Fans are constructed from many different materials, and like everything else, there are pros and cons to any option.
- Heavy solid-blade fans may experience extended starting times due to the high inertia load that the motor must accelerate to speed. It is not unusual to have electric motor overloads trip from extended starting times when applied on high inertia propeller fans. With increasing market pressures of energy conservation, fans generally are applied with less of a safety factor between motor size and fan requirements.
- Hollow construction fan blades offer thick airfoils that move large volumes of air, yet the blades are lightweight and relatively stiff.
- Fiberglass construction provides corrosion resistance for applications in chemically aggressive environments.
- In today’s market, blades are being constructed from aluminum alternatives. For example, many heat exchangers and cooling towers operating near the ocean now use aluminum fans. Several manufacturers offer fan blades constructed of marine-grade aluminum, which provides good corrosion resistance. For more aggressive environments, special coatings can be applied to aluminum blades, offering acceptable fan life at a lower initial cost compared to fiberglass blade construction.
- Wet cooling towers can have airborne water droplets that are surprisingly abrasive to a fiberglass fan blade’s leading edge. Most molded blades have a parting line at the leading edge. This area is especially susceptible to erosion damage due to trimming material that may be caught in the mold’s parting line during blade fabrication.
- Blades constructed with epoxy resins have more natural erosion resistance than blades using vinyl ester or polyester resins, but epoxy resins are less desirable to work with from a manufacturing perspective. Make sure fiberglass fan blades with vinyl ester or polyester resin materials have a barrier material – either external or internal to the fan blade – to protect against erosion at the leading edge.
- A high operating temperature generally is not a limiting factor for wet cooling towers, but it can be a factor for induced-draft air-cooled heat exchangers and other equipment. Fiberglass fan blades begin to experience reduced mechanical properties at much lower temperatures than aluminum fan blades. A recommended maximum temperature for fiberglass construction might be 180°F (82°C) while aluminum blades can perform satisfactorily at 300°F (149°C). Be sure to consider the maximum temperature the fan can experience when the process is operating but the fan is off.
Fan Construction for Explosive
Environments fan applications may involve the handling of potentially explosive or flammable particles, fumes or vapors. Such fan applications require careful consideration of all system components to insure the safe handling of such gas streams. The following should be noted:
- All parts of the fan in contact with the air or gas being handled shall be made of nonferrous material. Steps must also be taken to assure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components.
- The fan shall have a nonferrous impeller and nonferrous ring about the opening through which the shaft passes. Ferrous hubs, shafts, and hardware are allowed provided construction is such that a shift of impeller or shaft will not permit two ferrous parts of the fan to rub or strike. Steps must also be taken to assure the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components.
- The fan shall be so constructed that a shift of the impeller or shaft will not permit two ferrous parts of the fan to rub or strike.
- No bearings drive components or electrical devices shall be placed in the air or gas stream unless they are constructed or enclosed in such a manner that failure of that fan component cannot ignite the surrounding gas stream.
- The user shall electrically ground all fan parts.
- The use of aluminum or aluminum alloys in the presence of steel which has been allowed to rust requires special consideration. Research by the U.S. Bureau of Mines and others has shown that aluminum impellers rubbing on rusty steel may cause high intensity sparking. The exact method of construction and choice of alloys is the responsibility of the air blower manufacturer; however, the customer must accept both the type and design with full recognition of the potential hazard and the degree of protection required.
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