The exhaust flow of the multi-blades centrifugal fan is too big and it needs to be reduced. Three kinds of retrofit schemes are put forward for policymakers to choose and use in this paper. Scheme one, one change was made between the motor and fan transmission speed to reduce the air displacement. Scheme two, the rotor blades are shielded to reduce the exhaust flow of the multi-blades centrifugal fan. Frequency conversion technology is adopted to reduce the fan speed in the third scheme, which also can reduce the air displacement. The first two schemes are simple and practicable. They save retrofit cost, but not bring long-time power saving benefits. Using the frequency conversion technology to reduce the fan speed needs a high retrofit cost, but the motor power consumption can be saved and the fan noise can also be reduced.%多翼式离心通风机排气流量太大，需要降低排气量，这里提出三种改造方案，以供决策者改造选择使用。方案之一改变电动机和风机之间传动转速比降低排气量；方案之二遮挡风轮叶片减少多翼式离心通风机排气量；方案之三用变频技术降低风机转速减少风机排气量。这三种改造方案里前两者简单易行，节约改造费，但没有长期的节电效益，用变频技术降低风机转速虽然改造投入较大，但可以节约电机耗电量，也能降低风机噪音。
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