ASHRAE CH-06-9-2-2006 An Analysis of the Effects of Ceiling Height on Air Distribution in Data Centers《空气分布数据中心对楼底高度影响分析》.pdf
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1、CH-06-9-2 , An Analysis of the Effects of Ceiling Height on Air Distribution in Data Centers Vali Sorell, PE Member ASHRAE Yousef Abougabal, PE Associate Member ASHRAE Viralkumar Gandhi Associate Member ASHRAE ABSTRACT In this study, computationalfluid dynamics (CFD) models are used to evaluate how
2、ceiling height afects the overall performance of the air distribution system within a data center environment. First, an underfloor air delivery system is exam- ined while keeping all operational parameters constant and varying only the clear space height. Then, the performance of the same system is
3、 evaluated with a dropped ceiling while vary- ing the clear space height. Finally, an overhead air distribution system is considered, keeping all operational parameters constant while varying only the clear space height. Theperfor- mance of each system is evaluated using dimensionlessparam- eters de
4、veloped previously by others. These indices, supply heat index (SHr) and return heat index (RHI), when used to examine the temperatures at the inlets to the equipment inside the cabinets and at the air handler or computer room air- conditioning unit (CRAC) inlets, provide a measure of how much recir
5、culation and bypass occur within the data space. INTRODUCTION Users and designers of data centers and telecommunica- tions equipment rooms have often debated the relative merits of providing a ceiling over the equipment space. Sometimes a ceiling is provided for acoustical or aesthetic purposes alon
6、e; at other times the ceiling is provided in order to use the space above the ceiling as a path for the return air. Recent practice for many new data spaces has been to use the ceiling as a return air plenum; this has been accomplished by extending a return duct from the inlets of the computer room
7、air-conditioning (CRAC) units to the ceiling. This appears to follow the gener- ally accepted notion that using the ceiling plenum as an active participant in the return air system improves the overall perfor- Kishor Khankari, PhD Member ASHRAE Aas h is h Watve Associate Member ASHRAE mance of the d
8、ata space by placing a physical barrier between the hot and cold airstreams. Although this seems intuitively correct, there are no published data to support this approach. In fact, some have even questioned whether a ceiling is neces- sary. The question raised is whether it is possible to provide en
9、ough free area in the ceiling directly above the hot aisles to allow the capture of all of the hot aisle air into the return plenum without spillover (or recirculation) into the cold aisle. If there is recirculation, one needs to ask how many more ceil- ing tiles need to be removed in order to avoid
10、 recirculation. And if more tiles are removed, is there still value in having a ceiling at all? Another important consideration in the development of a data space is the overall height of the space. Obviously, many designs are limited to the clear heights available within the space designated to be
11、converted to a data space. Usually, space height is used as a selection criterion only to the extent necessary to ensure that equipment cabinets, IT cable manage- ment systems, lighting, ductwork (if used), and a raised floor system (if used) can fit within the space. Likewise, when new construction
12、 is pursued, the design for the height of the build- ing usually considers only that all the required equipment and systems fit into the space. In both cases, consideration is usually not likely to be given to optimizing the air distribution system. Although there may be a substantial cost associate
13、d with raising the roof several feet, such cost should be assessed considering that a taller space may improve the airflow dynamics of the space, thus allowing servers or equipment to be placed in cabinets where the environmental conditions would otherwise not have been favorable. This study examine
14、s these space and ceiling height parameters to address two important site selection andor Vali Sorell and Yousef Abougabal are with Syska Hennessy Group, Inc., San Francisco, Calif. Kishor Khankari is lead consulting engineer with Fluent, Inc., Ann Arbor, Mich. Viralkumar Gandhi and Aashish Watve ar
15、e with Fluent Inc., Lebanon, NH. 02006 ASHRAE. 623 design issues: (1) the optimal conditions for the use ofa ceiling as a return air plenum and (2) the benefits of selecting as tall a space as possible. These parameters are examined by applying indices developed by Sharma et al. (2002). The indices
16、provide a means of evaluating how much hot air recirculates from the hot aisle into the cold aisle and how much cold air bypasses the cold aisle to the return at the CRAC unit. Applying these indi- ces using computational fluid dynamics (CFD) modeling allows one to evaluate the performance of variou
17、s designs before they are built. The CFD models enable the designer to test alternative designs by extracting any number of relevant temperatures from the domain space of each tested design. SPACE HEIGHT%, FT(MJ ALTERNATIVE DATA CENTER DESIGNS UNDERFLOOR WITHOUTCEIUNG UNDERFLOOR WITH CEILING OVERHEA
18、D WITHOUT CEIUNG TRIALWI TRIAI 87 TRIAL93 TRIAL #I TRIAL U7 TRIAL83 TRIALWI TRIALU2 TRIALff3 120 140 160 I20 140 160 120 140 160 (366) (427) (4861 (366) (427) (468) (366) (427) (488) The Starting Point-“A Shell Space“ The mechanical design criteria for the modeled spaces were set similar to designs
19、currently found in many data spaces. The overall load density was set to 100 W/ft2 (1076 W/m2) in a space of approximately 9,540 ft2 (886 m2). The space is 159 fi (48.5 m) long and 60 fi (1 8.2 m) wide. The ceiling height (where used) and the roof height were kept as the variable. Where a raised flo
20、or was used, the raised floor height was set to 30 in. (0.76 m). The model used for this study is based on one used for a previous study that examined the overall performance of two air distribution methodologies as a function of total airflow (Sorell et al. 2005); that model is used here with some
21、slight modifications. In the interest of conserving space, the basic description of that model is simplified herein. CASE A UNDERFLOOR AIR DISTRIBUTION WITHOUT CEILING Three different configurations were developed within the CFD model. Each is summarized in Figure 1 and described in further detail a
22、s follows. Case A: Underfloor Air Distribution without Ceiling With the basic load and spatial parameters determined, the shell space was populated with servers, PDUs, and CRAC units such that the load over the entire space equated 100 W/ft2 (1076 W/m2). The equipment cabinets were modeled as though
23、 the flow across the perforated doors from front to back experiences a uniform airflow equivalent to a temperature rise of25“F (14C). This corresponds to an equip- ment airflow of approximately 120,000 cfm (56 m3/s) across the entire data space. Fifteen CRAC units were distributed along the perimete
24、r space, 13 of which were modeled as ON. (This represents the realistic condition in which two redundant units are kept in standby mode and consequently allows for a small amount of leakage from the underfloor plenum to the data space.) The total airflow through the 13 active CRAC units represents 1
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- ASHRAECH06922006ANANALYSISOFTHEEFFECTSOFCEILINGHEIGHTONAIRDISTRIBUTIONINDATACENTERS 空气 分布 数据中心 高度 影响
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