1、46.1CHAPTER 46AIR CLEANERS FOR GASEOUS CONTAMINANTSTerminology . 46.1Gaseous Contaminants 46.2Problem Assessment. 46.5Contaminant Reduction Strategies. 46.7Contaminant Removal by Ventilation Air Cleaning. 46.7Equipment 46.10Air Cleaner System Design 46.11Safety 46.16Installation, Start-Up, and Commi
2、ssioning . 46.16Operation and Maintenance. 46.17Environmental Influences on Air Cleaners 46.17Testing Media, Equipment, and Systems 46.18HE purpose of gas-phase (molecular) filtration is to removeTfrom the air contaminants that would adversely affect the occu-pants, processes, or contents of a space
3、. The effects are problematicat different concentration levels for different contaminants. Thereare four categories of harmful effects: toxicity, odor, irritation, andmaterial damage. In most cases, contaminants become annoyingthrough irritation or odor before they reach levels toxic to humans,but t
4、his is not always true. For example, the potentially deadly con-taminant carbon monoxide has no odor. More information on gas-eous contaminants and odors can be found in Chapters 11 and 12 ofthe 2013 ASHRAE HandbookFundamentals.Indoor gaseous contaminant levels can sometimes be reducedwith ventilati
5、on air drawn from outdoors, diluting the contaminantsto acceptable levels. However, available outdoor air may containundesirable gaseous contaminants at unacceptable concentrations. Ifso, it requires treatment by gaseous contaminant removal equipmentbefore being used for ventilation. In addition, mi
6、nimizing outdoorairflow, as specified in ASHRAE Standard 62.1s IAQ procedure, byusing a high recirculation rate and filtration is an attractive means ofenergy conservation. However, recirculated air cannot be madeequivalent to fresh outdoor air by removing only particulate contam-inants. Noxious, od
7、orous, and toxic contaminants must also beremoved by gaseous contaminant removal equipment, which is fre-quently different from particulate filtration equipment.This chapter covers design procedures for gaseous contaminantair-cleaning systems for occupied spaces only. Procedures dis-cussed are appro
8、priate to address odors and gaseous irritants.Removal of contaminants for the express purpose of protectingbuilding occupants (whether against deliberate attack or industrialaccidents) or to protect artifacts (such as in museums) requiresapplication of the same design principles, but applied more ri
9、gor-ously and with great emphasis on having specific design and per-formance data, providing redundancy, and added engineering safetyfactors. Design for protection is not a focus of this chapter, althoughpublished design guidance is included and referenced; for moredetail, see Chapter 59. Aspects of
10、 air-cleaning design for museums,libraries, and archives are included in Chapter 23, and removal ofgaseous contaminants from industrial processes and stack gases iscovered in Chapter 30 of the 2012 ASHRAE HandbookHVAC Sys-tems and Equipment.1. TERMINOLOGYThe terminology related to gaseous contaminan
11、t air-cleaningequipment is specific to the field, and the meaning of some terms fa-miliar from particle filtration is slightly different. In particular, gas-eous contaminant technology performance is a function of (1) thespecific contaminant, (2) its concentration, (3) airflow rate, and(4) environme
12、ntal conditions. Several methods of measuring theperformance of a gaseous air-cleaning device, some unique to thisapplication, are defined in the following.Absorption. Transport and dissolution of a sorbate into an absor-bent to form a homogeneous mixture having the characteristics of asolution. It
13、is important to distinguish absorption from the surfacephenomenon of adsorption, which is one of the most important pro-cesses in operation of air cleaners that remove gaseous contami-nants.Adsorbent. Any solid having the ability to concentrate signifi-cant quantities of other substances on its surf
14、ace.Adsorption, chemical (chemisorption). Binding of a contami-nant to the surface of a solid by forces with energy levels approxi-mately those of a chemical bond. This process is usually followed bychemical reaction that removes the contaminant from the airstream,but may add other gases to it. Chem
15、isorption is an irreversible process.Adsorption, physical. Attraction of a contaminant to the surface,both outer surface and inner pore surface, of adsorbent media byphysical forces (Van der Waals forces). Physical adsorption is areversible process.Activity. Mass of contaminant contained in a physic
16、al adsorbentat saturation, expressed as a percentage or fraction of the adsorbentmass (i.e., grams contaminant/grams adsorbent). Activity is an equi-librium property under particular challenge conditions, and is not afunction of airflow. (In most cases, commercial bed filters arechanged for efficien
17、cy reasons well before the adsorbent is satu-rated.) If a saturated adsorbent bed is then exposed to clean air, someof the adsorbed contaminant will desorb. Activity is generallygreater than retentivity.Breakthrough. Point in the operating cycle of a gas-phase air-cleaning device at which the efflue
18、nt contaminant concentrationbecomes measurable. Thereafter, the effluent concentration may riserapidly.Breakthrough curve. Plot of contaminant penetration throughthe air cleaner versus time for a particular challenge concentrationand airflow.Breakthrough time. Operating time (at constant operating c
19、on-ditions) before a certain penetration is achieved. For instance, the10% breakthrough time is the time between beginning to challengea physical adsorbent or chemisorbent and the time at which airdischarged contains 10% of the contaminant feed concentration.Continued operation leads to 50% and even
20、tually to 100% break-through, at which point a physical adsorbent is saturated. For a che-misorbent, the media is exhausted. (Some commercial devices aredesigned to allow some of the challenge gas to bypass the adsorbent.These devices break through immediately, and breakthrough time, asdefined here,
21、 does not apply.)Capacity. Amount (mass or moles) of a selected contaminant thatis contained in the media of a gas-phase air-cleaning device at giventest conditions.CAS number. An identification number unique to each individ-ual chemical, specified by the Chemical Abstracts Service (CAS), adivision
22、of the American Chemical Society (ACS).The preparation of this chapter is assigned to TC 2.3, Gaseous Air Contam-inants and Gas Contaminant Removal Equipment.46.2 2015 ASHRAE HandbookHVAC Applications (SI)Catalyst. Any substance that, when present in a small quantity,significantly affects the rate o
23、f a chemical reaction without itselfbeing consumed or undergoing a chemical change. Most catalystsaccelerate reactions, but a few retard them (negative catalysts, orinhibitors).Channeling. The greater flow of fluid (gas or liquid) throughpassages of lower resistance that can occur in fixed beds or c
24、olumnsof granular media because of nonuniform packing, irregular sizesand shapes of media, gas pockets, wall effects, and other causes.Challenge (air) stream. Test contaminant(s) of interest dilutedto the specific concentration(s) of the test prior to filtration.Concentration. Quantity of one substa
25、nce dispersed in a definedamount of another. Concentrations of contaminants in air are usu-ally expressed as parts per million by volume (ppmv) or as milli-grams of contaminant per cubic metre of air (mg/m3).Density, apparent (bulk density). Mass under specified condi-tions of a unit volume of a sol
26、id physical adsorbent or chemisorbent,including its pore volume and interparticle voids.Desorption. Process by which adsorbed molecules leave the sur-face of a physical adsorbent and reenter the fluid stream. This pro-cess is the opposite of adsorption.Efficiency. See Removal Efficiency.Efficiency c
27、urve. Plot of contaminant removal efficiency againsttime for a particular challenge concentration and airflow.HEPA filter. High-efficiency particle air filter.Mass transfer zone. Depth of physical adsorption or chemi-sorption media required to remove essentially all of an incomingcontaminant; depend
28、ent on type of media, media granule size, con-taminant nature, contaminant inlet concentration, and environmen-tal conditions.Mean particle diameter. Weighted average particle size, in mil-limeters, of a granular adsorbent; computed by multiplying the per-cent retained in a size fraction by the resp
29、ective mean sieveopenings, summing these values, and dividing by 100.Media. Granular or pelletized physical adsorbent (or chemisor-bent) used in gaseous contaminant air-cleaning equipment. Alsoused to refer to a material (e.g., a nonwoven) that contains a physicaladsorbent or chemisorbent.Penetratio
30、n. Ratio of contaminant concentration downstream ofthe media bed to the upstream (challenge) concentration, sometimesexpressed as a percentage. Related to fractional efficiency by theexpression Efficiency = (1 Penetration). Unlike particulate filters,physical adsorbents and chemisorbents both declin
31、e in efficiency asthey load. The decline can be very sudden, and is usually not linearwith time.Pressure drop. Difference in pressure between two points in anairflow system, caused by frictional resistance to airflow in a duct,filter, or other system component such as a media bed or air-cleaning dev
32、ice.Removal efficiency. Fraction or percentage of a challenge con-taminant that is removed by the air-cleaner media bed at a given time.Residence time. Theoretical time period that a contaminant mole-cule is within the boundaries of the media bed of a physical adsorbentor chemisorbent. The longer th
33、e residence time, the higher the effi-ciency, and the longer the bed life. For gaseous contaminant removalequipment, residence time is computed asResidence time = (1)For commercial gaseous contaminant air cleaners, residence timecomputation neglects the fact that a significant fraction of the volume
34、of the bed is occupied by the media. For example, a unitary adsorbercontaining trays totaling 4 m2media in a 25 mm deep bed, challengedat 1 m3/s, has a residence time of 0.1 s. Given this definition, adeeper media bed, lower airflow rate, or media beds in seriesincrease residence time and thus perfo
35、rmance. Because gaseouscontaminant air cleaners all tend to have approximately the samegranule size, residence time is a generally useful indicator of per-formance. In some engineering disciplines, the media volume issubtracted from the nominal volume of packed beds when calculat-ing residence time.
36、 This gives a shorter residence time value and isnot normally used for HVAC. A minimum 0.07 s residence time forphysical adsorbents, and 0.1 s for chemisorbents, will provide for aminimum 95% contact time.Different ways of arranging the media, different media, or differ-ent media granule sizes all c
37、an change the effective residence timebecause of their effect on the volume of the bed occupied by themedia. The geometry and packaging of some technologies makescomputation of residence time difficult. For example, the flow pat-tern in pleated fiber-carbon composite media is difficult to specify,ma
38、king residence time computation uncertain. Therefore, althoughresidence times can be computed for partial-bypass filters, fiber-adsorbent composite filters, or fiber-bonded filters, they cannot becompared directly and may serve more as a rating than as an actualresidence time. Manufacturers might pu
39、blish equivalent residencetime values that say that a particular physical adsorbent or chemi-sorbent performs the same as a traditional deep-bed air cleaner, butno standard test exists to verify such a rating.Retentivity. Measure of the ability of a physical adsorbent orgas-phase air-cleaning device
40、 to resist desorption of an adsorbate. Itis usually stated as a percentage or fraction of the adsorbent mass.Retentivity is generally less than activity.Saturation. State of a physical adsorbent when it contains all thecontaminant it can hold at the challenge concentration, temperature,and humidity
41、of operation.Vapor (vapor-phase contaminant). Substance whose vaporpressure is less than the ambient pressure at ambient temperaturebut is present in the gas phase by evaporation or sublimation.VOCs. Volatile organic compounds.2. GASEOUS CONTAMINANTSAmbient air contains nearly constant amounts of ni
42、trogen (78%by volume), oxygen (21%), and argon (0.9%), with varyingamounts of carbon dioxide (about 0.04%) and water vapor (up to3.5%). In addition, trace quantities of inert gases (neon, xenon,krypton, helium, etc.) are always present.Gases and vapors other than these natural constituents of air ar
43、eusually considered to be gaseous contaminants. Their concentra-tions are almost always small, but they may have serious effects onbuilding occupants, construction materials, or contents. Removingthese gaseous contaminants is often desirable or necessary.Sources of nonindustrial contaminants are dis
44、cussed in Chapter11 in the 2013 ASHRAE HandbookFundamentals. However, forconvenience, data on some of the contaminants in cigarette smoke(Table 1), and some common contaminants emitted by buildingmaterials (Table 2), indoor combustion appliances (Table 3), andoccupants (Table 4) are provided here.Ta
45、ble 5 gives typical outdoor concentrations for gaseous contam-inants at urban sites; however, these values may be exceeded if thebuilding under consideration is located near a fossil fuel powerplant, refinery, chemical production facility, sewage treatment plant,municipal refuse dump or incinerator,
46、 animal feed lot, or othermajor source of gaseous contaminants. If such sources have a sig-nificant influence on the intake air, a field survey or dispersionmodel must be run. Many computer programs have been developedto expedite such calculations.Using Source Data to Predict Indoor ConcentrationsSo
47、urce data such as those in Tables 1 to 5 provide the type of rawinformation on which air-cleaning system designs can be based asBed area exposed to airflow Bed depthAirflow rate-Air Cleaners for Gaseous Contaminants 46.3well as indicating which parameters to measure. Outdoor air con-taminants enter
48、buildings through the outdoor air intake and throughinfiltration. The indoor sources enter the occupied space air and aredistributed through the ventilation system. If measurements are notavailable, source data can be used to predict the contaminantchallenge to air-cleaning systems using building ai
49、r quality models.The following relatively simple published model is intended as anintroduction to the topic.Meckler and Janssen (1988) described a model for calculatingthe effect of outdoor pollution on indoor air quality, which is out-lined in this section and provided in the appendix of ASHRAE Stan-dard 62.1 and 62.2.A recirculating air-handling schematic is shown in Figure 1. Inthis case, mixing is not perfect; the horizontal dashed line repre-sents the boundary of the region close
