Architectural Engineering Senior Thesis
Architectural Engineering Senior Thesis:Centralized Plant Design of Direct Fired AbsorptionChiller-Heaters in a Low-Rise Office BuildingHITT Contracting Headquarters2900 Fairview Park Drive, Falls Church, VAPrepared By:Charles HaackMechanical OptionApril 9, 2009Prepared For:Dr. William P. Bahnfleth, Ph.D., P.E., ProfessorDepartment of Architectural EngineeringThe Pennsylvania State University
HITT CONTRACTING HEADQUARTERS2900 Fairview Park Drive Falls Church, VAProject OverviewNumber of Stories 3 above grade/4 total levelsSize - 135,000 Square FeetDelivery Method - Design-Bid-BuildCost - 30 MillionBuilding Use - Office, Fitness Center,Parking Garage, & Conference SpaceConstruction Dates Start - May 2008Completion - September 2009Project TeamOwner - HITT Contracting Fairfax, VACM - Urban, Ltd. Annandale, VALandscape Architect - Rhodeside & Harwell Inc. Alexandria, VAStructural - Fernandez & Associates Structural Engineers, P.C. Falls Church, VAMEP - KTA Group Herndon, VAArchitect - Noritake Associates Alexandria, VAMechanical(7) VAV Roof Top Units with Heat Exchangersproviding 148,500 CFM(3) Split-System AC Units supplement coolingloads for the Café & Fitness CenterVAV Terminal boxes distribute air to spaceStructuralSlab on grade foundation with footings10” 2-way concrete slab for floors 1-3Flat roof comprised of 10” 2-way concrete slabElectrical4000A 277/480v 3 phase 4 wire utility service2-75kVA Transformers per floor (6 Total)125kW 277/480v Stand-by Generator277v T5 fluorescent lightingArchitecturalPrecast concrete panel envelope with punched out glazingCovered parking deckFitness center and cafeSignature glazing spine divides buildingCharles HaackMechanical /2009/cjh5004/
Table of Contents Haack 3Thesis ReportTable of ContentsAbstract . 2Table of Contents . 3Acknowledgements. 5Executive Summary. 5Building Design Background . 8Existing Design Objectives . 8Structural System Background . 8Mechanical System Existing Conditions. 9Design Conditions & Assumptions . 9Airside Systems . 10Heating & Cooling Requirements . 12Existing Building Energy Usage Summary . 12Centralized Plant Design . 14Centralized Plant Objectives . 14Design Strategies. 14Absorption Chiller-Heater Design . 14Chiller-Heater Selection . 17Pumping Selection . 18Cooling Tower Selection . 18Air Handler Selection . 19Free Waterside Cooling Design . 20Centralized Plant Analysis . 21ASHRAE 90.1 Compliance . 22ASHRAE 62.1 Compliance . 24Usable Space Breakdown . 25Energy Analysis . 26Economic Analysis . 28HITT Contracting HeadquartersFalls Church, VA
Thesis ReportTable of Contents Haack 4Central Plant Conclusions . 30Sustainability Study: Rainwater Capturing System . 31Sustainability Objectives . 31Building Background . 31Calculation Process . 32Toilet System Water Demand . 32Tank Sizing Procedure . 33Equipment requirements . 34Economic Analysis . 36Conclusions . 37Structural Impact Study . 38Structural Objectives. 38Existing Design . 38Structural Analysis . 39Existing System Analysis . 39Air Handler Analysis . 40Cooling Tower Analysis . 40Economic Analysis . 41Structural Impact Conclusions . 42Report Conclusions . 42References . 43Appendices. 44Appendix A – Breakdown of Monthly Energy Consumption & Costs . 44Appendix B – Building Usage Schedules . 44Appendix C – Cooling Tower Design Sheets . 47Appendix D – Rainwater Tank Sizing Calculations . 49Appendix E – Structural Calculations . 49Width Graphical Outputs . 50Length Graphical Outputs . 53HITT Contracting HeadquartersFalls Church, VA
Thesis ReportAcknowledgements Haack 5AcknowledgementsThesis Advisor – William Bahnfleth, Ph. D, P.E., ProfessorRoss Rebraca from HITT ContractingKTA GroupDavid Noland LEED AP EIT from KTA GroupBrain Kopp from HC NyeMatt HaapalaDan HanleyMaster’s Thesis IntegrationThe topic of the absorption chillers was studied in AE 557 – Central Cooling Systems instructedby Professor Bahnfleth and is included in the mechanical system redesign described in thisreport.Executive SummaryHITT Contracting HeadquartersFalls Church, VA
Executive Summary Haack 6Thesis ReportHITT Contracting Headquarters is a four story, 135,000 square foot office building located nextto the Capital Beltway in Falls Church, Virginia. The building consists of a variety of spacesincluding office, conference rooms, server space, café, fitness center and covered, underbuilding parking. The current mechanical system was designed to achieve a LEED silver ratingand utilizes unitary rooftop units with DX cooling and electric resistance heating. The existingsystem is environmentally conscious and relatively efficient, but improvements could be madethrough a system redesign. It should be noted that economic and design constraints that wereplaced upon the design team were not taken into account in this report, as this report is notmeant to discredit the existing system.Three main studies were conducted for this report: Centralized redesign of mechanical systemSustainability study involving rainfall capturingStructural impact analysis studying the effects of the new systemsBefore describing the new centralized system, this report overviews the existing system. Thisnew centralized plant was designed with efficient systems including: absorption refrigerationand heating through the use of a chiller-heater, waterside free cooling, primary-secondarypumping, and cooling towers for heat rejection. The primary goal of the redesign was toimprove efficiency of the system with a lesser focus on economic first costs. It was found thatthe new system had a simple payback of seventeen years when compared to the existingsystem and reduced annual energy usage by approximately ten percent. The new system alsodiversifies the building energy sources by using both natural gas and electricity; the existingsystem only uses electricity.The sustainability study described in this report was implemented to provide additional waterfor non-potable uses from rainwater that would normally be treated as a waste and expelledfrom the building. A tank was sized to capture rainwater from the roof and to provide the fullamount of water for the toilet systems 25 percent of the time, with supplemental volumes therest of the year. An economic analysis concluded that the new rainwater capturing systemwould have a simple payback of approximately 20 years. As in the mechanical redesign,economic payback was not the primary concern of the study. Offsetting of potable water usewas the main goal of the study.A structural impact analysis was also performed to account for the new loads that themechanical redesign and sustainability study created. The study achieved its goals ofredesigning the structural system to the new loads and calculating the cost difference. Coolingtowers were added and unitary rooftop units were replaced with air handling units. Minorchanges to the roof structure were observed, including a net reduction in the amount ofHITT Contracting HeadquartersFalls Church, VA
Thesis ReportExecutive Summary Haack 7reinforcing steel. The cost change between the existing and new was a minimal savings of 614.51.The redesign of HITT Contracting Headquarters increases the overall system first cost whilereducing the annual operating costs of the building from 2.52 per square foot to 2.38 persquare foot. The overall energy usage and amount of potable water consumed by the toiletsystem were also reduced greatly by the new system. These modifications would be best forlong term solutions for the building when considering a life cycle of over 20 years.HITT Contracting HeadquartersFalls Church, VA
Building Design Background Haack 8Thesis ReportBuilding Design BackgroundHITT Contracting Headquarters is a four story, 135,000 square foot office building located nextto the Capital Beltway in Falls Church, Virginia. The building consists of a variety of spacesincluding office, conference rooms, server space, café, fitness center and covered, underbuilding parking. HITT Contracting is a general contractor based in Northern Virginia and theircurrent headquarters is located in Fairfax, Virginia.Existing Design ObjectivesThe design team was given with the task to devise a building that included office, conference,storage, plotting and printing, and fitness spaces, all while aiming for LEED silver certification.The design of the mechanical systems for HITT Contracting Headquarters had the followingrequirements: Occupant controllability of the systemMinimal use of usable square footage for mechanical systemsEnergy efficiency (LEED requires improvement upon baseline case)Meeting ASHRAE 62.1-2004 (LEED credit EQ 1 Minimum IAQ Performance)Meeting ASHRAE 90.1-2004 (Minimum Energy Efficiency Standards)Structural System BackgroundThe structural system of HITT Contracting Headquarters is of reinforced concrete design. Thefoundation system consists of a slab on grade 5” thick reinforced with W/ 6x6 W2.1xW2.1 WWFcentered in slab depth that is placed upon 4” of VA DOT #56 Gravel . Floors one to three arecomprised of 10” thick 2-way reinforced concrete slabs with 4 ½” deep drop panels aroundconcrete columns. The columns are spaced in approximately a 16’ by 16’ grid. The roof systemconsists of a 10” thick 2-way concrete slab that forms a flat roof for the structure. The buildingfaçade is a curtain wall structure comprised of precast concrete panels with punched outfenestration hung from each floor.HITT Contracting HeadquartersFalls Church, VA
Mechanical System Existing Conditions Haack 9Thesis ReportMechanical System Existing ConditionsDesign Conditions & AssumptionsSince the building is currently under construction, no usage data could be obtained. As this wasthe case, Trane Trace 700 was used model the building heating and cooling loads and energyconsumption rates. Listed in Tables 1, 2 and 3 below are the design condition and loadassumptions used to create the energy model of HITT Contracting Headquarters.Table 1 – ASHRAE Design ConditionsASHRAE Outdoor Air Conditions (99.6% and 0.4%)Washington, DCWinter Dry BulbSummer Dry BulbSummer Wet BulbTemperature F159578Table 2 – ASHRAE Indoor Design ConditionsIndoor DesignCooling Supply Dry BulbCooling Drift pointHeating Supply Dry BulbHeating Drift pointRelative HumidityTemperature F78F90F72F55F50%Table 3 – Load Calculation AssumptionsLoad Calculation AssumptionsLoad TypeLightingMisc. LoadsPeopleOccupancy DensityLoads1.1 Watts/SF3.46 Watts/SF250 Btu/Person Sensible250 Btu/Person Latent114 SF/Person (Office)50 SF/Person (Conference)20 SF/Person (Fitness)50 SF/Person (Cafe)HITT Contracting HeadquartersFalls Church, VA
Thesis ReportMechanical System Existing Conditions Haack 10Airside SystemsHITT Contracting Headquarters has seven 50 Ton AAON air-cooled packaged rooftop units withenergy recovery wheels serving the four occupied floors; three above ground and one belowgrade. Each above ground floor has at total of two units that serve the North and Southsections respectively. Parallel, series, and shut-off fan-powered Variable-Air-Volume (VAV)terminal units control the final supply temperature and flow to individual zones throughout thebuilding. Three split-system air-conditioning units provide air for loads in fitness and caféspaces. See Figure 1 for a schematic of a typical existing rooftop unit.Figure 1 – Schematic of Typical Existing Rooftop UnitPowered Roof Ventilators (PRV) provide exhaust for restroom and locker spaces throughout thebuilding. Additional exhaust for storage and trash rooms is provided by ceiling mountedexhaust fans. Exhaust fans also exist in entry rooms from the parking garage to expel harmfulvapors that enter from the parking area. See Figure 2 for a schematic of the entire existingairside system. Figure 3 is a rendering of the existing rooftop with rooftop units and screening.HITT Contracting HeadquartersFalls Church, VA
Thesis ReportMechanical System Existing Conditions Haack 11Figure 2 – Schematic of existing airside systemFigure 3 – Rendering of Existing RooftopHITT Contracting HeadquartersFalls Church, VA
Mechanical System Existing Conditions Haack 12Thesis ReportHeating & Cooling RequirementsThe peak Heating and cooling loads were calculated by Trane Trace 700 and are listed in Table 4below for each of the rooftop air handling units and the supplemental air handlers for the caféand fitness areas.Table 4 – Cooling and Heating LoadsCooling and Heating LoadsAHU 1-1AHU 1-2AHU 2-1AHU 2-2AHU 3-1AHU 3-2AHU 92.4264.191.3129Existing Building Energy Usage SummaryThe monthly energy consumption as calculated by Trace 700 is displayed in Figure 4 below. Theschedules noted in Appendix B were used for the energy consumption modeling. On peakdemand was set to occur between the hours of 10am – 10pm from June to September and7am-10pm from October to May. This, along with increased demand to satisfy the coolingloads, accounts for the spike in the off-peak demand during the summer months. The existingannual electricity consumption of HITT Contracting Headquarters was modeled to be 3,769,755kWh or 27.9 kWh/ft2. This usage leads to an annual energy cost of 340,748 or 2.52 per squarefoot with a cooling cost of 0.50 per square foot. Figure 5 below describes how the totalenergy usage is broken down by type.HITT Contracting HeadquartersFalls Church, VA
Mechanical System Existing Conditions Haack 13Thesis Report400,000Kilowatt Hours350,000300,000250,000On re 4 – Existing Monthly Electricity UsageHeating, 1.9%Cooling, 19.6%Fans, 5.9%Misc, 55.3%Lighting, 17.3%Figure 5 – Breakdown of Existing Electricity Consumption by UseHITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 14Thesis ReportCentralized Plant DesignThe change of the heating and cooling systems to a centralized plant design was chosen for thisanalysis due to the combination of design choices available in a centralized plant system overan all electric direct expansion design. The economic constraints that were placed upon thedesign team were not considered in this design project and the comparison is for educationalreasons only, not to point out flaws in the base building design.Centralized Plant ObjectivesThe objective of the centralized plant design has three main goals: Overall reduction in energy consumption over existing systemDecrease life cycle cost of mechanical systems over existing systemEducational interest in Absorption chiller & centralized plant designThe discussion of achievements of these goals is discussed in the conclusion section ofcentralized plant design.Design StrategiesThe new mechanical system will incorporate a centralized chiller-heater and waterside freecooling. These changes will require the removal of the existing Unitary DX cooling and electricheating rooftop units and the addition of air handlers, cooling towers, heat exchangers, pumpsand an absorption chiller-heater. The following sections outline design criteria and selection forthis new equipment.Absorption Chiller-Heater DesignChiller-heaters have three operating modes: cooling-only, heating-only, and simultaneousheating and cooling. The direct-fired type of absorption chiller utilizes natural gas or liquidpropane to provide heat for the high temperature generator used in the absorptionrefrigeration cycle. The primary advantage of this system is that there is only one unit thatserves in place of the traditional separate boiler and chiller plants.The cooling-only mode operates as a typical double effect absorption chiller would with a gasfired high temperature generator and absorber replacing the compressor, see Figure 6 below.HITT Contracting HeadquartersFalls Church, VA
Thesis ReportCentralized Plant Design Haack 15Figure 6 – Cooling-only mode of a Double Effect Direct-fired Absorption ChillerThe heating-only mode bypasses the condenser used in cooling and utilizes the mainevaporator as a condenser. A changeover and downtime is required to switch from coolingonly to heating-only mode because of this. See Figure 7 below for a schematic of the chillerheater in heating-only.Figure 7 – Heating-only mode of a Double Effect Direct-fired Absorption ChillerHITT Contracting HeadquartersFalls Church, VA
Thesis ReportCentralized Plant Design Haack 16The simultaneous heating and cooling mode operates as a typical double effect absorptionchiller would with a gas-fired high temperature generator, but a heat exchanger is added inparallel between the high and lower temperature generators to produce hot water. See Figure8 below for a schematic of the simultaneous heating and cooling mode.Figure 8– Simultaneous heating and cooling mode of a Double Effect Direct-firedAbsorption ChillerSince these systems can provide simultaneous heating and cooling, the chiller-heater cannot besized based solely upon the peak cooling load. This simultaneous process reduces the effect ofboth the heating and cooling capabilities since the heat exchanger used to provide hot waterreduces the generator heat output in the absorption refrigeration cycle displayed in the Figure8 above. Because of this combined operation, the chiller-heater should be sized to meet thepeak cooling load at approximately 80% of its total capacity to provide excess capacity forproducing hot water at part load conditions. See capacity chart in Figure 9 below for an idea ofhow this tradeoff works.HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 17Thesis ReportFigure 9 – Simultaneous Heating and Cooling Capacities Based on Energy Input fromCarrier’s Absorption Design GuideChiller-Heater SelectionThe peak cooling load was calculated to be 367 tons in Trane Trace 700. Based upon thiscalculation and the method described above, the plant size that would best fit the heating andcooling loads would be a cooling design load of approximately 458 tons. Two 240 ton chillerheaters (230 tons actual) were used in the new centralized plant for two main reasons: System redundancyAbility to meet base load with one chiller-heater.The design day 24 hour cooling demand profile is graphed in Figure 10 below. It is shown thatthe base load is approximately 60 tons of cooling in summer conditions. One 240 ton chiller candrop down to 30% of its total capacity to meet this base load, whereas if the system consistedof one 480 ton chiller, it would have to drop to 15% of its total capacity. This low capacity is notrecommended due to very low efficiencies.HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 18Thesis ReportFigure 10 – Daily Cooling Plant Demand Profile (tons)Pumping SelectionSince there are nine heating and cooling coils that the chiller-heaters are supplying, a four-pipeprimary/secondary pumping system will be utilized to distribute the hot and chilled water. Avariable primary flow system was considered but dismissed due to complications with modelingvariable flow rates in the evaporator of a chiller-heater system. So a primary secondary systemwas chosen. See Figure 13 for a schematic of the centralized plant system.Cooling Tower SelectionThe cooling towers were selected using Marley UPDATE cooling tower selection software. Table5 displays the numbers used in selecting each of the cooling towers. See Appendix C for datasheets on the cooling tower selection. The towers were set to have two speed fans to achieveperformances similar to variable speed fans, with less cost.Table 5 – Cooling Tower Selection CriteriaCooling Tower Selection Criteria# of Towers2GPM Range45010 FFan Type50/50 2 speedHITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 19Thesis ReportAir Handler SelectionThe air handlers for the centralized system serve the same zones as the existing unitary systemto provide necessary heating and cooling. This was unchanged due to the variability in peakload between the zones, as they are on different ends of the building. This design makes thefirst cost of the equipment smaller and can reduce the amount of energy used by the system.The zones are divided into two zones per floor for floors one to three and one zone for thecellar level. See Figure 11 for a graphic displaying the zones and levels described.3rd Floor2nd Floor1st FloorCellarFigure 11 – Zone Layout Schematic – All FloorsThe air handlers replace the existing DX unitary rooftop units and provide a reduction in weightand cost. The new air handlers are VAV rooftop air handlers with total enthalpy wheels andpowered exhaust. The basis of design is an AAON RN 40 Air handler. See Table 6 for anoverview of the air handler specifications.HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 20Thesis ReportTable 6 – Air Handler RequirementsAir Handler RequirementsAir AHU-C-2Café 1 & 3920Cooling (tons)52.142.552.139.350.64263.311.910.6Heating e Waterside Cooling DesignDuring cool weather, the outside ambient wet bulb temperature can help save energy insystems that utilize cooling towers. The temperature of water coming from the cooling towercan be used with a heat exchanger to provide cooling for the chilled water returning to thechilled water plant without running the thermal compressor of the absorption chiller. Freecooling can be used to save energy whenever the outside wet-bulb temperature drops belowthe required chilled water set-point of approximately 46 degrees Fahrenheit. The heatexchanger specifications are listed in Table 7. Figure 12 is an example of a plate and frame heatexchanger.Table 7 – Free Waterside Heat Exchanger RequirementsLMTD CalculationThotin 85 FThotoutTcoldin 95 F 65 FTcoldoutLMTD 4634.3 F FNTUhotNTUcoldhhothcoldΔP 0.29 0.55 750 75015U 219.5psigbtuh/ft2HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 21Thesis ReportFigure 12 – Plate and Frame Heat ExchangerCentralized Plant AnalysisThe new centralized plant will require a new piping system to deliver hydronic heating andcooling to the rooftop air handling unit along with condenser water to the cooling towers onthe rooftop. Space for the absorption chiller heater and plate and frame heat exchanger forfree cooling will also have to be made inside the building. See Figure 13 below for a schematicof both heating and cooling systems in the central plant. Only the secondary pumps are shownon the schematic for clarity.HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 22Thesis ReportFigure 13 – Centralized Plant SchematicThe long term flexibility of the central plant system is also a benefit to the building owner;when technologies become more efficient and available the building can be easily retrofittedfor a new system after the life cycle of the current system. The centralized chiller-heater withcooling tower was chosen for its anticipated improvement in energy efficiency, smaller shaftspace requirements, diversification of primary energy sources and for educational purposes.The system will maintain its ability to simultaneously heat and cool in different parts of thebuilding, provide adequate thermal comfort to building occupants, and provide minimumventilation.ASHRAE 90.1 ComplianceASHRAE 90.1-2007 prescribes minimum requirements for the building envelope, HVAC systems,service water heating, power, lighting and electric motor efficiency. The compliancecalculations below are applied to the equipment in the newly design chiller-heater plant. Thelocation of the building falls into climate zone 5A. Tables 8, 9 and 10 test these requirements.HITT Contracting HeadquartersFalls Church, VA
Centralized Plant Design Haack 23Thesis ReportTable 8 – Equipment ComplianceMinimum Efficiencies - AHSRAE 90.1 Section imumCOP1.00Pass/FailPassSystem TypeAbsorption double effe
The peak Heating and cooling loads were calculated by Trane Trace 700 and are listed in Table 4 below for each of the rooftop air handling units and the supplemental air handlers for the café and fitness areas. Table 4 - Cooling and Heating Loads Cooling and Heating Loads Cooling (tons) Heating (MBH) AHU 1-1 52.1 309.7 AHU 1-2 42.5 262.8
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