Properly Designed Filtration Systems Keep Plant Air Quality High
With the variety of contaminants that exist in most manufacturing plants, a single type of filtration system or filter medium seldom is universally effective.
Understand contaminant capture methods, filtration technologies, and maintenance needs before selecting equipment.
Capture pollutants at the source with enclosures, hoods, mechanical arms, or downdraft tables.
Use one of three types of common systems to filter airborne contaminants: centrifugal separators, electrostatic precipitators, or mechanical systems.
With the variety of contaminants that exist in most manufacturing plants, a single type of filtration system or filter medium seldom is universally effective. However, once the materials and processes used in the operation are known, types of pollutants in the air can be identified and an effective air filtration system designed to improve plant indoor air quality.
The first article in this two-part series on maintaining quality air in the plant (PE, October 1997, p 66, File 2540/7550/7040) looked at controlling contaminants, assessing airborne pollutants, and understanding terminology and regulations that govern them. In this article, specific contaminant capture technologies and inplant air filtration equipment are discussed.
Pollutants are captured at the source with four primary devices: enclosures, hoods, mechanical arms, and downdraft tables.
A well-designed machine enclosure can be the most important consideration for optimum containment of mist or dust. Good design achieves cross-ventilation through the enclosure. The filter system inlet should be as far as possible from the opening. When the door is opened, air should rush into the opening, pulling air away from the operator's breathing zone. An enclosure makes an excellent drop-out box or settling chamber, allowing all heavy material to drop out of the airstream. Fewer contaminants hitting the filters means less filter maintenance and maintenance costs.
If enclosing the machine completely is not possible or economical, hoods can be used to capture the majority of airborne pollutants before they leave the work area.
Sometimes space restrictions or the need for flexibility require that movable mechanical arms be used to bring a capture hood directly to the source of the pollutants.
Downdraft velocity air bench tables provide a slotted work surface that pulls dirty air away from the operator, improving air quality tremendously. A well-designed system can capture virtually 100% of contaminants. A well-designed system means all pollution generation sources are equipped with collectors or collection hoses, and all enclosures or pickup hoods are designed and used properly.
When contaminants cannot be contained sufficiently for source capture systems to work, ambient air filtration systems offer an option. Such free-hanging, unducted installations establish a closed-loop, recirculating airflow pattern above the contaminated area.
Filtration units are typically suspended from the ceiling, arranged in a pattern that creates a push-pull or race-track-style circular airflow. Cleaned exhaust air from each collector blows rising contaminants to the inlet of the next unit in the loop.
Collection efficiency of free-hanging systems depends greatly on the number and location of the units, and how well disturbances to the circular air pattern can be avoided. Cleaning efficiency never reaches 100% because some contaminant is always rising toward the airstream and some more blowing around the pattern hasn't yet been drawn into a collector. Most installations achieve a 50% to 90% particle count reduction.
Filtration system technologies
Once airborne contaminants are properly entrained, filtration equipment can be selected to handle it. Three types of filtering systems are commonly used: centrifugal separators, electrostatic precipitators, and mechanical filter systems. No single system is best for all applications. Each type has advantages and limitations. The type selected should be based on the kind and quantity of contaminants in the air.
Centrifugal mist collection
Coolant mist from machining operations presents a special filtration problem that can often be solved by centrifugal separation of submicron particulate right at the source. Centrifugal mist collectors work effectively with petrol-based or synthetic coolants and usually are mounted directly on machines.
In a typical system, mist drawn through a perforated, rapidly spinning drum is coalesced into droplets which pass through a filter lining into a collection trap. A centrifugal separator can reach an efficiency of 98% at 1 micron. Adding a HEPA afterfilter can raise this level to 99.97% at 0.3 micron. Self-purging and recycling of liquid maintains high efficiency for years with minimal maintenance.
Industrial situations involving smoke, fumes, or other submicron suspended airborne contaminants require an electrostatic precipitator (ESP) to control inplant air pollution. In a typical, two-stage precipitator, particles are first ionized as they pass by charged wires in the airstream. In the collection section, charged particles are attracted to collector plates and removed from the airstream.
ESPs work well when high airflow rates are required. They are especially effective at removing smoke and other submicron contaminants that may clog a cartridge filter. Extremely efficient units can be configured with a variety of options. These include impingers for mist removal, double pass or tandem units for severe smoke or fumes, inlet flanges for higher capture zones (free handling systems), or inlet plenums (for ducted systems). A cleaning rapper mechanism can remove dry contaminants from the collection plates to minimize routine maintenance.
Mechanical filter systems
A great many sizes and shapes of mechanical filter systems are available. All use a disposable or cleanable filter medium suspended in an airstream driven by a fan. Types of disposable and cleanable filters are discussed in detail in the section, "Sorting out cleanable, disposable filters."
Finding the right solution
Choosing a filtration system to do a particular job at the lowest overall cost can be difficult and may require expert consultation. Designing an efficient pollution control system involves carefully identifying the contaminants being collected, understanding how the filter system operates, and performing the required maintenance.
Knowing the types of pollutants present in the air helps determine the kind of filtration system needed. For example, removing mist requires knowledge of the kind of mist -- and there are many -- involved.
A centrifugal separator might be the choice for a turning application in which large chips of metal fall out while coolant oil is recirculating. However, a multi-element filtration system with successively finer filter elements would be better for a grinding application in which dust particles remain in the coolant oil mist.
Although filtration and collection systems differ widely, their costs of operation are actually similar. With mechanical filters, most of the maintenance costs are in materials (disposable filters) and only a small amount is for labor. With ESPs, the reverse is true: the main issue is the cost of labor to clean the unit.
Maintenance is the key to an effective system. Because choosing the wrong system could tax the maintenance department, filter system cost can quickly boil down to a discussion of maintenance.
Sizing the system
Sizing the system also is a critical factor. Choosing a system larger than anticipated can pay off in reduced maintenance. A larger unit can run longer before requiring service. The larger the surface area of the filter and lower the air speed through the filter, the higher the filtration efficiency will be. Some experts say a system can not be too big.
Too small a unit, on the other hand, can greatly increase maintenance costs. The larger the surface area of the filter, the longer the maintenance cycle. An industry rule of thumb states: "Double the filter area, triple the filter life." In addition, a larger system has a stronger blower to pull air through the filters as they begin to fill up, extending maintenance cycles even longer.
Each plant must determine how much maintenance responsibility it is willing to assume. Some want long maintenance cycles and just want to throw away the filter elements when they are dirty. Other plants want to pay labor to clean filters frequently but save on the cost of the elements.
What systems work best?
The selection decision may be simple if only a single contaminant is involved. Mixtures of smoke, dust, oil, and fumes, however, require a combination of systems. Chances of success are greatest by working with a supplier who offers a broad array of products. Finding the right solution becomes a matter of following the guidelines outlined in this article.
Minimizing maintenance costs means choosing the right type of filter for the contaminants being generated and specifying a system large enough to meet the need. Design of the capture approach is a key to system efficiency. An improperly designed capture hood may result in a much larger than necessary system.
Finding the right air filtering system requires a balancing of many factors. Choosing carefully can result in an economical system that provides years of satisfactory service.
-- Edited by Jeanine Katzel, Senior Editor, 847-390-2701, firstname.lastname@example.org
Sorting out cleanable, disposable filters
Cleanable filter systems are available for a variety of applications. Fabric filters, or bag filters as they are typically known, come in two types. An envelope-type bag is cleaned by a shaker system. Long tube-style filters, such as those used in industrial baghouse filter systems, are cleaned by a reverse jet pulse of air. Modular bag-type dust collectors are an economical solution to many industrial air contaminant problems. Premium bag filters can remove up to 99% of airborne particles from operations such as grinding, buffing and polishing, sanding and turning, chemical crushing, and fertilizer screening. Portable air cleaners can be taken right to a contaminant-generating site. Cartridge filters provide an economical, easy-to-maintain solution for extracting smoke, dust, dirt, and weld fumes. A collection hopper is typically used to keep large particulate away from filters and allow easy cleanout. A self-cleaning, pulse-jet blowdown system keeps filters clean, extending maintenance cycles. HEPA-like filter cartridges provide 99.9% filtration efficiency. A variety of cartridge system design styles are available. Large, central units can be ducted to achieve collection throughout a plant. Smaller units can be floor or ceiling-mounted with ducts running to individual stations. Free-hanging systems provide an economical solution for multiple workstations. Portable air cleaners can be moved directly to the source of contaminants, providing maximum flexibility when workstations must be relocated periodically. Filter cartridges that have been specially surface-treated or made from one of the new high-tech materials offer improved particulate release and enhanced filter life. Washable cartridges can be cleaned with solution after prolonged use. Washable, 100% polyester cartridges may last 2 to 10 yr. Disposable paper cartridges typically last 1 to 2 yr. Washable filters, such as cartridges, metal (aluminum) chevron impingers, and plastic or metal mesh filters are used to remove larger particulate before it reaches the more expensive filters that trap smaller particles. Typical use is as the first stage in a multistage system for filtering mist from wet machining operations. An impinger removes large particles of mist, droplets, spray, etc., as well as grinding swarf (small metal particles) that might otherwise quickly clog a high-efficiency filter.
Disposable filter systems are typically used when the nature of the contaminants (sticky or greasy) prohibit the use of cleanable filters. Prefilters are low-efficiency, typically 1-in. thick fiber glass or polyester mesh units similar to furnace filters and suitable for removing large particles. Multi-vee filters are intermediate efficiency and consist of media pleated from 1 to 4-in. deep to get more filter area in a smaller space. Typical efficiency of these filters in industrial applications is 30% to 40%. Vee-bag filters are fiber glass/polyester scrim-backed vee-bags for capturing dust and other airborne particulate with moderately high efficiency. They are typically used when sticky or oily dust is involved. These applications include grinding materials containing glue or working with greasy metal. Suggested efficiency for industrial applications should be around 95%. High-efficiency particulate air (HEPA) filters are used to remove extremely fine particles. HEPA filters use ultrafine fibers to entrap airborne pollutants as small as 0.3 micron with 99.97% efficiency. Prefilters are often used in conjunction with a HEPA filter to remove larger particles that would otherwise quickly clog the small pores of the HEPA filter.