Fans often serve over a wide range of operating conditions, many of which can be below the maximum design capacity. This requires the regulation of flow during times of low demand. Various approaches can be used for flow regulation, including fan speed control, use of inlet vanes, and use of inlet and outlet dampers. These approaches have their respective advantages and drawbacks, and optimal choice depends on the operating conditions prevailing in the system.
Dampers provide flow control by changing the restriction in the path of an airstream. By increasing system resistance, dampers force fans to operate against higher back-pressure, which reduces their output forces them to operate away from their best efficiency points. These conditions increase both operation and maintenance costs. However, use of dampers can als help improve system efficiency by isolating parts of the system, or by providing more favorable start-up conditions for fans.
Fan rotational speed adjustments provide the most efficient means of controlling air flow. By reducing fan rotational speed, less energy is imparted to the air- stream, which means less energy must be dissipated by the system airflow-control devices. Variable speed drives (VSDs) are one of the two methods that can be used to control fan rotational speed – the other one being less flexible and in times more costly option of using multiple-speed motors.
VSDs allow fan rotational speed adjustments over a continuous range, avoiding the need to jump from speed to speed as required by multiple-speed fans. VSDs include several different types of mechanical and electrical systems. Mechanical VSDs include hydraulic clutches, fluid couplings, and adjustable belts and pulleys. Electrical VSDs include eddy current clutches, wound rotor motor controllers, and variable frequency drives (VFDs). VFDs are by far the most popular type of VSD, largely because of their proven effectiveness in reducing energy costs. VFDs decrease energy losses by lowering overall system flow. By slowing the fan and lessening the amount of unnecessary energy imparted to the airstream, VFDs offer substantial savings with respect to the cost-per-unit volume of air moved. However, use of VFDs may not be suitable (e.g. high static pressure requirements) or may not offer the best solution certain conditions (e.g. systems with small and infrequent demand fluctin) (DOE, 2003. p.44-46)