1、28 2008 ASHRAEABSTRACT Sustainability for datacom installations means providingthe necessary cooling while using a minimum of energy, a mini-mum of water, and with a minimum release of pollutants to theenvironment. Designing a system that maximizes the use ofwaterside economizers may be a way to ach
2、ieve significantenergy savings. This paper will examine three choices in evap-orative cooling (open cooling towers, closed-circuit coolers,and hybrid water/air systems) and the benefits and issues withtheir operations in waterside economizer mode. In addition,advancements in non-chemical water treat
3、ment of the openloop in evaporative cooling equipment will be discussed.INTRODUCTIONThe cooling load profile in datacom centers is signifi-cantly different from the load profile for a comfort-coolingbuilding system. The most significant differences are the highwintertime cooling load required and le
4、ss humidity generatedwith datacom installations. The high winter loading presentsan opportunity for energy reduction by maximizing the use ofwaterside economizers. In waterside economizer mode, theenergy required to remove heat is in the 0.1 kW/ton rangecompared to the 0.55 to 0.75 kW/ton energy req
5、uirementswhen running a chiller. Even when the condensing watertemperature runs as low as 60F (a practical minimum temper-ature for chiller operation), chillers will still consume about0.35 kW/ton. Most HVAC cooling towers are designed to produce aspecified amount of cool water under a specific desi
6、gn-maxi-mum ambient condition. A waterside economizer is oftenadded almost as an afterthought. For comfort-cooling HVACsystems, this is a reasonable approach since the wintertimecooling load is drastically lower than the summertime load andit is the summertime loading that drives the cooling towerde
7、sign requirements. For data centers, the load requirementsare more constant over the year, and there can be significantcooling requirements even when outside ambient tempera-tures are quite cold. In addition, the heat loading for a data-center has a much higher percentage of sensible heat than mostH
8、VAC installations. This means that there is less moisturegenerated by a datacenter load than an equivalently sizedHVAC installation. Moisture is removed by providing verycold water (45-55F) to the cooling system and having themoisture condense on the cold-water coils. With less moisturein the system
9、, warmer cold water (55-65F) might be accept-able, resulting in more efficient operation of both the chiller(less lift) and waterside economizer (more hours of operation).This paper will not consider the merits between airsideeconomizers and waterside economizers; also, this paper willnot discuss th
10、e use of multiple small chillers coupled with anevaporative condenser that operates efficiently at low headpressure; rather it will focus on how different evaporativecooling systems respond to operation during winter condi-tions. We hope that, for a specific environment, this informa-tion may lead t
11、o a design solution with overall lower energyuse and improved operational reliability. Three types of evap-orative cooling systems will be reviewed with the benefits andissues for each system. The different systems will be evaluatedfor their ability to operate in a waterside economizer modeboth reli
12、ably and over a broad range of ambient conditions.EVAPORATIVE COOLINGCooling systems transfer heat from water to air. Evapo-rative (latent) cooling systems are more efficient than dryEvaporative Cooling Choices to Maximize Waterside Economizer Use in Datacom Installations John Lane Daryn ClineAssoci
13、ate Member ASHRAE Associate Member ASHRAEJohn Lane is a Vice President of Water Treatment Systems and Daryn Cline is a Senior Manager of Environmental Technologies for Evapco,Inc., Taneytown, MD.NY-08-0062008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashra
14、e.org). Published in ASHRAE Transactions, Volume 114, 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 29(sensible) cooling systems for two reasons. Firs
15、tly, in evapo-rative cooling, a cooling tower can typically cool system waterto within 5F of the ambient wet bulb temperature, this isdefined in the industry as the “approach” to wet bulb temper-ature. Air cooled heat exchangers can only cool system waterto within approximately 15F of the ambient dr
16、y bulb temper-ature. This inherent advantage means that you can always coolto a lower leaving water temperature with respect to theoutside ambient dry bulb or in other words achieve closeapproach temperatures with evaporative cooling. The secondadvantage, which is a little less intuitive, is that th
17、e amount ofheat that can be dumped into a given volume of air is muchgreater with evaporation than with sensible (dry) cooling.A pound of air occupies the volume of about two 55-gallon drums. To raise the temperature of this pound of airfrom 77F to 78F dry bulb Fahrenheit requires about Btu.To raise
18、 this same pound of air from 77F wet bulb to 78F wetbulb requires about one Btu, four times as much heat. Thereason for this difference is that when raising the wet bulb,water evaporates into the air. The latent heat from the evapo-ration of water requires the input of additional Btus. Sincecooling
19、towers cool water by raising the wet bulb of ambientair, the volume of air is required for evaporative cooling(using wet bulb temperatures) than for dry cooling. Thisallows for much smaller equipment and fan energy to be usedwith evaporative cooling than dry cooling. An additionaladvantage is that t
20、here are few times and places in populatedareas where the wet bulb exceeds 78F, while dry bulb temper-atures in excess of 100F are not uncommon.All of these advantages of evaporative cooling diminish atlow temperatures. The amount of water that can be held in coldair is much less at low temperatures
21、 than at higher tempera-tures. While dry heating a pound of air still is about Btu atlower temperatures, to heat a pound of air from 44F wet bulbto 45F requires Btu, only about one-half of the amount at77F. In addition, while at summer conditions the differencebetween wet bulb and dry bulb temperatu
22、re can often be 10 to15F or greater, at cold temperatures the difference is oftenmuch less. These two factors, lower quantity of water that canbe held by cool air and the smaller amount of wet bulb depres-sion (difference between wet bulb and dry bulb temperature),make designing an evaporative syste
23、m to cool water to 45Fvery different from designing a system to cool water to 85F.THEORETICAL AIRFLOW REQUIREMENTSTo “dry-cool” (no water added) 100 gpm of water from95F to 85F using 78F dry bulb 75% RH (72F WB) willrequire heating an airflow of about 27,600 CFM.1To cool thesame amount of water with
24、 the same ambient conditions warm and muggy using evaporative cooling would requireonly 4,200 CFM 85% less airflow. Since the size of thesystem is related to the amount of air that must be moved, evenunder these high humidity conditions, an air-cooled systemwould need to be 7 times larger than a wat
25、er-cooled system.The reason that water-cooling is so much more efficient is thatat these temperatures, air can hold a great deal of moisture andthe incoming wet bulb is lower than the incoming dry bulb.The more moisture the air can gain the more heat can beremoved per volume of air by evaporation. I
26、f the ambient airwere drier, the difference would be even more dramatic.At cooler temperatures, there are still advantages withevaporative cooling, though not as great. In waterside econo-mizer mode, to cool 100 gpm of water from 55F to 45F using38F dry bulb 75% R.H. (35F WB) air, seems to be simila
27、r tothe previous 95/85 example. For dry cooling, it is quite similar;dry cooling would require 25,600 CFM airflow, about thesame as needed at the higher temperatures, to cool the water.However, evaporative cooling at low temperatures isdifferent. At 38F 75% RH, 10,300 CFM of airflow would berequired
28、 to cool 100 gpm from 55F to 45F. While this is stillless than of the airflow required with dry cooling, it is 2times the airflow required for similar evaporative cooling atthe higher temperature. The reason is that both the amount ofmoisture held in the air is lower and the difference betweenwet bu
29、lb and dry bulb temperature is lower. To provide thesame amount of cooling in waterside economizer mode withevaporative cooling will require either larger equipment(higher initial capital cost) or cooler ambient temperatures(less hours of operation under economizer mode).As a last comment, if the co
30、ld-water temperature require-ments could be raised there would be several benefits.ASHRAEs Thermal Guidelines for Data Processing Environ-ments (ASHRAE 2004) recommended operating temperaturefor Class 1 and Class 2 environments is 68-77F. If this condi-tion can be met by operating the chilled water
31、in a range of 55-65F, then not only would additional days of waterside econ-omizer operation be available, but evaporative cooling equip-ment could be smaller, there would be less energy used tooperate the fans, and the risk of freezing in cold ambient condi-tion would be reduced. Figure 1 shows the
32、 theoretical minimum airflow to lowerthe temperature of 100 gpm of water 10F from a chiller at itsdesign maximum and from waterside economizers at variousoperating ranges. Under economizer mode, the temperaturerange was changed to reflect the 25% reduction in coolingrequired by the tower with the ch
33、iller by-passed (the 25%approximates the parasitic heating load from chiller operationand reduced general cooling needs). The relative airflowrequirements will approximate the relative box size and horse-power requirements of the equipment required to perform thecooling. 1.This and all of the exampl
34、es in this section assume that the inletdry bulb temperature is 7F below the exit water temperature, theambient relative humidity is 75%, and the exit air temperature isthe same as the entering (warm) water temperature. The details onthese calculations are shown in the Appendix.30 ASHRAE Transaction
35、sTYPES OF EQUIPMENTThe preceding has outlined the theoretical limitationswithin which waterside economizer must operate. This sectionwill describe some of the evaporative systems that arecommercially available. This paper will consider three generaltypes of evaporative cooling systems. These systems
36、 are:1. An open cooling tower with a plate and frame heatexchanger as a waterside economizer.2. A finned fluid cooler using glycol as the closed-loopfluid.3. A hybrid wet/dry cooler using glycol as the closed-loopfluid.Figure 2 shows a schematic of an open cooling systemwith a plate and frame econom
37、izer. During winter operation,the tower water feeds a plate and frame heat exchanger, thuscompletely bypassing the chiller. Typical leaving watertemperatures that can be provided by an open tower sized foreconomizer mode are in the 42F45F range. During summeroperation, warm water from the condenser
38、side of the chilleris cooled by the tower. A typical specification is 95F/85F/78F. This means that the tower is designed to cool a specifiedflow rate of 95F water from the chiller to 85F when the ambi-ent wet bulb is 78F. These would be the “design conditions”to specify the tower and the specified t
39、ower would have a 7Fapproach to wet bulb. When the ambient conditions are lower,the load is lower, or because of redundancy there is morecapacity, then cooler temperature water can be provided by thetower. Many chiller designs can take advantage of coolercondenser water down to 65F, with a rule-ofth
40、umb of 1 %chiller energy savings with every 1F decrease in incomingcondenser water temperature (Clark 2004). Some systemsutilize this effect by specifying cooling towers with a closeapproach to wet bulb. A 93F/83F/78F would have a 5-degree approach and under design-load conditions, wouldproduce cool
41、er water to the chiller, and allow the chiller tooperate more efficiently in many ambient conditions.Figure 3 shows a closed-circuit fluid cooler configured foreconomizer operation (chiller is valved off). With this system,glycol circulates in a closed loop in the fluid cooler coils. Duringnormal op
42、eration air flows up, around the coils and water spraysdown over the coils. Cooling occurs by evaporation. A typicalspecification would be 95F/85F/78F, the same design condi-tions as with an open tower for cooling. Often there will be anemergency dry bulb requirement for operation in the event ofcom
43、plete loss of make up water. With the additional dry-oper-ation specification of 110F/100F/85F, in the event of acomplete loss of water, the fluid cooler would still be able tocool condenser water to 100F at an 85F ambient dry bulb. Formore dry cooling capacity, the coils can be built with optionalf
44、ins (see Figure 4). During wintertime operation the chiller isby-passed and the glycol connects with the building chilled-water (in this case glycol) loop. For summertime operation, thissystem is less efficient than an open tower, both because of theadditional heat transfer step (through the tube wa
45、lls) and the useof glycol. For wintertime operation, both an open system witha plate and frame heat exchanger and a closed-circuit coolerhave the same number of heat transfer steps.Water treatment programs are simpler with a fluid cooler.The chiller condenser sees only closed-loop fluid; the open-lo
46、op water is restricted to the fluid cooler basin and exterior ofthe coil. Most chiller condensers now use enhanced tubes forimproved heat transfer. These tubes are much more suscepti-ble to waterside fouling than smooth tubes (Webb 1994).When fouled, the improvement in heat transfer from theenhancem
47、ents is lost. Use of a fluid cooler eliminates foulingon the chiller condenser and restricts the requirements ofopen-loop water treatment to the outside of the coils and basin.The open loop water volume of a fluid cooler is much smallerand water treatment is significantly easier. Figure 5 shows a hy
48、brid cooler. This is a combination ofan evaporativly-cooled fluid cooler and an air-cooled fluidcooler. Controls will automatically switch between wet cool-ing, dry cooling, or combined wet/dry cooling depending onthe ambient condition. Glycol is used for the closed loop. Ahybrid cooler will operate
49、 similarly to an evaporative fluidcooler but has enhanced dry cooling capabilities. The evapo-rative and dry sections connect in series, with the dry coolerremoving some of the heat load and the evaporative-coolercompleting the work. Hybrids will use less water than either anopen system or a closed-circuit cooler, because in coolweather some if not all of the cooling can be accomplished ina dry (sensible) mode. Thus, with a hybrid system you get mostof the water savings of dry-cooling, but with the thermal effi-ciency
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