Recip compressor with VFD

[iainuts]

When compressing a gas using a recip with a VFD, are there any significant issues regarding inlet and discharge valves on the cylinder? Valves can be tuned to a specific frequency of operation (ie: the RPM the compressor is running at) but if RPM is reduced by 50% or more, I suspect the valves will have a tendancy to flutter which may cause damage over a prolonged period.

Is this a significant issue that needs to be addressed when designing or specifying a compressor?

Have you ever seen problems with recips (valves) being driven at widely varying RPM's, such as accelerated wear on rubbing parts or on valve faces or any impact areas?

What, if any, features should such a valve have to minimize wear on valves that may flutter?

 

[dcasto]

I've never seen what you are talking about, even with belt drive, running motors with 1/2 the rpm of the maximum compressor rpm.

Having said this, most compressor valves have the ability to change the springs and the manufacturer will help you specify thier K valve (strengt or firmness). You will have to fiquire out the trade off. A stronger tension spring would decrease flutter, as you describe it and increase life, but it will cost more energy in the system by the amount of excess pressure it will take to open the valve. So its the valves that will be addressed, not the compressor.

There are two things you will to address with compressor and a VFD. At lower RPM's the compressors oil pump may not turn fast enough to supply the proper amount. The coupling may have some dynamics that could be affected by rpm ranges, including a criticle speed that produces a harmonic. The coupling issue is very rare.

There may be other issues with gas cooling or something, just some thoughts.

 

[zdas04]

To echo those points, it is really rare in real life to select a significantly different valve on a machine in service unless the valves start coming apart. You tune a set of valves for a very narrow range of differential pressure, specific mass flow rates, temperature, etc. All of those parameters change drastically with a fairly minor change in rpm. I've never run a VFD on a recip, but the effect of changing engine rpm can be significant and unpleasant.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[iainuts]

Hi dcasto,
You're right about the oil pump if it's driven off the crankshaft, but that's a side issue. Let's disregard.

I'm going to require a recip compressor that can turn down by a factor of 10 or more due to process conditions.

My concern is that the valves will run well at one speed, but not at another RPM, so I'm wondering what can be done about this. Some manufacturers will tell you there's not a problem, but that seems optimistic. Calculations I've done indicate the valve will open and shut rapidly at low RPM, so there must be some potential for accelerated wear both on sliding surfaces and the sealing surface.

 

[iainuts]

Thanks for the feedback, David.

 

[pmover]

to supplement other postings and without knowing process conditions and HP requirements, the pulsation study (if required) will be indepth and voluminous resulting from the changing rpm.

just curious, will an oil-flooded rotary screw with slide valve be more applicable for this application? these units are used in applications (i.e. with turndown). visit:

http://www.mycomcpc.com/prod_how.html

for further details.

fyi, the oil carryover concern can easily be resolved.

good luck!
-pmover

 

[dcasto]

Combine the VFD with unloaders.