Abstract:
Rack assemblies and methods for cooling one or more rack-based computer systems, as well as data center configurations that utilize the rack assembly. The rack assembly comprises a rack providing support for multiple columns of heat-generating electronic devices and device fans for moving air from an air inlet side of the rack through the devices and through an air outlet side of the rack. The rack assembly also comprises a unitary door having a support frame spanning the air outlet side of the rack and hingedly coupling the door to a rear vertical edge of the rack. The door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet must pass through the heat exchanger panel. The air outlet passage has a cross-sectional area that is substantially equal to or greater than the cross-sectional area of the multiple columns of heat-generating devices.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to cooling computer systems and to the configuration of a datacenter according to how computer equipment is cooled. 
   2. Description of the Related Art 
   Computer systems use electrical energy and produce heat as a byproduct of electrical resistance. Rack-based computer systems include many rack-mounted components in a high-density arrangement, which can produce a considerable amount of heat. Excess heat must be removed from the rack to control internal temperatures and to maintain system reliability, performance, and longevity. In a conventional rack-based computer system, rack-mounted fans move cool air through the rack to remove the excess heat and cool the components. The heated exhaust air must then be transported to a computer-room air conditioner (“CRAC”) that cools the air before returning the cooled air to the data center. 
   In a conventional datacenter layout, racks in a data center are commonly arranged in an organized hot-aisle/cold-aisle layout to minimize the likelihood of appreciable volumes of heated exhaust air from directly re-entering the racks. A hot-aisle/cold-aisle layout may include alternating hot aisles and cold aisles, with the front of each rack sharing a cold aisle with one adjacent rack and the rear of each rack sharing a hot aisle with another adjacent rack. The CRAC supplies the cooled air to the cold aisles. The air from the cool aisle is drawn into the front of each rack and the heated air is exhausted through the rear of the rack to the hot aisle. The heated exhaust air recirculates through the CRAC to be cooled and returned back to the cold aisles. 
   SUMMARY OF THE INVENTION 
   One embodiment of the invention provides a rack assembly for cooling a computer system. The rack assembly comprises a rack providing support for one or more columns of heat-generating electronic devices and device fans for moving air from an air inlet side of the rack through the devices and through an air outlet side of the rack. The rack assembly also comprises a unitary door having a support frame spanning the air outlet side of the rack and hingedly coupling the door to a rear vertical edge of the rack. The unitary rear door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet side of the rack must pass through the heat exchanger panel. The air outlet passage has a cross-sectional area that is substantially equal to or greater than the cross-sectional area of the multiple columns of heat-generating devices. 
   Another embodiment of the invention provides a data center comprising a plurality of rack assemblies for cooling a computer system. Each rack assembly comprises a rack providing support for one-or-more columns of heat-generating electronic devices and device fans for moving air from an air inlet side of the rack through the devices and through an air outlet side of the rack, and a unitary door having a support frame spanning the air outlet side of the rack and hingedly coupling the door to a rear vertical edge of the rack. The unitary door includes an air-to-liquid heat exchanger panel spanning an air outlet passage inside the support frame so that substantially all of the air passing through the air outlet side of the rack must pass through the heat exchanger panel. Furthermore, the air outlet passage has a cross-sectional area that is substantially equal to or greater than the cross-sectional area of the one-or-more columns of heat-generating devices. 
   Yet another embodiment of the invention provides a method comprising the steps of arranging electronic devices into at least two columns within a rack, operating the electronic devices within the rack, drawing air through an inlet to the rack to withdraw heat from the electronic devices, and passing the air through a heat exchanger panel spanning substantially across the at least two columns of the rack to remove substantially all of the heat that the air withdrew from the electronic devices before exhausting the air from and outlet the rack. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective front view of a double-wide rack having an integrated rear-door heat exchanger according to one embodiment of the invention. 
       FIG. 2  is a front partial assembly view of the double-wide rack illustrating how various modules may be supported on the double-wide rack. 
       FIG. 3  is a schematic partial assembly view of an exemplary chassis receiving the two compute modules. 
       FIG. 4  is a perspective view of a 3U chassis with the top panel removed. 
       FIG. 5  is a rear elevation view of the double-wide rack with a partially cut-away view of the rear-door heat exchanger. 
       FIG. 6  is a perspective view of the rear-door heat exchanger from below. 
       FIG. 7  is a sectioned view of a portion of the fin tube assembly used in the rear-door heat exchanger. 
       FIG. 8  is a plan view of the double-wide rack with the rear-door heat exchanger in a closed position. 
       FIG. 9  is a plan view of the double-wide rack with the rear-door heat exchanger pivoted to a partially open position. 
       FIG. 10  is a graph indicating some representative performance curves for a combination double-wide rack and rear-door heat exchanger. 
       FIG. 11  is a plan view of one exemplary new datacenter configuration enabled by the improved cooling performance of the double-wide rack with integrated rear-door heat exchanger. 
       FIG. 12  is a plan view of another exemplary new datacenter configuration. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   One embodiment of the invention is a double-wide rack with an integrated liquid-coolant rear-door heat exchanger having a cooling performance on the order of 100% heat removal. A plurality of modules is mounted in a chassis from the front of the double-wide rack. The modules are organized within two sets of high power (“primary”) zones and adjacent low power (“secondary”) zones. The high power zones contain primarily processor-intensive modules such as compute modules. Modules in the adjacent low power zones are low power devices such as network switches and power distribution units (PDUs) for supporting the modules in the high power zones. The modules in the low power zones are interconnected with the modules in the high power zones by cables either at the front of the double-wide rack or at the back of the rack just inside the rear-door heat exchanger. 
   The chassis include fans that drive airflow through the double-wide rack from the front to the rear. The efficiency and performance of the fans is enhanced by using fans that have a relatively large diameter within the constraint of the chassis physical dimensions. The rear-door heat exchanger has a low resistance to airflow, and the capacity of the chassis fans is sufficient to drive airflow exhausted from the double-wide rack through the rear-door heat exchanger without the use of external “booster” fans. The absence of booster fans results in a less expensive, quiet, energy-efficient cooling solution, with fewer parts which could potentially fail. Transferring the heat generated by the modules in the double-wide rack directly to the liquid coolant in the rear-door heat exchanger is more energy efficient than withdrawing heated air to a CRAC located at a distance from the double-wide rack to be cooled and returning the cooled air to the data center. The increased economy resulting from the air-to-liquid heat exchanger and the absence of booster fans results in a lower total cost of ownership than conventional cooling solutions, such as CRACs or heat exchangers which require booster fans. 
   Other embodiments of the invention are directed to improved datacenter layouts and flexibility enabled by the exceptional cooling performance of the double-wide rack and integrated rear-door heat exchanger. The double-wide racks with integrated rear-door heat exchangers are not limited to being organized in a hot-aisle/cold-aisle layout. Rather, the double-wide racks may be arranged in a more space-efficient arrangement, such as in series, such that airflow exhausted by one double-wide rack and chilled by that double-wide rack&#39;s rear-door heat exchanger may be directly provided as the inlet airflow to the next double-wide rack in series. The cooling performance of the rear-door heat exchanger provided to each double-wide rack may even be sufficient to cool the air in the data center, reducing the size and operational expense of the CRAC. As explained further below, by using liquid coolant at sufficiently low temperatures, such as water on the order of 16 degrees Celsius at a rate of 15 gallons per minute (gpm), the cooling performance of the rear-door heat exchanger may even exceed 100%, by removing more heat from the air than was transferred to the air by the heat-generating electronic devices supported on the double-wide rack. This exceptional cooling performance provided by the rear-door heat exchanger on the double-wide racks may be sufficient to cool the data center and eliminate the use of one or more remotely located conventional CRACs entirely. Overall, one or more of the embodiments of the invention may result in a lower total cost of ownership than conventional rack systems. 
     FIG. 1  is a perspective front view of a double-wide rack having an integrated rear-door heat exchanger  100  according to one embodiment of the invention. The double-wide rack  10  supports a plurality of modular electronic components (“modules”) and provides access to the modules primarily from the front  12  of the double-wide rack  10 . The rear-door heat exchanger  100  can be opened to provide access to some of the equipment in the double-wide rack  10  from the rear  14  of the double-wide rack  10 . However, the equipment is accessible from the front  12  so that the rear-door heat exchanger  100  can remain closed in a cooling mode, so that substantially all of the airflow driven through the double-wide rack  10  from the front  12  to the rear  14  exits through the rear-door heat exchanger  100 . The various modules are arranged with comparatively high power modules in two vertical columns referred to as “high power zones”  16 ,  18 , and comparatively low power modules in adjacent vertical columns referred to as the “low power zones”  20 ,  22 . The modules in the high-power zones  16 ,  18  are typically processor-intensive “compute modules” having a motherboard and one or more processors. The high power modules consume a large amount of electrical power as compared to the low power modules, and generate a correspondingly large amount of heat that must be removed from the double-wide rack  10  by the rear-door heat exchanger  100 . The modules in the low power zones  20 ,  22  are typically devices for supporting the high-power modules, such as network switches for providing network connectivity and power distribution units (PDUs) for distributing power to the modules in the high-power zones  16 ,  18 . The low power modules typically contribute a relatively small amount to the overall heat production of the double-wide rack  10 . 
   The term “double-wide” is applied to the rack  10  because the modules are arranged in two vertical groupings  15 ,  17  of complimentary high-power and complimentary low-power zones. This double-wide rack configuration gives the rack  10  a unique form factor having approximately twice the width of a conventional rack having a single column of high-power modules. The rack  10  is also approximately half the depth of a conventional rack, resulting in approximately the same overall footprint. As discussed further below, the double-wide rack configuration increases the area of the airflow through the matching rear-door heat exchanger  100  for greater cooling efficiency. Although a double-wide rack  10  having multiple columns of modules is discussed in this embodiment, it should be recognized that another embodiment of the invention may have only one column of modules. 
     FIG. 2  is a front, partial assembly view of the double-wide rack  10  illustrating how various modules may be supported on the double-wide rack  10 . Multiple chassis such as exemplary chassis  32 ,  34  are supportable on longitudinal rails  40  at different vertical locations to establish numerous “chassis bays.” The rails  40  are spaced apart at different vertical positions to accommodate a number of chassis of the same or different sizes, and the vertical positions of the rails  40  may be individually adjustable. The first exemplary 2U (two unit) chassis  32  is shown being inserted into a chassis bay  36  of the high-power zone  16  on the left. A second, 3U (three unit) chassis  34  is shown being inserted into a chassis bay  38  in the high-power zone  18  on the right. The rails  40  are spaced at a first vertical distance to accommodate the 2U chassis  32  and a second, larger vertical distance to accommodate the 3U chassis  34 . The lower left side rail  40  of each chassis bay secures an AC electrical connector  48  arranged to blind dock with a chassis power supply. The electrical connector  48  is aligned with a mating connector on the power supply so that complete insertion of the chassis into the respective chassis bay completes the connection and supplies electrical power to the respective chassis power supply. No access from the rear  14  of the double-wide rack  10  is necessary to complete this connection. 
   Each chassis has a number of openings (typically more than one) referred to as “module bays” for receiving a corresponding number of modules. The 2U chassis  32  is shown independently receiving two 1U compute modules  46 . The 3U chassis  34  is shown receiving a compute module  46  in a lower 1U module bay and has already received twelve 3.5 inch disk drives  44  that are installed into drive bays that occupy the equivalent of 2U space and which are a permanent part of the chassis  34 . Additional chassis having additional modules bays may be mounted on the double-wide rack  10 , such that the double-wide rack  10  supports numerous modules. The modules may be selectively interconnected with cable connections from the front  12  of the double-wide rack  10 . Some of the modules, such as compute modules and hard drive modules disposed in the high power bays  16 ,  18 , may be interconnected within their common chassis. 
   A plurality of low power module bays  50  are provided in the low power zone  20  immediately adjacent to the high power zone  16 , and in the low power zone  22  immediately adjacent to the high power zone  18 . The low power module bays  50  suitably receive various low power modules, such as network switches and PDUs. The close positioning of the low power zones  20 ,  22  to the respective high power zones  16 ,  18  facilitates cable connections between the low power modules and the high power modules they support. For example, a network switch may be positioned in one of the low power module bays  50  in the low power zone  20  and connected to a compute module  46  at the same vertical position in the high power zone  16  to connect that compute module  46  to the network switch. The close positioning of the network switch to the compute module  46  minimizes the physical length of network connections made between the network switch and the compute module  46 , and avoids interfering with other modules located elsewhere in the double-wide rack  10 . Having these cables and connections in the front of the rack makes configuration easier and does not require access to the back of the rack. Still further, positioning the lower power module bays  20 ,  22  consistently to one side of the respective high power zones  16 ,  18  make cabling even more convenient and manageable. 
     FIG. 3  is a schematic partial assembly view of the exemplary chassis  32  receiving the two compute modules  46 , wherein the chassis and upper compute module each have a top cover removed to show the components therein. The 2U chassis  32  includes a power supply  52  having two front-facing connectors  54  for direct blind docking with mating connectors  56  on the compute modules  46 . The two compute modules  46  are preferably independently aligned and inserted into the chassis  32 . Optionally, a single compute module may be installed or two compute modules may be installed separately. Each individual compute module  46  includes a tray  64 , a rearward facing power connector  56 , a motherboard  66 , a hard disk drive  68 , an input/output panel  70 , and a PCI slot  72 . The motherboard  66  is preferably an industry standard motherboard, which may include a pair of processors  74 , a plurality of memory modules  76 , a riser card  78  and a PCI card  80 . Other components known in the art may also be included with the motherboard  66 . The input/output panel  70  includes standard network connectors, such as Ethernet connectors  82 , which can be used to connect the motherboard  66  to a network switch disposed in one of the low power module bays  50  (see  FIG. 2 ) using an Ethernet cable. For purposes of this disclosure it is assumed that each compute module is similarly equipped although the number and types of components may vary. Upon insertion into the chassis  32 , the compute modules  46  are guided rearward along the side walls  84  of the chassis  32  until a rearward facing power connector  56  on each the two compute modules  46  have blind docked with one of the two front-facing connectors  54  on the power supply  52 . The vertical and lateral spacings of the front-facing connectors  54  and the rearward-facing connectors  56  are the same to facilitate their connection. Accordingly, the motherboard  66 , hard disk drive  68  and other components of each compute module  46  are supplied with power. 
   Each chassis in the double-wide rack  10  (see  FIG. 1 ) may include a fan assembly for generating airflow through that chassis. The term “fan” includes “blowers” and other devices for producing a current of air. Referring again to  FIG. 3 , the 2U chassis  32  includes five fans  58  (including one fan for the power supply), four of which are secured in a fan assembly  60  having an air intake grill  62 . The fan assembly  60  may be directly powered and controlled by the power supply  52  according to thermal sensor data passed to it from the compute module  46 . Although the number of fans may vary, the 2U chassis  32  can accommodate larger diameter fans than a 1U chassis due to the 2U height. An even larger chassis can support even larger fans. For example,  FIG. 4  is a perspective view of the 3U chassis  34  with the top panel removed for clarity. The 3U chassis  34  has four fans  96  (also including the fan in the power supply) that may each be larger than the fans  58  in the 2U chassis of  FIG. 3 . Even larger diameter fans could be used in larger chassis (e.g. a 4U chassis and larger), within the limits of the chassis physical dimensions. The use of larger fans may provide more efficient air flow, even in cases wherein the larger fan diameter dictates the use of fewer fans. The fan assembly in each chassis generates airflow for cooling that chassis. Collectively, the numerous fans included among the plurality of chassis in the double-wide rack  10  provide sufficient airflow to cool the individual chassis and also to move the air through the rear-door heat exchanger  100  to cool the airflow exiting the double-wide rack  10  of  FIG. 1  without the use of any additional, outboard booster fans. The absence of booster fans results in quieter and more economical operation of the rack, with fewer parts which could ultimately fail. 
     FIG. 5  is a rear elevation view of the double-wide rack  10  with a partially cut-away view of the rear-door heat exchanger  100 . A hinged edge  110  of the rear-door heat exchanger  100  is pivotably connected to the double-wide rack  10  by a hinge  107  at a hinged end, and may be opened like a door from a free end  112 . The fans included with the plurality of chassis (see the discussion of  FIGS. 3 and 4 ) drive airflow from the front of the double-wide rack  10  through the rear  14  of the double-wide rack  10 , and out through the rear-door heat exchanger  100 . A supply hose  102  supplies the rear-door heat exchanger  100  with a liquid coolant, such as water, from a coolant distribution unit (“CDU”). As heated air exiting the double-wide rack  10  passes through the rear-door heat exchanger  100 , heat from the air is transferred to the coolant, which cools the air exiting the rear-door heat exchanger  100 . The heated coolant is returned to the CDU  116  via a return hose  104 . The fluid circulates back through the CDU  116  to chill the coolant before the coolant is returned to the rear-door heat exchanger  100  through the supply hose  102 . The hoses  102 ,  104  are routed under the rear-door heat exchanger  100  so that the rear-door heat exchanger  100  extends along substantially the entire height H of the double-wide rack  10 , to prevent any airflow losses (e.g. impedance or leakage) that may otherwise result were the hoses  102 ,  104  to be routed in front of some of the modules. 
   The cutaway portion of the double-wide rack in  FIG. 5  reveals the double-wide arrangement of the modules with the two vertically-oriented high power zones  16 ,  18  and the two adjacent, vertically-oriented low power zones  20 ,  22 , as viewed from the rear  14 . The rear-door heat exchanger  100  spans the entire width W of the double-wide rack, resulting in a heat-exchanger with an effective width (i.e., the width that is actively involved in cooling) that is more than twice the effective width of a heat exchanger on a conventional “single wide” rack having a single vertical column of high power modules. This increased width of the rear-door heat exchanger  100  in combination with the reduced depth of the rack  10  contributes to improved cooling capacity as compared with conventional means of cooling racks. The coolant flow rate through the rear-door heat exchanger  100  may be increased accordingly (e.g. doubled) to account for the increased width of the rear-door heat exchanger  100 . As further explained below, the rear-door heat exchanger  100  may remove up to 100% or more of the quantity of heat that was added to the airflow by the modules in the double-wide rack  10 . 
     FIG. 6  is a perspective view of the rear-door heat exchanger  100  from below. The supply hose  102  and return hose  104  are coupled to an inlet manifold  122  and an outlet manifold  124  along the bottom edge of the rear-door heat exchanger  100  via a corresponding pair of schematically-represented “quick-connect” couplers  103 ,  105 . The quick connect coupler  103  places the supply hose  102  in fluid communication with the inlet manifold  122 , and the quick-connect coupler  105  places the return hose  104  in fluid communication with the outlet manifold  124 . The rear-door heat exchanger  100  includes a fin tube assembly  114  through which airflow passes. The inlet manifold  122  provides chilled coolant flow to the fin tube assembly  114 , and the outlet manifold  124  receives heated coolant after it has passed through the fin tube assembly  114 . 
     FIG. 7  is a sectioned view of a portion of the fin tube assembly  114 . The fin tube assembly  114  includes a plurality of tubes  126  (only one complete tube  126  shown) that pass through the rear-door heat exchanger  100  in a serpentine fashion, circulating chilled coolant from the inlet manifold  122  to the outlet manifold  124  under the force of an external pump (not shown) within the CDU. The inlet  122  and outlet manifolds  124  include multiple parallel tube branches having their own inlets and outlets as depicted in  FIG. 7 . The tubes are in direct thermal communication and contact with a plurality of cooling fins  128  that collectively provide a large surface area in contact with the airflow that passes through the rear-door heat exchanger  100 . 
     FIG. 8  is a plan view of the double-wide rack  10  with the rear-door heat exchanger  100  in a closed position. In the closed position, the rear-door heat exchanger  100  is flush with the rear  14  of the double-wide rack  10 , and the direction of the airflow through the fin tube assembly  114  is generally perpendicular to the plane of the rear-door heat exchanger  100 , as generally indicated by the arrow  130 . It should be recognized that the rack  10  will typically include a rear door in order to provide security, give the rack a more desirable appearance, and provide electromagnetic compatibility (EMC) shielding. The rear-door heat exchanger  100  of the embodiment shown is provided within the dimensions of the rack door and does not cause an increase in the rack footprint. 
   A seal  132 , such as a gasket, may be provided at least near the free end  112  of the rear-door heat exchanger  100 . The seal  132  may extend along a portion or the entirety of the perimeter of the rear-door heat exchanger  100 , between the rear-door heat exchanger  100  and the double-wide rack  10 . The seal  132  helps constrain the airflow exiting the double-wide rack  10  to flow substantially entirely through the rear-door heat exchanger  100  by preventing or reducing airflow leakage that may otherwise result in decreased cooling performance. All horizontal and vertical openings in the rack are filled with either the electronic modules or with filler panels so that hot air exiting the rear of the electronics modules can not recirculate, i.e. traverse to the front of the rack and re-enter the electronics. 
   As shown in the rear view of  FIG. 8 , the secondary zones  20 ,  22  that produce less heat are positioned to the left of the primary zones  18 ,  16 , respectively, that produce greater amounts of heat. Although this arrangement was previously described as providing a convenient cabling arrangement along the front  12 , there are additional advantages in the back  14 . First, this arrangement aligns the left-most secondary zone  22  with the heat exchanger manifolds  122 ,  124  which do not have fins. In this manner, an area of the heat exchanger that performs less cooling is aligned to receive the air flow from an area of the rack where components are generating less heat. Second, this arrangement also positions the most commonly serviced modules, such as PDUs and network switches, closer to the opening side of the door (farthest from the hinge), relative to each zone of modules  16 ,  18 . 
     FIG. 9  is a plan view of the double-wide rack  10  with the rear-door heat exchanger  110  pivoted to a partially open position. The rear-door heat exchanger  100  may be opened like a door from the free edge  112  opposite the hinged edge  110 , by pivoting the rear-door heat exchanger  100  on the hinge  107 . The rear-door heat exchanger  100  may be opened, for example, to access some components of the double-wide rack  10  from the rear  14  of the double-wide rack  10 . However, as explained above, access to the various modules supported on the double-wide rack  10  is primarily provided from the front of the double-wide rack  10  to minimize the need to open the rear-door heat exchanger  100 . 
   Opening the rear-door heat exchanger  100  as shown in  FIG. 9  temporarily reduces the cooling of the air in the datacenter because airflow through the double-wide rack  10  is not constrained to flow through the rear-door heat exchanger  100  when opened. Thus, before opening the rear-door heat exchanger  100  for an extended period of time, such as for more than a few minutes, it may be recommended to induce a reduced power consumption mode, at least for the modules in the high power zones  18 ,  20 . For example, a sleep state or throttled condition may be induced in the processors of the various modules mounted in the double-wide rack  10 , or some or all of the modules may be turned off completely. The reduced power consumption mode may be manually induced by datacenter personnel or automatically induced, such as in response to an optional switch  133  being activated or deactivated as a result of opening the rear-door heat exchanger  100 . The switch  133  may be in electronic communication with a management module (not shown) for controlling the power state of the various modules. 
   While inducing a reduced power mode for some of the equipment is generally recommended when the rear-door heat exchanger  100  is open, it should be noted that the numerous on-board cooling fans (see  FIGS. 3 and 4 ) will provide sufficient airflow to cool the double-wide rack  10  even while the rear-door heat exchanger  100  is open. The concern with operating the modules while the Rear-door heat exchanger  100  is open is related more to the potential effect on the ambient air temperatures. As discussed further below, the double-wide rack cooling performance provided by the embodiments of the invention presented herein enable new datacenter configurations wherein multiple double-wide racks are arranged in series, such that air exhausted by the double-wide rack  10  and cooled by the rear-door heat exchanger  100  may be provided as inlet airflow to an adjacent double-wide rack. Thus, once a datacenter has been designed in reliance upon the cooling of the heat exchanger, operating the double-wide rack  10  while the rear-door heat exchanger  100  is open may adversely affect the cooling of adjacent double-wide racks by elevating the intake temperature of airflow to the adjacent double-wide racks. 
   The double-wide door provides more than twice the width of a single-side door, because the effective cooling area of the heat exchanger does not include the door frame. As the width of the door is increases, the door frame represents a decreasing proportion of the door. For example, a double-wide rack could be fitted with two single-wide rear door heat exchangers. However, the two single wide doors would have non-cooling door frames extending along all sides of each door. This would put two non-cooling vertical frames right in the middle of the rack. By using a single, double-wide rear door heat exchanger, the area that would be blocked by the non-cooling vertical frames is replaced by cooling fins. This represents a significant increase in the amount of cooling that the rear door heat exchanger can achieve. In addition, using a single door (rather than two side-by-side doors) provides better access into the rack in the sense that when the door is opened and blocking an aisle, the user can still walk along the aisle to and from the back of the rack from one side. It should be recognized that the rear door heat exchanger can be hinged on either side of the rack, including the left side (not shown), but the arrangement of the rack zones would preferably also be reversed to maintain the beneficial access and cooling relationships described above. 
     FIG. 10  is a graph indicating some representative performance curves for a combination double-wide rack and rear-door heat exchanger according to an embodiment of the invention. Water is used as the coolant in this example. Five exemplary performance curves  181 ,  182 ,  183 ,  184 ,  185  are shown. Each performance curve plots the cooling performance (expressed as percentage heat removal) versus airflow rate for a particular water coolant temperature ranging from 12 Celsius (12 C.) to 20 C. as indicated. The cooling performance values exceed 100% at several points in the graph, indicating that more heat is removed from the airflow by the rear-door heat exchanger than was added to the airflow passing through the double-wide rack. 
   Achieving a cooling performance approaching or exceeding 100% heat removal results in the rear-door heat exchanger exhausting air back into the data center at temperatures near or even less than the mean ambient temperature of the data center. The use of the air-to-liquid heat exchanger on each rack, therefore, significantly reduces the thermal load on the CRACs, and possibly eliminates the need for the CRACs. The use of the air-to-liquid heat exchanger on each rack provides sufficient cooling for more equipment in a data center and/or for more equipment on each rack, such that rack density may be increased without increasing the capacity of the CRAC. 
   The enhanced cooling performance of the rear-door heat exchanger also enables new datacenter layouts and configurations that may depart from a conventional cold-aisle/hot-aisle layout. Data center cooling challenges may be overcome with the addition of a rear-door heat exchanger to one or more of the racks in the data center, and the resulting freedom to reconfigure a data center layout without constructing a facility from scratch. Racks with integrated rear-door heat exchangers according to an embodiment of the invention can individually replace selected racks throughout the data center, as needed. For example, a rack with integrated rear-door heat exchanger may be individually substituted for a conventional rack, where the rack and/or the data center have a history of cooling difficulties. The cooling difficulties with the conventional rack may arise, for example, due to having a high power density (in terms of the number of modules per rack), or due to “hot spots” within the data center where the three-dimensional distribution of chilled air from a CRAC within the data center makes cooling the rack difficult. Alternatively, groups of racks or even all of the racks in the entire data center can be replaced. Data centers often have sections associated with very high power density and those with much lower power density. Hence, one or more racks with an integrated rear-door heat exchanger can replace regions of the data center having racks with high power density. 
     FIG. 11  is a plan view of one exemplary new data center configuration  150  enabled by the improved cooling performance of the double-wide rack with integrated rear-door heat exchanger  10 . The data center configuration  150  includes a plurality of CRACs  152  and CDUs  153  around the perimeter of a data center  160 . Fewer CRACs are needed than in a conventional data center configuration. A raised floor  154  is provided having perforated tiles known in the art. Air is circulated from the CRACs  152 , underneath the raised floor  154 , up through perforated tiles in a central cool-air region  156 . Water is circulated from the CDUs  153 , underneath the raised floor  154 , up through cutouts in the tiles to the rear door heat exchangers. A plurality of the double-wide racks  10  with integrated heat exchangers are arranged in rows disposed around the central cool-air regions  156 . The rows are labeled  161 ,  162 ,  163  in order of increasing distance from the central cool-air regions  156 . Three rows  161 - 163  are shown on each side of the data center  160 , by way of example, though a different number of rows may be included. 
   The double-wide racks  10  in the row  161  nearest the cool air regions  156  have their front (inlet side)  12  facing the cool-air regions  156  to receive cooled air from the cool-air regions  156 . Because the air exhausted from the outlet sides  14  of the double-wide racks  10  are cooled by the rear-door heat exchanger integrated with the double-wide racks  10 , the air exiting the outlet sides  14  of the double-wide racks  10  is sufficiently cooled by the rear-door heat exchangers to supply air to the inlet side  12  of the subsequent double-wide rack in series. Thus, contrary to conventional hot-aisle/cold-aisle configurations, the rows  161 - 163  are arranged in series such that the air inlet side  12  of one double-wide rack faces the rear (air outlet side)  14  of another double-wide rack  10 . For example, the air inlet sides  12  of the double-wide racks  10  in the row  162  face and receive air exhausted from the air outlet sides  14  of the double-wide racks  10  in row  161 , and the air inlet side  12  of double-wide racks  10  in the row  163  face and receive air exhausted from the air outlet side  14  of the double-wide racks  10  in the row  162 . The air exhausted from the double-wide racks  10  in rows  163  enters the CRACs  152 , in case the air exhausted from row  163  has been progressively warmed by the double-wide racks  10 . If the exhaust air has not been progressively warmed by the time it exhausts from the row  163 , the CRACs  152  may be operated in a pure recirculation mode to simply recirculate the air to the cool air regions  156  without cooling the air. 
   For equivalent heat removal capacities, the electrical energy required to run a CDU is dramatically lower than that for a CRAC. A CDU consumes only about a third of the power consumed by a CRAC for the equivalent cooling capacity. Hence, the electrical power requirements and associated cooling costs are incrementally reduced with each CRAC that is replaced by a CDU in the data center. Although this describes a data center configuration with air and water entering from beneath the floor, this configuration could also be used with overhead delivery of the air and water. 
     FIG. 12  is a plan view of another exemplary new data center configuration  200  built on a slab-floor  202  instead of a raised floor. The more expensive raised floor has been eliminated in this embodiment by virtue of the improved cooling performance of the double-wide racks  10  with integrated heat exchangers. The double-wide racks are serially arranged in long columns  211 ,  212  along the length of the data center  210 . The optional CRACs  152  are shown in case some level of optional cooling is desired. The two columns  211 ,  212  each have, by way of example, fifteen rows of the double-wide racks  10 . The double-wide racks  10  in column  211  are arranged serially from one end of the room to the other, with the exhaust side  14  of each double-wide rack facing the inlet side  12  of the next double-wide rack  10  in series. Thus, air is circulated along the column  211  all the way across the room, and then circulated in the reverse direction along the column  212 , creating a generally clockwise air circulation in the data center. An optional barrier, curtain or lightweight wall  158  may be positioned to separate the two columns. Also, optional fans  173  may be located at each end to facilitate the turning of the air. Because the cooling performance of the rear-door heat exchanger provided with each double-wide rack  10  approaches or exceeds 100% heat removal, the air may be continuously circulated in the data center, without necessarily being transported to the CRACs  152  optionally provided. Eliminating the need for the CRACs eliminates the need for the raised floor environment. Similarly with reduced CRAC requirements the CRAC air can be provided overhead, also eliminating the need for the raised floor environment. The slab floor  202  is much more economical to construct than a raised floor. Although this describes a data center configuration with air and water entering from overhead, this configuration could also be used with under the floor delivery of the air and water. 
   The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention. 
   The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but it not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.