This essay is written with dust collector application in mind. A bagfilter with airflow capacity of more than 600,000m3/hr and higher may require that it be driven by a combination of 2 or more centrifugal fans instead of single bigger fan. A dust collector will typical operate at static pressure of 500mmWG, and at high range airflow application, it is virtually impossible to configure a centrifugal fan that achieves this specifications (airflow and static pressure combination) without compromising the fan operational stability. An inexperienced engineer may select a bigger sized fan that operates at slower speed. This may appeal in qualitative sense, but is likely to generate higher static pressure than is required, sending precious power into pressure factor more than is necessary. This in turn makes it harder to achieve the desired airflow.
Two centrifugal fans operate in parallel are actually equivalent to a Double Inlet Double Wheel (DIDW) centrifugal fan. The fan wheel is doubled only because a single impeller with oversized blade width makes the fan unstable to operate. If a single fan is a must, in theory it should be fan with same diameter and blade leading and trailing angles, yet with double fan width. Only then that the fan can churn out double airflow, while maintaining same static pressure that one of the fan can generate.
Engineers tend to believe the myth that it is possible to achieve combined airflow and static pressure by manipulating fan size and speed. This is only true up to certain range of airflow. Massive sized centrifugal fans are bound to give out higher static pressure, only suitable in applications where the high static pressure is demanded.