ASHRAE HVAC APPLICATIONS SI CH 57-2015 ROOM AIR DISTRIBUTION.pdf

1、57.1CHAPTER 57ROOM AIR DISTRIBUTIONIndoor Air Quality and Sustainability . 57.1Application Guidelines 57.2Mixed Air Distribution. 57.2Fully Stratified Air Distribution. 57.6Partially Mixed Air Distribution 57.9Terminal Units 57.10Fan Selection 57.15Chilled Beams 57.18OOM air distribution systems, li

2、ke other HVAC systems, areR intended to achieve required thermal comfort and ventilationfor space occupants and processes. Although air terminals (inletsand outlets), terminal units, local ducts, and the rooms themselvesmay affect room air distribution, this chapter addresses only airterminals and t

3、heir effect on occupant comfort. This chapter is in-tended to help HVAC designers apply air distribution systems to oc-cupied spaces, providing information on characteristics of variousair distribution strategies, and tools and guidelines for applicationsand system design. Naturally ventilated space

4、s are not addressed;see Chapter 16 of the 2013 ASHRAE HandbookFundamentals fordetails. Also see Chapter 20 of the 2013 ASHRAE HandbookFun-damentals for more information on space air diffusion; Chapter 20of the 2012 ASHRAE HandbookHVAC Systems and Equipment forinformation on room air distribution equ

5、ipment; and Chapter 48 ofthis volume for sound and vibration control guidance.Room air distribution systems can be classified by (1) their pri-mary objective and (2) the method by which they attempt to accom-plish that objective. The objective of any air distribution system canbe classified as one o

6、f the following: Conditioning and/or ventilation of the space for occupant thermalcomfortConditioning and/or ventilation to support processes within thespaceA combination of theseAs a general guideline, the occupied zone in a space is any loca-tion where occupants normally reside, and may differ fro

7、m project toproject; it is application-specific, and should be carefully defined bythe designer. The occupied zone is generally considered to be theroom volume between the floor level and 1.8 m above the floor. Stan-dards and guidelines, such as ANSI/ASHRAE Standards 55 and62.1, further define the o

8、ccupied zone (e.g., Standard 55 exemptsareas near walls).Occupant comfort is defined in detail in ANSI/ASHRAE Standard55-2004. Figure 5.2.1.1 of the standard shows acceptable ranges oftemperature and humidity for spaces. As a general guide, 80% ofoccupants in typical office spaces can be satisfied w

9、ith thermal envi-ronments over a wide range of temperatures and relative humidities.Designers often target indoor dry-bulb temperatures between 22 and25C, relative humidities between 25 and 60%, and occupied zoneair velocities below 0.25 m/s. ANSI/ASHRAE Standard 113describes a method for evaluating

10、 effectiveness of various room airdistribution systems in achieving thermal comfort.Room air distribution methods can be classified as one of thefollowing:Mixed systems (e.g., overhead distribution) have little or no ther-mal stratification of air in the occupied and/or process space.Full thermal st

11、ratification systems (e.g., thermal displacementventilation) have little or no air mixing in the occupied and/or pro-cess space.Partially mixed systems (e.g., most underfloor air distributiondesigns) provide limited air mixing in the occupied and/or processspace.Task/ambient air distribution (e.g.,

12、personally controlled deskoutlets, spot conditioning systems) focuses on conditioning onlypart of the space for thermal comfort and/or process control.Because task/ambient design requires a high degree of individualcontrol, it is not covered in this chapter; see Chapter 20 of the 2013ASHRAE Handbook

13、Fundamentals for details. Limited designguidance is also provided by Bauman and Daly (2003).Figure 1 illustrates the spectrum between the two extremes (fullmixing and full stratification) of room air distribution strategies.1. INDOOR AIR QUALITY AND SUSTAINABILITYAir distribution systems affect not

14、only indoor air quality (IAQ)and thermal comfort, but also energy consumption over the entirelife of the project. Choices made early in the design process areimportant. ANSI/ASHRAE/IESNA Standard 90.1 provides energyefficiency requirements that affect supply air characteristics.The U.S. Green Buildi

15、ng Councils (USGBC) Leadership inEnergy and Environmental Design (LEED) Green Building RatingSystem was originally created in response to indoor air qualityconcerns, and has evolved to include prerequisites and credits forincreasing ventilation effectiveness and improving thermal comfort(USGBC 2009)

16、. These requirements and optional points are rela-tively easy to achieve if good room air distribution design principles,methods, and standards are followed.Environmental tobacco smoke (ETS) control is a LEED prereq-uisite. Banning indoor smoking is a common approach, but if indoorsmoking is to be a

17、llowed, ANSI/ASHRAE Standard 62.1-2010requires that more than the base non-ETS ventilation air be providedwhere ETS is present in all or part of a building. Rock (2006) pro-vides additional advice on dealing with ETS.Ventilation effectiveness is affected directly by the room air dis-tribution system

18、s design, construction, and operation, but is verydifficult to predict. Many attempts have been made to quantify ven-tilation effectiveness, including ASHRAE Standard 129. However,this standard is only for experimental tests in well-controlled labo-ratories and should not be applied directly to real

19、 buildings.Because of the difficulty in predicting ventilation effectiveness,ASHRAE Standard 62.1 provides a table of typical values that weredetermined through the experiences of its Standard Project Commit-tee and reviewers or extracted from research literature; for example,well-designed ceiling-b

20、ased air diffusion systems produce near-per-fect air mixing in cooling mode, and yield an air change effective-ness of almost 1.0. More information on ASHRAE Standard 62.1 isavailable in its users manual (ASHRAE 2011).Displacement and underfloor air distribution (UFAD) systemshave the potential for

21、values greater than 1.0. More information onceiling- and wall-mounted air inlets and outlets can be found in Rockand Zhu (2002). Performance of displacement systems is describedby Chen and Glicksman (2003), and UFAD is discussed in detail byBauman and Daly (2003).The preparation of this chapter is a

22、ssigned to TC 5.3, Room Air Distri-bution.57.2 2015 ASHRAE HandbookHVAC Applications (SI)Air terminals, such as diffusers or grilles, may become unsightlyover time because of accumulation of dirt on their faces (smudging).Instead of replacing air terminals, and thus requiring new materialsand energy

23、 for manufacturing, they can often be cleaned in place torestore their appearance. Those that cannot be cleaned and must bereplaced should be recycled, not discarded, to recover the variousmetals and other desirable materials of construction.2. APPLICATION GUIDELINESDesign ConstraintsSpace design co

24、nstraints affect room air distribution systemchoices and how air inlets and outlets are used. Space constraintsmay include the following:DimensionsHeat gain and loss characteristicsUseAcoustical requirementsAvailable locations for air inlets and outletsInlet and outlet characteristics are discussed

25、in Chapter 20 of the2012 ASHRAE HandbookHVAC Systems and Equipment. Thischapter discusses more specific application considerations for airinlets and outlets.SoundSound emitted from inlets and outlets is directly related to theairflow quantity and free area velocity. The airflow sound intensityin a s

26、pace also depends on the rooms acoustical absorption and theobservers distance from air distribution devices, For more informa-tion, see Chapter 48 of this volume and Chapter 8 in the 2013ASHRAE HandbookFundamentals.Inlet Conditions to Air OutletsThe way an airstream approaches an outlet is importan

27、t. Forgood air diffusion, the inlet configuration should create a uniformdischarge velocity profile from the outlet, or the outlet may not per-form as intended.The outlet usually cannot correct effects of improper ductapproach. Many sidewall outlets are installed either at the end ofvertical ducts o

28、r in the side of horizontal ducts, and most ceiling out-lets are attached either directly to the bottom of horizontal ducts orto special vertical takeoff ducts that connect the outlet with the hor-izontal duct. In all these cases, devices for directing and equalizingthe airflow may be necessary for

29、proper direction and diffusion ofthe air.Return Air InletsThe success of a mixed air distribution system depends pri-marily on supply diffuser location. Return grille location is farless critical than with outlets. In fact, the return air intakeaffects room air motion only immediately around the gri

30、lle. Mea-surements of velocity near a return air grille show a rapid decreasein magnitude as the measuring device is moved away from the grilleface. Table 1 shows recommended maximum return air grille veloc-ities as a function of grille location. Every enclosed space shouldhave return/transfer inlet

31、s of adequate size per this table.For stratified and partially mixed air distribution systems, thereare advantageous locations for return air inlets. For example, anintake can be located to return the warmest air in cooling season.If the outlet is selected to provide adequate throw and directedaway

32、from returns or exhausts, supply short-circuiting is normallynot a problem. The success of this practice is confirmed by theavailability and use of combination supply and return diffusers.3. MIXED AIR DISTRIBUTIONIn mixed air systems, high-velocity supply jets from air outletsmaintain comfort by mix

33、ing room air with supply air. This airmixing, heat transfer, and resultant velocity reduction should occuroutside the occupied zone. Occupant comfort is maintained notdirectly by motion of air from outlets, but from secondary airFig. 1 Classification of Air Distribution StrategiesTable 1 Recommended

34、 Return Inlet Face VelocitiesInlet Location Velocity Across Gross Area, m/sAbove occupied zone 4In occupied zone, not near seats 3 to 4In occupied zone, near seats 2 to 3Door or wall louvers 1 to 1.5Through undercut area of doors 1 to 1.5Room Air Distribution 57.3motion from mixing in the unoccupied

35、 zone. Comfort is maximizedwhen uniform temperature distribution and room air velocities ofless than 0.25 m/s are maintained in the occupied zone.Maintaining velocities less than 0.25 m/s in the occupied zone isoften overlooked by designers, but is critical to maintaining com-fort. The outlets selec

36、tion, location, supply air volume, dischargevelocity, and air temperature differential determine the resulting airmotion in the occupied zone.Principles of OperationMixed systems generally provide comfort by entraining room airinto discharge jets located outside occupied zones, mixing supplyand room

37、 air. Ideally, these systems generate low-velocity airmotion (less than 0.25 m/s) throughout the occupied zone to provideuniform temperature gradients and velocities. Proper selection of anair outlet is critical for proper air distribution; improper selectioncan result in room air stagnation, unacce

38、ptable temperature gradi-ents, and unacceptable velocities in the occupied zone that may leadto occupant discomfort.The location of a discharge jet relative to surrounding surfaces isimportant. Discharge jets attach to parallel surfaces, given sufficientvelocity and proximity. When a jet is attached

39、, the throw increasesby about 40% over a jet discharged in an open area. This differenceis important when selecting an air outlet. For detailed discussion ofthe surface effect on discharge jets, see Chapter 20 of the 2013ASHRAE HandbookFundamentals.Space Ventilation and Contaminant RemovalThese syst

40、ems are intended to maintain acceptable indoor airquality by mixing supply and room air (dilution ventilation). Supplyair is typically a conditioned mixture of ventilation and recirculatedair. Outlet type and discharge velocity determine the mixing rate ofthe space and should be a design considerati

41、on. The rooms return orexhaust air carries away diluted air contaminants. Space air ventila-tion rates are mandated under ASHRAE Standard 62.1-2010, butsupply airflow rates are often higher because of thermal loads.Benefits and LimitationsBenefits of fully mixed systems include the following:Most of

42、fice applications can use lower supply dry-bulb temper-atures, for smaller ductwork and lower supply air quantities.Air can be supplied at a lower moisture content, possibly elimi-nating the need for a more complex humidity control system.Vertical temperature gradients are lower for cooling applicat

43、ionswith high internal heat gains, which may improve thermal comfort.Mixed systems are the most common design for distribution sys-tems, because designers and installers are familiar with the re-quired system components and installation.Limitations of mixed systems include the following:Partial-load

44、 operation in variable-air-volume (VAV) systems mayreduce outlet velocities, reducing room air mixing and compro-mising thermal comfort. Designers should consider this whenselecting outlets.Cooling and heating with the same ceiling or high-sidewall dif-fuser may cause inadequate performance in heati

45、ng mode and/orexcessive velocity in cooling mode.Ceilings more than 4 m high may require special design consid-erations to provide acceptable comfort in the occupied zone. Careshould be taken to select the proper outlet for these applications.Because mixed systems typically use high-velocity jets of

46、 air, anyobstructions in the space (e.g., bookshelves, wall partitions, fur-niture) can reduce comfort.Lighter-than-air contaminants are uniformly mixed in the spaceand typically result in higher contaminant concentrations, whichmay compromise indoor air quality.Mixed air systems typically use eithe

47、r ceiling or sidewall outletsdischarging air horizontally, or floor- or sill-mounted outlets dis-charging air vertically. They are the most common method of airdistribution in North America.Horizontal Discharge Cooling with Ceiling-Mounted OutletsCeiling-mounted outlets typically use the surface eff

48、ect to trans-port supply air in the unoccupied zone. The supply air projectsacross the ceiling and, with sufficient velocity, can continue downwall surfaces and across floors, as shown in Figure 2. In this appli-cation, supply air should remain outside the occupied zone until it isadequately mixed a

49、nd tempered with room air. Air motion in theoccupied zone is generated by room air entrainment into the supplyair (Nevins 1976).Overhead outlets may also be installed on exposed ducts, inwhich case the surface effect does not apply. Typically, if the outletis mounted 300 mm or more below a ceiling surface, discharge airwill not attach to the surface. The unattached supply air has a shorterthrow and can project downward, resulting in high air velocities inthe occupied zone. Some outlets are designed for use in exposedduct applications. Typical outlet performance data pre