Vacuum units, roots or vane

[viper]

I did not even know you could vary the rpm with a VFD on a vacuum cleaner motor or AC/DC motor. huh....

I will surely give the roots blower a shot. I am still in lala land on getting my readings but I know we will go all electronic with it and data log on a PC, would you have any preference on bench styles with this in mind? I know I had built the bench before to create a sealed box under the test port to read differential pressure but I am not what else I will need to extract all my data. We had talked about a velocity probe in the air stream but I don't think that will work.

[Tony]

A VFD adjusts BOTH the motor voltage and the frequency together.

An ac/dc brush type motor is only sensitive to motor voltage. Connected to a VFD it will certainly control motor speed, but it is only the variation in voltage that matters in this case.

[viper]

Your full of good info Tony.


Did you have any input on my electronic bench design? Is all this already on the forums. lol

[Tony]

[bruce]

[gtpumps]

I am using a roots blower driven by a 7.5HP AC motor controlled by a Siemens VSD. I will soon change over to a 10HP motor. And use 4" pipe, 3" is too small

[viper]

What blower are you using and what kind of performance are you seeing? Can I assume that by wanting to go to 4in pipe, you have too much depression and not enough flow?

[2seater]

I have the old 8V92 roots blower and I had pretty much decided to look into a large centrifugal blower instead, but, what Viper is doing has made me take a second look at it. I had planned to switch to sealed bearings, so I pulled one end from the blower. I made a tracing of the end of the clearance area between the rotor and the casing using 1/5" graph paper. As near as I can calculate, the void area is 4.12 sq. inches (103 1/5" squares). Two three-lobe rotors x the actual 16" rotor length yields .26 cu.ft per revolution. Does that sound about right? That sounds pretty close to the 8V71 blower that was mentioned but I would "think" the 92 series would be larger unless it is the same blower but just spun faster?

[Tony]

The advantage of a big centrifugal blower, is that basically it is a constant pressure device.

A roots blower is a constant volume per revolution device.

As we are trying to maintain a fairly constant test pressure over a very wide flow range, a centrifugal will be much easier to control. The pressure will not spike as much either, if flow s accidentally blocked.

While a big roots blower will make a truly splendid flow bench, especially if you already have one, it would not be my first choice if starting out with nothing.

[gtpumps]