ASHRAE FUNDAMENTALS SI CH 16-2013 Ventilation and Infiltration.pdf

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1、16.1CHAPTER 16 VENTILATION AND INFILTRATIONBasic Concepts and Terminology 16.1Tracer Gas Measurements 16.5Driving Mechanisms for Ventilation and Infiltration . 16.6Indoor Air Quality 16.10Thermal Loads 16.11Natural Ventilation 16.12Residential Air Leakage 16.14Residential Ventilation 16.18Residentia

2、l IAQ Control 16.20Simplified Models of Residential Ventilation and Infiltration 16.22Commercial and Institutional Air Leakage . 16.25Commercial and Institutional Ventilation . 16.28Office Building Example . 16.29Symbols . 16.32ROVIDING a comfortable and healthy indoor environment forPbuilding occup

3、ants is the primary concern of HVAC engineers.Comfort and indoor air quality (IAQ) depend on many factors,including thermal regulation; control of internal and external sourcesof pollutants; supply of acceptable air; removal of unacceptable air;occupants activities and preferences; and proper constr

4、uction, oper-ation, and maintenance of building systems. Ventilation and infiltra-tion are only part of the acceptable indoor air quality and thermalcomfort problem. HVAC designers, occupants, and building ownersmust be aware of and address other factors as well. Further informa-tion on indoor envir

5、onmental health may be found in Chapter 10.Changing ventilation and infiltration rates to solve thermal comfortproblems and reduce energy consumption can affect indoor air qual-ity and may be against code, so any changes should be approachedwith care and be under the direction of a registered profes

6、sionalengineer with expertise in HVAC analysis and design.HVAC design engineers and others concerned with building ven-tilation and indoor air quality should obtain a copy of ASHRAEStandard 62.1 or 62.2. These standards are reviewed regularly andcontain ventilation design and evaluation requirements

7、 for commer-cial (62.1) and low-rise residential (62.2) buildings, respectively. Indesign of a new building or analysis of an existing building, the ver-sion of Standard 62 that has been adopted by the local code authoritymust be determined. An existing building may be required to meetcurrent code,

8、or allowed to comply with an older code. If a projectinvolves infiltration in residences, then ASHRAE Standards 62.2and 136 should be consulted. The last chapter of each yearsASHRAE Handbook (Chapter 39 of this volume) has a list of currentstandards.This chapter addresses commercial and institutiona

9、l buildings,where ventilation concerns usually dominate (though infiltrationshould not be ignored), and single- and multifamily residences,where infiltration has always been considered important but ventila-tion issues have received increased attention in recent years. Basicconcepts and terminology

10、for both are presented before moreadvanced analytical and design techniques are given. Ventilation ofindustrial buildings is covered in Chapter 31 of the 2011 ASHRAEHandbookHVAC Applications. However, many of the fundamen-tal ideas and terminology covered in this chapter can also be appliedto indust

11、rial buildings.Sustainability Rating SystemsGood indoor air quality is necessary for maintaining health andhigh productivity. Consequently, green and sustainable buildingrating systems, such as the U.S. Green Building Councils (USGBC)Leadership in Energy and Environmental Design (LEED) program,place

12、 great importance on creating and maintaining acceptable IAQ.In fact, the LEED rating system was first developed to address IAQconcerns, and roughly one-third of the available credit points for newcommercial buildings are still IAQ-related. Preparers of such ratingsystems, like others, have struggle

13、d with how to characterize com-plex ventilation and infiltration issues; many portions of this chapter;separate ASHRAE design guides, manuals, books, and standards;and the references cited address these issues in detail and providemethods for demonstrating the effectiveness of various HVAC sys-tems

14、and techniques in providing good IAQ in residential, commer-cial, and other buildings.BASIC CONCEPTS AND TERMINOLOGYOutdoor air that flows through a building is often used to diluteand remove indoor air contaminants. However, the energy requiredto condition this outdoor air can be a significant port

15、ion of the totalspace-conditioning load. The magnitude of outdoor airflow into thebuilding must be known for proper sizing of the HVAC equipmentand evaluation of energy consumption. For buildings withoutmechanical cooling and dehumidification, proper ventilation andinfiltration airflows are importan

16、t for providing comfort for occu-pants. ASHRAE Standard 55 specifies conditions under which 80%or more of the occupants in a space will find it thermally acceptable.Chapter 9 of this volume also addresses thermal comfort. Addition-ally, airflow into buildings and between zones affects fires and them

17、ovement of smoke. Smoke management is addressed in Chapter 53of the 2011 ASHRAE HandbookHVAC Applications.Ventilation and InfiltrationAir exchange of outdoor air with air already in a building can bedivided into two broad classifications: ventilation and infiltration.Ventilation is intentional intro

18、duction of air from the outdoorsinto a building; it is further subdivided into natural and mechanicalventilation. Natural ventilation is the flow of air through open win-dows, doors, grilles, and other planned building envelope penetra-tions, and it is driven by natural and/or artificially produced

19、pressuredifferentials. Mechanical (or forced) ventilation, shown in Figure 1,is the intentional movement of air into and out of a building usingfans and intake and exhaust vents.Infiltration is the flow of outdoor air into a building throughcracks and other unintentional openings and through the nor

20、mal useof exterior doors for entrance and egress. Infiltration is also knownas air leakage into a building. Exfiltration, depicted in Figure 1, isleakage of indoor air out of a building through similar types of open-ings. Like natural ventilation, infiltration and exfiltration are drivenby natural a

21、nd/or artificial pressure differences. These forces are dis-cussed in detail in the section on Driving Mechanisms for Ventila-tion and Infiltration. Transfer air is air that moves from one interiorspace to another, either intentionally or not.The preparation of this chapter is assigned to TC 4.3, Ve

22、ntilation Require-ments and Infiltration.16.2 2013 ASHRAE HandbookFundamentals (SI)Ventilation and infiltration differ significantly in how they affectenergy consumption, air quality, and thermal comfort, and they caneach vary with weather conditions, building operation, and use.Although one mode ma

23、y be expected to dominate in a particularbuilding, all must be considered in the proper design and operationof an HVAC system.Ventilation AirVentilation air is air used to provide acceptable indoor air quality.It may be composed of mechanical or natural ventilation, infiltra-tion, suitably treated r

24、ecirculated air, transfer air, or an appropriatecombination, although the allowable means of providing ventilationair varies in standards and guidelines.Modern commercial and institutional buildings normally havemechanical ventilation and are usually pressurized somewhat toreduce or eliminate infilt

25、ration. Mechanical ventilation has thegreatest potential for control of air exchange when the system isproperly designed, installed, and operated; it should provide accept-able indoor air quality and thermal comfort when ASHRAE Stan-dard 55 and 62.1 requirements are followed. Mechanical ventilatione

26、quipment and systems are described in Chapters 1, 4, and 10 of the2012 ASHRAE HandbookHVAC Systems and Equipment.In commercial and institutional buildings, natural ventilation(e.g., through operable windows) may not be desirable from thepoint of view of energy conservation and comfort. In commercial

27、and institutional buildings with mechanical cooling and ventilation,an air- or water-side economizer may be preferable to operable win-dows for taking advantage of cool outdoor conditions when interiorcooling is required. Infiltration may be significant in commercialand institutional buildings, espe

28、cially in tall, leaky, or partially pres-surized buildings and in lobby areas.In most of the United States, residential buildings have histori-cally relied on infiltration and natural ventilation to meet their ven-tilation air needs. Neither is reliable for ventilation air purposesbecause they depen

29、d on weather conditions, building construction,and maintenance. However, natural ventilation, usually throughoperable windows, is more likely to allow occupants to control air-borne contaminants and interior air temperature, but it can have asubstantial energy cost if used while the residences heati

30、ng or cool-ing equipment is operating.In place of operable windows, small exhaust fans should be pro-vided for localized venting in residential spaces, such as kitchensand bathrooms. Not all local building codes require that the exhaustbe vented to the outside. Instead, the code may allow the air to

31、 betreated and returned to the space or to be discharged to an atticspace. Poor maintenance of these treatment devices can make non-ducted vents ineffective for ventilation purposes. Condensation inattics should be avoided. In northern Europe and in Canada, somebuilding codes require general mechani

32、cal ventilation in resi-dences, and heat recovery heat exchangers are popular for reducingenergy consumption. Low-rise residential buildings with low ratesof infiltration and natural ventilation, including most new build-ings, require mechanical ventilation at rates given in ASHRAEStandard 62.2.Forc

33、ed-Air Distribution SystemsFigure 2 shows a simple air-handling unit (AHU) or air handlerthat conditions air for a building. Air brought back to the air handlerfrom the conditioned space is return air (RA). The return air eitheris discharged to the environment exhaust air (EA) or is reused re-circul

34、ated air (CA). Air brought in intentionally from the environ-ment is outdoor air (OA). Because outdoor air may need treatmentto be acceptable for use in a building, it should not be called “freshair.” Outdoor and recirculated air are combined to form mixed air(MA), which is then conditioned and deli

35、vered to the thermal zone assupply air (SA). Any portion of the mixed air that intentionally orunintentionally circumvents conditioning is bypass air (BA). Be-cause of the wide variety of air-handling systems, the airflows shownin Figure 2 may not all be present in a particular system as definedhere

36、. Also, more complex systems may have additional airflows.Outdoor Air FractionThe outdoor airflow introduced to a building or zone by an air-handling unit can also be described by the outdoor air fraction Xoa,which is the ratio of the volumetric flow rate of outdoor air broughtin by the air handler

37、to the total supply airflow rate:Xoa= (1)When expressed as a percentage, the outdoor air fraction is calledthe percent outdoor air. The design outdoor airflow rate for abuildings or zones ventilation system is found by applying therequirements of ASHRAE Standard 62.1 to that specific building.The su

38、pply airflow rate is that required to meet the thermal load.The outdoor air fraction and percent outdoor air then describe thedegree of recirculation, where a low value indicates a high rate ofrecirculation, and a high value shows little recirculation. Conven-tional all-air air-handling systems for

39、commercial and institutionalbuildings have approximately 10 to 40% outdoor air.100% outdoor air means no recirculation of return air throughthe air-handling system. Instead, all the supply air is treated outdoorair, also known as makeup air (KA), and all return air is dischargeddirectly to the outdo

40、ors as relief air (LA), via separate or central-ized exhaust fans. An air-handling unit that provides 100% outdoorair to offset air that is exhausted is typically called a makeup airunit (MAU).Fig. 1 Two-Space Building with Mechanical Ventilation, Infiltration, and ExfiltrationFig. 2 Simple All-Air

41、Air-Handling Unit with Associated AirflowsQoaQsa-QoaQma-QoaQoaQca+-=Ventilation and Infiltration 16.3When outdoor air via mechanical ventilation is used to provideventilation air, as is common in commercial and institutional build-ings, this outdoor air is usually delivered to spaces as all or part

42、ofthe supply air. With a variable-air-volume (VAV) system, the out-door air fraction of the supply air may need to be increased whensupply airflow is reduced to meet a particular thermal load. In someHVAC systems, such as the dedicated outdoor air system (DOAS),conditioned outdoor air may be deliver

43、ed separately from the waythe spaces loads are handled (Mumma and Shank 2001).Room Air MovementAir movement within spaces affects the diffusion of ventilationair and, therefore, indoor air quality and comfort. Two distinct flowpatterns are commonly used to characterize air movement in rooms:displace

44、ment flow and entrainment flow. Displacement flow,shown in Figure 3, is the movement of air within a space in a piston-or plug-type motion. Ideally, no mixing of the room air occurs,which is desirable for removing pollutants generated within a space.A laminar-flow air distribution system that sweeps

45、 air across a spacemay produce displacement flow.Entrainment flow, shown in Figure 4, is also known as conven-tional mixing. Systems with ceiling-based supply air diffusers andreturn air grilles are common examples of air distribution systemsthat produce entrainment flow. Entrainment flow with very

46、poormixing in the room has been called short-circuiting flow becausemuch of the supply air leaves the room without mixing with roomair. There is little evidence that properly designed, installed, andoperated air distribution systems exhibit short circuiting, althoughpoorly designed, installed, or op

47、erated systems may short-circuit,especially ceiling-based systems in heating mode (Offermann andInt-Hout 1989).Perfect mixing occurs when supply air is instantly and evenlydistributed throughout a space. Perfect mixing is also known ascomplete or uniform mixing; the air may be called well stirred or

48、well mixed. This theoretical performance is approached by entrain-ment flow systems that have good mixing and by displacement flowsystems that allow too much mixing (Rock et al. 1995). The outdoorair requirements given in Table 6.1 of ASHRAE Standard 62.1assume delivery of ventilation air with perfe

49、ct mixing withinspaces. For more detailed information on space air diffusion, seeChapter 20.Underfloor air distribution (UFAD or UAD), as shown inFigure 5, is a hybrid method of conditioning and ventilating spaces(Bauman and Daly 2003). Air is introduced through a floor plenum,with or without branch ductwork or terminal units, and delivered toa space by floor-mounted diffusers. These diffusers encourage airmixing near the floor to temper the supply air. The combined airthen moves vertically through the space, with reduced mixin