Patent Publication Number: US-8113776-B2

Title: Reduced-impedance cooling system with variable pitch blade and hot-swappable spare

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cooling computers and other electronic systems. 
     2. Background of the Related Art 
     One aspect of thermally managing computer systems involves properly cooling computer equipment to ensure the reliability and proper performance of that equipment. Properly cooling computer equipment typically requires generating airflow through computer equipment to remove heat generated by the computer equipment and maintain the temperature of various components within a suitable operating range. Innovations in computer technology have led to an increase in the power consumption and power density of computer equipment, along with increase in the airflow rates that are required to cool the computer equipment. As a result, the cost of operating computer equipment is also increasing. For example, it is now common for the costs associated with powering a server during the server&#39;s lifetime to exceed the original purchase price of the server. Total cost of ownership of computer equipment is an important consideration in the design of computer systems, and minimizing the total cost of ownership is therefore desirable. 
     Various fan configurations are available for cooling computer systems. Current systems and methods include the use of redundant, serially-arranged cooling fans. For example, US Patent Application 20070081888 to Harrison discloses the simultaneous use of two serially-arranged fans with variable-pitch blades that collectively induce airflow through the computer at rates sufficient to cool the computer. In the event of a failure of the primary fan, the primary fan blades move from an oblique position to a coaxial position, which aligns the blades with the airflow to present lower impedance as seen by the secondary fan. Harrison further discloses a control system to sense failure of the primary fan and increase the speed of the remaining secondary fan accordingly, in order to ensure that a minimum airflow requirement is met until the defective primary fan can be replaced. 
     Each individual fan in a typical set of serially-arranged fans will contribute to the overall airflow. Typically, each individual fan is unable to supply the cooling needs of the system as a whole, particularly at higher loads. Thus, multiple fans are operated simultaneously to achieve the airflow necessary to cool the system. 
     The problem with operating multiple fans is that each fan presents airflow impedance to the other serially-arranged fans. This impedance represents a loss component that substantially undercuts the efficiency of the cooling system. Even though Harrison teaches the use of variable-pitch fan blades having the capacity to be moved to a lower-impedance position, the pitch of a fan is only moved to the lower-impedance position in the event of a failure of that fan. During normal operation, however, each of the multiple fans is operated in the higher-impedance position in order to provide sufficient airflow to cool the system. Thus, no appreciable efficiency benefit directly results from the presence of variable-pitch fans in the configuration taught by Harrison. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the present invention provides a method of operating variable pitch fans in series to cool an operating computer system. The method comprises running a first variable pitch fan with blades positioned at an operational pitch to induce airflow through the computer system and through a second variable pitch fan disposed in series with the first fan, and, simultaneously, not running the second variable pitch fan with blades positioned for minimal impedance to the airflow. In response to detecting a failure condition of the first fan, the method includes running the second fan with blades positioned at an operational pitch to induce airflow through the computer system and through the first fan, and, simultaneously, not running the first variable pitch fan with blades positioned for minimal impedance to the airflow. Preferably, the method includes locking the rotor of a fan that is not running, and locking the blades of the non-running fan in a minimal impedance position. 
     Another embodiment of the present invention provides a method of operating a computer system disposed in a chassis having a plurality of fan pairs, each fan pair including first and second variable pitch fans disposed in series. The first variable pitch fan of each fan pair is run with blades positioned at an operational pitch to induce airflow through the computer system and through the second variable pitch fan of the fan pair, and, simultaneously, the second variable pitch fan of each fan pair is turned off with blades positioned for minimal impedance to the airflow. The first fan of each fan pair is monitored for a failure condition. In response to detecting a failure condition in the first fan of at least one fan pair, the second fan of the at least one fan pair is run with blades positioned at an operational pitch to induce airflow through the computer system and through the first fan of the at least one fan pair, and, simultaneously, the first fan of the at least one fan pair is turned off with blades positioned for minimal impedance to the airflow. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic side view of two fans with adjustable-pitch blades arranged in series. 
         FIG. 2  is a front view of a first fan having blades positioned at an operational pitch. 
         FIG. 3  is a front view of a second fan having blades positioned for minimal airflow impedance. 
         FIG. 4  is a flow chart detailing a method for operating two fans with adjustable-pitch blades arranged in series. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention provides a method of operating variable pitch fans in series to cool an operating computer system. The method comprises running a first variable pitch fan with blades positioned at an operational pitch to induce airflow through the computer system and through a second variable pitch fan disposed in series with the first fan, and, simultaneously, not running the second variable pitch fan with blades positioned for minimal impedance to the airflow. In response to detecting a failure condition of the first fan, the method includes running the second fan with blades positioned at an operational pitch to induce airflow through the computer system and through the first fan, and, simultaneously, not running the first variable pitch fan with blades positioned for minimal impedance to the airflow. Accordingly, there is normally only one fan running and the other series fan is turned off and positioned for minimal impedance to airflow. Running the serially-arranged, variable pitch fans in this manner minimizes any loss of efficiency in the fan system, while also providing redundant fans that can continue to cool a computer system in the event of a fan failure. 
     Preferably, the method includes locking the rotor of a fan that is not running, locking the blades of the non-running fan in the minimal impedance position, or both. Each of these optional steps further reduces or minimizes the airflow impedance of the non-running fan. 
     In another embodiment, each of the fans can individually generate airflow, through the computer system and through the second fan with blades positioned for minimal impedance, that is sufficient to cool the computer system across a full range of anticipated workloads. Accordingly, the fans are fully redundant and the failure of one of the fans does not require any throttling or shut down of the computer system. Furthermore, since it is only necessary and desirable to run one fan at a time, the power consumption of the series fans is substantially the same as the power consumption of a single fan. 
     In yet another embodiment, the performance of the running fan is continuously monitored to detect failure conditions. Such performance monitoring may include predictive failure analysis. A failure condition may be characterized in many ways, for example when the actual fan speed that falls below a desired fan speed by more than a setpoint amount. Although some fans are designed to run at a constant speed and other are designed to run at various speeds, such as various different incremental speeds, the present example of a failure condition only occurs when the actual speed deviates from the intended speed that the fan. Alternatively, a failure condition could be detected by a change in the voltage or current to the fan motor. 
     In a further embodiment, the method may include occasionally running both of the fans with blades positioned at an operational pitch to identify whether either of the first and second fans is experiencing a failure condition. The benefit of this step is to test whether the non-running or “backup” fan is in good working order so that it can reliably take over if needed. 
     In an alternative embodiment, the method may include occasionally alternating which of the series fans is run with blades positioned at an operational pitch to induce airflow and which of the series fans is not run with blades positioned for minimal impedance to the airflow. In this manner, it is possible to test whether the non-running or “backup” fan is in good working order without operating both fans simultaneously. Furthermore, this step may provide more even wear on the two fans that disposed in series, and lengthen the time to failure. 
     In a still further embodiment, each of the variable pitch fans may allow incremental variation of the blade pitch, such that the operational pitch of blades on the running fan may be varied to optimize airflow efficiency at multiple fan speeds. Accordingly, this embodiment requires a variable pitch fans that supports multiple operational pitches, rather than simply an operational pitch and a minimum impedance pitch. 
     In yet another embodiment, the method includes, in response to detecting a failure condition, sending an alert signal to indicate that the first fan has failed. For example, an alert signal may be audible, visible or electronic, such as a beep, flashing light or alarm display, or a electronic communication to a workstation designated for a system administrator, respectively. Still further, an alert signal may be communicated to other hardware or software components for taking further action. Preferably, if the first fan has a failure condition, the method will further include replacing the first fan with a third variable pitch fan while the second fan is running. So long as the second fan is running, the third variable pitch fan remains turned off with blades positioned for minimal impedance to the airflow. Alternatively, the third fan may be temporarily tested or allowed to run while the second fan shut down with blades set to the minimal impedance position. The second and third fans are operated in the same manner as the original first and second fans. Most preferably, the first fan is replaced with the third fan without shutting down the computer system. 
     Another embodiment of the present invention provides a method of operating a computer system disposed in a chassis having a plurality of fan pairs, each fan pair including first and second variable pitch fans disposed in series. The first variable pitch fan of each fan pair is run with blades positioned at an operational pitch to induce airflow through the computer system and through the second variable pitch fan of the fan pair, and, simultaneously, the second variable pitch fan of each fan pair is turned off with blades positioned for minimal impedance to the airflow. The first fan of each fan pair is monitored for a failure condition. In response to detecting a failure condition in the first fan of at least one fan pair, the second fan of the at least one fan pair is run with blades positioned at an operational pitch to induce airflow through the computer system and through the first fan of the at least one fan pair, and, simultaneously, the first fan of the at least one fan pair is turned off with blades positioned for minimal impedance to the airflow. It should be recognized that each fan pair may be operated according to any one or more of the foregoing embodiments as described for a first and second fan disposed in series. 
       FIG. 1  is a schematic side view of an assembly  10  comprising two fans  20 ,  30  with variable-pitch blades arranged in series. A first variable pitch fan  20  has a housing  21  and a low airflow impedance support structure  22  for centrally supporting a fan motor  23 . The fan motor  23  is electrically connected to a source of electricity (not shown) and may utilize electrical current to turn a rotor and shaft  24 . A hub  25  couples a plurality of fan blades  26  to the end of the shaft  24  so that the blades are rotated about an axial center of the motor  23 . In order to vary the pitch of the blades  26 , each blade  26  has an individual blade shaft  27  that is received in the hub  25 . Various electromechanical designs are available for causing rotation of each blade shaft  27  so that the pitch of each blade  26  is varied. For example, the shaft  24  may include coaxial members, wherein the extension of one member relative to the other member causes rotation of each blade shaft  27 . By maintaining this relative extension, the blade pitch can be maintained and/or changed even while the motor  23  is run to cause the rotor and shaft  24  to rotate. Other electromechanical designs for a variable pitch fans will be recognized by those having ordinary skill in the art. 
     A second variable pitch fan  30  is disposed in series with the first fan  20  such that the air flow induced by one fan flows through the other fan. As shown in  FIG. 1 , the first fan  20  is running with blades in an operational pitch and the second fan  30  is not running and has its blades set parallel to the axis of the fan. The second fan  30  has a housing  31  and a low airflow impedance support structure  32  for centrally supporting a fan motor  33 . The fan motor  33  is electrically connected to a source of electricity (not shown) and may utilize electrical current to turn a rotor and shaft  34 . A hub  35  couples a plurality of fan blades  36  to the end of the shaft  34  so that the blades are rotated about an axial center of the motor  33 . In order to vary the pitch of the blades  36 , each blade  36  has an individual blade shaft  37  that is received in the hub  35 . As discussed above, various electromechanical designs are available for causing rotation of each blade shaft  37  so that the pitch of each blade  36  is varied. The present invention is not directed to the design of variable pitch fans, and the invention is not limited to any particular design of a variable pitch fan. 
     The two series-arranged fans  20 ,  30  of assembly  10  are typically positioned in a chassis that includes a computer system. Specifically, the computer system may be disposed on either the intake or outlet end of the assembly  10 , but it most commonly disposed on the intake end such that air is drawn through the computer system. 
       FIG. 2  is a front view of the first fan  20  having blades  26  positioned at an operational pitch. As shown, counter-clockwise rotation or the blades  26  draws air into the fan  20 . 
       FIG. 3  is a front view of the second fan  30  having blades  36  positioned for minimal airflow impedance. As shown from the front, the blades  36  leave the air flow passage substantially open with substantially only the thickness of the blades  36  providing any impedance to air flow. Optionally, rotor may be locked in a position where the blades  36  are aligned with the support structure  32  to further minimize the overall airflow impedance of the fan. It should be recognized that, in accordance with various embodiments of the present invention, the second fan  30  may be run with blades at an operational pitch (as shown in  FIG. 2 ) and the first fan  20  may be turned off with blades in a minimal impedance pitch (as shown in  FIG. 3 ). 
       FIG. 4  is a flow chart detailing a method  40  for operating two fans with adjustable-pitch blades arranged in series. This is just one exemplary method of the present invention. Those having ordinary skill in the art will, after gaining an appreciation of the invention as described herein, recognize other variations of the method. 
     In step  42 , both of the serially-arranged fans are started (run) at a preferred rotational speed and with the blades at a preferred operational pitch. The rotational speed may selected on the basis of the cooling requirements of the computer system at its present workload or it may be selected for the purpose of analyzing the fan performance. In step  44 , it is determined whether both fans are operating within one or more specified parameters. For example, the specified parameter may include a minimum achievable fan speed or current/voltage limitations. 
     If both fans are determined to be operating properly in step  44 , then in step  46  one of the two fans is shut down (turned off) and its blades are set in a minimum impedance position (See  FIG. 3 ). Also in step  46 , the other of the two fans is run with its blades at an operational pitch (See  FIG. 2 ). In step  48 , the performance of the operating (running) fan is monitored. If, in step  50 , fan is operating within specified parameters (i.e., no failure condition), then the operating fan continues to be monitored. However, it the fan is operating outside of the specified parameters, then the method continues to step  52 . In step  52 , the poorly operating fan is shut down (turned off) with its blades set in the minimum impedance position (See  FIG. 3 ) and the other fan is started (run) with blades set at an operational pitch (See  FIG. 2 ). Next, a fan controller or other components of the computer system notifies a user, in step  56 , that the particular fan has failed and should be replaced. Once the system detects that the fan has in fact been replaced, in step  58 , then the method returns to step  42 . 
     Returning to step  44 , if it is determined that both fans are not operating within specified parameters, then, in step  54 , the bad fan is shut down (turned off) with its blades set in the minimum impedance position (See  FIG. 3 ) and the other fan is run with blades set at an operational pitch (See  FIG. 2 ). After step  54 , the user is also notified about the bad fan and the need to replace the fan. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention. 
     The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but it not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.