Mine fan application and operational basics

****This article was first published by Phil Blankenship on the 15th of January 2018 on Linkedin.

Mine fan application and operational basics matching the fan to the mine air system

The fan will perform as the system allows

Fan performance, which is defined as the possible combinations of flow and pressure that a given fan can produce, can be graphed, conventionally, with flow on the horizontal and pressure on the vertical axis. This graph represents the fan characteristic.

The characteristic of the system, which in most cases takes the form of a parabola where pressure is proportional to the square of the flow, can also be drawn on this graph.

The point where these two curves intersect is the only one at which the fan will (if not can) operate with this system. To vary the flow through a mine’s air system, either the fan characteristic or the system characteristic must be altered.

  • Written by

    Howden

  • Date posted

    22nd January 2018

Fan Characteristics versus System Characteristics

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

      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:

      • Varying the rotational speed of the fan impeller (rotor) – Example Curves Below
      • 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)

      • 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
      • 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.

      • Employing an inlet control device such as inlet vane control (common to centrifugal fans, but not generally recommended for axial flow fans)
Ventilation on Demand Control room
Ventilation on Demand Control room
Ventilation on Demand Control room
Ventilation on Demand Control room