May 1995, Volume 3

Problems with abrasion to fan impellers and housings were common 10 years ago when air pollution control regulations were less stringent. Because air pollution control devices are more efficient and stack emissions are less tolerable to particulate concentration rates, wear problems with I.D. fans have almost disappeared.

Unless a fan serves as a material handling device for intentionally conveying material, a process or boiler fan should not experience abrasion if the upstream air pollution control device is properly maintained. There will be process applications where I.D. fans are mounted upstream of air pollution control devices, and in these cases, the fan must be designed to counter the abrasive action of the coarse particles entrained in the gas stream. This protection generally constitutes installing abrasion resistant steel liners on the blades and housing scroll.

If for some reason, a fan is not protected by an air pollution control device such as a precipitator, scrubber or baghouse, it is common practice to install a cyclone or multiple tube mechanical collector upstream of the fan for protective purposes. A properly maintained and operated mechanical collector will effectively collect the coarse particles but allow the fine particles to remain in the gas stream where they eventually are collected by the air pollution control device. Fine particles are those particles less than 10 microns in size, with a micron being defined as 1/10,000 of an inch. It is the particles above 10 microns in size which cause abrasive action to the fan and housing. Because the multitube mechanical collector is not capable of collecting the fine particles, it is not categorized as an air pollution control device, but rather is defined as a precleaner for protection of downstream equipment.

A recent installation of a mechanical collector was on a cement kiln in the Northeast. This mechanical collector is designed to protect the I.D. fan and to lower the dust loading entering the precipitator. Because this plant is operated at it's maximum product capability, the dust loadings to the precipitator have escalated and the mechanical collector actually improves the precipitator performance.

The sketch shown above illustrates how the cyclonic operation can remove coarse particles and prevent wear on downstream equipment.

If you are experiencing wear on an I.D. fan which is protected by a multiple tube mechanical collector, the following are troubleshooting recommendations:

  1. Check to make sure the hoppers seal adequately.
  2. Use the Process Equipment slide rule or other device for estimating the flue gas flow rate to determine if the collector is adequately sized. If you need a slide rule, please call or E-mail.
  3. Inspect the collector for any pluggage or internal leakage.
  4. Be sure that the inlet velocity profile at the mechanical collector allows for uniform distribution of dust and gas to individual tubes.

At your request, Process Equipment will, at no charge, send a more detailed technical report on the operation and maintenance of mechanical collectors. An adequately sized and maintained collector should prevent any serious wear problems to downstream equipment and protect the I.D. fan from serious wear.

For additional information on articles appearing in this newsletter, contact Process Equipment Company at (205) 663-5330 or E-mail to: information@process-equip.com


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