Dust Collection: Expense or Investment?
Well-engineered dust control systems not only improve shop air quality but also boost productivity, prolong machine life and save energy.
By Karen Wood, Contributing Writer | February 2008
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.
Source: Dynabrade Inc.
Point-source capture: This self-contained CFS AirWall dust collection unit (center photo, this page) uses two articulated arms to collect dust from two workstations. For hand tools (top) such as these from Dynabrade (Clarence, N.Y.), source-point capture can be handled with custom-made shrouds and vaccum hose attachments. For nonvacuum sanders and grinders, a down-draft table, such as this Dynabrade model (bottom photo, this page), can suffice.
“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.”
Source: Envirosystems LLC
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.
Source: Dynabrade Inc.
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.
Source: Frees Inc.
Contamination control booth: This Dust-Free CCB built by Frees Inc. (Shreveport, La.) for tooling manufacturer Janicki Industries (Sedro-Woolley, Wash.) not only removes grind dust and fumes but also provides temperature control for this large 5-axis mill. The “push-pull” system uses a bank of positive air blowers (at upper right above the booth’s right sidewall) to direct airflow toward the collection dampers at floor level on the opposite sidewall.
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.
