1、98 2010 ASHRAEABSTRACTApplying airside and waterside economizer to data centerprojects requires additional consideration to fulfill the uniquedesign criteria that is not common to other commercial build-ing types. Waterside economizer design considerationsinclude proper equipment sizing, cooling tow
2、er freeze protec-tion, pump design for parallel and series configuration, andcontrol strategies for system stability. Airside economizerdesign considerations include humidification, particulate con-tamination, gaseous contamination, fire prevention, smokedetection, supply and return air balancing.IN
3、TRODUCTIONData center facilities house computer servers and associ-ated components, such as networking equipment, telecommu-nication and archiving storage servers. Data centers with largefootprints or high power densities are typically standalonefacilities; whereas data centers with small footprints
4、 or lowpower densities could be a tenant within a building. Datacenters can be distinguished by their exceptionally highenergy consumption compared to other types of facilities. Forinstance, typical office buildings consume an average of 5 to10 W/ft2power density, as compared to modern data centers,
5、which are now designed to consume an average of 150 to750 W/ft2of electricity input (Patterson and Fenwick 2008).This is roughly equal to 600 to 1800 W/ft2of the server foot-print.Typically, a data center is served by dedicated mechani-cal, electrical and fire protection infrastructure that is inde-
6、pendent from the systems that serve other portions of thebuilding. Similar to other building types, data center infra-structures are designed to meet local, state and federal build-ing codes, as well as design standards and guidelines set forthby industry organizations such as the American Society o
7、fHeating, Refrigerating and Air-Conditioning (ASHRAE),National Electrical Code (NEC) and National Fire ProtectionAgency (NFPA). Additionally, data center design is oftenrequired to meet certain levels of redundancy, reliability,maintainability, fault tolerance, scalability and flexibility. Theindust
8、ry-recognized topologies for infrastructure design andsite resiliency are established by the Uptime Institute, aconsortium of companies and industry leaders devoted tomaximizing uptime for data center operations.ENERGY CONSUMPTION TRENDSEnergy consumption by data center servers and relatedinfrastruc
9、ture equipment has doubled from 2000 to 2005 in theUnited States and worldwide (Koomey 2007). As of 2006,energy consumption for computer severs in data center facil-ities and related electrical and mechanical supporting infra-structure equipment in the United States is estimated to be 61billion kilo
10、watt-hours (KWh). This is equal to 1.5% of totalU.S. electrical consumption. The same amount of electricity issufficient to support averaged 7.1 million four-person house-hold (U.S. EPA). Following the same trend, Koomey predictsthe energy consumption for the data center sector will doubleonce again
11、 from 2006 to 2011. It is up to the data center indus-try stakeholders, including owners, design engineers, contrac-tors, operators, and IT professionals, to reduce the energyconsumption from this point forward.The ASHRAE mission critical technical committee 9.9,mission critical facilities, released
12、 a supplementary designguideline in 2008 to update the thermal parameters that wereestablished by The Thermal Guidelines for Data ProcessingWaterside and Airside EconomizersDesign Considerations for Data Center FacilitiesYury Y. Lui, PEAssociate Member ASHRAEYury Y. Lui is a senior mechanical engine
13、er and worldwide LEED practice leader at HP Critical Facilities Services, delivered by EYP MCF.OR-10-012 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional
14、reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 99Environments in 2004 (ASHRAE 2008a). Refer to Table 1 for2004 and 2008 recommended data center thermal designparameters comparison. The new the
15、rmal parameter guidelineexpands the recommended ranges for both temperature andhumidity. These changes have increased the energy savingspotential of different energy saving measures, includingwaterside and airside economizer systems.In the last decade, energy conservation was not the pri-mary concer
16、n for most data center facility owners and designengineers. Design efforts were focused on achieving maxi-mum “uptime.” It is not uncommon that the energy efficiencywas sacrificed for better reliability and redundancy. However,this atmosphere has changed in the last few years. With risingenergy cost
17、s and the increasing demand to provide sustainabledesign and operation, the need for more energy efficient datacenters is gaining momentum and has become one of the toppriorities for the owners. The industry is currently experienc-ing high demand for efficient data center design. Many EnergyConserva
18、tion Measures (ECM) common to commercial build-ing are incorporated to the design of new data centers. Thisincludes improving building envelope performance, usinghigh efficiency lighting fixtures and equipment, using variablespeed drives, and enhanced lighting control. However, theaforementioned ECM
19、s do not directly improve the issue oflarge quantities.Typically, data center cooling is provided by floormounted air conditioners with direct expansion refrigerantcooling (DX) or chilled water cooling (CW). Refer to Figure1 for conventional data center air conditioning flow diagram.The fan inside t
20、he computer room air conditioner (CRAC)moves the hot air across the cooling coil and transfers the heatinto the chilled water loop. The heat is then exchanged to thecondenser water loop inside the chiller through the refrigerantvapor compression cycle. Eventually, the heat is rejected intothe atmosp
21、here by cooling tower. As compare to the conven-tional data center design using vapor compression chillers,cooling towers, and CRAC, the use of waterside and airsideeconomizers to generate free cooling to offset the computerheat dissipation has become a popular design concept toachieve higher buildi
22、ng energy efficiency.The purpose of this paper is to discuss the design consid-erations applicable to waterside and airside economizer sys-tems and thus the decision-making involved in choosingwhich to use. Each type has its advantages, so the decisionmust be made with the specifics of a project in
23、mind. A datacenter design team may use this document as a roadmap to de-velop and perform critical analysis at the project conception,such that the entire design team can better select and imple-ment a mechanical system that is very reliable, efficient, low-cost, and suitable energy conservation mea
24、sures without neg-atively impacting the project schedule. This report describeseach kind of system, explains how to perform energy savinganalysis for specific projects, and then summarizes the advan-tages and disadvantages of the two systems.Table 1. Comparison of 2004 and 2008Recommended Data Cente
25、rThermal Design Parameters (ASHRAE 2008a)2004 Version 2008 VersionLow-end temperature 20C (68F) 18C (64.4F)High-end temperature 25C (77F) 27C (80.6F)Low-end moisture 40% RH 5.5C dew point (41.9F)High-end moisture 55% RH 60% RH and15C dew point (59F)Figure 1 Conventional data center air conditioning
26、flow diagram. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitte
27、d without ASHRAEs prior written permission. 100 ASHRAE TransactionsWATERSIDE ECONOMIZER INTRODUCTIONThe basic principle of waterside economizer is to pre-coolsome or all of the return water in a chilled water loop with thecooling tower, substantially reducing or even eliminating theneed of mechanica
28、l cooling. Through the use of plate andframe heat exchangers, building heat is transferred from thechilled water loop into the cooling tower loop and eventuallydissipated into the atmosphere. As a result, the building cool-ing demand is partially or entirely satisfied without the use ofmechanical re
29、frigeration cooling system. Refer to Figure 2 forwaterside economizer flow diagram.There are two types of waterside economizers. “ParallelWaterside Economizer (PWSE)” and “Series Waterside Econ-omizer (SWSE)” are defined by whether the heat exchanger isinstalled in parallel or in series with respect
30、 to the chiller. Referto Figures 3 and 4 for typical PWSE and SWSE flow diagramsin normal and economizer operation.EQUIPMENT SIZINGBoth PWSE and SWSE are capable of providing fullwaterside economizer. However, only SWSEs can providepartial waterside economizing, which is benefit from thecomplicated
31、piping and control valve configuration. Duringpartial economizing, the cooling tower is operated in conjunc-tion with the chiller. The cooling tower pre-cools the buildingreturning chilled water before it enters the chiller and thusreduce the chiller cooling load. The wet-bulb switchovertemperature
32、for full and partial waterside economizer can bedetermined by the following equations.Partial Pre-Cooling ConditionAmbient Wet-Bulb Temperature Chilled Water Return Temperature Cooling Tower Approach Temperature Heat Exchanger Approach TemperatureFull Free Cooling ConditionAmbient Wet-Bulb Temperatu
33、re Chilled Water Supply Temperature Cooling Tower Approach Temperature Heat Exchanger Approach TemperatureThe first step of a waterside economizer design is tocalculate the cooling tower size. The typical design procedureis to match the cooling tower capacity with the chiller opera-tion in summer de
34、sign conditions. Since evaporation is theprimary operating principle for cooling towers, the ability forambient air to absorb moisture reduces as the wet bulb temper-ature drops. In other words, cooling tower approach tempera-ture increases as the wet bulb temperature deceases. Coolingtowers selecte
35、d for summer operation include the chillercompressor heat rejection. When a cooling tower operates inthe waterside economizer mode, the chiller is off and rejectsless heat to the cooling tower. Since the cooling tower pumpstays at the same flow rate, the overall cooling tower waterdifferential tempe
36、rature increases and the cooling towerwinter operation improves. If it is desired to increase thewaterside economizer run time, the cooling tower can be over-sized to optimize during winter operation. For example, Table2 illustrates two cooling tower selections that are optimized tosummer and winter
37、 operation. A slight increase in heightallows the cooling tower to generate additional full and partialFigure 2 Waterside economizer flow diagram. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116,
38、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 101WSE operation hours. As a result, a payback analysis isneeded to justify the additional initial inve
39、stment.COOLING TOWER FREEZE PROTECTIONBoth PWSE and SWSE concepts require operating thecooling tower in winter. As ambient temperature continues todrop, the cooling tower water supply temperature also dropsproportionally. During partial waterside economizer opera-tion, cooling tower water temperatur
40、e cannot drop below thechiller minimum condenser water entering temperaturesetpoint, as defined by the chiller manufacturer. During fullwaterside economizer operation, the colder than desiredcondenser water temperature also promotes ice formation atFigure 3 Typical parallel waterside economizer (PWS
41、E) flow diagram in normal operation (left) and economizer operation(right).Figure 4 Typical series waterside economizer (SWSE) flow diagram in normal operation (left) and economizer operation(right). 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.or
42、g). Published in ASHRAE Transactions 2010, 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. 102 ASHRAE Transactionsthe cooling tower. Hence, the following feature
43、s are recom-mended to control the cooling tower operation. First, the cool-ing tower should be equipped with VFD to slow down the fanas required. Second, the cooling tower fan motor should havethe capability to run in reverse direction to allow ice thawingat the air intake areas. Third, a bypass val
44、ve is suggested todivert the cooling tower water back to the sump. Finally, aremote cooling tower water sump with large reservoir volumecan also serves as a buffer to reduce cooling tower shortcycling. A waterside economizer design employs all thesedesign features should be able to prevent cooling t
45、ower freez-ing problem.PUMP SIZINGFor SWSE piping configuration, the hydrostatic paththrough the chiller and heat exchanger in series during thepartial waterside economizer generates the highest pressuredrop, compared to the operation of mechanical cooling bychiller and full waterside economizer. Th
46、e primary chilledwater pump sized according to the highest pressure dropbecomes oversized for other operations. Variable FrequencyDrives (VFD) should be considered for the primary chilledwater pump. With VFD, the pump can be balanced to operateat different speeds for different operational modes.For
47、PWSE piping configuration, pressure drop throughthe chiller is usually higher than the pressure drop through theheat exchanger. Hence, chilled water and cooling tower pumpsthat are sized for the chiller operation are acceptable to fullwater side economizer operation. Even if the pumps are notequippe
48、d with VFD, they will adapt to the new pressure dropwithout affecting the pump performance. Nevertheless, themechanical engineer may elect to size the heat exchanger withpressure drop identical to the chiller to avoid any difference inpressure drop. The resulting heat exchanger is usually smallerin
49、physical size and cheaper in cost.PLATE AND FRAME HEAT EXCHANGER SIZINGA plate and frame heat exchanger is essential equipmentfor both PWSE and SWSE systems. The major function of aheat exchanger is to transfer the energy between two fluidswithout cross contamination. The heat transfer characteristicof a heat exchanger is determined by the Log Mean Temper-ature Difference (LMTD). And the LMTD is directly propor-tional to the Approach Temperature, which is the hot fluidleaving temperature minus cold fluid incoming temperature.A heat exchanger with the lowest approac
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