Patent Publication Number: US-11659694-B2

Title: Computer system with external bypass air plenum

Description:
This application is a continuation of U.S. patent application Ser. No. 16/698,467, filed Nov. 27, 2019, which is a continuation of U.S. patent application Ser. No. 15/798,272, filed Oct. 30, 2017, now U.S. Pat. No. 10,499,546, which is a continuation of U.S. patent application Ser. No. 14/301,271, filed Jun. 10, 2014, now U.S. Pat. No. 9,807,911, which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Organizations such as on-line retailers, Internet service providers, search providers, financial institutions, universities, and other computing-intensive organizations often conduct computer operations from large scale computing facilities. Such computing facilities house and accommodate a large amount of server, network, and computer equipment to process, store, and exchange data as needed to carry out an organization&#39;s operations. Typically, a computer room of a computing facility includes many server racks. Each server rack, in turn, includes many servers and associated computer equipment. 
     Some computer systems, which can include servers, typically include a number of components that generate waste heat. Such components include printed circuit boards, mass storage devices, power supplies, and processors. For example, some computers with multiple processors may generate 250 watts of waste heat. Some known computer systems include a plurality of such larger, multiple-processor computers that are configured into rack-mounted components, and then are subsequently positioned within a rack system. Some known rack systems include 40 such rack-mounted components and such rack systems will therefore generate as much as 10 kilowatts of waste heat. Moreover, some known data centers include a plurality of such rack systems. 
     Some servers include a number of components that are mounted in an interior of the server. The components, which can include printed circuit boards (for example, a motherboard) and mass storage devices, can support one or more components that generate waste heat, referred to hereinafter as “heat-producing components”. For example, a motherboard can support a central processing unit, and mass storage devices can include hard disk drives which include motors and electronic components that generate heat. Some or all of this heat must be removed from the components to maintain continuous operation of a server. The amount of heat generated by the central processing units, hard disk drives, etc. within a data room may be substantial. Heat may be removed from the heat-producing components via an airflow flowing through a server. 
     In some cases, cooling systems, including air moving systems, may be used to induce airflow through one or more portions of a data center, including airflow through a rack-mounted server that includes various heat-producing components. However, some servers direct airflow through an interior that includes multiple heat-producing components, so that air removes heat as it passes through the interiors, so that air passing over heat-producing components in a downstream portion of the server has a reduced heat removal capacity relative to air passing over heat-producing components in an upstream portion of the server. As a result, less heat can be removed from downstream heat-producing components than upstream heat-producing components. In some cases, the downstream heat-producing components are more sensitive to heat than the upstream heat-producing components, which can result in a suboptimal configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a schematic diagram illustrating a perspective view of a computer system that includes heat-producing components installed in upstream portions and downstream portions of the computer system chassis interior, according to some embodiments. 
         FIG.  1 B  is a schematic diagram illustrating a side view of a computer system that includes heat-producing components installed in upstream portions and downstream portions of the computer system chassis interior, according to some embodiments. 
         FIG.  1 C  is a graphical representation of a temperature profile of an airflow through a computer system chassis interior that removes heat from heat-producing components installed in an upstream portion and heat-producing components installed in a downstream portion of a computer system chassis interior, according to some embodiments. 
         FIG.  2 A  is a schematic diagram illustrating a side view of a computer system that includes a bypass air plenum, according to some embodiments. 
         FIG.  2 B  is a graphical representation of a temperature profile of an inlet airflow through a computer system chassis interior that removes heat from heat-producing components installed in an upstream portion of the chassis interior, mixes with a bypass airflow downstream of the upstream heat-producing components, and removes heat from heat-producing components installed in a downstream portion of the chassis interior, according to some embodiments. 
         FIG.  3    is a schematic diagram illustrating a side view of an upstream portion of a computer system chassis interior that includes a bottom vent supplying air to the bypass air plenum, according to some embodiments. 
         FIG.  4    is a schematic diagram illustrating a side view of an upstream portion of a computer system chassis interior that includes a discharge vent that discharges bypass air from a bypass air plenum to mix with the inlet airflow, according to some embodiments. 
         FIG.  5 A-C  are schematic diagrams illustrating side views of an upstream portion of a computer system chassis interior that includes a component that can be installed in various elevations in the interior to control the cross-sectional areas of the bypass air plenum and the inlet air plenum, according to some embodiments. 
         FIG.  6    is a schematic diagram illustrating a side view of a computer system chassis interior that includes an adjustable flow control element, a temperature sensor device, and an air moving device that are communicatively coupled to a controller, according to some embodiments. 
         FIG.  7    is a schematic diagram illustrating a perspective view of a computer system that includes partitions that direct separate inlet air flows and bypass air flows through separate sets of air moving devices, according to some embodiments. 
         FIG.  8    is a schematic diagram illustrating a perspective view of a computer system that includes bypass vents installed in a base plate of the computer system chassis, according to some embodiments. 
         FIG.  9    is a schematic diagram illustrating a perspective view of a computer system that includes bypass vents installed in a side surface of the computer system chassis, according to some embodiments. 
         FIG.  10    illustrates a method of providing a bypass airflow in a computer system interior, according to some embodiments. 
     
    
    
     The various embodiments described herein are susceptible to various modifications and alternative forms. Specific embodiments are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the disclosure to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to. 
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Various embodiments of computer systems, and systems and methods for performing computing operations and removing waste heat from various heat-producing components in computer systems, are disclosed. According to one embodiment, a system for storing data includes one or more rack-mountable computer systems mounted in a rack, where the computer systems include a chassis that encompasses a chassis interior. A circuit board, which is installed in an upstream portion of the chassis interior that is near an air inlet vent, has an upper surface upon which an upstream heat-producing component is installed. An inlet air plenum directs an inlet air flow from the inlet end of the chassis through at least the upstream portion of the chassis interior to remove heat from the upstream heat-producing component installed on the upper surface of the circuit board. A bypass air plenum extends beneath the circuit board and is unencompassed by the inlet air plenum. The bypass air plenum directs a bypass air flow that is isolated from the inlet air flow to mix with the inlet air flow downstream of the upstream heat-producing component to establish a mixed air flow. The mixed air is directed into a downstream portion of the chassis interior to remove heat from a downstream heat-producing component installed in the downstream portion. 
     According to one embodiment, a computer system includes a chassis, having an inlet end, which at least partially encompasses a chassis interior, an inlet air plenum, and a bypass air plenum. The inlet air plenum directs an inlet air flow through an upstream portion of the chassis interior to remove heat from an upstream heat-producing component installed in the upstream portion. The bypass air plenum extends through a portion of the chassis interior that is unencompassed by the inlet air plenum and directs a bypass air flow that is isolated from the inlet air flow to mix with the inlet air flow downstream of the upstream heat-producing component. 
     According to one embodiment, configuring a computer system to direct a bypass air flow to bypass from flowing in heat transfer communication with a heat-producing component installed in the computer system interior and to mix with a separate inlet air flow in a portion of the computer system interior that is downstream of the heat-producing component in the computer system interior includes partitioning the upstream portion of the interior into separate air passages, where one passage directs the inlet air flow to remove heat from the heat-producing component and another separate passage directs a bypass air flow to mix with the inlet air flow in the portion of the computer system interior that is downstream of the at least one heat-producing component in the computer system interior. The separate air passage is unencompassed by the particular air passage that directs the inlet air flow. 
     According to one embodiment, a system for storing data includes a rack, multiple computer systems mounted on the rack, and at least one bypass air plenum that extends through a gap space between two adjacent computer systems. The computer systems include a chassis with an inlet and outlet end that encompasses an interior, a circuit board in an upstream portion of the interior and includes an upstream heat-producing component on an upper surface, and an inlet air plenum that extends through the interior above the circuit board and directs an inlet air flow that removes heat from the upstream heat-producing component. The bypass air plenum is unencompassed by the chassis interior of either of the adjacent computer systems and directs a bypass air flow that is isolated from the inlet air flow to mix with the inlet air flow in the inlet air plenum of one of the adjacent computer systems downstream of the upstream heat-producing component to establish a mixed air flow. 
     According to one embodiment, a computer system includes a chassis, having an inlet end, which at least partially encompasses a chassis interior, and inlet air plenum, and a bypass air vent. The chassis at least partially bounds, on an external surface of at least one end that is separate from the inlet end, a gap space external to the chassis interior. The inlet air plenum directs an inlet air flow from the inlet end through an upstream portion of the chassis interior to remove heat from an upstream heat-producing component installed in the upstream portion of the chassis interior. The bypass air vent is included in the at least one end that is separate from the inlet end. The bypass air vent receives a bypass airflow from a portion of the gap space to mix with the inlet air flow in a location in the chassis interior that is downstream of the upstream heat-producing component. 
     According to one embodiment, configuring a computer system to direct a bypass air flow of air that bypasses flowing in heat transfer communication with at least one heat-producing component installed in an interior of the computer system to mix with a separate inlet air flow in a portion of the computer system interior that is downstream of the at least one heat-producing component in the computer system interior comprises installing a bypass air vent in an end of the chassis and mounting at least one structural element to establishing a gap space external to the computer system. The end of the chassis in which the bypass air vent is installed is downstream of the at least one heat-producing component in the computer system interior. The bypass air vent is in flow communication with an external environment. The at least one structural element is mounted proximate to an outer surface of the end of the chassis. The gap space is at least partially encompassed by the outer surface of the end of the chassis and the at least one structural element. The bypass air vent directs the bypass air flow from the gap space into the chassis interior to mix with the inlet air flow in the portion of the computer system interior that is downstream of the at least one heat-producing component in the computer system interior. 
     As used herein, “air moving device” includes any device, element, system, or combination thereof that can move air. Examples of air moving devices include fans, blowers, and compressed air systems. 
     As used herein, “backplane” means a plate or board to which other electronic components, such as mass storage devices, circuit boards, can be mounted. In some embodiments, mass storage devices, which can include one or more hard disk drives, are plugged into a backplane in a generally perpendicular orientation relative to the face of the backplane. In some embodiments, a backplane includes and one or more power buses that can transmit power to components on the backplane, and one or more data buses that can transmit data to and from components installed on the backplane. 
     As used herein, “circuit board” means any board or plate that has one or more electrical conductors transmitting power, data, or signals from components on or coupled to the circuit board to other components on the board or to external components. In certain embodiments, a circuit board is an epoxy glass board with one or more conductive layers therein. A circuit board may, however, be made of any suitable combination of materials. A circuit board can include a printed circuit board. 
     As used herein, “chassis” means a structure or element that supports another element or to which other elements can be mounted. A chassis may have any shape or construction, including a frame, a sheet, a plate, a box, a channel, or a combination thereof. In one embodiment, a chassis is made from one or more sheet metal parts. A chassis for a computer system may support circuit board assemblies, power supply units, data storage devices, fans, cables, and other components of the computer system. 
     As used herein, “computing” includes any operations that can be performed by a computer, such as computation, data storage, data retrieval, or communications. 
     As used herein, “computer system” includes any of various computer systems, computing devices, or components thereof. One example of a computer system is a rack-mounted server. As used herein, the term computer is not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a processor, a server, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit, and other programmable circuits, and these terms are used interchangeably herein. In the various embodiments, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM). Alternatively, a compact disc—read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, additional input channels may include computer peripherals associated with an operator interface such as a mouse and a keyboard. Alternatively, other computer peripherals may also be used that may include, for example, a scanner. Furthermore, in the some embodiments, additional output channels may include an operator interface monitor and/or a printer. 
     As used herein, “data center” includes any facility or portion of a facility in which computer operations are carried out. A data center may include servers dedicated to specific functions or serving multiple functions. Examples of computer operations include information processing, communications, testing, simulations, power distribution and control, and operational control. 
     As used herein, to “direct” air includes directing or channeling air, such as to a region or point in space. In various embodiments, air movement for directing air may be induced by creating a high pressure region, a low pressure region, or a combination both. For example, air may be directed downwardly within a chassis by creating a low pressure region at the bottom of the chassis. In some embodiments, air is directed using vanes, panels, plates, baffles, pipes or other structural elements. 
     As used herein, “member” includes a single element or a combination of two or more elements (for example, a member can include two or more sheet metal parts fastened to one another. 
     As used herein, a “rack” means a rack, container, frame, or other element or combination of elements that can contain or physically support one or more computer systems. 
     As used herein, “room” means a room or a space of a building. As used herein, “computer room” means a room of a building in which computer systems, such as rack-mounted servers, are operated. 
     As used herein, a “space” means a space, area or volume. 
     As used herein, “shelf” means any element or combination of elements on which an object can be rested. A shelf may include, for example, a plate, a sheet, a tray, a disc, a block, a grid, or a box. A shelf may be rectangular, square, round, or another shape. In some embodiments, a shelf may be one or more rails. 
       FIG.  1 A  is a schematic diagram illustrating a perspective view of a computer system that includes heat-producing components installed in upstream portions and downstream portions of the computer system chassis interior and an airflow through the interior, according to some embodiments.  FIG.  1 B  is a schematic diagram illustrating a side view of the computer system.  FIG.  1 C  is a graphical representation of a temperature profile of an airflow through the computer system interior that removes heat from heat-producing components installed in the upstream portion and heat-producing components installed in the downstream portion, according to some embodiments. 
     Computer system  100  includes a chassis  108  that at least partially encompasses a chassis interior  101 , upstream heat-producing components  122 , supported by a circuit board  120 , that are coupled to the chassis  108  in an upstream portion of the interior  101 , downstream heat-producing components  134  coupled to the chassis  108  in a downstream portion  104  of the interior, and an array  106  of air moving devices  107  coupled to the chassis in the chassis interior  101 . Chassis  108  includes at least a base plate  110  and an inlet plate  112 . Components coupled to the chassis in a portion of the chassis interior can include components that are installed in the chassis interior, and installing a component in the chassis interior can include coupling the component to a portion of the chassis, such that the component is mounted in a particular position in the chassis interior. 
     In some embodiments, components mounted in a particular position in the chassis interior may partition a portion of the chassis interior into various interior spaces, which can include one or more air plenums through which air is directed to cool heat-producing components in the chassis interior. In the illustrated embodiment, for example, circuit board  120  is installed in an elevated position in an upstream portion of the chassis interior  101 , where the upstream portion comprises a portion of the interior  101  that is proximate to the inlet end  180  of the chassis. As a result of installing the circuit board  120  in the elevated position, the upstream portion of the interior  101  is partitioned into an inlet air plenum  102 , through which cooling air received into the plenum  102  as “inlet air” flows to cool various heat-producing components in the interior  101 , and a space  129  is established between the lower surface of the circuit board  120  and an upper surface of the base plate  110  of the chassis  108 . 
     The space  129  established between the circuit board  120  and the base plate  110  can mitigate contact by the base plate  110  with various elements that may protrude from the lower surface of the circuit board  120 , including capacitor elements, stiffening brackets under a central processing unit, connector tails soldered to the circuit board  120  that protrude through the lower surface, some combination thereof, or the like. The spacing  129  between the circuit board  120  and the base plate  110  can mitigate the risk of electrical shorting of the circuit board  120  via contact with the base plate  110 . 
     In some embodiments, portions of the chassis interior are partitioned based at least in part upon various components coupling with various portions of the chassis to establish interior spaces that are bounded by the components and portions of the chassis. In the illustrated embodiment, for example, the circuit board  120  is coupled to inlet plate  112  of the chassis  108 . In some embodiments, the circuit board is coupled at least partially to the base plate  110  and rests upon one or more vertical posts that support the circuit board  120  in an elevated position above space  129 . 
     In some embodiments, a computer system includes an inlet end that is configured to face into an aisle space when the computer system is mounted in a rack. The aisle space can include a cold aisle that supplies cooling air into the computer system to remove waste heat from heat-producing components in the computer system interior. The cooling air can enter the computer system interior, which can include a chassis interior  101 , via one or more inlet air vents installed in an inlet end of the computer system. In the illustrated embodiment, for example, chassis  108  includes an inlet plate  112  at the inlet end  180  of the computer system  100 , where the inlet plate  112  includes inlet vents  114  through which cooling air passes into the interior  101 . Such cooling air, referred to hereinafter as an “inlet air flow”  116 , can flow through the various portions of the interior  101  that are in flow communication with the inlet vents  114  and remove heat from the various heat-producing components installed in positions that are in heat transfer communication with the air flow  116 . For example, as plenum  102  is established via the upper surface of circuit board  120  and portions of the inlet plate  112  that include the air vents  114 , the air flow  116  directed into the plenum  102  via air vents  114  can remove heat from upstream heat-producing components  118 ,  122 , which can each include one or more heat-producing components, installed in heat transfer communication with the plenum  102 . 
     A component in heat transfer communication with an air plenum can include a component which is installed in a position that exposes one or more portions of the component to an air flow through the air plenum, such that the air flow can remove heat from the component. In some embodiments, a component in heat transfer communication with an air plenum includes a component that is at least partially installed within a portion of the air plenum through which the air flow is directed. 
     Air moving device  107  can supply the inlet airflow  116  that has passed through the inlet air plenum  102 , via ducting  111  and fans  109 , to a downstream portion  104  of the interior  101  in which downstream heat-producing components  134 , which can include mass storage devices, are installed. A heat-producing component may be referred to as an “upstream heat-producing component” or “downstream heat-producing component” based at least in part upon the heat-producing component being coupled to the chassis within the upstream portion or downstream portion of the chassis interior, respectively. 
     As the space  129  is not in flow communication with an air vent  114 , the air in space  129  can be stagnant. In the illustrated embodiment, for example, space  129  is established by base plate  110 , the lower surface of circuit board  120 , and a portion of inlet plate  112  that does not include inlet air vents  114 . As a result, space  129  may be unencompassed by, and isolated from, the plenum  102  and the airflow  116  passing through it. 
     In some embodiments, as the inlet air flow  116  passes from the inlet end  180  towards the exhaust end  190  and removes heat from heat-producing components coupled to the chassis in various portions of the chassis interior  101 , the inlet air flow  116  progressively increases in temperature and loses heat removal capacity.  FIG.  1 C  shows a graph  139  of a relationship  150  between the temperature  140  of the inlet air flow  116  and the distance  142  through which the air flow  116  passes in the chassis interior  101  illustrated in  FIG.  1 A-B . As shown in  FIG.  1 C , the inlet air flow  116  entering the chassis interior  101  at the inlet end  180 , also illustrated as point X=0, has a temperature  145  that corresponds to the temperature of the cooling air external to the computer system  100 . As the inlet air flow  116  passes through the inlet air plenum  102  that is proximate to the inlet end  180 , the inlet air flow  116  removes heat from one or more heat-producing components included in upstream heat-producing components  118 ,  122  installed in heat transfer communication with the plenum  102 . As further shown in the graph  139  of  FIG.  1 C , as the inlet air flow  116  removes heat from upstream heat-producing components  118 ,  122 , the temperature  140  of the air flow  116  rises, so that the temperature of the airflow  116  at a location “X 1 ” along the distance  142  through the interior  101 , where X 1  is a location in the interior  101  that is downstream of the upstream heat-producing components  118 ,  122 , is at a temperature  152  that is elevated over the initial temperature  145  of the air flow  116 . The rise in temperature can be due to the air flow  116  removing heat from upstream heat-producing components  118 ,  122 . 
     As the air flow  116  passes through the air moving device  107  and into the downstream portion  104  of the chassis interior  101 , the air flow  116  removes heat from various downstream heat-producing components  134  installed in the downstream portion  104 . As a result, as shown in  FIG.  1 C , the temperature of the air flow  116  continues to increase as it passes across the heat-producing components  134 , to temperature  154  at location “X 2 ” and to temperature  156  at location “X 3 ” downstream of the heat-producing components  134  proximate to the exhaust end  190 . 
     In some embodiments, various components installed in the computer system interior  101  have various maximum operating temperature thresholds. A maximum operating temperature threshold can include a maximum operating temperature of the component, above which the component may incur damage due to heat. In some embodiments, components to be installed in the chassis interior that have lower maximum temperatures, also referred to as components with relatively greater heat sensitivity, than other components to be installed in the chassis interior can be mounted proximate to the inlet end  180 , so that the inlet air flow  116  across those components has a lower temperature and greater heat removal capacity relative to the air flow passing through the downstream portion of the chassis interior. As a result, the components installed in the upstream portion of the computer system can operate at a lower temperature than components installed in the downstream portion. 
     In some embodiments, certain components are installed in the upstream portion of the chassis interior despite having lower heat sensitivities than components installed in the downstream portion. For example, in the illustrated embodiment of  FIG.  1 B , computer system  100  includes an I/O slot card  118  and circuit board  120  supporting upstream heat-producing component  122 , where upstream heat-producing component  122  can include a central processing unit (CPU). The illustrated configuration of components in the upstream portion, in some embodiments, enables certain connections to the computer system  100  on the inlet end  180 , including power connections, communication connections, etc. 
     However, as illustrated in  FIG.  1 C , the upstream heat-producing component  122  installed in the upstream portion, which can include a CPU, has a maximum operating temperature  144  that is higher than the maximum operating temperature  146  of the downstream heat-producing components  134 , which can include mass storage devices, installed in the downstream portion  104 . Because the CPU  122  is installed in the upstream portion, the operating temperature  152  of the CPU can be significantly less than the maximum operating temperature for that heat-producing component, as illustrated by margin  162 , based at least in part upon the lower temperature of the inlet air flow  116  in the plenum  102  and the higher maximum operating temperature  144  of the CPU  122 . 
     In contrast, because the mass storage devices  134  have a lower maximum operating temperature  146 , and the devices  134  are installed in the downstream portion  104 , the inlet air flow  116  that flows across the devices  134  has already removed heat from the CPU  122  and thus already has an elevated temperature and reduced heat removal capacity relative to the inlet air flow that initially enters through the inlet end at X=0. The resulting operating temperatures  154 ,  156  of the devices  134  can be relatively close to the maximum operating temperature  146 , with minimal temperature margin  164 ,  166  between the operating temperatures and the maximum operating temperature to avoid thermal damage. 
     In some embodiments, air flow through a computer system is controlled to be optimized against the maximum operating temperatures of the components installed therein. Optimized airflow can include an airflow that maintains an operating temperature of some or all of the components installed in the computer system approximately at their maximum operating temperatures. As a result, a given component receives a sufficient airflow at a sufficiently low temperature to avoid thermal damage, without expending excess airflow across other components. 
     However, where components with high heat sensitivity are installed in a downstream portion of a computer system, excess airflow may be supplied through the computer system to ensure that the downstream heat-producing components do not incur damage. In the illustrated embodiment of  FIG.  1 C , for example, while just enough inlet air flow  116  is supplied through the interior  101  to maintain the operating temperature  156  of the exhaust end-proximate mass storage device  134  with a minimal margin  166  beneath the maximum operating temperature  146 , and the preceding device  134  operates at a similar temperature  154  with slightly more margin  164 , the CPU  122  operates at a temperature  152  that is significantly less than the maximum operating temperature  144  for that heat-producing component, as shown by margin  162 . Such a significant temperature margin  162  indicates that excess air is being passed across CPU  122  in order to ensure that the mass storage devices  134  receive sufficient cooling. 
     Furthermore, while the rate of inlet air flow  116  is controlled by operation of the air moving devices  107  in the computer system interior  101 , and the devices  107  can adjust the inlet air flow  116  through the entire interior  101  by adjusting operating speed, such adjustment equally affects flow rate across upstream heat-producing components  118 ,  122  and downstream heat-producing components  134 . As a result, increasing the air flow  116  to provide sufficient cooling to downstream heat-producing components  134  provides excess air flow  116  to upstream heat-producing components  122 , while reducing air flow  116  to optimize air flow across upstream heat-producing components  122  can starve the downstream heat-producing components  134  from receiving sufficient cooling to prevent thermal damage. 
       FIG.  2 A  is a schematic diagram illustrating a side view of a computer system that includes heat-producing components installed in upstream portions and downstream portions of the computer system chassis interior and a bypass air plenum that extends beneath a circuit board and directs a bypass airflow to mix with an inlet airflow downstream of an upstream heat-producing component installed in the upstream portion of the computer system chassis interior, according to some embodiments.  FIG.  2 B  is a graphical representation of a temperature profile of an inlet airflow through the chassis interior that removes heat from upstream heat-producing components installed in an upstream portion of the chassis interior, mixes with a bypass airflow downstream of the upstream heat-producing components, and removes heat from downstream heat-producing components installed in a downstream portion of the chassis interior, according to some embodiments. 
     Computer system  200  includes a chassis  208  that at least partially encompasses a chassis interior  201 , upstream heat-producing components  222  supported by a circuit board  220  installed in an upstream portion of the interior  201 , downstream heat-producing components  234  installed in a downstream portion  204  of the interior  201 , and an air moving device  207  installed in the interior  201 . Chassis  208  includes at least a base plate  210  and an inlet plate  212 . 
     In some embodiments, a bypass air plenum directs a bypass air flow that is isolated from the inlet air flow to mix with the inlet air flow downstream of one or more heat-producing components from which the inlet air flow has removed heat. The bypass air flow mixes with the heated inlet air flow to provide a mixed air flow that is cooler than the heated inlet air flow. The mixed air flow can cool components installed in a downstream portion of the computer system. 
     In the illustrated embodiment, computer system  200  includes a bypass air plenum  270  that extends beneath the circuit board  220  in the upstream portion of the interior  201 . The plenum  270  can be at least partially established by a portion of the circuit board  220 , including a lower surface of the circuit board  220 , and a portion of the base plate  210 . Bypass air vent  272  supplies cooling air into the plenum  270  as a bypass air flow  274 . In some embodiments, bypass air plenum  270  is established by installing bypass air vent  272  in a computer system that includes a space beneath an installed circuit board, such as illustrated and discussed above with reference to space  129  of computer system  100  in  FIG.  1 A-B . The bypass air vent  272  enables air flow through the space that comprises the plenum  270 . 
     In some embodiments, the plenum  270  is unencompassed by the inlet air plenum  202  through which an inlet air flow  216  is directed to remove heat from one or more heat-producing components included in one or more upstream heat-producing components  222  installed in the upstream portion of the computer system interior  201 . In the illustrated embodiment, for example, plenum  270  does not extend through plenum  202  via ducting, but is rather isolated from the plenum  202  via the circuit board  220 . Heat transfer through circuit board  220  to the bypass air flow  274  from heat-producing components  222  may be significantly less than heat transfer from components  222  to inlet air flow  216 . As a result, the bypass air flow  274  may reach a point in the interior  201  that is downstream from components  222  with a reduced temperature and greater heat removal capacity than the inlet air flow  216  that has reached the same point downstream of components  222 . Inlet air that has passed in heat transfer communication with one or more upstream heat-producing components, and has removed heat from same, is referred to hereinafter as “heated inlet air”. 
     In some embodiments, a bypass air flow is directed, via a bypass air plenum, to mix with heated inlet air to provide a mixed air flow that is cooler than the heated inlet air. The bypass air flow can be directed into the inlet air plenum  202 , for example, at a location in the plenum  202  that is downstream of one or more upstream heat-producing components  222 . The bypass air flow can be directed into the inlet air plenum via a gap between various components in the computer system that allows a portion of the inlet air plenum to receive a bypass air flow from the bypass air plenum. In the illustrated embodiment, for example, a gap  276  between circuit board  220  and air moving device  207  enables the bypass air flow  274  to flow from plenum  270  into plenum  202  at a point downstream of the heat-producing components  222 , such that the bypass air flow  274  does not flow in heat transfer communication with the heat-producing components  222  and does not remove heat from the heat-producing components  222  due to such flow. In some embodiments, the bypass air flow  274  and the heated inlet air flow  216  can mix at a point downstream of each of the plenums  202 ,  270 . For example, the air flows  216 ,  274  can mix in a midstream portion  209  of the interior  201 , which may include one or more air moving devices  207 . In some embodiments, the operation of the air moving device  207  facilitates mixing of the air flows  216 ,  214  in the midstream portion  209 . For example, air moving device  207  can at least partially induce the bypass air flow  274  and inlet air flow  216  and, by drawing both air flows  216 ,  270  through the air moving device  207 , facilitates mixing of the separate air flows. 
     In some embodiments, the mixed air flow provided by the mixing of a bypass air flow and a heated inlet air flow is directed through a downstream portion of the chassis interior to cool various downstream heat-producing components installed in the downstream portion. In the illustrated embodiment of  FIG.  2 A , for example, mixed air flow  236  flows from midstream portion  209  and through downstream portion  204  to remove heat from downstream heat-producing components  234  installed in the downstream portion. Downstream heat-producing components  234  can include mass storage devices, including hard disk drives (HDDs). 
     In some embodiments, mixing the inlet air flow with the bypass air flow at a point downstream of at least one upstream heat-producing component provides a mixed air flow that has a greater heat removal capacity and lower temperature than the inlet air flow alone downstream of the upstream heat-producing component. For example, as the inlet air flow  216  passes from the inlet end  280  towards the exhaust end  290  and removes heat from heat-producing components  222  installed in the inlet air plenum  202 , the inlet air flow  216  progressively increases in temperature and loses heat removal capacity.  FIG.  2 B  shows graph  139  of a relationship  250  between the temperature  240  of the inlet air flow  216  and the distance  242  through which the air flow  216  passes in the computer system  200  illustrated in  FIG.  2 A . As shown in  FIG.  2 B , the inlet air flow  216  entering the computer system  200  at the inlet end  280 , also referred to as point X=0, has a temperature  245  that corresponds to the temperature of the cooling air external to the computer system  200 . As the inlet air flow  216  passes through the plenum  202  that is proximate to the inlet end  280 , the inlet air flow  216  removes heat from one or more heat-producing components  222  installed in heat transfer communication with the plenum  202 . As further shown in the graph  239  of  FIG.  2 B , as the inlet air flow  216  removes heat from heat-producing components  222 , the temperature  240  of the air flow  216  rises, so that the temperature of the airflow  216  at a location “X 1 ” along the distance  242  through the interior  201 , where X 1  is a location that is downstream of the heat-producing components  222 , is at a temperature  252  that is elevated over the initial temperature  245  of the air flow  216 . The rise in temperature can be due to heat removal from heat-producing components  222 . 
       FIG.  2 B  further illustrates a relationship  255  between the temperature  240  of the bypass air flow  274  and the distance  242  through the interior  201 . The illustrated relationship  255  shows that a bypass air flow  274  through the bypass air plenum  270  maintains an air temperature that approximates the initial temperature  245  of the cooling air, as the bypass air flow  274  bypasses from removing heat from heat-producing components  222  in the inlet air plenum  202 . 
     As the air flows  216  and  274  pass downstream of the upstream heat-producing components  222 , the bypass air flow  274  can be directed, via a gap  276  between surfaces establishing the plenum  270 , via directing by one or more elements through a midstream portion  209 , etc., to mix with the inlet air flow  216 . As a result, as shown in  FIG.  2 B , the characteristics of the two air flows mix to provide characteristics of the mixed air flow  236 . As the mixed air flows pass through the midstream portion  209 , from location “X 1 ” to “X 1 . 5 ”, the temperature of the mixed air flow  236  reaches a point  253  that is less than the temperature point  252  of the inlet air flow  216  after removing heat from heat-producing components  222  and prior to mixing and is more than the approximate temperature point  245  of the bypass air flow  274 . As shown, the mixed air flow  236  may have a greater heat removal capacity and lower temperature than the heated inlet air flow  216  alone. 
     As the mixed air flow  236  passes through the downstream portion  204  of the computer system interior  201 , the air flow  236  removes heat from various downstream heat-producing components  234  installed in the downstream portion  204 . As a result, as shown in  FIG.  2 B , the temperature of the air flow  236  increases as it passes across the heat-producing components  234 , to temperature  254  at location “X 2 ” and to temperature  256  at location “X 3 ” downstream of the downstream heat-producing components  234  proximate to the exhaust end  290 . 
     In some embodiments, air flow through a computer system that includes a bypass air plenum enables at least some heat-producing components in both upstream and downstream portions of the chassis interior to operate approximately at the respective maximum operating temperatures for the respective components, regardless of whether the downstream heat-producing components have greater heat sensitivity than the upstream heat-producing components. In the illustrated embodiment, where the downstream heat-producing components  234  have a lower maximum operating temperature  246  than the maximum operating temperature  244  of the upstream heat-producing components  222  by a certain margin  264 , the mixed air flow  236  has sufficiently flow and sufficiently reduced temperature to maintain the most exhaust-proximate heat-producing component  234  at a temperature  256  that approximates the maximum operating temperature  246  for that heat-producing component. A temperature of a component may be considered to approximate the maximum operating temperature when the temperature is less than the maximum operating temperature by no more than a certain temperature margin. As a result, air flow is optimized for that heat-producing component  234 . 
     Furthermore, as shown in  FIG.  2 B , the inlet air flow  216  entering plenum  202  via inlet end  280  is of sufficient flow rate and sufficiently low temperature  245  to maintain heat-producing components  222  at a temperature  252  that approximates the maximum operating temperature  244  for that heat-producing components  222 . As a result, air flow is optimized for those heat-producing components  222  as well. Thus, the air flow  216  through plenum  202  is optimized for heat removal from heat-producing components  222 , while the bypass airflow  274  enables optimizing of the airflow  236  through downstream portion  204  for heat removal from downstream heat-producing components  234 , thereby optimizing the airflow through the entire computer system  200 . Excess air flow through computer system  200  is minimized, such that, while some components may operate at a small margin  264  below the maximum operating temperature  246  for that respective component, the combined inlet airflow  216  and bypass airflow  270  is minimally sufficient to preclude thermal damage to components in both the upstream portion and the downstream portion of the computer system. In some embodiments, an optimized combined inlet air flow  216  and bypass airflow  270  enable maximization of the number of heat-producing components  222 ,  234  in the computer system that are operating approximately at their respective maximum operating temperatures. 
       FIG.  3    is a schematic diagram illustrating a side view of an upstream portion of a computer system interior that includes a bottom vent supplying air to the bypass air plenum, according to some embodiments. Computer system  300  includes a chassis  308  that includes a base plate  310  and an inlet plate  312 , a circuit board  320  supporting a heat-producing component  322  in heat transfer communication with an inlet air plenum  302 , and a bypass air plenum  370  extending beneath at least a portion of the circuit board  320 . In some embodiments, computer system  300  is at least partially comprised in at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     In some embodiments, a bypass air vent can be established in various faces of a bypass air plenum. The bypass air vent may be established in a portion of a chassis that is separate from another portion of the chassis in which one or more inlet air vents are established. For example, in the illustrated embodiment, the computer system  300  includes a bypass air plenum  370 , established between a chassis base plate  310 , inlet plate  312 , and circuit board  320 , that receives a bypass air flow  374  via a bypass air vent  372  that is established in the base plate  310 , rather than the inlet plate  312  in which the inlet air vents  314  that supply the inlet air flow  316  to the inlet air plenum  302  are established. In some embodiments, where the computer system  300  is stacked vertically adjacent to one or more external components, the bypass air vent  372  can supply air into the plenum  370  from a gap between the computer system  300  and an external component mounted beneath the computer system  300 . 
     In some embodiments, bypass air vent  372  supplies bypass air flow  374  from a separate air source than the source from which inlet air vents  314  supply inlet air flow  316 . For example, the inlet air flow  316  may be directed into plenum  302  from a cold aisle that is faced by inlet plate  312 , where the cold aisle circulates ambient air received into the cold air from a free-cooling system, while the bypass air flow  374  may be directed through bypass air vent  372  from a mechanical cooling system that sensibly chills the bypass air flow. In some embodiments, the bypass air flow  374  is supplied into plenum  370  via a conduit that couples with the bypass air vent  372 . 
       FIG.  4    is a schematic diagram illustrating a side view of an upstream portion of a computer system chassis interior that includes a discharge vent that discharges bypass air from a bypass air plenum to mix with the inlet air flow, according to some embodiments. Computer system  400  includes a chassis  408  including a base plate  410  and an inlet plate  412 , a circuit board  420  supporting an upstream heat-producing component  422  in heat transfer communication with an inlet air plenum  402 , and a bypass air plenum  470  extending beneath at least a portion of the circuit board  420 . In some embodiments, computer system  400  is at least partially comprised in at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     In some embodiments, a discharge air vent can be established in one or more portions of a component that establishes at least one boundary of the bypass air plenum. Where a boundary of the bypass air plenum is established by a circuit board, for example, a discharge air vent can be established in the circuit board. The discharge air vent enables a bypass air flow to pass through the circuit board, from the bypass air plenum extending along one surface of the circuit board, to mix with an inlet air flow in a space that is bounded by the opposite surface of the circuit board. 
     For example, in the illustrated embodiment of  FIG.  4   , where air moving device  406  is mounted on a structure  409  in physical contact with circuit board  420  to establish the boundaries of bypass air plenum  470  that are not established by plates  410 ,  412  of the chassis, circuit board  420  can include a discharge vent  476  that communicatively couples the bypass air plenum  470  with the inlet air plenum  402  at a point in the plenum  402  downstream of at least one heat-producing component  422  installed in heat transfer communication with the plenum  402 . As a result, bypass air  474  passing through the bypass air plenum  470  from the bypass air vent  472  can be discharged from the bypass air plenum into the inlet air plenum  402  via one or more discharge vents  476  established in the circuit board  420 . 
       FIG.  5 A-C  are schematic diagrams illustrating side views of an upstream portion of a computer system chassis interior that includes a component that can be installed in various elevations in the interior to control the cross-sectional areas of the bypass air plenum and the inlet air plenum, according to some embodiments. Computer system  500  includes a chassis  508  that includes a base plate  510  and inlet plate  512 , a circuit board  520  supporting an upstream heat-producing component  522  in heat transfer communication with an inlet air plenum  502 , a bypass air plenum  570  extending beneath the circuit board  520 , and an air moving device  507  that induces the inlet air flow  516  and bypass air flow  574  through at least the respective air plenums  502 ,  570 . In some embodiments, computer system  500  is at least partially comprised in at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     In some embodiments, one or more components in a computer system can be adjustably configured to be installed in the chassis interior in various positions to provide adjustable control of the relative flow rates of the inlet air flow and the bypass air flow through the computer system interior. Such one or more components can be referred to as flow control elements, and can be installed in the interior in various positions to provide variable cross-sectional areas of the inlet air plenum and bypass air plenum. A cross-sectional area of a plenum can refer to a cross-section of a plenum that extends perpendicular to the direction of air flow through the plenum. In some embodiments, a cross-sectional area of a plenum influences the rate of airflow, including a volumetric flow rate, mass flow rate, etc., that can be induced through the plenum. For example, increasing a cross-sectional area of the plenum can increase the airflow that can be induced through the plenum, and reducing the cross-sectional area of the plenum can decrease the airflow that can be induced through the plenum. 
     In some embodiments, one or more components can be installed in an interior of the chassis  508  in one or more various positions to enable various cross-sectional areas of the inlet air plenum and the bypass air plenum. The components can be installed in various positions to implement complementary changes to each of the inlet air plenum and bypass air plenum cross-sectional areas, where an increase in the cross-sectional area of the bypass air plenum corresponds with a decrease in the cross-sectional area of the inlet air plenum, and vice versa. In some embodiments, the one or more components can include a circuit board supporting one or more upstream heat-producing components, where the circuit board partitions a portion of the chassis interior to establish respective boundaries of the inlet air plenum and bypass air plenum. One or more of the chassis and the circuit board can be configured to install the circuit board in various positions in the chassis interior, such that various relative cross-sectional areas of the inlet air plenum and bypass air plenum are established. 
     Installing the circuit board in different positions in the chassis interior can enable different relative air flows through the inlet air plenum and bypass air plenum, which can enable different rates of cooling of various heat-producing components installed in the computer system and, therefore, different operating temperatures of the various components. For example,  FIG.  5 A-C  shows that a circuit board  520  that supports a heat-producing component  522  in an inlet air plenum  502  of the computer system  500  can be installed, via coupling to inlet plate  512  of the chassis  508 , in various positions that correspond to various elevations above the base plate  510  of the chassis  508 . The position at which the circuit board  520  is coupled to the inlet plate  512  can establish the respective cross-sectional areas  592 ,  594  of the bypass air plenum  570  and the inlet air plenum  502 , which can establish the relative rates of flow of the respective bypass air flow  574  and inlet air flow  516 . 
       FIG.  5 A  illustrates the circuit board  520  being coupled to inlet plate  512  at a particular position  580  in the upstream portion of the chassis interior. Because the opposite surfaces of circuit board  520  establish respective boundaries of the inlet air plenum  502  and bypass air plenum  570 , circuit board  520  establishes at least a portion of the respective cross-sectional areas  594 ,  592  of the respective air plenums  502 ,  570 . 
     As shown in  FIG.  5 B , circuit board  520  can be coupled to inlet plate  512  in a position that is elevated relative to the particular position  580  at which the circuit board  520  is coupled to inlet plate  512  in  FIG.  5 A . As a result, because the circuit board  520  is installed in an elevated position, the space beneath the circuit board that comprises the bypass air plenum  570  in  FIG.  5 B  is increased in volume relative to  FIG.  5 A . In addition, the cross-sectional area  592  of the bypass air plenum is increased. As a result, the flow rate of the bypass airflow  574  through the bypass air plenum  570  in  FIG.  5 B  can be greater than the bypass airflow  574  through the bypass air plenum  570  in  FIG.  5 A . Furthermore, because the upper surface of circuit board  520  establishes a lower boundary of the inlet air plenum  502 , the elevated installed position of the circuit board  520  in  FIG.  5 B  results in an inlet air plenum  502  with a reduced volume relative to  FIG.  5 A . In addition, the cross-sectional area  594  of the inlet air plenum  502  is reduced. As a result, the flow rate of the inlet airflow  516  through the inlet air plenum  502  in  FIG.  5 B  can be less than the inlet airflow  516  through the inlet air plenum  502  in  FIG.  5 A . 
     As shown in  FIG.  5 C , circuit board  520  can be coupled to inlet plate  512  in an installed position that is lowered relative to the particular position  580  at which the circuit board  520  is coupled to inlet plate  512  in  FIG.  5 A . As a result, because the circuit board  520  is in a lowered position, the space beneath the circuit board that comprises the bypass air plenum  570  is decreased in volume relative to  FIG.  5 A . In addition, the cross-sectional area  592  of the bypass air plenum  570  is decreased. As a result, the flow rate of the bypass airflow  574  through the bypass air plenum  570  in  FIG.  5 B  can be less than the bypass airflow  574  through the bypass air plenum  570  in  FIG.  5 A . Furthermore, because the upper surface of circuit board  520  establishes a lower boundary of the inlet air plenum  502 , the lowered installed position of the circuit board  520  in  FIG.  5 B  relative to the position  580  in  FIG.  5 A  results in an inlet air plenum  502  with an increased volume. In addition, the cross-sectional area  594  of the inlet air plenum  502  is increased. As a result, the flow rate of the inlet airflow  516  through the inlet air plenum  502  in  FIG.  5 B  can be greater than the inlet airflow  516  through the inlet air plenum  502  in  FIG.  5 A . 
     In some embodiments, where the circuit board  520  couples to the inlet plate  512 , and where the inlet plate includes air vents  514 ,  572  that can supply air to one or more of the separate plenums, the particular position at which circuit board  520  is installed in the computer system  500  can determine whether an air vent in the inlet plate  512  is an inlet air vent  514  that directs air into the inlet air plenum  502  or a bypass air vent  572  that directs air into the bypass air plenum  570 . Coupling the circuit board  520  to the inlet plate  512  at a position that is above an air vent  514  may put the air vent in flow communication with the bypass air plenum  570  and partition the air vent from being in flow communication with the inlet air plenum  502 , thereby rendering the air vent as a bypass air vent  572 . Conversely, coupling the circuit board  520  to the inlet plate  512  at a position that is below an air vent  514  may put the air vent in flow communication with the inlet air plenum  502  and partition the air vent from being in flow communication with the bypass air plenum  570 , thereby rendering the air vent as an inlet air vent. In some embodiments, one or more components, including a circuit board  520 , can be coupled to the inlet plate  512  to partition a portion of a vent in the inlet plate  512  between directing air to one plenum  502  and directing air to another plenum  570 . In some embodiments, including the embodiment illustrated in  FIG.  5 B , the circuit board  520  can be coupled to the inlet plate  512  at a position that at least partially obstructs an air vent. 
     Because the circuit board  520  establishes boundaries of both the inlet air plenum  502  and the bypass air plenum  570 , installing the circuit board  520  in the computer system  500  in various different positions can facilitate complementary changes in the cross-sectional areas of both plenums. For example, as shown in  FIG.  5 B , installing the circuit board  520  in an elevated position both increases the cross-sectional area  592  of the bypass air plenum  570  and decreases the cross-sectional area  594  of the inlet air plenum  502 , thereby enabling bypass air flow  574  to be increased while inlet air flow  516  is decreased. Similarly, as shown in  FIG.  5 C , installing the circuit board  520  in a lowered position both decreases the cross-sectional area  592  of the bypass air plenum  570  and increases the cross-sectional area  594  of the inlet air plenum  502 , thereby enabling bypass air flow  574  to be reduced while inlet air flow  516  is increased. 
     Because the flow rate of inlet air flow  516  can affect the cooling of heat-producing components  522  installed in heat transfer communication with the inlet air plenum, and the flow rate of bypass air flow  574  can affect the cooling of heat-producing components installed in a downstream portion (not shown in  FIG.  5 A-C ) of the chassis interior, installing the circuit board  520  in various positions can result in adjusting the rate of cooling of heat-producing components in both upstream and downstream portions of the chassis interior. For example, installing the circuit board  520  in the elevated position shown in  FIG.  5 B  can result in increased cooling of heat-producing components installed in a downstream portion of the computer system, as more bypass air that does not remove heat from one or more upstream heat-producing components is directed to mix with the inlet air flow, resulting in a mixed air flow that has more heat removal capacity, while also resulting in reduced cooling of heat-producing components  522  installed in heat transfer communication with the inlet air plenum  502 . Converse results can be achieved through installing the circuit board  520  in a lowered position as shown in  FIG.  5 C . As a result, the circuit board  520  can be installed in various positions to implement complementary control of the cooling of upstream and downstream heat-producing components. 
     In some embodiments, the installing of a circuit board  520  at one of various positions, as shown in  FIG.  5 A-C , can be implemented during coupling of the circuit board  520  to the chassis  508 , for example during manufacturing, assembly, etc. of some or all of the computer system  500 . The particular position at which the circuit board  520  is installed may be determined based at least in part upon the heat sensitivities, cooling requirements, etc. associated with the various upstream and downstream heat-producing components that are to be installed in the computer system  500 . In some embodiments, the circuit board can be adjustably positioned dynamically, during performance of computing operations and cooling operations in the computer system  500 , to enable dynamic control of relative cooling of upstream and downstream heat-producing components. 
       FIG.  6    is a schematic diagram illustrating a side view of a computer system chassis interior that includes an adjustable flow control element, a temperature sensor device, and an air moving device that are communicatively coupled to a controller, according to some embodiments. In some embodiments, system  600  includes a computer system  601  that at least partially comprises at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     In some embodiments, a flow control element can be adjustably positioned to adjust a cross-sectional area of one of the inlet air plenum and bypass air plenum, which can adjust the relative flow rates of air through each of the plenums. The flow control element can, in some embodiments, be dynamically adjusted to provide dynamic control of the relative air flow rates through separate air plenum to enable dynamic control of relative cooling of upstream and downstream heat-producing components in a computer system. 
     In the illustrated embodiment, system  600  includes a computer system  601  that itself includes a chassis  608  with inlet plate  612  and base plate  610 , air moving device  607 , upstream components  620 ,  622  installed in an upstream portion of the computer system  600 , and downstream heat-producing components  634  installed in a downstream portion  604  of the computer system  601 . The upstream portion includes an inlet air plenum  602  and bypass air plenum  670 . Inlet air vents  614  in the inlet plate  612  direct an inlet air flow  616  through the inlet air plenum  602  to cool upstream heat-producing component  622  installed in heat transfer communication with the inlet air plenum  602 . Bypass air vents  672 ,  673 , one or more of which can be installed on a different portion of a chassis  608  from the inlet air vents  614 , as shown with reference to vent  673 , direct a bypass air flow  674  through bypass air plenum  670  to mix with the inlet air flow  616  downstream of the upstream heat-producing component  622  to establish a mixed air flow  636  that removes heat from downstream heat-producing components  634  in the downstream portion  604 . 
     As shown in the illustrated embodiment, a flow control element  680  can be installed in the bypass air plenum  670 . In some embodiments, a flow control element comprises a flap that can be adjusted to pivot into the interior space of an air plenum to adjust a cross-sectional area of the air plenum between the flow control element and a boundary of the air plenum. For example, as shown in the illustrated embodiment, flow control element  680  can be coupled to the base plate  610  that establishes a lower boundary of the bypass air plenum  670  and can be controlled to adjustably pivot into and out of the bypass air plenum  670  to adjust a cross-sectional area  682  of the plenum  670  between the flow control element  680  and the lower surface of component  620  that establishes an upper boundary of the bypass air plenum  670 . Component  620  can comprise a circuit board, including a motherboard, that supports an upstream heat-producing component  622 , which can include a CPU, in heat transfer communication with the inlet air plenum  602 . 
     In some embodiments, a flow control element installed in a particular air plenum that can adjust a cross-sectional area of that plenum can be controlled to adjust the relative air flows through multiple air plenums. For example, when air moving device  607  maintains a particular fixed air flow rate through the computer system  601  chassis interior, the total flow rate through both the inlet air plenum  602  and the bypass air plenum  670  may remain approximately fixed. Adjusting the cross-sectional area  682  of the bypass air plenum  670  via control of the flow control element  680  may adjust the flow rate of the bypass air flow  674 , which can result in a complementary change in the inlet air flow  616  rate through the inlet air plenum  602 . For example, where the bypass air flow  674  rate is reduced by 1 cubic foot per minute, based upon the flow control element  680  being controlled to reduce the cross-sectional area  682  of the bypass air plenum  670 , the inlet air flow  616  rate may increase by approximately 1 cubic foot per minute. Because the inlet air flow  616  removes heat from the upstream heat-producing component  622  and the bypass air flow  674  does not, reducing the cross-sectional area  682  may result in increased cooling of heat-producing component  622  due to an increased flow rate of inlet air flow  616 . In addition, because the bypass air flow  674  mixes with the inlet air flow  616  to provide a mixed air flow  636  that has greater heat removal capacity than the inlet air flow  616  alone, reducing the cross-sectional area  682  may result in reduced cooling of heat-producing components  634  due to reduced mixing of the heated inlet air flow  616  with bypass air  674 , so that the mixed air flow  636  has a reduced heat removal capacity. 
     In some embodiments, one or more flow control elements  680  can be installed in both plenums  602 ,  670 , where each flow control element  680  installed in each plenum can be separately controlled to separately adjust a cross-sectional area of a separate one of the plenums. 
     In some embodiments, one or more flow control elements can be controlled based on sensor data indicating one or more environmental condition measurements in one or more portions of a computer system. The environmental conditions can include one or more of temperature, pressure, flow rate, relative humidity, some combination thereof, or the like. The environmental conditions can be measured by one or more sensor devices installed in one or more portions of the chassis interior of the computer systems, and the sensor data indicating the measurements of the environmental conditions can be processed by a controller to adjust one or more flow control elements in the computer system. The controller may adjust one or more flow control elements to adjust relative flow rates through two or more plenums, may adjust one or more flow control elements to adjust total flow rates through the two or more plenums, some combination thereof, or the like. 
     In the illustrated embodiment, for example, system  600  includes a controller  690  that is communicatively coupled to flow control element  680  installed in the bypass air plenum  670 , an air moving device  607 , a temperature sensor device  692  installed in the inlet air plenum  602 , and a temperature sensor device  694  installed in the downstream portion  604  of the chassis interior of the computer system  601 . Controller  690  can be implemented by one or more computer systems, computing devices, etc., and can adjustably control one or more of the flow control element  680  and the air moving device  607  based on sensor data generated by sensor devices  692 ,  694 . For example, where sensor device  692  indicates that heat-producing component  622  is operating at a temperature that has significant margin below the maximum operating temperature for the heat-producing component  622 , while sensor device  694  indicates that heat-producing components  634  are operating approximately at the maximum operating temperature for those heat-producing components  634 , controller  690  may adjust flow control element  680  to increase cross-sectional area  682 , thereby increasing the bypass air flow  674  and providing a complementary decrease in the inlet air flow  616 . In addition, controller  690  may adjust air moving device  607  to reduce the total flow rate through the computer system, so that both the inlet air flow  616  and bypass airflow  674  are reduced. The resulting airflow through the computer system may cause heat-producing components  622  and  634  to each operate approximately at their respective maximum operating temperatures, thereby optimizing airflow distribution through computer system  601 . 
       FIG.  7    is a schematic diagram illustrating a perspective view of a computer system that includes partitions that direct separate inlet air flows and bypass air flows through separate sets of air moving devices, according to some embodiments. In some embodiments, computer system  700  is at least partially comprised in at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     Computer system  700  includes a chassis  701  in which various upstream heat-producing components  722  are installed in an inlet air plenum  702  in an upstream portion of the chassis interior, and various downstream heat-producing components  734  are installed in a downstream portion  704  of the chassis interior. Heat-producing components  722 , in some embodiments, include memory devices and processor unit assemblies, and heat-producing components  734  can include mass storage devices. Heat-producing components  722  are supported by a circuit board  720  which is installed to position the heat-producing components  722  in heat transfer communication with the inlet air plenum  702 , and a bypass air plenum  770  extends beneath the circuit board  720  from a bypass air vent  740 . 
     In some embodiments, a computer system includes an array of air moving devices, and one or more sets of air moving devices in the array can be configured to supply airflow exclusively from a particular separate air plenum into the downstream portion of the computer system. A set of air moving devices can be configured thusly based at least in part upon installing partition elements in the chassis interior of the computer system that partition an inlet of each air moving device of the set of air moving devices from plenums other than the particular separate air plenum. As a result, mixing of air flow from separate air plenums can occur downstream of the air moving devices, and separate air flows from separate air plenums can be supplied through a dedicated set of air moving devices. The set of air moving devices dedicated to airflow through a particular plenum may be adjusted, separately from another set of air moving devices dedicated to airflow through another separate plenum, to adjust the airflow through the particular plenum independently of the airflow through the other plenum. 
     In the illustrated embodiment, for example, computer system  700  includes an array  706  of air moving devices  707 ,  709  that includes two separate sets of air moving devices that each supply air from a separate air plenum into the downstream portion  704  of the chassis  701  interior. One set of air moving devices  709  are partitioned, on an inlet side of the air moving devices, from the bypass air plenum  770  by various partition elements  730 , so that the set of air moving devices  709  exclusively supply the inlet air flow  716  from the inlet air plenum  702  to the downstream portion  704 . Conversely, another set of air moving devices  707  are partitioned, on an inlet side of the air moving devices  707 , from the inlet air plenum  702  by various partition elements  730 , so that the set of air moving devices  707  exclusively supply the bypass air flow  774  from the bypass air plenum  770  to the downstream portion  704 . Each set of air moving devices  707 ,  709  can be separately controlled to provide independent control of the respective air flows  716 ,  774 . For example, one set of air moving devices  707  can be controlled, independently of air moving devices  709 , to increase the bypass air flow  774 , while the other set of air moving devices  709  maintain a fixed inlet air flow  716 . As a result, the mixed air flow  736  through the downstream portion is increased and includes an increased proportion of bypass air flow  774  relative to inlet air flow  716 , which can result in an increased cooling of heat-producing components  734  in the downstream portion while maintaining a particular amount of cooling of upstream heat-producing components  722  installed in heat transfer communication with the inlet air plenum  702 . 
     In some embodiments, one or more partition elements  730  can be reversibly installed in various portions of the computer system  701  to change which air moving devices are included in particular sets of air moving devices. For example, some partition elements  730  can be moved from the illustrated mounting configuration in the interior of computer system  700  to partition one or more of the air moving devices  709  to receive air exclusively from the bypass air plenum  770  and to partition one or more of the air moving devices  707  to receive air exclusively from the inlet air plenum  702 . 
     In some embodiments, a computer system includes a bypass air plenum that extends along a side of the computer system chassis that is opposite from the inlet air plenum. The plenum may be bounded on at least one side by a portion of the chassis and may be bounded on other sides by elements that are external to the computer system, including portions of other computer systems. The chassis can include discharge vents that enable a bypass air flow through the bypass air plenum to be discharged into the inlet air plenum, at a point that is downstream of one or more upstream heat-producing components in the inlet air plenum, to mix with the inlet air flow. 
       FIG.  8    is a schematic diagram illustrating a perspective view of a computer system that includes bypass vents installed in a base plate of the computer system chassis, according to some embodiments. In the illustrated embodiment, rack computing system  800  includes a computer system  801  where the bypass air plenum  870  extends beneath the base plate  810  of a chassis of the computer system  801 . The bypass air plenum  870 , which may be referred to hereinafter as an “external bypass air plenum,” is established based at least in part by the lower surface of the base plate  810 , a baffle element  853 , and an upper surface of an external element  842 . The external element  842 , in some embodiments, is another computer system that is separate from computer system  801 . For example, in an embodiment where rack computing system  800  includes computer systems installed in a rack, computer system  801  and external element  842  (which may be another computer system) may be installed in a vertical stack in the rack, where gap  843  is present between adjacent computer systems and baffle element  853  substantially partitions the gap space  843  to establish the bypass air plenum  870  of the computer system  801 . In some embodiments, system  800  includes a computer system  801  that at least partially comprises at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     In some embodiments, including the illustrated embodiment, discharge vents  876  installed in the base plate  810  direct the bypass air flow  874  out of the bypass air plenum  870  and into an upstream portion  802  of the computer system  801  chassis interior to mix with the inlet air flow  816  at a point in the computer system  801  chassis interior that is downstream of the one or more upstream components  820  installed in the computer system  801 . Upstream components  820  can include one or more circuit boards supporting one or more heat-producing components, including central processing units, which generate heat. In the illustrated embodiment, the discharge vents  876  direct the bypass air flow  874  into one or more air moving devices  807  in an array  806  of air moving devices, where the air moving devices  807  supply both the bypass air flow  874  and the inlet air flow  816  into the downstream portion  804  of the computer system  801 . As a result, the bypass air flow  874  and inlet air flow  816  are mixed at least partially by the air moving devices  807  to provide a mixed air flow  836  that cools downstream heat-producing components  834  that are installed in the downstream portion  804 . In some embodiments, a flow control element, as illustrated and discussed above with reference to  FIG.  6   , can be installed in one or more of the bypass air plenum  870  and the upstream portion  802 . 
     In some embodiments, a baffle element prevents air recirculation into the bypass air plenum. For example, where mixed air  836  exits the downstream portion  804  of the computer system into an external exhaust air space, and the gap space  843  extends from an inlet end of the rack computing system  800  to an exhaust end that is in communication with the external exhaust air space, baffle element  853  partitions that gap space  843  to prevent exhaust air from recirculating back through the gap space  843  from the exhaust space to heat the bypass air flow  874 . Baffle  853  can, in some embodiments, include an insulating material that mitigates heat transfer to the bypass air plenum  870 . 
       FIG.  9    is a schematic diagram illustrating a perspective view of a computer system that includes bypass vents installed in a side portion of the computer system chassis, according to some embodiments. In the illustrated embodiment, computer system  900  includes a bypass air plenum  970 , which may be referred to hereinafter as an “external bypass air plenum,” that extends alongside a side plate  909  of a chassis of the computer system  900 , where the chassis includes at least the side plate  909  and a base plate  910 . The bypass air plenum  970  is established based at least in part by the outer surface of the side plate  909 , a baffle element  953 , and an adjacent surface of an external element  942 . The external element  942 , in some embodiments, includes another computer system, rack member, etc. that is separate from computer system  900 . For example, in an embodiment where computer system  900  is installed in a rack computing system that includes computer systems installed in a rack, element  942  may be a rack member, where gap  943  is present between the computer system  900  installed in the rack and the rack member  942 . Baffle element  953  substantially partitions the gap space  943  to establish the bypass air plenum  970  of the computer system  900 . Baffle element  953  can prevent air recirculation into the bypass air plenum  970 . In some embodiments, computer system  900  is at least partially comprised in at least computer system  200 , illustrated and described above with reference to  FIG.  2 A-B . 
     Discharge vents  976  installed in the side plate  909  direct the bypass air flow  974  out of the bypass air plenum  970  to mix with the inlet air flow  916  at a point  902  in the computer system  900  that is downstream of the one or more upstream heat-producing components  920 . The discharge vents  976  direct the bypass air flow  976  into one or more air moving devices  907 . 
       FIG.  10    illustrates a method of providing a bypass airflow in a computer system interior, according to some embodiments. The method can be implemented using various embodiments of a computer system, including one or more of the embodiments illustrated and described above with reference to  FIGS.  1 - 9   . 
     At  1002 , a chassis of a computer system is established, where the chassis at least partially encompasses a chassis interior. The chassis interior can be established by coupling various members together to establish the chassis, where the various members encompass a portion of the chassis interior. An example of chassis members can include a base plate that encompasses one or more lower portions of the chassis interior, side plates that encompass one or more side portions of the chassis interior, inlet plates that encompass one or more side portions of the chassis interior on an end of the chassis that is configured to receive at least an inlet air flow into the chassis interior, etc. 
     The chassis interior can include an inlet end and an exhaust end. The inlet end of the chassis is configured to face an external source of cooling air, and the exhaust end is configured to face an external exhaust air space. The chassis interior can include some or all of an “upstream” portion that is proximate to the inlet end, a “downstream” portion that is proximate to the exhaust end, and a “midstream” portion that is located between the upstream and downstream portions. 
     At  1004 , one or more heat-producing components are coupled to the chassis to install the heat-producing components in the chassis interior. The various heat-producing components can be included in one or more various components, including one or more mass storage devices, circuit boards, central processing units, etc. For example, one or more sets of mass storage devices, including one or more hard disk drives, can be installed in the downstream portion of the chassis interior. The mass storage devices can be coupled to one or more backplanes that are themselves coupled to the chassis to install the mass storage device. In another example, one or more circuit boards, which can include one or more motherboards supporting central processing units (CPUs), memory cards, etc., can be coupled to the chassis in an upstream portion of the chassis interior to install the circuit boards in the upstream portion. 
     At  1006 , a bypass air plenum is established in at least a portion of the chassis interior. The bypass air plenum is established as a space that is at least partially bounded by one or more portions of the chassis and one or more components coupled to the chassis. In some embodiments, the bypass air plenum is at least partially established by one or more surfaces of a circuit board installed in the upstream portion, where the circuit board is coupled to the chassis to install the circuit board in a particular position that partitions the upstream portion of the chassis interior into separate air plenums. 
     One plenum can extend above the circuit board and in heat transfer communication with various heat-producing components, including a CPU, installed on an upper surface of the circuit board, while the other plenum extends beneath the circuit board. The one air plenum can include an inlet air plenum, and an inlet air flow through the inlet air plenum can remove heat generated by the various heat-producing components installed in the upstream portion of the chassis interior. The circuit board can be installed in the particular position to enable the bypass air plenum to extend beneath the circuit board to provide at least sufficient spacing between the circuit board and a base plate of the chassis to mitigate the risk of contact between the base plate and various elements of the circuit board, including capacitor elements, stiffening brackets under a CPU, connector tails projecting through the lower surface of the circuit board, etc. In some embodiments, an additional element, including Mylar sheeting, is installed between the circuit board and the base plate to mitigate the risk of electrical shorting. 
     A bypass air plenum can be established based at least in part upon establishing an air vent in one or more surfaces bounding a space that is unencompassed by the inlet air plenum, so that an air flow can pass through the space in isolation from the inlet air flow and can mix with the inlet air flow at a location in the chassis interior that is downstream from one or more of the upstream heat-producing components installed in heat transfer communication with the inlet air plenum. Air vents can be established in one or more various members comprising the chassis. For example, some air vents can be established in an inlet plate of the chassis, some air vents can be established in a bottom plate of the chassis, and some air vents can be established in a side plate of the chassis. 
     In some embodiments, one or more components, which can include one or more circuit boards, are coupled to the chassis in one or more various positions to establish one or more various cross-sectional areas of at least the bypass air plenum. Coupling the component in a particular position, which establishes a particular cross-sectional area of the bypass air plenum, can enable a particular flow rate of the bypass air flow through the bypass air plenum. In some embodiments, the one or more components, including one or more circuit boards, can be dynamically adjusted in the chassis interior between various installed positions to dynamically adjust the bypass air flow rate via adjustment of the bypass air plenum cross-sectional area. 
     At  1008 , one or more air moving devices are coupled to the chassis to install the air moving devices in the chassis interior. The air moving device can be coupled to the chassis downstream of both the inlet air plenum and the bypass air plenum in the upstream portion of the chassis interior, so that the inlet air flow and the bypass air flow can at least partially mix upstream of the air moving device. For example, the air moving device can be installed in a midstream portion of the chassis interior that is spaced downstream from a circuit board that isolates the bypass air plenum from the inlet air plenum, so that a gap between the circuit board and the air moving device enables the bypass air flow to be directed from the bypass air plenum to mix with the inlet air flow from the inlet air plenum. In some embodiments, the air moving device is coupled to the chassis in contact with the circuit board, and one or more discharge vents are established in the circuit board to enable the bypass air flow to pass out of the bypass air plenum, through the circuit board, to mix with the inlet air flow. 
     In some embodiments, a chassis is free from including air moving devices, and the inlet and bypass air flows through the chassis interior can be induced based at least in part upon a pressure gradient through the chassis interior that is induced by an external source, including an externally-mounted air moving device. 
     At  1010 , one or more flow control elements are coupled to the chassis in heat transfer communication with one or more of the inlet air plenum and the bypass air plenum to enable adjustable control of the relative air flow rates through the plenums. The flow control elements can be coupled to one or more surfaces establishing one or more of the boundaries of a plenum to install the flow control elements in the plenum. In some embodiments, the flow control elements can be controlled to adjust a cross-sectional area of the plenum to control air flow rates through at least that particular plenum. In a computer system with a constant total flow rate maintained through the chassis interior, adjusting flow rate through one of the bypass air plenum and the inlet air plenum can induce a complementary change in flow rate in the other air plenum. 
     In some embodiments, partition elements are installed in the chassis interior to configure one or more air moving devices to supply air received exclusively from one or more particular plenums into a downstream portion of the chassis interior and to configure one or more other air moving devices to supply air received from another particular plenum into the downstream portion. For example, one air moving device can be partitioned to receive air exclusively from the bypass air plenum, and another air moving device can be partitioned to receive air exclusively from the inlet air plenum. 
     At  1012 , the air moving device and flow control element are operated to optimize utilization of the airflow through the chassis interior to cool the various components installed in various portions of the chassis interior. The control of one or more of the flow control elements and air moving devices coupled to the chassis can be implemented by a controller, which can be implemented at least partially by one or more computer systems. The controller can adjustably control one or more of the flow control elements and air moving devices to control cooling of various heat-producing components coupled to the chassis. For example, a flow control element coupled to the chassis in the bypass air plenum can be controlled to change the relative flow rates of the inlet air flow and the bypass air flow, and an air moving device coupled to the chassis can be controlled to change the total induced flow rates of the inlet air flow and the bypass air flow. 
     The controller, in some embodiments, adjustably controls the air moving devices and flow control elements based at least in part upon sensor data generated by one or more sensor devices installed in the chassis interior. One sensor device may provide temperature measurements proximate to heat-producing components in the inlet air plenum, and another sensor may provide temperature measurements proximate to heat-producing components in the downstream portion of the chassis interior. The controller may control the flow control elements and air moving devices to adjust the rate of cooling of the upstream heat-producing components and downstream heat-producing components, so that at least some of the upstream heat-producing components and the downstream heat-producing components are operating at respective temperatures that are within a certain margin of their respective maximum operating temperatures. Where the number of components in the chassis interior that are operating within the margins of their respective maximum operating temperatures, the utilization of cooling air from an external source can be maximized, such that the minimal air flow rate required to provide sufficient cooling to the components to avoid thermal damage is provided. 
     The various methods as illustrated in the Figures and described herein represent example embodiments of methods. The methods may be implemented in software, hardware, or a combination thereof. The order of method may be changed, and various elements may be added, reordered, combined, omitted, modified, etc. 
     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.