Wednesday, March 31, 2010

Industrial Fan Applications: Pickle Line

The manufacting of sheet steel uses a pickling process after they have rolled the steel. This is a heated high strength hydrocholric acid bath that the sheet steel goes through to remove any slag inclusions on the surface of the steel. If the slag inclusions are not removed, rust can start at that location.

The hydrochloric bath gives off acid fumes, so it must be contained in a sealed environment with a negative pressure to ensure acid fumes do not leak out into the environement. To maintain this negative pressure, an industrial fan is used. This fan sucks air out of the hydrocholric bath hood, and through a scrubber to remove the acid out of the air. Idealy, the scrubber removes all the acid from the airstream so that only clean air goes up the stack. Unfortunately, scrubbers are not perfect and occasionally acid is still entrained in the airstream as it enters the fans.

The acid in the airstream is very corrosive on the fan components. For this reason extra precautions must be taken in the choice of fan materials to protect against corrosion. The main areas of conern are the fan casing, fan impeller, and fan shaft.

To ensure the fan casing withstands corrosion, it can be made out of fiberglass with a c-veil to seal the airstream surface of the fibreglass. However, ,one of the concern about using fibreglass is the strength of the casing. Casing strength is considered for three primary reasons. First, the casing must withstand the static pressure differential between the inside and outside of the fan so as not to deflect excessively. Secondly, the aerodynamic turbulence within the fan casing can cause casing vibrations if the casing does not have sufficient stiffness. Thirdly, in the event of a impeller failure, it is good to have a casing that can contain the pieces of the impeller rather than allowing them to pierce through the casing and damage people and equipment. For these reasons, fibreglass casings should only be considered for lower speed fans.

For higher speed fans, the casing can be contructed out of steel and lined with a corrosive resistant barrier. The best barrier is rubber as it does not get damaged easily and during the application of the rubber, the applicator can do spark tests to ensure that a complete seal is obtained. By using a steel housing with rubber lining, the housing has all the strength of the steel with appropriate stiffeners.

The second fan component requiring special consideration is the impeller. For low speed fan applications, it is possible to use a rubber coated impeller. Note that this is only applicable for very slow fans, since the centrifugal forces generated by higher speed fans along with the aerodaynamic forces in the impeller will cause the rubber to be ripped off the impeller. The best solution for the impeller is to not coat the impeller, but rather use a material which can withstand the corrosion while demonstrating suitable mechanical properties. The two primary materials satisfying these requirements are titanium and Alloy C276. Alloy C276 tends to be lower cost then titanium and has a lower corrosion resistance, but is often sufficiently corrosion resistant as long as it is inspected regulary.

The third fan component is the shaft. By using an overhung arrangement, the only component of the shaft potentially exposed to the airstream is the part between the back of the hub and the casing opening. This can be protected by a shaft sleeve that is welded to the back side of the hub and extends through the back side of the casing.

By making special considerations for the casing, impeller, and shaft, an appropriate fan can be designed for an acidic environment.

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