VFD for Blower

[bks2000]

Does anyone have a suggestion on how to prevent a blower or compressor from going into surge in a VFD application? Since the machine operates at different speeds, just looking at one pressure won't be enough. Do we also have to match speed to the pressure (or flow) at each speed curve? There must be an easier way! Can anyone help?

 

[mharris]

A couple of things to think about. What is your VFD designed to do? Control flow or pressure. Depending on this, you may need to add a small slip stream back to the inlet on a PID control loop that helps maintain your desired parameters.

 

[bks2000]

It's designed to control flow. One additional item: I would rather use blowoff than inlet guide vane throttling, since the IGVs have been disabled for a long time.

 

[vanstoja]

To avoid surge in centrifugal compressors and blowers always operate at at a flowrate which is higher than the "surge-line" flow. The surge line is generally determined by impeller incidence angle and is more or less independent of rotational speed. For example, if surging is initiated at 30% rated flow at maximum running speed, it will occur at or near 30% of the corresponding rated flow at lower running speeds where rated flow at any running speed is directly proportional to the speed fraction.
Surge in compressible flow turbomachines is generally preceded by blade stall as flow is reduced from rated flow. Since airfoil blade stalling usually occurs at inlet positive incidence angles of roughly 12 to 18 degrees, surge can be expected at incidence angles higher than that.

 

[imok2]

bks2000
Surging is the result of the machine exceding it's rated pumping head, you either have to reduce the head pressure or increase the suction (back) pressure.

 

[fredt]

vanstoga's description of what happens in the blower is correct. The nature of the problem depends on the characteristics of the system resistance. The maximum pressure or the point at which the blower will stall is proportional to the square of the speed.

If you have a system where the pressure is more or less constant as flow rate changes then the blower will stall at some point as you reduce speed. Unfortunately in these cases VFD is simply an inappropriate control. If this applies in this case will have to reinstate the IGV system.

VFD is ideal where the system resistance varies as the square of flow but are definitely not suitable in cases where the pressure does not drop off substantially with flow.