It may be necessary to decide between two or more fan types when sizing a fan based on its operating characteristics. If the operating point (BP) does not optimally match the design point of the fan, another, more efficient fan in the BP is often preferable. There are, of course, other criteria for selecting a fan type. These include the medium to be conveyed, the acoustics and the installation space.
In the past, adjustments in fan characteristics were typically bypassed, e.g., by throttling or bypass control. In modern times, speed control or individually adjustable fan arrays are increasingly used (also referred to as fan wall), which can be used to precisely control the operating points.
An additional throttling to the system-plant characteristic as described in figure 4.2. as well as a bypass control do not change anything on the fan itself, but only influence the steepness of the system characteristic. Since both types of control involve losses, they should only be used for fans of lower 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: Adjusting the operating point