Where the user is unsure of the nature of the chemicals involved, or of the corrosive effect of the combination of chemicals involved, it is advisable to insert resin test coupons, as well as coupons of possible alternate materials of construction, into the gas stream for observation. All polyesters and vinyl esters are corrosion resistant to some degree.
Industrial process applications involve a wide variety of corrosive gas streams. Selecting the best materials of construction for air handling equipment can be difficult. This article provides information about the corrosion resistance of the resins used to manufacture standard FRP fans.
Fumes are the dry vapors evolved from acids, solvents, etc. An example is the dry acid vapor scavenged from a process using acid. As a generalization, fumes are not as corrosive as aerosols. Aerosols are suspensions of liquids or solids in a gas stream. For the purpose of this discussion, aerosols are considered as quite dilutebeing wet. Water as fog is an example of an aerosol. Another example is the mist of acid present in air scavenged from a process where acid is being used as a spray wash. As a generalization for the purpose of estimating corrosiveness, aerosols in fan driven systems can be considered as being dilute concentrations of the chemicals composing the aerosols. An example of the distinction between fumes and aerosols is a system where sulfuric acid fumes are collected by hoods and scrubbed. The dry fumes entering the scrubber could be quite concentrated but have a relatively mild corrosive effect on the hood and duct material. On the other hand, the wet gas down streamfrom the scrubber could be quite dilute but more corrosive because of the scrubber’s converting the fumes to an aerosol.
Standard FRP fan construction does not include the use of surface veil. Years of service prove this construction to be cost effective and functionally successful. However, the general approach to the design of most FRP chemical process equipment, such as storage tanks, is to use surface veil. Synthetic veil such as Nexus polyester veil protects the glass structure from attack by chemicals that are particularly aggressive toward glass. The system designer should have sufficient knowledge of, or experience with, the application to select the appropriate resin or alternate material.
FRP is inherently non-sparking and the electrical resistance of FRP may be considered infinitely high since it is essentially a non-conductive or non-metallic material. Because FRP is nonmetallic, the physical contact of two FRP parts or a metallic part with an FRP part will not produce a spark. However, FRP does have the tendency to hold a charge of static electricity. This charge can be generated by a dry gas or airstream passing over FRP. The fan can ultimately become a capacitor capable of discharging high-voltage, low-amperage sparks.
The static electricity or charge which builds up on the airstream surface of the FRP part must be eliminated in applications where the fumes are potentially explosive. This can be accomplished by making the surface electrically conductive, providing an electrical path to dissipate the relatively low-current static charge. FRP fans can be effectively grounded for the removal and control of static electricity by incorporating graphite in the airstream layer of resin. Static grounding by graphite impregnation does not interfere with the corrosion-resistant properties of the fan. Graphite is extremely corrosion resistant. However, the addition of the graphite makes the surface softer than normal and prevents the normal checking of the surface for Barcol-hardness readings.
FRP fans are often the best alternative for those applications which require the handling of explosive, as well as corrosive gas fumes. However, care must be taken to realize that there can be no guarantees against possible sparking or ignition in such airstreams. All aspects of the application, the system components, and even the potential for sparks resulting from “tramp” or “foreign” elements in the airstream must be considered to ensure the safety of the installation.
For additional information please refer to http://www.buffalofan.com/faninquiry.html
Buffalo Fan Co.
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Oleg Tchetchel, Ph. D., Senior Industrial Process Ventilation Specialist
Buffalo Fan Co. - Fans and Blowers People
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