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Benefits and Design Tipsfor Chillers

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Design tips for chillers
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  Optimal Air Systems – Benefits And Design Tips  While low temperature air distributionsystems are not new, their use hasgenerally been limited to grocery storesand other temperature and humidity sensitive applications. However, asgreater emphasis is placed on reducing building construction costs andequipment room and ceiling plenumspace, many architects and building owners/developers are becoming interested in the technology for officesand other commercial buildings. What Is Optimal Air? Most air conditioning designs are basedon supplying 55 ° F air to the space.Cooling supply air to 55 ° F generally provides the required humidity ratio tomaintain space conditions at 75 ° F and50% Relative Humidity. In short,cooling air to 55 ° F provides reasonablehumidity control. Warmer supply airtemperatures will lead to a humid orclammy environment when it is humidoutdoors.Cooling supply air below 55 ° Foffers the potential for significantcapital savings in many applications. As the supply air temperature isreduced, the supply air volume isreduced proportionally. That is, a 10% increase in supply air delta T(space setpoint minus the supply airtemperature) will result in a 10%drop in required supply air volume.This allows a 10% reduction induct and air handler face areas, and upto a 23% reduction in supply fanmotor BHP.The downside is that the colder supply air temperature requires morerefrigeration work and reduces thenumber of hours in a year wheneconomizer operation can be used. Forexample, lowering the supply airsetpoint from 55 ° F to 50 ° F removesthe opportunity to cool the building  with outdoor air when the ambientdrybulb is between 55 ° F and 50 ° F. With integrated economizers, somecooling effect can be gained, butsupplemental mechanical cooling willbe required.The Optimal Air or Balance Point isthe lowest supply air temperature thatcan be used without increasing theannual operating cost of the building. While it is typically 48 ° F to 52 ° F, every building is different and annual energy analysis is required. ENGINEERING T his issue of Engineering System Solutions considers whether one of the HVAC industry’s most valued design parameters is truly the best  parameter for all buildings. While the use of 55  ° F is based onmaintaining acceptable humidity control in a building, it does not necessarily offer the best annual energy performance or capital cost.In many cases, a lower supply air temperature is at least as efficient and will offer significant capital savings. This article is about finding the best balance point and designing the system.  McQuay also has an ApplicationGuide (AG 31-005) on Optimal Air Design that can be downloaded from www.mcquay.com or ordered from your local McQuay Representative.Your local McQuay Representative has the Energy Analyzer  ™  software used to perform the annual energy analyses used in this newsletter and they are willing to assist you in any analysis you may require on your next building project.  Hugh Crowther  Director of Applications McQuay North America SYSTEM SOLUTIONS Edition No. 12July 2002 Figure 1 –   Annual HVAC Energy Usage  Fans48%Chillers38%Pumps12%Towers2% Continued on next page.  Optimal Air Advantages To illustrate the benefits of Optimal Air systems, let ’ s consider a 10 story,200,000 ft 2 office building in Chicago.The HVAC system is VAV with a chiller plant. The design supply airtemperature is 55 ° F. In this examplethe fan work actually exceeds thechiller work on an annual basisbecause the fans must operate whenever the building is occupied. Table 1shows the annual energy analysisas the supply air temperature is loweredin 1 ° F increments down to 45 ° F. Thefollowing adjustments were made tomore accurately reflect changes inequipment requirements orperformance because the supply airtemperature is being lowered: ã The total static pressure is slowly increased to offset the deeper coilsrequired for colder air. ã The chiller plant efficiency slowly decreases.Reviewing the total annual work showsthe building load staying level until49 ° F. Beyond that point, the loss of economizer hours causes the energy useto increase. Comparing the 55 ° F supply air design to the 49 ° F supply air designleads to the following conclusions: Fan Energy and Duct Sizing The 49 ° F supply air design shows a 23% reduction in design supply airvolume. This represents a very largecapital savings in reduced duct sizes,air handling units and fan motors. Refrigeration The refrigeration work increased asexpected due to the lowered suctionpressure required by the chillers, theincreased load from the ventilation air(which must be cooled to a lowerenthalpy than 55 ° F supply airsystems) and the increased operating hours to offset the reducedeconomizer operation.The design refrigeration load actually  went up slightly. This is due toincreased enthalpy load from theventilation air, offset slightly by thereduced fan motor heat gain. Space Design Temperature andRelated Comfort The above example is based onmaintaining 75 ° F in the occupiedspace. Because the supply air has beenlowered to 49 ° F, the space RelativeHumidity (RH) has also been lowered. With a 90% sensible heat ratio, thespace RH is now 40%. At this RH,the space temperature can be raised to77 ° F while still maintaining acceptableconditions. ASHRAE Standard 55, Thermal Conditions for HumanOccupancy  , covers in detail the correcttemperature and humidity ranges sothat 80% of the occupants engaged inlight office work should be satisfied. Table 2 shows an annual energy analysisfor the same 10-story office building inChicago based on a 49 ° F supply airtemperature and a space temperatureranging from 75 ° F to 77 ° F. Raising thespace temperature to 77 ° F provides an8% reduction in design supply airvolume. It also lowers the annual energy usage by 6%. The energy savings are dueto lowered cooling loads (the indoorversus outdoor temperature difference issmaller) and an increase in the amountof available economizer hours. Sound Because of the reduced supply airvolume in Optimal Air systems, thefan(s) and motor(s) can be smaller, which lowers the sound power levels of the air handling units. Lower soundpower levels require less attenuation, which can lower both the staticpressure drops and capital costs. Indoor Air Quality Using the Optimal Air approach doesnot require any special requirements tomeet ASHRAE Standard 62.1. Lowerspace relative humidity can actually helpreduce the possibility of mold growth. ASHRAE Standard 90.1 Compliance  As shown in the above example, thegoal is to not increase the annualenergy usage in the building so thatOptimal Air systems comply withStandard 90.1 requirements. Section6.3 of ASHRAE Standard 90.1 allowsa credit for fan motor brakehorsepower when using the Optimal Air concept. SATTSPSupply AirDesign CWSTPerfor-RoomChillerFanTotalVolumeCoolingmanceSetpointWorkWorkWorkF w.c.cfmTonsFkW/tonFkWh/yrkWh/yrkWh/yr 553152,686 473 440.5575219,605 202,736 422,341 543.02145,415 475 43.60.55475226,594 194,061 420,655 533.04138,805 477 43.20.55875236,352 186,175 422,527 523.06132,770 479 42.80.56275241,228 178,975 420,203 513.08127,238 481 42.40.56675248,661 172,375 421,036 503.1122,149 483 420.5775255,777 166,303 422,080 493.12117,451 485 41.60.57475261,285 160,698 421,983 483.14113,101 486 41.20.57875269,044 155,508 424,552 473.16109,061 488 40.80.58275276,512 150,689 427,201 463.18105,301 490 40.40.58675286,605 146,202 432,807 453.2101,791 492 400.5975292,832 142,014 434,846 Table 1 –   Annual Energy Analysis (55  ° F to 45  ° F supply air temperature range) SATTSPSupply AirDesign CWSTPerfor-RoomChillerFanTotalVolumeCoolingmanceSetpointWorkWorkWorkF w.c.cfmTonsFkW/tonFkWh/yrkWh/yrkWh/yr 493.12117,451 485 41.60.57475261,285 160,698 421,983 493.12112,549 48041.60.57476255,231153,695408,926493.12107,997475 41.60.57477249,350147,438396,788 Table 2 –   Annual Energy Analysis (75  ° F to 77  ° F space temperature range)  Design Considerations When Using Optimal Air Unlike low temperature air systems,Optimal Air requires very little changesin design methodology versusconventional HVAC systems. Buildings with high sensible heat ratios are excellentcandidates for Optimal Air. Load Calculations The first step in designing an Optimal Air system is to identify the supply airbalance point, which will become thesupply air temperature. This requiressome form of annual energy analysis.McQuay  ’ s Energy Analyzer ™  can beused to quickly find the balance point.Most buildings can be evaluated in lessthan an hour. Once the supply airtemperature is identified, the rest of thebuilding load analysis is the same as in a conventional design. Secondary System Selection Conventional secondary system design will work fine for Optimal Air.Standard VAV is the most common,but fan assisted VAV boxes are alsoused. This is often predicated on theneed to address skin heat loss during  winter months. Blowthrough Or Drawthrough The main objective of Optimal Air is toincrease the temperature differencebetween the supply air and the spacetemperature to reduce the requiredsupply air volume. The supply airtemperature is the temperature of theair as it leaves the air handling unit andenters the ductwork –  not as it leavesthe coil. This is a very important considerationbecause a supply fan will add enoughheat to raise the supply air temperatureabout 2 ° F to3 ° F. Because blowthroughair handling units have the supply fanupstream of the cooling coil, their leaving air temperature off the cooling coil is thesame as the supply air temperature as itenters the ductwork. On the other hand,drawthrough units add the fan heatdownstream from the cooling coil. Tocompensate, the coil leaving airtemperature must be 2 ° F to 3 ° Flower than the supply airtemperature. Both drawthrough andblowthrough arrangements will work in Optimal Air systemsand both have advantages. Thesensible heat ratio provided by blowthrough equipment is a good match for buildings withhigh sensible heat ratios (such asoffice buildings). Air Distribution and DiffuserSelection Introducing colder air does affectair distribution and diffuserselection. Since Optimal Air is typically only 5 ° F to 8 ° F less than conventionalsystems, standard diffusers will work butthey will perform a little differently.Linear diffusers tend to offer the bestperformance since they have highersupply air velocities than lay-in typediffusers. A key parameter that must be monitoredis the separation distance relative tothrow (see Figure 3). For conventionalsystems, most designers treat these twoas the same. However, as the supply airtemperature is lowered and the supply air density increases, the separationdistance will decrease from the throw.Figure 4 shows the change in separationdistance as the supply air temperature islowered. The goal is to have anacceptable separation distance at bothmaximum and minimum air flow. Primary System Selection In an Optimal Air system, the primary system must be capable of providing thelow supply air temperature. While this isnot a problem with chiller and airhandling systems, it can present someunique challenges to rooftop or verticalself-contained units. Typical unitary equipment cannot provide the lowersupply air temperatures. However,applied products such as McQuay  ’ sRoofPak  ™  rooftop units or the SWPindoor vertical self-contained units havethe necessary flexibility in DX coils andrefrigeration components to meet thelower supply air temperaturerequirement. Other Design Considerations Duct condensation is a perceivedconcern because of the colder supply airtemperature. However, because the spaceRH is also lower, the difference betweenthe duct surface temperature and thespace dewpoint is about the same forboth conventional and Optimal Airsystems. Infiltration is a bigger concern forOptimal Air systems. Moist, warm airleaking into the building is more likely to condense on Optimal Air supply airductwork than a conventional design.Duct heat gain is another issue. As a result, many applications will requiremore duct insulation. 1.201.000.800.600.400.200.00 55 54 53 52 51 50 49 48 47 46 45 Supply Air Temperature (F)    P  e  r  c  e  n   t   D  r  o  p   I  n   S  e  p  a  r  a   t   i  o  n   D   i  s   t  a  n  c  e Separation Distance Figure 3 –   Separation Distance Figure 4 –   Separation Distance vs. Supply Air Temperature  Continued on back page.  For comments or suggestions, please call or write:Chris Sackrison, EditorMcQuay International13600 Industrial Park BoulevardMinneapolis, MN 55441Phone: (763) 553-5419E-mail: chris.sackrison@mcquay.comFor more information on McQuay products and services, or to speak with your local representative, call (800) 432-1342, or visit our web page at www.mcquay.com . ©2002 McQuay International continued from page 3. Conclusions  While 55 ° F supply air works well as a design parameter, it may not be the mostefficient operating point or provide thelowest capital cost. Lowering the designsupply air temperature until the annualenergy usage starts to climb is a goodmethod for optimizing an HVAC systemdesign. Reviewing the design load calculationsonly will not provide the answer. Anannual energy analysis must beperformed. The rest of the HVAC designis similar to a conventional design using standard air diffusion products. Appliedunitary products are recommendedbecause the lowered supply airtemperature requires more flexibility fromthe HVAC equipment.McQuay has produced an ApplicationGuide on Optimal Air Design (AG31-005) which provides detailed designconsiderations for Optimal Air systems.Contact your McQuay Representative fora copy of this and other ApplicationGuides or visit www.mcquay.com. Inaddition, the annual energy analysis canbe performed by your local McQuay Representative using McQuay’s Energy  Analyzer™.
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