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power increase rather than a power reduction if the fan is throttled with an outlet damper. Fan performance at 100% capacity is typically reduced slightly due to pressure losses across the dampers in the wide-open position.

Inlet box damper (IBD) control decreases air volume at constant speed with a resistance effect, as well as a flow-modifying effect. Like outlet dampers, the maximum turndown rate is limited by leakage through the damper at the fully closed position. As shown in Figure 32-10, some power reduction can be achieved when inlet box dampers are applied to a fan with a positively sloping power characteristic. The spin produced by the IBD also provides a greater reduction in input power than is achieved using outlet box dampers.

Variable inlet vanes (VIV) are designed to spin the air in the direction of fan rotation, providing better fan efficiency at lower flow rates than simple damper control. Like inlet box dampers, VIVs can be considered to have a resistance effect as well as a flow-modifying effect, although VIVs are more effective as they are usually placed close to the impeller. As shown in Figure 32-11, VIVs usually save more power than do IBDs.

Variable blade pitch control is used for axial-flow fans. Flow is controlled at constant speed by varying the blade angle of the fan to create the optimal

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