Data center cooling system

[00002] Embodiments of the present invention are directed to cooling of rack-mounted devices, and more particularly to a data center infrastructure having a cooling system.

[00003] Communications and information technology equipment is commonly designed for mounting to racks and for housing within enclosures. Equipment racks and enclosures are used to contain and to arrange communications and information technology equipment, such as servers, CPUs, internetworking equipment and storage devices, in small wiring closets as well as equipment rooms and large data centers. An equipment rack can be an open configuration and can be housed within a rack enclosure. A standard rack typically includes front-mounting rails to which multiple units of equipment, such as servers and CPUs, are mounted and stacked vertically within the rack. The equipment capacity of a standard rack relates to the height of the mounting rails. The height is set at a standard increment of 1.75 inches, which is expressed as “U” units or the “U” height capacity of a rack. A typical U height or value of a rack is 42 U. A standard rack at any given time can be sparsely or densely populated with a variety of different components as well as with components from different manufacturers.

[00004] Most rack-mounted communications and information technology equipment consumes electrical power and generates heat. Heat produced by rack-mounted equipment can have adverse effects on the performance, reliability and useful life of the equipment components. In particular, rack-mounted equipment housed within an enclosure is particularly vulnerable to heat build-up and hot spots produced within the confines of the enclosure during operation. The amount of heat generated by a rack is dependent on the amount of electrical power drawn by equipment in the rack during operation. Heat output of a rack can vary from a few watts per U unit of rack capacity up to 250 watts per U unit, depending on the number and the type of components mounted to the rack. Users of communications and information technology equipment add, remove, and rearrange rack-mounted components as their needs change and new needs develop. The amount of heat a given rack or enclosure can generate, therefore, can vary considerably from a few tens of watts up to about 10,000 watts.

[00005] Rack-mounted equipment typically cools itself by drawing air along a front side or air inlet side of a rack or enclosure, drawing air through its components, and subsequently exhausting air from a rear or vent side of the rack or enclosure. Air flow requirements to provide sufficient air for cooling, thus, can vary considerably as a result of the number and the type of rack-mounted components and the configurations of racks and enclosures.

[00006] Equipment rooms and data centers are typically equipped with an air conditioning or cooling system that supplies and circulates cool air to rack-mounted equipment and enclosures. Many air conditioning or cooling systems, such as the system disclosed in U.S. patent application Publication No. 2001/0029163 A1, application Ser. No. 09/784,238, require that an equipment room or data center have a raised floor construction to facilitate the system's air conditioning and circulation functions. These systems typically use open floor tiles and floor grills or vents to deliver cool air from the air passageway disposed below the raised floor of an equipment room. Open floor tiles and floor grills or vents are typically located in front of equipment racks and enclosures, and along aisles between rows of racks and enclosures arranged side-by-side.

[00007] The cooling systems and methods that require a raised floor construction typically do not efficiently meet the cooling requirements of rack-mounted equipment. In particular, racks that include high power equipment having a thermal exhaust air output above 5,000 watts and up to 10,000 watts present a particular challenge for such systems and methods. A raised floor construction typically provides an open floor tile or a floor grill or vent having a venting area of about 12 by 12 inches and is configured to deliver from about 200 cfm to about 500 cfm of cool air. A rack of high power equipment drawing up to 10,000 watts and requiring an air flow of approximately 1,800 cfm, therefore, would need about 3.5 to about 5 open floor tiles, grills or vents disposed around the rack's perimeter to supply sufficient cool air to meet its cooling requirements. Such a floor configuration would be difficult to achieve in equipment rooms crowded with racks and enclosures, and impossible to implement if racks and enclosures are arranged side-by-side in rows. Air cooling systems and methods that incorporate raised floor configurations, thus, are typically only used with racks and enclosures spaced apart to provide sufficient floor area to accommodate multiple open floor tiles, grills or vents. For typical rack spacing, this places a limit on the density of equipment that can be achieved. When a raised floor is not used, the problem of distributing cool air from one or more centralized air conditioning systems is even greater, as the cool air typically must be distributed across a room containing rows of racks.

[00008] Equipment rooms and data centers are often reconfigured to meet new and/or different equipment needs that require individual racks and enclosures to be relocated and/or replaced. In this context, raised floor air cooling systems and methods are inflexible and can typically only be reconfigured and/or retrofitted to service rearranged, relocated and/or newly installed equipment racks at considerable cost. Raised floor configurations cannot easily and inexpensively accommodate the manner by which users typically deploy equipment racks and reconfigure equipment rooms and data centers to meet their new or changing needs.

[00009] In addition, cooling systems and methods that require raised floor construction lack physical flexibility and portability to operatively account for a wide variation in electrical power consumption between different racks and enclosures in an equipment room, and, in particular, between racks and enclosures located in the same row. Cooling systems and methods that rely upon raised floor air passageways and open floor tiles, grills or vents to supply cool air cannot easily and inexpensively vary or concentrate cool air to those high power racks that consume relatively large amounts of electrical power and have a high thermal air exhaust output. In addition, newly installed equipment may draw more electrical power than replaced or existing equipment to create thermal problem areas in functioning equipment rooms.

[00010] Further, a particular problem with existing air conditioning solutions is that hot spots can develop in a room due to a lack of proper recirculation of exhaust air from racks to the return side of a room air conditioner. This can cause racks to undesirably draw warm air into the racks. To attempt to overcome air circulation problems, many room air conditioners are designed to provide very cool air of approximately 58 degrees F. and receive return air having a typical temperature of approximately 78 degrees F. One problem with such air conditioners is that with an output air temperature of 58 degrees F., it is often necessary to add a humidification system to increase moisture in the air in a data center. Such humidification systems can be expense to install and operate.

[00011] Therefore, it is desirable to provide a system and method for cooling rack-mounted communications and information technology equipment such that equipment cooling requirements are met efficiently and economically, both for data centers that have a raised floor for data centers that do not have a raised floor. A rack cooling system and method that is inexpensive, and able to support groups of particularly high power racks and/or enclosures, or to overcome local thermal problem areas in an equipment room or data center is desirable.

[00012] A first aspect of the present invention is directed to a modular data center. The modular data center includes a plurality of racks, each of the racks having a front face and a back face, wherein the plurality of racks is arranged in a first row and a second row, such that the back faces of racks of the first row are facing the second row, and the back faces of the racks of the second row are facing the first row. The data center also includes a first end panel coupled between a first rack of the first row and a first rack of the second row, the first end panel having a bottom edge and a tope edge. Further, the data center includes a second end panel coupled between a second rack of the first row and a second rack of the second row, the second end panel having a top edge and a bottom edge, and a roof panel is included to couple between the top edge of the first panel and the top edge of the second panel.

[00013] The modular data center can be designed so that the roof panel is coupled to a top portion of at least one rack of the first row and to a top portion of at least one rack of the second row, such that the roof panel, the first end panel, the second end panel, and the first and second rows of racks form an enclosure around an area between the first row of racks and the second row of racks. The plurality of racks can further include cooling equipment that draws air from the area, cools the air and returns cooled air out of the front face of one of the racks. At least one of the first end panel and the second end panel can include a door. Further, at least a portion of the roof panel can be translucent. The modular data center can have at least one rack that includes an uninterruptible power supply to provide uninterrupted power to equipment in at least one other rack of the plurality of racks. The first row of racks in the modular data center can be substantially parallel to the second row. In addition, the modular data center can be designed such that one of the plurality of racks includes cooling equipment that draws air from an area between the first row and the second row, cools the air and returns cooled air out of the front face of one of the racks.

[00014] Another aspect of the present invention is directed to a method of cooling electronic equipment contained in racks in a data center. The method includes arranging the racks in two rows, including a first row and a second row that is substantially parallel to the first row, with a back face of at least one of the racks of the first row facing towards a back face of at least one of the racks of the second row. The method also includes forming an enclosure around an area between the first row and the second row, and drawing air from the area into one of the racks and passing the air out of a front face of the one of the racks.

[00015] The method can include a further step of cooling the air drawn into the one of the racks prior to passing the air out of the front face. The step of forming an enclosure may include coupling first and second side panels and a roof panel between the first row and the second row. At least one of the first side panel and the second side panel may include a door and the roof panel can include a translucent portion. Additionally, the method can include using an uninterruptible power supply to provide power to equipment in the racks.

[00016] Yet another aspect of the present invention is directed to a modular data center that includes a plurality of racks, each of the racks having a front face and a back face, wherein the plurality of racks is arranged in a first row and a second row, such that the back faces of the racks of the first row are facing the second row, and the back faces of the racks of the second row are facing the first row. The modular data center further includes means for enclosing a first area between the first row and the second row, and means for drawing air from the enclosed area, cooling the air, and returning cooled air to a second area.

[00017] The means for drawing air can further include means for passing cooled air through the front face of one of the racks. The modular data center can also be comprised of means for providing uninterruptible power to equipment in the racks. Access means for allowing access into the first area may also be a design feature of the modular data center.

[00018] Still another aspect of the invention is directed to a modular data center that includes a plurality of equipment racks, each of the equipment racks being configured to draw cooling air from a first area and to provide exhaust air to a second area, and at least one enclosure panel coupled between a first rack and a second rack of the plurality of equipment racks. At least one of the equipment racks includes cooling equipment configured to draw exhaust air from the second area and to provide cool air to the first area, and the plurality of equipment racks and the at least one enclosure panel are arranged to substantially enclose the second area.

[00019] The at least one enclosure panel can be a roof panel coupled from a roof of one equipment rack to a roof of another equipment rack. The data center can further include at least one end panel disposed between one of the plurality of equipment racks and another one of the plurality of equipment racks, the at least one end panel including a door that provides access from the first area to the second area. At least a portion of the roof panel can be translucent, and at least one of the plurality of equipment racks can include an uninterruptible power supply.

[00020] Another aspect of the invention is directed to a method of cooling equipment in a plurality of equipment racks. The method includes drawing cooling air from a first area into at least one of the equipment racks and providing exhaust air from the at least one of the equipment racks into a second area, providing an enclosure around the second area, drawing exhaust air from the second area into a second one of the plurality of equipment racks, cooling the exhaust air to produce cooled air, and providing the cooled air into the first area. The method can also include arranging the plurality of equipment racks to form two rows with the second area being between the rows.

[00021] The invention will be more fully understood after a review of the following figures, detailed description and claims.

[00026] Embodiments of the invention provide a data center infrastructure having a cooling system for cooling rack-mounted electronic equipment. Embodiments of the invention provide a modular data center for rack-mounted equipment, wherein the modular data center provides power distribution, cooling and structural support for the rack-mounted equipment. The power distribution unit and cooling is provided in some embodiments using redundant systems to prevent downtime due to electrical or mechanical failures. As understood by those skilled in the art, other embodiments are within the scope of the invention, such as embodiments used to provide infrastructure for equipment other than electronic equipment.

[00027] A system for providing power distribution for rack-mounted equipment, which can be used with embodiments of the present invention, is described in U.S. patent application Ser. No. 10/038,106, entitled, “Adjustable Scalable Rack Power System and Method,” which is herein incorporated by reference.

[00028] Referring to FIG. 1, a perspective view of a modular data center 10 is shown. The modular data center 10 includes a power distribution unit 14, a power protection unit 12, a floor mounted cooling unit 16, equipment racks 18, and a hot room 22. The modular data center 10 also has a door 52 having a window 54, a roof 56, a cooling water supply and return 60, and a voltage feed 58. The cooling water supply and return 60 can consist of condenser water in the event that the cooling unit 16 is of the liquid cooled direct expansion variety, chilled water if cooling unit 16 is of the chilled water variety, or refrigerant supply and return if cooling unit 16 is of the air cooled direct expansion variety. The data center 10 is a modular unit comprised of the power distribution unit 14, the power protection unit 12 the floor mounted cooling unit 16, and equipment racks 18 positioned adjacent to each other to form a row 32 and a row 34. Row 32 and row 34 are substantially parallel. The power distribution unit 14 and the power protection unit 12 can be located directly adjacent to one another, and can be located at the end of one of the rows. The floor-mounted cooling unit 16 may be located and positioned adjacent to the power distribution unit 14. Remaining enclosures forming the at least one additional row in the data center 10 are equipment racks 18. The hot room 22 is located between row 32 and row 34, and rows 32 and 34 comprise two of the perimeter walls of the modular data center 10.

[00029] The power distribution unit 14 typically contains a transformer, and power distribution circuitry, such as circuit breakers, for distributing power to each of the racks in the modular data center 10. The power distribution unit 14 provides redundant power to the racks 18 and can monitor the total current draw. An uninterruptible power supply can provide uninterruptible power to the power distribution unit 14. Preferably, the power distribution unit 14 includes a 40 kW uninterruptible power supply having N+1 redundancy, where the ability to add another power module provides N+1 redundancy. In one embodiment of the invention, input power to the power distribution unit 14 is received through the top of the rack from a voltage feed 58. In one embodiment, the voltage feed 58 is a 240 volt feed coupled to the power distribution unit 14 that enters through the roof panel 56. Alternatively, the input power may be received from underneath the rack, as through a raised floor, or through the back of the rack.

[00030] The power protection unit 12 provides redundant power protection for centralized information technology equipment, as is contained in the equipment racks 18. The power protection unit 12 can have individual power modules and battery modules that can be individually added or removed to accommodate different load requirements. The use of multiple power modules and battery modules provides redundancy by allowing continued operation despite the failure of any one power module or battery module. For example, the power protection unit can include a Symmetra PX® scalable, uninterruptible power supply having a three-phase input and a three-phase output, available from American Power Conversion Corporation, of West Kingston, R.I., or the power protection unit can include one of the uninterruptible power supplies described in U.S. Pat. No. 5,982,652, titled, “Method and Apparatus for Providing Uninterruptible Power,” which is incorporated herein by reference.

[00031] The floor mounted cooling unit 16 provides heat removal by use of a chilled water supply, which enters the unit through supply line 60. Alternatively, the cooling units can provide heat removal using DX compressorized cooling via use of a direct expansion refrigerant-based unit, which can be in the unit itself. The cooling unit may contain a primary chilled water coil and secondary direct expansion coil within the same frame. The cooling unit can be configured for air, water or glycol use. Cooled air can be released through the bottom of the unit or the top of the unit. In one embodiment of the invention, cool air is released from the cooling unit 16 out its front face, so that the air flow is from the back of the rack and out the front of the rack. The cooling unit 16 can further be configured as one, two or three modules. In the embodiment shown in FIG. 1, a three-module cooling unit is used.

[00032] In the embodiment of FIG. 1, each of row 32 and row 34 is comprised of six racks. In embodiments of the invention, the number of racks and the function of the equipment in the racks can vary. In one embodiment of the invention, the racks 18 are modified standard19 inch racks, such as those available from American Power Conversion Corporation of West Kingston, R.I., under the trade name NETSHELTER VX Enclosures®.

[00033] The back face of each of the power distribution unit 14, the power protection unit 12, the floor mounted cooling unit 16, and the equipment racks 18 faces the interior of the modular data center 10, or the hot room 22. Essentially, the back faces of the racks in row 32 face the back faces of the racks in row 34. In one embodiment, the equipment racks 18 have their rear doors removed so that each rack 18 remains open to the inside of the hot room 22. In the embodiment shown, the modular data center 10 contains seven equipment racks 18. Alternatively, in another embodiment, six equipment racks 18 complete the rows, but more than seven equipment racks 18 can complete the rows contained in the data center 10 and can be adjacent to one another or adjacent to other enclosures in the data center 10, such as the power distribution unit 14, the power protection unit 12, or the floor mounted cooling unit 16.

[00034] The door 52 located at the end of the row of racks is attached with hinges 53 to a detachable frame 55. The detachable frame 55 is located behind the power protection unit 12. The detachable frame may be positioned behind any one of the power protection unit 12, the power distribution unit 14, or the equipments racks 18, depending on which of the units are positioned at the end of a row in the data center 10. The detachable frame 55 allows the door 52 to be quickly removed for replacement of the power protection unit 12 if necessary. The hot room is accessible by the door 52 and can be monitored through the observation window 54. Preferably, a door 52 is located at each end of the hot room 22. Generally, the door 52 is a 2×36 inch insulated, lockable steel door having an insulated observation window 54.

[00035] The water or refrigerant supply and return 60 can enter the hot room through supply pipes into the roof 56 or directly into the roofs of the racks. The voltage feed 58 can also enter through the roof 56 or through the roofs of the racks. Alternatively, the water or refrigerant supply and return 60 and the voltage feed 58 enter the hot room through a raised floor on which the modular data center rests or from another location outside of the room and into the racks, such as into the sides of the racks.

[00036] The roof panel 56 is preferably a semi-transparent plexiglass roof panel supported by steel supports 62 that are positioned at intervals along the length 72 of the data center 10. The roof 56 extends to cover the top of the hot room 22 located in the middle of the rows of racks. The roof 56 can be easily detachable to allow for removal of racks 18 or the power protection unit 12 when necessary. A roof panel 56 constructed of semi-transparent plexiglass allows room light to enter the space defining the hot room 22. Additionally, the plexiglass roof 56 is preferably substantially airtight.

[00037] The hot room 22 is completely enclosed and has walls formed by the backside of the racks 18 and walls comprised of the door 52 attached at each end of the hot room 22. Alternatively, panels without doors can be the walls that complete the hot room. The hot room 22 is a substantially airtight passageway when the roof panel 56 is in place. Thus, the modular data center 10 is an enclosed computer infrastructure defined on its outside perimeter by the front face of each of the racks 18, power protection unit 12, power distribution unit 14, and cooling unit 16, and having a hot room 22 in its midsection. The outside walls of the hot room formed by the doors 52 are a portion of two of the outside walls of the modular data center 10.

[00038] Referring to FIG. 2, a top view of a modular data center 10 in one embodiment of the invention is shown. The modular data center of FIG. 2 is similar to that of FIG. 1, but has five racks in each of row 32 and row 34, rather than the six racks in each row of FIG. 1. With like numbers referring to like embodiments, the modular data center 10 of FIG. 2 is comprised of the power distribution unit 14, the power protection unit 12, the floor mounted cooling unit 16, the equipment racks 18, and the hot room 22. The power protection unit 12 is positioned directly adjacent to one side of the power distribution unit 14, while a floor-mounted cooling unit 16 is positioned on the other side of the power distribution unit. A service clearance area 20 surrounds the modular data center 10. In FIG. 2, an embodiment of the invention is shown having six equipment racks 18 and a cooling unit 16 having two modules.

[00039] The dimensions of the modular data center 10 depend on the number of racks included in each of the rows of racks. For example, and referring again to FIG. 1, a data center 10 having six equipment racks 18 can have a width of 120″, indicated by arrow 28, a length of 120″, indicated by arrow 29, and a height of 36″, indicated by arrow 24. The height 24 of the data center can be 36″, while the service clearance is preferably 36″ in width 26. With the inclusion of the service clearance 20, the floor surface area for the data center 10 is, preferably, a length 30 of 192″ and a width 30 of 192″. Alternatively, and referring to FIG. 2, a data center 10 having seven equipment racks 18 can have a width of 120″ and a length of 144″, while the height of the data center 10 is 36″. With the inclusion of the service clearance 20, the floor surface area for an alternate data center is 192″ by 216″. The dimensions of the modular data center are given only as examples, but can vary significantly depending upon the type and size of racks used to design the data center.

[00040] The modular data center 10 is operational when provided with a source of chilled water, condensor water or refrigerant piping 60 and a voltage feed 58. The data center can include a number of different power input designs, but is preferably a 40 kW design, allowing 6.7 kW/rack in a system having six equipment racks 18, or 5.7 kW/rack in a system having seven equipment racks 18, for example. Cooling water or refrigerant enters the floor mounted cooling units 16 via supply lines 60. A common supply line 60 can provide cooling water to one or more cooling units simultaneously, as the cooling units 16 are connected to the common supply 60 with flexible hose that is easily disconnected.

[00041] The modular data center 10 provides cooling for equipment in the data center as follows. Air from the room, or ambient air, filters through the front of the racks 18 to cool the equipment stored in the racks 18. Air enters through the front of the racks 18 and is expelled out of the backside of the racks 18. As the air passes through the equipment racks 18, the temperature of the air rises. The respectively warmer air is expelled into the hot room 22. The hot room 22 contains the warm air and prevents the warm air from mixing with air in the surrounding room. The cooling unit 16 draws warm air from the hot room and returns cool air to the room outside the data center 10. The warm air enters the cooling units 16 directly from the hot room 22. The cooling unit acts to lower the temperature of the air, and the cooled air is then released into the surrounding area. The air is recycled to the surrounding room at a substantially cooled temperature. For example, the cooling unit 16 generally receives air from the hot room at 95° F. and cools it to a temperature of approximately 72° F. before it is released into the area surrounding the data center 10. The floor mounted cooling unit 16 operates at substantially higher supply and return temperatures, allowing realization of high capacity without latent heat removal.

[00042] Referring to FIG. 3, with further reference to FIGS. 1-2, the data center 10 is configured to perform a process of cooling equipment stored in enclosed racks using an infrastructure having independent power and coolant supplies. The process 100 includes the stages shown, although the process 100 may be altered, e.g., by having stages added, deleted, or moved relative to the stages shown.

[00043] The process 100 of FIG. 3 includes stage 102, wherein power is supplied from a power distribution unit to a plurality of equipment racks 18. The equipment racks 18 may contain a variety of electronic equipment that requires a consistent power supply to avoid system downtime. A voltage feed 58 is connected to the power distribution unit 14, and a power protection unit 12 is installed adjacent to the power distribution unit 14 to ensure redundant power supply.

[00044] At stage 104, the racks 18 draw cool air from the surrounding room through the front face of the racks 18. There may, for example, be an air distribution unit within the racks and/or within equipment contained in the racks that draws the room air into the rack 18 and distributes the air throughout the rack to cool components contained in the rack. As the air passes through the rack 18, the air increases in temperature.

[00045] At stage 106, the racks 18 expel the air at an increased temperature into the hot room 22. The air is expelled out of the backside of the racks 18. As described above, in one embodiment, the racks 18 do not have rear doors. In other embodiments, rear doors may be included on the racks with the warm air being expelled into the hot room through vents in the doors. Air is held in the hot room 22 at an increased temperature and mixing of the warm air with the surrounding ambient air is prevented. In one embodiment of the invention, the modular data center is designed to maintain an air pressure in the hot room that is approximately equal to the air pressure outside the hot room. This allows one of the doors to be opened without allowing cool air to enter the hot room. In one such embodiment, the cooling unit provides 160 cfm/kW.

[00046] At stage 108, the cooling unit draws the warm air from the hot room 22. The cooling unit 16 uses the cold water from the cold water supply 60 to cool the air from the hot room. At stage 110, the cooled air is released from the cooling unit into the surrounding room, which completes the cooling cycle. The air in the surrounding room is then drawn into the racks 18 once again, and the cycle continues.

[00047] Other embodiments are within the scope and spirit of the appended claims. For example, air could be forced up through the equipment racks 18. Air moved through the racks 18 could be of varying temperatures, including hot air. The data center 10 may be configured to distribute gases other than air. Additionally, a refrigerant or other coolant may be used rather than cold water. Further, a controller can be coupled to the data center 10 to monitor air temperatures and flow rates, as well as power supply so that each rack is provided adequate power consistently. A data center may contain a single equipment rack 18 having a single cooling unit 16 creating an individual data center, whereby power is distributed to a single data center 10 or multiple single-rack data centers simultaneously.

[00048] In one embodiment of the present invention, one or more cooling units are centrally located in the modular data center to try to equalize the draw of hot air from each of the racks into the cooling unit. In other embodiments, the cooling units may placed in other locations, and in one embodiment, one or more cooling units may be positioned to be closest to a rack or racks that generate the greatest heat in the modular data center.

[00049] Further, in embodiments of the present invention, the roof over the hot area may include a number of fans that are controlled to exhaust air from the hot area in the event of a failure of an air conditioning unit in the modular data center, and/or when air temperature in the hot area exceeds a predetermined limit or if air pressure in the hot area exceeds a predetermined limit.

[00050] In embodiments of the invention described above, racks of modular data centers are described as being arranged in two rows. In other embodiments, the racks can be arranged in other geometrical configurations. Further, on sides of a modular data center, one or more racks can be used in addition to or in place of one or both side panels.

[00051] Embodiments of the present invention provide one or more of the following advantages: mixing of exhaust air with cool air in a data center is reduced; hot spots around groups of high power racks can be reduced by containing such high power racks in a modular data center as described herein; and the use of localized cooling allows air conditioning units in a data center, including within modular data centers, to operate more efficiently and produce cool air at higher temperatures, thereby negating the need for humidifying systems.

[00052] Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the scope and spirit of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention's limit is defined only in the following claims and the equivalents thereto.