centrifugal and PD blowers in parallel

[aquabueno]

I'm looking for the best way to improve energy efficiency for a wastewater treatment plant activated sludge aeration system. The system currently consists of 3 identical constant speed centrifugal blowers (2100 scfm @ 6.0 psig), typically operating 2 at any given time. These can not be turned down, because they would not provide the required static pressure to aerate the basins.

My question is, how difficult would it be to operate one of these fans in parallel with a variable speed positive displacement blower, that would be rated for the same conditions?

Thanks in advance for any feedback.

 

[ccw]

Hi aquabueno,

I think more information is needed here. When you say parallel, are you on separate but parallel delivery and intakes all the way down to and including the basin discharge outlets?

Are you trying to trim airflow delivery with the variable speed positive displacement blower? Why not do this trim with delivery diverters and/or throttling valves off of the centrifugal blowers?

 

[aquabueno]

The blowers would discharge to the same header. The purpose of having a VFD on the PD blower would be to provide a variable airflow that would be controlled with a dissolved oxygen feedback loop from the aeration basins. At times the PD blower would be able to provide all of the aeration required. During peaks one of the centrifugals would provide a relatively constant "base" airflow, and I'd want the PD to vary output to satisfy DO in the basins.

The reason I don't want to throttle the existing centrifugals is that this will save relatively little energy. I'm looking for the most energy efficient solution.

I'm looking for someone that knows if PD blowers and centrifugal blowers can be used simultaneously to discharge to the same network. I'm looking for pitfalls to avoid, control issues, overall feedback on the technical feasibility of doing this.

 

[ccw]

Hi aquabueno,

I am not the "someone". But, for what it is worth, I think you are on the right track in your assessment. It seems one would have a cleaner shot at success if the VFD/PDB discharge was on a separate manifold alway out to and including the basin discharge, ie. a separate sparger. Dumping the PDB into the centrifugal's manifold could be problematic. How do you keep the 2nd and 3rd Centrifugal isolated when it is not online?