A direct link between the size of particles and their potential for causing health problems is what has the U.S. Environmental Protection Agency (EPA; Washington, D.C.; epa.gov) concerned about particulate matter (PM) that is 10µm in diameter or smaller. Renewed concerns have prompted the chemical process industries (CPI) to seek out more efficient dust-control systems that can handle even submicron particles, and equipment suppliers are responding.
“Regulations prod this business,” says Rick Davies, V.P. of sales at Dustex Corp. (Kennesaw, Ga.). Dustex uses a dry-scrubbing technology to agglomerate and remove small particles. Mr. Davies went on to say that in addition to following regulations, many corporations are just “doing the right thing” referring to the trend he sees of an increased demand for tighter dust-control measures on finer particles.
In 1987, the EPA introduced standards to regulate particles that are 10 µm or smaller because those are the particles that generally pass through the throat and nose and enter the lungs. In 1997, a revision set separate standards for fine particles, which are defined as those smaller than 2.5 µm (PM2.5). Suppliers of dust-collection equipment expect that the focus will hone in on even smaller particles as more scientific information is reviewed and new regulations are set.
“More and more, people are going to really high-efficiency recirculating systems,” for ambient temperature, nuisance-dust collection says Lee Morgan, General Manager at Farr Air Pollution Control (Farr APC; Jonesboro, Ark.). Morgan’s firm has noticed an increased demand for pleated cartridge filters due to their high efficiency and small footprint. Farr APC offers an innovative cartridge-filter design that gives extended service life and lower pressure drop as compared to standard dust-collector cartridges, Morgan says. Called the HemiPleat, this filter uses synthetic beads to hold the pleats of the cartridge open with wide pleat spacing to maximize media use, and is available up to a MERV 16 efficiency rating (see more about this rating system below).
One of the main challenges in filtration, particularly of fine particles, is keeping a “balance between efficiency and sufficient airflow, ” says Tim Grafe, Director of Product Development for Industrial Filtration Solutions at Donaldson Torit (Minneapolis, Minn.). Donaldson has met this challenge by developing a nanofiber technology, which is incorporated into their Ultra-Web cartridge filters. The nanofibers are a cellulose/synthetic composite media with fibers, 0.2–0.3 µm in diameter, that form a web-like net to capture dust on the surface of the filter media. This surface-loading technology results in a lower pressure drop as compared to other commodity types of media and it allows dust to be removed from the filter more easily for cleaning, says Grafe’s company. The Ultra-Web can filter submicron particles at a MERV 13 efficiency level.
Rating the filters
The MERV (minimum efficiency reporting value) system is based on a standard for rating filters that was published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE; Atlanta, Ga.; ashrae.org). MERV values range from 1 to 20, and the values of 17–20 are generally associated with HEPA (high-efficiency particulate air) filters. Donaldson’s Grafe explains that his company wanted a technical-based system of comparison for filter efficiencies and that the MERV scale was the best standard currently available to do that. Independent laboratories conduct the testing that results in the MERV rating.
Jim Orr, National Sales Manager for Micro Air (Wichita, Kan.), says that the move to the MERV rating is the “biggest change” that he has seen most recently in the area of filtration. Other suppliers of dust collection systems are also using the MERV system. Airguard (Louisville, Ky.), for example, has introduced their X-Treme pleated panel filters with a MERV 7 rating (minimum efficiency of 50% for 3–10 µm).
Farr’s Morgan says that his company is currently using the MERV system because it is the only one that exists. He cautions, however, that the MERV rating system is based on clean efficiency measurements — meaning that the measurements are done on a clean filter — and does not reflect the performance of a filter in a realistic dust-collection system where pulsing technology is commonly used. Morgan currently serves as Chairman of the ASHRAE Technical Committee (TC 5.4) for industrial air cleaning. He and his committee have been striving towards the development of a publishable ASHRAE standard that would more realistically represent the performance of dust collectors. Such a standard could be directly related to mass emissions and thereby used in relation to EPA regulations. Morgan is happy to say that very recently, AHSRAE has funded the research to develop this dust-collector standard.
Filter media are commonly cleaned by blowing the collected dust off with pulses of compressed air. Several variations to this method are offered by manufacturers. Scientific Dust Collectors (Alsip, Ill.) has developed converging/diverging nozzles that are mounted on pulse pipes to induce more cleaning air. These nozzles greatly increase the velocity at a filter bag’s throat orifice, thereby resulting in better cleaning, says the company. Scientific Dust Collectors offers a brochure, which gives more details, on their website (see box).
Precision AirConvey’s (Newark, Del.) PAC reverse pulse-jet Models 12 and 16 have been recently designed with an oversized, compressed-air cannon and reservoir that permit high volumes of air bursts to clean filter cartridges at frequent intervals. The PAC dust collectors include a slide-out filter-cartridge system, which ensures that occasional filter removal for replacement may be done entirely from outside the dust collector.
Airlanco (Falls City, Nebr.) offers its proprietary Venturi Assemblies at each filter-bag opening to draw in extra air during the pulse cycle and enhance the removal of material from the surfaces of the filter bags.
Meanwhile, Vac-U-Max (Belleville, N.J.) provides a Teflon-coated filter in their Model 2120 Heavy Duty Central Vacuum System, which collects dust directly into a 55-gal drum. The slick filter surface, says the company, allows debris to release easily and prevent material buildup.
Diverging from the conventional reverse-pulse systems, Micro Air uses a Roto-Pulse cartridge cleaning system where air is pulsed into a stainless-steel rotating tube with pre-drilled holes. As it incrementally rotates, the tube distributes the pulse of air across the length of the inside surface of the cartridge. The number of increments per revolution and the time between pulses can be adjusted for flexibility.
Better filters are not the only solution for removal of fine particles. Electrostatic precipitators and baghouses are often considered for fine-particle control (For a related article, see Controlling Particulates with ESPs, CE December 2004, pp. 17–20), and new technologies abound.
“The trend in dust or particulate collection is to treat smaller particles,” concurs Rod Gravley, Technical Director, CCS Systems at Tri-Mer Corp. (Owosso, Mich.). Gravley expects that collection of the submicron fraction of particulates will receive even more focus as new health-based regulations are set. Tri-Mer has approached this challenge by developing a new technology called the Cloud Chamber Scrubber (CCS). This technology is particularly suitable for submicron particles, such as those found in smokes from combustion or other high-temperature processes.
In the CCS, the process gas enters a preconditioning chamber where particles larger than 10 µm are removed, and ultrafine particles are grown to a few tenths of a micron. This preconditioned gas is then introduced into a second vessel where it mixes with a cloud of charged water droplets. These charged droplets, which capture and remove submicron particles, are core to the CCS technology. For even higher performance, the gas stream can be passed through a second cloud chamber containing oppositely charged droplets. Gas scrubbing occurs concurrently in the system. This new technology, explains Gravley, was in development for six years, and the first commercial unit was installed in 2000. The technology has since been implemented for many applications. Because of its effectiveness on submicron particles, CCS technology is gaining attention for fine-particle applications, such as for diesel smoke.
Another trend in dust collection is in the area of monitoring and control. “Customers want to monitor multiple filters from one computer station,” explains Micro Air’s Orr. In response, his firm has added the capability for remote monitoring of their equipment.
Dustex’s Davies describes the same trend by saying that he has had requests for more instrumentation for both emissions detection and to improve maintenance response time.
Indeed, reduced maintenance costs and a solution for compliance regulations are a couple of the benefits sited by FilterSense (Beverly, Mass.) for its B-PAC Series of dust-collector diagnostic controllers. These units can operate as standalone controllers or they can be networked with existing PLCs and DCS systems.
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