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Recirculation ventilation in paint spray booths: New insights
Recirculation Ventilation in Paint Spray Booths: New Insights Jacqueline Ayer, Air Quality Services, Newport Beach, Calif.
F
or most metal-finishing operations, the cost of controlling volatile organic compound (VOC) emissions from coating processes is exorbitant. The primary reason for these high emission control costs is that typical coating operations are associated with high volumetric exhaust flow rates coupled with low VOC concentrations. This article focuses on safe and efficient methods for reducing these flow rates, which in turn will facilitate more cost-effective VOC emission control from paint spray booth and other coating operations.
BACKGROUND The Federal Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association (NFPA) require that minimum ventilation flow rates be maintained through paint booth enclosures and other coating operations to ensure a safe working environment. Conforming with these safety requirements has the adverse effect of generating high process exhaust volumes. Because the EPA and other agencies recognize that the cost of installing coating process VOC emission abatement systems is often prohibitive, their efforts on reducing emissions have historically focused on limiting the usage rate and the VOC content of the coatings that are employed. Under this system, add-on VOC emission controls are only required if a facility elects to use coatings that do not conform with the required VOC content limits, or to use more coatings than their permitted allocation. With the advent of the 1990 Amendments to the Clean Air Act (CAAA), the whole scene has changed. Implementation of Title I and Title III of the CAAA is likely to bring many metalfinishing facilities face to face with VOC emission control requirements. How, then, can facilities cost-effectively comply with these VOC emisMETALFINISHING
. DECEMBER
1995
sion control regulations? Coating manufacturers are hard at work to create new low- and no-VOC formulations, and equipment manufacturers are working equally hard to develop less expensive air pollution control systems. What can the facility do? The easiest and most cost-effective course of action is simply to reduce the flow rate exhausted from the coating process. THE BENEFITS OF FLOW REDUCTION Typical air pollution control system (APCS) size and operating requirements are directly related to the process exhaust flow rate. Thus, the effect of reducing the exhaust volume flow rate on emission control costs is dramatic. For example, if the flow rate from a large coating operation is reduced from 100,000 to 50,000 cfm, the corresponding installation cost for a rotary concentrator/oxidizer system can be reduced from $1.7 million to less than $1 million, which represents a 40% reduction. Naturally, the emission control system operating costs are reduced significantly (up to 50%) as well. This is particularly true if a thermal or catalytic emission control system is installed, because far less thermal energy is required to heat the process air to the requisite temperature. Moreover, the cost of installing a flow reduction system is generally an order of magnitude less than the incremental cost incurred to install an APCS, which is sized to process the entire ventilation flow rate. Thus, the flow reduction system installation payback period is almost zero! Flow reduction is applicable to, and will result in, reduced APCS installation and operating costs for all coating operations; however, for small coating processes (10,000 cfm) the payback period is somewhat longer, perhaps up to 1 year. Of course, for typical industrial operations, a 1-year payback is quite reasonable.
By reducing the process exhaust flow rate, a number of additional benefits are derived that are not directly related to APCS installation and operating cost reductions. First, facility heating, ventilation, and air conditioning (HVAC) costs are significantly reduced. This is particularly important for facilities operating in areas that experience temperature extremes such as high heat and humidity in the summer and/or dry, cold conditions in the winter. Many coating processes must operate within narrowly defined temperature and humidity limits to ensure that the coating is applied and cured correctly. A second benefit is the reduction in energy costs that occurs as a result of downsizing the exhaust fan. This may be significant if long distances separate coating processes from facility exhaust points. Another benefit is accrued to facilities that employ intake filters to ensure that only dust-free air is introduced into the coating process. The various flow reduction strategies described below will increase the lifetime of these filters.
FLOW REDUCTION STRATEGIES Most ventilated coating operations operate in a single-pass mode in which ventilation air is introduced into the process area, where it picks up overspray particulate and/or solvent vapors, and is then exhausted to atmosphere. As discussed previously, this single-pass mode of operation generates a high exhaust air flow rate. By implementing an appropriate flow reduction strategy that is tailored to the particular coating operation, these flow rates are significantly reduced. The simplest method for reducing paint booth exhaust flow rates is to install a return air system, which recirculates a portion of the exhaust air back into the booth. The portion that is not recirculated is vented to an APCS. Prior to reentering the booth, the recirculated 21
OFTIONAL SAFEIY BYPASS DUCT
FRESH
DUCT
)-L
FILTER FACE
/
Figure 1. Simple recirculation ventilation. stream is mixed with fresh make-up air that is required to replace the exhaust stream vented to the APCS. This ventilation strategy, referred to as simple recirculation, is illustrated in Figure 1. The flow reduction that is achieved from simple recirculation is a function of the percent recirculation rate that may be safely employed. Naturally, safety limits are placed on the percent recirculation that is permitted for a particular process operation. These limits, discussed in more detail below, relate to the concentration of hazardous constituents that are present in the recirculated stream. There are hvo variations of simple recirculation that may be employed to further reduce exhaust volumetric flow rates. These two recirculation enhancement strategies are referred to as splifJowlrecirculation and dynamically optimized recirculation.
Split-Flow/Recirculation By inspection of Figure 1, it may be observed that the concentration of hazardous constituents in the recirculated stream (point A) generated by simple recirculation is the same as the concentration found in the exhaust stream (point B). It, therefore, follows that recirculation can be further enhanced (i.e., the recirculation rate increased) if the ventilation system can be configured to generate a higher constituent concentration in the exhaust stream than in the recirculation stream. This may be achieved by taking advantage of the natural stratification that occurs 22
in some coating process enclosures. This stratification tends to create zones within the enclosure that have relatively high constituent concentrations, and zones that have relatively low concentrations. For instance, in some cross draft paint spray booths, the upper regions of the booth generally have lower VOC and overspray particulate levels than the lower regions of the booth. This is often evidenced by the higher deposition of paint overspray on the filter panels situated at the lower region of the exhaust face. For coating operations that demonstrate stratification, the concentration in the exhaust stream can be increased by selectively venting air from only the high concentration zones to the exhaust duct. Correspondingly, only air from the low concentration zones within the enclosure is recirculated. This achieves higher constituent concentrations in the exhaust stream, and, therefore, further enhances recirculation system operation. A schematic diagram illustrating a simple cross flow booth, which demonstrates stratification characteristics and for which the split-flow/recirculation concept is applicable, is provided in Figure 2. In this configuration, paint overspray particulate and solvent vapors tend to remain at or below the height at which they are released under the influence of air flow dynamics and gravity. Under these conditions, exhaust face constituent concentrations at the lower levels tend to be much higher than at the higher levels. By actively splitting the exhaust
plenum into two zones (via the indicated split partition), the constituent stratification is preserved. There are a number of safety and equipment issues that must be considered in designing and installing a safe split-flow/ recirculation system; these issues are discussed below.
Dynamically Optimized Recirculation The most innovative, cost effective, and universally applicable enhancement of the recirculation concept is known as dynamically optimized recirculation. As illustrated in Figures 1 and 2, both simple recirculation and split-flowkirculation rely on fixed volume flow rates in the exhaust and recirculation ducts. Conversely, dynamically optimized mcirculation allows these flow rates to vary to ensure that the maximum level of recirculation is continually achieved on a mal-time basis. For example, a dynamically optimized system installed on a paint spray booth will increase the recirculation rate during periods in which the spray guns are turned off (i.e., while the worlcpiece dries between coatings or during color change), and will reduce the recirculation rate when painting is resumed. This allows the facility continually to achieve the lowest possible exhaust flow rate from a coating process operation. Correspondingly, emission control costs, HVAC costs, and make-up air filtration requirements are also continually reduced to the lowest levels. In addition to achieving the lowest possible operating costs, the dynamically optimized recirculation system is completely independent of booth configuration and is, therefore, universally applicable. A simple application of the dynamically optimized recirculation system is illustrated in Figure 3. In the contiguration indicated, a monitor that continuously analyzes constituent concentrations in the recirculation duct provides input data to a central controller, which adjusts the exhaust and recirculation rates as appropriate. When the monitor detects that the constituent concentrations in the recirculation duct ate below a setpoint level, the central controller reduces the exhaust flow rate by adjusting a variable frequency drive (VFD) exhaust fan, and adjusts the dampers in the make-up air and krculation ducts such that the recirculation rate is increased, METAL FINISHING
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1995
OPTIONALSAFETY BVPASS DUCT
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Figure 2. Split-flow/recirculation ventilation. ADJUSTABLE_ DAMPERS
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Figure 3. Dynamically optimized recirculation.
and the make-up air flow rate is decreased. Dampers are used in the recirculation and make-up air ducts because a fixed drive intake fan is employed rather than a VFD fan. There are several advantages of this configuration; not only is a fixed drive fan less expensive, but it is adequate for ensuring that the minimum ventilation flow rate required by OSHA is delivered through the paint booth enclosure. Like the simple recirculation and split-flow/recirculation systems, an APCS that is operated in conjunction with a dynamically optimized recirculation system must be sized according
to the maximum anticipated exhaust flow rates; therefore, the dynamically optimized recirculation strategy does not reduce APCS installation costs per se. It does, however, significantly reduce APCS operating costs, as well as other facility costs. RECIRCULATION SYSTEM DESIGN AND INSTALLATION ISSUES In 1990, OSHA furnished a de minimis ruling that permits the use of recirculation to reduce VOC emission control costs, provided that hazardous
constituent concentrations in the booth intake stream remain below permissible exposure levels (PELs). For example, the established 8-hour time weighted average (TWA) and shortterm exposure level (STEL) for xylene (a common coating solvent ) are 435 mg/m3 and 655 mg/m3, respectively. These levels are recommended by the National Institute of Occupational Safety and Health (NIOSH); however, because most coating operations involve mixtures of solvents and other organic and inorganic hazardous materials, the additive effects of these materials must also be considered. The key to designing a safe and effective recirculation system is to maximize the percent recirculation while ensuring that constituent concentrations remain below acceptable levels. The initial part of this section focuses on general design issues that should be addressed to achieve this objective safely and to obtain appropriate system operating permits successfully. In general, the following items should be considered in the design and installation of any recirculation system: Interlocks: The installation of system interlocks should be considered to ensure that the APCS is functioning properly and that the ventilation system is fully operational and delivering the proper ventilation rates prior to initiating any process operations. This is actually applicable for nonrecirculating systems as well. Safety Control System: A strategy for ensuring safe recirculation system operation is to install a safety monitoring system that analyzes the recirculation stream constituent concentrations and automatically converts the booth to single-pass operation in the event excessive concentrations are found. It may be reasonable to discontinue use of this interlock system after an appropriate period of time has passed during which the monitor records no excursions above the setpoint level; however, the monitoring system should always be reactivated if new coating formulations are employed, to ensure continued safe operation. Although the dynamically optimized system already incorporates a recirculation stream continuous monitor, the inclusion of a single-pass conversion option provides an additional level of safety. METAL FINISHING
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1995
Enclosed Operation: Although it is technically feasible to design and install a recirculation system that is not completely enclosed (i.e., with one open side), such open contigurationsare susceptible to fluctuations in ventilation air flow patterns due to operating conditions in the surrounding area (as when an exterior roll-up door is opened, or the periodic activation of an intermittently operated building air handling system). Such fluctuations will affect the ventilation flow rate in the booth, which in turn will have an impact on the recirculation system operation. It is, therefore, recommended that any work area that employs recirculation ventilation be fully enclosed. Moreover, open-sided operations tend to have far higher volumetric flow rates than fully enclosed systems in order to maintain a minimum-required linear velocity at the openings. Thus, open-sided operations tend to defeat the whole purpose of recirculation in the first place. Finally, open work areas often have significant fugitive emission problems. For these reasons and others, it is generally recommended that coating operations (both recirculating and nonrecirculating) be enclosed to the maximum extent possible. Retrofit versus New Construction: All of the recirculation technologies discussed in this article are applicable to both retrofit and new construction activities. In some ways, retrofit systems are more easily engineered and designed because there usually exists a reasonable database relating to target configurations and coating usage data. This information is necessary to design and install a safe and efficient recirculation system. Air Pollution Control Agency Permitting: Irrespective of whether a new or retrofit system is considered, it is likely that the construction activities will require some level of permitting and compliance review by an air pollution control agency. With respect to operation of the coating enclosure (as opposed to the APCS), permitting agencies are generally most concerned with the issue of pollutant capture efficiency, which defines the ability of the ventilation system to collect and vent pollutants to a control device (such as a particulate filtration system and a VOC APCS). As with any coating operation, a well-designed enclo26
sure that employs recirculation operates under negative pressure. Thus, all material released in the enclosure (such as paint overspray particulate or VOCs) is captured and vented, rather than escaping as fugitive emissions. There is little or no difference between recirculation ventilation and any other coating operation ventilation system from a permitting perspective. In fact, because recirculation systems are generally enclosed, they have much higher capture efficiencies than other systems that are only partially enclosed. Unfortunately, application of recirculation technologies is not yet widespread; as such, regulatory agencies may scrutinize a recirculation system permit somewhat more closely. This is overcome by including comprehensive design information in the permit application package such as flow rates, pressures, filtration system data, and other information, which demonstrates that the necessary capture efficiency and filtration efficiency levels are met. Other Permitting Requirements: As with any construction activity, it is likely that the installation of a recirculation ventilation system will entail various building permits and perhaps even safety inspections by an in-house safety manager or an outside agency. If appropriate, calculations and supporting information employed to determine the recirculation flow rate may be referenced during these inspections. The remainder of this section focuses on specific design issues that relate to each of the three flow reduction strategies considered.
Simple Recirculation System Design Considerations Many of the components employed in typical metal-finishing operations are classified as potentially hazardous by OSHA; therefore, the additive effects of each component that is present in the coating must be carefully considered in designing a safe and efficient recirculation system. The appropriate recirculation flow rate is calculated based on several parameters, including coating material usage rates, coating material components, and ventilation flow rates through the process area. A conservative approach, which relies on worst-case assumptions, should be adopted in determining the recircu-
lation rate. For example, the highest possible material usage rate should be considered in the analysis to ensure that OSHA constituent concentration limits are not exceeded in the recirculation stream. This can be accomplished for paint spray operations by considering the maximum number of paint spray guns that may be employed, coupled with their respective coating release rates. Another important consideration stems from the fact that coating operations that employ dry filters to remove exhaust stream particulate typically rely on fixed-drive fans to provide the necessary process ventilation air flow. Because these fans are not capable of overcoming the gradual increase in pressure drop that occurs across the filter as it becomes loaded with particulate, the flow rate through the process area tends to decrease slowly over time. This unavoidable flow rate decrease must also be considered in determining the appropriate recirculation rate.
Split-Flow/Recirculation System Design Considerations The added complexity of the splitflow recirculation system has associated with it a number of design constraints that must be considered in addition to those design issues discussed above. The location of the split partition dictates the volumetric flow rate of the partitioned streams and, therefore, dictates the system recirculation rate. The split partition location must be carefully derived based on a number of variables, including the coating process area configuration; the concentration profile of the vapor phase components at the exhaust face; the concentration profile of the solid phase components at the exhaust face; and the particulate control efficiency of the exhaust face filtration system. The solid- (paint overspray) and vapor-phase concentration profiles must also be identified separately because solid- and vapor-phase components do not respond equally to the effects of flow dynamics and gravity, thus it may not be appropriate to assume they have similar profiles. These parameters are very important, because they ultimately dictate the concentration of hazardous constituents that are present in the recirculated stream. METAL FINISHING
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Another important issue that must be considered is that the exhaust stream is selectively pulled from zones within the work area having relatively high concentrations; thus, it is likely that the filters through which the exhaust air passes will become loaded with particulate much faster than the filters through which the recirculation stream passes. If not properly handled, this selective blinding effect on the filters will cause ventilation air that ordinarily passes into the exhaust duct to be diverted into the recirculation duct. This can cause unexpectedly high constituent concentrations in the recirculation stream, and significant flow profile variations across the exhaust filter face. To avoid this occurrence, the filters through which the exhaust air passes should be replaced more frequently than the filters through which the recirculation air passes. The rate of filter replacement should be determined using separate pressure drop monitors (such as incline manometers) across the exhaust filter face and the recirculation filter face. To increase filter life, the facility may elect to install VFD fans that operate in conjunction with flow monitors placed in the recirculation and exhaust streams. The VFD fans will deliver a constant flow rate despite filtration system loading problems; however, there are some very important design issues that must be considered in deciding whether or not VFD fans are appropriate. The first is that the exhaust VFD fan operation must be very carefully integrated with APCS operation. The second factor is that the VFD fans must never be driven to the point where the maximum pressure drop across the filter face or in the exhaust plenum is reached. Such an occurrence can cause the filter elements to be dislodged and pulled through the fan, or cause the plenum system and possibly the duct work to collapse. As discussed above, the split-flow/ recirculation configuration establishes a fixed split-height and a corresponding recirculation rate based on the stratification conditions in the booth enclosure. Of course, not all booth configurations can be adapted to employ split-flow/recirculation. Configurations in which split-flow/recirculation may not be applicable include the following. METAL FINISHING
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Booths that have a high variability in workpiece configuration such that any stratification pattern that may exist is either inconsistent or not repeatable. This is critical because the split-height and recirculation rate should be determined based on worst-case conditions that would result in the highest recirculation stream concentrations. Thus, if worst-case conditions occur only once a month, yet the split-height and recirculation rate is established based on these conditions, the booth operate under nonoptimal circumstances most of the time. For this type of operation, a dynamically optimized recirculation strategy is more applicable. Booths that employ a particulate filtration system, which does not preserve the stratification pattern, for example, a water wash particulate filtration system will destroy the stratification pattern before the ventilation air reaches the split partition. Booths that have ventilation air flow patterns characterized by highly turbulent conditions, either due to the orientation and characteristics of the target workpiece, or nonstandard intake and exhaust face configurations. For example, booths in which the ventilation air is brought in through the top of the booth, and vented out the sides or back of the booth, may not be reasonable candidates for split-flow/ recirculation. Dynamically Optimized Recirculation System Design Considerations As indicated previously, a dynamically optimized system is the most versatile of the recirculations systems, because it is independent of booth configuration, and can be designed to avoid some of the filter blinding problems associated with the split-flow/recirculation strategy. The system illustrated in Figure 3 indicates a VFD fan in the exhaust stream; as such, this configuration is subject to constraints similar to those identified for splitflow/recirculation. Alternatively, a fixed-drive fan operated in conjunction with a damper could be employed rather than a VFD fan to achieve the necessary exhaust flow rate adjustments; however, this configuration
may not necessarily yield the most energy efficient system. As in the case of simple recirculation and, for that matter, split-flow/ recirculation, an APCS that is operated in conjunction with a dynamically optimized recirculation system must be sized according to the maximum anticipated exhaust flow rates; therefore, many of the dynamically optimized design issues that must be addressed are similar to the design issues that relate to simple recirculation. The dynamic system, however, also requires the installation and operation of a real-time constituent concentration monitor coupled with a flow rate control system. The control system must be integrated on some level with the APCS to ensure appropriate handling of the exhaust flow rate adjustments.
CONCLUSIONS The advantages of employing recirculation flow reduction strategies for achieving cost-effective VOC emission control are significant, and their importance is becoming increasingly apparent as we face the reality of more stringent emission control regulations. Recirculation ventilation provides a relatively inexpensive means of significantly reducing the installation and operating costs of a VOC emission control system, as well as other process operating costs associated with air handling, cooling, and heating. Although this article has focused on paint spray booth operations, the technologies described herein are applicable to a number of other processes including chemical stripping (depainting), and clean room operations. Selection and design of an appropriate recirculation system for a particular facility must be given careful consideration. The design and installation of a recirculation system is not difficult if proper attention is paid to key aspects relating to system safety and operation. Many recirculation systems have been installed across the country, and although most of them probably operate within established safety limits, it is this author’s opinion that a number of them were installed with insufficient regard for issues relating to safety and system operating efficiency. This article provides a brief overview of key issues that a facility should address in 27
determining the applicability of recirculation. It should also be referred to by a facility wishing to install a recirculation technology to ensure that the consultants who are retained to design and install the system perform adequately. If a consultant does not appear to have sufficient regard for various aspects relating to safety and system integration, run, don’t walk, to a consultant who does. You will be rewarded
with a safely operating recirculation system that is efftciently operated and properly integrated with the APCS. Disclaimer The recirculation ventilation systems described in this article are protected under patent infringement regulations, and may not be employed without permission of the patent owners or their designated licensees.
ELECTROPLATING OF ZINC DIE CASTINGS bySK. Jalota 225pages hardcover $YO.OO This book provides a comprehensive coverage of plating on zinc die castings starting with design and production with emphasis on its relation to plating. Subsequent chapters treat polishing, buffing and mass finishing, cleaning and pretreatment and the equipment for plating. The balance of the book runs the gamut of the various metals that can be deposited. Each chapter devotes a portion to troubleshooting-causes of defects and remedies. Concluding chapters concern coloring, lacquering, stripping of deposits and analysis and testing.
ZINC PLATING byH. Geduki SGOpages hardcover $100.00 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.
ELECTROLESS NICKEL PLATING by Worfgng Riedel 328page.s hardcokr $109.00 This book is a translation of the German Edition updated through 1989. It contains 17 chapters covering all aspects from pretreatment to post-treatment including brief discussions of he market size, stripping specifications and waste treatment. The history and chemistry of electroless plating are covered in more detail as are set-up, operations and applications; however, the heart of this volume is concerned with the structure and properties of the deposits, which make up nearly one:hird of this volume. A valuable reference for finishers.
It is the intention of the author to provide background information relating to recirculation technologies only. The design and safety issues described herein should not be considered of sufficient detail to adequately design a recirculation system without supplemental information. MC
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The first complete what-to-do and how-to-do-it manual on powder coating methods and procedures. Every aspect of powder coating and powder coating technology is discussed, in a logical sequence, in depth: types and characteristics of powder materials; cleaning and preparation of substrate metals prior to application of a powder coating; powder application equipment;pretreatment equipment; ovens; conveyor systems; hanging furtures; staging; feasibility studies; installation, maintenance; and purchasing of equipment. For any organization seeking to capitalize on the advances in powder coating materials and equipment, this is the definite guide.
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28
METAL FINISHING
. DECEMBER
1995
F
or most metal-finishing operations, the cost of controlling volatile organic compound (VOC) emissions from coating processes is exorbitant. The primary reason for these high emission control costs is that typical coating operations are associated with high volumetric exhaust flow rates coupled with low VOC concentrations. This article focuses on safe and efficient methods for reducing these flow rates, which in turn will facilitate more cost-effective VOC emission control from paint spray booth and other coating operations.
BACKGROUND The Federal Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association (NFPA) require that minimum ventilation flow rates be maintained through paint booth enclosures and other coating operations to ensure a safe working environment. Conforming with these safety requirements has the adverse effect of generating high process exhaust volumes. Because the EPA and other agencies recognize that the cost of installing coating process VOC emission abatement systems is often prohibitive, their efforts on reducing emissions have historically focused on limiting the usage rate and the VOC content of the coatings that are employed. Under this system, add-on VOC emission controls are only required if a facility elects to use coatings that do not conform with the required VOC content limits, or to use more coatings than their permitted allocation. With the advent of the 1990 Amendments to the Clean Air Act (CAAA), the whole scene has changed. Implementation of Title I and Title III of the CAAA is likely to bring many metalfinishing facilities face to face with VOC emission control requirements. How, then, can facilities cost-effectively comply with these VOC emisMETALFINISHING
. DECEMBER
1995
sion control regulations? Coating manufacturers are hard at work to create new low- and no-VOC formulations, and equipment manufacturers are working equally hard to develop less expensive air pollution control systems. What can the facility do? The easiest and most cost-effective course of action is simply to reduce the flow rate exhausted from the coating process. THE BENEFITS OF FLOW REDUCTION Typical air pollution control system (APCS) size and operating requirements are directly related to the process exhaust flow rate. Thus, the effect of reducing the exhaust volume flow rate on emission control costs is dramatic. For example, if the flow rate from a large coating operation is reduced from 100,000 to 50,000 cfm, the corresponding installation cost for a rotary concentrator/oxidizer system can be reduced from $1.7 million to less than $1 million, which represents a 40% reduction. Naturally, the emission control system operating costs are reduced significantly (up to 50%) as well. This is particularly true if a thermal or catalytic emission control system is installed, because far less thermal energy is required to heat the process air to the requisite temperature. Moreover, the cost of installing a flow reduction system is generally an order of magnitude less than the incremental cost incurred to install an APCS, which is sized to process the entire ventilation flow rate. Thus, the flow reduction system installation payback period is almost zero! Flow reduction is applicable to, and will result in, reduced APCS installation and operating costs for all coating operations; however, for small coating processes (10,000 cfm) the payback period is somewhat longer, perhaps up to 1 year. Of course, for typical industrial operations, a 1-year payback is quite reasonable.
By reducing the process exhaust flow rate, a number of additional benefits are derived that are not directly related to APCS installation and operating cost reductions. First, facility heating, ventilation, and air conditioning (HVAC) costs are significantly reduced. This is particularly important for facilities operating in areas that experience temperature extremes such as high heat and humidity in the summer and/or dry, cold conditions in the winter. Many coating processes must operate within narrowly defined temperature and humidity limits to ensure that the coating is applied and cured correctly. A second benefit is the reduction in energy costs that occurs as a result of downsizing the exhaust fan. This may be significant if long distances separate coating processes from facility exhaust points. Another benefit is accrued to facilities that employ intake filters to ensure that only dust-free air is introduced into the coating process. The various flow reduction strategies described below will increase the lifetime of these filters.
FLOW REDUCTION STRATEGIES Most ventilated coating operations operate in a single-pass mode in which ventilation air is introduced into the process area, where it picks up overspray particulate and/or solvent vapors, and is then exhausted to atmosphere. As discussed previously, this single-pass mode of operation generates a high exhaust air flow rate. By implementing an appropriate flow reduction strategy that is tailored to the particular coating operation, these flow rates are significantly reduced. The simplest method for reducing paint booth exhaust flow rates is to install a return air system, which recirculates a portion of the exhaust air back into the booth. The portion that is not recirculated is vented to an APCS. Prior to reentering the booth, the recirculated 21
OFTIONAL SAFEIY BYPASS DUCT
FRESH
DUCT
)-L
FILTER FACE
/
Figure 1. Simple recirculation ventilation. stream is mixed with fresh make-up air that is required to replace the exhaust stream vented to the APCS. This ventilation strategy, referred to as simple recirculation, is illustrated in Figure 1. The flow reduction that is achieved from simple recirculation is a function of the percent recirculation rate that may be safely employed. Naturally, safety limits are placed on the percent recirculation that is permitted for a particular process operation. These limits, discussed in more detail below, relate to the concentration of hazardous constituents that are present in the recirculated stream. There are hvo variations of simple recirculation that may be employed to further reduce exhaust volumetric flow rates. These two recirculation enhancement strategies are referred to as splifJowlrecirculation and dynamically optimized recirculation.
Split-Flow/Recirculation By inspection of Figure 1, it may be observed that the concentration of hazardous constituents in the recirculated stream (point A) generated by simple recirculation is the same as the concentration found in the exhaust stream (point B). It, therefore, follows that recirculation can be further enhanced (i.e., the recirculation rate increased) if the ventilation system can be configured to generate a higher constituent concentration in the exhaust stream than in the recirculation stream. This may be achieved by taking advantage of the natural stratification that occurs 22
in some coating process enclosures. This stratification tends to create zones within the enclosure that have relatively high constituent concentrations, and zones that have relatively low concentrations. For instance, in some cross draft paint spray booths, the upper regions of the booth generally have lower VOC and overspray particulate levels than the lower regions of the booth. This is often evidenced by the higher deposition of paint overspray on the filter panels situated at the lower region of the exhaust face. For coating operations that demonstrate stratification, the concentration in the exhaust stream can be increased by selectively venting air from only the high concentration zones to the exhaust duct. Correspondingly, only air from the low concentration zones within the enclosure is recirculated. This achieves higher constituent concentrations in the exhaust stream, and, therefore, further enhances recirculation system operation. A schematic diagram illustrating a simple cross flow booth, which demonstrates stratification characteristics and for which the split-flow/recirculation concept is applicable, is provided in Figure 2. In this configuration, paint overspray particulate and solvent vapors tend to remain at or below the height at which they are released under the influence of air flow dynamics and gravity. Under these conditions, exhaust face constituent concentrations at the lower levels tend to be much higher than at the higher levels. By actively splitting the exhaust
plenum into two zones (via the indicated split partition), the constituent stratification is preserved. There are a number of safety and equipment issues that must be considered in designing and installing a safe split-flow/ recirculation system; these issues are discussed below.
Dynamically Optimized Recirculation The most innovative, cost effective, and universally applicable enhancement of the recirculation concept is known as dynamically optimized recirculation. As illustrated in Figures 1 and 2, both simple recirculation and split-flowkirculation rely on fixed volume flow rates in the exhaust and recirculation ducts. Conversely, dynamically optimized mcirculation allows these flow rates to vary to ensure that the maximum level of recirculation is continually achieved on a mal-time basis. For example, a dynamically optimized system installed on a paint spray booth will increase the recirculation rate during periods in which the spray guns are turned off (i.e., while the worlcpiece dries between coatings or during color change), and will reduce the recirculation rate when painting is resumed. This allows the facility continually to achieve the lowest possible exhaust flow rate from a coating process operation. Correspondingly, emission control costs, HVAC costs, and make-up air filtration requirements are also continually reduced to the lowest levels. In addition to achieving the lowest possible operating costs, the dynamically optimized recirculation system is completely independent of booth configuration and is, therefore, universally applicable. A simple application of the dynamically optimized recirculation system is illustrated in Figure 3. In the contiguration indicated, a monitor that continuously analyzes constituent concentrations in the recirculation duct provides input data to a central controller, which adjusts the exhaust and recirculation rates as appropriate. When the monitor detects that the constituent concentrations in the recirculation duct ate below a setpoint level, the central controller reduces the exhaust flow rate by adjusting a variable frequency drive (VFD) exhaust fan, and adjusts the dampers in the make-up air and krculation ducts such that the recirculation rate is increased, METAL FINISHING
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OPTIONALSAFETY BVPASS DUCT
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Figure 3. Dynamically optimized recirculation.
and the make-up air flow rate is decreased. Dampers are used in the recirculation and make-up air ducts because a fixed drive intake fan is employed rather than a VFD fan. There are several advantages of this configuration; not only is a fixed drive fan less expensive, but it is adequate for ensuring that the minimum ventilation flow rate required by OSHA is delivered through the paint booth enclosure. Like the simple recirculation and split-flow/recirculation systems, an APCS that is operated in conjunction with a dynamically optimized recirculation system must be sized according
to the maximum anticipated exhaust flow rates; therefore, the dynamically optimized recirculation strategy does not reduce APCS installation costs per se. It does, however, significantly reduce APCS operating costs, as well as other facility costs. RECIRCULATION SYSTEM DESIGN AND INSTALLATION ISSUES In 1990, OSHA furnished a de minimis ruling that permits the use of recirculation to reduce VOC emission control costs, provided that hazardous
constituent concentrations in the booth intake stream remain below permissible exposure levels (PELs). For example, the established 8-hour time weighted average (TWA) and shortterm exposure level (STEL) for xylene (a common coating solvent ) are 435 mg/m3 and 655 mg/m3, respectively. These levels are recommended by the National Institute of Occupational Safety and Health (NIOSH); however, because most coating operations involve mixtures of solvents and other organic and inorganic hazardous materials, the additive effects of these materials must also be considered. The key to designing a safe and effective recirculation system is to maximize the percent recirculation while ensuring that constituent concentrations remain below acceptable levels. The initial part of this section focuses on general design issues that should be addressed to achieve this objective safely and to obtain appropriate system operating permits successfully. In general, the following items should be considered in the design and installation of any recirculation system: Interlocks: The installation of system interlocks should be considered to ensure that the APCS is functioning properly and that the ventilation system is fully operational and delivering the proper ventilation rates prior to initiating any process operations. This is actually applicable for nonrecirculating systems as well. Safety Control System: A strategy for ensuring safe recirculation system operation is to install a safety monitoring system that analyzes the recirculation stream constituent concentrations and automatically converts the booth to single-pass operation in the event excessive concentrations are found. It may be reasonable to discontinue use of this interlock system after an appropriate period of time has passed during which the monitor records no excursions above the setpoint level; however, the monitoring system should always be reactivated if new coating formulations are employed, to ensure continued safe operation. Although the dynamically optimized system already incorporates a recirculation stream continuous monitor, the inclusion of a single-pass conversion option provides an additional level of safety. METAL FINISHING
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Enclosed Operation: Although it is technically feasible to design and install a recirculation system that is not completely enclosed (i.e., with one open side), such open contigurationsare susceptible to fluctuations in ventilation air flow patterns due to operating conditions in the surrounding area (as when an exterior roll-up door is opened, or the periodic activation of an intermittently operated building air handling system). Such fluctuations will affect the ventilation flow rate in the booth, which in turn will have an impact on the recirculation system operation. It is, therefore, recommended that any work area that employs recirculation ventilation be fully enclosed. Moreover, open-sided operations tend to have far higher volumetric flow rates than fully enclosed systems in order to maintain a minimum-required linear velocity at the openings. Thus, open-sided operations tend to defeat the whole purpose of recirculation in the first place. Finally, open work areas often have significant fugitive emission problems. For these reasons and others, it is generally recommended that coating operations (both recirculating and nonrecirculating) be enclosed to the maximum extent possible. Retrofit versus New Construction: All of the recirculation technologies discussed in this article are applicable to both retrofit and new construction activities. In some ways, retrofit systems are more easily engineered and designed because there usually exists a reasonable database relating to target configurations and coating usage data. This information is necessary to design and install a safe and efficient recirculation system. Air Pollution Control Agency Permitting: Irrespective of whether a new or retrofit system is considered, it is likely that the construction activities will require some level of permitting and compliance review by an air pollution control agency. With respect to operation of the coating enclosure (as opposed to the APCS), permitting agencies are generally most concerned with the issue of pollutant capture efficiency, which defines the ability of the ventilation system to collect and vent pollutants to a control device (such as a particulate filtration system and a VOC APCS). As with any coating operation, a well-designed enclo26
sure that employs recirculation operates under negative pressure. Thus, all material released in the enclosure (such as paint overspray particulate or VOCs) is captured and vented, rather than escaping as fugitive emissions. There is little or no difference between recirculation ventilation and any other coating operation ventilation system from a permitting perspective. In fact, because recirculation systems are generally enclosed, they have much higher capture efficiencies than other systems that are only partially enclosed. Unfortunately, application of recirculation technologies is not yet widespread; as such, regulatory agencies may scrutinize a recirculation system permit somewhat more closely. This is overcome by including comprehensive design information in the permit application package such as flow rates, pressures, filtration system data, and other information, which demonstrates that the necessary capture efficiency and filtration efficiency levels are met. Other Permitting Requirements: As with any construction activity, it is likely that the installation of a recirculation ventilation system will entail various building permits and perhaps even safety inspections by an in-house safety manager or an outside agency. If appropriate, calculations and supporting information employed to determine the recirculation flow rate may be referenced during these inspections. The remainder of this section focuses on specific design issues that relate to each of the three flow reduction strategies considered.
Simple Recirculation System Design Considerations Many of the components employed in typical metal-finishing operations are classified as potentially hazardous by OSHA; therefore, the additive effects of each component that is present in the coating must be carefully considered in designing a safe and efficient recirculation system. The appropriate recirculation flow rate is calculated based on several parameters, including coating material usage rates, coating material components, and ventilation flow rates through the process area. A conservative approach, which relies on worst-case assumptions, should be adopted in determining the recircu-
lation rate. For example, the highest possible material usage rate should be considered in the analysis to ensure that OSHA constituent concentration limits are not exceeded in the recirculation stream. This can be accomplished for paint spray operations by considering the maximum number of paint spray guns that may be employed, coupled with their respective coating release rates. Another important consideration stems from the fact that coating operations that employ dry filters to remove exhaust stream particulate typically rely on fixed-drive fans to provide the necessary process ventilation air flow. Because these fans are not capable of overcoming the gradual increase in pressure drop that occurs across the filter as it becomes loaded with particulate, the flow rate through the process area tends to decrease slowly over time. This unavoidable flow rate decrease must also be considered in determining the appropriate recirculation rate.
Split-Flow/Recirculation System Design Considerations The added complexity of the splitflow recirculation system has associated with it a number of design constraints that must be considered in addition to those design issues discussed above. The location of the split partition dictates the volumetric flow rate of the partitioned streams and, therefore, dictates the system recirculation rate. The split partition location must be carefully derived based on a number of variables, including the coating process area configuration; the concentration profile of the vapor phase components at the exhaust face; the concentration profile of the solid phase components at the exhaust face; and the particulate control efficiency of the exhaust face filtration system. The solid- (paint overspray) and vapor-phase concentration profiles must also be identified separately because solid- and vapor-phase components do not respond equally to the effects of flow dynamics and gravity, thus it may not be appropriate to assume they have similar profiles. These parameters are very important, because they ultimately dictate the concentration of hazardous constituents that are present in the recirculated stream. METAL FINISHING
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Another important issue that must be considered is that the exhaust stream is selectively pulled from zones within the work area having relatively high concentrations; thus, it is likely that the filters through which the exhaust air passes will become loaded with particulate much faster than the filters through which the recirculation stream passes. If not properly handled, this selective blinding effect on the filters will cause ventilation air that ordinarily passes into the exhaust duct to be diverted into the recirculation duct. This can cause unexpectedly high constituent concentrations in the recirculation stream, and significant flow profile variations across the exhaust filter face. To avoid this occurrence, the filters through which the exhaust air passes should be replaced more frequently than the filters through which the recirculation air passes. The rate of filter replacement should be determined using separate pressure drop monitors (such as incline manometers) across the exhaust filter face and the recirculation filter face. To increase filter life, the facility may elect to install VFD fans that operate in conjunction with flow monitors placed in the recirculation and exhaust streams. The VFD fans will deliver a constant flow rate despite filtration system loading problems; however, there are some very important design issues that must be considered in deciding whether or not VFD fans are appropriate. The first is that the exhaust VFD fan operation must be very carefully integrated with APCS operation. The second factor is that the VFD fans must never be driven to the point where the maximum pressure drop across the filter face or in the exhaust plenum is reached. Such an occurrence can cause the filter elements to be dislodged and pulled through the fan, or cause the plenum system and possibly the duct work to collapse. As discussed above, the split-flow/ recirculation configuration establishes a fixed split-height and a corresponding recirculation rate based on the stratification conditions in the booth enclosure. Of course, not all booth configurations can be adapted to employ split-flow/recirculation. Configurations in which split-flow/recirculation may not be applicable include the following. METAL FINISHING
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Booths that have a high variability in workpiece configuration such that any stratification pattern that may exist is either inconsistent or not repeatable. This is critical because the split-height and recirculation rate should be determined based on worst-case conditions that would result in the highest recirculation stream concentrations. Thus, if worst-case conditions occur only once a month, yet the split-height and recirculation rate is established based on these conditions, the booth operate under nonoptimal circumstances most of the time. For this type of operation, a dynamically optimized recirculation strategy is more applicable. Booths that employ a particulate filtration system, which does not preserve the stratification pattern, for example, a water wash particulate filtration system will destroy the stratification pattern before the ventilation air reaches the split partition. Booths that have ventilation air flow patterns characterized by highly turbulent conditions, either due to the orientation and characteristics of the target workpiece, or nonstandard intake and exhaust face configurations. For example, booths in which the ventilation air is brought in through the top of the booth, and vented out the sides or back of the booth, may not be reasonable candidates for split-flow/ recirculation. Dynamically Optimized Recirculation System Design Considerations As indicated previously, a dynamically optimized system is the most versatile of the recirculations systems, because it is independent of booth configuration, and can be designed to avoid some of the filter blinding problems associated with the split-flow/recirculation strategy. The system illustrated in Figure 3 indicates a VFD fan in the exhaust stream; as such, this configuration is subject to constraints similar to those identified for splitflow/recirculation. Alternatively, a fixed-drive fan operated in conjunction with a damper could be employed rather than a VFD fan to achieve the necessary exhaust flow rate adjustments; however, this configuration
may not necessarily yield the most energy efficient system. As in the case of simple recirculation and, for that matter, split-flow/ recirculation, an APCS that is operated in conjunction with a dynamically optimized recirculation system must be sized according to the maximum anticipated exhaust flow rates; therefore, many of the dynamically optimized design issues that must be addressed are similar to the design issues that relate to simple recirculation. The dynamic system, however, also requires the installation and operation of a real-time constituent concentration monitor coupled with a flow rate control system. The control system must be integrated on some level with the APCS to ensure appropriate handling of the exhaust flow rate adjustments.
CONCLUSIONS The advantages of employing recirculation flow reduction strategies for achieving cost-effective VOC emission control are significant, and their importance is becoming increasingly apparent as we face the reality of more stringent emission control regulations. Recirculation ventilation provides a relatively inexpensive means of significantly reducing the installation and operating costs of a VOC emission control system, as well as other process operating costs associated with air handling, cooling, and heating. Although this article has focused on paint spray booth operations, the technologies described herein are applicable to a number of other processes including chemical stripping (depainting), and clean room operations. Selection and design of an appropriate recirculation system for a particular facility must be given careful consideration. The design and installation of a recirculation system is not difficult if proper attention is paid to key aspects relating to system safety and operation. Many recirculation systems have been installed across the country, and although most of them probably operate within established safety limits, it is this author’s opinion that a number of them were installed with insufficient regard for issues relating to safety and system operating efficiency. This article provides a brief overview of key issues that a facility should address in 27
determining the applicability of recirculation. It should also be referred to by a facility wishing to install a recirculation technology to ensure that the consultants who are retained to design and install the system perform adequately. If a consultant does not appear to have sufficient regard for various aspects relating to safety and system integration, run, don’t walk, to a consultant who does. You will be rewarded
with a safely operating recirculation system that is efftciently operated and properly integrated with the APCS. Disclaimer The recirculation ventilation systems described in this article are protected under patent infringement regulations, and may not be employed without permission of the patent owners or their designated licensees.
ELECTROPLATING OF ZINC DIE CASTINGS bySK. Jalota 225pages hardcover $YO.OO This book provides a comprehensive coverage of plating on zinc die castings starting with design and production with emphasis on its relation to plating. Subsequent chapters treat polishing, buffing and mass finishing, cleaning and pretreatment and the equipment for plating. The balance of the book runs the gamut of the various metals that can be deposited. Each chapter devotes a portion to troubleshooting-causes of defects and remedies. Concluding chapters concern coloring, lacquering, stripping of deposits and analysis and testing.
ZINC PLATING byH. Geduki SGOpages hardcover $100.00 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.
ELECTROLESS NICKEL PLATING by Worfgng Riedel 328page.s hardcokr $109.00 This book is a translation of the German Edition updated through 1989. It contains 17 chapters covering all aspects from pretreatment to post-treatment including brief discussions of he market size, stripping specifications and waste treatment. The history and chemistry of electroless plating are covered in more detail as are set-up, operations and applications; however, the heart of this volume is concerned with the structure and properties of the deposits, which make up nearly one:hird of this volume. A valuable reference for finishers.
It is the intention of the author to provide background information relating to recirculation technologies only. The design and safety issues described herein should not be considered of sufficient detail to adequately design a recirculation system without supplemental information. MC
USERS GUIDE TO VACUUM TECHNOLOGY by J.F. O’Hanlon 481 pages hardcover $75.00 This is a’comprehensive review of vacuum technology written for today’s system users. The presentation is clear and practical.
ELECTROPLATING & RELATING PROCESSES byJ. B. Mohler 311 pages hardcover $55.00 This is a good introductory volume to the industry. The explanations are clear and the material is uncluttered with verbiage or meaningless illustrations. The reader wastes no time while preparing the groundwork for further study.
POWDER COATING SYSTEMS by W. Lebr 255pages hardcover $55.00
The first complete what-to-do and how-to-do-it manual on powder coating methods and procedures. Every aspect of powder coating and powder coating technology is discussed, in a logical sequence, in depth: types and characteristics of powder materials; cleaning and preparation of substrate metals prior to application of a powder coating; powder application equipment;pretreatment equipment; ovens; conveyor systems; hanging furtures; staging; feasibility studies; installation, maintenance; and purchasing of equipment. For any organization seeking to capitalize on the advances in powder coating materials and equipment, this is the definite guide.
MODERN ELECTROPLATING byF. Louwaheim 801 pages hardcover $145.00
This is the third Edition of this valuable textbook.
Numerous authorities have contributed summaries on the principles of plating and metal finishing, control, maintenance, deposit properties and troubleshooting.
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METAL FINISHING
. DECEMBER
1995