PET reciprocating compressor

[vorda]

Hi everybody

Does anyone of You have some experiance with capacitity controll of reciprocating compressor by changing speed with frequancy converter?
Is it possible to controll from 1-100%?
Vibration problems?

Thank You for any suggestion!

Davor

 

[dcasto]

typically you do not have 1 to 100% control because of lubrication of the bearings. If you could set up a seperate lubrication system, then, I suspect you could get down to near zero RPM. During start up a recip with a VFD, they start and ramp to at least 50% of their maximum RPM.

To obtain more capacity, valve unloaders are typically used.

 

[nickelkid]

lmf.at has a PET package that is energy saving oriented 55-100% control.


dcasto, your suspicions on a seperate lube system is correct. I started up a recip with a DC motor that had variable speed capacity control. This type of design (VFD or DC) requires a comprehensive torsional analysis.

 

[dcasto]

RCHandy. yep on the torsional analysis. I had a hurry up job that called for a VFD. We found the Ariel compressors and a VFD with a 4 week delivery. The torrsional analysis said the VFD would only be good for 950 rpm to 1200 rpm, not much of a range. Given more time, we could have matched a machine better.

 

[vorda]

Thank You people for Your answers!

My aplication is 3 stage 40 bars reciprocating compressor driven by 200 kW electric motor with capacity control by load/unload.
When I use this compressor for blowing small format of bottels, motor run more than 70% unloaded.
So I intention is to install VSD for controling RPM of motor to have better compressor effitiancy.

I am electritian so I preffer electrical solution for my aplication if it is possible.

Can You give me some web source where I can find more information about my application

Thank You

 

[dcasto]

I'd talk to your compressor manufacturer first thing. Then the motor manufacturer because will most likely need a new motor.

 

[nickelkid]

you will need a torque effort curve of the compressor to size the VSD correctly.

I think you will find that the frame size of your new motor will need to be increased to have higher fan capacity for motor cooling, you may also may need to have additional air source for cooling the motor in low rpm's.

 

[sdra2]

Some thoughts, and I'm happy to be shot down on any of them..

* Does your compressor never switch off - is it always either loaded or unloaded, but running? If true, this would seem to be one easy inefficiency to eliminate, limited by the number of starts permissible per hour possibly?

* Any change to the running speed will change the mechanical loadings on the compressor - the gas forces will remain similar but the inertial forces will change - can the design permit this? Not only bearings to consider here, I would suggest.

* A new motor, plus inverter drive is liable to be very expensive: plus, how much power is required by the inverter itself? How much saved money/increased efficiency is there likely to be in reality, and what will your payback be? You may find this is none/infinite - you may not.

* You are right to be concerned about vibrations at certain speed ranges. How are you going to find this out before purchasing and installing an expensive VFD system?

* The compressor cylinder valves will most likely be optimised to some degree or other for operation at the speed your compressor runs at currently. Will running at different speeds cause valve inefficiencies to outweigh any potential benefits of VFD?

* There are a multitude of possibilities for mechanical capacity control that would be a lot less capital intensive. Whether any of them would lead to any efficiency improvements for you (in the loosest sense) is a moot point, however.

* My opinion - do not do anything without a buy-in from the compressor manufacturer.

 

[crjones]

A word of caution on lubrication. In many compressors there is a minimum piston velocity to get lubrication on the wrist pin in the piston. On some units, this is thru a hole in the connecting rod. So you need to get with manufacturer of your equipment otherwise ad hoc redesigns could have you replacing cylinders.

 

[dcasto]

Here is another warning. Some VFD fire 4 to 8 times per cycle. This step wave causes a reciprocating compressor to also "step". We had some 5000 HP compressors that could not be operated at max RPM because the gas pulsation tore up the bottles. The same compressor driven by a gas engine didn't have this problem.

 

[Sam654]

With the cost of a new VFD and motor along with the cost of installation, you may be better off with buying a smaller compressor if you're running 70% unloaded. Do the economics.

crjones is correct about lubrication. Depending on design, some components depend on slash lubrication.