1、2010 ASHRAE 365This paper is based on findings resulting from ASHRAE Research Project RP-1254.ABSTRACTThis paper provides an overview of the results of ASHRAEResearch Project RP-1254, Evaluating the Ability of UnitaryEquipment to Maintain Adequate Space Humidity Levels;Phase II: Simulations, Summary
2、 and Development of Guide-lines. Whole-building energy simulations were used to performa parametric analysis of eighteen HVAC system types in sevencommercial building types (small office, large retail, 9 monthclassroom,12 month classroom, restaurant dining area, smallhotel/motel guest room, and thea
3、ter) using two sets of venti-lation rates (ASHRAE Standards 62-2001 and 62.1-2004) in10 locations. The system types included single-path and dual-path direct expansion (DX) with and without enhancementssuch as enthalpy wheel, demand controlled ventilation, desic-cant dehumidifier, subcool reheat, ho
4、t gas reheat, and air-to-air heat exchangers around the cooling coil. The relativeperformance of each system type is compared on the basis ofhumidity control (occupied hours 65% RH) and annualenergy use, including heating energy. The systems are alsocompared for life cycle costs using approximate in
5、stalledequipment costs and HVAC annual energy costs.INTRODUCTIONANSI/ASHRAE Standard 62-2001 (ASHRAE 2001),Ventilation for Acceptable Indoor Air Quality, increased venti-lation requirements in all commercial buildings and iscurrently referenced by many model building codes. Theseincreased ventilatio
6、n requirements imposed greater heatingand cooling loads on HVAC equipment and in many climatesincreased the dehumidification load which in turn impactedthe ability of HVAC systems to control humidity within condi-tioned spaces. ASHRAE Research Project RP-1254completed Phase 2 of a two-phase effort t
7、o evaluate severalmeans to improve an HVAC systems dehumidificationperformance and provide better humidity control in commer-cial buildings. The Phase 1 effort (Brandemuehl et al. 2001)developed a plan to guide the evaluation effort which wascarried out as part of Phase 2. This technical paper prese
8、nts theresults of Phase 2 which was targeted at:Comparing various unitary air conditioning systemhumidity control configurations for application to com-mercial buildings in terms of humidity control perfor-mance, operating costs, and life cycle costs to each otheras well as to conventional unitary e
9、quipment, andDeveloping guidelines to help HVAC engineers andpractitioners identify the important application charac-teristics and climate factors that determine which optionis most appropriate.For unitary products, many options exist today for betterdehumidification technologies including:Evaporato
10、r coils with more rows and lower airflowrates,Lower airflow rates,Air-to-air heat exchangers to precool and reheat airentering and leaving the cooling coil,Condenser reheat coil in series with evaporator,Prevention of evaporation from wet cooling coil bycycling the supply air fan,Evaluating the Abil
11、ity of Unitary Equipment to Maintain Adequate Space Humidity LevelsMichael J. Witte, PhD Robert H. HenningerMember ASHRAEMichael J. Witte and Robert H. Henninger are principal engineers at GARD Analytics, Inc., Arlington Heights, IL.OR-10-040 (RP-1254) 2010, American Society of Heating, Refrigeratin
12、g 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 permitted without ASHRAEs prior written permission. 366 ASHRAE Transa
13、ctionsBypassing a fraction of the airflow around the coolingcoil,Pre-drying using active desiccant prior to the coolingcoil,Enthalpy recovery wheel using exhausted return air,Dedicated preconditioning DX system for outside air,Separate outside air conditioning dual path system,Enthalpy recovery whee
14、l with a separate outside air con-ditioning system,Air-to-air heat exchangers with a separate outside airconditioning system,Separate outside air conditioning with desiccant system,Reducing ventilation by using carbon dioxide monitor-ing, andSeparate outside air conditioning with reducing the ven-ti
15、lation quantity by carbon dioxide monitoring.Given the large number of options, it is difficult to choosewhich enhanced unitary technology is appropriate for a givenapplication. This ASHRAE research project was initiated toimprove our knowledge about these choices by providing aquantitative study ac
16、ross many of the available options. Byexamining these applications from the perspective of what isthe most cost effective way to control indoor humidity levels,designers will be able to use the guidance provided to assist inselecting the equipment configuration for a specific building.DESCRIPTION OF
17、 BUILDINGS AND HVAC SYSTEMSA range of commercial building types, ventilation rates,locations and HVAC equipment options were chosen for anal-ysis as described below through use of a whole-buildingenergy simulation tool:Seven commercial building typesSmall OfficeLarge RetailClassroom 9-monthClassroom
18、 12-monthRestaurant Dining AreaSmall Hotel (Motel) Guest RoomTheaterTwo ventilation ratesBased on Standard 62-2001 which is referenced by many building codesBased on Standard 62.1-2004 which was the current version of the standard at the time of this workTen locationsAtlanta, GA New York, NYChicago,
19、 IL Portland, ORDallas/Fort Worth, TX Shreveport, LAHouston, TX St. Louis, MOMiami, FL Washington, D.C.Eighteen HVAC equipment optionsCase 0Conventional DX System (typical HVAC design practice)Case 1Base DX System (good dehumidification design practice)Case 2DX Design for Improved Dehumidification (
20、modified coil, compressor, etc.)Case 3Base DX System with Lower AirflowCase 4DX System with Air-to-Air Heat Exchanger (AAHX)Case 5DX System with Subcooling Reheat CoilCase 6Base DX System with No Latent DegradationCase 7DX System with Airflow Control Using Bypass DamperCase 8Hybrid DX and Desiccant
21、System (condition outdoor air)Case 9DX System with Enthalpy Recovery WheelCase 10DX System with Outdoor Air PreconditioningCase 11DX Dual Path System (separate systems for outdoor and recirculated air)Case 12DX Dual Path with Enthalpy RecoveryCase 13DX Dual Path with AAHXCase 14DX Dual Path System w
22、ith Desiccant (desiccant system for outdoor air)Case 15DX System with Demand Controlled VentilationCase 16DX Dual Path System with Demand Controlled VentilationCase 17Base DX System with Free Reheat (hot gas reheat)The commercial building models used in the analysiswere constructed based on informat
23、ion presented in the Phase1 Evaluation Plan (see Table 1) with some modifications. Theprototypical building physical characteristics were taken froma study done by Huang and Franconi (Huang and Franconi1999). The theater was added and its characteristics weretaken from a set of design drawings for a
24、n actual theater builtwithin the last several years. Its envelope construction wasconsidered to be similar to the large retail building. Internalload levels for each building type for lights and equipmentwere also set as prescribed in the Phase 1 Evaluation Plan. Allof the building models were a sin
25、gle zone, representing theparticular space type of interest for each application. For theclassroom and motel room, the zone was modeled as a centerspace on the ground floor with one exterior wall facing south,and conditioned spaces on the other three sides and above.The significant differences betwe
26、en the various HVACsystems described above are summarized in Table 2.VENTILATION RATESThe Phase 1 Evaluation Plan called for two differentlevels of occupant densities and ventilation rates to be 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). P
27、ublished 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. ASHRAE Transactions 367Table 1. Prototype Building CharacteristicsCharacter
28、isticSmallOfficeLargeRetail ClassroomRestaurant Dining RoomMotelRoom TheaterFloor area, ft2(m2) 6600 (613) 79,000 (7340) 1000 (93) 5250 (488) 350 (33) 9000 (836)No. floors 1 1 2 1 2 1Ceiling height, ft (m) 10 (3.0) 15 (4.6) 10 (3.0) 10 (3.0) 8 (2.4) 24 (7.3)Percent glass, % 15 15 18 15 21 0Window R-
29、value, hft2F/Btu (m2K/W) 1.6 (0.282) 1.7 (0.299) 1.7 (0.299) 1.5 (0.264) 1.71 (0.301) 1.7 (0.299)Window SC 0.75 0.76 0.73 0.80 0.76 0.76Wall insulation R-value, hft2F/Btu (m2K/W) 5.6 (0.986) 4.8 (0.845) 5.7 (1.004) 4.9 (0.863) 5.32 (0.937) 4.8 (0.845)Roof insulation R-value, hft2F/Btu (m2K/W) 12.6 (
30、2.219) 12.0 (2.113) 13.3 (2.342) 13.2 (2.325) 13.16 (2.318) 12.0 (2.113)Wall material Masonry Masonry Masonry Masonry Masonry MasonryRoof material Built-up Built-up Built-up Built-up Built-up Built-upLighting power, W/ft2(W/m2) 1.7 (18.3) 1.6 (17.2) 1.8 (19.4) 2.1 (22.6) 1.06 (11.4) 1.0 (10.8)Equipm
31、ent power, W/ft2 (W/m2) 0.5 (5.4) 0.4 (4.3) 0.8 (8.6) 0.0 (0.0) 0.69 (7.4) 0.0 (0.0)Table 2. HVAC System CharacteristicsCaseDescriptionNominal AirflowComments/Features cfm/ton L/s/kW0 Conventional DX System 400 53.7 Typical HVAC design practice1 Base DX System 350 47.0 Better dehumidification practi
32、ceDifferent equipment from Case 02 DX System with Improved Dehumidification 300 40.3 Modified coil, compressor, etc3 Base DX System with Lower Airflow 300 40.3 Same coil and compressor as Case 14 DX System with Air-to-Air Heat Exchanger 350 47.0 Wrap around heat exchangerSensible Eff. = 0.4No latent
33、 transferSingle stage coil5 DX System with Subcooling Reheat Coil 350 47.0 Normal mode same as Case 1Enhanced dehumidification modeSwitch modes if 50% RH setpoint not met6 Base DX System with No Latent Degradation 350 47.0 Normal mode same as Case 1Fan off short time when compressor cycles offNo moi
34、sture evaporation from wet coil, modeled by turning off latent degradation in DX coil model7 DX System with Bypass Damper 350 47.0 Normal mode same as Case 1300 cfm/ton in bypass mode50 cfm/ton bypassed as neededSwitch modes if 50% RH setpoint not met8 Hybrid DX System with Desiccant 400 53.7 Desicc
35、ant dehumidifier with sensible heat recoveryDesiccant system conditions outside airMixed air to cooling coilControl to meet 50% RH setpoint9 DX System with Enthalpy Recovery Wheel 350 47.0 Enthalpy heat recovery OA and exhaustBypassed when not beneficial 0.91 sensible effectiveness 0.85 latent effec
36、tiveness10 DX System with Outdoor Air Preconditioning 350 main coil 47.0 main coil Evaporator in OA stream580 DX precond.77.8 DX precond.Condenser in relief air streamRun DX preconditioner coil firstRun main coil as needed11 DX Dual Path System OA system: 300 OA system: 40.3 OA system: 2 DX coils in
37、 series,150 overall 20.1 overall 2 stages each,RA system: 400 RA system: 53.7 7.22C (45F) min supply temperatureRA system: 1 DX coil,1 stage (last stage of cooling) 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transaction
38、s 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. 368 ASHRAE Transactionsanalyzed. In consultation with the RP-1254 Project Monitor-ing Subcommittee, it wa
39、s decided to evaluate the occupantdensities and ventilation rates as specified in ANSI/ASHRAEStandards 62-2001 (ASHRAE 2001b) and 62.1-2004(ASHRAE 2004). Standard 62-2001 is currently referencedby many model building codes. Standard 62.1-2004 is thelatest version of Standard 62 for commercial buildi
40、ngs. Table3 compares the requirements of these two standards for theapplications studied as part of this research project. There aresignificant differences between the two Standards.METHOD OF ANALYSISCooling design loads were calculated based on simulationof the 0.4% dry-bulb design day weather for
41、each location(ASHRAE 2001). The peak sensible cooling load that oc-curred was increased by a 10% oversizing factor. Supply air-flow was then calculated based on each systems rated sensiblecfm/ton as determined from catalog data as summarized inTable 4 below. For dual-path systems, the 100% OA system
42、cooling capacity was determined by taking the OA flow rateand applying the rated sensible cfm/ton and rated SHR. Anyremaining sensible capacity requirement was then applied tothe return air system and the supply airflow rate was thencalculated by applying the rated sensible cfm/ton. For theenthalpy
43、wheel systems, the design sensible capacity wasreduced by the enthalpy wheel sensible capacity at design con-ditions based on the outside airflow rate and dry bulb temper-ature.Using this methodology, a different supply airflow ratewas computed for every system type for every combination ofbuilding
44、and location. The constant across all systems was thenet sensible capacity. For dual-path systems, this was thecombined net sensible capacity of the main (OA) system andthe secondary return air system with a few exceptions. For dualpath systems, the sizing method was to first take the requiredOA flo
45、w rate and size the OA system according to its ratedflow per sensible capacity cfm/Scap (L/s/Scap). The return airunit was sized to meet the remaining sensible capacity12 DX Dual Path System with Enthalpy Recovery OA system: 350 OA system: 47.0 OA system: 1 DX coil2 stages of cooling, enthalpy heat
46、recoveryRA system: 400 RA system: 53.7 OA and exhaust,7.22C (45F) min supply temperatureRA system: 1 DX coil,1 stage (last stage of cooling)13 DX Dual Path System with Air-to-Air Heat ExchangerOA system: 300 OA system: 40.3 OA system: 2 DX coils in series,150 overall 20.1 overall 1 stage each,RA sys
47、tem: 400 RA system: 53.7 AAHX 0.4 sensible effectiveness,No latent transfer,7.22C (45F) min supply temperatureRA system: 1 DX coil, 1 stage (last stage of cooling)14 DX Dual Path System with Desiccant OA system: 350 OA system: 47.0 OA system: 1 DX coil, RA system: 400 RA system: 53.7 2 stages of coo
48、ling, desiccant dehumidifier with sensible heat recovery to exhaust air,desiccant control 50% RH7.22C (45F) min supply temperatureRA system: 1 DX coil, 1 stage (last stage of cooling) 15 DX System with Demand Controlled Ventilation 350 47.0 Case 1 equipmentMin OA tracks occupancy schedule16DX Dual P
49、ath System with Demand Controlled VentilationOA system: 300 OA system: 40.3 Case 11 equipment150 overall 20.1 overall Min OA tracks occupancy scheduleRA system: 400 RA system: 53.7OA system is constant volume, unit processes mixed air when DCV reduces OA flow17 Base DX System with Free Reheat 350 47.0 Case 1 equipmentCondenser heat available for reheatReheat capacity 100% of cool capacity plus compressor powerControlled for sensible and humidityDehumidify only when sensible cooling loadTable 2. HVAC System Characteristics (continued)CaseDescriptionN
