Variable Frequency Drive for Dust Extractor Fans

[vinnygalbo]

I have heard different stories about variable frequency drive (VFD) for extractor fans. How do they work?

Sellers I’ve researched about the drive claim it will reduce electricity costs. I suppose it will reduce noise as well. I have spoken to two suppliers of ducting. They both say that it causes an increased risk of fire as shavings tend to build up in the ducting. They also say that extraction fans are designed to run at one frequency and varying the frequency makes the fan very inefficient.

Is this true?

 

[petersonra]

I have heard different stories about variable frequency drive (VFD) for extractor fans. How do they work?

Sellers I’ve researched about the drive claim it will reduce electricity costs. I suppose it will reduce noise as well. I have spoken to two suppliers of ducting. They both say that it causes an increased risk of fire as shavings tend to build up in the ducting. They also say that extraction fans are designed to run at one frequency and varying the frequency makes the fan very inefficient.

Is this true?

The reality is that the amount of energy used is dependant on how much work the fan does. If the fan is engineered so that at full speed it does the exact amount of work required so that a VFD would run at full speed all the time, there would be no benefit to a VFD.

It is also true that running a fan at lower speeds reduces the noise level. That is all good and well except you still need to move a certain amount of air. If you can do that at a lower frequency a VFD is often a good choice.

There is generally no truth to the idea that varying the motor speed adversely affects the fan's efficiency. It is probably possible to come up with a situation where it might (like running a fan at 5% of rated frequency) but generally that would not be something one would do.

The shavings thing might or might not be true. If one needs a certain volume of air to get enough air velocity to remove the shavings you can't run the fan below that speed.

 

[Tom the lectrician]

My thoughts exactly. Yes you can slow your motor down and adjust parameters for starting torque and amp draw, but in the end will it still perform what you need it to going slower? It was probably designed to move a certain volume of air in a time frame. Cubic feet per minute. I mean just the start up and stopping abilities can save you on wear of the equipment also though, so u have to weigh all of that out.

 

[Ingenieur]

I've seen them in applications where one fan draws from multiple snorkels
when not in use the snorkel damper is closed
a variable volume set up

a pressure sensor modulates the vfd to maintain a static setpoint
as snorkels open pressure drops vfd ramps up, and the converse

 

[drcampbell]

They could also be set up to maintain adequate performance with clean or dirty filters. If a speed is selected to be adequate with dirty filters, it will be much faster than necessary with clean filters.

 

[Tony S]

I have heard different stories about variable frequency drive (VFD) for extractor fans. How do they work?

Sellers I’ve researched about the drive claim it will reduce electricity costs. I suppose it will reduce noise as well. I have spoken to two suppliers of ducting. They both say that it causes an increased risk of fire as shavings tend to build up in the ducting. They also say that extraction fans are designed to run at one frequency and varying the frequency makes the fan very inefficient.

Is this true?

Just a comment on noise. Running a fan at slower speed doesn’t necessarily mean it will be quieter. Centrifugal fans will have a resonant frequency, hit that speed and all hell breaks loose. Phone calls from angry neighbours being just one of the problems.

 

[iwire]

The reality is that the amount of energy used is dependent on how much work the fan does. If the fan is engineered so that at full speed it does the exact amount of work required so that a VFD would run at full speed all the time, there would be no benefit to a VFD.

That is a great on point answer.

 

[iwire]

Just a comment on noise. Running a fan at slower speed doesn’t necessarily mean it will be quieter. Centrifugal fans will have a resonant frequency, hit that speed and all hell breaks loose. Phone calls from angry neighbours being just one of the problems.

Man you always seem to have the worst luck. Is there a thunderstorm always over you?

In 50+ years on the planet I have never seen / heard a fan that did not get quieter when slowed down.

It seems pretty straightforward that if you did happen to hit a resonant frequency you would adjust the speed up or down a bit to get away from it but maybe that is too simple?

 

[Haji]

Just a comment on noise. Running a fan at slower speed doesn’t necessarily mean it will be quieter. Centrifugal fans will have a resonant frequency, hit that speed and all hell breaks loose. Phone calls from angry neighbours being just one of the problems.

Your concern is legitimate. However, I think some VFD makes can be programmed to bypass that resonant frequency.

 

[Jraef]

To further Bob's comments, you can only save energy that is wasted. So if, right now, you ARE ALREADY varying the air flow using some mechanical means, such as dampers or inlet guide vanes, then REPLACING that system by varying the motor speed can eliminate a portion of the losses inherent in that other system, as long as the machine is a centrifugal (variable torque) application. You need BOTH factors though; a centrifugal machine AND the need to vary the flow.

It is true that there can be a slight decrease in overall motor efficiency as the load on it decreases. This however is only because there are fixed energy costs in turning a motor into a motor at all, so as a percentage of total energy consumed, that fixed amount represents a larger percentage as the driven load energy decreases, which translates to being a "lower" throughput efficiency. Many people interpret this incorrectly, and also, remember that it the goal was energy savings, it has to be a centrifugal load and variable flow system. So when properly applied the energy you are saving in varying the speed more than makes up for the slight change in efficienc. The other important fact is that if you NEVER vary the speed, there are fixed losses inside of the VFD, about 3% max., that become realized in the system. If you want to permanently change flow, change the blower.

In my opinion, the primary consideration here is whether or not varying the flow is even a viable option in the fist place. If not, adding a VFD will just add those 3% losses for no benefit.

 

[Tony S]

Man you always seem to have the worst luck. Is there a thunderstorm always over you?

In 50+ years on the planet I have never seen / heard a fan that did not get quieter when slowed down.

It seems pretty straightforward that if you did happen to hit a resonant frequency you would adjust the speed up or down a bit to get away from it but maybe that is too simple?

Maybe I just pick the wrong places to work.

If the PID control decides that the resonant speed is the speed required to match the process what then?

I’ve only come across this three times in forty years.

48x30HP fans in a quarry. The quarry face acted as a sound reflector.
2x2500HP fans, the building acted as a sounding board.
1x2200HP fan, the 250Ft stack was like a tuning fork.

All very expensive to put right before the environment agency shut us down. OK they are large installations, but any variable speed fan can cause resonant vibration. It is the most god awful noise and you can’t escape it.

The quarry episode was weird. The load was cyclic, as the load moved between each unit the focal point of the noise moved and so did the direction of the sound.

Read up on fan resonance vibration.

 

[Haji]

I have heard different stories about variable frequency drive (VFD) for extractor fans. How do they work?

Sellers I’ve researched about the drive claim it will reduce electricity costs. I suppose it will reduce noise as well. I have spoken to two suppliers of ducting. They both say that it causes an increased risk of fire as shavings tend to build up in the ducting. They also say that extraction fans are designed to run at one frequency and varying the frequency makes the fan very inefficient.

Is this true?

Try to get supportive evidence from them before coming to a conclusion and taking a decision.

 

[iwire]

Maybe I just pick the wrong places to work.

Maybe.

If the PID control decides that the resonant speed is the speed required to match the process what then?

You change one of the variables, it is not rocket science.

 

[vinnygalbo]

Thank you for all the great advice.

I talked to good tech support engineers at Lenze & Red Lion Controls. I have a better understanding now. Thank you.

 

[petersonra]

one other thing to note about VFD usage.

there are often unknown factors that might affect the optimal speed of a motor. it is often more cost effective to add a VFD to account for those factors rather than do a design that accommodates those factors, especially since you might miss one in the rush that we often experience these days to get projects going.

 

[Besoeker]

Maybe I just pick the wrong places to work.

If the PID control decides that the resonant speed is the speed required to match the process what then?

I’ve only come across this three times in forty years.

48x30HP fans in a quarry. The quarry face acted as a sound reflector.
2x2500HP fans, the building acted as a sounding board.
1x2200HP fan, the 250Ft stack was like a tuning fork.

All very expensive to put right before the environment agency shut us down. OK they are large installations, but any variable speed fan can cause resonant vibration. It is the most god awful noise and you can’t escape it.

The quarry episode was weird. The load was cyclic, as the load moved between each unit the focal point of the noise moved and so did the direction of the sound.

Read up on fan resonance vibration.

Cement works in the Midlands maybe?

 

[Tony S]

Cement works in the Midlands maybe?

The ID and EP fans made one hell of a racket when the frequency was just so :thumbsup:

The others were at different locations

 

[Besoeker]

The ID and EP fans made one hell of a racket when the frequency was just so :thumbsup:

What type of variable speed drive was the 2200HP unit?

 

[Tony S]

Some form of Kramer.

 

[Besoeker]

Some form of Kramer.

Possibly one I designed.
Where was it?