Standard for Critical Speed Margin of Upright Mixers 2

[kotridge]

Dear Gentlemen,

Is there any standard that we can use as reference for critical speed margin of upright mixers?
For example, our vendor claims that it is 10% bellow and above Nc (critical speed), i.e. 90%<Nc<110%, while other source from internet mention 30%.

This of course creates disputes while there is no firm standard that can be used as reference.

Kind regards,

 

[BigInch]

No way I'd buy a +/- 10% margin on anything called critical.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Latexman]

My vendor, a major agitator supplier, recommends the operating speed not exceed 65% of the first lateral natural frequency of the system if no stabilizers are used. 80% if stabilizers are used. This is for production size equipment and there are a few other constraints they have on max. runout, balancing, and max. operating speed.

Good luck,
Latexman

 

[MJCronin]

kotridge...

I agree with Latexman....

My observations and understanding of agitator design is that the drive shafts are very robust in order to ensure that operation is far below critical speed.

The common vibration problem with mixers/agitator applications is that the device begins a slow "walk" back and forth if there is too much flexibility in the tank mounting. This can be due to loose fasteners or a poor support design.

But these common problems occur at frequencies far below that of the "critical shaft speed"

Give us details of your installation (ie shaft diameter, configuration, speed, length and horsepower)

 

[moltenmetal]

A lot of small agitators are designed to operate ABOVE first critical speed but below 2nd. That begs the question: how do you start them up? The answer is VERY FAST- you need to get them up to full operating speed before they shake themselves to pieces. But it's actually tough to find a small agitator which is designed for below 1st critical. Forget about putting a VFD on these: even with block-out ranges in the VFD, you still can have problems during start-up or shut-down.

 

[Latexman]

kotridge,

What size is your application (hp or kW)?

Good luck,
Latexman

 

[Compositepro]

The mixed fluid often provides enough damping that critcal speed of the mixing shaft is only an issue while running a mixer when it is empty.

 

[kotridge]

Hai, sorry for the late response,
@Latexman,
here is the detail of one of the agitator:

Impeller diameter: 400 mm
Revolution : 200 /min
Shaft Diameter : 40 mm
Shaft Length : 2400 mm
Pitch : 600 mm
Modulus Longitudinal Elasticity : 1.97 x 10E6 kgf/cm2
Impeller Weight 1: 2.5 kg
Impeller Weight 2: 2.5 kg
Shaft Specific Gravity: 7980 kgf/m3
Shaft Weight : 24.07kg
Motor Cap: 1.5 kW 380V 50Hz 6P

Result of Calculation:
NC = 224.76 /min
Therefore: (N/NC)X100 = 89%..SCARRY RIGHT?

So?is there any standard for the margin?

Kind regards,

 

[kotridge]

@Compositepro..

If you would kindly explain the influence of viscosity of the mixed fluid as damper..
We are mixing 3000 cp fluid (polymer) using above mentioned mixer.

Kind regards,

 

[Latexman]

Viscosity dampening works for sure, HOWEVER one of these days the agitator will run while tank is empty. No matter how hard you try to prevent the agitator running when tank is empty, it will happen. When it does, you don't want agitator to destroy itself.

Good luck,
Latexman

 

[Compositepro]

3000 cP polymer would be highly effective at damping any oscillation that occurs at critical speed. I've also seen weight added to the blade end of the shaft to lower the critical speed. At low speed the out-of balance forces are low, so operating at critical speed is not damaging. I've worked mostly with variable speed mixers where it is hard to avoid passing through critical speed.