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Introduction to Fan Systems

Fans¹ are widely used in industrial and commercial applications. From shop ventilation to material handling to boiler applications, fans are critical for process support and human health. In the manufacturing sector, fans use about 78.7 billion kilowatt-hours² of energy each year. This consumption represents 15 percent of the electricity used by motors.³ Similarly, in the commercial sector, electricity needed to operate fan motors composes a large portion of the energy costs for space conditioning.

Performance may range from "free air" to several pounds per square inch gage (psig)⁴, with airflow from a few cubic feet per minute (cfm) to more than 1 million cfm. Pressures above 15 psig generally require air compressors, which are addressed in a separate sourcebook titled Improving Compressed Air System Performance, A Sourcebook for Industry.

In manufacturing, fan reliability is critical to plant operation. For example, where fans serve material handling applications, fan failure will immediately create a process stoppage. In industrial ventilation applications, fan failure will often force a process to be shut down (although there is often enough time to bring the process to an orderly stoppage). Even in heating and cooling applications, fan operation is essential to maintain a productive work environment. Fan failure leads to conditions in which worker productivity and product quality declines. This is especially true for some production applications in which air cleanliness is critical to minimizing production defects (for example, plastics injection molding and electronic component manufacturing).

In each case, fan operation has a significant impact on plant production. The importance of fan reliability often causes system designers to design fan systems conservatively. Concerned about being responsible for under-performing systems, designers tend to compensate for uncertainties in the design process by adding capacity to fans. Unfortunately, oversizing fan systems creates problems that can increase system operating costs while decreasing fan reliability.

Fans that are oversized for their service requirements do not operate at their best efficiency points. In severe cases, these fans may operate in an unstable manner because of the point of operation on the fan airflow-pressure curve. Oversized fans generate excess flow energy, resulting in high airflow noise and increased stress on the fan and the system. Consequently, oversized fans not only cost more to purchase and to operate, they create avoidable system performance problems. The use of a "systems approach" in the fan selection process will typically yield a quieter, more efficient, and more reliable system.

¹ For the purposes of this sourcebook, the term "fan" will be used for all air-moving machines other than compressors.

² United States Industrial Electric Motor Systems Market Opportunities Assessment, U. S. Department of Energy, December 1998.

³ Ibid.

⁴ At standard conditions, a column of water 27.68 inches high exerts 1 psig of pressure. Equivalently, 1 inch of water gage = 0.036 psig.