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!

 

[itsmoked]

Given the know how and correct sensor for feedback a VFD can control the pressure.

Are all the blowers going to be running in parallel or do they run their own aerators?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Brian2903]

When VFD reduces the speed of the blower, isn't the DP will also reduced? Since the process requires constant discharge pressure, I was thinking VFD alone cannot meet the goal.

The plant has total of 4 blowers, 2 aerators. Each blower has an independent run/cycle timer. They typically run one blower in each aeration tank continuously, and then maybe cycle a second blower on/off during the day.

 

[zdas04]

The fluid mechanics within a blower is really complex. The short answer is the discharge pressure is controlled by the discharge resistance to flow. If there is a backpressure valve (for example, this scenario is easier to get your head around than pipe friction creating the required backpressure) on the outlet of the blower that maintains a constant blower-discharge pressure regardless of the flow rate then the blower will overcome that pressure if its driver has enough hp. So if you slow it down, it will move less gas at the same discharge pressure (because of the backpressure valve)--it does this by packing more gas into the process.

A VFD can be very effective on a PD machine. The reduction in power consumption is much less than it would be with a dynamic machine (like a fan or centrifugal compressor), but it can still be substantial.

David

 

[Brian2903]

Thanks David. Thus, in short, the PD blower requires a VFD and some kind of inlet or outlet valve to mainten the required discharge pressure.

A VFD itself by slowing down the speed of the blower motor would not satisfy the discharge pressure requirement. Am I correct?

 

[btrueblood]

"A VFD itself by slowing down the speed of the blower motor would not satisfy the discharge pressure requirement. Am I correct?"

Answer is "maybe, but maybe not", the depth of the aerator outlet and/or its opening hole sizes may do more to limit the discharge pressure than anything the blower does, to some limiting lower speed limit for the blower. Get a good set of operating curves and/or software from the blower manufacturer and run various cases to see how far down (in RPM) you can run it and still make bubbles come out of the pipes.

 

[MartinSr00]

You need to develop a speed torque curve for the VFD/motor combination and then work out the speed/torque curve on the pump side operating in the system.

The VFD manufacturer can probably give you a good idea on the motor side.

On the pump side you, the blower vendor can't tell you this without knowledge of your system.

Without knowing the information above, success will be a crap shoot.

 

[zdas04]

You misundertook me. I was using a backpressure valve as an example of something that would maintain a constant discharge pressure, not as a design recommendation.

Let's say that the reason you need a particular pressure is to overcome hydrostatic pressure as you blow air into the bottom of a pond for aeration. The blower will overcome the pipe friction and the hydrostatic head at any speed, but the friction component will change dramatically as you change your flow rate. It is still the same thing, the compressor will keep increasing its discharge pressure until either: (1) it overcomes resistance to flow; (2) it runs out of hp; or (3) it lifts a PSV (or breaks something). One of those three things has to happen at every speed.

David

 

[BigInch]

Brian, I believe your comment/question,

"When VFD reduces the speed of the blower, isn't the DP will also reduced? Since the process requires constant discharge pressure, I was thinking VFD alone cannot meet the goal.", shows you have a reasonable handle on the logic and effects of installing a speed control and that itis totally correct. We engineers should know that you don't get a free lunch; everything has a disadvantage that comes along with its supposed advantage, so don't let anybody change your mind without solid calculations to back it up.

A speed control (taken by itself) has a tendency to reduce BOTH discharge pressure and flow. If you need both of those functions, there could be a big advantage to using a VFD. If you don't, there may be only disadvantageS. If your system curve shows that it also reduces its pressure requirements when flow reduces, it could be a good candidate for speed control. If you system curve shows that you need constant pressure when flow reduces, be very weary of speed controls.

If you install a vane (valve) with a speed control, in effect you will have to run at a higher rpm to increase the pressure by at least an amount equal to what the vane will lose. Essentially that is the same manner that a fixed speed blower without speed control works, the only difference may be that you might be able to run at a somewhat lesser speed than blower's rated fixed speed. Maybe not. You might wind up having to turn up the speed to almost the same or even higher than fixed speed, if you're near your max flowrates. Two big disadvantages! If you need to run at nearly fixed speed with the vane installed, you may save nothing. That's the question you need to answer, however with fixed pressure requirements for all flows, the likelyhood you will save power and money with speed controls is very doubtfull.

Check it carefully.




**********************
"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]

if you talking about a positive displacement blower,
the discherge pressure isn't defined by the rpm, but rather defined by the system back pressure (provided flow>internal leakage).

 

[zdas04]

That's what I said above, seems like the rest of the people discussing this topic believe in magic.

David

 

[BigInch]

Please kindly explain the difference in discharge pressure and system backpressure and how they are not one and the same.

**********************
"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/

 

[BigInch]

are you distinguishing between the maximum cylinder or diaphram pressure and the pressure in the container just before the cylinder gas volume is discharged into it?

**********************
"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]

discharge pressure (pump) needs to be bigger than system pressure.If so flow is established.So for a blower to deliver air,it needs to overcome the system pressure
flow is oriented away from pump discharge,system pressure is oriented towards pump discharge pressure,hence system back pressure,pump discharge pressure.

 

[BigInch]

Considering a short segment of pipe immediately following the PD pump discharge into the system, the resulting pressure in that segment is a function of three things, the initial pressure in that segment, the mass of gas leaving and the mass of gas entering during the PD cycle time.

During the time of one PD cycle, if certain conditions are met, a specific mass of gas is ejected into the system segment. Yes, pressure in the PD must be slightly higher than system inlet pressure in order for the discharge valve to open. The critical question becomes, how much higher.
Well, the pressure must be higher by 1.) enough to open the valve; we'll say a pretty small amount there, and 2.) The cylinder pressure must be high enough such that, if the gas is ejected into the system segment, the resulting pressure of the segment equals the initial downstream pressure plus the increase in pressure due to the entering mass of gas from the compressor, minus the decrease in pressure due to the mass of gas exiting the segment to downstream points. So, yes, the mass of gas coming from the PD's cylinder affects the system pressure. And we also note that the downstream pressure affects the compressor's discharge pressure. Just as you say. However, we must also look at the mass of gas entering the system segment from the compressor, as we have shown that it affects the system pressure too. So, is not the mass of gas entering the system a function of the PD's cycle volume, and the mass of gas entering the system will increase with decreased cycle time? And, therefore isn't the mass of gas entering the system hence also a function of RPM? There is no way for the compressor to create an internal pressure greater than what is needed for the valve to open and eject its gas into the system downstream. It can't create any pressure higher than that, because the compressor's outlet valve is open by then. If it tries to increase pressure more than that, mass simply escapes the cylinder into the downstream segment. It is possible that the downstream segment pressure tends to increase, if and only if the downstream segment's net mass at the end of the timestep is greater than the original mass at t0, or decrease if net mass is less. If incoming mass equals outgoing mass, the system remains at the same operating point, so flowrates and pressures remain as they began at t0.

Don't try to separate the compressor from the system, or the system from the compressor. They function together as a unit. Each depends on the other equally, except for the small differential pressure needed to open the valve and pack the first segment of pipe. The length of pipe making up that segment that we need to consider is the same distance that the pressure wave will travel, at the velocity of sound inside the pipeline, during the time of one compressor cycle.



**********************
"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/

 

[btrueblood]

Even if you replace BigInch's compressor with a lobe-type PD blower (e.g. Roots blower), and the outlet valve with a plenum, the arguements still hold. Dunno how that is magical thinking.

 

[btrueblood]

Here are a set of performance curves for some random lobe-type blowers.

http://www.allproducts.com/machine/trundean-1/13-curve_diagraph_th.html

Draw a straight line across any of the charts, to show the operating line for a blower discharging to a constant system backpressure. Reducing RPM drops the power required, as volumetric flow is decreased. The power decrease isn't linearly proportional to flow, due to slip, and certainly is not as dramatic as the square-law decrease in power if pressure is allowed to decrease, but it's not insignificant.

Here's another view, using a set of data tables:

http://www.omega-inds.com/roots-blowers.htm

Take the Whispair 44 blower, one that I'm quite familiar with, operating at 5 psig. At 1500 rpm, it puts out 82 cfm, consuming 3.5 bhp. Drop the speed to 900 rpm, and it puts out 28 cfm at 2.1 bhp. I.e. the power dropped proportionally to the rpm decrease, but the cfm decreased a lot more.

Understanding how much power savings could be achieved would require more info. than currently given. But it wouldn't be zero, unless the system is designed in the top right corner of the blower's operating envelope.

Sha-zam!

 

[btrueblood]

Ah, finally. The Roots website is slow to navigate, but if you drill long enough and deep enough, it has the data also.

http://www.rootsblower.com/documents/RCS_J/rcsj_drwg_pcurve_imp.pdf

 

[CH5OH]

a VFD opens a lot of possebilities to better control a blower:
since the machine itself is a reliable piece of equipment
(provided with a sync gear)
on a constant speed machine, there is a requirement for an unloading valve, to coope with flow fluctuations.
if VFD can keep motor torque within shaft working envelope,
the unloader valve as a failing mechanism could well be proven obsolete.

 

[CH5OH]

if the blowers are tend to be operated at low speeds and concernes should be given to provide forced cooling (separare driven cooling fan) for cooling drive motor.