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.
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.
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.
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.
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.
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:
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.