Air Filtration Fan RPM?

[Trent Flemming]

Hello Creekers,

Thinking about building an air filtration system. I have the squirrel cage and a seperate 220v 1/3 hp motor. The motor is 3 speed and the RPM rating is 1075. I think this speed is a bit slow. What do you think?

Trent

[glenn bradley]

Never paid attention till you asked. Mine is 3450 for a 14" prop. A squirrel probably has different requirements. Does your cage give a max rpm rating? I wouldn't care for my ambient filter to be excessively noisy or blow my plans off the bench but I would want it to move enough air to do me some good as long as I'm going to pay to run it. Your cage probably commits to a certain CFM at a given speed. I'd shoot for that. If the cage has no tag and you don't know the rating you could compare units of similar size online for your figures.

[Randy Denby]

Here's some poop on blower motors taken out of HVAC units...Speed of the blower wheel is usually based on standardized motor sizes and rpms of these motors. So since most of these small blower manufacturers work with a "constant" rpm for hi speed, then the speed has less to do with CFM output and is more to do with the size and pitch of the blade. Most squirrel cage blowers operate at 1075 rpm, no matter if its a 1.5 ton unit or 5 ton unit. They get the airflow from different sizes and pitch of the squirrel cage, which will affect horespower to maintain that 1075rpm. Saying all that, even a 1.5 ton blower will drastically outperform the overhead air filter machines designed for our hobby shops...such as the Jet, JDS, etc. The blowers in these are tiny comparitevly.


EDit: this info is for direct drive blowers...belt drive will usually have a 1750rpm motor, that is then slowed down to required cfm/amp draw of the motor via pulleys /sheaves

[Rod Sheridan]

Trent, the fan motor will be a certain horsepower, and you cannot exceed that rating.

Your fan will most likely have very little static pressure, so the fan speed will have to be relatively low to avoid overloading the motor.

I would suggest that you build it and initially set your drive ratio at 2:1, put all the filers in, and measure the motor current. If it's too high, increase your drive ratio.

Note that fan horsepower is proportional to the cube of the fan speed, so a small change in fan speed produces large power changes.

Regards, Rod.