ASHRAE FUNDAMENTALS SI CH 20-2013 Space Air Diffusion.pdf

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1、20.1CHAPTER 20SPACE AIR DIFFUSIONIndoor Air Quality and Sustainability. 20.1Applicable Standards and Codes 20.1Terminology 20.2Principles of Jet Behavior. 20.3Symbols . 20.7OOM air distribution systems are intended to provide thermalR comfort and ventilation for space occupants and processes.Althoug

2、h air terminals (inlets and outlets), terminal units, localducts, and rooms themselves may affect room air diffusion, thischapter addresses only air terminals and their direct effect on occu-pant comfort. This chapter is intended to present HVAC designersthe fundamental characteristics of air distri

3、bution devices. For infor-mation on naturally ventilated spaces, see Chapter 16. For a discus-sion of various air distribution strategies, tools, and guidelines fordesign and application, see Chapter 57 in the 2011 ASHRAE Hand-bookHVAC Applications. Chapter 20 in the 2012 ASHRAE Hand-bookHVAC System

4、s and Equipment provides descriptions of thecharacteristics of various air terminals (inlets and outlets) and termi-nal units, as well as selection tools and guidelines.Room air diffusion methods can be classified as one of the fol-lowing as shown in Figure 1: Mixed systems produce little or no ther

5、mal stratification of airwithin the space. Overhead air distribution is an example of thistype of system.Fully (thermally) stratified systems produce little or no mixingof air within the occupied space. Thermal displacement ventila-tion is an example of this type of system.Partially mixed systems pr

6、ovide some mixing within the occupiedand/or process space while creating stratified conditions in the vol-ume above. Most underfloor air distribution and task/ambient con-ditioning designs are examples of this type of system.Task/ambient conditioning systems focus on conditioning onlya certain porti

7、on of the space for thermal comfort and/or processcontrol. Examples of task/ambient systems are personally con-trolled desk outlets (sometimes referred to as personal ventilationsystems) and spot-conditioning systems.As shown in Figure 1, local temperature and carbon dioxide(CO2) concentration have

8、similar profiles, although their rates usu-ally differ.Air distribution systems, such as thermal displacement ventila-tion (TDV) and underfloor air distribution (UFAD), that deliver airin cooling mode at or near floor level and return air at or near ceilinglevel produce varying amounts of room air s

9、tratification. For floor-level supply, thermal plumes that develop over heat sources in theroom play a major role in driving overall floor-to-ceiling air motion.The amount of stratification in the room is primarily determined bythe balance between total room airflow and heat load. In practice, theac

10、tual temperature and concentration profile depends on the com-bined effects of various factors, but is largely driven by the charac-teristics of the room supply airflow and heat load configuration.For room supply airflow, the major factors areTotal room supply airflow quantityRoom supply air tempera

11、tureDiffuser typeDiffuser throw height (or outlet velocity); this is associated withthe amount of mixing provided by a floor diffuser (or room con-ditions near a low-sidewall TDV diffuser)For room heat loads, the major factors areMagnitude and number of loads in spaceLoad type (point or distributed

12、source)Elevation of load (e.g., overhead lighting, person standing onfloor, floor-to-ceiling glazing)Radiative/convective splitFor pollutant concentration profiles, whether pollutants are asso-ciated with heat sourcesINDOOR AIR QUALITY AND SUSTAINABILITYAir diffusion methods affect not only indoor a

13、ir quality (IAQ) andthermal comfort, but also energy consumption over the buildingslife. Choices made early in the design process are important. The U.S.Green Building Councils (USGBC 2009) Leadership in Energy andEnvironmental Design (LEED) rating system, which was originallycreated in response to

14、indoor air quality concerns, now includes pre-requisites and credits for increasing ventilation effectiveness andimproving thermal comfort. These requirements and optional pointsare relatively easy to achieve if good room air diffusion design prin-ciples, methods, and standards are followed (see Cha

15、pter 57 of the2011 ASHRAE HandbookHVAC Applications).Air change effectiveness is affected directly by the room air dis-tribution systems design, construction, and operation, but is verydifficult to predict. Many attempts have been made to quantify airchange effectiveness, including ASHRAE Standard 1

16、29. However,this standard is only for experimental tests in well-controlled labo-ratories, and should not be applied directly to real buildings.ANSI/ASHRAE Standard 62.1-2010 provides a table of typicalvalues to help predict zone air distribution effectiveness. For exam-ple, well-designed ceiling-ba

17、sed air distribution systems producenear-perfect air mixing in cooling mode, and yield an air changeeffectiveness of 1.0.Displacement and underfloor air distribution (UFAD) systemshave the potential for values greater than 1.0. More information onceiling- and wall-mounted air inlets and outlets can

18、be found in Rockand Zhu (2002). Displacement system performance is described inChen and Glicksman (2003). Bauman and Daly (2003) discussUFAD in detail. (These three ASHRAE books were produced by re-search projects sponsored by Technical Committee 5.3.) More infor-mation on ANSI/ASHRAE Standard 62.1-

19、2010 is available in itsusers manual (ASHRAE 2010).APPLICABLE STANDARDS AND CODESThe following standards and codes should be reviewed whenapplying various room air diffusion methods:The preparation of this chapter is assigned to TC 5.3, Room Air Distribu-tion.20.2 2013 ASHRAE HandbookFundamentals (S

20、I)ASHRAE Standard 55 specifies the combination of indoor ther-mal environmental factors and personal factors that produce ther-mal acceptability to a majority of space occupants.ASHRAE Standard 62.1 establishes ventilation requirements foracceptable indoor environmental quality. This standard is ado

21、ptedas part of many building codes.ASHRAE Standard 70 is a method of test for performance of airoutlets and inlets.ASHRAE/IES Standard 90.1 provides energy efficiency require-ments that affect supply air characteristics.ASHRAE Standard 113 defines a repeatable method of testingsteady-state air diffu

22、sion performance of an air distribution sys-tem in occupied zones of buildings. This method is based on airvelocity and air temperature distributions at specified heating orcooling loads and operating conditions.ASHRAE Standard 129 specifies a method for measuring air-change effectiveness in mechani

23、cally ventilated spaces. Thisstandard is only for experimental tests in well-controlled labora-tories, and should not be applied directly to real buildings.ASHRAE Standard 170 defines ventilation system designrequirements that provide environmental control for comfort,asepsis, and odor in health car

24、e facilities.Local codes should also be checked to see how they apply to eachof these subjects.TERMINOLOGYAspect ratio. Ratio of length to width of opening or core of agrille.Attached jet. A supply air jet affected by surfaces because of theCoanda effect.Axial jet. A supply air jet with a conical di

25、scharge profile.Coanda effect. Effect of a moving jet attaching to a parallel sur-face because of negative pressure developed between jet and sur-face.Coefficient of discharge. Ratio of area at vena contracta to areaof opening.Core area. Area of a register, grille, or linear slot pertaining tothe fr

26、ame or border, whichever is less.Diffusion. Dispersion of air within a space.Distribution. Moving air to or in a space by an outlet discharg-ing supply air.Draft. Undesired local cooling of a person caused by air move-ment.Drop. Vertical distance that the lower edge of a horizontally pro-jected airs

27、tream descends between the outlet and the end of itsthrow.Effective area. Net area of an outlet or inlet device throughwhich air can pass; equal to the free area times the coefficient of dis-charge.Entrainment. Movement of space air into the jet caused by theairstream discharged from the outlet.Entr

28、ainment ratio. Volume flow rate of total air (primary plusentrained air) divided by the volume flow rate of primary air at agiven distance from the outlet.Free area. Total minimum area of openings in an air outlet orinlet through which air can pass.Free jet. An air jet not obstructed or affected by

29、walls, ceiling, orother surfaces.Induction. Movement of space air into air outlet device.Induction ratio. Volume flow rate of induced air divided by vol-ume flow rate of primary air.Inlet. A device that allows air to exit the space (e.g., grilles, reg-isters, diffusers)Isothermal jet. A jet in which

30、 supply air temperature equals sur-rounding room air temperature.Linear jet. A supply air jet with a relatively high aspect ratio.Neck area. Nominal area of duct connection to air outlet or inlet.Nonisothermal jet. A jet in which supply air temperature doesnot equal surrounding room air temperature.

31、Occupied zone. The volume of space intended to be comfortconditioned for occupants (see ANSI/ASHRAE Standard 55).Outlet. A device discharging air into the space (e.g., grilles, reg-isters, diffusers). Classified according to location and type of dis-charge.Outlet velocity. Average velocity of air em

32、erging from outlet,measured in plane of opening.Primary air. Air delivered to an outlet by a supply duct.Radial jet. A supply air jet that discharges 360 and expands uni-formly.Spread. Divergence of airstream in horizontal and/or verticalplane after it leaves an outlet.Fig. 1 Classification of Air D

33、iffusion MethodsSpace Air Diffusion 20.3Stratification height. Vertical distance from floor to horizontalplane that defines lower boundary of upper mixed zone (in a fullystratified or partially mixed system).Stratified zone. Zone in which air movement is entirely drivenby buoyancy caused by convecti

34、ve heat sources. Typically found infully stratified or partially mixed systems.Terminal velocity. Maximum airstream velocity at end of throw.Throw. Horizontal or vertical axial distance an airstream travelsafter leaving an air outlet before maximum stream velocity isreduced to a specified terminal v

35、elocity (e.g., 0.25, 0.5, 0.75, or1.0 m/s), defined by ASHRAE Standard 70.Total air. Mixture of discharged and entrained air.Vena contracta. Smallest cross-sectional area of a fluid streamleaving an orifice.PRINCIPLES OF JET BEHAVIORAir Jet FundamentalsAir supplied to rooms through various types of

36、outlets can be dis-tributed by turbulent air jets (mixed and partially mixed systems) orin a low-velocity, unidirectional manner (stratified systems). The airjet discharged from an outlet is the primary factor affecting room airmotion. The jet boundary contours are not well defined, are billowyand e

37、asily affected by external influences. Baturin (1972), Chris-tianson (1989), and Murakami (1992) have further information onthe relationship between the air jet and occupied zone.If an air jet is not obstructed or affected by walls, ceiling, or othersurfaces, it is considered a free jet. When outlet

38、 area is small com-pared to the dimensions of the space normal to the jet, the jet may beconsidered free as long asX 1.5 (1)whereX = distance from face of outlet, mAR= cross-sectional area of confined space normal to jet, m2Characteristics of the air jet in a room might be influenced byreverse flows

39、 created by the same jet entraining ambient air. If thesupply air temperature is equal to the ambient room air temperature,the air jet is called an isothermal jet. A jet with an initial tempera-ture different from the ambient air temperature is called a non-isothermal jet. The air temperature differ

40、ential between suppliedand ambient room air generates thermal forces (buoyancy) in jets,affecting the jets (1) trajectory, (2) location at which it attaches toand separates from the ceiling/floor, and (3) throw. The significanceof these effects depends on the ratio between the thermal buoyancyof the

41、 air and jet momentum.Jet Expansion Zones. The full length of an air jet, in terms of themaximum or centerline velocity and temperature differential at thecross section, can be divided into four zones:Zone 1 is a short core zone extending from the outlet face, inwhich the maximum velocity and temper

42、ature of the airstreamremains practically unchanged.Zone 2 is a transition zone, with its length determined by the typeof outlet, aspect ratio of the outlet, initial airflow turbulence, etc.Zone 3 is of major engineering importance because, in mostcases, the jet enters the occupied area in this zone

43、. Turbulent flowis fully established and may be 25 to 100 equivalent air outletdiameters (i.e., widths of slot air diffusers) long. The angle ofdivergence is well defined. Typically, free air jets diverge at a con-stant angle, usually ranging from 20 to 24, with an average of22. Coalescing jets for

44、closely spaced multiple outlets expand atsmaller angles, averaging 18, and jets discharging into relativelysmall spaces show even smaller angles of expansion (McElroy1943). The angle of divergence is easily affected by externalinfluences, such as local eddies, vortices, and surges. Internalforces go

45、verning this air motion are extremely delicate (Nottageet al. 1952a).Zone 4 is a zone of jet degradation, where maximum air velocityand temperature decrease rapidly. Distance to this zone and itslength depend on the velocities and turbulence characteristics ofambient air. In a few diameters or width

46、s, air velocity becomes lessthan 0.25 m/s.Centerline Velocities in Zones 1 and 2. In zone 1, the ratio Vx/Vois constant and ranges between 1.0 and 1.2, equal to the ratio of thecenter velocity of the jet at the start of expansion to the averagevelocity. The ratio Vx/Vovaries from approximately 1.0 f

47、or roundedentrance nozzles to about 1.2 for straight pipe discharges; it hasmuch higher values for diverging discharge outlets.Experimental evidence indicates that, in zone 2,(2)whereVx= centerline velocity at distance X from outlet, m/sVo= Vc/CdRfa= average initial velocity at discharge from open-e

48、nded duct or across contracted stream at vena contracta of orifice or multiple-opening outlet, m/sVc= nominal velocity of discharge based on core area, m/sCd= discharge coefficient (usually between 0.65 and 0.90)Rfa= ratio of free area to gross (core) areaHo= width of jet at outlet or at vena contra

49、cta, mKc= centerline velocity constant, depending on outlet type and discharge pattern (see Table 1)X (1/KcHo)1/2= distance from outlet to measurement of centerline velocity Vx, mThe aspect ratio (Tuve 1953) and turbulence (Nottage et al.1952a) primarily affect centerline velocities in zones 1 and 2. Aspectratio has little effect on the terminal zone of the jet when Hois greaterthan 100 mm. This is particularly true of nonisothermal jets. WhenHois very small, induced air can penetrate the core of the jet, thusreducing centerline velocities. The difference in performancebet