Calculating tip speed?

[acfm]

Can somone assist me with information on how to calculate the tip speed of a vacuum pump? I'm trying to determine the slowest speed that I can operate the vacuum pump without the liquid ring collasping? The actual capacity is not important.

Thanks!

 

[quark]

I don't know what is tip speed. But to check the minimum speed you can equate centrifugal force on liquid ring to it's weight. This is what is done with Centrifuges also.

If m is the mass of the fluid inside the pump then

mg = mV²/ro

Where g = acc. due to gravity
V = Speed of liquid ring (consider it equal to that of pump speed, ideally at no slip condition)
ro = Outer radius of the ring (so that you will get maximum minimum speed) For more accuracy you can do with (ro+ri)/2

If you know the volume of water and outer radius, it is easy to calculate inner radius by concentric cylinder volume formulae.


So V = (rg)^1/2

But I fear this speed is for zero discharge.

Note: This is just a simple idea and I bear no responsibility. :-(

Cheers!

 

[MortenA]

assuming that you mean tip of impeller/rotor then it should be easy:

You need to know the rotor diameter and the rpm.

Then tip speed=PI*diam*rpm/60)

Use preferred unit for speed in your "diam". Eg use meters and you get m/s

Best Regards

Morten

 

[PierreSTRAUCH]

Dear acfm,

A tip speed of min. 15 m/s is required to obtain a stable liquid ring (when using water or similar density, with atmospheric discharge pressure and for 50–100mbarA suction pressure).

Smaller liquid ring vacuum pumps are designed to be direct driven with tip speeds of 20-22m/s for cast iron and 22-25m/s for stainless steel.

Suction capacity V is (approx.) proportional to rotating speed : V1=V0.(n1/n0) while the power absorbed P will be (approx.) P1=P0.(n1/n0)1,4

hope it helps,
Pierre

http://www.vacuum-guide.com