First adopted by the semiconductor industry, fluoropolymer-coated stainless-steel exhaust duct has spread to many industries where corrosive vapors are generated
By Vincent Rocca, Fab-Tech, Inc.
The EPA’s Ruling 40 CFR1 and the National Fire Protection Association’s (NFPA) Standard 318 for Cleanrooms are affecting the way plant operators specify process ventilation systems. Manufacturers of one of the key components of these systems, the duct itself, are touting new materials of construction and innovations, which have been developed over the past fifteen years.
The evolution of one such specialized product, fluoropolymer-coated stainless-steel (FCSS) duct, began in the semiconductor industry in the early 1990s. Fires spreading through manufacturing buildings via ventilation systems made of combustible materials resulted in a string of catastrophic losses. Much of the work done at these sites was conducted in cleanrooms where corrosive chemicals were used and hazardous vapors were generated.
The role of the exhaust system is to safely transport these harmful vapors to devices where they can be rendered harmless and safe for discharge into the environment. The use of FCSS duct has now become the material of choice for this industry as it meets the extreme demands of both safety and corrosion resistance. And it is being discovered by other industrial sectors as well.
A material for broad applications
The choice of materials for a vent exhaust system is a function of the corrosive processes employed within the building. It is the role of the design engineering firm, working in conjunction with the architect and client, to choose the highest-performing, safest, yet most cost-effective materials.
Based on published data, ethylene-tetrafluoroethylene (ETFE) and ethylene-chlorotrifluoroethylene (ECTFE) coatings far surpass other materials in chemical resistance. And the stainless-steel tubing will not burn, collapse or leak. FCSS duct is 300 series stainless steel coated with a two-part (primer and top coat) fluoropolymer system then heated and “baked” to form a chemical and mechanical bond with the stainless substrate (see Fig. 1). Some manufacturers utilizing ETFE have developed proprietary primer technology that assures superior adhesion to the stainless steel, resulting in a coating that will not delaminate.
In evaluating the level of corrosiveness of a given application, it’s vital to consider the entire range of chemicals to which the duct system will be exposed. Even more important is the potential impact of chemical combinations. “It is also important to evaluate complex systems with several different types of process streams (e.g., reactor, storage tank, relief stream) discharging into a common header. Where vents from several sources are combined, it’s necessary to carefully consider all possible interactions between the different streams in terms of both chemical reactivity and flammability.”1
Fluoropolymers have a wide corrosion-resistant envelope to handle most chemical situations. Additional reasons for considering FCSS duct include robust mechanical strength; light weight; ease and speed of installation using rotating van stone flanges or EZ-type clamps; the ability to be field-modified (e.g., it can be shortened; nozzles or drain ports can be added); and it doesn’t need painting. Unlike FRP and other “glued” plastic systems, there is no need for grinding, sanding, prep work or the use of malodorous epoxies and heating blankets.
FCSS duct is priced competitively compared to other materials such as FRP, composite materials and PTFE-lined pipe. It’s important not only to compare per-foot costs but also to evaluate the total installed cost of the system, including the mechanical contractor’s labor and any related piping and sprinkler costs.
Even among the manufacturers of FCSS duct, there’s debate regarding the selection of ETFE vs. ECTFE. Both polymers fall into the range of partially fluorinated polymers, but ECTFE has a chlorine atom substituted for fluorine-a difference in molecular composition that has an effect on performance. Some believe fluoropolymer ETFE has a performance edge over ECTFE. “ETFE has better chemical resistance and higher temperature resistance, as determined empirically and supported by a sound basis in chemical principles. These primary advantages not only provide an extra margin of performance in chemical service, but also impart more consistent quality in daily production as well as allowing greater production flexibility for better customer service.”2
Safety benefits: Fire and smoke ratings
As in the semiconductor industry, hazards in other manufacturing segments that also use chemicals include fire or explosion due to the use of solvents, flammable liquids or dust, and the resulting contamination of production, storage, and cleanroom areas by smoke or other substances released by the fire. Corrosive fumes that can attack and penetrate ductwork can allow fugitive emissions and corrosive liquid to enter clean areas, resulting in an unhealthy and hazardous work environment for employees.
Many building codes and insurance companies require ducting made of stainless steel or other noncombustible materials. In some instances, fiberglass and other combustible duct materials may be used but require the use of internal sprinklers. As stated in the NFPA Standard 318, “Exhaust duct systems shall be constructed of non-combustible materials or protected with internal sprinklers in accordance with 2-1.2.6. Exception: Ducts approved for use without automatic sprinklers.”3
The NFPA standard continues with a hierarchy of material preferences: “Considering fire protection issues only, duct materials listed in descending order of preference are: (a) metallic, (b) approved coated metallic or nonmetallic not requiring fire sprinklers, (c) combustible with internal automatic sprinkler protection.” The standard notes that “although most NFPA standards are not laws, they are widely accepted industry standards with considerable legal standing. Failure to comply with them can potentially put manufacturers in serious liability.” Factory Mutual (FM) approved stainless-steel duct, coated internally with a fluoropolymer, satisfies this requirement without the use and costs of an internal sprinkler system. FM is an affiliate of the insurance company FM Global and conducts extensive research in loss prevention.
Manufacturers of FCSS duct use FM Research’s services to earn the FM Approval mark, certifying the reliability of their products. FCSS duct is regulated and approved by FM Research Standard Numbers 4922 and 4910. With FCSS duct, structural integrity is maintained in the event of a fire. With extremely low flame and smoke characteristics (flame spread under 10, smoke generation under 15), these systems will not burn, melt or generate large quantities of smoke, an extremely important consideration in cleanroom and laboratory environments.
Chemical and life sciences
In 1998, EPA Ruling 40 CFR imposed strict new standards to reduce the quantity of air toxins released from chemical and pharmaceutical manufacturing sites. “The agencies rule was intended to reduce emissions of a number of air toxins and hazardous air pollutants (HAPs), including methylene chloride, methanol, toluene, and HCl. It was estimated at the time that the ruling would reduce air toxins by approximately 24,000 tons or 65% from contemporaneous levels.”4
Various technologies to comply with this ruling are available, but the two leading methods for reducing air pollutants are the use of either a regenerative thermal oxidizer (RTO) or a caustic scrubber system. In some instances, both technologies are used in tandem: a thermal oxidizer capable of incinerating a variety of emissions, including methylene chloride, acetone, ethanol, isopropyl alcohol, methanol and mineral spirits, and then a scrubber that can remove the resulting hydrochloric acid emissions from the thermal oxidizer’s outlet. FCSS duct is increasingly being used to safely carry these hazardous pollutants.
In chemical synthesis and formulation processes, the materials of construction for reactor vessels, process equipment and process piping include borosilicate glass, glass-lined steel, exotic alloys, and fluoropolymer. The equipment is usually an ASME pressure-rated type, capable of containing process liquids and vapors up to 150 PSIG, that incorporates a vent nozzle or relief device connection.
The manufacturing suite may have a general exhaust (or snorkel vent at individual equipment stations) for the area, but increasingly, pharmaceutical and biopharm facilities also incorporate large walk-in reactor enclosures and fume hoods that are tied into the process vent system. In applications where higher pressures and full vacuum are customary, fluoropolymer-coated, schedule 10, stainless-steel pipe (150 PSIG-rated) would be the choice for corrosive vent lines.
Where pressures involved are lower (to negative 18 inches of water gauge) for use with cleanroom exhausts, fume hoods, snorkels, walk-in reactor enclosures or biological safety cabinets (BSC) where corrosive fumes are present, less expensive FCSS duct, rather than pipe, would be the appropriate product choice. These low-pressure, low-vacuum duct systems are governed by SMACNA HVAC guidelines (see Fig. 2).
Additional areas for use of FCSS duct in pharma/biopharm applications include “clean air” supply duct systems feeding the cleanroom. Hot, corrosive, ultrapure water for injection (WFI) is used to set incoming air humidity levels. Some manufacturing facilities have experienced corrosion in their galvanized and even stainless-steel supply-air ducting from these hot WFI vapors. Coated duct is also impervious to harsh chemicals used to disinfect and sterilize cleanrooms and ducting. Chemicals used can range from sodium hypochlorite (bleach) to quaternary ammonia to phenols to formaldehyde-based products. Steam temperatures for sterilization can often exceed 120°C (248°F).
Medical devices
Another industry that has benefited from the use of FCSS duct is the medical device industry, specifically, manufacturers of drug-eluting stents. Stent manufacturing employs many of the same chemistries and protocols used in semiconductor production. For example, the etchants and rinsing agents used to fabricate semiconductor devices are very similar to those used to fabricate stainless-steel coronary stents. These hazardous materials must be monitored and safely controlled to prevent accidental release.
Stent manufacturer Boston Scientific’s newest facility, Weaver Lake 3, in Maple Grove, MN, is a two-story building incorporating a “mid-story” interstitial level for handling utilities. Long used in semiconductor facilities, interstitial floors are now also gaining acceptance in pharmaceutical, biotech and medical device manufacturing.
 Figure 3: Coated duct was chosen for use as main exhaust lines, manufacturing area and lab hood exhausts in Boston Scientific’s Weaver Lake 3 facility. |
In the Maple Grove project, the interstitial space was equipped with an exhaust system with variable speed fans. FCSS duct was selected for the safe handling of corrosive exhaust. Approximately 2,000 feet of coated duct was installed in sizes ranging from 12- to 40-inch diameters for use as main exhaust lines, manufacturing area and lab hood exhausts (see Fig. 3).
Photovoltaics
Several photovoltaic (PV), or solar cell, manufacturers have also installed FCSS exhaust duct in their facilities to handle their corrosive exhaust requirements. One company has unveiled a new type of solar cell technology using thousands of tiny silicon spheres. Spheral Solar (Cambridge, Ontario) debuted its new manufacturing plant in June 2004. It is Canada’s first full-scale solar cell manufacturing facility.
Solar cell manufacturing also employs many of the same materials and protocols used in semiconductor production. For example, the etchants and rinsing agents used to fabricate semiconductor devices are very similar to those used to fabricate PV cells. Since PV manufacturing processes involve the use of highly corrosive, combustible and toxic chemicals and gases, double-contained chemical areas, hazardous-vapor sniffers and many cutting-edge safety devices were incorporated into the design of the new facility. For the safe handling of corrosive exhaust, FCSS duct was selected.
University nanotech and science centers
On university campuses, from Albany, NY, to Berkeley, CA, billions of dollars are being spent on construction of new nano-facilities. And it is also at these new and expanding nano-sites where manufacturers of FCSS duct have seen a significant increase in the demand for corrosion-resistant product.
Major corporations such as IBM, Fujitsu and Intel are also pouring vast sums of money into nanotechnology research, and the U.S. government, which lagged behind other nations, is now spending billions to catch up on the nanotechnology racetrack. Spurred on and financed in great part by the National Nanotechnology Initiative’s (NNI) $2 billion 2006 budget,5 more than 50 universities are either building or expanding facilities, or conducting research into nanotechnology.
 Figure 4: CNSE has installed several thousand feet of FCSS duct on three nanotech cleanroom projects. |
One of the nation’s top nanotechnology sites is the College of Nanoscale Science and Engineering (CNSE) at the University at Albany. CNSE is the first college in the world devoted exclusively to the research, development and deployment of innovative nanoscience, nanoengineering, nanobioscience and nanoeconomic concepts. Its Albany NanoTech complex-a $3.5 billion, 450,000-square-foot facility-is the most advanced research complex of its kind at any university in the world.
 Figure 5: CNSE has installed several thousand feet of FCSS duct on three nanotech cleanroom projects. |
Nanotechnology research employs many of the same manufacturing protocols as microprocessor manufacturing. For example, it requires that research and manufacturing be conducted in a cleanroom, and as in semiconductor manufacturing, nano-facilities use chemicals to harden an unexposed photoresist and then chemically etch it to selectively strip off an oxide layer where no photoresist protects it. Through the end of 2006, CNSE had installed several thousand feet of FCSS duct on three nanotech cleanroom projects (see Figs. 4 and 5).
Conclusion
Although FCSS duct is new to many industries, it has been successfully used in manufacturing for more than 17 years. It meets all the required design and operating criteria for extreme service when applied and installed according to the parameters for its intended use. If corrosive vapors and fire safety are a concern, FCSS could be the ideal solution. Professionals who design, build or manage laboratory and manufacturing sites that contend with corrosive and hazardous vapors can benefit by doing additional research into this new type of product.
Vincent Rocca is industrial sales manager for Fab-Tech Inc. (Colchester, VT). Previously, he held management positions with DeDietrich, Pfaudler and Resistoflex. He is a graduate of Seton Hall University in South Orange, NJ.
References
- Ennis, Tony, “Collect and Destroy Emissions Safely,” Chemical Engineering Progress, May 2004.
- Obals, W. Douglas, “Teflon Finishes in the Semiconductor Industry,” Cleanroom Technology, July 1999.
- National Fire Protection Agency (NFPA) 318 Standard for the Protection of Cleanrooms.
- Kudronowicz, Jeff, “RTOs Leave Nothing to HAP-penstance,” Pollution Engineering, April 1, 2005.
- For more information, see http://www.nano.gov/.