Fan speed limitations 1

[Kaplin]

I have a 24" Centrifugal Fan currently operating at 1985 RPM. I would like to increase the RPM in the future (change from belt driven to direct drive). How do I determine what the RPM limit on the fan is? Is there one? I am not able to find a fan curve.

More specific details below:

24 Af. Centrifugal Fan D.W.
CW rotation down blast discharge.
Class 3 Arr. 3
Fan performance: 18000 SCFM
8" SP, OV: 2900 FPM, BHP: 27.45, Service Factor 1.3

 

[KenRad]

I can't think of any way to determine fan RPM limitation without consulting the fan manufacturer (maybe someone else can help here).

Since you gave motor info, I assume that you are trying to use the same motor. Any time you want to increase fan speed, you need to look at the motor amperage at the current speed, and using the fan laws, determine where your amperage will be relative to full load amps after the speed increase. You may find that you need a higher horsepower motor.

Can you possibly accomplish the desired RPM increase by changing out the motor sheave? That would be a whole lot simpler than converting to direct drive. Another option might be a variable frequency direct drive, which would give you the most flexibility in choosing speed.

But if we are talking about a significant increase in fan speed, and in the absence of any manufacturer's data, I would go the safe route -- buy a new fan that is designed to give me the exact performance I need, with room for any future speed changes that might be necessary.

 

[ChasBean1]

Gabe (Welcome back Mr. Kott-ier, Sir.!) - I will venture to guess that the motor sheave right now is about 11 percent bigger than the fan sheave. If you go to direct drive (motors are usually ~1785 RPM) you'll lose flow.

Go to the manufacturer. One needed given you forgot to give was make (e.g., Loren Cook) and model. They'll supply you the curve. It's not automatic that increasing RPM on a centrifugal fan (DW - double-wide, just like my trailor!... kidding - just means that fan sucks in air on both sides) will deliver more volume by increasing RPM. Usually you find you are limited by motor and amperage, as stated by KenRad, although spinning the fan faster usually does provide more flow.

Main point: changing to direct drive will probably hurt more than help. Increase motor sheave (or decrease fan sheave) if amperage permits. But first off, get the manufacturer's curve and look at the current state.

 

[windy1]

Tip speed of class 3 fan is approx 16000 fpm about 2400 rpm for the fan yoiu discribe Horsepower will increase to approx 40 at 2400 rpm . to Direct drive at 2 pole (3500 rpm)appox 50 hp approx 22500 fpm tipspeed . Best reference for this Buffalo Forge Handbook Oops I mean Howden Forge . Think this is fairly accurate
Hope it helps

 

[kaiyiuwong]

The most common failure mode of motor is "overheat"
I think the limit of your fan is the temperature limiation of the stator and rotor you use.