When fiberglass is the selected material for an air-handling system, it is logical that the fan also be made of FRP. For example, the acids used in the pickling of stainless steel are necessarily those that attack stainless steel. In such a system, the acid resistant tanks, fume control hoods, ducts, air and gas scubbers, and fans are often made of FRP because FRP resists acid corrosion and costs less than metal alloys having comparable resistance.
Fiberglass reinforced plastic (FRP) made from chemical grade polyester or vinyl ester resin resists corrosion as well as, or in some cases better than, high-priced materials such as titanium or nickel alloys. In general, FRP (also known as RTP, or reinforced thermoset plastic) is widely used in handling the fumes of acids and of many inorganic and organic chemicals where service temperatures do not exceed 250°F.
Potential applications for FRP fans include any process in which corrosive fumes must be captured, moved, cleaned, or vented. FRP fans are most often used in fume-scrubber systems where the scrubber itself may be constructed of FRP or an exotic alloy, but where FRP is the preferred fan material. Galvanizing and etching processes often have FRP exhaust hoods and ducts, and many of the fans used to convey fumes in such systems are also built of FRP. Waste water treatment plants and laboratory exhaust systems are other applications for which FRP fans are being used with increasing frequency.
Fiberglass fans may be an economical alternative to stainless steel or other metal-alloy fans when corrosion is a concern and temperature is below 250°F. In addition to the economic advantage, FRP fans often provide better performance than special alloys in handling airstreams that are particularly corrosive to metals.
A comparison of the corrosion resistance and economics of fans made of various materials leads to these generalizations:
Coated steel fans vary greatly in the degree of corrosion protection provided and cost. Costs for coated fans run from about one-third that of FRP fans for the least chemicl resistant coated steel fans to about half the cost for the baked phenoliccoated fans. Coated fans, regardless of the inherent corrosion resistance of the coating, have the potential of coating failure and resultant rapid deterioration of the base metal. Failures occur when coatings are physically damaged, and when corrosive attack permeates the coating to attack the metal.
Stainless steel is susceptible to attack by chlorides and resultant physical failure by stress cracking. Residential hot water heaters are never made of stainless steel because the combination of small amounts of chlorine in the water, modest temperatures, and the stresses caused by changes in water pressure results in rapid failure of the stainless steel. Stainless steel is also much more susceptible to corrosive attack by most acids than is FRP.
Fans made of rigid polyvinyl chloride (PVC) have good allaround corrosion resistance and generally cost less than fans made of FRP. However, PVC has two significant physical weaknesses that severely limit its use in fans: PVC becomes quite brittle at temperatures below freezing, and PVC loses its strength so rapidly with increasing temperatures that even ordinary summer rooftop operating conditions are marginal. Wheels sag and go out of balance and strike housings. PVC is a thermoplastic material that "remembers" its original shape at about 150°F., and needs to reach only about 300°F. for it to have the zero strength needed for vacuum forming. Numerous users have disavowed the use of PVC fans because of their experiences with failures resulting from PVC's low temperature brittleness and high-temperature weakness.
The use of PVC equipment involves some safety considerations as well. PVC does not burn, but because it is a low temperature thermoplastic it collapses early in a fire and will drip molten PVC. Thus, rather than containing a potential fire within the duct system, as fire retardant FRP will do, PVC tends to expand the fire into other areas, even though it is not inherently combustible. In addition, PVC releases highly toxic hydrochloric acid fumes when exposed to flame even though it is a self-extinguishing material. PVC, like FRP, is an insulating material and inherently spark resistant. However, unlike FRP, it cannot be made electrically conductive to control static electricity.
The term FRP describes a broad spectrum of fiber-reinforced plastic materials. For example, cabinets for office machines might be made of non-corrosion-resistant plastics reinforced with mica and loosely called FRP. However, the FRP used in making process vessels and equipment such as fans is composed of about 30% by weight of glass or other fibers that have been given a coating (sizing) to enhance their bonding with the resin, and about 70% by weight of corrosion resistant polyester or vinyl ester resin.
The fibers provide physical strength, and the resin provides the corrosion resistance and rigidity that make FRP a workable solid. Sometimes, non glass-fiber materials are used in FRP to impart special properties. For example, graphite fibers add tensile strength, and aramid fibers add toughness. But FRP for process equipment usually has glass fibers because they are more economical and easier to work with; graphite fibers, for example, are more difficult to handle and do not bond as well as glass. Glass fibers are available in a variety of forms, including continuous-strand roving, woven roving, continuous -strand mat, chopped-strand mat, chopped fibers, and milled fibers.
The corrosion resistance of FRP depends on the resin. Resins used in FRP for process equipment are formulated for maximum corrosion resistance, and are consequently two or three times as costly as those used in everyday products such as boat hulls or auto body parts.
FRP fan manufacturers normally use two types of resin in the construction of their products. Polyester is the resin of choice for non-moving components such as housings and inlet cones because it provides excellent corrosion resistance for most FRP applications at a relatively low cost. Unfortunately, this type of resin cannot withstand the dynamic stresses inherent in rotating parts such as wheels. Therefore, FRP wheel construction dictates the use of vinyl ester resins which are much stronger and more flexible than polyester resins. The strength and elasticity of vinyl ester resins enable FRP wheels to achieve maximum safe speeds comparable to similarsized steel wheels at 70°F. As with steel and other alloys, the strength and flexibility of vinyl ester is compromised at elevated temperatures, resulting in safe speed derate factors above 150°F. Refer to specific product bulletins for maximum safe speeds and applicable derate factors.
Resins for chemical duty can be made fire retardant by formulating the resins to include adequate molecularly bound halogens, such as chlorine or bromine, or by the use of smaller amounts of halogens but with the addition of antimony trioxide. The first method is more costly but provides a clear resin that improves quality control of the product being manufactured since the workers and the inspectors can see into the finished product. Antimony trioxide is a white pigment which produces an opaque product that reduces the ability to visually check product quality. Further, antimony additives can reduce the corrosion resistance and strength of the resin.
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Canadian Process Design Engineer
Buffalo Fan Co.
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Canadian Process Design Engineer
Buffalo Fan Co.
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