ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 20-2012 ROOM AIR DISTRIBUTION EQUIPMENT.pdf

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1、20.1CHAPTER 20ROOM AIR DISTRIBUTION EQUIPMENTSUPPLY OUTLETS . 20.1Fully Mixed Systems 20.1Fully Stratified Systems 20.3Partially Mixed Systems. 20.3Types of Supply Air Outlets 20.4RETURN AND EXHAUST AIR INLETS 20.7Types of Inlets. 20.7Applications . 20.7TERMINAL UNITS. 20.8Chilled Beams 20.9Fan-Coil

2、 Unit Systems 20.10UPPLY air outlets and diffusing equipment introduce air intoSa conditioned space to obtain a desired indoor atmospheric envi-ronment. Return and exhaust air are removed from a space throughreturn and exhaust inlets (inlet and outlet are defined relative to theduct system and not t

3、he room, as shown in Figure 1). Various typesof air outlets and inlets are available as standard manufactured prod-ucts. This chapter describes this equipment, details its proper use,and is intended to help HVAC designers select room air distributionequipment applicable to the air distribution metho

4、ds outlined inChapter 57 of the 2011 ASHRAE HandbookHVAC Applications.Room air distribution systems can be classified according to theirprimary objective and the method used to accomplish that objective.The objective of any air distribution system is to condition and/orventilate the space for occupa

5、nts thermal comfort, or to supportprocesses within the space, or both.Methods used to condition a space can be classified as one of thefollowing:Mixed systems have little or no thermal stratification of air withinthe occupied and/or process space. Overhead air distribution is anexample of this type

6、of system.Full thermal stratification systems have little or no mixing of airwithin the occupied and/or process space. Thermal displacementventilation is an example of this type of system.Partially mixed systems provide limited mixing of air within theoccupied and/or process space. Most underfloor a

7、ir distributiondesigns are examples of this type of system.Task/ambient air distribution focuses on conditioning only aportion of the space for thermal comfort and/or process control.Examples of task/ambient systems are personally controlled deskoutlets and spot-conditioning systems. Because task/am

8、bientdistribution requires a high level of individual control, it is notcovered in this chapter, but is discussed in Chapter 20 of the 2009ASHRAE HandbookFundamentals. Additional design guid-ance is also provided in Bauman (2003).Figure 2 illustrates the spectrum between the two extremes (fullmixing

9、 and full stratification) of room air distribution strategies.The following publications should be reviewed when selectingsystems and equipment for room air distribution:ANSI/ASHRAE Standard 55-2010 establishes indoor thermalenvironmental and personal factors for the occupied space.ANSI/ASHRAE Stand

10、ard 62.1-2010 specifies ventilation re-quirements for acceptable indoor environmental quality. Thisstandard is adopted as part of many building codes.ANSI/ASHRAE/IESNA Standard 90.1-2010 provides mini-mum energy efficiency requirements that affect supply air char-acteristics.ANSI/ASHRAE Standard 113

11、-2009 defines a method for testingthe steady-state air diffusion performance of various room air dis-tribution systems.Chapter 48 of the 2011 ASHRAE HandbookHVAC Applicationsrecommends ranges for HVAC-related background noise in vari-ous spaces.Local codes should also be checked for applicability to

12、 each ofthese subjects.Other useful references on selecting air distribution equipmentinclude Chapter 20 of the 2009 ASHRAE HandbookFundamen-tals, Chapter 57 of the 2011 ASHRAE HandbookHVAC Applica-tions, as well as Bauman (2003), Chen and Glicksman (2003), Rockand Zhu (2002), and Skistad et al. (20

13、02).SUPPLY OUTLETSFULLY MIXED SYSTEMSIn fully mixed systems, supply air outlets, properly sized and lo-cated, control the air pattern to obtain proper air mixing and temper-ature equalization in the space.Accessories used with an outlet regulate the volume of supply airand control its flow pattern.

14、For example, an outlet cannot dischargeair properly and uniformly unless the air enters it in a straight anduniform manner. Accessories may also be necessary for proper airdistribution in a space, so they must be selected and used accordingto the manufacturers recommendations.Primary airflow from an

15、 outlet entrains room air into the jet. Thisentrained air increases the total air in the jet stream. Because themomentum of the jet remains constant, velocity decreases as themass increases. As the two air masses mix, the temperature of the jetapproaches the room air temperature (Rock and Zhu 2002).

16、 Outletsshould be sized to project air so that its velocity and temperaturereach acceptable levels before entering the occupied zone.Outlet locations and patterns also affect a jets throw, entrain-ment, and temperature equalization capabilities. Some general char-acteristics include the following:Th

17、e preparation of this chapter is assigned to TC 5.3, Room Air Distribution.Fig. 1 Designations for Inlet and Outlet20.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)When outlets are located close to a surface, entrainment may berestricted, which can result in a longer throw.When the air patter

18、n is spread horizontally, throw is reduced.Outlets with horizontally radial airflow patterns typically haveshorter throws than outlets with directional patterns.Ceiling or sidewall outlets in cooling applications are most com-monly selected with supply air temperatures at or above 11C.Special high-i

19、nduction outlets are available for use with low-temperature air distribution systems (i.e., those with supply airtemperature below 11C). These outlets include special features thatrapidly mix cold supply air with room air at the outlet and effec-tively reduce the temperature differential between the

20、 supply androom air. For further information, designers can consult ASHRAEsCold Air Distribution System Design Guide (ASHRAE 1996).Outlet Selection ProcedureThe following procedure is generally used in selecting and locat-ing an outlet in a fully mixed system. More details and examples areavailable

21、in Rock and Zhu (2002).1. Determine the amount of air to be supplied to each room. (SeeChapters 17 and 18 of the 2009 ASHRAE HandbookFunda-mentals to determine air quantities for heating and cooling.)2. Select the type and quantity of outlets for each room, consider-ing factors such as air quantity

22、required, distance available forthrow or radius of diffusion, structural characteristics, andarchitectural concepts. Table 1, which is based on experienceand typical ratings of various outlets, may be used as a guide forusing outlets in rooms with various heating and cooling loads.Special conditions

23、, such as ceiling height less than 2.4 or greaterthan 3.7 m, exposed duct mounting, product modifications, andunusual conditions of room occupancy, should be considered.Manufacturers performance data should be consulted to deter-mine the suitability of the outlets used.3. Outlets may be sized and lo

24、cated to distribute air in the space toachieve acceptable temperature and velocity in the occupied zone.4. Select the proper size outlet from the manufacturers performancedata according to air quantity, neck and discharge velocity,throw, distribution pattern, and sound level. Note manufacturersrecom

25、mendations with regard to use. In an open space, the inter-action of airstreams from multiple air outlets may alter a singleoutlets throw, air temperature, or air velocity. As a result,manufacturers data may be insufficient to predict air motion ina particular space. Also, obstructions to the primar

26、y air distribu-tion pattern require special study.Factors that Influence SelectionCoanda (Surface or Ceiling) Effect. An airstream moving adja-cent to or in contact with a wall or ceiling creates a low-pressure areaimmediately adjacent to that surface, causing the air to remain incontact with the su

27、rface substantially throughout the length of throw.This Coanda effect, also referred to as the surface or ceiling effect,counteracts the drop of a horizontally projected cool airstream.Round and four-way horizontal-throw ceiling outlets exhibit ahigh Coanda effect because the discharge air pattern b

28、lankets theceiling area surrounding each outlet. This effect diminishes with adirectional discharge that does not blanket the full ceiling surfacesurrounding the outlet. Sidewall grilles exhibit varying degrees ofCoanda effect, depending on the spread of the particular air patternand the proximity a

29、nd angle of airstream approach to the surface.When outlets are mounted on an exposed duct discharging intoa free space, the airstream entrains air around the entire perimeterof the jet. As a result, a higher rate of entrainment is obtained andthe isothermal throw is shortened by approximately 30%. W

30、henoutlets are installed on exposed ducts for cooling applications, thesupply air tends to drop. Outlets can be selected to counteract thiseffect.Multiple parallel jets in close proximity tend to combine into asingle jet, increasing the throw distance of the combined jet. Moreinformation on parallel

31、 jets can be found in Chapter 20 of the 2009ASHRAE HandbookFundamentals.Temperature Differential. The greater the temperature differ-ential between the supply air projected into a space and the air inthe space, the greater the buoyancy effect on the path of the supplyairstream. Because heated, horiz

32、ontally projected air rises andcooled air falls, this effect should be considered during outlet selec-tion. See Chapter 20 of the 2009 ASHRAE HandbookFundamen-tals for further discussion of the buoyancy effect.Low-temperature supply air or cold building start-up in a humidenvironment may cause conde

33、nsation. Consideration should begiven to the effect of condensation on outlet and space surfaces dur-ing outlet selection.Sound Level. The sound level from an outlet is largely a functionof its discharge velocity and transmission of system noise. For aFig. 2 Classification of Air Distribution Strate

34、giesRoom Air Distribution Equipment 20.3given air capacity, a larger outlet has a lower discharge velocity andcorresponding lower generated sound. A larger outlet also allows ahigher level of sound to pass through the outlet, which may appearas outlet-generated noise. High-frequency noise can result

35、 fromexcessive outlet velocity but may also be generated in the duct bythe moving airstream. Low-frequency noise is generally mechanicalequipment sound and/or terminal box or balancing damper soundtransmitted through the duct and outlet to the room.The cause of the noise can usually be pinpointed as

36、 outlet or sys-tem sounds by removing the outlet core during operation. If thenoise remains essentially unchanged, the system is the source. If thenoise is significantly reduced, the outlet is the source. The noisemay be caused by a highly irregular velocity profile at the entranceto the outlet. The

37、 velocity profile should be measured. If the velocityvaries less than 10% in the air outlet entrance neck, the outlet iscausing the noise. If the velocity profile at the entrance indicatespeak velocities significantly higher than average, check the manu-facturers data for sound at the peak velocity.

38、 If this value approxi-mates the observed noise, the velocity profile in the duct must becorrected to achieve design performance.Smudging. Smudging is the deposition of particles on the airoutlet or a surface near the outlet. Particles are entrained into theprimary discharge jet and impinged into th

39、e device or ceiling sur-face in areas of lower pressure. Smudging tends to be heavier inhigh-traffic areas near building entrances, where particulates arebrought into a space on the bottom of occupants shoes. In well-maintained systems, filtered supply air contributes little to ceilingsmudging. Smud

40、ging is typically more prevalent with ceiling-mounted outlets and linear outlets that discharge parallel to themounting surface than with outlets that discharge perpendicular tothe surface.Variable Air Volume. Outlet(s) should be selected based on thetotal range of airflow for the space served. Outl

41、et performance char-acteristics should be evaluated at both the minimum and maximumflow. More information regarding selection of outlets can be found inChapter 57 of the 2011 ASHRAE HandbookHVAC Applications.FULLY STRATIFIED SYSTEMSStratified room air distribution systems generally rely on supplyout

42、lets with very low discharge velocities (0.25 to 0.36 m/s based ontotal face area) to produce minimal room air entrainment so thatmuch of the temperature difference between supply and ambient airis preserved. Thus, cool supply air accumulates in the lower levelsof the space. Horizontal movement of a

43、ir in the space occurs at min-imal velocities that are insufficient to produce mixing with room air;thus, the supply airstream maintains its thermal integrity. Heatsources in the space create convection plumes that originate aroundthe boundaries of the heat source and rise naturally because of their

44、buoyancy. If these sources are near the supply airstream, supply airis entrained to fill the void of the rising convection plume.Although the supply airstream is several degrees cooler than theroom air when it enters the occupied zone, the temperature differen-tial between supply and room air is gen

45、erally less than that com-monly used for fully mixed systems. Stratified systems used intransient spaces such as transportation terminals, lobbies, and indus-trial spaces may, however, use air temperature differentials similar tothose in fully mixed systems.Outlet Selection ProcedureSupply outlets u

46、sed in stratified air systems tend to be mounted inlow sidewall or floor locations. To produce adequately low dischargevelocities, the outlets also tend to be quite large. Because dischargevelocities are very low, the supply airstream produces little momen-tum, and obstacles (other than heat sources

47、) in its path have little orno effect on its travel. Selection and application of air outlets forthese systems is based primarily on the following considerations:Maintaining vertical temperature gradients within the occupiedspace that conform to ASHRAE Standard 55-2010. Further guid-ance on designin

48、g for conformance to this standard is presented inChapter 57 of the 2011 ASHRAE HandbookHVAC Applications.Maintaining a near zone adjacent to the outlets that is acceptableto the use and occupancy of the space.Providing acoustical performance that conforms to the require-ments of the space.Supply ou

49、tlets used in fully stratified systems are typicallyselected for a maximum face velocity of 0.25 to 0.36 m/s. Limitationof the face velocity is determined by noise requirements and prox-imity of occupants to the outlet. Where space noise requirements arenot so stringent and stationary occupants are far away from diffus-ers, higher face velocities can be used. It is also important that thesupply air outlet be designed to distribute airflow evenly across itsentire discharge area to avoid excessive velocity deviations.The area adjacent to the supply outlet where local v