1、24 2010 ASHRAEABSTRACT Traditional raised floor cooling is not designed toadequately cool the high heat densities now being seen in manydata centers. There are certain baseline measures that can betaken to optimize the functioning of the traditional coolingsystem. However, adding cooling solutions s
2、pecificallydesigned for high heat density is often the most efficient solu-tion to cool the hot racks and zones that occur in high densityenvironments and generate a sizable amount of heat.This paper presents the baseline measures that can betaken to optimize the traditional cooling and it provides
3、anoverview and the pros and cons of cooling fluids and systemarchitectures for high heat density cooling. The paper alsoprovides information to guide decisions about the most appro-priate cooling technologies for particular data center appli-cations.INTRODUCTIONAccording to ASHRAEs publication “Data
4、com Equip-ment Power Trends and Cooling Applications”, by 2010,computer and communications rack heat loads are projected toreach 15 to 48 kW heat load per rack.Driving this trend is the fact that advances in technologyare allowing more and more computing power to be placedinto smaller and smaller pa
5、ckages. Other contributing factorsinclude the trend of businesses to reduce capital costs byputting virtualized servers in smaller spaces, and consolida-tion of multiple remote data centers into centralized mega datacenters. This compaction increases power requirementsthereby generating more heat.BA
6、SELINE STRATEGIES TO INCREASE COOLING EFFICIENCIESCertain changes can be made to the physical infrastruc-ture to increase the efficiency of the cooling system, whichwill help better manage the heat generated by high densityequipment. These include properly sealing the data center andoptimizing the a
7、ir flow within the data center.Seal the Data Center EnvironmentCooling system efficiency is reduced when air is leakingthrough floors, walls and ceilings, or when humidity is trans-ferred from (or to) outside the critical facility. Therefore, thedata center should be isolated from the general buildi
8、ng andoutside environment as much as possible.Doors should be kept closed at all times and vapor sealsshould be used to isolate the data center atmosphere. Thevapor seal is one of the most important methods for controllingthe data center environment.Without a good vapor seal, humidity will migrate i
9、nto thedata center during the hot summer months and escape duringthe cold winter months. In ASHRAEs publication “DesignConsiderations for Datacom Equipment Centers”, theexpanded recommended relative humidity level for Class 1and Class 2 data center environments is 41.9F (5.5C) dewpoint to 60% RH and
10、 59F (15C) dew point. Computer roomprecision air conditioners (CRACs) control humidity throughhumidification or dehumidification as required. An effectivevapor seal can reduce the amount of energy expended onhumidification or dehumidification.System Architectures and Fluids for High Heat Density Coo
11、ling SolutionsLennart StahlMember ASHRAELennart Stahl is senior marketing manager for Liebert Cooling Products, Emerson Network Power in McKinney, TX.OR-10-004 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 201
12、0, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 25Optimize Air FlowOnce the room is sealed, the next step is to ensure efficientair moveme
13、nt. The goal is to move the maximum amount ofheat away from the equipment, utilizing a minimum amount ofenergy. Optimizing air flow requires an evaluation of how rackarrangement, CRAC placement/air distribution and cablemanagement, might be impacting the air flow in the room.Rack Arrangement. Most e
14、quipment manufacturedtoday is designed to draw in air through the front and exhaustit out the rear. This allows equipment racks to be arranged tocreate hot aisles and cold aisles. This approach positions racksso that rows of racks face each other, with the front of eachopposing row of racks drawing
15、cold air from the same aisle(the “cold” aisle). Hot air from two rows is exhausted into a“hot” aisle, raising the temperature of the air returning to theCRAC unit and allowing it to operate more efficiently (Figure1). This principle is called a hot-aisle/cold-aisle configuration.Blanking Panels/Rack
16、s. To implement an effective hot-aisle/cold-aisle configuration, it is vital that the hot air not mixwith the cold air. Therefore, perforated floor tiles should beremoved from hot aisles and used only in cold aisles. Blankingpanels should be placed in the open spaces in racks to preventhot air from
17、being drawn back through the rack. Even emptyspaces between racks should be filled with blanking panels orracks to prevent the mixing of hot and cold air.Seal Raised Floor. Some type of cabling grommet/sealshould also be used in the cable penetrations in the raised floorto prevent the cold air from
18、entering the space through cableopenings, which are typically at the rear of the rack. Also theseparation between the under floor plenum and adjacentrooms should be sealed so cold air does not leak from the pres-surized raised floor into adjacent rooms.CRAC Placement. When using the hot-aisle/cold-a
19、isleconfiguration, CRAC units should always be placed perpen-dicular to the hot aisle to reduce air travel and prevent hot airfrom being pulled down into the cold aisles as it returns to theair conditioner. If the CRAC units cannot be placed perpen-dicular to the hot aisle, the return ceiling plenum
20、 can be effec-tive in minimizing the mixing of hot and cold air (Figure 2).Cable Management. The growing increase in thenumber of servers that data centers need to support has createdcable management challenges in many facilities. If not prop-erly managed, cables can obstruct air flow through perfor
21、atedfloor tiles and prevent air from being properly exhausted outthe rear of the rack. The under-floor plenum should be checkedto determine if cabling (or piping) is obstructing air flow.Overhead cabling is becoming an increasingly popular meansto eliminate the potential for obstruction. Deeper rack
22、s arealso now available to allow for increased airflow. Sometimesexisting racks can even be equipped with expansion channelsto add depth for cables and airflow.It is also recommended to investigate the option of bring-ing high-voltage 3-phase power as close to the IT equipmentas possible and increas
23、ing the voltage of the IT equipment.These steps will minimize the quantity and size of the powercable feeds under the floor. This can sometimes be accom-plished by using high-voltage 3-phase managed power stripswithin the rack, but may also require the use of multiple-poledistribution panels or PDUs
24、 located within the row of ITFigure 1 Raised floor cooling with racks in hot aisle/coldaisle configuration.Figure 2 Hot air return to CRACs via ceiling plenum. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 201
25、0, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 26 ASHRAE Transactionsequipment racks. If racks have extensive server cabling in therear that obstructs the ho
26、t air exhaust from the servers, fans canbe added to the rear of racks to help draw the hot air out of therack. In a similar way, fans can be added to the front/bottomof the rack to improve the cold air distribution to the servers inthe rack. However, it is important to remember that these fansconsum
27、e energy and generate additional heat that must beremoved from the room.HIGH HEAT DENSITY COOLINGIn typical installations with 12 to 24 in. (0.3 to 0.6 m)raised floor heights, the raised-floor cooling becomes lesseffective as rack densities exceed approximately 5 kW andload diversity across the room
28、 increases. At higher densities,equipment in the bottom of the rack may consume so muchcold air that remaining quantities of cold air are insufficient tocool equipment at the top of the rack. The height of the raisedfloor creates a physical limitation on the volume of air that canbe efficiently dist
29、ributed into the room, so adding more roomair conditioners may not solve the problem. Adopting thebaseline strategies described above is a good place to beginwhen faced with increasing heat loads in the data center.However, they may not be enough to effectively remove theheat generated by high densi
30、ty equipment. In that case, addi-tional actions are recommended. The actions can generally bedivided in 2 groups:FluidBringing the cooling fluid (typically water, re-frigerant or air) closer to the heat source.ArchitectureSelecting Open, Closed or Semi-closed/open architectureIn most cases, the best
31、 action is a combination of the twomeasures.Cooling FluidsHigher density applications can benefit from liquid-cool-ing brought closer to the heat loads to effectively remove thehigh concentrations of heat being generated. By bringing acooling liquid closer to the heat source, the amount of energytyp
32、ically required for air movement is reduced considerably.The capacity and efficiency of the cooling system is alsoincreased because the temperature of the air entering the cool-ing coil is now higher.The liquid choices available for cooling are mainly water,refrigerant and dielectric fluid. Table 1
33、highlights key thermalproperties. Because dielectric fluid is substantially less effi-cient and more costly when compared to both water and refrig-erant, it will not be considered further in this paper.Water. Water has several positive attributes as a coolingfluid, including low cost, non-toxicity,
34、plentiful availability,and it can be used in virtually any size room. Also, water hasbeen used in data center cooling many years. Conversely,water can introduce a host of issues to the data center, espe-cially when it is distributed closer to the heat load. Water is aconductive liquid, so cooling sy
35、stem leaks can be electricallydisastrous. It is also corrosive and requires careful engineeringof the materials used in system construction. When water isused as a cooling fluid, it is typically not recommended for usewith overhead piping or when cooling units are located abovethe electronic equipme
36、nt, even if the water circuit has controlsthat keep the water temperature above the dew point in theroom.Refrigerant. By contrast, refrigerants such as R-134aand R-744 (CO2) are non-conductive and exist in a vapor stateat room conditions. They are nontoxic, non-flammable, envi-ronmentally friendly (
37、Ozone Depletion Potential of zero) andfully approved for use as a coolant. However, at data centeroperating temperatures, R-744 has an operating pressure thatis approximately 10 times higher than the typical operatingpressure for R-134. Therefore, the piping, connections andunits in the R7-44 based
38、system must be designed for thisconsiderably higher pressure.R-134a provides very high performance heat transfer intwo-phase operation. Compared with water, required flowrates for water based systems tend to be four to eight timeshigher than R134 two-phase refrigerant and pressure drops inthe coolin
39、g system are significantly lower in refrigerantsystems than for water systems. (Hannemann 2007).From an efficiency perspective, refrigerant performsbetter than water for high-density cooling because greater heatabsorption capacity of two phase refrigerant requires lowerfluid volumes to remove compar
40、able heat. Refrigerant for highheat density cooling can be used in either Direct Expansion orPumped versions. In the pumped refrigerant version, there isTable 1. Key Coolant Properties (ASHRAE Best Practices for Datacom Facility Energy Efficiency)CoolantFreezing Point, F (C)Thermal Conductivity, Btu
41、/hftF (W/mK)Specific heat,Btu/lbF (J/Kkg)Density, lb/ft3 (kg/m3)Latent Heat of Vaporization, Btu/lb (kJ/kg)Dielectric, FC-87 175 (115) 0.033 (0.057) 0.251 (1050) 103.6 (1659) 44 (102)Water 32 (0) 0.347 (0.6) 1.004 (4203) 62.3 (998) 1058 (2460)Ethylene glycol/water (50:50 v/v)36 (38) 0.215 (0.372) 0.
42、788 (3299) 67.8 (1086)R-134a 154 (103) 0.048 (0.083) 0.337 (1410) 76.4 (1223) 93 (216)R-744 70 (57) 0.049 (0.085) 0.815 (3412) 48.4 (775) 66 (153) 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116,
43、Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 27no compressor operating in the circuit, unlike a direct expan-sion refrigeration system (Figure 3). T
44、his allows the pumpedrefrigerant circuit to operate at a considerably lower pressureand, because no oil is needed in the pumped refrigerant circuit,oil traps and other oil-related issues are avoided.In the pumped refrigerant version, the refrigerant ispumped in the piping system as a liquid, becomes
45、 a gas withinthe distributed cooling units when the heat from electronicequipment is transferred into the fluid circuit, and then isreturned to either a pumping unit or a chiller. In the pumpingunit/chiller, the heat is emitted from the fluid circuit as the gasis condensed back to a fluid before it
46、is pumped back to thecooling unit. This phase change of the fluid contributes togreater system efficiency than water-based systems.Since refrigerants are non-conductive and exist as a vaporat room conditions, refrigerant piping and cooling units can beplaced above the racks if the controls have a fu
47、nction that keepthe fluid temperature above the dew point in the room. Thiscan save floor space.ArchitectureCooling can be brought closer to the load through eithera closed, open or semi-closed/open architecture. The mainadvantage with the closed and semi-closed/open architecturesis that they have t
48、he ability to separate the hot and cold air andtherefore increase the capacity and efficiency of the coolingsystem. Nevertheless, even in an open architecture environ-ment the capacity and efficiency can be increased if the fluidis brought close to the heat source so the possibilities for thehot and
49、 cold air to mix are minimized.Open Architecture. By definition, the open architecturehas the active cooling source outside the enclosure. Typicallythis means that the cooling units are placed at the perimeter ofthe room and supply cold air to the front of the racks via araised floor (Figure 1). The open architecture utilizes the roomair volume as a thermal storage to ride through short poweroutages. In open architecture for high heat density, wheredistributed cooling units are on or near racks, but not part of anenclosure, room air is used as a buffer in the ev
