Fan Speed Increase
Fan Speed Increase
(OP)
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.
-> (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.





RE: Fan Speed Increase
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.
RE: Fan Speed Increase
There is another one at htt
RE: Fan Speed Increase
Your sound in your figures.
Andy W.
RE: Fan Speed Increase
RE: Fan Speed Increase
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.
RE: Fan Speed Increase
100% of what?
RE: Fan Speed Increase
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.
RE: Fan Speed Increase
RE: Fan Speed Increase
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.
RE: Fan Speed Increase
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.
RE: Fan Speed Increase
RE: Fan Speed Increase