Increase Fan Speed 2

[TCPhoenix]

I have a belt driven hot air fan that is currently operating at 1288 rpm. We would like to increase the speed to 1400 rpm. The motor driving the fan has a nameplate that states 100 Hp/575V/60Hz/89.6 Amp/1785 rpm. The motor has a sheave that is 10.9” diameter and the fan has a 15” sheave. We measured the load of the motor during operation and it pulls 80 Amps.

We are considering the following options to increase the speed of the fan.

1. Increase motor sheave to 11.75” diameter
2. Decrease fan sheave to 13.89” diameter
3. Decrease motor and fan sheaves (this seems ideal because a smaller sheave on the motor will require less torque. Correct?)

The following are items we will consider:

Check fan curve to see effects of increased speed. Can the fan take the faster speed, will speeding up the fan increase air flow.
Check the amount of amps that will be drawn due to different sheaves (if the motor pulls too many amps we will consider a higher Hp motor).
Are there any other items I should consider/check?
Any suggestions on how to calculate the increased amperage due to larger sheaves?


TCPhoenix

http://www.phoenix-engineer.com

 

[DickDV]

You are doomed to fail regardless of which option you choose. A centrifugal fan hp curve follows the speed cubed so, 1400/1288^3 = 1.284 or a hp increase of 28.4%

Your motor current will increase slightly less than 28.4% to about 101amps. This is way over the motor FLA.

Basically, you've run out of hp before you get to 1400rpm.

My quick calculations show that you will be able to reach about 1335 before you are at the motor's limit.

 

[itsmoked]

Looks like you've thought of everything. Nicely stated too.

If you can look up the fan curve you can see at what speed it needs 100HP if that would be 1400RPM or more - no sweat.

If you use bigger pulleys the belts last longer.

If the motor has a service factor over 1 (SF) you can thrash it a bit worse especially if it has benefit of the air flow.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hot air fan, hot air is less dense. If the fan ever has to operate with cool air, you may need the extra capacity that you now appear to have.

Why are you speeding the motor up?
Can you get a spec sheet for the fan? That would be the place to find the increased air flow Vs increased speed and the power required.
If the specs are not available I would be searching online for specs for similar fans. Specs for other fans cannot be trusted completely but if you compare specs for several fans you should get a good feel for the probable result.
Based on Dick's figures you should be looking at 150 Hp.
That will also give you a safety margin should you ever have to start the fan with cool air.
For a given ratio, a smaller diameter pulley will have more load tension and less contact area to develop the needed friction. A lose, lose situation. I would suggest increasing the size of the motor pulley.

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

 

[TCPhoenix]

DickDV, interesting comment. At one point a 125 Hp motor was installed with this fan.

waross, we are speeding up the fan to get more flow (cfm). The mfg is working to get me the fan curve, so that will help me with the power required. Smaller diameters yields less contact area, I didn't think of that.

Thanks,

TCPhoenix

http://www.phoenix-engineer.com

 

[rockman7892]

What about a motor that is operating at a fixed speed and has the airflow controlled with a damper? I have seen a speed vs tourqe curve for a fan controlled by a damper that has two curves plotted, one for an open damper and one for a closed damper. I'm assuming that where the open damper curve intersects with the fixed speed on this type of curve will be the maximum torque required for this motor application. The torqure can then be translated into hp.

We once had to try to estimate %load hp between the damper closed and open position. We basically took the difference between the two torques where the intersected the motor speed and broke it into increments based off of 100%. We could then estimate the required hp for the motor when the dampers were say 30% open, 40% open etc...