Fan Speed Increase 1

[macmet]

I have a fan that runs at ~1600 rpm requiring 28.5HP. The fan has a 40 HP motor. If I were to switch the fan and motor sheaves I'd be left with a speed of ~2100 rpm. Based on fan laws I get the new horsepower to be

-> (28.5 hp) * (2100rpm/1600rpm)^3 = ~65 HP

that seems way too high.

If I were to look at the SP, the fan would produce significantly more than required at this higher speed.

Can I use the fan laws accurately in a situation like this? Or are they not applicable. If not, why not?

Is there any problem with having the motor sheave larger than the fan sheave?

Cheers.

 

[gepman]

Yes you can use the affinity laws reasonably well. Power increases with the cube of the speed, static pressure with the square of the speed, and flow linearly with speed.

The ratio of the fan sheaves is not an issue but you need to be certain that your fan is rated for the increased centrifugal force of the increased speed. Most fans will have an upper rpm limit marked on them.

You can always buy different sheaves or use a VFD. If using a VFD make certain that you check the inertial load during slowing so it does not turn the fan into a generator and trip the VFD on high DC bus voltage. You can always set the decel parameter to a large value.

 

[gepman]

I forgot to mention that there is a fan and pump savings calculator for using VFD's at

http://www.abb.us/product/ap/seitp322/24b03100d005c31ac1256e040043f4c1.aspx

There is another one at

http://www.bpa.gov/Energy/N/projects/industrial/audit/index.cfm

 

[Wareagl487]

gepman made a GREAT point - I was looking at increased H.P. in lieu of if the fan could handle the increased performance.

Your sound in your figures.

Andy W.

 

[walkes]

You did mention "switch" the fan, which would invalidate the affinity laws. Did you mean you are changing the fan out or just changing the speed?

 

[macmet]

I meant just changing the speeds not the fan. We are trying to get maybe 15% more capacity out of it.

I guess I should have mentioned that this fan is already on a VFD and constantly runs at 100%.

Would that change anything? Because the motor is relatively large to what is required I figured I have some room to play before having to increase the fan itself.

I sent a note to the manufacturer last week about whether or not the fan is physically able to handle a higher load. Still waiting to hear back though.

 

[MintJulep]

and constantly runs at 100%

100% of what?

 

[gepman]

Running a fan, pump, or anything using a VFD and running at 100% speed (i.e. 60Hz in the United States) is a waste of energy. The VFD is probably about 97-98% efficient so you are wasting that 2-3% energy. There could be a slight increas in inefficiency in the motor due to the VFD harmonics.

You should check the actual motor amperage draw from the VFD display. The VFD can actually drive the motor faster than 60 Hz synchronous speed, usually up to about 90HZ. You cannot get more than 40hp out of the VFD but since you are not drawing that much now you will be able to increase the speed up to the point where the motor draws 40hp.

Again make certain that the fan and other components are rated for that speed.

 

[DrRTU]

Macmet, how do you know the motor/fan were at 28.5 hp? When you need to alter the flow, most techs look at FLA, max amps, service factor and the fan curve. Most HVAC drives have the ability to read current on the drive. If not, use a true RMS meter. Your sheave selection seems too aggressive. You should be able to raise the fan speed in smaller bumps. Review the Browning catalog. Gepman is correct, you should be able to raise the output “max HZ” until you are at your flow or are at max motor amps. Do not change the VFD max speed until you have checked with the fan, motor and VFD manufactures. Read the VFD book very closely. A typical drive has hundreds of setting and it very confusing. Look for max amps and max HZ. If you do not know what the setting mean don’t change it. The drive mfg. may have a service company that can help you but they will not know the limits of the fan or motor.

 

[macmet]

Alright, well to answer the questions...

The fan is running at 60Hz. I was not aware that you could increase the freq above this so I will talk to our controls guy about this further. The intent was never to run this fan at 100% but b/c of some system changes the air capacity has been increased and this fan is maxed. To correct this we have to get more capacity or throttle back the system.

I get the 28.5HP from the fan curves and other data supplied when it was originally purchased. I talked to the end user and none of the sheaves have been changed and it still has the original motor.

DrRTU, what is a proper sheave selection? Can you point me to a reference that would have some information on this. Generally we do not select the sheave sizes our self but leave it up to the manufacturer.

And lastly, I will not be making any changes until I speak to the fan manufacturer to make sure the unit can take the increase.

 

[MintJulep]

Seems like you have plenty of options open.

While the fan laws are fully applicable, since you don't know the system curve they really are not much help to you in arriving at a solution because you don't know where the fan curve and the system curve will intersect.

After confirming with the fan manufacturer what the allowable max rpm of the fan is, I would start by cranking up the frequency on the drive until you get the airflow that you need or protection on the drive trips or the current to the motor goes over the max RLA listed on the motor.

If you can get the necessary flow without overloading anything you can either simply change the drive settings and leave it running at whatever speed works, or you can play with sheave sizes and turn the drive down to a speed that makes you happy.

 

[DrRTU]

Your control guy my not know what you are trying to do. In most cases, automation contractors feed the drive a 0-10v or 4-20ma signal and trust the drive is configured correctly. You should have your test & balance firm or the mechanical contractor re-configure the drive for output past 60HZ operation. It may take a few runs to find the dive parameters needed for your increased flow. The drives have small displays and are sometimes installed in the air stream or you may need to run the unit with the doors open. I like to use the flexibility of the drive to speed up the fan then work backwards with a sheave selection to bring the motor back to name plate rpm when the system is at 100%. Find the part numbers on the fan & motor pulley then use the sheave manufacture catalog to pick a new fan or motor sheave with the same belts. If the unit is new, check with the equipment manufacture. If the new sheave selection is acceptable by the OEM, they may give you sheave if they get the old one back.

http://www.emerson-ept.com/

 

[RossABQ]

I have run into a number of installations where VFD's are "overdriving" or "overclocking" a motor to as much as 72 Hz, but usually only to provide for "redundancy" in multi-fan installations. I was surprised to find that controls guys are for the most part unaware of this capability, and as DrRTU says, they had the control signals corresponding to 100% at 60 Hz when they need to go above that.