Head loss through freewheeling blower?

[arnermd]

Hello All,

Was wondering if anybody knows of a method to estimate head loss through a freewheeling centrifugal blower given the performance curve when the blower is running?

Seems to me there should be a relationship or rule of thumb to relate this to the powered performance curve.

Thanks for any guidance,

Arnermd

 

[BigInch]

Haven't done this before, so I'm just TOL (thinking out loud), but it might be found by using affinity laws, if the density of the gas does't change much between H2 and H1,

H2 = H1 * N2^2 / N1^2

N1 is blower running speed
N2 is freewheel speed

H2 should be something like = rho v^2/2/g
where v = freewheel tangential velocity

Let me know how that works out.


Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BronYrAur]

Please clarify freewheeling. Do you mean that the fan is in series with another, and air is being blown through? Do you mean that a damper is closed and the fan is just turning and moving no air? Or do you mean that there isn't enough static, and the fan is "running wild"?

 

[BigInch]

I just assumed freewheeling is turning with no power input from the motor. I don't think it matters what freewheeling is other than that. In all cases, it should correspond with the above relationships. Think of it as a wind turbine. The head flow relationship contained in the original curve is independent of power applied and dependent only on angular velocity of the fan. Power is what you have to add to it to get air out of it, but if air is going into it without applied power, power is what you can get out of it. Shut-off "head" is what's needed to overcome friction and spin at rated rpm, so I'd reduce the original curves "shut-off head" with the speed change, by..what... the speed cubed, and then you have the new freewheeling curve at the freewheeling speed.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[arnermd]

All,

Thanks for your thoughts.

Freewheeling means pretty much what BigInch said. In this particular case I am looking at a draft blower downstream of a micro turbine. The sole function of the draft fan is to make up for ducting losses, normally the micro turbine does not require a blower on the exit side but in this case we have a lot of duct work and associated pressure drop. I am trying to estimate what the backpressure will be if the blower does not turn on and to do this I need to know what it's pressure drop characteristic is.

BigInch: Interesting thought. Density won't change much, I am talking about inches of water for pressure rise. Only problem is I don't know what the freewheeling speed will be.

 

[BigInch]

Looks like you gotta' know either the tangential velocity or the air flowrate at the other speed (or the other speed), to get the head.

Can't you just go spin one?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Robfitter]

I'm not sure what the problem is. All you need is a strobe light, to check how fast the shaft turns when it freewheels. Put a mark on the shaft, then observe when the blower is freewheeling. When your mark doesn't move, read your rpms off the strobe.

 

[arnermd]

I didn't specify this originally but..... I don't have a blower to spin, I am designing a system that doesn't exist yet.

When I said "given the performance curve when the blower is running" I literally meant a piece of paper with a curve on it, not that I had a blower up and running that i could measure performance.

If I had a system up and running I would shut the blower off and measure the gas flow through it with a pitot or high temp anemometer. Unfortunately I don't have that luxury right now.