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VFD Can Replace Regulating Valve and Air Damper

Regulating valve and baffle are installed in the process pipelines to change the sectional area of pipe, so as to control the flow of fluid in the pipeline. When VFD is used to drive the motor and then drive the fan and water pump, the flow of fluid can also be changed as the rotation speed of equipment changes. In a word, the two have a same function, but they use different methods to control the fluid flow. For pressure loss, variable frequency drive is better than the regulating valve. Therefore, VFD can be used to replace the regulating valve and air damper in the constant flow control applications, such as fan and pump.

When applying VFD on the boiler blower and induced draft fan, the air dampers on original blower and induced draft fan can be replaced.

Regulating valve is generally used to control the water supply in the automatic water supply of boiler, a stand-by pump is also prepared. For such a single boiler, it is feasible to use the VFD to control the water supply pump and replace the original water supply control valve.

Question
VFD can drive water pump in two ways: one to one or one to two. In actual application, if the air supply requirement is not huge and the water pump may run at 20Hz, can it ensure water supply?

Answer
Certainly. When replacing the regulating valve with VFD in a 10t/h-75t/h boiler, lower frequency of the pump VFD was originally set to be 10Hz for safety consideration. But when it was put in use, the controller still fed water to the boiler when it didn’t have any output under a small air supply load due to its low frequency of 10Hz. Seriously, water level of the boiler could even be 100% or higher. Later, when the lower frequency was set to be 0Hz, the water has been normally supplied for many years while the water level keeps stable at the setting value. Actually, the full stroke time of an electric control valve (DN50-DN80mm) is between 16-25s, and the acceleration and deceleration time of pump VFD can be set to be lower than above time, so there’s no need to worry about water supply. On the premise of guaranteeing water supply, the acceleration and deceleration time of VFD should be set longer, so as to help the pipeline to overcome water hammer effect, which occurs in fast acceleration time, and the cavitation phenomena which occurs in fast deceleration time.

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