Install VFD on positive displacement blower?? 1

[Brian2903]

My client is using 4 positive displacement blowers with timer control in his wastewater treatment plant to provide oxygen to the process. Recently, he proposed to install VFD on these blowers to control the amount of air flow and reduce energy consumption. Since the blowers are supplying constant discharge pressure to the process, it seems to me that it is impossible to adjust the speed of the blower by using the VFD. I was thinking about installing a inlet vane in additional to VFD so that the blower can be reduce in speed as well as maintaning the specific discharge pressre. Would any one has comments to this setup? Thanks in advance!

 

[BigInch]

We had unloading valves on IC engine drive compressors. They're variable speed.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[CH5OH]

the unloader valves for IC motor driven compressor are there to cover 0 to idler speed IC motor
(to keep motor running with 0 flow)

 

[BigInch]

We did't use them like that.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[MikeHalloran]

One possible snag: If the system is equipped with reactive silencers, a VFD may change its noise signature.



Mike Halloran
Pembroke Pines, FL, USA

 

[MartinSr00]

The problem with positive displacement pumps/blowers, etc., is that the flow remains relatively constant, while the pressure, horsepower and torque are all dependent on the system.

If you take a positive displacement pump with minimal slip and dead head it, it will increase pressure until the motor stalls or something breaks.

You need to determine how motor maximum torque is reduced as speed is reduced to get a pump curve. Then you need to determine how system pressure behaves with decreasing blower speed.

This second item changes depending on what you do. If you have a backpressure valve controlling pressure, maybe system pressure stays constant meaning that torque increases as the blower motor slows down. If there's a fixed orifice or normal system resistance then pressure drops as the square of the speed, and the torque reduces. I'm fairly confident that at decreased VFD speed, maximum motor torque will decrease. So, a backpressure valve may not work well with a VFD. A fixed orifice probably work quite nicely.

Until you work this out, I doubt you can definitively answer the qustion "will it work" without an after-the-fact test.

 

[BigInch]

Flow is dependent on the system.
Both centrifugal and PDs have pump "curves", see attached, that can vary significantly with RPM. Of course the curve in a PD pump is more of a straight line with negative slope.
While it is true that a pd pump running at constant speed has relatively little variation in flow (when talking about a "normal" system curve), varying RPM affects flow in a centrifugal pump, a pd pump and the system.
The operating point depends on BOTH pump and system characteristics for either type of pump.
The only problem with a PD pump is there is no problem.
Draw the curves for variable speeds and you will see that you can get from 0 to runout flows in either type of pump.

Because of the relatively limited flow range with a fixed speed PD pump, a flow control valve on a fixed speed PD is virutally useless, which is why most drives for PDs are variable. A variable speed device combined with a control valve is belt and suspenders in most cases. Simply not needed. RPM can be varied to accomplish the same thing. jThey might be of use, if system head changes radically and fast, where a valve might be adjusted more quickly than the motor rpm, but I would think that circumstance is probably rare. A pressure control valve (used as a relief valve), does make sense.

It would also make little general sense to use a valve with a centrifugal pump with a VFD. Belt and suspenders again.
Flow control valves with a fixed speed centrifugal pump does make sense, due to the high flow variations possible, but used a a pressure control valve, probably is not necessary, as long as the system will tolerate the pump's shutoff pressure.

The secret to knowing what type of pump to use and what valve, if any, or if you need VFD or not, is to look at BOTH the system curve(s) and the pump curve(s), whether its a centrifugal or PD pump, the answer is there, right at the operating point(s) <where both curves intersect>.

If you look at the attachment of curves for a generic PD and centrifugal pump, combined with the flowrates you know that you must be able to run, everything becomes quite apparent.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ischgl99]

In my plant, we use VFDs on blowers to maintain sufficient pressure in the system, but use control valves in each aeration basin to control the DO level. Basically the valves have their own PID loop to provide the correct amount of air and the VFD controls to a pressure setpoint to ensure there is enough pressure in the header. There is also a look ahead loop tied into the VFD in case the valves need to open quickly to adjust for changes, the VFD can start increasing the pressure sooner.

If your customer has one aeration basin, then controlling only the VFD should work, but if he has more then one and wants to independantly control the level in each tank, I think using the control valve with the VFD controlled blower would work better.

 

[BigInch]

If you have a widely varying air requirement, I would agree with that application. Apparently you do, since you operate several independelnt tanks off of one pump. Once again, the key requirement shows up ... the widely varying flow.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/