4.2.1. Throttling and Bypass Control

Additional throttling or bypass control to the system-plant characteristic curve does not change anything on the fan itself, but only influences the slope of the system characteristic curve (cf. Figures 4.2.1.a and 4.2.1.b). Since both types of control involve losses, they should only be used for fans of smaller capacities or reductions. These methods are recommended if frequent or continuous power adjustment of the fan due to changing operating conditions, e.g. by speed control or pole-changing motor, or by changing the fan impeller / housing would be very costly and thus uneconomical. Whether throttling or bypass is to be selected depends on the course of the power requirement between the operating points.

Figure 4.2.1a: Adjusting the operating point (forward curved fan)

 
If the volume flow is to be throttled by q_{VI} to  q_{VII} without reducing the fan speed, an additional resistor in the form of a throttle valve, orifice plate or similar component is usually installed in the system.
From the figure it can be seen that a pressure drop of the size \Delta p_{tot} must be installed in the system in order to reduce the volume flow to  q_{VII}. The plant characteristic curve I thus changes to II. The actual operating point is thus shifted from B_I to  B_{II}. The gridded area between the points  B_I and  B_{II} as well as the ordinate is a measure of the throttling loss that must be additionally applied by the fan's motor.

Figure 4.2.1b: Adjusting the operating point (backward curved fan)