ASHRAE HVAC APPLICATIONS IP CH 45-2015 BUILDING AIR INTAKE AND EXHAUST DESIGN.pdf

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1、45.1CHAPTER 45BUILDING AIR INTAKE AND EXHAUST DESIGNExhaust Stack and Air Intake Design Strategies. 45.1Geometric Method for Estimating Stack Height . 45.5Exhaust-to-Intake Dilution or Concentration Calculations 45.7Other Considerations 45.10RESH air enters a building through its air intake. Likewis

2、e,Fbuilding exhausts remove air contaminants from a building sowind can dilute the emissions. If the intake or exhaust system is notwell designed, contaminants from nearby outside sources (e.g., vehi-cle exhaust, emergency generator, laboratory fume hoods on nearbybuildings) or from the building its

3、elf (e.g., laboratory fume hoodexhaust) can enter the building with insufficient dilution. Poorlydiluted contaminants may cause odors, health impacts, and reducedindoor air quality. This chapter discusses proper design of exhauststacks and placement of air intakes to avoid adverse air qualityimpacts

4、. Chapter 24 of the 2013 ASHRAE HandbookFundamen-tals more fully describes wind and airflow patterns around buildings.Related information can also be found in Chapters 8, 17, 32, 33, and34 of this volume, Chapters 11 and 12 of the 2013 ASHRAE Hand-bookFundamentals, and Chapters 29, 30, and 35 of the

5、 2012ASHRAE HandbookHVAC Systems and Equipment.1. EXHAUST STACK AND AIR INTAKE DESIGN STRATEGIESStack Design StrategiesThe dilution a stack exhaust can provide is limited by the disper-sion capability of the atmosphere. Before discharge, exhaust con-tamination can be reduced by filters, collectors,

6、and scrubbers ifneeded to maintain acceptable air quality. The ultimate goal of thestack design is to specify the lowest flow, exhaust velocity, and stackheight that ensures acceptable air quality at all locations of concern.This also ensures that energy consumption is minimized.Central exhausts tha

7、t combine flows from many collecting sta-tions should always be used where safe and practical. By combiningseveral exhaust streams, central systems dilute intermittent bursts ofcontamination from a single station. Also, the combined flow formsan exhaust plume that rises a greater distance above the

8、emittingbuilding. If necessary for air quality or architectural reasons, addi-tional air volume can be added to the exhaust near the exit with amakeup air unit to increase initial dilution and exhaust plume rise.This added air volume does not need heating or cooling, and theadditional energy cost is

9、 lower. A small increase in stack height mayalso achieve the same benefit but without any added energy cost.In some cases, separate exhaust systems are mandatory. Thenature of the contaminants to be combined, recommended industrialhygiene practice, and applicable safety codes need to be considered.S

10、eparate exhaust stacks could be grouped in a tight cluster to takeadvantage of the larger plume rise of the resulting combined jet. Also,a single stack location for a central exhaust system or a tight clusterof stacks allows building air intakes to be positioned as far as possiblefrom the exhaust. P

11、etersen and Reifschneider (2008) provide guide-lines on optimum arrangements for ganged stacks. In general, for atight cluster to be considered as a single stack (i.e., to add stackmomentums together) in dilution calculations, the stacks must beuncapped and nearly be touching the middle stack of the

12、 group.As shown in Figure 1, stack height hsis measured above the rooflevel on which the air intake is located. Wilson and Winkel (1982)demonstrated that stacks terminating below the level of adjacent wallsand architectural enclosures frequently do not effectively reduce roof-level exhaust contamina

13、tion. To take full advantage of their height,stacks should be located on the highest roof of a building.Architectural screens used to mask rooftop equipment adverselyaffect exhaust dilution, depending on porosity, relative height, anddistance from the stack. Petersen et al. (1999) found that exhaust

14、 dis-persion improves with increased screen porosity.Large buildings, structures, and terrain close to the emitting build-ing can adversely affect stack exhaust dilution, because the emittingbuilding can be within the recirculation flow zones downwind ofthese nearby flow obstacles (Wilson et al. 199

15、8a). In addition, an airintake located on a nearby taller building can be contaminated byexhausts from the shorter building. Wherever possible, facilitiesemitting toxic or highly odorous contaminants should not be locatednear taller buildings or at the base of steep terrain.As shown in Figure 2, sta

16、cks should be vertically directed anduncapped. Stack caps that deflect the exhaust jet have a detrimentaleffect on exhaust plume rise. Small conical stack caps often do notcompletely exclude rain, because rain does not usually fall straightdown; periods of heavy rainfall are often accompanied by hig

17、h windsthat deflect raindrops under the cap and into the stack (Changnon1966). A stack exhaust velocity Veof about 2500 fpm prevents con-densed moisture from draining down the stack and keeps rain fromentering the stack. For intermittently operated systems, protectionfrom rain and snow should be pro

18、vided by stack drains, as shown inFigure 2F to 2J, rather than stack caps.Recommended Stack Exhaust VelocityHigh stack exhaust velocity and temperatures increase plume rise,which tends to reduce intake contamination. Exhaust velocity Veshould be maintained above 2000 fpm (even with drains in the sta

19、ck)to provide adequate plume rise and jet dilution. Velocities above2000 fpm provide still more plume rise and dilution, but above 3000to 4000 fpm, noise, vibration, and energy costs can become an impor-tant concern. An exit nozzle (Figure 2B) can be used to increaseThe preparation of this chapter i

20、s assigned to TC 4.3, Ventilation Require-ments and Infiltration.Fig. 1 Flow Recirculation Regions and Exhaust Parameters(Wilson 1982)45.2 2015 ASHRAE HandbookHVAC Applicationsexhaust velocity and plume rise. Many laboratory fume hood sys-tems use variable-volume fans that reduce flow from hoods whe

21、nthey are closed. Stack exhaust velocity calculation must be based onthe minimum total flow rate from the system, not the maximum.Exceptions to these exhaust velocity recommendations includewhen corrosive condensate droplets are discharged. In this case, avelocity of 1000 fpm in the stack and a cond

22、ensate drain are recom-mended to reduce droplet emission. At this low exhaust velocity, ataller stack may be needed to counteract downwash caused by lowexit velocity. Another exception is when a detailed dispersion mod-eling analysis is conducted. Such an analysis can determine the min-imum exit vel

23、ocity needed to maintain acceptable dilution versusstack height. Generally, the taller the stack, the lower the requiredexit velocity and hence fan energy.Stack wake downwash occurs where low-velocity exhausts arepulled downward by negative pressures immediately downwind ofthe stack, as shown in Fig

24、ure 3. Veshould be at least 1.5 times thedesign speed UHat roof height in the approach wind to avoid stackwake downwash. A meteorological station design wind speed Umetthat is exceeded less than 1% of the time can be used. This value canbe obtained from Chapter 14 of the 2013 ASHRAE HandbookFun-dame

25、ntals, or estimated by applying Table 2 of Chapter 24 of thatvolume to annual average wind speed. Because wind speed increaseswith height, a correction for roof height should be applied for build-ings significantly higher than 30 ft, using Equation (4) and Table 1 ofChapter 24 of the 2013 ASHRAE Han

26、dbookFundamentals.Other Stack Design StandardsMinimum heights for chimneys and other flues are discussed inthe International Building Code (ICC 2006). For laboratory fumehood exhausts, American Industrial Hygiene Association (AIHA)Standard Z9.5 recommends a minimum stack height of 10 ft abovethe adj

27、acent roof line, an exhaust velocity Veof 3000 fpm, and astack height extending one stack diameter above any architecturalscreen; National Fire Protection Association (NFPA) Standard 45specifies a minimum stack height of 10 ft to protect rooftop work-ers. Toxic chemical emissions may also be regulat

28、ed by federal,state, and local air quality agencies.Contamination SourcesSome contamination sources that need consideration in stack andintake design include the following.Toxic Stack Exhausts. Boilers, emergency generators, and lab-oratory fume hoods are some sources that can seriously affect build

29、-ing indoor air quality because of toxic air pollutants. These sources,especially diesel-fueled emergency generators, can also producestrong odors that may require administrative measures, such as gen-erator testing during low building occupancy or temporarily closingthe intakes.Automobile and Truck

30、 Traffic. Heavily traveled roads andparking garages emit carbon monoxide, dust, and other pollutants.Diesel trucks and ambulances are common sources of odor com-plaints (Smeaton et al. 1991). Intakes near vehicle loading zonesshould be avoided. Overhead canopies on vehicle docks do not pre-vent hot

31、vehicle exhaust from rising to intakes above the canopy.When the loading zone is in the flow recirculation region downwindfrom the building, vehicle exhaust may spread upwind over largesections of the building surface (Ratcliff et al. 1994). Garbage con-tainers may also be a source of odors, and gar

32、bage trucks may emitdiesel exhaust with strong odors.Kitchen Cooking Hoods. Kitchen exhaust can be a source ofodors and cause plugging and corrosion of heat exchangers. Greasehoods have stronger odors than other general kitchen exhausts.Grease and odor removal equipment beyond that for code require-

33、ments may be needed if air intakes cannot be placed far away.Evaporative Cooling Towers. Outbreaks of Legionnaires dis-ease have been linked to bacteria in cooling tower drift dropletsbeing drawn into the building through air intakes (Puckorius 1999).ASHRAE Guideline 12 gives advice on cooling tower

34、 maintenancefor minimizing the risk of Legionnaires disease, and suggests keep-ing cooling towers as far away as possible from intakes, operablewindows, and outdoor public areas. No specific minimum separa-tion distance is provided or available. Prevailing wind directionsshould also be considered to

35、 minimize risk. Evaporative coolingtowers can have several other effects: water vapor can increase air-conditioning loads, condensing and freezing water vapor can dam-age equipment, and ice can block intake grilles and filters. Chemi-cals added to retard scaling and biological contamination may beem

36、itted from the cooling tower, creating odors or health effects, asdiscussed by Vanderheyden and Schuyler (1994).Fig. 2 Stack Designs Providing Vertical Discharge and Rain ProtectionFig. 3 Reduction of Effective Stack Height by Stack Wake DownwashBuilding Air Intake and Exhaust Design 45.3Building Ge

37、neral Exhaust Air. General indoor air that is ex-hausted will normally contain elevated concentrations of carbondioxide, dust, copier toner, off-gassing from materials, cleaningagents, and body odors. General exhaust air should not be allowedto reenter the building without sufficient dilution.Stagna

38、nt Water Bodies, Snow, and Leaves. Stagnant waterbodies can be sources of objectionable odors and potentially harm-ful organisms. Poor drainage should be avoided on the roof orground near the intake. Restricted airflow from snow drifts, fallenleaves, and other debris can be avoided in the design sta

39、ge with ele-vated louvers above ground or roof level.Rain and Fog. Direct intake of rain and fog can increase growthof microorganisms in the building. AMCA (2009) recommendsselecting louvers and grilles with low rain penetration and installingdrains just inside the louvers and grilles. In locations

40、with chronicfog, some outdoor air treatment is recommended. One approach isto recirculate some part of the indoor air to evaporate entrainedwater droplets, even during full air-side economizer operation(maximum outdoor air use).Environmental Tobacco Smoke. Outdoor air intakes should notbe placed clo

41、se to outdoor smoking areas.Plumbing Vents. Codes frequently require a minimum distancebetween plumbing vents and intakes to avoid odors.Smoke from Fires. Smoke from fires is a significant safety haz-ard because of its direct health effects and from reduced visibilityduring evacuation. NFPA Standard

42、 92A discusses the need for goodair intake placement relative to smoke exhaust points.Construction. Construction dust and equipment exhaust can bea significant nuisance over a long period. Temporary precondition-ing of outdoor air is necessary in such situations, but is rarely pro-vided. A simple so

43、lution is to provide room and access to theoutdoor air duct for adding temporary air treatment filters or otherdevices, or a sufficient length of duct so that such equipment couldbe added when needed. Intake louvers and outdoor air ducts alsorequire more frequent inspections and cleaning when constr

44、uctionoccurs nearby.Vandalism and Terrorism. Acts of vandalism and terrorism areof increasing concern. Louvers and grilles are potential points ofillegal access to buildings, so their placement and construction areimportant. Intentional introduction of offensive or potentially harm-ful gaseous subst

45、ances is also of concern. Some prudent initialdesign considerations might be elevating grilles and louvers awayfrom easy pedestrian access and specifying security bars and otherdevices. Also, unlocked stair tower doors required for roof accessduring emergency evacuations may limit use of rooftop air

46、 intakesin sensitive applications because individuals would have readyaccess to the louvers. For more information, see ASHRAEs (2003)Risk Management Guidance for Health, Safety, and EnvironmentalSecurity under Extraordinary Incidents.General Guidance on Intake PlacementCarefully placed outdoor air i

47、ntakes can reduce stack heightrequirements and help maintain acceptable indoor air quality. Rockand Moylan (1999) review recent literature on air intake locationsand design. Petersen and LeCompte (2002) also showed the benefitof placing air intakes on building sidewalls. ASHRAE Standard62.1-2010 hig

48、hlights the need to locate makeup air inlets andexhaust outlets to avoid contamination.Experience provides some general guidelines on air intake place-ment. Unless the appropriate dispersion modeling analysis is con-ducted, intakes should never be located in the same architecturalscreen enclosure as

49、 contaminated exhaust outlets. This is especiallythe case for low-momentum or capped exhausts (which tend to betrapped in the wind recirculation zone within the screen). For moreinformation, see the section on Influence of Architectural Screens onExhaust Dilution.If exhaust is discharged from several locations on a roof, intakesshould be sited to minimize contamination. Typically, this meansmaximizing separation distance. Where all exhausts of concern areemitted from a single, relatively tall stack or tight cluster of stacks,a possible intake location might be close to the base of th