1、424 2008 ASHRAE ABSTRACTThe major function of a general exhaust ventilation systemis used to supply enough clean air into the working environ-ments to dilute the air pollutants. Sometimes, local ventilatedhoods are combined with general ventilation systems for somespecific air contaminant control pu
2、rposes, such as electronicindustrial and chemical laboratory environments. The inter-action between general ventilation system and local exhausthood was always found in the kind of working environments.Usually, the airflow pattern consisted by different loca-tions of inlet/outlet diffusers may influ
3、ence the performance oflocal ventilated hoods. Therefore, an experimental study wasconducted to investigate air contaminant removal efficiency bya local ventilated hood installed inside a general ventilationenvironment. The location of inlet diffuser played a more domi-nant role than outlet diffuser
4、 in decision the air contaminantremoval efficiency by a local exhaust hood. No matter wherethe locations of inlet/outlet diffusers were installed, the higherturbulence intensity may result in a lower capture efficiency ofan exhaust hood located inside a general ventilation environ-ment. The experime
5、ntal results are also used to compare withthe computational fluid dynamic model. Referred to the exper-imental data, it seems that the computer model is not able topredict precisely around the high turbulence intensity area.INTRODUCTIONThe primary objective of a ventilation system is toimprove indoo
6、r air quality and remove contaminants in work-ing places. The general ventilation system achieves this bymechanically-driven air motion, where fresh air is delivered tothe occupied zone and contaminants are removed or diluted.Normally, general ventilation system forms simply of anexhaust fan pulling
7、 air out of the workplace and exhausting itto the outdoors. A general ventilation system, which removesair contaminated by gases, vapors or particulates not capturedby local exhausts, usually consists of one or more fans, plusinlets, ductwork and an air cleaner or filters 1. Local exhaustventilation
8、 system implies an attempt to remove the contam-inant at or near the point of release, thus minimizing the oppor-tunity for the contaminant to enter the workplace air. Theability of a local exhaust ventilation system to accomplish thistask depends on its proper design, construction, and operation.Th
9、e nominal local exhaust ventilation system includes anexhaust hood ducting, a fan, and an exhaust outlet. As withgeneral exhaust ventilation, additional components, such asreplacement air systems and air-cleaning devices, may beincluded. On the other hand, local exhaust ventilation systemsare used i
10、n a wide variety of settings, from research laboratoryhoods to commercial kitchens to foundries. Local exhaustventilation systems can be used in the vast majority of situa-tions in preference over general ventilation 2. Local exhaustventilation is widely used to remove contaminants at the pointof ge
11、neration and thus prevent contaminants from entering theworkers breathing zone. Recently, local supply air also hasbeen successfully used in combination with local exhaust tofurther reduce the exposure by providing a region of clean airaround the workers 3-5. However, many industrial ventila-tion sy
12、stems must handle simultaneous exposures to heat andhazardous substances. In these cases, the required ventilationcan be provided by a combination of local exhaust, generalventilation air supply, and general exhaust systems. Someuseful design guidelines were provided by ASHRAE 6.Many researchers 7,8
13、 have reported that both airflowrate and diffusers location are important design parametersPerformance of Local Ventilated Hood in a General Ventilation Working EnvironmentChung Kee-Chiang, PhD Tsai Kuo-Pao Wang You-HsuanAssociate Member ASHRAEChung Kee-Chiang is a professor in the Department of Mec
14、hanical Engineering, and Tsai Kuo-Pao is a doctoral student at the GraduateSchool of Engineering Science and Technology, National Yunlin University of Science and Technology, Yunlin, Taiwan. Wang You-Hsuanis an IRSD Researcher of the Architecture and Building Research Institute Ministry of the Inter
15、ior, Taiwan.NY-08-0522008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.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 perm
16、itted without ASHRAEs prior written permission.ASHRAE Transactions 425for obtaining higher general ventilation efficiency. Khan et al.9 examined both computationally and experimentallyseveral different inlet and exhaust locations and types to deter-mine optimum inlet and exhaust position for lower i
17、ndoorcontaminant concentration. They suggested that the exhaustslocated near ceiling resulted in lower concentrations than thecorresponding exhausts near the floor for wall jet inlet. Also,the exhausts located on the same wall as the inlet were rela-tively better than that of the opposite side wall.
18、 A comprehen-sive airflow pattern measurement was provided byBuggenhout et al. 10 using new airflow pattern sensorconcept. The characteristics of airflow in the occupied zone ofmechanically ventilated air spaces have been studied in build-ings designed for humans. The airflow pattern will influencet
19、he airflow trajectory in a ventilated room, Therefore, the airtemperature, air velocity and air contaminant distributionsdepended on the configuration of the inlet and outlet diffusersin a ventilated space. The ventilation efficiency of differentventilation patterns arranged by two inlet and two out
20、letdiffusers at different locations was investigated by Chung andHsu 11. They applied the tracer gas techniques to the exper-imental program; a concentration decay of CO2was used tocalculate the ventilation efficiency and air change rate of thetest chamber. The results indicated that the air exchang
21、e ratewas influenced greatly by the air supply volume and is insen-sitive to the locations of inlet/outlet diffusers. On the contrary,the location of inlet and outlet diffusers severely affects venti-lation efficiency in a general ventilation environment.However, no effects of a local exhaust hood w
22、ere investigatedin the report. Transport of airborne contaminants and exposurepotential depends on the air movements near the worker andexhaust openings. Therefore, it is essential to understand theairflow and contaminant distribution around the local exhausthood for evaluating the performance of a
23、local exhaust hood.Nevertheless, such design information and technique reportsare very limits. In order to evaluate the performance of localexhaust ventilation systems, an experimental study wasconducted in this paper. Also, CFD model predictions werethen compared with the experimental measurements,
24、 and onthe basis of the results the optimum control configurationsmay be determined.CAPTURE EFFICIENCY OF HOODIn order to analyze the transport properties of the contin-uum flow in a working environment, one of the most effectiveand straightforward methods is to analyze the hood captureefficiency. I
25、t should be relevant for determining the degree towhich fresh air is dispersed, recirculated and mixing within aventilated workplace, as well as for determining how ventilat-ing air interacts with pollutants. Local hood design plays a key role in the exhaust ofcontaminants dispersed around the conta
26、minant souse. Theconcepts of hood capture efficiency provide a useful tool ofevaluating the performance of an exhaust hood system in aworking environment. Usually, capture efficiency is defined asthe fraction of contaminant generated that is captured directlyby the hood. A number of different measur
27、es for capture effi-ciency have been proposed to provide a basis for local exhaustventilation system design during last few years. Peng et al.12 have shown capture efficiency to be a function of hood airflow, hood area, distance from the hood, crossdraft velocity,and source temperature. Several rese
28、archers have conducted empirical studies ofcapture efficiency. Flynn and Ellenbecker 13 developed amodel for capture efficiency for flanged circular hoods whichcombined their velocity field model with an empirically deter-mined spread parameter. Jankovic et al. 14 recognized thatvelocity is not the
29、only determinant of exhaust effectiveness.Using dimensional analysis, capture efficiency, being dimen-sionless, must be equated with some other non-dimensionalexpression. Some physical quantity or quantities other than airvelocity must be involved. Roach 15 also measured captureefficiency for a 100
30、mm diameter opening and perpendicularto the hood centerline. Diffusivities were calculated for thecase where the crossdraft was directed toward the hood open-ing. For the case of crossdraft perpendicular to the centerline,capture efficiency was plotted versus crossdraft velocity for aconstant releas
31、e point. Using steady-state condition, the rela-tive capture efficiency for a specific hood can be defined as: (1)where= the hood capture efficiencyCi= contaminant concentration in hoodCb= the background contaminant concentrationC100%= the 100% contaminant concentration value of the source emission
32、The relative efficiency of a local exhaust hood based onmeasured concentration levels in the test chamber and insidethe exhaust hood. The CO2gas was used as tracer gas in allexperiments. The background CO2concentration level is 350ppm approximately at the inlet diffuser. The indoor CO2concentration
33、was obtained using the average value from sixdifferent locations of CO2sensors.COMPUTATIONAL MODEL In order to assistance the industrial ventilation engineersto design an appropriate local ventilation system under theinteraction with general air supply system, a model analysiswas undertaken to compa
34、re with the experimental results. Athree-dimensional turbulence numerical package adopted inthis paper is a finite difference code developed by EngineeringSciences Inc. 16. The program solves the conservation equa-tions for continuity, momentum, and energy as well as theequations for turbulent kinet
35、ic energy and its dissipation rate.The basic equations of the code are the curvilinear-trans-formed multi-component conservation equations. The high-CiCbC100%Cb-x100%=426 ASHRAE TransactionsRenolds-number -turbulence closure is used in the program inconjunction with empirical wall functions. A gener
36、alized formof these equations based on eddy viscosity turbulence mixingconcept can be written as:(2)whereJ=Jacobian of coordinate transformationUi= volume-weighted contravariant velocitiesGif= diffusion metricsSq= the source term of the general fluid property The variable q is equal to 1, u, v, w, h
37、, k, and c for the conti-nuity, momentum, energy, turbulence model, and chemicalspecies transport equations, respectively. A pressure relax-ation method is used to satisfy the Poisson equation for massconservation. The dimensions used in numerical processes aresame as the full-scale experimental cha
38、mber. The calculatedgrid points depend on the geometry of inlet/outlet arrange-ment. The CO2gas was treated as an indoor contaminant andno chemical reaction between CO2and air was assumed. Thecalculation time step is one second. The third-order upwindnumerical algorithm was chosen in the computation
39、 andupwind damping parameter was set as 0.1.Total of calculated grid points are 16926 (21 x 26 x 31).Such calculated grid points exhibit sufficient convergentcondition for a 4 m x 3 m x 2.5 m small space. An isothermalboundary condition is used both for computing and testingcases. The velocity bound
40、ary condition near a wall is based ona power-law assumption of the velocity profile. The first realvelocity point, parallel to a wall surface is a reasonableassumption. The adjacent dummy node velocity is selectedbased on a one-sided finite difference approximation. If a one-seven power law is assum
41、ed, this fraction is 0.714. the bound-ary condition for assumes zero gradient at the wall, setting thedummy node value for to the real node value of. The conditionforis based on Prandtls mixing length hypothesis, thus settingboth the first real and dummy nodes to the following equation: (3)EXPERIMEN
42、TAL PROGRAMA full-scale experimental program was conducted in thisinvestigation to understand the performance of local ventilatedhoods in a general ventilation environment. A controlled envi-ronment chamber with dimensions of 4 m (length) x 3 m (width)x 2.5 m (height) was designed for the test. Thre
43、e different airchange rates (ACH) coupled with 8 different inlet/outlet diffus-ers arrangements are adopted for test. Capital A and B charac-teristics represent different locations of inlet diffusers. Numeralnumbers 1 to 4 indicates different locations of outlet diffusers. General Ventilation System
44、Two different locations for inlet diffusers (index as A andB) and four different locations for outlet diffusers (index as 1,2, 3 and 4) are chosen for creating different flow patterns. Eachdiffuser had a surface area of 0.4 m x 0.4 m. Total of eightdifferent combinations was arranged for test. Compl
45、ete sche-matic diagram for the inlet/outlet diffusers arrangement isshown in Figure 1. Three different power supply and volumet-ric fans were required for achieving the experimental goads.Each fan can deliver 110 to 3000 ft3/min airflow, correspond-ing to about 2 to 52 air changes per hour. Local Ex
46、haust HoodThe local ventilation system consists of a hood, appropri-ate ducts and a variable speed fan. The hood was installed uponthe ceiling located at 103 cm from the ceiling and 90 cm fromthe south wall. A hotwire thermal anemometer was installedinside the duct 25 cm away from the ceiling. In or
47、der tomeasure stable CO2concentration inside the duct, the CO2gascenser was installed into 45 cm of the latter portion of the duct.The entire test facility was depicted in Figure 1.Tracer Gas MeasurementThe tracer gas measurements were used to study captureefficiency of a local hood under different
48、indoor airflowpatterns. In the test, a CO2ejector was built referred toASHRAE 110 17 and installed at 35 cm under the openingof hood. The constant CO2 gas flow, 2.65 l/min was releasedat 150 mmaq gage pressure. The whole injection set wasshown in Figure 2. The measurement range of CO2 gas moni-tors
49、designed from zero to 5000 PPM and the operationaltemperature was ranged from 5 to 45C.The whole contaminant emission concentration (100%contaminant capture) was measured first to evaluate the effectof contaminant from source captured by hood. In the experi-mental program, the supply and exhaust fans are turn off forminimizing the general ventilation effects. Consequently, thelocal exhaust fan which connected to a contaminant capturehood provided 63.8 m3/hr air volume to exhaust the CO2released from CO2ejector. In order to
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