ASHRAE HVAC APPLICATIONS SI CH 46-2015 AIR CLEANERS FOR GASEOUS CONTAMINANTS.pdf
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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
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