ASHRAE FUNDAMENTALS SI CH 16-2017 Ventilation and Infiltration.pdf

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1、16.1CHAPTER 16VENTILATION AND INFILTRATIONBasic Concepts and Terminology 16.1Tracer Gas Measurements. 16.5Driving Mechanisms for Ventilation and Infiltration 16.7Indoor Air Quality . 16.11Thermal Loads . 16.11Natural Ventilation 16.13Residential Air Leakage. 16.15Residential Ventilation. 16.18Reside

2、ntial IAQ Control. 16.20Simplified Models of Residential Ventilation and Infiltration 16.23Commercial and Institutional Air Leakage 16.26Commercial and Institutional Ventilation 16.29Office Building Example 16.30Symbols 16.33ROVIDING a comfortable and healthy indoor environment forPbuilding occupant

3、s 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 construct

4、ion, oper-ation, and maintenance of building systems. Proper ventilation andinfiltration are only part of achieving acceptable indoor air qualityand thermal comfort. HVAC designers, occupants, and buildingowners must be aware of and address other factors as well. Furtherinformation on indoor environ

5、mental health may be found in Chap-ter 10. Changing ventilation and infiltration rates to solve thermalcomfort problems and reduce energy consumption can affect indoorair quality and may be against building code or other regulations, soany changes should be approached with care and be under the dire

6、c-tion of a registered professional engineer with expertise in HVACanalysis 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, or those for specific applications (e.g., Stan-dard 170 for health

7、care), whichever is most relevant to the project.These standards are reviewed regularly and contain ventilationdesign and evaluation requirements for commercial and institutional(Standard 62.1) and residential (Standard 62.2) buildings, respec-tively. When designing a new building or analyzing an ex

8、istingbuilding, check which version of Standard 62 has been adopted bythe local code authority. An existing building may be required tomeet the current version of the standard, or allowed to comply withan older version. The last chapter of each years ASHRAE Handbook(Chapter 39 of this volume) has a

9、list of current standards.This chapter addresses commercial and institutional buildings,where ventilation concerns usually dominate (though infiltrationshould not be ignored), and single- and multifamily residences,where infiltration has traditionally been considered most importantbut ventilation is

10、sues have received increased attention in recentyears. Basic concepts and terminology for both are presented beforemore advanced analytical and design techniques are given. Ventila-tion of industrial buildings is covered in Chapter 31 of the 2015ASHRAE HandbookHVAC Applications. However, many of the

11、fundamental ideas and terminology presented in this chapter canalso be applied to industrial buildings.Sustainable Building Standards and Rating SystemsGood indoor air quality is necessary for maintaining health andhigh productivity. Consequently, sustainable building standardssuch as ASHRAE Standar

12、d 189.1 and building rating systems, suchas U.S. Green Building Councils (USGBC) Leadership in Energyand Environmental Design (LEED) program, place great impor-tance on creating and maintaining acceptable IAQ. In fact, the LEEDrating system was first developed to address IAQ concerns, androughly one

13、-quarter of the available credit points for new commer-cial buildings are still IAQ related. Preparers of such rating systems,like others, have struggled with how to characterize complex venti-lation and infiltration issues. These issues are addressed in detail bymany portions of this chapter; separ

14、ate ASHRAE design guides,manuals, books, and standards; and the references cited; thesesources also provide methods to demonstrate the effectiveness ofvarious HVAC systems and techniques in providing good IAQ in res-idential, commercial, and other buildings. In all designs, care isneeded to eliminat

15、e excessive ventilation (e.g., beyond that neededfor IAQ or by an air-side economizer) to avoid inappropriately in-creasing energy use. Increasing the ventilation rate above that re-quired by Standard 62.1, for example, does not necessarily increasethe acceptability of the indoor air quality.1. BASI

16、C 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 portion of the totalspace-conditioning load. The magnitude of outdoor airflow into thebuilding

17、 must be determined to size the HVAC equipment properly,and to evaluate energy consumption (if required). For buildingswithout mechanical cooling and dehumidification, proper ventila-tion and infiltration airflows are important for providing acceptableIAQ and better thermal comfort for occupants. AS

18、HRAE Standard55 specifies conditions under which 80% or more of the occupants ina space will find it thermally acceptable. Chapter 9 of this volumealso addresses thermal comfort. Airflow into buildings and between zones also affects fires,smoke movement, and safe occupant egress. Smoke management is

19、addressed in Chapter 53 of the 2015 ASHRAE HandbookHVACApplications.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 introduction of air from the outdoorsinto a

20、 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. Mechanical (or forced) ventilation, shown in Figure 1, is theintentional movement of air into a

21、nd out of a building using fans,ductwork, intake louvers, and exhaust grilles, for example.Infiltration is the flow of outdoor air into a building throughcracks and other unintentional openings and through the normal useThe preparation of this chapter is assigned to TC 4.3, Ventilation Require-ments

22、 and Infiltration.16.2 2017 ASHRAE HandbookFundamentals (SI)of 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,

23、 infiltration and exfiltration are drivenby natural and/or artificial pressure differences. These forces arediscussed in detail in the section on Driving Mechanisms for Venti-lation and Infiltration. Transfer air is air that moves from one inte-rior space to another, either intentionally or not.Vent

24、ilation and infiltration differ significantly in how they affectenergy consumption, air quality, and thermal comfort, and can eachvary with weather conditions, HVAC system operation, and build-ing use. Although one mode may be expected to dominate in a par-ticular building, both must be considered i

25、n the proper design andoperation of an HVAC system. Seasonal weather and other transientfactors must be considered, as well.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 rec

26、irculated 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 intended to be pressurizedsomewhat to reduce or eli

27、minate infiltration. Mechanical ventilationhas the greatest potential for control of air exchange when the sys-tem is properly designed, installed, and operated; it should provideacceptable indoor air quality and thermal comfort when ASHRAEStandards 55 and 62.1s requirements are followed, although i

28、ssues(e.g., unusually strong pollutant sources) can still result in unaccept-able indoor environment conditions. Mechanical ventilation equip-ment and systems are described in Chapters 1, 4, and 10 of the 2016ASHRAE HandbookHVAC Systems and Equipment.In commercial and institutional buildings, natura

29、l ventilation(e.g., through uncontrolled use of manually operated windows) maynot be desirable from the points of view of energy conservation,comfort, security, or control of airborne pollen or other pollutants insome climates and locations. In commercial and institutional build-ings with mechanical

30、 cooling and ventilation, an automatically con-trolled air- or water-side economizer may be preferable to operablewindows for taking advantage of cool outdoor conditions when inte-rior cooling is required. When moderate outdoor temperaturesoccur, an air-side economizer control scheme may not only in

31、creasethe rate of ventilation but also operate the cooling equipment to opti-mize energy use (hybrid or mixed mode).Infiltration may be significant in commercial and institutionalbuildings too, especially in tall, leaky, or partially pressurized build-ings and in lobby and loading dock areas. The jo

32、int between roofdecking and outer walls is often particularly leaky in commercialand other large buildings, and should be properly detailed, con-structed, and inspected.In most of the United States, residential buildings have histori-cally relied on infiltration and natural ventilation to meet their

33、 ven-tilation air needs. Neither is reliable for ventilation air purposesbecause they depend on weather conditions, building construction,occupants, and maintenance. Natural ventilation, usually throughoperable windows and screened doors, is more likely to allow occu-pants to control indoor airborne

34、 contaminants and interior air tem-perature, but it can have a substantial energy cost if used while theresidences heating or cooling equipment is operating. Openedwindows and doors also may lead to security, noise, or other con-cerns.In place of or in addition to operable windows, small exhaustfans

35、 should be provided for localized venting of residential spaceswith high pollutant levels or moisture (e.g., kitchens, bathrooms).Not all local building codes require that such exhaust be vented tothe outdoors, but it is required by ASHRAE Standard 62.2. Instead,a local code may allow the air to be

36、treated and returned to the spaceor to be discharged to an attic space. Poor maintenance of theserecirculating treatment devices can make nonducted vents ineffec-tive for ventilation purposes. Warm exhaust air can hold much mois-ture, so condensation in attics should be avoided. If not alreadyrequir

37、ed by code, consider venting attached garages and other stor-age spaces to the outdoors, as well.Increasingly, building codes require general mechanical ventila-tion in residences. Heat recovery heat exchangers are popular forreducing energy consumption, especially in cold climates. Residen-tial bui

38、ldings with low rates of infiltration and natural ventilation,including most new buildings, require mechanical ventilation atrates given in ASHRAE Standard 62.2.Forced-Air Distribution SystemsFigure 2 shows a simple air-handling unit (AHU) or air handlerthat conditions air for a building. Air brough

39、t 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 reusedrecirculated air (CA). Air brought in intentionally from the envi-ronment is outdoor air (OA). Because outdoor air may need treat-ment to be acce

40、ptable for use in a building, it should not be called“fresh air.” Outdoor and recirculated air are combined to form mixedair (MA), which is then conditioned and delivered to the spacesserved as supply air (SA). Any portion of the mixed air that inten-tionally or unintentionally circumvents condition

41、ing is bypass air(BA). Because of the wide variety of air-handling systems, the air-flows shown in Figure 2 may not all be present in a particular systemas defined here. Also, more complex systems may have additionalairflows.Fig. 1 Two-Space Building with Mechanical Ventilation, Infiltration, and Ex

42、filtrationFig. 2 Simple All-Air Air-Handling Unit with Associated AirflowsVentilation and Infiltration 16.3In HVAC design, volumetric airflow rates Q are normally report-ed in litres per second (L/s) or cubic metres per second (m3/s). Theincorrect term “volume” should not be used to describe airflow

43、rates.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 Q of outdoor airbrought in by the air handler to the total supply airflow rate:Xoa= (1)When expr

44、essed as a percentage, the outdoor air fraction is calledthe percent outdoor air. The design outdoor airflow rate Qoafor abuildings or zones ventilation system is found by applying the re-quirements of ASHRAE Standard 62.1 or 62.2 to that specificbuilding, occupancy, and HVAC system. The supply airf

45、low rate Qsais that required to meet the thermal load. The outdoor air fractionand percent outdoor air then describe the degree of recirculation,where a low value indicates a high rate of recirculation, and a highvalue shows little recirculation. Conventional all-air air-handlingsystems for commerci

46、al and institutional buildings often have ap-proximately 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

47、ors as relief air (LA), via separate or central-ized exhaust fans or relief dampers and grilles. An air-handling unitthat provides exclusively 100% outdoor air to offset air that is ex-hausted is typically called a makeup air unit (MAU).When outdoor air via mechanical ventilation is used to providev

48、entilation air, as is common in commercial and institutional build-ings and increasingly in residences, this outdoor air is usuallydelivered to spaces as all or part of the supply air. With a variable-air-volume (VAV) system, the outdoor air fraction of the supply airmay need to be increased when su

49、pply airflow is reduced to meet aparticular thermal load. In some HVAC systems, such as a dedi-cated outdoor air system (DOAS), conditioned outdoor air may bedelivered separately from the way the 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:displacement flow and entrainment flow. Displacement flow,shown in Figure 3, is the movement of air within a space in a