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Air Cleaners for Gaseous Contaminants

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Air Cleaners for Gaseous Contaminants CHAPTER 47 AIR CLEANERS FOR GASEOUS CONTAMINANTS Terminology ............................................................................. 47.1 Air Cleaner System Design .................................................... 47.12 Gaseous Contaminants ............................................................ 47.3 Safety ...................................................................................... 47.17 Problem Assessment ................................................................ 47.7 Installation, Start-Up, and Commissioning ........................... 47.18 Contaminant Reduction Strategies .......................................... 47.8 Operation and Maintenance .................................................. 47.18 Contaminant Removal by Ventilation Air Cleaning ................ 47.8 Environmental Influences on Air Cleaners ............................ 47.19 Equipment .............................................................................. 47.11 Testing Media, Equipment, and Systems................................ 47.19 HE purpose of gas-phase (molecular) filtration is to remove Absorption. Transport and dissolution of a sorbate into an absor- Tair contaminants that could adversely affect the occupants, pro- bent to form a homogeneous mixture having the characteristics of a cesses, or contents of a space. The effects are problematic at different solution. It is important to distinguish absorption from the surface concentration levels for different contaminants. There are four cate- phenomenon of adsorption, which is one of the most important pro- gories of harmful effects: toxicity, odor, irritation, and material cesses in operation of air cleaners that remove gaseous contami- damage. In most cases, contaminants become annoying through irri- nants. tation or odor before they reach levels toxic to humans, but this is not Activated Carbon. Carbon, usually in the form of granules, always true. For example, the potentially deadly contaminant carbon treated to enhance its surface area and consequent ability to adsorb monoxide has no odor. More information on gaseous contaminants gases through a highly developed pore structure. and odors can be found in Chapters 11 and 12 of the 2017 ASHRAE Activity. Mass of contaminant contained in a physical adsorbent Handbook—Fundamentals. at saturation, expressed as a percentage or fraction of the adsorbent Indoor gaseous contaminant levels can sometimes be reduced mass (i.e., grams contaminant/grams adsorbent). Activity is an equi- with ventilation air drawn from outdoors, diluting the contaminants librium property under particular challenge conditions, and is not a to acceptable levels. However, available outdoor air itself may con- function of airflow. (In most cases, commercial bed filters are tain undesirable gaseous contaminants at unacceptable concentra- changed for efficiency reasons well before the adsorbent is saturat- tions. If so, it requires treatment by gaseous contaminant removal ed.) If a saturated adsorbent bed is then exposed to clean air, some of equipment before being used for ventilation. In addition, minimizing the adsorbed contaminant will desorb. Activity is generally greater outdoor airflow, as specified in ASHRAE Standard 62.1’s IAQ pro- than receptivity. cedure, by using a high recirculation rate and filtration is an attrac- Adsorbent. Any solid having the ability to concentrate signifi- tive means of energy conservation. However, recirculated air cannot cant quantities of other substances on its surface. be made equivalent to fresh outdoor air by removing only particulate Adsorption. Process in which the molecules of a gas or vapor ad- contaminants. Noxious, odorous, and toxic contaminants must also here by physical or chemical processes to the exposed surface of be removed by gaseous contaminant removal equipment, which is solid substances (both the outer surface and inner pore surface) with frequently different from particulate filtration equipment. which they come into contact. This chapter covers design procedures for gaseous contaminant Adsorption, Chemical (Chemisorption). Binding of a contami- air-cleaning systems for occupied spaces only. Procedures dis- nant to the surface of an adsorbent by forces with energy levels cussed are appropriate to address odors and gaseous irritants. approximately those of a chemical bond. Chemisorption is an irre- Removal of contaminants for the express purpose of protecting versible process. building occupants (whether against deliberate attack or industrial Adsorption, Physical. Attraction of a contaminant to the surface, accidents) or to protect artifacts (e.g., in museums) requires appli- both outer surface and inner pore surface, of adsorbent media by cation of the same design principles, but applied more rigorously physical forces (Van der Waals forces). Physical adsorption is a re- and with great emphasis on having specific design and performance versible process. data, providing redundancy, and added engineering safety factors. Adsorption Capacity. The amount (mass or moles) of a selected Design for protection is not a focus of this chapter, although pub- contaminant that can be contained in the media of a gas-phase air- lished design guidance is included and referenced; for more detail, cleaning device under given test conditions and end point. see Chapter 61. Aspects of air-cleaning design for museums, librar- Adsorbent, Regenerated. Adsorbent material which, after satu- ies, and archives are included in Chapter 24, and removal of gaseous ration, may be treated to recover its adsorption properties and thereby contaminants from industrial processes and stack gases is covered enabled for reuse. in Chapter 30 of the 2016 ASHRAE Handbook—HVAC Systems and Adsorption Isotherm. A curve obtained by plotting, at constant Equipment. temperature, the quantity of adsorbate against the concentration of the substance in the original gas or solution. 1. TERMINOLOGY Breakthrough. Point in the operating cycle of a gas-phase air- cleaning device at which the effluent contaminant concentration be- In particular, gaseous contaminant control technology perfor- comes measurable. Thereafter, the effluent concentration may rise mance is a function of (1) the specific contaminant, (2) its concen- rapidly. tration, (3) airflow rate, and (4) environmental conditions. The Breakthrough Curve. Plot of contaminant penetration through terminology related to gaseous contaminant air-cleaning equipment the air cleaner versus time for a particular challenge concentration is specific to the field, and some terms that are familiar from particle and airflow. filtration differ slightly in this context. Breakthrough Time. Operating time (at constant operating con- ditions) before a certain penetration is achieved. For instance, the The preparation of this chapter is assigned to TC 2.3, Gaseous Air Contam- 10% breakthrough time is the time between beginning to challenge inants and Gas Contaminant Removal Equipment. a physical adsorbent or chemisorbent and the time at which air 47.1 47.2 2019 ASHRAE Handbook—HVAC Applications discharged contains 10% of the contaminant feed concentration. Pressure Drop. Difference in absolute (static) pressure between Continued operation leads to 50% and eventually to 100% break- two points in an airflow system, caused by frictional resistance to through, at which point a physical adsorbent is saturated. For a airflow in a duct, filter, or other system component such as a media chemisorbent, the media is exhausted. (Some commercial devices bed or air-cleaning device. are designed to allow some of the challenge gas to bypass the adsor- Reactivation. Treatment of carbon with elevated temperature bent. These devices break through immediately, and breakthrough and steam to remove the volatiles that have been adsorbed on the time, as defined here, does not apply.) carbon, so that the adsorbent can be reused. Capacity. See Adsorption Capacity. Regeneration. The process of treating carbon thermally or CAS Number. An identification number unique to each chemically to extend its life. individual chemical, specified by the Chemical Abstracts Service Removal Efficiency. Fraction or percentage of a challenge con- (CAS), a division of the American Chemical Society (ACS). taminant that is removed by the air-cleaner media bed at a given time. Catalyst. Any substance that, when present in a small quantity, Removal efficiency is also known simply as “efficiency.” significantly affects the rate of a chemical reaction without itself Residence Time. Theoretical time period that a contaminant being consumed or undergoing a chemical change. Most catalysts molecule is within the boundaries of the media bed of a physical accelerate reactions, but a few retard them (negative catalysts, or adsorbent or chemisorbent. The longer the residence time, the inhibitors). higher the efficiency. For gaseous contaminant removal equipment, Channeling. The disproportionate or uneven flow of fluid (gas or residence time is computed as liquid) through passages of lower resistance that can occur in fixed Bed area exposed to airflow Bed depth beds or columns of granular media because of nonuniform packing, Residence time (s) = -------------------------------------------------------------------------------------------------- (1) irregular sizes and shapes of media, gas pockets, wall effects, and Airflow rate other causes. For commercial gaseous contaminant air cleaners, residence time Challenge (Air) Stream. Test contaminant(s) of interest
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