fan’s input horsepower is converted into airborne sound during the creation of airflow and pressure. Older fans or those that have been modified can create higher sound levels. What’s more, population growth creates neighborhood noise issues related to issues such as sound direction and distance from property lines. Physical sound barriers, fan rooms, earth berms and reduced hours of operation have all resulted from regulatory issues related to noise.

Where multiple units have been installed at a single plant, sound level readings can become complex and create secondary sound sources. Cases also exist in which fans have created higher sound levels at reduced load operating points. These situations can be caused by fan inlet vane damper settings and changes in rotor blade pass frequencies due to speed changes. Further study of this phenomenon is possible with new acoustical modeling software. Due to laboratory restrictions fan manufacturers could not measure very low frequency sound data until recently. Additional study and field measurements are advisable for these problems.

Standard absorptive silencers can be supplied with multiple transmissivity barriers that further reduce sound levels radiated to the environment. Noise canceling systems have been developed that can damper fan noise levels at remote locations. Applying this type of new technology is available as part of a study for replacement of low efficiency fan types that can also have a positive affect on sound issues.

combustion system. Energy upgrades and maintenance cost reductions called for the highest efficiency airfoil centrifugal fans and the removal of the high-maintenance whizzers.

Many fluidized bed boilers that burn a variety of fuel inputs are reaching a stage where performance modifications are necessary. Most of these installations were supplied with high efficiency airfoil blade fans that provide combustion air and fluidizing air to the main combustion system. The fans resemble coal pulverizing mill units but are usually much higher-pressure pieces of machinery. The addition of acoustical treatment and air preheat systems have changed fan design conditions. In most cases, these alterations require new fans that are the highest efficiency possible. Once again, consulting with an original system designer is recommended to create the most efficient fan selections.

Hot Gas Recirculating Fans

Hot gas recirculating fans require some of the most unique design and fabrication talent in the fan industry. These fans must handle radical temperature changes along with material erosion possibilities. Special metallurgy along with sophisticated fabrication techniques are required to provide long life to these new, high-efficiency fans. Previous designs have used a slow speed radial blade fan with heavy armor cladding. Recent advances have made it possible to apply high efficiency airfoil and backward-curved blade types to this application. The study of wear patterns associated with various types of coal and biomass fuels has resulted in fan designs perfectly matched to individual applications.

Reducing overall fan noise by installing higher efficiency fans is another reason to consider replacement. A percentage of any

Resisting Abrasion

High-efficiency fans can be designed to provide long life and predictable wear in fly ash environments. A primary concern for operators of coal-fired boiler fans is fly ash erosion. Experience with abrasive dusts of all kinds has taught fan engineers how to armor the critical wear parts of high-efficiency airfoil blade fans to resist abrasion and erosion. Use of sacrificial thicknesses of hard surface materials, along with high-tech welding techniques result in extended fan wheel and casing life. Applying hard surface alloys or ceramics along with erosion disruption barriers have extended the life of many induced draft and hot gas recirculation fans. In some cases, heat treating and special welding technologies are recommended for stress relief in these types of abrasion resistant fan impellers.

The leading edges of axial fan blades can