Vibration with bitzer!!!!

[Georgis]

Hi everyone.I have serious vibration problem with Bitzer motors.I use the new ECOLINE SERIES motor and all the 2cylinder motors(e.x:2des-3y or 2fes-3y) are create big vibration.I also must tell you that i use the motors in multi systems(units) with 2or3 motors in paralel and inverter the first one and R404 for medium temperature(-10 to +45).I tried everything... changed the phases(rotation),chek valves in dishgarge line,pressures suction and dishgarge line.I gave a solution by changing the 2cylinder motors with 4cylinder(4des-4fes) and the unit was something else.Does somebody know if the 2cylinder motors have problem?Thank you very much George...

[Glenn Moore]

Hi Georgis
Is the vibration on the 2 cylinder speed controlled lead machine or on all the 2cyl machines (Fixed Speed)?

2 Cylinder compressors especially invertor speed controlled types are the worst type for starting and running due to the opposing piston forces. 4,6,& 8cylinder machines are easier to start and smoother to run due to the multi pistons giving an equalising effect to the motor shaft..
Is the vibration across the frequency range or just certain frequencies ?. If its only certain frequencies then you can set the invertor to miss the frequencies that cause the most vibration

[Georgis]

Thanks for answering Glenn.The vibration was on all the motors(variable speed and on/off).Of course in the inverter motor between 35Hz and 30Hz before stopping the vibration was bigger.Do you think that i will have the same problem if i use Bock or Copeland compressors?Because i will not use again Bitzer 2cylinder and i must find o solution for small units...

[Glenn Moore]

Hi georgis
Hav you tried the Copeland digital scroll or the Maneurop VTZ variable speed compressors as an alternative these machines are used extensively in packs.

[Peter_1]

We have a lot of Bitzers running on a VFD. Every compressor has a point where vibration will occur.
This is a phenomena related to every running machine but I don't know the correct English expression for it.
Purely translated Own vibration number.
Our experience is that the Bitzer compressor will do this around 30 to 33 Hz. It changes also a littlke bit with changing condensing pressures.
So, you must program in your VFD (most VFD's have this feature) a SKIP FREQEUNCY band so that it jumps from let's say 31 Hz tot 33 Hz. Try different settings to narrow this band.

[Grizzly]

We have a lot of Bitzers running on a VFD. Every compressor has a point where vibration will occur.
This is a phenomena related to every running machine but I don't know the correct English expression for it.
Purely translated Own vibration number.
Our experience is that the Bitzer compressor will do this around 30 to 33 Hz. It changes also a littlke bit with changing condensing pressures.
So, you must program in your VFD (most VFD's have this feature) a SKIP FREQEUNCY band so that it jumps from let's say 31 Hz tot 33 Hz. Try different settings to narrow this band.

A good point, well made Peter. The skip feature within controllers does indeed exist with most I have come across.
I often wonder when some do their Vibration analysis logs, whether they realise how important the Compressor load Point is?

To be fair Glenn covered the point well in the first reply.
Unnatural / increasing vibration is still a very useful and free Indicator of how well things are running.


Georgis
Your issue may have been improved by putting larger comps in place, but think of all that extra mass that is being moved. As with all plant designs there is a compromise, many will advocate the fitting of vsd's of everything!
Which is seen as a cost saving. Not So!
Fit a vsd on a chiller water pump circuit and watch the energy consumption rocket as the chiller loading chases the fluctuating water flows!

Most of my time is spent NOT getting plants to reach their full design load. BUT getting them to run smoothly as the load drops off!
As designers each add in a little more capacity to cover the original design speech. you can very quickly end up with an install that is 3 times the size it needs to be.
The issues you briefly describe are highly likely to be those of plant sizing.
Grizzly

[Carlos Barrantes]

Dear friend ,

You require a vibration analysis considering all compressor-tubing-supporting base-building , all of these components has its own vibration (frequency) and when two match resonance occurs . The compressor may resonate at a specified speed . To solve the problem must increase the mass of the support base filling it with concrete and avoid "critical speed" of the compressor. Remember that the compressor is not in a cloud.

Exitos 2015

[sterl]

These machines spring mounted or otherwise vibe isolated by a mat or similar? Are you cylinder unloading as well as speed unloading?

There is no practical way to change the natural frequency of the compressor/spring arrangement with speed.
What you did by adding motor size was increase the flywheel mass and diameter which would shift the natural frequency of the assembly down. Slowing the compressor also slows the oil pump and at some low speed you can anticipate you will run out of oil circulation or pressure.
The rotary compressors should handle this much easier though both screws and scrolls require velocity to shear lubricant in order to seal high pressure from low. So an AC scroll machine can be run fairly slow but a machine trying to make a larger pressure difference is going to loose its oil seal. A lower diameter scroll is going to have more pressure difference issues than a larger diameter. Sometimes the larger diameter scroll is accompanied by a larger diameter motor which will shift the nat frequency lower.