System Characteristics

System characteristic changes (deviation) from the design point result from:

• Adjusting UG system regulators (adjustment of regulators nearest to the fan has the greatest impact).
• Changes in the UG system, such as: added or deleted airways, airway deterioration, roof falls, water accumulation, etc.
• Errors in calculation of the system pressure loss (design duty) upon which the fan was selected.

  Changes in or deviation from the system characteristic at the design point may result in:

  Fan Characteristics

  Appropriate changes to the fan characteristic result in optimum fan efficiency and performance. Changes to the fan characteristic can be accomplished by one of or a combination of the following measures:

  • • Fan stalling (fatigue damage due to stalling accumulates with repeated or sustained stall and can result in catastrophic fan failure).
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    • Fan performance other than that anticipated when the fan was initially selected (less flow, higher pressure, greater power consumption due to reduced fan efficiency).

    Reducing or increasing the fan’s rotational speed respectively reduces or increases the height of the fan’s characteristic curve (as exhibited on the example curves below). The fan efficiency contours move in coincidence with the individual curve. With that, optimum efficiency is maintained across a broadened range of duty points.

    Air flow varies directly as the change in speed. That is:

    (RPM2 / RPM1) Air Volume 1 = Air Volume 2

    Ex.: (1800 RPM/900 RPM) 100,000 cfm = 200,000 cfm

    Pressure varies as the square of the speed change ratio. That is:

    (RPM2 / RPM1)² Pressure 1 = Pressure 2

    Ex.: (1800 RPM/900 RPM)² 4-in. w.g. = 16-in. w.g.

    Power (HP) varies as the cube of the speed change ratio. That is:

     

    RPM2 / RPM1)³ HP 1 = HP 2

    Ex.: (1800 RPM/900 RPM)³ 63 HP = 504 Air HP (exclusive of fan efficiency)

    Reducing or increasing the number of fan blades (altering solidity) respectively reduces or increases the height of the fan’s characteristic curve (as exhibited on the example curves below). The fan efficiency contours move in coincidence with the individual curve. With that, optimum efficiency is maintained across a broadened range of duty points.

    • Varying the rotational speed of the fan impeller (rotor) – Example Curves Below
    • Varying the blade angle (axial flow fans). – Exhibited on the example curves below.
    • Varying the number of fan blades (i.e. "half- blading" axial flow fans) – Example Curves Below
    • Employing an inlet control device such as inlet vane control (common to centrifugal fans, but not generally recommended for axial flow fans)