Choosing the Right Dust Collector
An effective solution might be to install a dust collection system that eliminates the problem entirely. Factors such as particle type, size, shape and volume, as well as the temperature, moisture content, specific gravity, flammability and the percentage of dust in the airstream, are critical in determining which type of system is ideal. Each type of collector comes with a variety of cost-benefit considerations that typically determine why one is a smarter purchase than another for a given application.
As health and safety data have confirmed the dangers of personnel exposure to process dust, the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) have continued mounting their air quality offensives while mandating worker protections and increasing enforcement. Preventing exposure to fine particulates in the workplace has never been of greater concern.
The facts are especially compelling for companies processing ceramics. Ceramic dust particles tend to be heavier and more dense than dust particles of wood or plastic. And the dust generated by hot processes is typically of the submicron size (1 micron = 1/25,000 of an inch)-almost invisible to the naked eye and easily inhaled. Breathing silica dust, for example, has been documented as a cause of the disease silicosis, and the presence of airborne silica dust concentrations above OSHA's permissible exposure limit is one of the administration's most commonly cited violations. In such cases, OSHA requires employers to take immediate action to protect employees from further exposure and eliminate the problem with an engineered solution. One way to quickly protect workers from dust is to outfit them with respiratory gear. However, National Institute for Occupational Safety and Health (NIOSH)-approved devices cost anywhere from $6 to more than $600 per employee, plus filter replacement costs, depending on the type of dust, the length of exposure and other factors. Ensuring that every employee actually wears the equipment every day is yet another challenge.
A more effective solution might be to install a dust collection system that eliminates the problem entirely. But with all of the different dust collectors sporting a variety of new and classic technologies, how can a company be sure to purchase the right system for its facility?
The three types of dust collectors commonly used in industry can be classified as wet, dry cyclonic (centrifugal) or static filter media (cloth) collectors. Though any can be designed to effectively remove dust, each type relies on a different method of cleaning. Factors including particle type, size, shape and volume, as well as the temperature, moisture content, specific gravity, flammability and the percentage of dust in the airstream, are critical in determining which type is ideal. Power consumption, maintenance requirements and other ongoing costs must also be considered. Each type of collector comes with a variety of cost-benefit considerations that typically determine why one is a smarter purchase than another for a given application.
Wet Collectors
Wet collectors use water or other liquid contact media to wet and then absorb particles to separate them from the airstream. Several different types of wet collectors are available, such as cyclonic, impingement and venturi-based collectors. Their efficiency is proportional to the energy consumed in removing the particles. The venturi-based wet collector, for example, can deliver an impressive 99.95% collection efficiency; however, this efficiency often comes at the cost of substantial energy consumption, and sometimes at a disturbingly high decibel reading.The primary drawback of these systems is that they collect the dust in a liquid slurry, which creates yet another disposal or treatment process to manage. Some processes, such as in the paper industry, can recycle the slurry. But for most ceramic processors-and especially those involved with volatile organic compounds (VOCs) or sulfur compounds-generating a foul slurry or hazardous wastewater as a solution to improving environmental conditions seems counterintuitive.
Dry Collectors
Dry cyclonic collectors use centrifugal force to separate particles from the airstream. The air is typically directed into a conical chamber, where it is spun around at high speeds. This action forces the heavy, solid particles to migrate to the vessel wall, where they agglomerate and drop into the receiving hopper.These versatile collectors deliver high collection rates (98.9% of 10-micron particles in some applications) without generating any liquid waste. But like the wet collectors, their efficiency is a function of pressure drop. In applications where the particles are small and/or where they are generated by a hot process, the elegant simplicity of the dry cyclonic collectors can become their undoing. In these and other more challenging conditions, static filter media collectors are often the most efficient and cost-effective solution.
Static Filter Media Collectors
Instead of using water or centrifugal force to separate dust from the airstream, static filter media collectors pass the contaminated airstream through a filter made of fiberglass, woven cloth, polyester, wool, Teflon, or other materials, depending on the application. The dust is captured as deposits on the filter surface and is periodically dislodged by the machine. The cleaned air is returned to the process or released to the atmosphere, depending on the application.The oldest of this style of media collector, called a "shaker baghouse," uses a series of woven cloth media filters shaped like a long bag. The filters are used to build up a cake of dust on the inside surface of the bag; as the cake grows, the system's collection efficiency improves. To maintain its efficiency, a shaker motor periodically shakes the filters free of the accumulated dust, which drops into a hopper.
Since this type of collector requires shutting down the filtering operation during the cleaning cycle to allow the dust to settle in the hopper, these collectors are effective when intermittent operation is acceptable. For continuous, automatic operation, the shaker collector requires a second compartment to filter while the other is shut down for cleaning. The result is a much larger collector fitted with a range of additional components, along with correspondingly higher maintenance costs. Though shaker collectors offer advantages in niche applications, such as sulfur dioxide removal, the pervasive need for an economical, compact and continuous-duty dust collector has driven the shaker's decline while giving rise to the modern pulse jet collector.
One of the pulse jet collector's greatest assets for plant and process engineers is its ability to function 24/7 without the constant interruptions characteristic of the shaker type baghouse. Rather than relying on building a dust cake, the pulse jet system collects dust on the outside of felt filter media, which are encased in a large baghouse. As dust accumulates on the filter media, periodic pulses of high-pressure air flowing in reverse of the airstream cause the dust to drop into a hopper. Since this process is performed automatically in a continuous cycle, one row of filter media at a time, the pulse jet collector is ideal for manufacturing with continuous production lines.
Though it has proven to perform in a wide range of conditions, the pulse jet collector baghouse often requires large steel encasements standing 10 ft or more off the floor on an extensive footprint that can consume precious square footage. Since maintaining a constant airflow temperature is critical to optimum performance, installing the system outside is not always an option. Further, pulse jet collectors require manual maintenance to evaluate and replace worn filter media. The rate of wear and frequency of maintenance depends on the type of dust, the dust load and other factors, and filter replacement typically requires two workers to physically open the housing and go inside the dust-laden baghouse. Besides consuming time and often triggering both production line downtime and OSHA space confinement issues, directly exposing workers to this dust is a potential hazard that is best avoided whenever possible.
The cartridge filter collector addresses and solves the shortcomings of the common pulse jet collector. Relying on the same type of pulse-based cleaning technology to collect and safely remove the dust, the cartridge filter collector often delivers the same collecting capacity as its ancestor but in a compact size and a footprint approximately 67% smaller. This is because its filters feature corrugated flue technology, which permits more cloth to fit into a smaller area. For example, 226 square feet of filter media can be packed in a single cartridge, while the same capacity in a pulse jet would require 24 filter bags set in a 16-ft tall baghouse. The comparatively smaller size also permits installation in tighter spaces and can eliminate the need for ductwork leading to the collector, which adversely impacts system pressure. Filter maintenance and replacement is easily done by a single person from outside the collector with zero exposure to dust and no special tools needed. The cartridge is slid out of the collector for inspection. If worn, the filter media is replaced and the new cartridge is slid into the collector, like closing a file drawer.
To capitalize on the advantages of these collectors, engineers have devised filter media that boost efficiency and capacity. Polyester, acrylic, ceramic, cotton, paper, Nomex, Teflon and a range of other materials are now specified, especially when moisture is present or the traditional cloth and felt materials are ineffective. When sticky dust particles are present, the type of filter media and the spacing between the filter media flues can be adjusted to better control the release characteristics of the cartridge.
Regardless of the type of filter media specified, pulse jet and cartridge-based filters do have a lifespan. Every time the system pulses to clean the filters of accumulated dust, the filters must flex and then return to position. Each cleansing pulse wears the material until it can no longer return to its rightful position, or it ruptures and needs to be replaced. This damage might not happen for two or three years, depending on whether the operation is continuous or intermittent, the temperature, dust material and other factors. When the pressure drop across the bags reaches 5-7 in. water column (WC) (a measure of the resistance the fan must overcome to deliver the desired airflow) or when dust is visible, replacement is needed. But since the cartridge-based unit stands at floor level (often about 39 in. tall), filter replacement is no longer performed at precarious heights. And with a smaller, less cumbersome system to manufacture, less material is needed, production is smoother, delivery is faster and pricing is often lower.
Process Gas Considerations
Collecting nuisance dust from grinding and polishing is easy, relatively speaking. The particles are typically large enough to be seen by the naked eye, and the operations are often performed at ambient or relatively low temperatures. Dust created from hot kilns and melting furnaces is far more challenging, since the elevated temperatures produce finer particles. Particles of carbon black dust, for example, are so fine they are measured in submicron sizes. If the filtering velocities are too high, particles like these often blow through the media without being collected. Such applications typically favor cartridge-based or pulse jet collectors for the variety of available filter media and their ability to collect fine particles.Hot process gases that contain acids or other corrosives, such as metal oxide-chlorine mixtures, present additional challenges. Should the gas be released into the atmosphere, it would condense as a dangerous particle mist. If it were to condense inside a dust collector, the system's housing would be attacked and would deteriorate from within. Even stainless steel housings would eventually corrode. For acid-laden gases, a scrubbing system with a cooling chamber is often specified to neutralize the contaminants. In the case of explosive materials, reducing the risk upstream with static attenuation devices is preferred, and relief vents in the outer casing are typically required by National Fire Protection Association (NFPA) codes and standards. Here again, the flexibility of the cartridge and pulse jet collectors supports their specification. Each type can accommodate carbon-impregnated bags or cartridges, which release electrical charges to a grounded circuit for enhanced overall safety.
A Balancing Act
A variety of different technologies are available for collecting dust and meeting government-mandated safety standards. Selecting the ideal solution for a specific process is a balancing act based as much on the art of experience as on the science of the material being processed.For more information about dust collectors, contact Precision AirConvey, Pencader Corporate Ctr., 210 Executive Dr. #6, Newark, DE 19702; (302) 999-8000; fax (302) 999-8510; e-mail get-facts@airconvey.com; or visit www.PrecisionAirConvey.com.
Editor's note: The comparisons and opinions presented in this article are those of the author and do not imply endorsement by Ceramic Industry of a specific equipment brand or type. Companies should carefully evaluate all products and related claims before making a purchase.
SIDEBAR: System Sizing
Figure 1 illustrates how a company might determine the air-to-cloth ratio needed to efficiently remove ceramic pigment dust from a bag dumping station exhausting 5000 cubic feet per minute (CFM) of air with a dust particle size of 10-40 microns and a dust loading of 20 grains/cubic foot of air at 701/4F in a continuous-duty operation for shaker-type, pulse jet and cartridge-based collectors.
Once the air-to-cloth ratio has been obtained, the company can use this data to compare the different collector systems and determine which type will provide the most efficient operation:
- Shaker collector: 5000 CFM divided by 1.84 air-to-cloth ratio equals 2718 square feet of filter area. Using a bag filter size of 4.5 in. in diameter x 8 ft long equals 9.425 square feet per filter. The number of filters required is obtained by dividing the square footage of the filter area by the square footage per filter-2718/9.425 = 288 filters. For continuous duty, double the number of filters would be needed (576) to permit one compartment to operate while the other cleans. At a typical center spacing of 71/2 in. between bags, a shaker collector would require 18 x 12 x 16 ft in height and would cost an estimated $22,000.
- Pulse jet collector: 5000 CFM divided by 7.38 air-to-cloth ratio equals 678 square feet of filter area. At 9.42 square feet per filter bag, 72 filter bags would be needed in a space requiring 5 ft 4 in. x 5 ft 4 in. x 16 ft in height. The system would cost an estimated $14,000.
- Cartridge-based collector: 5000 CFM divided by 1.53 air-to-cloth ratio equals 3268 square feet of filter area. Using a 12-in. diameter x 39-in. high cartridge with 226 square feet per cartridge will require 14.6 cartridges (16 to be used). This collector requires a space of 25 in. x 25 in. x 10 ft 4 in. in height, at an estimated cost of $11,500.
Based on these hypothetical parameters, the cartridge-based collector operates at the lowest air-to-cloth ratio, requires the least amount of space and is the lowest in price.
Tom Embley, P.E., is chief executive officer of Precision AirConvey Corp., Newark, Del., and Robert A. Betances is a process consultant located in Berlin, N.J.