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Vsd For Oxidation Aerators


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#1 epsngr

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Posted 24 July 2007 - 07:03 PM

I am currently in a dillema and I thought I should get some help. My client is interested in using VSD for surface aerators which are controlled by DO monitors. My colleagues think that it will be difficult. They would rather stick to fixed drives as we have normally done. I think, there may be some advantage for using VSD but my fears are?

Has it been done elsewhere? Can VSD aerators be controlled by DO monitors without have too much hunting or overheating? How do I size the drives such that a high DO level does not cause too low speed and increased torque.

I need any information I can get. I'll be grateful.

#2 jraef

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Posted 25 July 2007 - 05:08 AM

I have never seen it done, so I cannot speak from experience, but I see no technical reason which would make the concept impossible. There are a few issues however.
  1. What is it you think is the reason for doing it in the first place? Energy savings?
  2. What type of aerator is it? For instance, floating centrifugal? Paddle wheel? Spiral?
  3. How far will it be from the VFD to the motors?
  4. Do you have a PID controller to decide what the desired DO level is to be? (Hint, you will need one)

Assuming you mean a floating centrifugal, then if you want to maintain a DO level at some times that is lower than at other times, then yes it could be a viable energy saver. But is that really a viable idea? Correct me if I'm wrong, but the necessary dissolved oxygen for digestion is a constant, not really a variable parameter. So I don't see why changing the motor speed would affect that. You either have enough motor and surface agitation for the area you are working with, or you don't. Does perhaps the DO level of the influent vary greatly so that you can reduce the aeration to take advantage if it's already high to start with? If so, that might be practical then.
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#3 chaterpilar

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Posted 25 July 2007 - 06:53 AM

The concept is good and is viable if your influent has a large fluctuation in DO.

For example if you are aiming for x ppm of DO and your influent is y ppm less than what you desire then you need more aeration and consequently a higher speed.

If difference is lower then you require a lower aeration.

You may need a DO sensor/Controller which has a facility of 4 to 20 ma output facility which you can tap as analog input to the drive.

The drive can be progemmed to maintain the required setpoint.

We have similar application going for maintaining humidity although energy saving was not the prime objective for that, it was done for moisture control in corn flakes.

#4 epsngr

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Posted 25 July 2007 - 09:44 AM

QUOTE(chaterpilar @ Jul 25 2007, 07:53 AM) View Post

The concept is good and is viable if your influent has a large fluctuation in DO.

For example if you are aiming for x ppm of DO and your influent is y ppm less than what you desire then you need more aeration and consequently a higher speed.

If difference is lower then you require a lower aeration.

You may need a DO sensor/Controller which has a facility of 4 to 20 ma output facility which you can tap as analog input to the drive.

The drive can be progemmed to maintain the required setpoint.

We have similar application going for maintaining humidity although energy saving was not the prime objective for that, it was done for moisture control in corn flakes.


Thanks for comment so far. The driving factor for this is purely energy savings!

The DO level of the influent varies but how much, I do not know just now. Also, the fluactuation of the DO levels based on current fixed speed drives is not available to me but operators say the fixed speed drives stop and start at most twice in 24hrs. I assume that this means the DO fluactuation is not so much at full speed with fixed drives. So the benefit of VSDs may be challnege. The aerators are surface floating centrifuges and will be about 30 meters away from the motors.

A PID controller will be used with setpoints from the DO mA output for controller the VSD speed.

I assume that DO will increase as the pump speed increases and at some point the Speed should show some linearity with the DO levels subject to variations in influent DO levels. Is there any reason will this will not happen? Do you envisage any heating issues with the pump based on narrow DO range? Any torque issue that may affect the Pump?





#5 jraef

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Posted 25 July 2007 - 08:15 PM

The concept appears sound IF the DO levels of the influent do in fact vary widely. If the DO is relatively constant and the motors spend a significant amount of time at full speed anyway, you will actually WASTE more energy in that the VFDs are not 100% efficient themselves. So to accurately model the application you will first need a detailed analysis of the DO level fluctuations, plus an anaylsis of how motor speed affects DO levels. Once you know that, you should be able to use one of the free "energy savings anaylsis" software tools that most VFD manufacturers provide. They are typically set up to look at pum or fan applications, but with a llittle creative effort you should be able to adapt your data to "fool" the program into determining the viability of energy savings here.
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#6 epsngr

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Posted 25 July 2007 - 09:35 PM

Thanks for your input. Talking about efficiency. If I have two drives for this purpose and one is running at Full load speed (rated 15.5KW) while the other is running at 75% (rated 22KW). I reckon that the 75% drives will be more efficient. How about energy consumption in terms of Power Factor, does it matter?

#7 jraef

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Posted 26 July 2007 - 02:52 AM

Power factor becomes irrelevant in VFD applications, but besides, that has little to do with energy efficiency anyway.

But in truth, as far as PERCENTAGES of efficiency goes, the full speed one is going to be more efficient. But it will be more efficient on a higher total power, so the overall efficiency will end up lower. For example, based on my experience the throughput efficiency of the VFD is probably 97% at full speed, and maybe 90% at 1/2 speed, so let's say it is 94% eff. at 75% speed. In a centrifugal load, Power varies by the cube of the speed. So on your motor running at 75% speed, it is using .75 x .75 x .75 = 42% of the power than if it were running full speed. So at 94% eff. of 42% poower, the losses are roughly 2.5% of the consumed power, as opposed to 3% if the motor is running at full speed. So although the numbers may look as though a VFD is more efficient at higher speeds, in fact the opposite ends up being true most of the time.

Of course, it is also doing less work too! The chief benefit of using VFDs on centrifugal loads is in matching the motor speed to the required work load, as opposed to running the motor full speed and modulating the work after the fact. In your aerator application, the only chance you have for modulation is to turn the motors on or off. So back to your scenario, you might save more energy by running both motors at 87% speed!
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#8 epsngr

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Posted 26 July 2007 - 10:08 AM

Thanks very much. i will go now and piece this all together to see what justification I can come up with.




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