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Adjustment of Flow rate

The phenomenon of flow excess or insufficient often occurs in operation, there are many reasons in occurring such phenomenon. If the phenomenon of flow rate sometimes great and sometimes little occurs during operation, the main reason is due to resistance in pipe net sometimes strong and sometimes weak. If it is gradually reduced through a long period or suddenly reduced within a short time, the main reason is that the fan passage or pipe net is pl ed. The phenomenon of flow excess or insufficient will occur during normal running after installing, the main reason is as follows:

1.The actual and theoretic values k of resistance in pipe net is widely different. It was known normal pipe net characteristic equation H=KQ2, where: k is the coefficient of resistance, if the actual value k is less than the theoretic value k, the flow rate increases; vice versa, flow rate decreases, see Fig. 1 (b)

2.The effect of fan total pressure deviation △p have not been considered during selecting. when the actual total pressure of fan is a positive deviation, the flow rate increases, and when it is a negative deviation, the flow rate decreases, see fig 1 (a).

Fig.1  Pipe net characteristic curve

The flow rate excess or in sufficient occurred after fan newly installing and putting into operation, one of the following methods can be used to eliminate:

(1) Flow rate can be adjusted by damper.

(2) Increasing or decreasing the speed of the fan.

(3) Changeover a higher or lower pressure fan.

(4) Changeover pipe net to reduce the coefficient  k of resistance.

It must be pointed out: flow rate is normally adjusted by damper, but when the actual flow rate is much greater than the required, it wastes power, it is not economical, if the condition permits reducing fan speed or changing-over a lower pressure fan is normally used.

When the damper is full open, the flow rate is still insufficient, at this moment, the damper has lost its function, so try to reduce the resistance coefficient of pipe net, or increase the speed of the fan, or change-over a higher pressure fan, but the fan driven by motor and coupling can not be normally changed the speed, only the fan driven by belt, the speed can be increased and decreased by the means of changing the size of belt pulley diameter, the max. speed must be lower than the performance and optional part table required.

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