October 20, 2011
Metal manufacturers use wet filter systems for two reasons: to collect combustible metal dust and to filter particulate in heavy-sparking applications. Applications like deburring and grinding can involve both combustible metal particulate and heavy sparking--and for these applications wet dust collection systems can help mitigate inherent health and safety risks.
Metal dust can be a fabricator’s worst nightmare. If a grinding application causes dry aluminum and steel dust to mix, it can be a disaster waiting to happen.
The National Fire Protection Association (NFPA) has identified specific levels, in parts per million, of certain metal dusts that can be in the air before a serious risk of explosion arises. This is where proper dust collection helps. Dust collectors help prevent an explosion that can destroy a building and a business. Most important, they save lives.
Dust collectors come in wet and dry versions, and each suits certain operations. Dry filters can collect extremely small fines. Collection for these environments usually occurs either with a localized, arm-based system or filtration system far overhead. In all circumstances, the fume collection must ensure that any stray welding sparks dissipate well before they reach the dry filter medium.
Metal manufacturers use wet filter systems for two reasons: to collect combustible metal dust and to filter particulate in heavy-sparking applications. Common combustible metals are aluminum, lithium, magnesium, niobium, zirconium, and titanium. In fact, heavy use of titanium seems to be driving much of the aerospace sector toward wet filtration technology.
Across all sectors, applications like deburring and grinding can involve both combustible metal particulate and heavy sparking—and for these applications wet dust collection systems can help mitigate inherent health and safety risks.
In a typical wet dust collector, particulate goes into a water bath. The bath itself is stirred, forcing the material to settle. As the airstream passes the fixed baffles, particulate is separated by a heavy, turbulent curtain of water created by high-velocity air. The centrifugal force caused by the rapid changes in airflow direction forces the dust particles to penetrate the water and become entrapped. Dust, as sludge, settles to the collector bottom, and the water is reused. Clean, filtered air is then forced out the top of the unit.
Filtering particulate in water removes the ignition source, which eliminates the problem—that is, until the particulate dries out. The process effectively turns the particulate into sludge, but if that sludge is removed and allowed to dry, the dust still can be combustible and, thus, a serious hazard. This is why workers must remove the sludge and then mix it with something else, usually sand or diatomaceous earth (crushed sedimentary rock). The mixture varies depending on the kind of particulate in the collector. After this, workers can safely dispose the sludge mixture.
Wet filtration systems come in a variety of configurations. The simplest involves direct collection, essentially a “wet” downdraft table. In these workbench systems, the particulates are driven directly into the water below, and clean air is released. With the pool of water so close to the work area, these systems are simple and effective. However, the setup does dedicate one filter system to one work area. Each work area for grinding or other processes requires a separate filter system.
Some workbench setups don’t look like the typical downdraft table application, in which a worker grinds away at a small workpiece on a table. In some cases, downdraft workbenches are designed to accommodate very large parts. For instance, a table can be designed to be 8 ft. wide and 4 ft. deep, with the surface of the wet-filter tank just a little more than 2 ft. off the ground. Such a downdraft table configuration could handle fairly large parts. In these cases, lift assists can be integrated with the table so that workers don’t break their backs lifting heavy workpieces all day.
Consider one application involving a company that makes metal carts, consoles, and enclosures. The company grinds and polishes both cold-rolled steel and aluminum, and to avoid the dangerous mixing of metal fines, the shop separates the grinding areas by the metal type. Steel parts go to one area of the plant, aluminum parts go to another.
Just because fines don’t mix doesn’t mean the fire hazard goes away. In this case, the particulate produced from grinding one type of steel resembles steel wool lint. If that lint hits sparks emitted from the grinding wheel, the particulates could smolder and ignite.
This operation is high-mix, low-volume. Because the grinding department has a small number of workers, as opposed to dozens in a large plant, several wet downdraft tables may suit this operation best. The tables require a relatively simple configuration—again, because the metal dust goes directly into the wet filter below.
Of course, some applications involve dozens of work centers, and in these situations dedicating a filtration system to each may not be the most efficient approach. This is where custom, ducted systems can help. These can collect dust from multiple operations and send particulate through one central filtration unit. A plant may have a dozen grinders located in different areas of the plant, and particulates from each can be sent to one wet filtration unit.
However, manufacturers must ensure such systems are configured correctly, and this includes considering factors like static pressure. In basic terms, the narrower the ductwork, the higher the static pressure becomes, and higher static pressure means it will take more horsepower for a fan to effectively blow particulate at a specified CFM (cubic feet per minute)—usually about 3,500 CFM.
It’s a balancing act. If the blower and fan units don’t move particulate at a sufficient CFM, then the metal particulate will settle in the ductwork. If, say, titanium dust settles in the ductwork and then, several hours later, comes in contact with aluminum, a serious safety hazard is created. On the other hand, a system with too much horsepower will sound like an airplane taking off. Alleviating one hazard (fire) has introduced others, including worker hearing loss. Besides, the loud noise creates a very unpleasant working environment.
An optimized wet dust collection system will be designed so it produces the highest CFM with as little horsepower as possible. This happens by engineering the ductwork, blower, and blade design to move as much air with as little effort as possible.
A variant on ducted systems, a wet filtration booth can enclose a large work area; 30 ft. long isn’t uncommon. These booths work well for applications involving workpieces too large to fit on a typical downdraft table. Such systems can be flexible as well. For instance, crane slots allow the booths to be moved where needed, should the plant layout change to handle a different product mix.
Drawing dust away from the work area sometimes is only part of the picture. Other aspects are fixturing and workpiece access. Dust collection set-ups can be configured so the workpiece rotates, maintaining a constant point of worker access. In certain instances, this also results in more efficient dust collection. If a grinder contacts a workpiece at the same place time after time, the metal particulate efficiently flows into a duct system designed for the application.
So which is best? It depends, as always, on the application. Dry systems handle finer particulates as well as smoke, while wet filters handle slightly larger particulates that may be combustible. Wet filters are still very efficient, just not as efficient as dry filters. Regardless, a safe shop requires a comprehensive, carefully planned filtration strategy. The result makes the shop floor a safer, more pleasant work environment.
Like its dry counterparts, a wet filtration system must be maintained. As always, you should follow the manufacturer’s recommendations, but here are a few general preventive maintenance tips:
1. Periodically test the water’s pH level. If it is out of the specified range recommended by the manufacturer (6.5 to 8 pH is typical), the water should be drained and refilled. Too acidic or alkaline water can damage the filtration unit. You can contact a local water treatment service company for assistance, if required.
2. At regular intervals, totally drain and refill water in the tank, as determined by the application.
3. In wet downdraft tables, dust settles as sludge in the main dust collector compartment. Sludge should be removed from the tank either when the water is drained or as required by the application. Often, once 1 inch of sludge has accumulated at the bottom of the tank, it is time for cleaning.
4. Periodically inspect other elements of the unit for any built-up solids, including the impingers immersed in water.
The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.