1、29.1CHAPTER 29AIR CLEANERS FOR PARTICULATE CONTAMINANTSAtmospheric Dust. 29.1Aerosol Characteristics 29.1Air-Cleaning Applications . 29.2Mechanisms of Particle Collection . 29.2Evaluating Air Cleaners 29.2Air Cleaner Test Methods. 29.3Types of Air Cleaners. 29.4Filter Types and Performance 29.5Selec
2、tion and Maintenance 29.8Air Cleaner Installation . 29.10Safety Considerations. 29.10HIS chapter discusses removal of contaminants from bothTventilation and recirculated air used for conditioning buildinginteriors. Complete air cleaning may require removing of airborneparticles, microorganisms, and
3、gaseous contaminants, but thischapter only covers removal of airborne particles and briefly dis-cusses bioaerosols. Chapter 46 of the 2011 ASHRAE HandbookHVAC Applications covers the removal of gaseous contaminants.The total suspended particulate concentration in applications dis-cussed in this chap
4、ter seldom exceeds 2 mg/m3and is usually lessthan 0.2 mg/m3of air. This contrasts with flue gas or exhaust gasfrom processes, where dust concentration typically ranges from 200to 40 000 mg/m3. Chapter 26 discusses exhaust-gas control.Most air cleaners discussed in this chapter are not used in exhaus
5、tgas streams, because of the extreme dust concentration, high tem-perature, and high humidity that may be encountered in processexhaust. However, the air cleaners discussed here are used exten-sively in supplying makeup air with low particulate concentration toindustrial processes.ATMOSPHERIC DUSTAt
6、mospheric dust is a complex mixture of smokes, mists, fumes,dry granular particles, bioaerosols, and natural and synthetic fibers.When suspended in a gas such as air, this mixture is called an aero-sol. A sample of atmospheric dust usually contains soot and smoke,silica, clay, decayed animal and veg
7、etable matter, organic materialsin the form of lint and plant fibers, and metallic fragments. It mayalso contain living organisms, such as mold spores, bacteria, andplant pollens, which may cause diseases or allergic responses.(Chapter 11 of the 2009 ASHRAE HandbookFundamentals con-tains further inf
8、ormation on atmospheric contaminants.) A sampleof atmospheric dust gathered at any point generally contains mate-rials common to that locality, together with other components thatoriginated at a distance but were transported by air currents or dif-fusion. These components and their concentrations va
9、ry with thegeography of the locality (urban or rural), season of the year,weather, direction and strength of the wind, and proximity of dustsources.Aerosol sizes range from 0.01 m and smaller for freshly formedcombustion particles and radon progeny; to 0.1 m for aged cookingand cigarette smokes; and
10、 0.1 to 10 m for airborne dust, microor-ganisms, and allergens; and up to 100 m and larger for airbornesoil, pollens, and allergens.Concentrations of atmospheric aerosols generally peak at sub-micrometre sizes and decrease rapidly as the particulate size in-creases above 1 m. For a given size, the c
11、oncentration can vary byseveral orders of magnitude over time and space, particularly nearan aerosol source, such as human activities, equipment, furnishings,and pets (McCrone et al. 1967). This wide range of particulate sizeand concentration makes it impossible to design one cleaner for allapplicat
12、ions.AEROSOL CHARACTERISTICSThe characteristics of aerosols that most affect air filter perfor-mance include particle size and shape, mass, concentration, andelectrical properties. The most important of these is particle size.Figure 3 in Chapter 11 of the 2009 ASHRAE HandbookFunda-mentals gives data
13、 on the sizes and characteristics of a wide range ofairborne particles that may be encountered.Particle size in this discussion refers to aerodynamic particle size.Particles less than 0.1 m in diameter are generally referred to asultrafine-mode or nanoparticles, those between 0.1 and 2.5 m aretermed
14、 fine mode, and those larger than 2.5 m as coarse mode.Whereas ultrafine- and fine-mode particles may be formed together,fine- and coarse-mode particles typically originate by separatemechanisms, are transformed separately, have different chemicalcompositions, and require different control strategie
15、s. Vehicle ex-haust is a major source of ultrafine particles. Ultrafines are mini-mally affected by gravitational settling and can remain suspendedfor days at a time. Fine-mode particles generally originate fromcondensation or are directly emitted as combustion products. Manymicroorganisms (bacteria
16、 and fungi) either are in this size range orproduce components this size. These particles are less likely to be re-moved by gravitational settling and are just as likely to deposit onvertical surfaces as on horizontal surfaces. Coarse-mode particlesare typically produced by mechanical actions such a
17、s erosion andfriction. Coarse particles are more easily removed by gravitationalsettling, and thus have a shorter airborne lifetime.For industrial hygiene purposes, particles 5 m in diameter areconsidered respirable particles (RSPs) because a large percentageof them may reach the alveolar region of
18、the lungs. A cutoff of5.0 m includes 80 to 90% of the particles that can reach the func-tional pulmonary region of the lungs (James et al. 1991; Phalen et al.1991). Willeke and Baron (1993) describe a detailed aerosol sam-pling technique for RSPs, including the use of impactors. See alsothe discussi
19、on in the section on Sizes of Airborne Particles in Chap-ter 11 of the 2009 ASHRAE HandbookFundamentals.Bioaerosols are a diverse class of particulates of biological ori-gin. They are of particular concern in indoor air because of theirassociation with allergies and asthma and their ability to cause
20、 dis-ease. Chapters 10 and 11 of the 2009 ASHRAE HandbookFunda-mentals contains more detailed descriptions of these contaminants.Airborne viral and bacterial aerosols are generally transmitted bydroplet nuclei, which average about 3 m in diameter. Fungal sporesare generally 2 to 5 m in diameter (Whe
21、eler 1994). Combinations ofproper ventilation and filtration can be used to control indoor bio-aerosols. Morey (1994) recommends providing a ventilation rate of7 to 16.5 L/s per person to control human-shed bacteria. ACGIH(1989) recommends dilution with a minimum of 7 L/s per person. Italso reports
22、50 to 70% ASHRAE atmospheric dust-spot efficiencyThe preparation of this chapter is assigned to TC 2.4, Particulate Air Con-taminants and Particulate Contaminant Removal Equipment.29.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)filters can remove most microbial agents 1 to 2 m in diameter.Wh
23、eeler (1994) states that 60% ASHRAE atmospheric dust-spotefficiency filters remove 85% or more of particles 2.5 m in diame-ter, and 80 to 85% efficiency filters remove 96% of 2.5 m particles.AIR-CLEANING APPLICATIONSDifferent fields of application require different degrees of aircleaning effectivene
24、ss. In industrial ventilation, only removing thelarger dust particles from the airstream may be necessary for clean-liness of the structure, protection of mechanical equipment, andemployee health. In other applications, surface discoloration mustbe prevented. Unfortunately, the smaller components of
25、 atmo-spheric dust are the worst offenders in smudging and discoloringbuilding interiors. Electronic air cleaners or medium- to high-efficiency filters are required to remove smaller particles, especiallythe respirable fraction, which often must be controlled for healthreasons. In cleanrooms or when
26、 radioactive or other dangerous par-ticles are present, high- or ultrahigh-efficiency filters should beselected. For more information on cleanrooms, see Chapter 18 ofthe 2011 ASHRAE HandbookHVAC Applications.Major factors influencing filter design and selection include(1) degree of air cleanliness r
27、equired, (2) specific particle size rangeor aerosols that require filtration, (3) aerosol concentration,(4) resistance to airflow through the filter and (5) design face veloc-ity to achieve published performance.MECHANISMS OF PARTICLE COLLECTIONIn particle collection, air cleaners made of fibrous me
28、dia rely onthe following five main principles or mechanisms:Straining. The coarsest kind of filtration strains particlesthrough an opening smaller than the particle being removed. It ismost often observed as the collection of large particles and lint onthe filter surface. The mechanism is not adequa
29、te to achieve the fil-tration of submicrometre aerosols through fibrous matrices, whichoccurs through other physical mechanisms, as follows.Inertial Impingement. When particles are large or dense enoughthat they cannot follow the airstream around a fiber, they cross overstreamlines, hit the fiber, a
30、nd remain there if the attraction is strongenough. With flat-panel and other minimal-media-area filters havinghigh air velocities (where the effect of inertia is most pronounced),the particle may not adhere to the fiber because drag and bounceforces are so high. In this case, a viscous coating (pref
31、erably odorlessand nonmigrating) is applied to the fiber to enhance retention of theparticles. This adhesive coating is critical to metal mesh impinge-ment filter performance.Interception. Particles follow the airstream close enough to a fiberthat the particle contacts the fiber and remains there ma
32、inly because ofvan der Waals forces (i.e., weak intermolecular attractions betweentemporary dipoles). The process depends on air velocity through themedia being low enough not to dislodge the particles, and is thereforethe predominant capture mechanism in extended-media filters such asbag and deep-p
33、leated rigid cartridge types.Diffusion. The path of very small particles is not smooth buterratic and random within the airstream. This is caused by gas mol-ecules in the air bombarding them (Brownian motion), producing anerratic path that brings the particles close enough to a media fiber tobe capt
34、ured by interception. As more particles are captured, a con-centration gradient forms in the region of the fiber, further enhanc-ing filtration by diffusion and interception. The effects of diffusionincrease with decreasing particle size and media velocity.Electrostatic Effects. Particle or media el
35、ectrostatic chargecan produce changes in dust collection affected by the electricalproperties of the airstream. Some particles may carry a naturalcharge. Passive electrostatic (without a power source) filter fibersmay be electrostatically charged during their manufacture or (insome materials) by mai
36、nly dry air blowing through the media.Charges on the particle and media fibers can produce a strongattracting force if opposite. Efficiency is generally considered to behighest when the media is new and clean.EVALUATING AIR CLEANERSIn addition to criteria affecting the degree of air cleanliness, fac
37、-tors such as cost (initial investment, maintenance, and energy effec-tiveness), space requirements, and airflow resistance have led to thedevelopment of a wide variety of air cleaners. Comparisons of dif-ferent air cleaners can be made from data obtained by standardizedtest methods.The distinguishi
38、ng operating characteristics are particle size effi-ciency, resistance to airflow, and life-cycle capacity. Efficiencymeasures the ability of the air cleaner to remove particles from anairstream. Minimum efficiency during the life of the filter is themost meaningful characteristic for most filters a
39、nd applications.Resistance to airflow (or simply resistance) is the static pressuredrop differential across the filter at a given face velocity. The termstatic pressure differential is interchangeable with pressure dropand resistance if the difference of height in the filtering system isnegligible.
40、Life-cycle cost is the evaluation of device performancein the application in terms of overall cost along with filter servicelife, including element cost, energy consumption, maintenance, dis-posal, etc.Air filter testing is complex and no individual test adequatelydescribes all filters. Ideally, per
41、formance testing of equipmentshould simulate operation under actual conditions and evaluate thecharacteristics important to the equipment user. Wide variations inthe amount and type of particles in the air being cleaned make eval-uation difficult. Another complication is the difficulty of closelyrel
42、ating measurable performance to the specific requirements ofusers. Recirculated air tends to have a larger proportion of lint thandoes outdoor air. However, performance tests should strive to sim-ulate actual use as closely as possible.Arrestance. A standardized ASHRAE synthetic dust consistingof va
43、rious particle sizes and types is fed into the test air stream to theair cleaner and the mass fraction of the dust removed is determined.In the ASHRAE Standard 52.2 test, summarized in the segment onAir Cleaner Test Methods in this chapter, this measurement is calledsynthetic dust mass arrestance to
44、 distinguish it from other effi-ciency values.The indicated mass arrestance of air filters, as determined in thearrestance test, depends greatly on the particle size distribution ofthe test dust, which, in turn, is affected by its state of agglomeration.Therefore, this filter test requires a high de
45、gree of standardization ofthe test dust, the dust dispersion apparatus, and other elements oftest equipment and procedures. This test is particularly suited to dis-tinguish between the many types of low-efficiency air filters in theminimum efficiency reporting value (MERV) 1-4 categories. Theseare g
46、enerally roughing filters such as automatic rolls, metal wash-ables, or screen mesh filters used for gross concentration removal ofdebris and very large particles. It does not adequately distinguishbetween higher-efficiency filters.ASHRAE Atmospheric Dust-Spot Efficiency. This methodevaluated discol
47、oration (staining) of targets in upstream versusdownstream sampling. As of 2009, the dust-spot efficiency methodis no longer an ASHRAE standard of test, and was replaced withparticle-size-specific testing under Standard 52.2-2007. Note thatthere is no direct correlation between dust-spot efficiencie
48、s andMERV number values, because the basis for test is completely dif-ferent.Fractional Efficiency or Penetration. Defined-size particles arefed into the air cleaner and the percentage removed by the cleaner isdetermined, typically by a photometer, optical particle counter, orcondensation nuclei cou
49、nter. In fractional efficiency tests, the use ofAir Cleaners for Particulate Contaminants 29.3defined-particle-size aerosols results in an accurate measure of theparticle size versus efficiency characteristic of filters over a wideatmospheric size spectrum. This method has been used primarily inresearch, which led to the ASHRAE Standard 52.2 test, in which apolydispersed challenge aerosol such as potassium chloride ismetered into the test duct as a challenge to the air cleaner. Air sam-ples taken upstream and downstream are drawn through an opticalparticle counter or
