ASHRAE ST-16-005-2016 Simplified Procedure for Calculating Exhaust Intake Separation Distances.pdf

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1、46 2016 ASHRAEThis paper is based on findings resulting from ASHRAE Research Project RP-1635.ABSTRACTThe purpose of this research project is to provide asimple yet accurate procedure for calculating the minimumdistance required between the outlet of an exhaust systemand the outdoor air intake to a v

2、entilation system to avoidreentrainment of exhaust gases. The new procedureaddresses the technical deficiencies in the simplified equa-tions and tables that are currently in ANSI/ASHRAE Stan-dard 62.1-2016, Ventilation for Acceptable Indoor AirQuality (ASHRAE 2016a), and model building codes. Thisne

3、w procedure makes use of the knowledge provided inChapter45ofthe2015ASHRAEHandbookHVACAppli-cations (ASHRAE 2015), and was tested against variousphysical modeling and full-scale studies.Thestudydemonstratedthatthenewmethodismoreaccu-rate than the existing Standard 62.1 equation, which under-predicts

4、 and overpredicts observed dilution more frequentlythanthenewmethod.Inaddition,thenewmethodaccountsforthe following additional important variables: stack height,wind speed, and “hidden” intake. The new method also hastheoretically justified procedures for addressing heatedexhaust, louvered exhaust,

5、capped heated exhaust, and hori-zontal exhaust that is pointed away from the intake.INTRODUCTIONANSI/ASHRAE Standard 62.1-2016 (ASHRAE 2016a)hasairintakeminimumseparationdistancesspecifiedforvari-ous types of exhaust sources in Table 5.5-1 of the standard.Other codes and standards (e.g., Uniform Mec

6、hanical CodeIAPMO2015a,InternationalMechanicalCodeICC2012,Uniform Plumbing Code IAPMO 2015b, and ANSI/ASHRAE Standard 62.2 ASHRAE 2016b) also specifyminimum separation distances, all of which appear to be rule-of-thumb based with 1 to3m(3to10ft)being the magicnumbers for most exhaust types. The sepa

7、ration distances canbe both far too lenient and far too restrictive depending on thetype of exhaust and exhaust and intake configurations.Both code and Standard 62.1 requirements are overlysimplistic and fail to account for significant variables such asthe exhaust airflow rate, the enhanced mixing c

8、aused by highexhaustdischargevelocity,theorientationofthedischarge,orthe height of the exhaust relative to intake. Standard 62.1includes an Informative Appendix F that outlines a procedureto account for exhaust airflow rate, velocity, and exhaustorientation to achieve target dilution levels. The app

9、endix isnotmandatorybutisgivenasanexampleofhowtouseanalyt-ical techniques to show that separation distances other thanthose in Table 5.5-1 are acceptable.Thepurposeofthisresearchprojectistoprovideasimpleyet accurate procedure for calculating the minimum distancerequired between the outlet of an exha

10、ust system and theoutdoor air intake to a ventilation system to avoid reentrain-ment of exhaust gases. The procedure addresses the technicaldeficiencies in the simplified equations and tables currently inStandard 62.1. This new procedure makes use of the knowl-edge provided in Chapter 45 of the 2015

11、 ASHRAE Hand-bookHVAC Applications (ASHRAE 2015) and variouswind tunnel and full-scale studies discussed herein.The new methodology is suitable for standard HVACengineering practice and has exhaust outlet velocity, exhaustairvolumetricflowrate,exhaustoutletconfiguration(capped/Simplified Procedure f

12、or CalculatingExhaust/Intake Separation DistancesRon L. Petersen, PhD Jared RitterFellow ASHRAE Associate Member ASHRAERon L. Petersen is the vice president and Jared Ritter is an engineer at CPP, Inc., Fort Collins, CO.ST-16-005 (RP-1635)Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE T

13、ransactions 47uncapped and position/orientation relative to intake), desireddilution ratio, and ambient wind speed as independent vari-ables. The current Standard 62.1 Informative Appendix Fmethod includes some of these factors but does not includevariable wind speed, stack height, plume rise effect

14、 caused byexhaust velocity, or hidden intake reduction factors. The newmethod discussed herein takes into account all of these vari-ables. The new method also has theoretically justified proce-duresforaddressingheatedexhaust,louveredexhaust,cappedheated exhaust, and horizontal exhaust that is pointe

15、d awayfrom the intake.The research started out with an objective to develop twonew procedures from existing and new research with thefollowing characteristics:Procedure 1. A general procedure suitable for standardHVAC engineering practice that has exhaust outletvelocity, exhaust air volumetric flow

16、rate, exhaust outletconfiguration (capped/uncapped/horizontal/louvered)and position (vertical separation distance), exhaustdirection, desired dilution ratio, hidden intakes (buildingsidewall), and ambient wind speed as independent vari-ables. Other factors, such as location relative to wallsand edge

17、 of building, geometry of the exhaust dischargeand inlets, etc., are reduced to fixed assumptions that arereasonable yet somewhat conservative.Procedure 2. A regulatory procedure suitable for Stan-dard 62.1, Standard 62.2, and model building codes thathas only exhaust outlet velocity, exhaust air vo

18、lumetricflow rate, desired dilution ratio, and a simple way toaccount for orientation relative to the inlet as indepen-dent variables. All other variables are reduced to fixedassumptions that are reasonable yet conservative.In the end, one simple procedure was developed that metthe overall objective

19、s of the study and is appropriate for thefollowing exhaust types:Toilet exhaust from rain-capped vents or dome exhaustfansGrease and other kitchen fan exhaustsCombustion flues and vents with either forced or naturaldraft discharge in horizontal or vertical direction, withand without flue caps (this

20、includes diesel generators)Diesel vehicle emissionsBuilding exhaust at indoor air temperature through lou-vered or hooded ventsPlumbing ventsCooling towersThe method does not address laboratory and industrialventilation process exhausts; large, industrial-sized combus-tion flues and stacks; or packa

21、ged units that have integralexhaust and intake locations.Asecondaryobjectiveofthisprojectistoaddressdilutiontargets, a necessary parameter for calculating the separationdistance calculation. Accordingly, minimum dilution factorswere reviewed and updated for various types of exhausts asappropriate,es

22、peciallythosewithknownemissionsandhealthimpacts such as combustion exhaust. The results of thatresearch are not discussed herein but can be found in theresearchbyPetersenetal.(2015).Table1providesasummaryof the minimum recommended dilution factors from thatresearch.The following sections provide a r

23、eview of the Standard62.1 equation, discussion of databases that were used to testandcomparetheStandard62.1equationandthenewequation,developmentofthenewequation,anevaluationofthenewandStandard 62.1 equations against observations, and a discus-sion of the new methodology.EVALUATION OF EXISTINGSTANDAR

24、D 62.1 EQUATIONThe development of the Standard 62.1 equation can befound in Appendix N of the August 1996 Public Review Draftof ASHRAE Standard 62, which will be referred to as 62-1989R (ASHRAE 1996). The equation development beginswith the minimum dilution equation Dminfound in the 1993ASHRAE Handb

25、ookFundamentals, Chapter 14 (ASHRAE1993) and in the research by Wilson and Lamb (1994):Table 1. Summary of RecommendedMinimum Dilution FactorsExhaust Type Minimum Dilution Factor (DF)Class 1 air exhaust/relief outlet 5Class 2 air exhaust/relief outlet 10Class 3 air exhaust/relief outlet 50Class 4 ai

26、r exhaust/relief outlet 300Wood-burning kitchen exhaust 700General boilers, natural gas andfuel oil, based on NOx*ppmfactor, p in percent*NOx= nitrous oxides (NO and NO2)If the NOxppm is 10, p = 10 and DF = 28.2.8 pGarage entry, automobileloading area, or drive-in queue(light-duty gasoline vehicles)

27、50Diesel generators, diesel truckloading area or dock, diesel busparking/idling areae = 1 efficiency of the odor filter. For example, if the filter is 80% efficient, e = 0.2and DF = 400.2000 eCooling tower exhaust(based on chemicals used fortreatment)10Published in ASHRAE Transactions, Volume 122, P

28、art 2 48 ASHRAE Transactions(1)(2)(3)whereDo= initial jet dilutionDs= dilution that occurs versus separation distances = “stretched string” distance measured along a trajec-toryUH= wind speed at the roof levelVe= discharge velocityQe= volume flow rate = factor that relates the nature of discharge ou

29、tlet; equals1fortheverticaldischargeand0foracapped(or downward) discharge62-1989RstatesthatC1rangesfrom1.6to7and1rangesfrom 0.0625 to 0.25 (C2in 62-1989R).The minimum separation distance is defined as the short-est “stretched string” distance from the closest point of theoutlet opening to the closes

30、t point of the outdoor air intakeopening or operable window, skylight, or door opening alonga trajectory as if a string were stretched between them.To develop the Standard 62.1 equation, Equations 1, 2,and 3 were first rearranged to solve for s (L in the Standard62.1 equation), which results in(4)Th

31、e equation is then simplified by assuming (ASHRAE1996) the following:The 1 term is insignificantVe= 0 for capped or non-vertical stacksUH= 2.5 m/s (500 fpm) average wind speedC1= 1.7 (on the low end of the range, giving less creditfor dilution due to the discharge velocity, which tends toincrease th

32、e separation distance)1= 0.25 (on the high end of the range, giving maxi-mum credit for dilution due to separation, and tends toreduce separation distance, and is non-conservative)Using the above assumptions, the Standard 62.1 equationthen results, or(5)(6)whereQe= exhaust air volume, L/s (cfm)D = d

33、ilution factor for the exhaust type of concernVe= exhaust air discharge velocity, m/s (fpm)Veis positive when the exhaust is directed away from theoutdoor air intake at a direction that is greater than 45 fromthe direction of a line drawn from the closest exhaust point tothe edge of the intake.Vehas

34、 a negative value when the exhaust is directedtoward the intake bounded by lines drawn from the closestexhaust point the edge of the intake.Veis set to 0 for other exhaust air directions regardless ofactualvelocity. Veisalsosetto0forventsfromgravity(atmo-spheric) fuel-fired appliances, plumbing vent

35、s, and othernonpoweredexhausts,oriftheexhaustdischargeiscoveredbya cap or other device that dissipates the exhaust airstream.For hot gas exhausts such as combustion products, aneffective additional 2.5 m/s (500 fpm) upward velocity isadded to the actual discharge velocity if the exhaust stream isaim

36、ed directly upward and unimpeded by devices such as fluecaps or louvers.Equation 4, from which Equations 5 and 6 were devel-oped, has the following problems:The equation is only valid for flush vents and does notaccount for stack height or height difference betweenthe stack and the air intake.Even t

37、hough an exit velocity term is included, it doesnot adequately account for high-velocity exhaust sys-tems. The velocity term accounts for the added dilutiondue to a higher exit velocity but does not account for theadditional plume rise.The assumed value for the constant C1(1.7), while con-servative,

38、 is not supported by the research. According toWilson and Chui (1994) and ASHRAE (1993, 1997),values of 7 and 13 are more appropriate.The assumed value for the constant 1(0.25) is non-conservative and is not supported by the research.According to Wilson and Chui (1994) and ASHRAE(1993, 1997), values

39、 ranging from 0.04 to 0.08 are moreappropriate.For vertical stacks, a wind speed higher than 2.5 m/s(500 fpm) may be critical because plume rise willdecrease as wind speed increases, while at low windspeed the plume rise will be very large. For flush ventsand capped stacks, a wind speed lower than 2

40、.5 m/s (500fpm) will most likely be the critical case. Speeds as lowas 1 m/s (200 fpm) can occur a significant fraction of thetime (Perkins 1974).Setting Veequal to a negative number when the exhaustis directed away from the intake, while intuitively cor-rect, cannot be derived from the original equ

41、ation usedto develop the Standard 62.1 approach.DminDo0.5Ds0.5+2=Do1 C1VeUH-2+=Ds1s2UHQe-=sQe1UH-0.5D0.51 C1VeUH-2+0.5=s 0.04Qe0.5D0.5Ve2- (in metres)=s 0.09Qe0.5D0.5Ve400-(infet)=Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 49ToevaluatetheStandard62.1equation,itneedstobe

42、rear-ranged so dilution can be predicted for comparison with thedilution values recorded in the databases discussed the “Dilu-tion Databases” section. The rearranged equation is providedas follows:(I-P) (7)(SI) (8)Overall,thissectionshowssomeoftheproblemswiththecurrent 62.1 equation and confirms the

43、 need for an improvedequation.DILUTION DATABASESIn order to evaluate the existing Standard 62.1 equationand New4, existing wind tunnel and full-scale data wereassembled and reviewed. Only those wind tunnel databasesthat meet the criteria outlined in the Environmental ProtectionAgencys(EPA)Guidelinef

44、orFluidModelingofAtmosphericDiffusion (Snyder 1981) were used in this study. Some of theimportant criteria considered are as follows: a boundary-layerwindprofilerepresentativeoftheatmospherewasestablished,the approach turbulence profile was representative of theatmosphere, and Reynolds number indepe

45、ndent flow wasestablished.For the relevant databases, data were entered into aMicrosoftExcel spreadsheet in a form that would expeditecomparisons with Standard 62.1 Informative Appendix Fequations and the new method. The following subsectionsdiscuss each database.Database 1Wilson and Chui (1994)The

46、following summarizes the important aspects of thisdatabase:1:500 and 1:2000 scale model tests were conductedBuilding Reynolds numbers exceeded 104 to meet theReynolds number independence criterion by Snyder(1981)A wind power law exponent of 0.25 was established andwind speeds at building heights of

47、5.9 to 12.1 m/s (1200to 2400 fpm) were setEleven model building configurations were tested at sixdifferentexhaustvelocityratios(M = Ve/UH).Exhaustparam-eterswereaflushcircularventwithexhaustdensityratiovary-ing from 0.14 to 0.38. Velocity ratios varied from 0.8 to 1.5.Buildingheighttowidthratiosvari

48、edfrom1to12.WilsonandChui (1994) showed that Equations 1, 2, and 3 with 1= 0.625and C1= 7 provided a lower bound to the observed dilutionvaluesforseveralbuildingconfigurations.Thisdatabaseisnotuseddirectlytoevaluatetheperformanceofthenewequation;rather, the predicted lower bound using Equations 1, 2, and 3with recommended constants are used as a lower boundprediction for comparison purposes.Wilson and Chui (1994) show comparisons of predicted(Equations 1, 2, and 3) and observed dilution versus normal-ized distance.Database 2Wilson and Lamb