Backward curved FAN

[Kooler]

Hello,

First of all, thank you so much for reading this e-mail, and I hope you can help me.

I have one doubt, about centrifugal fans. A colleague has told me that if you rotate a backward curved fan in the opposite direction as the one it has been designed for, no so much change in the fan performance is obtained. For me this is something unbelievable.

So, the question is... does anyone now what happens when rotating a backward curved fan in the opposite direction as the one it has been designed? does the same effect take place in the case a forward curved fan?

Thank you so much!

Best regards,

jon

 

[btrueblood]

You should notice a difference in the low-flow behavior. The backward-curved blower (fans are axial devices, centrifugals are usually called blowers to distinguish them) will have a very "soft" stall characteristic, the forward-curved blower will probably exhibit stall, noise, and unpredictable flow at higher flows than the former.

 

[insult2injury]

For some designs, you may not notice much difference; however, with most design you will most likely be limiting your operating window and experiencing higher levels of vibration, noise, etc. With forward curved designs, mach number effects can be a problem. Is there a reason you want to spin the fan backward? Buy the wrong motor?

I2I

 

[Kooler]

Thanks for the info. It is what I think, but I didn't knew if trust or not my colleague.

The reason I am asking this is that one time, because of a wrong connection on the motor of the fan, we started running it in the wrong direction. We realized it was wrong so we changed it, but without measuring anything. So we don’t really know what would have happened, and we had this question in mind.

btrueblood, what do you mean with “unpredictable flow at higher flows than the former”, I have never heard this nomenclature.

insult2injury, this is what I think also. For me, changing the sense of rotation, the obtained air-flow would be much lower, limiting as you say the operating window.

 

[btrueblood]

When a fan or blower stalls, the flow and pressure can fluctuate in an unsteady manner. Also, depending on whether the operating point of the fan is approached from the low flow side or the high flow side, forward-facing blades can sometimes give different outlet pressure.

 

[rmw]

Backwardly inclined fan designs (sorry, but fans are fans in my world, and most of them are centrifugal) tend to be non overloading meaning that the motor HP curve is relatively flat. Forward curve fans have a much steeper HP curve meaning that the faster you run them, or the more you load them, the more HP they require.

In your case, since you didn't trip the motor, other parts of the system limited the throughput so that the motor didn't overload. If you had suddenly opened the output up however, who knows...

rmw

 

[btrueblood]

"fans are fans in my world"

Enjoy it.

 

[Kooler]

Hi again!

This post is becoming quite funny, haha.

I agree that this is what would happen. Maybe if I have time, I will make this test in order to obtain real values.

Also, for me "fans" are all model of fans. I know that some literature calls them blower, but all of them are fans.

This is not in order to create a small fight between us with those terms.

Thank you all!

 

[btrueblood]

Oh, you started a fight all right. I am not a fan of calling blowers fans...and don't bring up mixed-flow designs, or you'll really be fanning (blowing?) the fire.

Rmw either knows I'm yanking his chain, or he is cowering.

 

[rmw]

btrueblood,

I am cowering, but it has nothing to do with the fun going on in this thread. I am typing this reply on a laptop powered with a generator in my driveway via a laptop connect card that has finally gotten a data signal during the daytime.

All of the above is a result of Hurricane Ike. I live in the Houston area and have been without power since last Friday evening and the current projections are next Monday for power restoration both in my residence and at our office, so due to all this, I wasn't able to "fan" the flames.

My only comment is that Chicago Blower Company built some mighty fine fans.

rmw

 

[btrueblood]

I know. Sorry about your troubles, rmw, we had a 2-week outage due to a big windstorm last winter, it's no fun.