Ventilation and air pollution control

VENTILATION

AND AIR POLLUTION

CONTROL

by Arthur N. Mabbett Mabbett

& Associates

PURPOSE

Inc., Bedford,

OF EXHAUST

Mass.

SYSTEMS

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 may be 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 which 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 plant components and equipment. New facilities should be designed to control these contaminants in the workplace through proper ventilation. Existing facilities should be evaluated for acceptable work environment. This article will describe 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 Practicesfor Ventilation and Operation of Open Surface Tanks, by the American National Standards Institute (ANSI Z9.1.1977), and Industrial Ventilation: A Manual of Recommended Practices, by the American Conference of Governmental Industrial Hygienists (ACGIH), Committee on Industrial Ventilation, Lansing, MI. The Sheet Metal and Air Conditioning Contractors’ National Association, Inc. (SMACNA), Arlington, VA, has also published several manuals useful for specifying engineers and installers. TYPES

OF SYSTEMS

The two methods of handling contaminant exhaust are local exhaust ventilation, and general dilution (Fig. 1). Local exhaust ventilation means controlling contamination at its source, while dilution ventilation means treating the workroom as if it were a “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 accommodate 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 which require exhaust ventilation. The exhausts from most, if not all, of the process baths can usually be combined into a single exhaust system; however, segregation into multiple systems is desirable in certain situations. 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 off 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

705

5,000 5,000

CFM

t G.4

t

CONCENTRATION RELATIVELY

Fig.

CFM

.

I’ ’ RELATIVELY LOW BREATHING ZONE CONCENTRATION OEPENOING UPON HOOD CAPTURE EFFICIENCY

HIGH

1. Left: general dilution ventilation-room local exhaust ventilation-localized ing zone contaminant concentrations as general dilution ventilation.

behaves as a mixing chamber; right: capture provides lower room and breathusing the same amount of exhaust air

prevent formation of ammonium chloride, a fine white particulate which will cause a vrsible plume, and is difficult for conventional air pollution control equipment to remove. Oxides of nitrogen from fuming nitric acid tanks present a particularly difficult emission control problem. If strict control of oxides of nitrogen (NO,) emissions is indicated by plant siting considerations, or by air quality authorities, the NO, 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 problems. 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 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 to 50 micrometers in diameter, having the same chemical make-up as the tank. Gases and vapors. characterized as molecular forms having diameters less than 0.01 micrometers, are emitted from the evaporation of tank components; such emissions can include hydrogen cyanide, hydrogen fluoride, hydrogen chloride in water based solutions as well as 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. 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 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

707

A. UPWARD

PL ENIJAI

8. DOWNWARD

PLENUM

C. CENTRAL

Fig.

SL 07

2. Lateral

slot hoods.

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 emitted. Maximum permissible exposure limits (PEL’s) are mandated by the U.S. Occupational Safety and Health Administration (OSHA) and guideline threshold limit values (TLV’s) are recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). Acceptable levels in the air can range from 0.1 to 50 parts per million (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 has 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 hood is to sweep emissions from the tank surface, in the most energy efficient manner. The amount required, depends on the particular plating or metal it can be physically enclosed. Enclosure is desirable ence and reduces the amount of open surface area full or partial covers, or with booth type arrangements is also partly achievable by floating spheres or surface 708

exhaust hood. The intent of the exhaust away from the workers’ breathing zone, of exhaust flow, and therefore energy finishing process and the degree to which because it minimizes cross draft interferto be controlled. It is best achieved with over the tank surface. Surface enclosure 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. temperature and size of operations, to name a few. 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 rates. 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 PEL’s. (Refer to OSHA General Industry Standards, 29 CFR 1910.94[dl 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. LATERALEXHAUST 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”, slots on both sides are desirable. Where W exceeds 36”. slots on both sides are necessary. If W exceeds 48”, lateral exhaust is not usually practical unless enclosure or push-pull arrangements are used. Variations of the basic lateral slot hood are depicted in Figs. 2-b and 2-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 less than 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 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, 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 x 6 ft heated tank equipped with anode bars, where crossdrafts were 75 fpm. Push air was furnished through a 1/4” slot at 35 to 45 cfm per foot of tank length, and exhaust flow was 50 to 75 710

Exhaust Push

nozzle

flow

plenum

(0,)

JExhaust

opening

height(h)

hood fOP Push

nozzle as =a.

supply x L

Fig.

3.

exhaust

Push-pull

hood.

cfm/fta of tank area. 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 the fact 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 20th Edition of the ACGIH manual or other references. OTHER

EXHAUST

HOODS

The canopy hood is a receiving type hood, and is most where vertical convection currents move contaminants toward best when side curtains are furnished approaching the booth without curtains is not recommended for cold processes due to divert contaminants away from the hood. COMPARING

EXHAUST

appropriate for hot processes the hood. Canopy hoods work configuration. A canopy hood the ease with which crossdrafts

OPTIONS

For comparison purposes, Table I indicates the minimum recommended exhaust flow for a 2% x 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. Table

I. Comparative

Exhaust

Flow

Rates for Various

Hood

Types

Single

Lateral

Exhaust

Push-

Open

Enclosing

Slot

Slots

at Front

Pull

Canopy

Booth

andBack

Control Velocity CFMiFt’ CFM Notes:

712

100 175

100 150

100

2190

1875

1250

175

175 -

8135

750

Single slot fishtail type hood. Lateral slots located at inside tank edge. Double slot fishtail type hood on rear, downdraft on front. Canopy hood located 2 ft above process. Enclosing booth with 2 ft high by 5 ft long opening on one side. 12.5 square foot tank area.

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 which will keep air contaminants in suspension during transport to the scrubber fan, but 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 to 1,500 fpm. For open surface tanks, and the contaminants generated, a duct velocity of 2,000 to 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 American Conference of Governmental Industrial Hygienists Indusfrial Ventilation Manual for details. Each design has advantages and disadvantages. The self balancing system has the advantage of being relatively tamperproof and will ensure 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 off. Conversely, the blast gate method is subject to tampering, but offers greater operational flexibility. Accordingly, self balancing 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” water column (WC) for systems without scrubbers, and 3 to 5” for systems with scrubbers. Slot hoods designed as previously described will produce a pressure drop ranging from approximately 0.5 to 1” WC. The typical duct system will add an additional 0.5 to 1” WC. Exhaust gas scrubbers have pressure drops typically ranging from 0.5 to 3” 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 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 build-up in the ductwork of liquids and solids left behind by evaporation of the liquids. Cleanouts should be furnished at specified intervals on straight horizontal runs and at each elbow. Duct should be sloped to a low point and drains with traps or isolation valves 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 soume 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, TX. 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 resistance 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 714

process tanks, typically caused by electric immersion heaters in dry tanks, are a fairly common occurrence. The tank tire spreads to the plastic hood, with tbe potential for involving the entire exhaust system and adjacent building components. Some insurance underwriters require plastic ductwork to be furnished with internal sprinklers, while others will accept the system without sprinklers provided that materials do not exceed a certain thickness, e.g. i/4” 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 tire retardant 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.

EXHAUSTFANTYPES

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 arid FRP are more commonly used for exhaust fans handling the highly corrosive fumes 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 achieved in vane axial fans (up to about 6” 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, limits their usefulness as industrial process exhaust fans. Centrifugal fans can be classified into three main types: forward curved blade, backward inclined blade and radial blade. A forward curved blade, or squirrel cage, fan has blades that curve toward the direction of fan rotation. They deliver low 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 streams. A backward inclined (BI) blade centrifugal fan has blades which 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 build-up of material on the fan blades. Radial blade centrifugal fans have blades similar to those of a paddle wheel. This blade orientation limits material build-up 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.

716

CLEAN

GAS

MISTA ELIMINATOR

WATERSPRAY

Fig.

4. Spray

chamber

I

. ,, \ I’,_ ‘;Njs I>

---t

I--

WATER INLET

scrubber. DIRTY -GAS --

+ 1

DIRTY WATER OUTLET

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 FRM fan costing nearly twice the epoxy-steel. Air Movement and Control Association (AMCA) classes Types A, B & C are for spark proof fans and are required for exhausting flammable vapors such as non-halogenated 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 which 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. Vinyl ester based FRP is generally used for fan wheels because of its strength and ductility. Fan housings are typically made of polyester based FRP. Since 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 directly with the fan speed, static pressure with the square of the fan speed and power required 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 717

GAS

OUT I

-

-\ ’

.LIQUID DISTRIBUTOR

REDISTRIBUTOR TELLERETTE

~_ -m:-

Fig.

5. Packed

LIQUID

OUT

bed scrubber

, .‘I), (3y ex d, BERL SADDLE

with common

/=-A--I c/T-INTALOX

packing

SADDLE

types.

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 which 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 build-up of dirt and cleaned, if necessary. Any dynamic imbalance caused by excessive corrosion should be corrected to prevent damage to the bearings. While 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 modem 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 in 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. Cross-drafts 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 threshold limit value (TLV). 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 which 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 susbstantial 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 both the Federal and state levels

720

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, impact 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 last 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. 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/particuIates and vapors. The most common sources of VOC in metal finishing come from solvents contained in paints and 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 these 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 volatiles. The carbon is then stripped of the adsorbed VOC by heat and the organics 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

721

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 scrubber’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, 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 a system where the absorbing liquid is collected at the bottom of the chamber then 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 to 0.3 gallons per minute (gpm) per thousand cubic feet per minute (cfm) of exhaust air. The absorbing liquid recirculation is approximately 5 to 15 gpm per 1000 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 are typically fabricated of rigid polyvinyl chloride (PVC) or fiberglass reinforced plastic (FRP). Internals are usually a combination of PVC. FRP, polypropylene, and sometimes stainless steel materials. If water recirculating scrubbers are located outdoors, remote indoor sumps constructed of corrosion resistant materials are required for freeze protection. Wet scrubbers are not suitable devices for volatile organic compound (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 which contain less 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 non-cyanide operations eliminates cyanide emissions. Process changes can be a difficult course to follow in emission reductions because of the myriad of choices sometimes available, as well as the scarcity of guidance offered by regulatory officials.

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EXHAUST

DISCHARGE

After the exhaust fan and/or control device the exhaust gas is vented to 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 they are 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 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 height above ground should be at least one and one-half to two times that of the tallest adjacent structures, where practical.

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