other appliance devices 
     In one embodiment, the stack of IT components  205 A- 205 B are positioned between front panel  201  and back panel  202  to form a front rack aisle  211  (e.g., first rack aisle) and a back rack aisle  212  (e.g., second rack aisle). In addition, a cooling unit  210  having at least one heat exchanger is disposed underneath or below IT components  205 A- 205 B. As a result, even if there is leakage of liquid from a heat exchanger of cooling unit  210 , it would not significantly damage IT components  205 A- 205 B due to the leakage. In one embodiment, in addition to one or more heat exchangers, cooling unit  210  includes one or more fans or air propellers (not shown) to push cool air into front rack aisle  211  upwardly and to draw hot air from back rack aisle  212  downwardly into cooling unit  210 . Front rack aisle  211  is also referred to as a rack cool aisle (RCA), while back rack aisle  212  is also referred to as a rack hot aisle (RHA). 
     Electronic rack  110  further includes one or more fans or air propellers  215  mounted on a side of IT components  205 A- 205 B to direct cool air from rack aisle  211  flowing through the airspace between IT components  205 A- 205 B into rack aisle  212 . While flowing through the airspace between IT components  205 A- 205 B, the cool air exchanges with the heat generated from IT components  205 A- 205 B to transform or become hot or warm air in rack aisle  212 . That is, due to the heat exchange, the airflow flowing into rack aisle  212  has a temperature higher than the temperature of the airflow received in rack aisle  211 . Such a heat exchange of an airflow leads to a temperature reduction of IT components  205 A- 205 B. 
     Note that electronic rack  110  may represent any of electronic racks  110 A- 110 N as shown in  FIGS. 1A-1C . Also note that the configuration as shown in  FIGS. 2A-2C  are described for the purpose of illustration only. Other configurations may also be applied. For example, a front side or front panel may be configured as a back side or back panel, or vice versa in another configuration. Further, fans  215  may be mounted on the other side of the IT components, as long as they can direct the cool air travelling through the airspace between the IT components. 
       FIGS. 3A-3B  are block diagrams illustrating a data center system according to another embodiment of the invention. In this example,  FIG. 3A  shows a top view of data center system  300 , while  FIG. 3B  shows a side or cross view  350  of data center system  300 . Data center  300  may represent data center system  100  of  FIGS. 1A-1C . Referring to  FIGS. 3A-3B , in this example, data center system  300  includes first row  101  of electronic racks  301 A- 301 N (collectively referred to as electronic racks  301 ) and second row  102  of electronic racks  302 A- 302 N (collectively referred to as electronic racks  302 ). Rows  101 - 102  are positioned aligned with or against the opposite inner walls of a container or housing of data center system  300 . Rows  101 - 102  form pod aisle  103  between rows  101 - 102  within the container. 
     In one embodiment, pod aisle  103  allows an IT personnel or operator to walk into the housing to access the IT components from one side of the electronic racks, such as inspection, configuration, repairs, or any other management operations. An IT personnel can also access the IT components from the other side of the electronic racks via an exterior wall of the container of data center system  300 . As described above, an electronic rack includes a front panel and a back panel as part of a corresponding rack enclosure or housing that encloses the IT components therein. The front panel and the back panel operate as a panel door or window for access, which may be controlled (e.g., opened or closed) by a controller or by an IT personnel manually. 
     According to one embodiment, data center system  300  further includes at least one cooling unit (e.g., cooling units  361  and  362  as shown in  FIG. 3B ) that includes at least one heat exchanger configured to provide cool air to exchange heat generated by electronic racks  301 - 302 . The cooling unit may be disposed at a position that is lower than the position at which electronic racks  301 - 302  are disposed. As a result, if there is a leakage from the cooling unit, the IT components stored within electronic racks  301 - 302  would not be significantly damaged due to the leakage. 
     According to one embodiment, each of electronic racks  301 - 302  includes a stack of IT components. The stack of IT components is positioned within a corresponding rack housing (also referred to as rack container, rack enclosure). For example, the rack housing may include a stack of electronic component shelves or slots (e.g., horizontal slots) to allow an individual IT component to be inserted into or removed from the shelf. The stack of IT components is positioned in a manner, such that an airspace between the rack housing and the stack of IT components forms a RCA (e.g., a first rack aisle) and a RHA (e.g., a second rack aisle). 
     Referring to  FIGS. 3A-3B , according to one embodiment, an RCA of each of electronic racks  301 - 302  is formed and positioned at a near side with respect to pod aisle  103  (e.g., adjacent to pod aisle  103 ). An RHA of each of electronic racks  301 - 302  is formed and positioned at a far side from pod aisle  103  (e.g., adjacent to the wall of the container, away from pod aisle  103 ). In one embodiment, an RCA (e.g., RCAs  311 A and  321 A) is configured to direct cool air  304  generated from a cooling unit upwardly. Cool air  304  may then be directed flowing across (e.g., horizontally) the airspace between IT components within the corresponding electronic rack outwardly with respect to pod aisle  103 , for example, using one or more fans mounted on a side of the IT components (e.g., fans  215  of  FIG. 2A ). Cool air  304  exchanges the heat generated from the IT components and transforms into hot or warm air  303  to reach an RHA. In one embodiment, an RHA (e.g., RHAs  312 A and  322 A) is configured to direct hot air  303  to the cooling unit downwardly to allow the cooling unit (e.g., cooling units  361 - 362 ) to recycle the hot air back to cool air. 
     In one embodiment, each of electronic racks  301 - 302  is associated with or includes an individual cooling unit positioned underneath or below the corresponding stack of IT components. The individual cooling unit operates independently with respect to other cooling units of other electronic racks. One of the advantages in this configuration is that each electronic rack becomes an independent modular unit that can inserted and removed (e.g., together with the corresponding cooling unit) from the data center system without affecting other electronic racks. In the event that any malfunction occurs within one electronic rack, the malfunctioning electronic rack can be individually shut down and replaced. For example, if an individual cooling unit of a particular electronic rack leaks, such a leakage would not cause significant damage to other electronic racks due to liquid damage or overheat because of malfunctioned individual cooling unit. Similarly, an individual cooling unit can also be individually replaced, for example, without having to remove or replace the associated electronic rack. 
     Another advantage is that the liquid flow volume or speed can be independently controlled by a controller (not shown) based on the operating temperature of the corresponding electronic rack, which may be monitored by one or more sensors (not shown). Each electronic rack may operate under a different condition or operating environment (e.g., workload, or number of active IT components) and thus, it may operate with a different operating temperature. As a result, the work load and/or power of the cooling unit and/or the corresponding chiller system can be balanced or reduced. In one embodiment, the flow control of liquid can be performed on a per electronic rack basis, a per row basis, a per data center unit basis, or a combination thereof. 
     Alternatively according to another embodiment, each of rows  101 - 102  is associated with one or more cooling units that provide cool air to all electronic racks of the corresponding row. That is, one or more cooling units may be shared by the entire row of electronic racks. Furthermore, both rows  101 - 102  may share one or more cooling units. A combination of the above configurations may be implemented dependent upon the specific requirement of a data center (e.g., cost benefit). For example, a data center having multiple data center systems or units may include a first data center system with an individual cooling unit for each electronic rack. The data center may further include a second data center system with one or more cooling units shared by each row of electronic racks. The data center may further include a third data center system with one or more cooling units shared by both rows of electronic racks, etc. 
     Further, even within a data center system, one row can be configured to have an individual cooling unit for each electronic racks, while the other row has one or more cooling units shared by all electronic racks of the same row. Similarly, even within the same row, some of the electronic racks may share one or more cooling units, while other electronic racks may have an individual or designated cooling unit. Such configurations may be configured statically or on demand, for example, using programmable controllers, switches, and/or controllable valves. Other configurations may also be applied. 
     A heat exchanger of a cooling unit (e.g., cooling units  361 - 362 ) is a device used to transfer heat between one or more fluids or liquid. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. For efficiency, heat exchangers are designed to maximize the surface area of the wall between the fluids, while minimizing resistance to fluid flowing through the exchanger. The exchanger&#39;s performance can also be affected by the addition of fins or corrugations in one or both directions, which increase a surface area and may channel fluid flow or induce turbulence. Similar to an air conditioning system, a chiller system such as chiller system  120  is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air. Dependent upon the specific requirement of cool air, a control system (not shown) may be configured to control or adjust cool air with desired temperature and/or moisture. 
     Referring back to  FIGS. 3A-3B , according to one embodiment, a cooling unit generates and directs a first portion  304  (e.g., approximately 85-90 percent) of the cool air into an RCA, while it directs a second portion  305  (e.g., approximately 10-15 percent) of the cool air into pod aisle  103 . As a result, the overall temperature within pod aisle  103  is lower than the temperature within an RHA in generate. The temperature of pod aisle  103  may be similar to slightly lower than the temperature of an RCA. For the purpose of illustration only, pod aisle  103  is also referred to as pod cool aisle (PCA) since it has a cooler temperature than an RHA. The cooler air in PCA  103  may be utilized to cool the IT components of an electronic rack when a corresponding cooling unit malfunctions or fails to provide adequate cool air. 
       FIGS. 4A-4B  are block diagrams illustrating a data center system according to another embodiment of the invention. In this example, similar to  FIGS. 3A-3B ,  FIG. 4A  shows a top view of data center system  400 , while  FIG. 4B  shows a side or cross view  450  of data center system  400 . Data center  400  may represent any of the data center systems as described above. Referring to  FIGS. 4A-4B , as described above, some of the cool air generated by one or more cooling units may be directed or provided to PCA  103  as part of cool air  305 . 
     For the purpose of illustration, it is assumed that a cooling unit associated with electronic rack  302 C fails to provide adequate cool air to cool the IT components contained within electronic rack  302 C. In one embodiment, such a failure can be detected based on the temperature detected by one or more sensors or detectors deployed at various locations of electronic rack  302 C (not shown). For example, when a temperature of electronic rack  302 C exceeds a predetermined threshold, it can be inferred that the corresponding cooling unit fails to provide adequate cool air. Alternatively, a detector may detect there is a leakage of the corresponding cooling unit as a sign of malfunction of the cooling unit. 
     According to one embodiment, in response to such a detection, a controller (not shown) is configured to unlock and open one or more windows or openings, or the entire panel door on the interior side of electronic rack  302 C (e.g., the back panel or inner panel adjacent to PCA  103 ) to allow the cooler air  305  of PCA  103  to enter electronic rack  302 C to cool the IT components of electronic rack  302 C. That is, the cool air flowing through the airspace of IT components of electronic rack  302 C is drawn from cool air  305  of PCA  103 , instead of or additionally from cooling unit  362  associated with electronic rack  302 C. 
     In one embodiment, there may be a magnetic switching mechanism deployed on a window or panel door of each electronic rack that is controlled by a controller. In response to a detection of a failed cooling unit received from a sensor or detector, the controller sends a signal to the magnetic switching mechanism to unlock the window or door, which causes the window or door to be opened to allow cool air  305  to enter the electronic rack. 
     Note that in this example, cooling unit  362  associated with electronic rack  302 C does not have to stop providing cool air in order to allow cool air  305  to enter electronic rack  302 C. Rather, as long as it is detected that there is insufficient cool air to be circulated within electronic rack  302 C (e.g., a temperature below a predetermined threshold or liquid leakage), its window or panel door can be unlocked and opened to allow additional cool air  305  to enter electronic rack  302 C. This configuration can be applied to a scenario in which the IT components of a particular electronic rack cause a higher operating temperature, for example, due to an unusually high workload/demand or processing power/resources. 
     Meanwhile cool air  305  is continuously provided by other cooling units associated with other electronic racks. As a result, the IT components of electronic rack  302 C can at least be temporarily cooled to reduce the probability of overheating, while waiting for a replacement of the corresponding failed cooling unit or the entire electronic rack. The data services provided to the clients would not be significantly interrupted or disturbed. 
       FIGS. 5A-5B  are block diagrams illustrating an example of a data center system according to a further embodiment of the invention. Data center system  500  may be implemented as part of data center system  100  as shown in  FIGS. 1A-1C . In this example,  FIG. 5A  is shown as a top view while  FIG. 5B  is shown as a cross view of a data center system. Referring to  FIGS. 5A-5B , data center system  500  includes a floor platform that is raised from the ground, referred to herein as a raised floor  503 . All of the electronic racks, in this example, electronic racks  301 - 302  of rows  101 - 102 , are disposed or positioned on the raised floor  503 . Data center system  500  includes one or more cooling units  501  positioned underneath raised floor  503  and below PCA  103 . The centrally located cooling unit(s)  501  may be configured to provide cool air  502  to be supplied to both rows  101 - 102  of electronic racks  301 - 302 . 
     In one embodiment, the space between raised floor  503  and the ground floor forms hot air channels or tunnels  511 - 512 . In addition, raised floor  503  includes an array of outlet ports  505  disposed within the area of PCA  103  and an array of inlet ports  504  disposed within the areas of the RHAs, in this example, RHA  312 A and  322 A. There may be one or more fans or air propellers mounted within a proximity of outlet ports  505  to push or force cool air  502  generated from cooling unit  501  upwardly into PCA vie outlet ports  505  disposed on raised floor  503 . Cool air  502  is then directed outwardly into electronic racks  301 - 302  of rows  101 - 102 . Cool air  502  flows across the airspace between the IT components of rows  101 - 102  for the purpose of heat exchange, which transforms cool air  502  into hot air  303  within the RHAs. Hot air  303  is then directed downwardly within the RHAs and into hot air channels  511 - 512  via the corresponding inlet ports  504  to return to cooling unit  501 . 
       FIG. 6  is a block diagram illustrating a data center system according to another embodiment of the invention. System  600  represents an alternative design with respect to  FIGS. 5A-5B . Referring to  FIG. 6 , the design is similar to the design as shown in  FIGS. 5A-5B , except that the air flow is configured to flow an opposite direction as of  FIGS. 5A-5B . In this embodiment, cooling unit  501  generates cool air that will flow upwardly into RCS  311  and  321 , where pod aisle  103  becomes a pod hot aisle (PHA). The cool air travels inwardly from RCAs  311  and  321  through the airspace between the IT components of electronic racks  301 - 302 . Cooling unit  501  has the same or similar structure as of  FIGS. 5A-5B . 
     In one embodiment, cooling unit  501  includes one or more heat exchangers  602 - 603  that can be individually plugged or removed for replacement purpose. In addition, cooling unit  501  includes a coolant distribution system  601  to distribute the coolant to cool the hot air received from PHA  103 . Cooling unit  501  further includes liquid containment unit  604  to contain or collect any potential liquid that may be leaked from heat exchangers  602 - 603 , such that the leaked liquid would not damage the IT components in the electronic racks  301 - 302 . 
       FIGS. 7A and 7B  are block diagrams illustrating an example of a cooling unit according to one embodiment of the invention. Referring to  FIGS. 7A-7B , cooling unit  700  represents any of the cooling units described above, such as cooling units  210  and  361 - 362 . In this embodiment, cooling unit  700  (also referred to as a cooling module) includes one or more heat exchangers  701  and one or more power modules  702 . Power modules  702  are configured to provide power to IT components of an electronic rack via power bus or interconnect  704 . Heat exchangers  701  are coupled to a pair of liquid supply lines via liquid connectors  703 , including an inlet connector to receive cool/cold liquid from an inlet liquid supply line from a chiller system or a central distribution unit (CDU) and an outlet connector to transmit warm/hot liquid to the chiller system or the CDU. In one embodiment, power modules  702  only occupy a portion of airspace within a housing of cooling unit  700  to allow air flow  710  to flow through and also provide cooling to power modules  702  as shown in  FIG. 7B . Cooling unit  700  may further include one or more fans (not shown) to propel the air flow for heat exchange. 
     In one embodiment, cooling unit  700  is configured to be a modular or portable unit that can be swapped in and out from a shelf or slot within an electronic rack, also referred to as a cooling slot. The power connectors  704  and liquid connectors  703  can be hot swappable to allow cooling unit  700  to be individually replaced and/or repaired. Cooling unit  700  can be inserted into any of the suitable slots within an electronic rack, as long as a proper liquid containment mechanism is configured to prevent any potential liquid leaks from damaging the IT components. One or more of cooling unit  700  can be placed anywhere within an electronic rack, dependent upon the specific configuration or demand. Further, heat exchange(s)  701  can also be individually swappable from cooling unit  700  via connectors  703 . 
       FIG. 8  is a block diagram illustrating a data center system according to another embodiment of the invention. Data center system  800  may represent any of the data center systems described above. Referring to  FIG. 8 , system  800  includes an array of electronic racks  801 - 801  (collectively referred to as electronic racks  801 ), which may represent any of the electronic racks described above. Electronic racks are coupled to central distribution unit (CDU)  805  via a respective pair of liquid supply lines  803 A- 803 N (collectively referred to as liquid supply lines  803 ). Each of electronic racks  801  includes or is associated with one of cooling units  802 A- 802 N (collectively referred to as cooling units  802 ). Although only one cooling unit is shown, however, multiple cool units can be implemented within an electronic rack. 
     As described above, each of cooling units  802  includes at least one heat exchanger, also referred to as a local heat exchanger as it is located locally with respect to the associated IT components within an electronic rack. The local heat exchangers are coupled to one of heat exchangers  804 A- 804 N of CDU  805  (collectively referred to as centralized heat exchangers  804 ) via liquid supply lines  803 , respectively. Each of the local heat exchangers of cooling units  802  is a liquid-air heat exchanger to exchange heat from hot air received from the IT components with cool/cold liquid received from CDU  805 . Each of centralized heat exchangers  804  is a liquid-liquid heat exchanger to exchange heat from warm/hot liquid received from a corresponding one of cooling units  802  with cool/cold liquid received from chiller unit  820 . 
     In one embodiment, data center system  800  is further associated with or includes controller  810  to control (e.g., turning on/off, liquid flow rate) the operations of heat exchangers  804  and/or the heat exchangers within cooling units  802 . Controller  810  may be implemented as part of CDU  805 . Controller  810  can individually control the operations of heat exchangers  804  and/or the heat exchangers of cooling units  802 . As described above, each of cooling units  802  can be individually swappable for replacement or repair. When any one of the cooling units  802  breaks down, the cooling unit can be individually shut off and disconnected from the respective pair of liquid supply lines  803  without impacting the remaining cooling units and their respective pairs of liquid supply lines. For example, before an old unit is taken offline, the liquid has to be flushed out from the lines. When a new cooling unit or local heat exchanger is connected, an individual test/purge operation can be performed on the corresponding liquid supply lines, while the remaining cooling units continue working without interruption. For example, additional air bubbles have to be flushed out before turning on a new cooling unit and/or a local heat exchanger. Similarly, centralized heat exchangers  804  can also be individually replaced. Note that CDU  805  may be implemented within a housing of data center system  800  or external to data center  800 . CDU  805  may also be shared by multiple data center systems. 
     Note that the cooling techniques described above can be applied to a variety of different types of data centers, such as, for example, traditional colocation data centers and greenfield data centers. A colocation data center is a type of data center where equipment, space, and bandwidth are available for rental to retail customers. Colocation facilities provide space, power, cooling, and physical security for the server, storage, and networking equipment of other firms,and connect them to a variety of telecommunications and network service providers with a minimum of cost and complexity. A greenfield data center refers to a data center that is built and configured in a location where none exists before. The techniques described above can also be applied to or work in conjunction with a performance optimized data center (POD), or portable on-demand or container data center, where racks of servers are housed in one or more individual containers, modular rooms, or modular housings. 
     The processes or methods depicted in the preceding figures may be performed by processing logic that comprises hardware (e.g. circuitry, dedicated logic, etc.), software (e.g., embodied on a non-transitory computer readable medium), or a combination of both. Although the processes or methods are described above in terms of some sequential operations, it should be appreciated that some of the operations described may be performed in a different order. Moreover, some operations may be performed in parallel rather than sequentially. 
     In the foregoing specification, embodiments of the invention have been described with reference to specific exemplary embodiments thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of the invention as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.