Calculating mechanical efficiancy of an axial fan

[physicsdude17]

We are trying to improve the efficiency of a fan for an engineering project. However, we first need to figure out what the efficiency of the fan is now.

Since we know how many watts the fans motor is using at each setting, we figured we could just find how much power is in the air the fan is moving using P=1/2(rho)A(v^3), however this didn't work(maybe we're just all really bad at math?). It could be because we tried to calculate velocity based on cfm.

What is the best way for us to calculate the efficiency of the fan? We know how much power the motor is using and how many cfm of air it is moving. Thank you

 

[russ_watters]

The kinetic energy of the air is not the only energy: that equation doesn't capture the pressure energy of the air. A simple thumb-rule equation which you can derive from the definition of work (w=f*d) is HP= SP*CFM/(6356*eff)

...Where SP is the total static pressure generated by the fan.

 

[minger]

Efficiency is pretty vague. Unfortunately that "can" mean different things to different people. At my work, we have a script that will spit out three different efficiency numbers for a turbine stage. Some are based on work, some torque, some whatever whatever.

The important thing is to find a parameter or something you feel important to performance and make sure you just keep using the same definition.

 

[physicsdude17]

How can we measure the static pressure of the fan?

What is another way we could go about doing this project. Basically, we have to make the fan do more work using the same power or the same work using less power.

Thanks

 

[minger]

If you could "pipe" the fan output into a turbine that creates electricity, you could measure the drop in voltage to determine an efficiency. Of course, you would need the efficiency of the downstream turbine.

Conversely, don't be stuck on this idea of an efficiency. The purpose of a fan is to move air, so measure airflow.

 

[Jobrag]

The simplest way of increasing a fan's efficiency is to polish the blades.
ASME PTC 11 will tell you how to test axial flow fans.