blower/vacuum pumps roots or centrifugal?

[viper6383]

I was building a flow bench and planned to use the little vac motors but I decided I need a " real" bench and my newer ideas will be MUCH eaier to implement. I plan to use a blower unit coupled to a 3phase motor and use a VFD to vary and reverse my system. My biggest question is roots or centrifugal style? We can get ahold of some Detroit truck blowers cheap! However, I have no idea on specs of the pumps or what it will take to power them. They are the 6-71 and 8-71.
We are also building a small vacuum hold down table for our CNC mill and need serious vacuum for that. We want about 15in/Hg on that one. Anyone have a roots or centrifugal style rig that can help me out here? It would seem that a centrifugal blower for the flow bench would be better suited dueto lower vacuum and higher cfm. Dunno

B

[Tony]

Centrifugal is better in many ways being smaller, and with reasonably constant output pressure over a wide flow range.

But if you could get your hands on a really cheap big diesel roots blower, it should work especially well with a VFD and three phase motor. The VFD could also make automatic test pressure correction possible. Most modern VFDs now have PID closed loop software included. With that, and a pressure transducer (automotive MAP sensor) it becomes possible to do very accurate closed loop test pressure adjustment and control.

I have never thought of this before, but a VFD could also reverse the direction of a roots blower, and that would simplify flow bench construction A LOT.

[viper6383]

Sounds like you are up on the electronics of a setup like this. We are still trying to find an easy way to monitor flow rates. Are you saying to use the map sensor to read the flow by monitoring the pressure differential in the flow tube? Or are you talking about using the map sensor output simply to have the VFD correct to a specific negative pressure?

[Tony]

No, but a MAP sensor could measure the test pressure applied to the cylinder head and, adjust the blower speed to hold it constant over a range of valve lifts.

Accurate flow measurement is an entirely different requirement.