How are ID Fans Loaded?

[HandsOnEngineer]

Hi to all,

I have a Milling circuit in my plant and have witnessed an interesting characteristic. First, here is the arrangement of our Milling circuit: 1. Ball Mill 2. Dynamic Seperater (then coarse return to Mill) 3. Cyclones (x4 of) 4a. Cyclone coarse product goes to Star/Rotary feeder and then to a Blower 4b. Clyclone fines go to a Catridge Filter then are fed into the Blower delivery line by Screw Conveyors 5. The Blower deliveres to Storage Silos. The whole system (save the Blower line) is air swept by the ID fan.

Last week we observed a decrease in the running amps of the ID fan. Then the old operators suggested we check the filter because it could be blocked, I agreed. But first I did a check, I measured the pressure differecial across the filter at low amps, then later when the amps went even much lower I measured again. I found that the relationship is: When the Pressure difference increases (blockage) the amps decrease. We then opened the filter, cleaned and replaced some catridges and now all is running perfectly.

Question: Why do amps drop when there's a blockage? When I put my hand behind my wife's hair dryer the amps go up, the dryer spins faster. Why is it defferent with the ID fan?

I look forward to your comments.

Thanks in advance.

 

[JJPellin]

You are exactly backwards. If you block the inlet to your wife's hair dryer and it speeds up, that means the load went down and the amps went down. I usually work with pumps, but the underlying principle is the same. If you pinch on the outlet of a typical centrifugal pump, the flow will drop and so will the brake horsepower. If you pinch on the inlet to a blower or centrifugal compressor, the mass flow rate through the blower will drop. If it is moving less mass, it will be using less power. I suspect someone may point out exceptions to these general statements. There are pumps that have a very steep head curve where lower flow results in higher horsepower. There may be blowers that also exhibit this tendency. But, in general, lower flow means lower brake horsepower. And, for a motor driven machine, this means lower amps.

Johnny Pellin

 

[MintJulep]

Moving less air --> doing less work --> need less power.

 

[MachinaMan]

Agreed, this can be confirmed by looking at your ID fan curve.

 

[HandsOnEngineer]

Thanks a lot for your explanation input. I think Im clear.

 

[electricpete]

Yes, as mentioned there are exceptions. In general radial flow machines act as described above (increasing power with increasing flow), axial flow machines are the opposite (increasing power with decreasing flow) and mixed flow machines are a combination (not necessarily monotonically changing)
faq237-1543

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(2B)+(2B)' ?