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Jeamon (Civil/Environmental) (OP)
12 Feb 10 14:18
Hi,

We are working on the design of the aeration tanks for a conventional activated sludge plant and we are considering using a common blower system using either turbo blowers or PD blowers.  Our control philosophy will be:
 - Blower speed is controlled by pressure in header of common manifold.
- Air supply to aeration basins will be controlled using control valves, which are modulating air supply based on DO in basin.

I'm wondering if anyone has any experience with this style of operation and if they can recommend a type of control valve to vary the air flow rate in the basins?  I'd like to use something other than a butterfly valve to give better flow control (i.e. globe valve or v-notch ball valve).

Thanks
 
bimr (Civil/Environmental)
12 Feb 10 17:01
This subject was discussed in thread 161-256801.

Unfortunately, at the low pressures (10 psig or less) in aeration systems, the control of air using a valve is difficult. That is because air is compressible and you can not take enough pressure differential across the valve to control the flow.

For example, if you are controlling water flow with a valve such as a globe or a v-notch, you would need to have a pressure drop across the valve of 2-3 psig to obtain precise flow control.

Valves in this application will be limited by the operating conditions to on/off operations, not flow control.



 
oldengineer1949 (Civil/Environmental)
16 Feb 10 15:08
Jeamone,

I would suggest controling flow from the inlet side of the blowers.  You will not be wasting energy by pumping air to the discharge manifold and then resticting it to control volume.
sjohns4 (Civil/Environmental)
18 Feb 10 9:23
Jeamon,

I was going through the same thing a few months ago. From what I've gathered, bimr is correct, it's difficult to do but not impossible. Butterfly valves are the most common way of throttling air flow. Make sure you spec a butterfly valve that is designed to take the heat from the blowers, not your typical AWWA valve. Also, consider the actuators, the wrong actuator connected to the wrong PLC program will wear itself out in a very short time.

I would make sure your integrator has a long list of good verifiable references for similar projects. Require this in the bid documents and call the references. I ran across a company called NE Controls a local rep put me in contact with. Blower and DO control is what they do. You can Google them, they are in Syracuse NY, ask for Harvey. He had spotless references on new instllations and fixing old ones, all over the country.

Since your designing new tanks consider this - can you use one small blower per tank? This was suggusted to me by the PD rep, his qoute for blowers and drives for six small blowers was less than three large blowers. My project was a retrofit so I had to work with what was there, and the piping was not arranged for this to work. Danfoss VFD's will accept a direct signal from a DO probe and have a DO set point, so you can eliminate the PLC. Kaeser PD blowers come in all weather enclosures so you can mount them outside. I'd have them in a single location with dedicated discharge pipes all interconnected with valves for redundancy. I'm sure there are other VFD and blower manufacturers that can do the same, those are just the two I'm familiar with.  

PD vs. single stage turbo - I base bid PD's with VFD's and had single stage turbos with soft starts as an alternate. The mud slinging from each side got down right ugly. We have nearly the cheapest power in the whole country around here, so the PD's one on the life cycle cost. If you find yourself doing the same analysis look hard at the maintainence requirements of the single stage. Talk to people that have had them a while. I believe there is some normal maintainence that is required that a factory rep will have to do so account for those costs. I didnt look that hard into it b/c the PD's had already emerged as the most cost effective in our case.  

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