Fan Selection and Control

Many fan designs are designated based on their typical field of application. Examples include:

• Ventilating fans, are typically designed for clean-air service at normal temperatures. Heavy-duty ventilating fan designs may be selected for application under severe conditions.

• Industrial exhausters are typically designed with a focus on simplicity, durability, and the ability to withstand corrosive environments.

• Pressure blowers are designed to withstand the high tip speeds required to develop high pressures.

• Mechanical draft fans are designed for the capacities, temperatures, and environmental conditions encountered in a variety of combustion systems.

The fan specification should provide the supplier with necessary information regarding performance, service, arrangements, duct layout, size of the connecting ductwork, and any unique site-specific conditions. The fan output must be sufficient to overcome the losses caused by the various system elements. In an open system, differences in kinetic energy between inlet and outlet must also be considered. The effects of leakage, heat transfer, pressurization, and other fans operating in the system must be taken into account. Factors that affect air or gas density, such as barometric pressure, temperature, and relative humidity, must be considered, as well as any entrained material such as dust.

Fans can operate at any flow rate, from zero to maximum (free air delivery). Performance of a fan in a given application depends on the system characteristics and the fan pressure characteristics. Fan performance rating is a procedure, based on the fans laws, that permits the determination of variables such as flow rate, specific output, gas density, fan size, speed, input power, sound power level, and

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