Estimating fan RPM 2

[bvanhiel]

I have a large military surplus vane axial fan that I'd like to run, but I do not know it's max RPM. Any tips on how to estimate it?

My current plan is to measure the section of one of the blades (they are cast aluminum) and extimate a safe speed based on a safety factor over the yeild stress.

Is there a better way? Is blade strength the determining factor or should I be looking at tip speed or something else?

The specs that I know are 11" H20 static pressure and 9-18k cfm. The outlet is 24". The blade diameter is somewhat smaller.

Thanks,

-b

 

[Bribyk]

Can you get to the bearings? If they are a commercially available bearing, their maxed rated speed will give you some idea where the fan is meant to run. Keep in mind the bearings are likely not meant to be run at their max speed but it'll give you an upper bound.

If it's belt driven you should be able to find/calculate some kind of max safe speed based on the belt drive as well.

 

[bvanhiel]

I'll check the bearings.

It's belt driven, but I don't have the belt dimensions. The shaft is huge: 1-13/16" with a half inch keyway. I'm not sure if that's indicative of the torque or the fact that it's military (it's the cooling fan from a mobile howitzer).

-b

 

[MintJulep]

Spin it until it explodes, then back off 10%.

Find a data sheet for a fan of similar construction and capacity. For the most part, fans is fans, so if you can find something that is about the same you fan's performance won't be too different.

 

[fredt]

A fan engineer with access to appropriate data can reverse engineer it from the pressure, flow and diameter to determine its original speed. Blade stress or blade attachment probably will be the limiting factor but calculating it accurately is not that simple.