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Ventilation and air pollution control
VENTILATION AND AIR POLLUTION CONTROL by Arthur N. Mabbett Mabbett & Associates Inc., Bedford, Mass. The primary function of an industrial exhaust system is to protect workers against potentially toxic and irritating airborne contaminants generated in the workplace: contaminants from metal-finishing tanks are generally irritants to human tissue. Insufficient ventilation m a y b e manifested by worker complaints or obvious eye, nose, or throat irritation upon entering the work environment. Air sampling may be necessary to evaluate exposure conditions where substances with-poor warning properties are in use. Substances with poor warning properties are those that do not produce noticeable odor or irritation even at unacceptable levels. A properly functioning exhaust system will have the benefits of maintaining a safe comfortable work environment, as well as prolonging the life of corrosion-susceptible plan components and equipment. New facilities should be designed to control these contaminants in the workplace through proper ventilation. Existing facilities should be evaluated for acceptability of the work environment. This article describes the basics of ventilation design and the use of air pollution control equipment to remove contaminants before discharge to the outside air.
CODE OF COMPLIANCE The quality of workroom air is regulated by the federal Occupational Safety and Health Administration (OSHA) and, to some extent, by state bureaus of occupational hygiene. Copies _ of applicable regulations should be obtained prior to initiating design activities. Two generally accepted reference documents for designers are Practices for Ventilation and Operation of Open Surface Tanks, by the American National Standards Institute (ANSI) (Z91-1977), and Industrial Ventilation: A Manual of Recommended Practices, by the American Conference of Governmental Industrial Hygienists (ACGIH), Committee on Industrial Ventilation, Lansing, Mich. The Sheet Metal and Air Conditioning Contractors' National Association, Inc. (SMACNA), Chantilly, Va., has also published several manuals that are useful for specifying engineers and installers.
TYPES OF SYSTEMS The two methods o f handling contaminant exhaust are local exhaust ventilation and general dilution (Fig. 1). Local exhaust ventilation means controlling contamination at its source, whereas dilution ventilation means treating the workroom as if it were 9 "mixing box" or contamination sink. In general, dilution ventilation is not as effective for health hazard control as is local ventilation. Dilution ventilation requires greater air flows to achieve the same effect as local exhaust ventilation and cannot protect workers who must work close to the process operation. Although there are occasional circumstances where dilution ventilation must be used because the operation or process cannot acconmaodate local exhaust, local exhaust ventilation is by far the most cost-effective and generally accepted method of contaminant control. Many plating and metal-finishing shops have a number of different open surface tank processes that require exhaust ventilation. The exhausts from most, if not all, o~fthe process baths can usually be combined into a single exhaust system; however, segregation into multiple systems is desirable in certain situations.
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Figure 1. Left: general dilution ventilation--room behaves as a mixing chamber; right: local exhaust ventilation--localized capture provides lower room and breathing zone contaminant concentrations using the same amount of exhaust air as general dilution ventilation.
Layout or duct-sizing considerations may indicate the need for separate systems. If a process is infrequently used, a separate exhaust system, which can be turned oft', will save energy. When both acid- and cyanide-containing solutions are to be vented, separate systems should be used, if practical, to prevent the possible formation of hydrogen cyanide gas. Similarly, ammonia- and chlorine-containing exhaust streams should be kept separate to prevent formation of ammonium chloride, a fine white particulate that will cause a visible plume, which is difficult for conventional air pollution control equipment to remove. Oxides of nitrogen (NOx) from fuming nitric acid tanks present a particularly difficult emissioncontrol problem. If strict control of NO x emissions is indicated by plant siting considerations or by air-quality authorities, the NO x exhaust stream should be kept separate to minimize treatment costs. Generally, the advantages of separate systems are partially offset by the higher overall capital cost of air-moving and cleaning equipment, and by potential make-up air distribution control problem. NATURE
OF
CONTAMINANTS
Not all finishing operations are identical in emissions of air contaminants. Air contaminants from open surface tanks vary in type, intensity, and toxicity depending upon the operation. All of these factors are significant in the determination of ventilation and air pollution control requirements for a facility. The type of emission is very important when designing a ventilation system. There are two types of air contaminants from open surface tanks: mists and gases, typically, tanks containing water-based solutions emit mist droplets in the range of 0.5-50 /xm in diameter, having the same chemical make-up as the tank. Gases and vapors, characterized as molecular forms having diameters <0.01 /xm, are emitted from the evaporation of tank components; such emissions can include hydrogen cyanide, hydrogen fluoride, hydrogen chloride in water-based solutions, as well a s solvents such as methanol, naphtha, or chlorinated hydrocarbons. These types of emissions can be captured using local ventilation. The distinction becomes important when considering effective capture by a local exhaust, transport in ductwork, and collection in air pollution control devices. 780
The rate of evolution of air contaminants will, in part, determine the ventilation requirements and is dependent on the type of finishing operation performed and on the specifics of an individual operation. Mists arise from the bursting of bubbles at the liquid surface, entraining tank components into the air. These bubbles can be caused by electrolysis, by nonelectrolytic chemical reactions with the work, and by physical actions such as air and mechanical agitation of the bath. Gaseous emissions can be affected by the same elements, as well as by temperature. In both cases, the rate of material processed is a major factor in emissions. Based on the rate of contaminant released by the tank, the rate of local exhaust is determined. Highly reactive metals or processes require high ventilation rates. The most important factor in local exhaust is the toxicity of the air contaminant. Toxicity is specific to the chemical emifled. Maximum permissible exposure limits (PELs) are mandated by OSHA and guideline threshold limit values (TLVs) are recommended by the ACGIH. Acceptable levels in the air can range from 0.1 to 50 ppm and are based on available data concerning health effects of exposure to the compounds. A local exhaust and ventilation system is designed based on the allowable or recommended exposure levels. These three factors, type, rate, and toxicity of contaminant release, are carefully considered when designing a local exhaust ventilation system. Data have been tabulated for each factor and, as is described in subsequent sections, ventilation systems can be designed from these compilations.
DESIGN Exhaust system design begins with the individual exhaust hood. The intent of the exhaust hood is to sweep emissions from the tank surface, away from the workers' breathing zone, in the most energy-efficient manner. The amount of exhaust flow and, therefore, of energy required, depends on the particular plating or metal-finishing process and the degree to which it can be physically enclosed. Enclosure is desirable because it minimizes crossdraft interference and reduces the amount of open surface area to be controlled. It is best achieved with full or partial covers, or with booth-type arrangements over the tank surface. Surface enclosure is also partly achievable by floating spheres or surface tension chemical agents in the liquid. Once a hood configuration has been selected, the next design step is to calculate the required exhaust flow. Exhaust flows are calculated based upon the individual processes and are dependent upon the type of contaminants to be controlled, the rate at which contaminants are evolved, the temperature, and the size of operations, among other factors. In general, exhaust flows should adequately control workroom air concentrations to within acceptable health levels. Potential corrosion of the workroom and workpieces is another concern. (Refer to the aforementioned documents for process specific ventilation rtes.) Exhaust flows in excess of those required for health considerations may be necessary to control product quality. It should also be noted that OSHA mandates the use of standard tables for flow calculation if employee exposure to air contaminants is found by air sampling to be in excess of the PELs. (Refer to OSHA General Industry Standards, 29 CFR 1910.94[d] and 1910.1000.)
HOOD TYPES The three basic hood designs are the lateral slot hood, the canopy hood, and the booth hood with one open side. The lateral and canopy types are most efficient when combined with baffles or curtains to approach the booth configuration having one open side. 782
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LATERAL EXHAUST Lateral slot hoods are depicted in Fig. 2. Although lateral hoods have the potential for requiring the greatest amount of exhaust flow among the various hood types, they are the type most commonly installed because they create minimum physical interference with the process. The slot entry must be sized to furnish a minimum-recommended entry velocity of 2,000 fpm. The recommended slot-entry velocity will ensure even distribution of flow across the slot opening. Slot velocity alone does not determine the control velocity achieved at the tank surface boundary: the slot's sole purpose is to distribute flow. The plenum behind the slot should have a depth at least twice the slot width to ensure distribution across the face. Where the effective width (W) over which the hood must pull air to operate exceeds 20 in., slots on both sides are desirable. Where W exceeds 36 in., slots on both sides are necessary. If W exceeds 48 in., lateral exhaust is not usually practical unless enclosure or push-pull arrangements are used. Variations of the basic lateral slot hood are depicted in Fig. 2B and C. Slots on two or more sides of the tank provide more favorable aspect ratios, allowing the use of less exhaust air relative to a single slot. Minimum-recommended flow rates are for tank locations with minimum crossdrafts, specifically crossdrafts <50 fpm. Lateral hoods are highly susceptible to crossdrafts caused by open windows or doors, particularly when make-up air is not mechanically furnished to the space. The distribution of make-up air to the workroom can also be a source of undue
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PUSH-PULL LATERAL EXHAUST For large open surface tanks, normal lateral exhaust is not feasible due to the enormous amount of exhaust needed to maintain capture velocities at the far end of the tank. Instead, a push-pull-type exhaust, as depicted in Fig. 3, can be used. Push-pull exhaust combines a lateral slot hood at one end of the tank with a jet of push air at the opposite end. Push air is typically furnished by a dedicated supply fan located in the workroom: compressed air is an unnecessarily expensive source of push air. Push-pull systems offer the potential advantage of achieving control with less exhaust air flow, particularly where crossdrafts are a problem, making exhaust of wide tanks more feasible. One recent study demonstrated a 97% contaminant capture efficiency for a 4 × 6-ft heated tank equipped with anode bars, where crossdrafts were 75 fpm. Push air was furnished through a 1A-in. slot at 35-45 cfm/ft of tank length, and exhaust flow was 50-75 cfm/ft2 of tank area. The design of push-pull systems is empirical, and such systems should be designed so that they can be easily modified or adjusted to obtain the desired results. The major drawback of push-pull systems is that obstructions, such as the work, anode bars, etc., may deflect the curtain of air and scatter it, along with air contaminants, into the work area; hence, the need for design flexibility. Design criteria can be found in the latest edition of the ACGIH manual or other references.
OTHER EXHAUST HOODS The canopy hood is a receiving-type hood and is most appropriate for hot processes where vertical convection currents move contaminants toward the hood. Canopy hoods work best when side curtains are furnished approaching the booth configuration. A canopy hood without curtains is not recommended for cold processes due to the ease with which crossdrafts divert contaminants away from the hood.
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Table I. Comparative Exhaust Flow Rates for Various Hood Types°
Control velocity, fpm Design velocity, fpm Flow rate, cfm
Single Slot
Lateral Exhaust Slots at Front and Back
Push-Pull
Open Canopy
Enclosing Booth
100 175 2,190
100 150 1,875
-100 1,250
175 -8,135
175 -750
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COMPARING EXHAUST OPTIONS For comparison purposes, Table I indicates the minimum-recommended exhaust flow for a 21/2 × 5-ft alkaline cleaning tank operating at 200°F, for each of the hood types discussed. The table illustrates the differences of one hood type or another in required air flows.
DUCT SYSTEMS Ducts are the link between the fan and hood to convey air contaminants. Industrial exhaust ducts are sized by the system designer to provide an air velocity that will keep air contaminants in suspension during transport to the scrubber fan, but which is not so great as to cause unnecessary friction losses and equipment strain. For gases and vapors, any transport velocity is acceptable; economic considerations result in a usual transport velocity of 1,000-1,500 fpm. For open surface tanks, and the contaminants generated, a duct velocity of 2,000-3,000 fpm is commonly recommended. There are two general approaches to duct design, the "self-balanced system" and "blast gate" methods. With the self-balanced system approach, ducts are sized and flows are adjusted so that the minimum-desired hood flows are achieved without the use of balancing dampers or blast gates. With the blast gate method, system balancing is achieved using strategically placed blast gates, or blast gates at each hood. (The reader is directed to the ACGIH Industrial Ventilation M a n u a l for details.) Eac h design has advantages and disadvantages. The self-balanced system has the advantage of being relatively tamperproof and of ensuring that design transport velocities are maintained. Flows cannot be altered without adjusting the fan speed or drive. The disadvantage is reduced flexibility because temporarily inactive hoods or branches cannot easily be shut oft'. Conversely, the blast gate method is subject to tampering, but offers greater operational flexibility. Accordingly, self-balanced systems are ideal for nonchanging processes and processes utilizing extremely hazardous components, where ventilation tampering is undesirable. Prior to selecting a fan from the manufacturers' performance tables, the actual pressure losses must be obtained by calculation of duct, hood, and air acceleration losses, and by contacting air pollution control equipment manufacturers. Typical fan static pressures for open surface tank exhaust systems are 2-in. water column (WC) for systems without scrubbers, and 3-5 in. for systems with scrubbers. Slot hoods, designed as previously described, will produce a pressure drop ranging from approximately 0.5 to 1 in. WC. The typical duct system will add an additional 0.5-1 in. WC. Exhaust gas scrubbers have pressure drops typically ranging from 0.5 to 3 in. WC. Smooth air flow is an important consideration for duct-system design to conserve energy. Good engineering fluid flow practices and general principles should be incorporated into the design and construction of a duct system. Flexible duct is commonly employed as an expediency, but its use should be minimized. Resistance to flow from flexible duct is 788
substantially greater than that for smooth duct. This results in excessive fan horsepower for systems designed for flexible duct, or inadequate flow on systems not specifically designed for flexible duct. Impaction of contaminant mists on duct surfaces invariably leads to buildup in the ductwork of liquids and solids left behind by evaporation of the liquids. Cleanonts should be furnished at specified intervals on straight horizontal runs and at each elbow. Ducts should be sloped to a low point and drains with traps or isolation valves should be furnished at this point. The most cost-effective duct material will vary from installation to installation. Duct selection, and therefore cost, depends upon corrosion and temperature requirements, desired lifetime, and insurance underwriter requirements. Several materials are commonly used for duct and hood construction. Manufacturers' tables of corrosion resistance should be consulted during selection of specific products. An excellent reference source of corrosion-resistant properties for numerous materials is the Corrosion Data Survey, metals and nonmetals sections, obtainable from the National Association of Corrosion Engineers, Houston, Texas. In general, duct service is less severe than the immersion ratings typically given in tables of corrosion resistance. Hoods and ductwork are typically constructed of galvanized, resin-coated or stainless steel, or plastics such as polypropylene, polyvinyl chloride (PVC), or fiberglass-reinforced plastic (FRP). The plastics are currently in widespread use because of their excellent corrosion-resistant properties. A prime consideration in the selection of duct and hood materials is combustibility. Polypropylene, PVC, and FRP are all combustible to varying degrees. Fires originating in process'tanks, typically caused by electric immersion heaters in dry tanks, are a fairly common occurrence. The tank fire spreads to the plastic hood, with the potential for involving the entire exhaust system and adjacent building components. Some insurance underwriters require plastic ductwork to be furnished with internal sprinklers, whereas others will accept the system without sprinklers provided that materials do not exceed a certain thickness, e.g., V4-in. maximum thickness for PVC. The basis for the thickness exemption is that thin walls will melt more quickly and interrupt flame spread. The FRP duct is usually assigned a flame spread rating, with better ratings achieved by the addition of fire retardants to the resin. A few proprietary FRP duct systems, which incorporate patented flame interrupters and fireretardant resins, have been approved for use without internal sprinklers. Where higher temperatures may be encountered due to the process, or on rooftops in southern climates, temperature-rated FRP or steel construction may be preferable to plastics, which soften at elevated temperatures. Open surface tank exhaust systems draw in large amounts of room air, consequently the duct is typically exposed to temperatures of 100°F or less. The relative cost of one material compared with others for a given duct system will depend upon the number and size of fittings, the availability of standard extruded or fabricated duct for the desired diameter, and the availability of qualified installers for specialty duct. Generally speaking, relative duct cost for an installed system is given in ascending order of cost as follows: plain steel, galvanized steel, resin-coated steel, PVC, polypropylene, FRP, proprietary flame-interrupting FRP, and stainless steel.
EXHAUST FAN TYPES The energy required to induce a flow of air through a local exhaust system is provided by an exhaust fan. There are two basic types of fans to choose from, axial and centrifugal. Among the available materials of construction, PVC and FRP are more commonly used for exhaust fans handling the highly corrosive fumes that are produced in metal finishing. Axial fans move air parallel to the fan shaft. The propeller type is commonly used to move air from one room to another or from outdoors to indoors. Vane axial and tube axial fans are both installed in ducts, with the main difference between them being the higher pressures
789
achieved in vane axial fans (up to about 6 in. water gauge) due to their air guide vanes. The relatively low static pressures obtained by axial fans, as well as their low resistance to abrasive dusts, limit their usefulness as industrial process exhaust fans. Centrifugal fans can be classified into three main types: forward curved blade, backward inclined (BI) blade, and radial blade. A forward curved blade, or squirrel cage fan has blades that curve toward the direction of fan rotation. They deliver tow to medium volumes at low pressures. Their low rotational speed for a given air volume makes them quiet in operation. The blade shape tends to collect dust and liquid, making these fans unsuitable for acid mists and other dirty particulate exhaust strealTiS.
A BI blade centrifugal fan has blades that are inclined away from the direction of rotation. They can deliver air against a high enough pressure drop to be used with fume scrubbers. Their high efficiency and nonoverloading characteristics make BI fans a good choice for many exhaust applications. Although they can handle a higher dust loading than forward curved fans, they are not the ideal choice for highly abrasive exhaust streams. The BI wheel fans are the most commonly used centrifugal exhaust fans for open surface tank ventilation. The exhaust fan is usually placed downstream of any scrubber or other pollution-control device being used to minimize buildup of material on the fan blades. Radial blade centrifugal fans have blades similar to those of a paddle wheel. This blade orientation limits material buildup and the moderate rotation speed means particles move along the blades at lower velocities than in all but forward curved fans. Radial blade fans are, therefore, widely used for industrial applications where heavily dust laden streams are being exhausted. This type of fan can provide a high static pressure and can be a low maintenance item because of its heavy duty construction. Radial fans have somewhat lower efficiency than other industrial fans, which is the trade-off for its advantages.
FAN SELECTION Fan selection for any exhaust application must be based on matching the fan flow characteristics to the desired flow and resistance of the system. The flow characteristics of a particular fan are determined empirically by the fan manufacturer. The desired flow and resistance are calculated in the exhaust system design, as discussed previously. The purchase price for a particular size and type of exhaust fan can vary considerably depending on the materials of construction. The greatest cost differential is between fans of FRP construction and those of epoxy-coated steel, with the FRP fan costing nearly twice as much as the epoxy steel fan. Air Movement and Control Association (AMCA) classes types A, B and C are for spark-proof fans and are required for exhausting flammable vapors such as nonhalogenated solvents. In the metal-finishing industry, where fumes and mists of highly corrosive natures are handled by the exhaust system, PVC or FRP are the two best materials available for the parts of the fan that come into contact with the gas stream. Often, the entire fan and housing, except of course the motor, will be made of either PVC or FRP. FRP has good corrosion resistance to many acids, ammonia, chlorine, sodium hydroxide, and other chemicals up to 180°F. Vinylester-based FRP is generally used for fan wheels because of its strength and ductility. Fan housings are typically made of polyester-based FRP. As PVC does not have the strength of FRP, its use for fan wheels is generally limited to PVC-coated steel or steel-reinforced PVC. Even though PVC has excellent abrasion resistance, it should not be used for exhaust air streams containing organic solvents, and it is limited to temperatures below 150°F. PVC is significantly less expensive than FRP for use in exhaust fans. Another choice made in selecting the proper exhaust fan is the use of a belt-driven or direct-driven motor. A belt drive allows for easily varying the fan speed with changing system resistance, within the limitations of the motor used. The fan laws govern the new operating point reached when the fan speed is changed. For a given duct system, the flow will vary
790
directly with the fan speed, static pressure will vary directly with the square of the fan speed, and power required will vary directly with the cube of the fan speed. When specifying an exhaust fan, the materials of construction, operating characteristics, testing procedures, and any special features, such as spark resistance, should be outlined. Reference should be made to any applicable standards, the standard for FRP fans, for example, is American Society for Testing and Materials (ASTM) D 4167 entitled "Standard Specification for Fiber-Reinforced Plastic Fans and Blowers." The exhaust fan standards of AMCA should also be consulted in this regard.Optional accessories such as cleanout doors, bottom drains, and weather covers should be considered when purchasing an exhaust fan. A bottom drain is especially important for fans handling hot exhaust streams with condensable vapors that would tend to collect in the fan casing. The mists vented from plating tanks or scrubbers collect in the exhaust fan and must be drained. The belt tension must be checked periodically to assure that the fan is rotating at the proper speed. Belts do break occasionally and must be replaced. Fan bearings must be adequately lubricated and aligned. The fan wheel should be checked regularly for buildup of dirt, and it should be cleaned if necessary. Any dynamic imbalance caused by excessive corrosion should be corrected to prevent damage to the bearings. Although adding flexibility to an exhaust fan, belt drives involve more maintenance. If properly selected, an exhaust fan need not be a high-maintenance item. Corrosion is the most common cause of fan failure, therefore, much of the effort in maintaining an exhaust fan should involve inspection of the fan wheel and casing for the effects of corrosion.
MAKE-UP AIR The air that is removed from a workplace by the exhaust ventilation system must be replaced with clean air. This can occur passively through infiltration or actively via a make-up air supply system. In modern industrial facilities, infiltration is almost never sufficient to provide the needed replacement air. When insufficient air reaches the workplace, the area becomes "air starved." This puts an added burden on the exhaust system and can reduce the quantity of air being exhausted to an unacceptably low level. Uncontrolled influx of replacement air to the plating area can cause air-distribution and temperature-regulation problems elsewhere in the plant. Compensating measures taken to maintain comfort in those areas ultimately lead to the use of more energy than would be required by a well-designed make-up air system. For these reasons, make-up air systems providing tempered air to replace the exhausted air are highly desirable. Make-up air is usually supplied by a roof-mounted supply fan equipped with air filters and some method of heating the air. Direct-fired heating of the air with gas is the most energy-efficient method. The combustion products are usually not a concern because high air flow rates provide sufficient dilution. Steam is a commonly used heating medium. Its disadvantages include freezing problems in the winter and relatively slow response to changing air temperatures. Natural gas is the most desirable heating fuel. The distribution of make-up air in the workplace is also important from the standpoint of worker comfort, as well as proper operation of the exhaust system. Drafts on workers should be avoided in winter, but may be desirable during the summer. This can be accomplished through the use of a louver arrangement, which allows for enough flexibility in air distribution. Crossdrafts over the tops of vented tanks should be avoided. Rather than provide 100% outside, air for make-up air, it is sometimes feasible to recirculate exhausted air into the workplace. This can reduce the energy requirements of the ventilation system, because heating or cooling of the recirculated air is usually not required. This technique is generally not advisable for the plating industry, however, because of the risk of reintroducing contaminated air into occupied areas. According to the ACGIH, the requirements for a safe, reliable recirculation system are: 1. A primary air cleaning system designed to reduce all contaminants in the recirculated air below the TLV.
791
2. A secondary air cleaning system of equal or greater efficiency in series with the primary system, or a reliable monitoring device to analyze a representative sample of the recirculated air. 3. A warning alarm system with provisions for immediate bypass of the recirculated air in the event that' contaminant concentrations above preset limits are detected by the monitor, or the air cleaning system requires attention. Recirculation from wet scrubbers employed for plating and metal finishing is generally not recommended because of difficulties associated with continuous direct monitoring of air contaminants at the scrubber discharge. Commercially available "recirculation packages" generally monitor scrubber performance only indirectly by monitoring fresh water feed rate, pressure drop, and/or scrubbing solution pH. Introduction of humidity into the workspace from wet scrubbers is also a concern. Although not generally recommended for the metal-finishing industry because of the inherent difficulties and hazards, there are certain conditions that may make a recirculation system more feasible. If a process is to be completely isolated from workers, recirculation of that isolated exhaust air directly back to the process could be considered. If exhaust contaminants are only of the nuisance type rather than being toxic, recirculation might be feasible. Partial recirculation could be utilized to gain some energy savings while minimizing the risk of contaminants being supplied to the workplace. Heat exchangers are often considered for recovering heat from exhaust air prior to discharge. Heat exchangers of any kind are rarely used for plating exhausts because of the severely corrosive nature of acidic exhausts. The use of protective coatings on heat transfer devices has met with limited success. Exhaust air is usually at room temperature, which limits the heat available for recovery. Maintenance and equipment-replacement costs associated with corrosion generally convert marginal economic feasibility into infeasibility. Local exhaust ventilation and make-up air systems require substantial space allocations, both overhead and on the roof. Careful consideration should be given to these requirements when designing the ventilation systems. Interferences between supply and exhaust ductwork can easily be avoided through proper planning, layout, and design. Locating the make-up air unit as far from the scrubber/exhaust fan as practical, for example, will minimize the possibility of large ducts (headers) crossing each other. This, in turn, may reduce the overall building height requirement and result in substantial cost savings. The distance between any make-up air intakes and exhaust gas discharges, and their orientations relative to the prevailing wind direction, should be such that there is no significant introduction of contaminants into the make-up air supply and subsequently into the building.
AIR POLLUTION CONTROL Air discharge limitations have become stricter in the last few years due to increasing concerns about the effects of air pollution. Regulations at the federal and state levels have reduced allowable emission rates of many compounds and will continue to be revised to regulate more sources of air pollution as deemed appropriate. Sources subject to emission limitations most often need to control a large part of their emissions with an air pollution control device. Sources not specifically subject to an emission limitation may also need to control emissions because of localized effects of emissions. Such effects can be equipment damage from long-term exposure of untreated discharges, impacts of emissions on especially sensitive areas, and excessive impact due to location of the source. The technologies and air pollution control methods that need to be considered when evaluating emission control can be one of three strategies: add-on controls, reformulation, or process modification. In determining the required level of control, all three of these must be considered as possible reduction techniques. Often, the latter two methods are overlooked, yet it is in the best interest of a source to consider all reduction techniques. Further, regulatory agencies will often require such a comprehensive evaluation.
792
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Figure 4. Spray chamber scrubber.
The traditional method of pollution control is the add-on control device. Some types of add-on devices for control are incinerators, condensers, carbon adsorbers, scrubbers, and fabric filters, to name a few. These units are placed on the exhaust stream from the process and have the advantage of requiring minimal changes in the process itself, Such devices, however, are often very expensive to purchase, operate, and maintain and can substitute one pollution problem (air) for another (contaminated water discharge or hazardous waste generation). However, these devices, often referred to as end-of-pipe control, are necessary in many cases. The type of pollution problem will dictate the type of add-on control device used. In the metal-finishing industry, pollutants of concern are volatile organic compounds (VOCs) and acid/alkali mists/particulates and vapors. The most common sources of VOCs in metal finishing are solvents contained in paints, coatings, and organic solvent cleaning operations. Acid/alkali mists are entrained in the local exhaust ventilation of many finishing operations. Potential also exists for other emissions, such as cyanide compounds, and to a small degree, metals. The available control devices for each of these problems can be divided into two categories, VOC control and particulate control. VOCs, the major focus of much air pollution regulation, require sophisticated control techniques. Incinerators or afterburners oxidize organics to carbon dioxide and water and are found on some coating operations. Complications arise from inefficient combustion or contaminants other than simple hydrocarbons in the exhaust stream. Depending on the VOC being controlled, the incinerator itself may need a control device to remove undesirable products of combustion (such as hydrogen chloride in the case of a chlorinated compound being incinerated), The advantage of incineration is that, under proper circumstances, there is no secondary disposal concern. In contrast, other VOC control devices do not destroy pollutants, but collect them for reuse or disposal. Condensers work by cooling the air stream to a point where the organics condense to a liquid and are collected. Carbon adsorbers pass the contaminated exhaust stream through a bed of activated carbon, which retains the 793
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OUT BERLSADDLE INTALOX SADDLE
Figure 5. Packed bed scrubber with common packing types.
volatiles. The carbon is then stripped of the adsorbed VOC by heat and the organics are collected. In the metal-finishing industry, wet collectors (also called scrubbers) are the most suitable pollution-control devices for removing acid/alkali pollutants. In the simplest scrubber, a spray chamber scrubber (Fig. 4), water droplets are continuously sprayed into the gas stream, where water-soluble contaminants are absorbed by the droplets. The fan pulls the air through the spray and a mist eliminator, and then releases it to atmosphere, free of the water-soluble contaminants. The mist eliminator traps water droplets and mists carried by the momentum of the air from being released through the stack. Packed bed scrubbers (Fig. 5) are the most efficient of the commonly used wet collectors. They are similar to spray chamber scrubbers, but are filled with a packing material that increases gas-liquid contact. The gas stream enters the chamber from the bottom and the absorbing liquid (typically water) is sprayed onto the packing material from the top. The water trickles downward through the packing, while the gas stream passes upward through the packing and water. The large surface area for gas-liquid contact makes packed bed scrubbers highly efficient for both water-soluble gas and mist removal. Although most scrubbers can readily achieve mist removal efficiencies of 90-99%, it is important to choose the proper scrubber to maximize its gaseous removal efficiency. The physical and chemical characteristics of the gas stream are important when deciding on the type of scrubber needed. Characteristics such as volume of collected air, contaminant solubility in water, stability of the contaminants in the gas stream, and droplets may indicate a preference for a particular type of scrubber. Once a scrubber has been chosen, certain features can be added Or changed that will increase the scubber's efficiency. Installation of high-pressure nozzles achieves smaller droplet size and higher droplet velocity, which increases gas-liquid contact and subsequently increases the removal efficiency of a spray chamber scrubber. However, high-pressure nozzles plug easily and are more expensive. Introducing the air flow to the spray chamber scrubbers tangentially will impose a spiral motion to the gas stream. The spiral motion will force a large percentage of the gas flow to the walls of the chamber where the spray is directed. The packing material in packed bed scrubbers can be changed to increase surface area and,
794
i
1.5 to 2 0 H
| l
,f PREFERRED High discharge stack relakive
to
building height,
air inlet on roof.
AVOID Figure 6. A high discharge stack relative to building height (top) is preferred. A low discharge stack relative to building height and air inlets" (bottom) should be avoided. These figures apply only to the simple case of a low building without surrounding obstructions on reasonably level terrain. Note that low pressure on the lee of a building may cause the return qf contaminants into the building through openings.
subsequently, gas-liquid contact. A water redistributor midway through the packing redisperses the water flow evenly to ensure uniform water flow throughout the scrubber. In any acid scrubbing system, maintaining an alkaline pH will also increase removal efficiency. Spray chamber scrubbers generally use ~i system where the absorbing liquid is collected at the bottom of the chamber and then is completely removed from the system. Packed bed scrubbers recirculate relatively large amounts of the collected absorbing liquid back into the spray stream. Contaminated absorbing liquid is removed from the recirculation loop at approximately 0.1-0.3 gpm/1,000 cfm of exhaust air. The absorbing liquid recirculation is approximately 5-15 gprrdl,000 cfm. Recirculation offers cost savings by reducing the quantity of fresh absorbing liquid required to operate the scrubber efficiently. Corrosion-resistant plastics are most commonly used for construction of scrubbers and exhaust-handling ducts in the plating and metal-finishing industry. Plastic scrubber housings
795
are typically fabricated of rigid PVC or of FRR Internals are usually a combination of PVC, FRR and polypropylene, and sometimes stainless steel materials. If water-recirculating scmbbers are located outdoors, remote indoor sumps constructed of corrosion-resistant materials are required for freeze protection. Wet scrubbers are not suitable devices for VOC control because most VOCs are insoluble in the water-based absorbing liquid. Exhausts from processes using VOCs should be separate from all aqueous process exhausts. State air-quality authorities may require control of significant quantities of VOCs by activated carbon adsorption, thermal oxidation, or process modifications. Initial cost should be one of a number of factors to consider when purchasing air pollution control equipment. The physical and chemical properties of the pollution problem, as well as the maintenance, service, and operational costs, must be matched with the available control technology to ensure the most efficient purchase. The most environmentally sound and often cost-effective method of pollution control is process modification. Sometimes, with moderate effort, less polluting ways of achieving the same result can be found. New formulations of many coatings and other VOC-containing compounds exist that have reduced or eliminated organic solvents. Many water-based paints are readily available with performance similar to conventional paints. There is constant development of new formulations that contain tess VOCs and are as easy to use as previous products. In actual plating operations, sometimes less polluting alternatives can be found. The replacement of cyanide plating operations with noncyanide operations eliminates cyanide emissions. Process changes can be a difficult course to follow in emission reductions because of the myriad choices sometimes available, as well as the scarcity of guidance offered by regulatory officials.
EXHAUST DISCHARGE After the exhaust fan and/or control device, the exhaust gas is vented to the atmosphere through a stack. Even if the exhaust gas has been cleaned by a control device, low concentrations of hazardous chemicals may be present. Ambient air currents mix and diffuse the gas as it is emitted from the stack. Air currents are affected by nearby buildings and equipment, and the wake effect created may cause pollutants from the stack to reach the ground at objectionally high concentrations (see Fig. 6). Stack design is critical to ensure adequate ambient dilution of the exhaust. The stack should be designed so that the exhaust momentum carries the gas straight up and above buildings or equipment that may disrupt the air currents. "Rain hats," or any other structure on the stack that disrupts the upward momentum are not recommended. Stack he!ght above ground should be at least one and one-half to two times that of the tallest adjacent structures, where practical.
Zinc Plating by tt. Ge&dd 360 pages $125.00 This book contains 15 chapters including a history, cleaning procedures, equipment and plant layout, cyanide, zincate and acid baths, post-plate treatments, troubleshooting, special operations such as mechanical and strip and wire continuous processing, control of solutions, and waste treatment. It is a practical volume written for the practitioner and chemist. Send Orders to: METALFINISHING,660 White Plains Rd., Tarrytown, N Y 105915153For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 Allbookordersmustbe prepaid.Pleaseinclude$5.00shippingandhandlingfordeliveryof eachbookvia UPSin the U.S., $10.00for eachbookshippedexpressto Canada;and $20.00for eachbookshippedexpressto all othercoimtries.
796
CODE OF COMPLIANCE The quality of workroom air is regulated by the federal Occupational Safety and Health Administration (OSHA) and, to some extent, by state bureaus of occupational hygiene. Copies _ of applicable regulations should be obtained prior to initiating design activities. Two generally accepted reference documents for designers are Practices for Ventilation and Operation of Open Surface Tanks, by the American National Standards Institute (ANSI) (Z91-1977), and Industrial Ventilation: A Manual of Recommended Practices, by the American Conference of Governmental Industrial Hygienists (ACGIH), Committee on Industrial Ventilation, Lansing, Mich. The Sheet Metal and Air Conditioning Contractors' National Association, Inc. (SMACNA), Chantilly, Va., has also published several manuals that are useful for specifying engineers and installers.
TYPES OF SYSTEMS The two methods o f handling contaminant exhaust are local exhaust ventilation and general dilution (Fig. 1). Local exhaust ventilation means controlling contamination at its source, whereas dilution ventilation means treating the workroom as if it were 9 "mixing box" or contamination sink. In general, dilution ventilation is not as effective for health hazard control as is local ventilation. Dilution ventilation requires greater air flows to achieve the same effect as local exhaust ventilation and cannot protect workers who must work close to the process operation. Although there are occasional circumstances where dilution ventilation must be used because the operation or process cannot acconmaodate local exhaust, local exhaust ventilation is by far the most cost-effective and generally accepted method of contaminant control. Many plating and metal-finishing shops have a number of different open surface tank processes that require exhaust ventilation. The exhausts from most, if not all, o~fthe process baths can usually be combined into a single exhaust system; however, segregation into multiple systems is desirable in certain situations.
778
5,000 CFM 5,000 CFM
t
t SUPPLY AIR ROOF~, ,,," __ %
f.
~,._
SUPPLY ROOF OR WALL EXHAUSTER ROOFqI AIR
,
AMBIENT EQUALIBRIUM-CONCENTRATION BREATHINGZOI;IE CONCENTRATIONS RELATIVELYHIGH
~'
t
w
' ;~
• "" ~, ." • ". ~ ~ ~'
" VERY lOW AMBIENT CONCENTRATION ,,~
,,. : __/'": RELATIVELYLOW BREATHING / ZONE CONCENTRATIONDEPENDING TANK UPON HOODCAPTUREEFFICIENCY
Figure 1. Left: general dilution ventilation--room behaves as a mixing chamber; right: local exhaust ventilation--localized capture provides lower room and breathing zone contaminant concentrations using the same amount of exhaust air as general dilution ventilation.
Layout or duct-sizing considerations may indicate the need for separate systems. If a process is infrequently used, a separate exhaust system, which can be turned oft', will save energy. When both acid- and cyanide-containing solutions are to be vented, separate systems should be used, if practical, to prevent the possible formation of hydrogen cyanide gas. Similarly, ammonia- and chlorine-containing exhaust streams should be kept separate to prevent formation of ammonium chloride, a fine white particulate that will cause a visible plume, which is difficult for conventional air pollution control equipment to remove. Oxides of nitrogen (NOx) from fuming nitric acid tanks present a particularly difficult emissioncontrol problem. If strict control of NO x emissions is indicated by plant siting considerations or by air-quality authorities, the NO x exhaust stream should be kept separate to minimize treatment costs. Generally, the advantages of separate systems are partially offset by the higher overall capital cost of air-moving and cleaning equipment, and by potential make-up air distribution control problem. NATURE
OF
CONTAMINANTS
Not all finishing operations are identical in emissions of air contaminants. Air contaminants from open surface tanks vary in type, intensity, and toxicity depending upon the operation. All of these factors are significant in the determination of ventilation and air pollution control requirements for a facility. The type of emission is very important when designing a ventilation system. There are two types of air contaminants from open surface tanks: mists and gases, typically, tanks containing water-based solutions emit mist droplets in the range of 0.5-50 /xm in diameter, having the same chemical make-up as the tank. Gases and vapors, characterized as molecular forms having diameters <0.01 /xm, are emitted from the evaporation of tank components; such emissions can include hydrogen cyanide, hydrogen fluoride, hydrogen chloride in water-based solutions, as well a s solvents such as methanol, naphtha, or chlorinated hydrocarbons. These types of emissions can be captured using local ventilation. The distinction becomes important when considering effective capture by a local exhaust, transport in ductwork, and collection in air pollution control devices. 780
The rate of evolution of air contaminants will, in part, determine the ventilation requirements and is dependent on the type of finishing operation performed and on the specifics of an individual operation. Mists arise from the bursting of bubbles at the liquid surface, entraining tank components into the air. These bubbles can be caused by electrolysis, by nonelectrolytic chemical reactions with the work, and by physical actions such as air and mechanical agitation of the bath. Gaseous emissions can be affected by the same elements, as well as by temperature. In both cases, the rate of material processed is a major factor in emissions. Based on the rate of contaminant released by the tank, the rate of local exhaust is determined. Highly reactive metals or processes require high ventilation rates. The most important factor in local exhaust is the toxicity of the air contaminant. Toxicity is specific to the chemical emifled. Maximum permissible exposure limits (PELs) are mandated by OSHA and guideline threshold limit values (TLVs) are recommended by the ACGIH. Acceptable levels in the air can range from 0.1 to 50 ppm and are based on available data concerning health effects of exposure to the compounds. A local exhaust and ventilation system is designed based on the allowable or recommended exposure levels. These three factors, type, rate, and toxicity of contaminant release, are carefully considered when designing a local exhaust ventilation system. Data have been tabulated for each factor and, as is described in subsequent sections, ventilation systems can be designed from these compilations.
DESIGN Exhaust system design begins with the individual exhaust hood. The intent of the exhaust hood is to sweep emissions from the tank surface, away from the workers' breathing zone, in the most energy-efficient manner. The amount of exhaust flow and, therefore, of energy required, depends on the particular plating or metal-finishing process and the degree to which it can be physically enclosed. Enclosure is desirable because it minimizes crossdraft interference and reduces the amount of open surface area to be controlled. It is best achieved with full or partial covers, or with booth-type arrangements over the tank surface. Surface enclosure is also partly achievable by floating spheres or surface tension chemical agents in the liquid. Once a hood configuration has been selected, the next design step is to calculate the required exhaust flow. Exhaust flows are calculated based upon the individual processes and are dependent upon the type of contaminants to be controlled, the rate at which contaminants are evolved, the temperature, and the size of operations, among other factors. In general, exhaust flows should adequately control workroom air concentrations to within acceptable health levels. Potential corrosion of the workroom and workpieces is another concern. (Refer to the aforementioned documents for process specific ventilation rtes.) Exhaust flows in excess of those required for health considerations may be necessary to control product quality. It should also be noted that OSHA mandates the use of standard tables for flow calculation if employee exposure to air contaminants is found by air sampling to be in excess of the PELs. (Refer to OSHA General Industry Standards, 29 CFR 1910.94[d] and 1910.1000.)
HOOD TYPES The three basic hood designs are the lateral slot hood, the canopy hood, and the booth hood with one open side. The lateral and canopy types are most efficient when combined with baffles or curtains to approach the booth configuration having one open side. 782
.%J
-
A. U P W A R D
~
PLENUM
Ff q]
m
W
"8. O O W N W A R O
t
PLENUM
t
,-
.
.._t C. C E N T R A L
,f l,,
I/L~S ~tSLOT ~
]
w J " 7 S e c h ~ A -A
Figure 2. Lateral slot hoods.
LATERAL EXHAUST Lateral slot hoods are depicted in Fig. 2. Although lateral hoods have the potential for requiring the greatest amount of exhaust flow among the various hood types, they are the type most commonly installed because they create minimum physical interference with the process. The slot entry must be sized to furnish a minimum-recommended entry velocity of 2,000 fpm. The recommended slot-entry velocity will ensure even distribution of flow across the slot opening. Slot velocity alone does not determine the control velocity achieved at the tank surface boundary: the slot's sole purpose is to distribute flow. The plenum behind the slot should have a depth at least twice the slot width to ensure distribution across the face. Where the effective width (W) over which the hood must pull air to operate exceeds 20 in., slots on both sides are desirable. Where W exceeds 36 in., slots on both sides are necessary. If W exceeds 48 in., lateral exhaust is not usually practical unless enclosure or push-pull arrangements are used. Variations of the basic lateral slot hood are depicted in Fig. 2B and C. Slots on two or more sides of the tank provide more favorable aspect ratios, allowing the use of less exhaust air relative to a single slot. Minimum-recommended flow rates are for tank locations with minimum crossdrafts, specifically crossdrafts <50 fpm. Lateral hoods are highly susceptible to crossdrafts caused by open windows or doors, particularly when make-up air is not mechanically furnished to the space. The distribution of make-up air to the workroom can also be a source of undue
784
Exhaust flow (0[) Push nozzle plenum Exhousf opening heigh|(h) j
L
~
Figure 3. Push-pull exhaust hood. turbulence. Increased exhaust flow, proportional to the increased magnitude of crossdraft velocity, is required to achieve the contaminant control necessary.
PUSH-PULL LATERAL EXHAUST For large open surface tanks, normal lateral exhaust is not feasible due to the enormous amount of exhaust needed to maintain capture velocities at the far end of the tank. Instead, a push-pull-type exhaust, as depicted in Fig. 3, can be used. Push-pull exhaust combines a lateral slot hood at one end of the tank with a jet of push air at the opposite end. Push air is typically furnished by a dedicated supply fan located in the workroom: compressed air is an unnecessarily expensive source of push air. Push-pull systems offer the potential advantage of achieving control with less exhaust air flow, particularly where crossdrafts are a problem, making exhaust of wide tanks more feasible. One recent study demonstrated a 97% contaminant capture efficiency for a 4 × 6-ft heated tank equipped with anode bars, where crossdrafts were 75 fpm. Push air was furnished through a 1A-in. slot at 35-45 cfm/ft of tank length, and exhaust flow was 50-75 cfm/ft2 of tank area. The design of push-pull systems is empirical, and such systems should be designed so that they can be easily modified or adjusted to obtain the desired results. The major drawback of push-pull systems is that obstructions, such as the work, anode bars, etc., may deflect the curtain of air and scatter it, along with air contaminants, into the work area; hence, the need for design flexibility. Design criteria can be found in the latest edition of the ACGIH manual or other references.
OTHER EXHAUST HOODS The canopy hood is a receiving-type hood and is most appropriate for hot processes where vertical convection currents move contaminants toward the hood. Canopy hoods work best when side curtains are furnished approaching the booth configuration. A canopy hood without curtains is not recommended for cold processes due to the ease with which crossdrafts divert contaminants away from the hood.
786
Table I. Comparative Exhaust Flow Rates for Various Hood Types°
Control velocity, fpm Design velocity, fpm Flow rate, cfm
Single Slot
Lateral Exhaust Slots at Front and Back
Push-Pull
Open Canopy
Enclosing Booth
100 175 2,190
100 150 1,875
-100 1,250
175 -8,135
175 -750
aSingle-slotfishtail-typehood.Lateral slotslocated at inside tank edge. Double-slotfishtail-typehoodon rear. downdrafton front. Canopyhoodlocated at 2 ft aboveprocess. Enclosingboothwith2-ft highby 5-ft longopeningon one side. 12LA-ftz tank area.
COMPARING EXHAUST OPTIONS For comparison purposes, Table I indicates the minimum-recommended exhaust flow for a 21/2 × 5-ft alkaline cleaning tank operating at 200°F, for each of the hood types discussed. The table illustrates the differences of one hood type or another in required air flows.
DUCT SYSTEMS Ducts are the link between the fan and hood to convey air contaminants. Industrial exhaust ducts are sized by the system designer to provide an air velocity that will keep air contaminants in suspension during transport to the scrubber fan, but which is not so great as to cause unnecessary friction losses and equipment strain. For gases and vapors, any transport velocity is acceptable; economic considerations result in a usual transport velocity of 1,000-1,500 fpm. For open surface tanks, and the contaminants generated, a duct velocity of 2,000-3,000 fpm is commonly recommended. There are two general approaches to duct design, the "self-balanced system" and "blast gate" methods. With the self-balanced system approach, ducts are sized and flows are adjusted so that the minimum-desired hood flows are achieved without the use of balancing dampers or blast gates. With the blast gate method, system balancing is achieved using strategically placed blast gates, or blast gates at each hood. (The reader is directed to the ACGIH Industrial Ventilation M a n u a l for details.) Eac h design has advantages and disadvantages. The self-balanced system has the advantage of being relatively tamperproof and of ensuring that design transport velocities are maintained. Flows cannot be altered without adjusting the fan speed or drive. The disadvantage is reduced flexibility because temporarily inactive hoods or branches cannot easily be shut oft'. Conversely, the blast gate method is subject to tampering, but offers greater operational flexibility. Accordingly, self-balanced systems are ideal for nonchanging processes and processes utilizing extremely hazardous components, where ventilation tampering is undesirable. Prior to selecting a fan from the manufacturers' performance tables, the actual pressure losses must be obtained by calculation of duct, hood, and air acceleration losses, and by contacting air pollution control equipment manufacturers. Typical fan static pressures for open surface tank exhaust systems are 2-in. water column (WC) for systems without scrubbers, and 3-5 in. for systems with scrubbers. Slot hoods, designed as previously described, will produce a pressure drop ranging from approximately 0.5 to 1 in. WC. The typical duct system will add an additional 0.5-1 in. WC. Exhaust gas scrubbers have pressure drops typically ranging from 0.5 to 3 in. WC. Smooth air flow is an important consideration for duct-system design to conserve energy. Good engineering fluid flow practices and general principles should be incorporated into the design and construction of a duct system. Flexible duct is commonly employed as an expediency, but its use should be minimized. Resistance to flow from flexible duct is 788
substantially greater than that for smooth duct. This results in excessive fan horsepower for systems designed for flexible duct, or inadequate flow on systems not specifically designed for flexible duct. Impaction of contaminant mists on duct surfaces invariably leads to buildup in the ductwork of liquids and solids left behind by evaporation of the liquids. Cleanonts should be furnished at specified intervals on straight horizontal runs and at each elbow. Ducts should be sloped to a low point and drains with traps or isolation valves should be furnished at this point. The most cost-effective duct material will vary from installation to installation. Duct selection, and therefore cost, depends upon corrosion and temperature requirements, desired lifetime, and insurance underwriter requirements. Several materials are commonly used for duct and hood construction. Manufacturers' tables of corrosion resistance should be consulted during selection of specific products. An excellent reference source of corrosion-resistant properties for numerous materials is the Corrosion Data Survey, metals and nonmetals sections, obtainable from the National Association of Corrosion Engineers, Houston, Texas. In general, duct service is less severe than the immersion ratings typically given in tables of corrosion resistance. Hoods and ductwork are typically constructed of galvanized, resin-coated or stainless steel, or plastics such as polypropylene, polyvinyl chloride (PVC), or fiberglass-reinforced plastic (FRP). The plastics are currently in widespread use because of their excellent corrosion-resistant properties. A prime consideration in the selection of duct and hood materials is combustibility. Polypropylene, PVC, and FRP are all combustible to varying degrees. Fires originating in process'tanks, typically caused by electric immersion heaters in dry tanks, are a fairly common occurrence. The tank fire spreads to the plastic hood, with the potential for involving the entire exhaust system and adjacent building components. Some insurance underwriters require plastic ductwork to be furnished with internal sprinklers, whereas others will accept the system without sprinklers provided that materials do not exceed a certain thickness, e.g., V4-in. maximum thickness for PVC. The basis for the thickness exemption is that thin walls will melt more quickly and interrupt flame spread. The FRP duct is usually assigned a flame spread rating, with better ratings achieved by the addition of fire retardants to the resin. A few proprietary FRP duct systems, which incorporate patented flame interrupters and fireretardant resins, have been approved for use without internal sprinklers. Where higher temperatures may be encountered due to the process, or on rooftops in southern climates, temperature-rated FRP or steel construction may be preferable to plastics, which soften at elevated temperatures. Open surface tank exhaust systems draw in large amounts of room air, consequently the duct is typically exposed to temperatures of 100°F or less. The relative cost of one material compared with others for a given duct system will depend upon the number and size of fittings, the availability of standard extruded or fabricated duct for the desired diameter, and the availability of qualified installers for specialty duct. Generally speaking, relative duct cost for an installed system is given in ascending order of cost as follows: plain steel, galvanized steel, resin-coated steel, PVC, polypropylene, FRP, proprietary flame-interrupting FRP, and stainless steel.
EXHAUST FAN TYPES The energy required to induce a flow of air through a local exhaust system is provided by an exhaust fan. There are two basic types of fans to choose from, axial and centrifugal. Among the available materials of construction, PVC and FRP are more commonly used for exhaust fans handling the highly corrosive fumes that are produced in metal finishing. Axial fans move air parallel to the fan shaft. The propeller type is commonly used to move air from one room to another or from outdoors to indoors. Vane axial and tube axial fans are both installed in ducts, with the main difference between them being the higher pressures
789
achieved in vane axial fans (up to about 6 in. water gauge) due to their air guide vanes. The relatively low static pressures obtained by axial fans, as well as their low resistance to abrasive dusts, limit their usefulness as industrial process exhaust fans. Centrifugal fans can be classified into three main types: forward curved blade, backward inclined (BI) blade, and radial blade. A forward curved blade, or squirrel cage fan has blades that curve toward the direction of fan rotation. They deliver tow to medium volumes at low pressures. Their low rotational speed for a given air volume makes them quiet in operation. The blade shape tends to collect dust and liquid, making these fans unsuitable for acid mists and other dirty particulate exhaust strealTiS.
A BI blade centrifugal fan has blades that are inclined away from the direction of rotation. They can deliver air against a high enough pressure drop to be used with fume scrubbers. Their high efficiency and nonoverloading characteristics make BI fans a good choice for many exhaust applications. Although they can handle a higher dust loading than forward curved fans, they are not the ideal choice for highly abrasive exhaust streams. The BI wheel fans are the most commonly used centrifugal exhaust fans for open surface tank ventilation. The exhaust fan is usually placed downstream of any scrubber or other pollution-control device being used to minimize buildup of material on the fan blades. Radial blade centrifugal fans have blades similar to those of a paddle wheel. This blade orientation limits material buildup and the moderate rotation speed means particles move along the blades at lower velocities than in all but forward curved fans. Radial blade fans are, therefore, widely used for industrial applications where heavily dust laden streams are being exhausted. This type of fan can provide a high static pressure and can be a low maintenance item because of its heavy duty construction. Radial fans have somewhat lower efficiency than other industrial fans, which is the trade-off for its advantages.
FAN SELECTION Fan selection for any exhaust application must be based on matching the fan flow characteristics to the desired flow and resistance of the system. The flow characteristics of a particular fan are determined empirically by the fan manufacturer. The desired flow and resistance are calculated in the exhaust system design, as discussed previously. The purchase price for a particular size and type of exhaust fan can vary considerably depending on the materials of construction. The greatest cost differential is between fans of FRP construction and those of epoxy-coated steel, with the FRP fan costing nearly twice as much as the epoxy steel fan. Air Movement and Control Association (AMCA) classes types A, B and C are for spark-proof fans and are required for exhausting flammable vapors such as nonhalogenated solvents. In the metal-finishing industry, where fumes and mists of highly corrosive natures are handled by the exhaust system, PVC or FRP are the two best materials available for the parts of the fan that come into contact with the gas stream. Often, the entire fan and housing, except of course the motor, will be made of either PVC or FRP. FRP has good corrosion resistance to many acids, ammonia, chlorine, sodium hydroxide, and other chemicals up to 180°F. Vinylester-based FRP is generally used for fan wheels because of its strength and ductility. Fan housings are typically made of polyester-based FRP. As PVC does not have the strength of FRP, its use for fan wheels is generally limited to PVC-coated steel or steel-reinforced PVC. Even though PVC has excellent abrasion resistance, it should not be used for exhaust air streams containing organic solvents, and it is limited to temperatures below 150°F. PVC is significantly less expensive than FRP for use in exhaust fans. Another choice made in selecting the proper exhaust fan is the use of a belt-driven or direct-driven motor. A belt drive allows for easily varying the fan speed with changing system resistance, within the limitations of the motor used. The fan laws govern the new operating point reached when the fan speed is changed. For a given duct system, the flow will vary
790
directly with the fan speed, static pressure will vary directly with the square of the fan speed, and power required will vary directly with the cube of the fan speed. When specifying an exhaust fan, the materials of construction, operating characteristics, testing procedures, and any special features, such as spark resistance, should be outlined. Reference should be made to any applicable standards, the standard for FRP fans, for example, is American Society for Testing and Materials (ASTM) D 4167 entitled "Standard Specification for Fiber-Reinforced Plastic Fans and Blowers." The exhaust fan standards of AMCA should also be consulted in this regard.Optional accessories such as cleanout doors, bottom drains, and weather covers should be considered when purchasing an exhaust fan. A bottom drain is especially important for fans handling hot exhaust streams with condensable vapors that would tend to collect in the fan casing. The mists vented from plating tanks or scrubbers collect in the exhaust fan and must be drained. The belt tension must be checked periodically to assure that the fan is rotating at the proper speed. Belts do break occasionally and must be replaced. Fan bearings must be adequately lubricated and aligned. The fan wheel should be checked regularly for buildup of dirt, and it should be cleaned if necessary. Any dynamic imbalance caused by excessive corrosion should be corrected to prevent damage to the bearings. Although adding flexibility to an exhaust fan, belt drives involve more maintenance. If properly selected, an exhaust fan need not be a high-maintenance item. Corrosion is the most common cause of fan failure, therefore, much of the effort in maintaining an exhaust fan should involve inspection of the fan wheel and casing for the effects of corrosion.
MAKE-UP AIR The air that is removed from a workplace by the exhaust ventilation system must be replaced with clean air. This can occur passively through infiltration or actively via a make-up air supply system. In modern industrial facilities, infiltration is almost never sufficient to provide the needed replacement air. When insufficient air reaches the workplace, the area becomes "air starved." This puts an added burden on the exhaust system and can reduce the quantity of air being exhausted to an unacceptably low level. Uncontrolled influx of replacement air to the plating area can cause air-distribution and temperature-regulation problems elsewhere in the plant. Compensating measures taken to maintain comfort in those areas ultimately lead to the use of more energy than would be required by a well-designed make-up air system. For these reasons, make-up air systems providing tempered air to replace the exhausted air are highly desirable. Make-up air is usually supplied by a roof-mounted supply fan equipped with air filters and some method of heating the air. Direct-fired heating of the air with gas is the most energy-efficient method. The combustion products are usually not a concern because high air flow rates provide sufficient dilution. Steam is a commonly used heating medium. Its disadvantages include freezing problems in the winter and relatively slow response to changing air temperatures. Natural gas is the most desirable heating fuel. The distribution of make-up air in the workplace is also important from the standpoint of worker comfort, as well as proper operation of the exhaust system. Drafts on workers should be avoided in winter, but may be desirable during the summer. This can be accomplished through the use of a louver arrangement, which allows for enough flexibility in air distribution. Crossdrafts over the tops of vented tanks should be avoided. Rather than provide 100% outside, air for make-up air, it is sometimes feasible to recirculate exhausted air into the workplace. This can reduce the energy requirements of the ventilation system, because heating or cooling of the recirculated air is usually not required. This technique is generally not advisable for the plating industry, however, because of the risk of reintroducing contaminated air into occupied areas. According to the ACGIH, the requirements for a safe, reliable recirculation system are: 1. A primary air cleaning system designed to reduce all contaminants in the recirculated air below the TLV.
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2. A secondary air cleaning system of equal or greater efficiency in series with the primary system, or a reliable monitoring device to analyze a representative sample of the recirculated air. 3. A warning alarm system with provisions for immediate bypass of the recirculated air in the event that' contaminant concentrations above preset limits are detected by the monitor, or the air cleaning system requires attention. Recirculation from wet scrubbers employed for plating and metal finishing is generally not recommended because of difficulties associated with continuous direct monitoring of air contaminants at the scrubber discharge. Commercially available "recirculation packages" generally monitor scrubber performance only indirectly by monitoring fresh water feed rate, pressure drop, and/or scrubbing solution pH. Introduction of humidity into the workspace from wet scrubbers is also a concern. Although not generally recommended for the metal-finishing industry because of the inherent difficulties and hazards, there are certain conditions that may make a recirculation system more feasible. If a process is to be completely isolated from workers, recirculation of that isolated exhaust air directly back to the process could be considered. If exhaust contaminants are only of the nuisance type rather than being toxic, recirculation might be feasible. Partial recirculation could be utilized to gain some energy savings while minimizing the risk of contaminants being supplied to the workplace. Heat exchangers are often considered for recovering heat from exhaust air prior to discharge. Heat exchangers of any kind are rarely used for plating exhausts because of the severely corrosive nature of acidic exhausts. The use of protective coatings on heat transfer devices has met with limited success. Exhaust air is usually at room temperature, which limits the heat available for recovery. Maintenance and equipment-replacement costs associated with corrosion generally convert marginal economic feasibility into infeasibility. Local exhaust ventilation and make-up air systems require substantial space allocations, both overhead and on the roof. Careful consideration should be given to these requirements when designing the ventilation systems. Interferences between supply and exhaust ductwork can easily be avoided through proper planning, layout, and design. Locating the make-up air unit as far from the scrubber/exhaust fan as practical, for example, will minimize the possibility of large ducts (headers) crossing each other. This, in turn, may reduce the overall building height requirement and result in substantial cost savings. The distance between any make-up air intakes and exhaust gas discharges, and their orientations relative to the prevailing wind direction, should be such that there is no significant introduction of contaminants into the make-up air supply and subsequently into the building.
AIR POLLUTION CONTROL Air discharge limitations have become stricter in the last few years due to increasing concerns about the effects of air pollution. Regulations at the federal and state levels have reduced allowable emission rates of many compounds and will continue to be revised to regulate more sources of air pollution as deemed appropriate. Sources subject to emission limitations most often need to control a large part of their emissions with an air pollution control device. Sources not specifically subject to an emission limitation may also need to control emissions because of localized effects of emissions. Such effects can be equipment damage from long-term exposure of untreated discharges, impacts of emissions on especially sensitive areas, and excessive impact due to location of the source. The technologies and air pollution control methods that need to be considered when evaluating emission control can be one of three strategies: add-on controls, reformulation, or process modification. In determining the required level of control, all three of these must be considered as possible reduction techniques. Often, the latter two methods are overlooked, yet it is in the best interest of a source to consider all reduction techniques. Further, regulatory agencies will often require such a comprehensive evaluation.
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CLEAN GAS
tL MIST~ ELIMINATOR WATER SPRAY
¢-
WATER INLET
DIRTY=.. GAS
DIRTY WATER OUTLET
Figure 4. Spray chamber scrubber.
The traditional method of pollution control is the add-on control device. Some types of add-on devices for control are incinerators, condensers, carbon adsorbers, scrubbers, and fabric filters, to name a few. These units are placed on the exhaust stream from the process and have the advantage of requiring minimal changes in the process itself, Such devices, however, are often very expensive to purchase, operate, and maintain and can substitute one pollution problem (air) for another (contaminated water discharge or hazardous waste generation). However, these devices, often referred to as end-of-pipe control, are necessary in many cases. The type of pollution problem will dictate the type of add-on control device used. In the metal-finishing industry, pollutants of concern are volatile organic compounds (VOCs) and acid/alkali mists/particulates and vapors. The most common sources of VOCs in metal finishing are solvents contained in paints, coatings, and organic solvent cleaning operations. Acid/alkali mists are entrained in the local exhaust ventilation of many finishing operations. Potential also exists for other emissions, such as cyanide compounds, and to a small degree, metals. The available control devices for each of these problems can be divided into two categories, VOC control and particulate control. VOCs, the major focus of much air pollution regulation, require sophisticated control techniques. Incinerators or afterburners oxidize organics to carbon dioxide and water and are found on some coating operations. Complications arise from inefficient combustion or contaminants other than simple hydrocarbons in the exhaust stream. Depending on the VOC being controlled, the incinerator itself may need a control device to remove undesirable products of combustion (such as hydrogen chloride in the case of a chlorinated compound being incinerated), The advantage of incineration is that, under proper circumstances, there is no secondary disposal concern. In contrast, other VOC control devices do not destroy pollutants, but collect them for reuse or disposal. Condensers work by cooling the air stream to a point where the organics condense to a liquid and are collected. Carbon adsorbers pass the contaminated exhaust stream through a bed of activated carbon, which retains the 793
GAS OUT ~-~ L,QU,DIN
/_. LIQUID ;"
DISTR,SUTOR
.
.
.
.
.
RESTRAINER RASCHIG RING PALLRING
I ' LIQU,D RED,STRI.UTOR ~:ii!!t r ~ L ~ : : l ~ / - PACKING LIQUID I N ~ E ~ : ~ ~ " SUPPORT ]~-~--~LIQUID
TELLERETTE /~
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OUT BERLSADDLE INTALOX SADDLE
Figure 5. Packed bed scrubber with common packing types.
volatiles. The carbon is then stripped of the adsorbed VOC by heat and the organics are collected. In the metal-finishing industry, wet collectors (also called scrubbers) are the most suitable pollution-control devices for removing acid/alkali pollutants. In the simplest scrubber, a spray chamber scrubber (Fig. 4), water droplets are continuously sprayed into the gas stream, where water-soluble contaminants are absorbed by the droplets. The fan pulls the air through the spray and a mist eliminator, and then releases it to atmosphere, free of the water-soluble contaminants. The mist eliminator traps water droplets and mists carried by the momentum of the air from being released through the stack. Packed bed scrubbers (Fig. 5) are the most efficient of the commonly used wet collectors. They are similar to spray chamber scrubbers, but are filled with a packing material that increases gas-liquid contact. The gas stream enters the chamber from the bottom and the absorbing liquid (typically water) is sprayed onto the packing material from the top. The water trickles downward through the packing, while the gas stream passes upward through the packing and water. The large surface area for gas-liquid contact makes packed bed scrubbers highly efficient for both water-soluble gas and mist removal. Although most scrubbers can readily achieve mist removal efficiencies of 90-99%, it is important to choose the proper scrubber to maximize its gaseous removal efficiency. The physical and chemical characteristics of the gas stream are important when deciding on the type of scrubber needed. Characteristics such as volume of collected air, contaminant solubility in water, stability of the contaminants in the gas stream, and droplets may indicate a preference for a particular type of scrubber. Once a scrubber has been chosen, certain features can be added Or changed that will increase the scubber's efficiency. Installation of high-pressure nozzles achieves smaller droplet size and higher droplet velocity, which increases gas-liquid contact and subsequently increases the removal efficiency of a spray chamber scrubber. However, high-pressure nozzles plug easily and are more expensive. Introducing the air flow to the spray chamber scrubbers tangentially will impose a spiral motion to the gas stream. The spiral motion will force a large percentage of the gas flow to the walls of the chamber where the spray is directed. The packing material in packed bed scrubbers can be changed to increase surface area and,
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i
1.5 to 2 0 H
| l
,f PREFERRED High discharge stack relakive
to
building height,
air inlet on roof.
AVOID Figure 6. A high discharge stack relative to building height (top) is preferred. A low discharge stack relative to building height and air inlets" (bottom) should be avoided. These figures apply only to the simple case of a low building without surrounding obstructions on reasonably level terrain. Note that low pressure on the lee of a building may cause the return qf contaminants into the building through openings.
subsequently, gas-liquid contact. A water redistributor midway through the packing redisperses the water flow evenly to ensure uniform water flow throughout the scrubber. In any acid scrubbing system, maintaining an alkaline pH will also increase removal efficiency. Spray chamber scrubbers generally use ~i system where the absorbing liquid is collected at the bottom of the chamber and then is completely removed from the system. Packed bed scrubbers recirculate relatively large amounts of the collected absorbing liquid back into the spray stream. Contaminated absorbing liquid is removed from the recirculation loop at approximately 0.1-0.3 gpm/1,000 cfm of exhaust air. The absorbing liquid recirculation is approximately 5-15 gprrdl,000 cfm. Recirculation offers cost savings by reducing the quantity of fresh absorbing liquid required to operate the scrubber efficiently. Corrosion-resistant plastics are most commonly used for construction of scrubbers and exhaust-handling ducts in the plating and metal-finishing industry. Plastic scrubber housings
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are typically fabricated of rigid PVC or of FRR Internals are usually a combination of PVC, FRR and polypropylene, and sometimes stainless steel materials. If water-recirculating scmbbers are located outdoors, remote indoor sumps constructed of corrosion-resistant materials are required for freeze protection. Wet scrubbers are not suitable devices for VOC control because most VOCs are insoluble in the water-based absorbing liquid. Exhausts from processes using VOCs should be separate from all aqueous process exhausts. State air-quality authorities may require control of significant quantities of VOCs by activated carbon adsorption, thermal oxidation, or process modifications. Initial cost should be one of a number of factors to consider when purchasing air pollution control equipment. The physical and chemical properties of the pollution problem, as well as the maintenance, service, and operational costs, must be matched with the available control technology to ensure the most efficient purchase. The most environmentally sound and often cost-effective method of pollution control is process modification. Sometimes, with moderate effort, less polluting ways of achieving the same result can be found. New formulations of many coatings and other VOC-containing compounds exist that have reduced or eliminated organic solvents. Many water-based paints are readily available with performance similar to conventional paints. There is constant development of new formulations that contain tess VOCs and are as easy to use as previous products. In actual plating operations, sometimes less polluting alternatives can be found. The replacement of cyanide plating operations with noncyanide operations eliminates cyanide emissions. Process changes can be a difficult course to follow in emission reductions because of the myriad choices sometimes available, as well as the scarcity of guidance offered by regulatory officials.
EXHAUST DISCHARGE After the exhaust fan and/or control device, the exhaust gas is vented to the atmosphere through a stack. Even if the exhaust gas has been cleaned by a control device, low concentrations of hazardous chemicals may be present. Ambient air currents mix and diffuse the gas as it is emitted from the stack. Air currents are affected by nearby buildings and equipment, and the wake effect created may cause pollutants from the stack to reach the ground at objectionally high concentrations (see Fig. 6). Stack design is critical to ensure adequate ambient dilution of the exhaust. The stack should be designed so that the exhaust momentum carries the gas straight up and above buildings or equipment that may disrupt the air currents. "Rain hats," or any other structure on the stack that disrupts the upward momentum are not recommended. Stack he!ght above ground should be at least one and one-half to two times that of the tallest adjacent structures, where practical.
Zinc Plating by tt. Ge&dd 360 pages $125.00 This book contains 15 chapters including a history, cleaning procedures, equipment and plant layout, cyanide, zincate and acid baths, post-plate treatments, troubleshooting, special operations such as mechanical and strip and wire continuous processing, control of solutions, and waste treatment. It is a practical volume written for the practitioner and chemist. Send Orders to: METALFINISHING,660 White Plains Rd., Tarrytown, N Y 105915153For faster service, call (914) 333-2578 or FAX your order to (914) 333-2570 Allbookordersmustbe prepaid.Pleaseinclude$5.00shippingandhandlingfordeliveryof eachbookvia UPSin the U.S., $10.00for eachbookshippedexpressto Canada;and $20.00for eachbookshippedexpressto all othercoimtries.
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