Friction charts

Charts have been published in various text books or the guides of the major institutions and societies which produced results without the need for tedious calculations. In former times they gave the frictional resistance in ins. w.g. per 100 ft of straight duct. Reading from volumetric flowrate in ft3/min across hori­zontally to the duct diameter line in ins., a vertical line projected down to the bottom scale gave the friction. The velocity in ft/min could also be determined.

More recent versions have been converted to the SI units with flowrates in m3/sec, duct diameters in mm or m, velocities in m/s and friction in Pa/m.

All these published charts look very similar, especially if the same units are used. Before accepting any particular version it is wise to check at small and large diameters to see what differ­ences are present. Note especially that there will be areas of the chart which are close to the stated formula whilst at the ex­tremities they are less accurate. Some charts show the pre­ferred areas shaded. That shown as Figure 3.54 is as good as any.

The author’s reaction is that, in an age of computers, it is just as easy to return to the classical formula, inserting the value of fric­tional coefficient appropriate to the relative roughness and Reynolds Number as obtained from the Moody chart in Figure 3.13. Table 3.1 has been compiled for a range of duct sizes and velocities. It will be noted that for all the velocities encountered in ventilation systems the flow quality is in the transitional zone where f is not a constant. The variation of f for both a constant duct size and a constant velocity is considerable.

Posted in Fans Ventilation A Practical Guide

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