RATIO OF OPEN AREA TO CLOSED AREA IN PANELS FOR ELECTRONIC EQUIPMENT ENCLOSURES

[0001] The present application is a U.S. nonprovisional patent application of, and claims priority under 35 U.S.C. §119(e) to, U.S. provisional patent application Ser. No. 60/725,511, filed Oct. 10, 2005, which provisional patent application is incorporated by reference herein.

[0002] All of the material in this patent document is subject to copyright protection under the copyright laws of the United States and of other countries. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.

[0003] 1. Field of the Present Invention

[0004] The present invention generally relates to methods and apparatus for thermal management in electronic equipment enclosures, and, in particular, to the effects of perforations in various walls and other panels of enclosures on temperature, air flow and other thermal phenomena.

[0005] 2. Background

[0006] Racks, frames and enclosures for mounting and storing computer and other electronic components or equipment (hereinafter referred to as “electronic equipment”) have been well known for many years. Racks and frames are typically simple rectangular frameworks on which electronic equipment may be mounted, or on which other mounting members, such as shelves or brackets, may be mounted which in turn may support the electronic equipment. Enclosures are typically frames on which panels or doors, or both, are hung to provide aesthetic improvement, to protect the equipment from external influences, to provide security for the equipment stored inside, or for other reasons.

[0007] Racks, frames and enclosures have been built in many different sizes and with many different proportions in order to best accommodate the equipment which they are designed to store. Electronic equipment stored in these enclosures may include audio and video equipment and the like, but quite frequently include computer equipment and related peripheral devices. These components typically include housings enclosing internal operative elements.

[0008] The electronic equipment housed in the enclosures generate large amounts of thermal energy. The enclosures frequently make use of fans and perforated panels, among other things, to help control the thermal energy and maximize the flow of cooling air through the enclosure and exhausting heated air therefrom. Such panels are perhaps most frequently used on doors, but may also be used in top, bottom, side, front and back walls and in interior surfaces as well, such as shelves or partitions.

[0009] The supply of cool air to the enclosure, and the transfer of thermal energy from the electronic equipment, is conventionally handled by a Computer Room Air Conditioner (“CRAC”). Airflow into the enclosure generally relies solely or at least primarily on the air pressure differential as measured between the entry point of air into the room and the ambient room. However, active means are often used to push or pull heated air out of the enclosures.

[0010] For a particular component, thermal energy is transferred from its housing using forced air convection. More specifically, internal fans draw or push air through the housing from front-to-rear over the heated internal elements within the housing. The air absorbs the thermal energy from the internal elements and carries it away as it exits the housing.

[0011] Airflow through a particular component housing is primarily controlled by the internal fan installed by the manufacturer. While it is possible to reduce this throughput by constricting air flow through an enclosure, it is difficult to appreciably increase the airflow through a component housing.

[0012] One way in which enclosure manufacturers have tried to increase airflow to an enclosure is to increase the number of perforations in an enclosure panel. For a panel, the ratio of the openings created by perforations therein to the total area of the panel can be expressed as the “percent open area” (or “percent open”) of the panel. Equipment manufacturers have sometimes specified a minimum percent open for third party enclosures being used to house their equipment. Typically, equipment manufacturers have specified a minimum percent open of 63%. Enclosures are presently available that provide panels with percent open values ranging up to 80% open. If no panel whatsoever is used, the resulting opening may be described as 100% open.

[0013] Despite the presence of perforated panels and fans, the ability to run an efficient data center can be a challenge especially when dealing with legacy installations while trying to plan for future applications. The principles of data center design for effective thermal management with high density data communications equipment heat loads are frequently violated. More often than not these violations come via an acquisition of previously developed space or habitation of mature space designed for equipment with lower heat loads.

[0014] There are a number of standard practices and a few creative patches that are used in an attempt to minimize or neutralize the resultant hot spots caused by these violations. These patches include adding high static pressure blowers to the bottom spaces of equipment enclosures; plugging all sources of bypass air; creating barriers to hot air recirculation, such as internal enclosure air dams, enclosure top return air isolation panels, and closed duct return air paths; and adding floor fans to deliver more cold air to the fronts of enclosures.

[0015] Another approach that has commonly been used is the removal of one or more panels, such as a front door or another easily-removed panel, from the enclosure itself, thus resulting in a “100% open” opening. This has been traditionally thought to maximize the amount of air-flow through the opening, even when the panel thus removed already had at least some perforations therethrough, and in particular, even when the percent open area created by the perforations was already as great as the minimum specified by equipment manufacturers. It has further traditionally been thought that maximizing airflow through the enclosure maximized removal of thermal energy from the enclosure. Unfortunately, the removal of such a panel is not an ideal solution, since it results in a security risk, is inconvenient for the user, and causes other problems.

[0016] In addition, it has traditionally been thought that providing a maximum percent open in a perforated panel maximizes the amount of airflow through and thus thermal energy removed from the enclosure. However, as percent open increases, the thickness of the panel is typically reduced. As the thickness of the perforated panel decreases, so to does the security and stability of the enclosure. Therefore, increasing the percent open of a perforated panel is also not always an ideal solution. This issue becomes particularly critical as the percent open is increased beyond about 63%.

[0017] Thus, a need exists for a perforation solution that maximizes the relative benefit achieved by airflow through the enclosure while still maintaining the various advantages provided by relatively thick panels rather than thin panels or uncovered openings. More particularly, the effectiveness of removing panels from an equipment enclosure or using a relatively thin perforated panel with a percent open greater than 63% relative to the benefits provided by relatively thicker perforated panels with a percent open of 63% or less has not previously been understood in the enclosure industry. As a result, the design process for panels for electronic equipment enclosures has been inexact, and designers have been forced to vary multiple design parameters for a particular panel during their design process in an effort to develop a design that satisfies the various criteria for the panel.

[0018] The present invention includes many aspects and features. Moreover, while many aspects and features relate to, and are described in, the context of methods and apparatus for thermal management in electronic equipment enclosures, the present invention is not limited to use only in thermal management in electronic equipment enclosures, as will become apparent from the following summaries and detailed descriptions of aspects, features, and one or more embodiments of the present invention.

[0019] Accordingly, one aspect of the present invention relates to a method of providing an electronic equipment enclosure having a plurality of panels, the method including: identifying at least one panel of the plurality of panels through which air is intended to flow; determining the design parameters of the at least one panel, including: for the identified panel, first establishing a maximum ratio of open area to closed area of between 60% and 70%, and determining one or more other parameters of the design of the identified panel in view of the established maximum ratio of open area to closed area; and providing an electronic equipment enclosure that includes the identified panel, constructed according to the determined design parameters.

[0020] In a feature of this aspect, the step of establishing a maximum ratio of open area to closed area includes establishing a maximum ratio of open area to closed area of about 63%. In another feature of this aspect, determining the design parameters includes maintaining the first-established maximum ratio while the one or more other parameters are determined, and wherein providing the enclosure includes providing an electronic equipment enclosure that includes the identified panel, constructed according to the determined design parameters, including the first-established maximum ratio.

[0021] In additional features, determining one or more other parameters of the design of the identified panel includes determining, in view of the established maximum ratio of open area to closed area, a material of which the identified panel is to be constructed, the thickness of the material, and the rigidity of the identified panel. In a further feature of this aspect, determining one or more other parameters of the identified panel includes determining at least two other parameters, and iteratively varying the at least two other parameters while maintaining the established maximum ratio of open area to closed area of between 60% and 70%.

[0022] Another aspect of the present invention relates to a method of providing an electronic equipment enclosure having a front panel and a rear panel, the method including: identifying one panel, of the front panel and the rear panel, through which air is intended to flow, wherein the identified panel defines a first panel, and wherein the other panel, of the front panel and the rear panel, defines a second panel; determining the design parameters of the first panel, including establishing a maximum ratio of open area to closed area of between 60% and 70%; determining the design parameter of the second panel, including establishing a ratio of open area to closed area of about 0%; and providing an electronic equipment enclosure, including: providing the first panel constructed according to its determined design parameters, providing the second panel constructed according to its determined design parameters, and providing an exhaust duct extending upward from a top of the enclosure.

[0023] In a feature of this aspect, the step of establishing a maximum ratio of open area to closed area includes establishing a maximum ratio of open area to closed area of about 63%, and while in another feature of this aspect, determining the design parameters of the first panel includes determining one or more other parameters of the design of the first panel in view of the established maximum ratio of open area to closed area for the first panel.

[0024] In other features of this aspect, the step of providing an exhaust duct includes providing an exhaust duct that extends from the top of the enclosure to a ceiling structure of a room in which the electronic equipment enclosure is disposed; the step of providing an exhaust duct includes connecting the exhaust duct to the ceiling structure such that the exhaust duct is in direct fluid communication with a ceiling-mounted return air duct of the room in which the electronic equipment enclosure is disposed; and the step of providing an exhaust duct includes providing an exhaust duct having a rectangular cross-section.

[0025] In still other features of this aspect, determining the design parameters of the first panel includes maintaining the established maximum ratio while the one or more other parameters are determined, and wherein providing the enclosure includes providing the first panel constructed according to its determined design parameters, including the established maximum ratio; and determining one or more other parameters of the design of the first panel includes: determining at least two other parameters of the design of the first panel, and iteratively varying the at least two other parameters while maintaining the established maximum ratio of open area to closed area of about 63%.

[0026] Yet another aspect of the present invention relates to a method of providing an electronic equipment enclosure having a plurality of panels, the method including: identifying a first panel and a second panel, of the plurality of panels, through which air is intended to flow; determining the design parameters of the first panel, including establishing a maximum ratio of open area to closed area of between 60% and 70%; determining the design parameters of the second panel, including establishing a maximum ratio of open area to closed area of between 60% and 70%; and providing an electronic equipment enclosure, including: providing the first and second panels constructed according to their determined design parameters, and providing an above cabinet barrier extending upward from a top of the enclosure.

[0027] In a feature of this aspect, the step of determining the design parameters of the first panel includes establishing a maximum ratio of open area to closed area of about 63%.

[0028] In additional features of this aspect, determining the design parameters of the first panel includes determining one or more other parameters of the design of the first panel in view of the established maximum ratio of open area to closed area for the first panel; determining the design parameters of the first panel includes maintaining the established maximum ratio while the one or more other parameters are determined, and wherein providing the enclosure includes providing the first panel constructed according to its determined design parameters, including the established maximum ratio; and determining one or more other parameters of the design of the first panel includes: determining at least two other parameters of the design of the first panel, and iteratively varying the at least two other parameters while maintaining the established maximum ratio of open area to closed area of about 63%

[0029] In still other features of this aspect, determining the design parameters of the second panel includes establishing a maximum ratio of open area to closed area of about 63%; the electronic equipment enclosure has a front and a rear, and wherein the first and second panels are the front and rear panels of the electronic equipment enclosure; and the above cabinet barrier extends substantially entirely from one side of the electronic equipment enclosure to the opposite side.

[0030] Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

[0031] Further features, embodiments, and advantages of the present invention will become apparent from the following detailed description with reference to the drawings, wherein:

[0032] FIG. 1 is an orthogonal view of a representative enclosure including a perforated front panel door in accordance with a preferred embodiment of the present invention;

[0033] FIG. 2 is a graphical illustration depicting prior art knowledge of pressure loss as a function of uniform impact velocity;

[0034] FIG. 3 is a tabular illustration depicting measured data for a test enclosure similar to the enclosure of FIG. 1 but with the door removed;

[0035] FIG. 4 is a tabular illustration depicting measured data for the test enclosure wherein a front panel door with 63% open area was attached thereto;

[0036] FIG. 5 is a table showing the flow reduction for the 63% open door panel of FIG. 4 calculated as a percentage of the flow reduction for the 100% open configuration of FIG. 3;

[0037] FIG. 6 is a graphical illustration depicting CPU temperature as a function of blade slot;

[0038] FIG. 7 is a graphical illustration depicting fan speed as a function of blade slot;

[0039] FIG. 8 is a tabular illustration depicting raw flow rate data and calculated data indicating simulated flowrate and percent flowrate reduction;

[0040] FIG. 9 is a graphical illustration depicting percent flowrate reduction for panels having a percent open area ranging from 0-100%;

[0041] FIG. 10 is a graphical illustration depicting percent flowrate reduction for panels having a percent open area ranging from 40-100%; and

[0042] FIG. 11 is a tabular illustration depicting calculated theoretical flowrate and cooling information based on measured flowrate data from a controlled flowrate test.

[0043] As a preliminary matter, it will readily be understood by one having ordinary skill in the relevant art (“Ordinary Artisan”) that the present invention has broad utility and application. Furthermore, any embodiment discussed and identified as being “preferred” is considered to be part of a best mode contemplated for carrying out the present invention. Other embodiments also may be discussed for additional illustrative purposes in providing a full and enabling disclosure of the present invention. Moreover, many embodiments, such as adaptations, variations, modifications, and equivalent arrangements, will be implicitly disclosed by the embodiments described herein and fall within the scope of the present invention.

[0044] Accordingly, while the present invention is described herein in detail in relation to one or more embodiments, it is to be understood that this disclosure is illustrative and exemplary of the present invention, and is made merely for the purposes of providing a full and enabling disclosure of the present invention. The detailed disclosure herein of one or more embodiments is not intended, nor is to be construed, to limit the scope of patent protection afforded the present invention, which scope is to be defined by the claims and the equivalents thereof. It is not intended that the scope of patent protection afforded the present invention be defined by reading into any claim a limitation found herein that does not explicitly appear in the claim itself.

[0045] Thus, for example, any sequence(s) and/or temporal order of steps of various processes or methods that are described herein are illustrative and not restrictive. Accordingly, it should be understood that, although steps of various processes or methods may be shown and described as being in a sequence or temporal order, the steps of any such processes or methods are not limited to being carried out in any particular sequence or order, absent an indication otherwise. Indeed, the steps in such processes or methods generally may be carried out in various different sequences and orders while still falling within the scope of the present invention. Accordingly, it is intended that the scope of patent protection afforded the present invention is to be defined by the appended claims rather than the description set forth herein.

[0046] Additionally, it is important to note that each term used herein refers to that which the Ordinary Artisan would understand such term to mean based on the contextual use of such term herein. To the extent that the meaning of a term used herein—as understood by the Ordinary Artisan based on the contextual use of such term—differs in any way from any particular dictionary definition of such term, it is intended that the meaning of the term as understood by the Ordinary Artisan should prevail.

[0047] Furthermore, it is important to note that, as used herein, “a” and “an” each generally denotes “at least one,” but does not exclude a plurality unless the contextual use dictates otherwise. Thus, reference to “a picnic basket having an apple” describes “a picnic basket having at least one apple” as well as “a picnic basket having apples.” In contrast, reference to “a picnic basket having a single apple” describes “a picnic basket having only one apple.”

[0048] When used herein to join a list of items, “or” denotes “at least one of the items,” but does not exclude a plurality of items of the list. Thus, reference to “a picnic basket having cheese or crackers” describes “a picnic basket having cheese without crackers”, “a picnic basket having crackers without cheese”, and “a picnic basket having both cheese and crackers.” Finally, when used herein to join a list of items, “and” denotes “all of the items of the list.” Thus, reference to “a picnic basket having cheese and crackers” describes “a picnic basket having cheese, wherein the picnic basket further has crackers,” as well as describes “a picnic basket having crackers, wherein the picnic basket further has cheese.”

[0049] Referring now to the drawings, in which like numerals represent like components throughout the several views, the preferred embodiments of the present invention are next described. The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0050] FIG. 1 is an orthogonal view of a representative enclosure 10 including a perforated front panel door 12 in accordance with a preferred embodiment of the present invention. The equipment enclosure 10 includes a four post frame structure having panels, more fully described hereinbelow, enclosing the enclosure, attached thereto. The four post frame structure may be of conventional design and construction. As shown and described, the four post frame structure includes four vertical support posts 14, upper and lower front cross members 16, upper and lower rear cross members 18 and two pairs of upper and lower side cross members 20. Each vertical support post 14 includes a plurality of cross member attachment apertures at each end. Two of the vertical support posts 14 are connected together at their upper and lower ends by the upper and lower front cross members 16, respectively, and the other two support posts 14 are connected together at their upper and lower ends by the upper and lower rear cross members 18, respectively. The front cross members 16 and their respective support posts 14 thus define a front frame, and the rear cross members 18 and their respective support posts 14 define a rear frame. The front and rear frames may then be connected together at their respective corners by the upper and lower side cross members 20.

[0051] Any known connection means may be used to join the various members together. Although not illustrated herein, at least one other example of conventional connection means is described in commonly-assigned U.S. Pat. No. 6,185,098, the entirety of which is incorporated herein by reference. Although likewise not illustrated herein, the precision and the stability of each of the corners of at least some types of four post frame structures may be enhanced by utilizing a self-squaring corner attachment bracket such as that disclosed by the commonly-assigned U.S. Pat. No. 5,997,117 entitled “RACK FRAME CABINET,” the entirety of which is hereby incorporated by reference.

[0052] It will be evident to the Ordinary Artisan that other structures may be used to form a frame structure on which panels may be mounted to form an enclosure. For example, in at least one embodiment (not illustrated), a frame structure may be formed from only two support posts.

[0053] The four post frame structure may further comprise one or more pairs of horizontal mounting rails 22. Each horizontal mounting rail 22 may include a slot running substantially its entire length. In addition, one or more pairs of vertical mounting rails are mounted to the horizontal mounting rails using suitable fasteners held in place in the slots of the horizontal mounting rails 22. Each vertical mounting rail preferably includes a series of threaded mounting apertures, arranged in evenly-spaced sets, extending along substantially its entire length for use in mounting electronic components, peripheral devices, cable brackets, additional mounting members, or the like thereto. It is contemplated that the number of horizontal and vertical mounting rails is variable. For example, an enclosure may include two horizontal mounting rails and two vertical mounting rails, three horizontal mounting rails and three vertical mounting rails, or the like. Further, although the number of horizontal mounting rails is equal to the number of vertical mounting rails in the two examples mentioned herein, it is not necessary that the number of mounting rails be equal. It is further contemplated that, alternatively, each horizontal mounting rail may include one or more rows of mounting apertures extending along its length.

[0054] In addition to the front panel 22, the illustrated enclosure 10 includes a right panel 24, a left panel 26, a top panel 28 and a back panel 30, all attached to the frame structure, which together form the walls of the enclosure. The right and left panels 24, 26 are similarly dimensioned and constructed. As noted previously, the front panel 22 is perforated or ventilated. The Ordinary Artisan will understand that either variation of a front panel (i.e., perforated or ventilated) is operative to provide a path for air to enter the enclosure 10 for cooling the components contained therein. It is preferred that the front panel 22 be perforated or ventilated with a percent open of between 60% and 70%, and more preferably about 63%, as further described herein below. The back panel 30 may be a lockable door, a pair of doors, a panel, or the like. Also, it will be evident to the Ordinary Artisan that alternatively other types of doors and panels may be substituted for the various panels, and that one or more of the illustrated panels (such as one or both side panels) may in some cases be omitted altogether (such as in a row of two or more adjoining enclosures). In at least some embodiments, it is preferred that the back panel be solid, i.e., substantially air impervious, while in others the back panel 30 may be perforated or ventilated like, or instead of, the front panel 22. Any known connection means may be used to join the panels to the frame structure.

[0055] As mentioned previously, the front panel of the enclosure 10 includes perforations providing a percent open of about around 63%. Front panels are commonly available that have a percent open ranging from a relatively low percent open, e.g., 40%, to a relatively high percent open, e.g., 80%. It is also possible to remove the front panel altogether, which would provide a percent open of 100%.

[0056] In the enclosure industry, the cooling effect of varying the percent open of a panel was not previously understood. That is, it was commonly assumed that an increase in the percent open of a front panel (or any other panel) would necessarily correlate to an increase in removal of thermal energy from the enclosure to which the front panel was attached, or removed, in certain situations. Therefore, the thought was that an enclosure having a perforated front panel with a relatively high percent open, such as 80% or more, or having no front panel at all would provide for greater removal of thermal energy than an enclosure having a front panel (or any other panel) with a relatively lower percent open. Unfortunately, increasing the panel percent open beyond around about 63% requires the use of a relatively thin material for the panel. In order for the perforations to be close enough to one another to increase the percent open to greater than around about 63%, the panel material must be thinner. Accordingly, a perforated panel with 80% open is thinner than a perforated panel with 63% open. Using the thinner material reduces the security and strength of the enclosure. Therefore, there is a potential trade off between a panel's percent open and the security of the enclosure to which the panel is attached.

[0057] As counterintuitive as it may be, using experimental tests described below, it has been determined that increasing the percent open area of an enclosure panel, such as the front door of an enclosure, over a certain threshold percent open area does not appreciably increase the cooling airflow through the panel. The threshold percent open area was determined to be around about 63%. This finding may be applied to any of a wide variety of enclosures, including those in the commonly-assigned U.S. Pat. Nos. 5,997,117 and 6,185,098 and the commonly assigned U.S. Pat. No. Des. 340,700, the entirety of each of which is incorporated herein by reference.

[0058] In the first of the experimental tests, an airflow and cooling audit was performed to determine whether corrective actions taken in a data center delivered the anticipated cooling benefit. More particularly, airflow through an enclosure with various percent open door configurations was monitored. To aid in understanding the experimental test, a general overview of the heat transfer principles and server cooling principles are provided herein.

[0059] First, there are various equations which may be used to describe the cooling that happens with server equipment in enclosures. One equation is the equation for forced air convective heat transfer. More particularly, convective heat transfer from a surface to an airflow is governed by the following equation:
Q=hA(T_W−T_f)

• • [0060] Where: Q=the heat transfer rate (BTU-hr)
    • [0061] h=the convection heat transfer coefficient (BTU/(h*ft²*° F.))
    • [0062] A=the surface area (ft²)
    • [0063] Tw=the temperature of the surface (° F.)
    • [0064] Tf=the temperature of the fluid (° F.)

[0065] The amount of cooling taking place inside of a particular piece of equipment, such as a server, may also be measured using the equation for sensible cooling. Sensible cooling is the removal of thermal energy that causes a change in temperature without a change in humidity. The equation for sensible cooling is as follows:
Cooling=ΔT* airflow *1.08

• • [0066] Where: Cooling=amount of cooling (BTU)
    • [0067] ΔT=the difference in air temperature between source and return air (° F.)
    • [0068] airflow=airflow rate (cubic feet per minute—CFM)

[0069] The sensible cooling equation can be rearranged and converted into metric units to create the following equation, which may be used to describe airflow in a typical datacenter environment. $\mathrm{airflow}=\frac{1.67W}{C_t}$

• • [0070] Where: airflow=airflow through server (CFM)
    • [0071] W=power (Watts)
    • [0072] Ct=Temperature rise across the equipment (° C.)

[0073] It is instructive to review what these equations are describing. When the heat transfer equation is applied to equipment enclosures, every factor except the fluid temperature (Tf) is essentially a constant defined by the equipment manufacturer. Therefore, the only variable which may be effectively varied is the temperature of the input air.

[0074] Similarly, in the sensible cooling equation, every factor is controlled by the equipment performance specification. More specifically, the airflow is controlled by the fans in the servers. The airflow may vary if the fans are variable speed fans or if the fans are choked by inadequate air supply, but the airflow is primarily dictated by the equipment manufacturer and its choice of fans. Temperature difference between source air and return air is a function of the equipment and the air crossing it. As with the heat transfer equation, temperature rise in the airflow equation is a constant defined internal to the server. While sensible cooling is variable at the room level and may be affected by various factors, there is nothing variable about sensible cooling at the equipment level. Based on the foregoing, it should be apparent that the only variable over which a user has any control, as far as equipment heat transfer is concerned, is the temperature of the air that is being drawn into the equipment housing.

[0075] As mentioned previously, an audit was performed in a data center having thermal management issues. As a part of this audit, experimental flowrate tests were performed on an enclosure having a perforated front door panel with 63% open area and on the same enclosure with the front door panel having been removed, i.e., 100% open area. Both tests were performed at a flow rate of approximately 2800 cubic feet per minute. The results from the acquired data suggest that increasing the percent open area of the door perforation above 63% does not significantly affect the net airflow rate through the server. Such a result was completely unexpected.

[0076] Because the results of the data center audit were so unexpected, a subsequent experimental test was performed in a more controlled setting to verify the anticipated poorer airflow performance in panels with percent open areas less than 63% and to validate the results from the data center test regarding airflow in panels with greater than 63% open area.

[0077] As previously described, server temperatures are affected by input air temperature and by server fan choking. If the server fans are not choked, input air temperature is left as the affecting factor for server temperature. As such, it is important to understand the effects of choking the server fans with front panel doors that are not 100% open. Therefore, in the subsequent test, the only variable considered was restriction to airflow, i.e., choking, caused by the front door panel not being 100% open.

[0078] The data from the second experimental test confirmed the findings from the less-controlled data center test and specifically indicated that only a minimal flow efficiency improvement can be expected when changing from a panel having 63% open area to no panel whatsoever, i.e., 100% open area. That is, increasing the percent open area above 63% does not significantly affect either (a) the airflow rate through the equipment or (b) the temperatures of the CPUs in the equipment.

[0079] This result runs completely counter to a widely held belief and to intuition regarding perforated metal enclosure doors. Based on the findings of the two experimental tests that were performed, perforated metal enclosure doors that have 63% open do not appear to be appreciably more restrictive to the flow of air through them than does an enclosure with its front completely open. Presently, it is common practice for technical personnel in data centers to remove perforated metal enclosure doors because it seems intuitive that removing the doors, thereby creating a state of 100% open area, would substantially improve the airflow through the rack-mounted equipment in the enclosures and thus lower the temperatures of the CPUs in the equipment. However, the experimental tests that were performed show that removing the doors does not, in fact, appreciably increase flow rate or thermal energy removal from an enclosure. Consequently, the results of these tests are important for designers, manufacturers, and users of enclosures with perforated metal doors used in contemporary data centers.

[0080] The Industrial Perforators Association has provided a chart, shown in FIG. 2, showing pressure loss in inches of water column as a function of uniform impact velocity in feet per minute. Although this chart provides pressure loss and flow rate information, it does not provide information that is readily usable and understandable to equipment enclosure designers and manufacturers. More specifically, it does not indicate whether increased percent open area provides increased flow rate, and more importantly, increased thermal energy removal.

[0081] The second experimental test mentioned above and described below was specifically designed to provide direct, precise, and easy-to-understand information for enclosure designers, manufacturers and users. In the test, the percent open area of a perforated metal door was varied and the effect on airflow rate through the door was measured.

[0082] During this test, the airflow rate was varied as is shown in FIG. 8, which is explained in more detail below. The range of flowrates were selected because the rate of airflow through perforated metal enclosure doors, expressed as Uniform Impact Velocity in feet-per-minute (fpm), is not found to be greater than approximately 1,000 fpm in contemporary data centers with high power densities.

[0083] An examination of the experimental data obtained provides confirmation that, over the expected range of airflow rates and using fans of the type commonly found in the rack-mounted equipment in data centers, the rate of cooling airflow through the door can only be increased by a maximum of approximately 1.6% by increasing the percent open of the perforated metal door panel above 63%. The test thus provides direct, precise, and easy-to-understand information to enclosure designers, which was previously unavailable.

[0084] As mentioned above, it is also important to understand whether increasing the percent open area in a front panel provides increased thermal energy removal for equipment in an enclosure. The second experimental test performed provided data that was used to calculate a theoretical operating temperature decrease for the various door configurations and flow rates that were used in the test. The results of the calculations indicate that the decrease in server operating temperature is not practically significant when comparing a front panel door with 63% open area to an open configuration wherein no door is present. Such result indicates that providing a perforated panel, more specifically a perforated panel door, with a percent open greater than 63% does not provide an appreciable increase in effective cooling for the equipment stored in the enclosure.

[0085] When designing an equipment enclosure, many factors are taken into consideration. Such factors include enclosure stability and security and thermal management of equipment stored in the enclosure. As mentioned previously, it is important that thermal energy is able to be removed from equipment in the enclosure. Typically the thermal energy is removed by air passing over the equipment in the enclosure. It is generally understood that an enclosure is more stable and secure if it has a front panel. In addition, a front panel constructed of a relatively thick material is more stable and secure than a front panel constructed of a relatively thin material.

[0086] Therefore, the idea that a perforated front panel having 63% open area provides effectively as much sensible cooling ability as a perforated panel with greater percent open area or no panel at all is extremely important to enclosure designers and manufacturers. Previously it was thought that stability and security provided by a front panel, e.g., front panel door, would have to be sacrificed in high density equipment configurations in order to provide the maximum amount of airflow through the equipment, and thus the maximum amount of sensible cooling for the equipment. However, the experimental tests performed and described herein provide results that counter this idea. The test results show that providing a perforated panel door having a percent open area of 63% essentially provides the same airflow and the same amount of sensible cooling to equipment in an enclosure as removing the door altogether.

[0087] Using the information discovered about how percent open area affects airflow through a panel, an enclosure designer may follow the following steps when designing an electronic equipment enclosure. The designer will identify at least one panel of the electronic equipment enclosure through which air is intended to flow. This panel may be the front panel or another panel of the enclosure. Further, the panel may be a door panel or a fixed panel. After identifying the panel through which air will flow, the designer will determine the design parameters of the panel. Based on the information provided herein, the designer should establish a maximum ratio of open area to closed area of about 63% for the identified panel. The designer can then determine one or more other parameters of the design of the identified panel in view of the established maximum ratio of open area to closed area. For example, the designer may select a particular material of construction or may select a panel of a particular thickness, i.e., of a particular rigidity. After the design decisions are made, the designer will provide a panel constructed according to the determined design parameters.

[0088] The designer may repeat the steps outlined above for designing multiple enclosures. In addition, the designer may then design and construct the entire enclosure based on the parameters that are selected for the panel. Using a panel with a maximum percent open of 63% allows the designer to set one parameter and then design other features of the enclosure, including other parameters of the panel, more freely. Ordinarily a designer would be continually trying to increase the percent open area of the selected panel at the expense of other design parameters of the enclosure. With the information provided herein, a designer does not have to continuously worry about trying to further increase the percent open area. The designer can start with a panel having a maximum of about 63% open area and design the rest of the enclosure using this fixed parameter.

[0089] It is further extrapolated from the results described herein that similar benefits may be obtained from panel designs having open area to closed area ratios lying between 60% and 70%. That is, a designer may first set a maximum percent open of between 60% and 70% for one or more panels of an enclosure and then design other parameters of the panel more freely.

[0090] Such a design scenario, and a perforated panel resulting from such a design scenario, may be particularly appropriate in developing and providing various electrical equipment enclosures wherein the flow of air is to be carefully planned and controlled for maximum efficiency and maximum use of passive means for achieving such efficiency. For example, commonly-assigned U.S. application Ser. No. 11/533,359, the entirety of which is incorporated herein by reference, discloses an electronic equipment enclosure that includes a frame structure formed from a plurality of support posts and at least partially enclosed by a plurality of panels, the panels including at least side, top and back panels defining an enclosure having a top, a bottom and a rear thereof, wherein the top panel includes an opening therethrough that is rectangular in shape; and an exhaust air duct extending upward from the top panel of the enclosure, wherein the exhaust air duct is rectangular in cross-section and is disposed in surrounding relation to, and in fluid communication with, the top panel opening; wherein the exhaust air duct is adapted to segregate hot air being exhausted from the enclosure from cool air entering the enclosure, thereby improving thermal management of the enclosure. In designing such an enclosure, it may be particularly useful to design a front panel with a maximum of about 63% open area and to design a 100% closed back panel. Using the design scenario of the present invention, air flow through the front panel and into the enclosure, and then out the top of the enclosure through the exhaust air duct, is maximized while permitting such design parameters of the front panel as construction material, thickness, reinforcement elements, and the like to be optimized without further regard to maximizing air flow.

[0091] Similarly, commonly-assigned U.S. application Serial No. 60/746,971, the entirety of which is incorporated herein by reference, discloses an electronic equipment enclosure above which is supported an above cabinet barrier to provide a physical barrier that prevents the recirculation of hot air over the top of the enclosure. As disclosed therein, such an arrangement may be particularly useful in maintaining a “hot aisle/cold aisle” approach to data center design, and as such it may be particularly useful to design both a front panel and a rear panel with a maximum of about 63% open area. Once again, using the design scenario of the present invention, air flow, from a cold aisle through the front panel, through the interior of the enclosure and through the rear panel into a hot aisle, is maximized while permitting such design parameters of the front and rear panels as construction material, thickness, reinforcement elements, and the like to be optimized without further regard to maximizing air flow.

[0092] Of course, the design methods of the present invention are equally applicable to a wide variety of other enclosure designs without departing from the scope of the present invention.

[0093] In a data center environment, airflow and temperature tests were performed on a 7 foot enclosure which was capable of containing up to twelve 8-blade server chassis. Each chassis occupied 3 rack mount units (RMU) and included four horizontally aligned fans. For the testing, the fans provided an effective flow rate of approximately 2800 CFM. The enclosure had the chassis installed in RMU 10 through 45.

[0094] The equipment stored in the enclosure require a peak inrush current of 10A; however, a nominal draw between 4.5 and 6.5A is typical for each chassis. The actual electrical current supplied to the enclosure could not be measured; however, based on the product literature, the enclosure load was estimated to be within the range of 6.5 kW and 9.4 kW.

[0095] Each equipment chassis included four fans, which provided the airflow for the enclosure. The temperature at the air intake and exhaust location for each fan of three exemplary chassis were measured. Additionally, the air velocity was measured at the exhaust location of each fan of the exemplary chassis. The exemplary chassis were as follows: one near the bottom of the enclosure (RMU locations 10-12), one near the middle of the enclosure (RMU locations 30-32) and one near the top of the enclosure (RMU locations 43-45). The airflow and temperature measurements were taken using a turbine-type blade anemometer at the rear of the enclosure and at the chassis inlets for each of the exemplary chassis.

[0096] The data collected during the test is summarized in FIGS. 3-5. FIG. 3 is a tabular illustration depicting measured data for a test enclosure similar to the enclosure of FIG. 1 but with the door removed. FIG. 4 is a tabular illustration depicting measured data for the test enclosure wherein a front panel door with 63% open area was attached thereto. FIG. 5 is a table showing the flow reduction for the 63% open door panel of FIG. 4 calculated as a percentage of the flow reduction for the 100% open configuration of FIG. 3. In the tables of FIGS. 3 and 4, “Delta T” is the difference in air temperature measured at the exhaust of the equipment relative to the inlet. By knowing the change in temperature coupled with the rate of air being transported through the equipment, the power dissipation rate can be calculated with the previously-described equations.

[0097] As can be seen from respective columns of the table in FIG. 5, the results show that the server exhaust temperatures did not decrease appreciably when the door panel was removed from the enclosure. In fact, some of the server exhaust temperatures were actually lower with the 63% open panel in place than when the front door was omitted altogether (100% open). This anomaly was unexpected and was attributed to the timing differential between tests coinciding with both the heat of the day and the heavier transaction load on the servers. Unfortunately, the temperature readings were not as instructive as desired because of the anomalous results. Nonetheless, as can be seen in FIGS. 3-5 and more directly in FIG. 5, the 63% open door panel had, at most, a very small effect on reducing cooling airflow. For each chassis, the percent reduction in flowrate was less than about 1%.

[0098] In addition, chassis temperatures and fan speeds were monitored directly through the chassis's operating system. FIGS. 6 and 7 show the results of these measurements. FIG. 6 is a chart showing CPU temperature as measured for each blade slot. FIG. 7 is a chart showing fan speed as measured for each blade slot. As is shown, the temperature and fan rate remained relatively constant at each RMU location when the 63% open panel was compared to the open configuration.

[0099] A controlled test was designed and executed to determine the effect on exhaust airflow and temperature of using front panel doors with varying open area percentages.

[0100] The test environment was the center section of an empty, multi-compartment enclosure. Four different front panel door configurations were tested: a vented plexiglass front panel door with 8% open area, a perforated metal front panel door with 40% open area, a perforated metal front panel door with 63% open area, and the front panel door being removed which provided 100% open area “open configuration”).

[0101] A plate was fabricated to hold twelve axial flow fans within the space (14 RMU) available in the center enclosure section. Axial flow fans were used because these fans are commonly found in rack-mounted equipment in contemporary data centers. Squirrel cage fans are also commonly found in rack-mount equipment. The conclusions drawn from this experimental test would have been the same if squirrel cage fans had been used because squirrel cage fans are less sensitive to flow restriction. Cardboard and duct tape were used to seal off any air leaks through the fans or the plate. Each of the fans was capable of ˜100 CFM in a free condition.

[0102] A horn was attached to a turbine anemometer at the rear of the enclosure in order to measure flow through one of the fans for each of the four various front panel door configurations in feet per second. Flowrate data was taken with varying numbers of fans activated to simulate varying airflow rates through a 45 RMU enclosure. The fans were activated as follows: 12, 10, 8 and 6 fans were activated in the enclosure to simulate flows of approximately 3600, 3000, 2400, and 1800 CFM, respectively. In addition, the inactive fans were covered with duct taped to prevent air leakage for the tests when fewer than twelve fans were active. Two data points were captured for each flow rate and door configuration combination. In addition, for each of the various fan activation levels and door configurations wherein a door was present, the flowrate was measured for the open configuration in order to be able to accurately compare each of door configuration wherein a door was present to the open configuration. The data points were averaged to estimate the flow rate in CFM.

[0103] FIG. 8 is a table providing raw flow rate data and calculated data indicating simulated flowrate and percent flowrate reduction. The percent flowrate reduction is in reference to a panel having 100% open area, i.e., no panel. FIGS. 8 and 9 use the data provided in FIG. 3 to show percent flowrate reduction as a function of percent open area. FIG. 9 is a chart showing percent flowrate reduction for panels having a percent open area ranging from 0-100%, and FIG. 10 is a chart showing percent flowrate reduction for panels having a percent open area ranging from 40-100%.

[0104] As is shown in FIGS. 8-10, the data confirms the findings from the less-controlled data center tests and specifically indicates that for a flowrate of 3600 CFM a maximum flow efficiency improvement of only 1.6% was seen in going from 63% open panel to 100% open configuration. At 1800 CFM, the improvement from a 63% open panel to a 100% open configuration dropped to 0.5%.

[0105] FIG. 11 is a table providing calculated theoretical flowrate and cooling information based on measured flowrate data from the controlled flowrate test. Using the flowrate information from the above-described test, it was desired to calculate a theoretical cooling amount for each of the door configurations. More particularly, the calculated percent reduction in flowrate for the maximum flowrate for each door configuration was used to calculate the theoretical increase in processor temperature due to door airflow restriction caused by each door configuration. The theoretical cooling for the open configuration was then compared to the theoretical cooling for each of the door configurations.

[0106] To begin the cooling calculations, known flowrate data was obtained from the equipment manufacturer of a commonly available blade chassis. The equipment manufacturer provides literature stating that each of the blade chassis requires an airflow of 520 CFM. A seven foot enclosure, such as the one used in the airflow test, is able to hold six of these blade chassis. Therefore, the air required for a seven foot enclosure filled with these blade chassis is 3120 CFM. Using the 1.6% flow reduction number calculated in the airflow test for the 63% open door panel, a theoretical airflow rate of 3070 CFM is calculated for a 63% open door panel. The calculated theoretical flowrates for the 8% open door panel and 40% open door panel are provided in FIG. 11.

[0107] A typical maximum power load for a 45 RMU enclosure with a typical type of equipment installed therein is 26.6 kW. Equipment manufacturers have exceeded this power requirement; however, it is a common maximum power load and is useful for the purpose of determining theoretical cooling herein. Using the airflow equation provided above, along with the typical power usage and calculated flowrate, a theoretical cooling effect was calculated for each of the door configurations tested. As is shown in FIG. 11, a 63% open panel door induces a processor temperature increase of only 0.23° C. less than the 100% open configuration. This difference in temperature is statistically and practically inconsequential. These results suggest that a door panel with 63% open area provides essentially the same cooling effect as not having a door panel present at all.

[0108] Such results are counter to what was previously believed. In addition, these results are beneficial for enclosure manufacturers and users. Based on these results, an enclosure manufacturer does not have to sacrifice thermal energy removal for security and stability in enclosure design. Both goals may be achieved without sacrificing one or the other.

[0109] Based on the foregoing information, it is readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those specifically described herein, as well as many variations, modifications, and equivalent arrangements, will be apparent from or reasonably suggested by the present invention and the foregoing descriptions thereof, without departing from the substance or scope of the present invention.

[0110] Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for the purpose of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended to be construed to limit the present invention or otherwise exclude any such other embodiments, adaptations, variations, modifications or equivalent arrangements; the present invention being limited only by the claims appended hereto and the equivalents thereof. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for the purpose of limitation.