VFD alternatives 3

[BruceTheEngineer]

Hello all,
Let me start out by saying I'm a chemical engineer by training, so please don't laugh at my lack of electrical engineering knowledge too hard.
The problem I'm encountering at work is that our positive displacement Roots blowers are blowing too much air into our pneumatic conveying lines. In my research and experiments, I've found that decreasing the amount of air (in CFM) by bleeding out excess air has dramatically improved our ability to convey product and reduce defects such as degradation, streamers, and pipe wear because our solids/air ratio has increased. The thing is, I really don't want to bleed off air because that's just a waste of 40 and 50 hp motor.
I have thought about installing VFD's, but they are expensive to install. I have about 6 motors (all between 40-100 hp) that are single speed AC motors. Are there any alternatives to VFD's I should consider? Like I said, I'm not that familiar with electrical products on the market today, so I don't know what's out there.
For four of the applications, I'm thinking about just changing out the sheaves because the product flow rate stays the same, but for two of the motors the product flow rate changes so I would need to vary the amount of air I use to convey product depending on how much product I have going into my pipe. The manufacturer of the blower said "NO ATTEMPT SHOULD EVER BE MADE TO THROTTLE INLET OR OUTLET AIR FLOW VIA A VALVE" so installing a valve is out of the picture.

 

[BrianPetersen]

A fixed belt ratio change is the way to go if the target flow rate is known and unvarying.

For varying the speed of an AC induction motor in this day and age, a VFD is the best choice. Nothing else will allow the speed of the motor to be changed. The only way to variably change the speed of the pump without allowing the speed of the motor to change is to put a CVT (continuously variable transmission) between them, and believe me, you do not want to do that.

 

[ScottyUK]

Mechanical variator would be an option, but expensive compared to a VFD and far less reliable. There's a reason no one uses them anymore.

A magnetic coupling could act as a variable speed controller with the right load characteristics, but they're hideously inefficient. They're robust and can work in high ambient temperatures, but that's about the only things in their favour.

 

[GroovyGuy]

HI Bruce,
After giving it some thought, the obvious solutions include;
1) A fixed belt ratio change (per Brian's reply above), or
2) Installing VFDs
Are the blowers currently belt driven? If so, then Option 1) above is likley your least expensive solution. If not, then Option 2) may be less expensive.
Have your condidered the energy costs of not doing anything? (ie the cost of bleeding off of 40-50hp)
GG




"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[BruceTheEngineer]

Yes sir, the blowers are belt driven. We have some mechanical variators on some motors in other applications at our plant, and we rarely use them because like ScottyUK said, they aren't very reliable. Besides Motovario, is there any other vendor that sells those?

 

[waross]

I'm not sure but I suspect that bleeding air from the discharge may not be as inefficient as you think.
Before going any further, have your electrician monitor the current to the motor while you open the bleed valve.
You may not be wasting nearly as much energy as you think.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bacon4life]

Is it common to prohibit valves on blowers?

Bill- Can you share a bit more of your hypothesis? Since the OP is measuring current rather than power, the excitation current would make it easy to underestimate the wasted energy.

 

[FreddyNurk]

I'm assuming the restriction on using a valve for throttling is based on the blowers being Positive Displacement. I'd also assume using a valve to bleed off pressure isn't the same thing.

waross is right, check the actual consumption of the motors, it might not be as bad as the nameplate suggests.

EDMS Australia

 

[waross]

Yes I agree that it is not linear, but the motor current is a pretty good indicator of motor load when above about 50% load.
If you see a drop in current, then the drop in real power will likely be more.
I think that you will find that bleeding air will lower the load on the motor. Check and let us know.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BruceTheEngineer]

We had what I'll just call a "process upset" at work, so all of our resources are trying to fix that right now. Once that storm passes, I'll have to get with an electrician to pull the amp measurements (they don't trust chemical engineers to open any panel more than 50 volts).

Is it best to take the measurements from the MCC (motor control center) bucket or right at the motor using a digital clamp multimeter? Whenever I've asked an electrician to take amp readings before, they've taken it from the MCC room (which can be 300 ft. away from the motor) but I always thought that because of the distance away from the motor and the voltage drop along that distance, the amp reading would be off from the actual. Once again, I'm just a dumb chemical engineer asking the question.

 

[edison123]

Amp readings both at MCC and motor head will be the same. Only the voltages at both ends would be slightly different because of voltage drop in the cable.

You don't need to risk opening the MCC or motor terminal box and measuring with clamps. Just read the amps at the MCC metering panel.

Muthu

http://www.edison.co.in

 

[BruceTheEngineer]

Unfortunately, these MCC buckets don't have metering panels, these were installed in the 90's and I don't want to conjecture on why they don't have a metering panel, but the only way for us to get an amp measurement is to open up the MCC bucket and use the clamp on multimeter.

 

[edison123]

Then you need electricians, chemical engineers not gonna cut it.

Muthu

http://www.edison.co.in

 

[EdStainless]

Bruce,
Roots style blowers have a limited range of differential pressure across them in order to keep them happy.
It depends on the design of the rotors.
I have seen some that were built to only support a few psi, and some that would handle 30psi.
Throttling valves can easily lead to exceeding the allowed pressure differential.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

Put a throttle on the inlet of the blower. The lobes will be spinning in a vacuum which takes the load off of the motor saving you electricity vs venting the discharge. This is standard practice for all continuously running positive displacement compressors.

Ed, roots blowers have no seals and experience a lot of slip if operated at high differential pressures. Number one, this is an efficiency problem but in this case we are trying to reduce the efficiency of the pump. I can see slip being an issue if the discharge pressure is very high due to heating of the rotors by recirculated air. However, if the inlet side is restricted there will be less mass to compress so there will be little heat generated.

 

[EdStainless]

And if the pressure differential exceeds the design you may well get clashing of rotors, or overloaded bearings, or start sucking or blowing oil out of the gearbox.
The stiffness of the rotors and the clearances are the key, some blowers will tolerate a lot, others not so much.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[TugboatEng]

Ok, I was thinking of a roots blower that sits between a throttle and an engine, the engine pulls a vacuum on the discharge side while the blower pulls a vacuum on the suction side. The net result is no differential across the blower and no load on the blower as it's rotating in a vacuum. If we were to throttle the inlet to maintain a fixed outlet in the current case differential would vary between the outlet pressure and outlet pressure+athmosphere.

I guess we're going to need a data sheet and discharge pressure requirements.

 

[oldfieldguy]

Voith makes variable speed hydraulic couplings for applications ranging from 0.1 to a lot of megawatts. We have several in service driving centrifugal gas compressors in the 22,000 horsepower range. In put RPM is fixed - synchronous motors - and throughput is varied by changing the output speed of the hydraulic transmission.

old field guy

 

[jraef]

For the variable flow requirements, your options are limited.
[ol 1]

[li]Vary the speed of the motor with a VFD[/li]
[li]Vary the blower speed with a mechanical vari-drive (tapered belt) system[/li]
[li]Vary the blower speed with hydraulic drive / fluid slip clutch[/li]
[li]Vary the blower speed with an Eddy Current drive (aka "Mag Drive")[/li]

[/ol]

The problem with a mechanical or hydraulic vari-drive, or even an eddy current drive system, is one of efficiency compared to a VFD. Even in the unlikely event that you can spend less up front, the energy losses in those types of systems will more than make up for it and in short order, your total cost of ownership will go up significantly, and permanently.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Cheetos]

is it possible to change it to a smaller fan? If you keep the motors the same and get smaller fans, your efficiency is probably not going to be that great because your motors may not run at its optimum range.