Fifteen years ago, dust management in composites manufacturing operations was somewhat unsophisticated. A common solution to the dust generated by cutting, trimming, sanding and grinding was to cut a hole in the shop wall and install an exhaust fan. Although simple and relatively inexpensive, this method did little but improve visibility. Today, governments mandate particulate emission control, both inside and outside the plant. Respirable dust, which is classified as less than or equal to 5 µm in diameter, is small enough to penetrate deep into the lungs, with serious health consequences. But the cost of uncontrolled dust goes beyond worker health. Inhalable dust, which averages 10 µm in diameter, not only can get trapped in the nose, throat, and upper respiratory tract and irritate eyes and skin, but it also can build up on machinery components, causing premature wear. Moreover, dust can impact product quality as well. In this respect, says Ken Abbott, managing member of Envirosystems LLC (Tucson, Ariz.), “the composites industry is unique. It’s very sophisticated in terms of materials and techniques, and with that sophistication comes an increased sensitivity to contamination.” If allowed to float freely through the air, dust from carbon fiber, for example, can corrode surrounding aluminum components, and a small amount of any contaminant on a faying surface can interfere with adhesion in bonded part assemblies. As a result, the industry’s overall perception of dust control has begun to change.
“Long perceived as simply a cost of doing business, companies are now realizing that dust collection equipment — when done right — can be an investment,” says Abbot. “Our customers expect dust collection to help improve product quality, reduce scrap due to contaminated parts, lower housekeeping and equipment maintenance costs, and boost worker productivity.”
MANDATE FOR DUST MANAGEMENT
Managing particulate as minute as one-twelfth the width of a human hair is challenging: The U.S. Occupational Safety and Health Admin. (OSHA), for example, requires that worker exposure levels for respirable dust be limited to just 5 mg/m³ averaged over an eight-hour period. Without dust control, “most people using a sander or grinder will quickly exceed the OSHA level,” says D. Scott McConnell, vice president, Dustcontrol Inc. (Wilmington, N.C.). The key is to investment in what dust control system designers call an engineered solution.
Today, dust control systems are rarely off-the-shelf products. Instead, each is customized to meet the requirements of the customer’s application, and there are many variations from which to choose. While a well-designed system can have a positive affect on the bottom line, the opposite also is true: “You can put an inexpensive system together and collect dust with it, but if it is not done correctly, it can be very expensive to operate,” warns Abbott. System design involves consideration of factors that impact the effectiveness of dust containment technology, including the dust collection method and vacuum systems (fan size, motor power rating and filter media) — the selection of which depends on careful calculation of application-specific process variables, such as air volume, capture velocity and static pressure.
ENGINEERING FOR EXTRACTION EFFICIENCY
One of the most important variables in dust control system design is air volume. To determine the air volume required for a particular application, the width of the space to be controlled is multiplied by the height, resulting in a room cross-section value expressed in square feet (ft²). This cross-sectional area is multiplied by the required speed of air movement through the room in feet per minute (fpm) to calculate air volume, as expressed in cubic feet per minute (cfm). Therefore, airflow speed of 50 fpm in a room that measures 40 ft wide by 10 ft high (12m by 3m) would require fan volume of 20,000 cfm.
When selecting a fan, says Abbott, “static pressure will determine whether or not the fan will perform the function for which it was chosen.” Static pressure (SP) — or resistance to airflow — essentially rates how much resistance to airflow can be introduced (by dust buildup, filter media and/or ductwork, for example) without affecting the air volume rating. “Using a fan with the incorrect SP rating will result in a system that, at best, will cost more than it should to operate or, at worst, won’t be able to do the job at all,” says Abbott.
“As an example, a fan with a rating of 10,000 cfm at 0.75-inch SP may only use a 5-hp motor to effectively move air, at that static pressure, through a paint booth or other type of low-resistance system,” says Abbott. If this same fan and paint booth were used to collect dust, however, “the fan will be all but useless before dust is even collected because a new filter provides 0.75-inch SP right out of the box,” he contends, noting that “a 10,000-cfm fan suitable for a typical dust collection system will need to achieve its full rated volume at a resistance closer to 3 inches SP or more to be effective and would require 10 hp or more.”
A large factor that affects system design is the size of ductwork that might be required to transport dust from the source to its collection point. Duct size in cross-section directly affects system performance and is based on what particulate will be collected and the volume of air that must be moved. According to Donaldson Torit (Minneapolis, Minn.), which offers cartridge- and bag-type dust collectors, ductwork that’s too small tends to restrict airflow, resulting in pressure loss. This reduces the air volume and increases energy use. If the ducts are too large compared to the air volume, air velocity is reduced. Dust capture will be poor and dust will not be pulled through the ductwork.
A key to system efficiency, then, is to minimize static pressure. Assuming an average cost of industrial power of approximately $0.08 (USD) per kilowatt hour (KwH), operating one 5-hp fan for a single shift, five days a week, for 52 weeks would cost $805 per year. If, due to ducting or other installation requirements, the fan needs 30 hp to move the same air volume, the cost would be $4,238 per year. “The most cost-effective method of eliminating airborne contamination is to confine it to one area where it can be isolated and filtered using the least amount of air,” says Abbott. Strategies include locating the dust collector as close as possible to the area it is filtering to reduce ductwork and, therefore, the fan’s horsepower requirement. Whenever possible, the filtered air should be exhausted back into the plant to retain conditioned air — heated air in the winter or chilled air in the summer — to minimize building heating and cooling costs.
SELECTING A COLLECTION STRATEGY
Given these design constraints, dust control system manufacturers have developed three basic collection strategies: whole-room, containment booth and source-point capture. Strategy selection is based on the size, type and number of the customer’s dust-generating machines.
Whole-room systems are often the only practical option when an individual piece of equipment is massive, such as a gantry-style CNC router. The whole-room approach typically involves a room built around a machine to reduce noise and dust. The dust collector, which can be located outside the building or inside, pulls air from the work area into an inlet device — typically mounted along the wall at the narrow end of the room. The air is directed through filter media where contaminants are trapped and clean air is exhausted back into the work area or outside the building. These systems can involve extensive ductwork or, in some cases, be free of ductwork.
Envirosystems’ trademarked AirWall dust collection equipment, for example, is self-contained, eliminating the need for ductwork and greatly reducing static pressure. “By saving 2 to 3 inches in static pressure with no ductwork, we can move the same amount of air with a 5-hp fan as a ducted system [can move] with a 40-hp fan,” Abbott claims.
The system reportedly removes more than 99.99 percent of airborne sub-micron particles (down to 0.5 micron in size), which surpasses current OSHA requirements. A high-velocity, reverse pulse-jet cleaning system automatically cleans cartridge filters (see “Filtration Facts” at the end of this article, on p. 3).
Given a room size of 20-ft by 30-ft by 10-ft (6.1m by 9.1m by 3m), a complete air change every minute would require air to be pulled through the room at 30 fpm and could demand a fan volume as high as 6,000 cfm in a ducted system. In a duct-free system, the same air volume reportedly can be achieved with a 5-hp fan. The average cost of the duct-free system would be about $15,000.
Where room size is larger than the dust-generating machinery, the latter can be located within a contamination control booth (CCB), a three-sided, ceilinged structure with integral lighting, open on the fourth side for easy access. “Booth sizes can be as small as 10-ft by 10-ft or as large as 130-ft by 50-ft [39.6m by 15.2m],” explains Ronnie Frees, president of Frees Inc. (Shreveport, La.). “Tub and shower manufacturing operations, for instance, typically require 40-ft by 40-ft [12.2m by 12.2m] containment rooms.”
An exhaust fan with relatively high airflow, typically in the 140 fpm to 160 fpm range, draws air out of the CCB, creating negative pressure within the CCB that draws air into the booth’s open end, preventing dust from escaping. A grinding booth for two to four workers measuring 22 ft wide by 7.5 ft high by 8 ft deep (6.7m by 2.3m by 2.4m) would require a 20-hp to 30-hp fan motor to pull the 160 fpm necessary to generate an air velocity of 22,000 cfm. The system would cost approximately $36,000.
Frees and other companies offer an additional “air curtain” feature that can be adapted to both large CCBs and room-size exhaust systems. A blower system is positioned at the front edges of the open booth or on the side opposite the collectors in room-size applications (see photo, this page). These blowers generate a high-velocity positive airflow angled toward the collectors to reinforce transport velocity within the booth.
To create its air curtain, Frees’ trademarked Dust-Free system uses a “push-pull” recirculation method. An exhaust fan draws dust into a dust-separation chamber at the back of the CCB where a filter tube sheet traps up to 99 percent of the airborne dust particles. The clean air then is channeled through ductwork to the open end of the chamber where it provides positive airflow, pushing the air inside the room back toward the dust collector inlet. The system uses digital direct control (DDC) to save energy. “When workers are in the containment area working, the system is on, and when they stop working, the system will gradually slow down until it is off or nearly off,” says Frees.
The source-point capture strategy can take several forms and becomes a practical option when the dust source can be localized and is especially useful when large volumes of dust are being generated by one source. “The source-point capture system offers advantages for operations where there are many different machines operating in a large space with no way to effectively group and enclose them for effective dust control,” says Abbott.
For stationary equipment — both large and small — source-point capture can be accomplished via an overhead or side-draft hood. For handheld tools, there are two options: The down draft table, which draws dust down through a perforated tabletop, does not impair tool use, but it is best used with smaller parts because large parts can block airflow and create pockets of dust-filled air. Capture also can be accomplished by affixing a suction casing, or shroud, and vacuum hose as near as possible to the dust-generating portion of the tool. (See photos, this page) Ductwork, typically located overhead, connects the dust collection unit to suction outlets from which individual hoses can be dropped down to the work area. Usually located near or along compressed air “drops,” hoses are typically looped with and run parallel to the pneumatic airline or electrical power cord for easy handling. Automatic valves can be used so that suction only occurs when the tool is actually in use.
A typical source-point capture system designed with four drops to accommodate two to four workers operating vacuum-assisted, heavy-duty sanders, for example, would require a 10-hp turbo pump and cost approximately $18,000.
MANAGING MULTIPOINT CAPTURE
Large plants often opt for multiple-point source capture systems that employ a centralized dust collection unit (fan housing, fan motor and filtering equipment) connected via ductwork to remote workstations. This arrangement frees the manufacturer from the need to confine multiple dust-generating tasks to a small area within a plant. Trimming, sanding, and similar tasks can take place where they best benefit production workflow. “There is a tremendous amount of flexibility in the layout of suction points with a source-point capture system,” says McConnell. But, as noted earlier, the need for sometimes significant lengths of ductwork necessarily increases fan/motor size and power requirements. According to Rob Retter, manager for U.S. industrial sales at Eurovac Inc. (Concord, Ontario, Canada), which manufactures source capture systems for stationary equipment and handheld tools in composites operations, it is essential to use metal ductwork (such as galvanized steel) rather than PVC pipe to guard against static charge buildup.
Multipoint systems can be very large in scale. Fiberblade, a subsidiary of Gamesa, employs a Dustcontrol-built source-point capture system with 120 drops for vacuum-assisted handheld tools used in polishing and sanding 85-ft to 140-ft (26m to 42.6m) long wind turbine rotor blades at its Ebensburg, Pa. plant (bottom photo, this page).
For small companies, or those for which it is easier to take the machine to the part than to take the part to the machine, portable systems offer enhanced flexibility. Dust control system specialist Clayton Associates Inc. (Lakewood, N.J.) has developed a line of highly portable dust collection solutions for handheld tools, including the ACE-1000 Hornet vacuum, a HEPA-filtered pneumatic vacuum designed for dustless sanding and foreign object damage (FOD) control. Originally built for the Air Force Advanced Composites Office (Hill Air Force Base, Utah), the Hornet weighs just 10 lb/22 kg and can be converted to a backpack configuration. According to the company, the vacuum consumes so little compressed air that both the sander and the vacuum can be powered by a single air line. The unit, which comes with a sander, hose, and consumables, sells for $3,045.
HANDLING HANDHELD TOOLS
One of the most important aspects of dust control system design involves the capture method for handheld tools. “A low-volume, high-velocity flow of air is required to capture the dust at the source of generation,” says McConnell. “Capture velocity of 180 to 200 ft/sec is required.” For example, a 7-inch sander at 6,000 rpm throws off dust particles at a tip velocity of 183 ft/sec (125 mph). To capture the respirable dust, air must travel at that speed or faster, he explains. While heavy particles might tend to escape the airflow, they are mechanically stopped by the edge of the shroud. “Typical air volumes required for two or three workers using 7-inch heavy-duty sanders with a source capture system are approximately 360 cfm,” says McConnell. “A 4-hp turbo pump will handle one 7-inch grinder, while a 10-hp unit can handle two or three 7-inch grinders or several smaller sanding tools at the same time,” he adds.
Because handheld tool design varies according to tool type, configuration and size, shroud design is a critical issue. The challenge here is to design a shroud for the tool that will efficiently extract dust but not interfere with tool operation. “Suction casings are designed to work with a particular tool at a given airflow rate,” says McConnell, noting that “not every tool can be effectively fitted with a suction casing for every application.”
To address tool differences, Clayton has developed a line of universal shrouds, which reportedly work with any brand of tool. “Our shroud is independent of the tool,” says Brad Clayton, VP of operations. “The shroud floats on the shaft or mandrel and does not obscure the working edges of the abrasive disc — a critical factor for tool acceptance.” Clayton and dust control system supplier DCM Clean-Air Products Inc. (Ft. Worth, Texas) both sell a full line of shrouded sanders, grinders, saws and drills.
DCM offers patented Postiv-Lok abrasive disks and disk holders for its trademarked Vacu-shroud system. Postiv-Lok is designed to consistently align the holes in the disk with those in the holder to optimize airflow. Recently, a DCM hand sander with integral dust capture shroud, disc/holder design, and HEPA-filtered vacuum system was used to repair the composite door of the Discovery Space Shuttle. The door is constructed of carbon fiber/epoxy skins over an aramid core and required a two-layer doubler to repair. The repair required precision sanding of five plies of carbon fiber/epoxy at 0.15 mm per ply.
Abrasive tool manufacturers also produce tools with custom-designed shrouds. A supplier of grinders, sanders and polishing equipment, Dynabrade Inc. (Clarence, N.Y.), for example, offers a line of air-powered sanding and grinding tools with integrated shrouds. The company also makes kits that can convert its nonvacuum air-operated tools for vacuum connection. Self-Generated Vacuum tools and conversion kits contain small vacuum units in the shroud and are connected via lightweight hose to portable dust collection units. Central vacuum tools and conversion kits are designed for use with centralized vacuum systems. Dynabrade also supplies two types of portable, wheeled dust collectors that can service its handheld tools. Its electrically powered, dry collectors (10 models) come with a stainless steel drum — 9.9 gal or 17 gal (38 liter or 64 liter) capacity — with HEPA filtration and disposable collection bag. A pneumatic system (for spark-free operation in potential explosive environments) has 10-gal (37.8-liter) capacity (with collection bag) and a washable filter element. Additionally, the company offers self-contained down draft table-type dust collection systems. For example, its Heavy-Duty Table with 48-inch by 96-inch (1,220-mm by 2,440-mm) tabletop (with optional casters), features two 1.5-hp fan motors. The system is self-contained, housing a large-area dual (blanket/paper filter) filtration system in its pedestal base.
Hooded, down-draft and handheld source-capture not only protects workers from harm but increases equipment service life. “Fabricators that use source capture systems have documented an increased life of 200 percent for tools and a 500 percent increase in the life of the abrasive,” says McConnell.
While today’s dust collection systems are typically customized to meet defined parameters, in many instances, dust system suppliers must be very creative. For example, workers at The Boeing Co.’s plant in San Antonio, Texas, must be lifted 70 ft/21.3m into the air to work on the tail section of the C-17 strategic airlifter (see photo, p. 36). Clayton designed a remote distribution unit (about 8-inches, or 203-mm, square), which is hung on the railing of the lift. As the lift rises, it pulls up an 80-ft/24.4m hose attached to a 245-cfm dust collection unit at the base. At the top, workers attach 10-ft/3m lightweight work hoses to the distribution box, which supplies vacuum and compressed air to their tools.
A similar system is in place at Boeing’s plant at the Long Beach Airport (Long Beach, Calif.) where workers who perform sanding operations from stands and moveable platforms wanted the convenience of a central vacuum system but the flexibility to detach quickly to allow the stands to be moved when aircraft enter or exit the hangar. Clayton built a system based on its DM-304 vacuum unit, which uses a cyclone separator and three-stage filtration (see sidebar, at right). Rigid, fixed piping was installed on each stand, and vacuum ports were installed at various locations around the stands. When the mobile stands are in place around the aircraft, the vacuums are rolled into place and joined to the piping system. Workers then plug their sanders’ lightweight work hoses into any available port on the stand.
When United Air Specialists Inc. (UAS, Cincinnati, Ohio) built a system for a 260,000-ft² (24,155m²) plant that produces 30 million fiberglass furnace filters per year — each of the plant’s seven production lines raised a thick haze of particulate where fiberglass rolls were cut — a central collection unit and ductwork would have been impractical. UAS specified its BDC-22T dust collection system and a custom-designed hood for each production line. Each self-contained system incorporates a short 30-gal discharge drum, a direct-drive blower and a silencer to reduce noise. Dust drawn into each compact collection system is filtered using 2-inch, 16-oz polyester felted bags, which are cleaned on demand and during downtime using a pulse-jet cleaning system.
DUST CONTROL PLUS
A recent trend is toward a multipurpose system — one that can handle both dust extraction and related tasks. A case in point is a recently installed Envirosystems-built 200,000-cfm system that includes several dual-purpose CCBs used for sanding composite surfaces as well as surface washdown. “To keep humidity levels down and speed drying times, the company wanted to ventilate the rooms during both dry and wet operations,” explains Abbott. During the dry mode, dampers in the center of the booth’s back wall open, and air is pulled through cartridges that filter out the dust and exhaust the clean air back into the building. During wet applications, the air velocity is reduced, inlets to the dust filter cartridges are closed to prevent moisture damage and two smaller dampers open to collect the moist air. These bypass the filters and direct air into a common exhaust plenum.
Envirosystems also is designing a wet/dry system that will combine dust collection and VOC/HAP removal. “Such a system allows one room to be used for both grinding/sanding and chopper gun applications,” says Abbott. “The cartridges would collect the airborne contamination, and the vented fumes … would go through an RTO [regenerative thermal oxidizer].”
Dual systems someday may be required. “The more complicated composite manufacturing becomes, the more there will be a need for systems that do both dust collection and VOC filtration,” says Frees.
In this case, however, Abbott notes that “a true dual system would probably cost several orders of magnitude more than just a dust collection system due to the high inherent cost of the required RTO unit.”
SPECIAL REPORT: FILTRATION FACTS
There are three main types of filters used in dust collection — cartridge, bag and pleated bag. Cartridge filters, which are cylindrical in shape and wrapped with pleated filtration media, are considered to be the most advanced form of filtration. In bag and cartridge formats, pleated filters are the most commonly used filtration media for industrial dust collection. Pleated filters, mostly fabric blends, can incorporate a variety of fibrous materials, including nanofiber technology.
HEPA. High efficiency particulate arrester (HEPA) filters are required in applications that involve known carcinogens. A HEPA-rated filter can capture 99.97 percent of particles down to 0.3 µm in diameter. Because these filters have a very fine weave, a prefilter is recommended to remove large particles that could constrict airflow and diminish the life of the HEPA filter.
Filter cleaning. Most dust control systems feature a filter cleaning system. Commonly used by major manufacturers of both cartridge and bag filters, pulse-jet cleaning systems blow air through the filter from the reverse (clean) side to blast out accumulated dust particles, which then fall down into a collection bin. Pulse-jet cleaning can be on demand or automatic and does not require the system to be taken off line. Mechanical or manual shaking is another option for filter cleaning, although this technique — often used to clean baghouse collectors — can be performed only when the system is off line.
Cyclone Preseparators. When the dust load is particularly heavy, preseparators are used to remove larger particles before they reach the filter, thus extending filter life. Air enters the top of the cylindrical collector and is forced downward in a circular motion, causing heavier dust particles to be thrown outward against the walls of the collector, at which point they slide down into a hopper below.