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Rotary Screw Versus Vane Compressors- which one is a good choice?

Jfarid (Mechanical) (OP)
20 Jul 11 12:18
Hi Folks,

Just a basic question about rotary compressors:

what are the advantages or distadvantages of a rotaty screw compressor over a rotary vane compressor or vice versa? we are using it for plant instrument air  application and need to decide on one type? Apparently, a vane unit is quite economical than a screw unit.

I would like to get feed back on performacne, machine life, maintenance issues, noise and ability to handle solid particles(although the application is air however would like to know if the service was for natural gas VRU applications)

Thanks for your time.

Jfarid
  
 
JJPellin (Mechanical)
20 Jul 11 14:04
You are probably going to need to provide more details to get much value from the response.  The terminology is not always consistent between different industries and in different countries.  So, please be specific.  

•    If this is an air machine, what capacity range are you looking for?  Some machines are more economic when they are large. What discharge pressure do you require?  Some machines work very, very well, but are limited to relatively low pressures.
•    Are you asking specifically about dry screw or flooded screw technology?  Perhaps you don't care. I have never heard of anyone using a dry screw compressor for air. But, if you had a great need for oil-free air, someone might consider it.
•    Are you talking about sliding vane or some other vane technology?  We only have one sliding vane unit in air service, but it is a very specialized non-lubricated model.

In plant air (instrument air) service, we use all of the following:

•    Oil lubricated reciprocating piston compressors.
•    Oil flooded screw compressors
•    High speed centrifugal compressors (multi-stage)
•    One non-lubricated sliding vane compressor

We have dry screw machines in off-gas service.  We have liquid ring machines in vacuum service.  We have oil lubricated sliding vane machines in flare gas recovery service.  We have many multi-stage centrifugal compressors in variety of services.  
 

Johnny Pellin

Jfarid (Mechanical) (OP)
20 Jul 11 17:08
Here are specific details on the unit:

capacity: 50 scfm.
Discharge pressure: 125 psig

I am refering to an oil-injected compressor and the compressor is a sliding vane unit.

Thanks

Jfarid
zdas04 (Mechanical)
20 Jul 11 18:46
Johnny,
The only place I've ever seen dry screws is in air service, it is funny to me that you've never seen them in that service.  I guess "normal" really is a function of the path you've taken.

Jfarid,
50 SCFM for an air compressor is less than 15 hp, or about $10/day in electric cost.  I wouldn't spend a lot of time chasing much effeciency improvement.

VRU's are the single hottest topic in my world right now.  It is an amazingly complex subject and probably should be its own thread.

David
JJPellin (Mechanical)
20 Jul 11 22:14
We have a number of oil flooded screw machines in plant air service running at about 125 psig.  In general, they all run very well and have been trouble free.  They are package units that came in complete, so installation was relatively simple.  There is a certain about of routine maintenance required for the filters, coolers and coalescers.  However, it seems to me to be a small price to pay.  The oldest of these machines has probably been running for about 15 years with no major repairs.  

We do not have any oil lubricated sliding vane machines in air service. But the ones we have in other services are much more trouble.  We tend to see wear on the vanes which requires a rather intensive PM program.  We have to remove and measure the vanes for wear and replace them frequently. This has improved since we changed to Kevlar vanes.  We tend to see wear in the bores of the cases (wash-boarding) that cannot be fully repaired.  After some number of years, the vibration and vane wear caused by the worn cases requires us to completely replace the case.  Our sliding vanes machines were not purchased as package units, so the installation was quite complex.  We have a circulating glycol system for cooling, an oil injection system for lubrication of the compressors and bearings.  We have gearboxes to achieve the needed speed which required another circulating oil system for the gearbox and motor.  

The flooded screw machines are much quieter than the vane machines.  The building that houses our largest pair of sliding vane compressors is one of only two areas in the plant that requires double hearing protection (plugs plus muffs).  The noise level is too high for ear plugs alone.

Given my experience, I would much prefer the flooded screw option for air service rather than the sliding vane machine.  I am not sure I know what you refer to as VRU, so I can't comment on that service.  

Johnny Pellin

zdas04 (Mechanical)
20 Jul 11 23:16
VRU is "Vapor Recovery Unit".  As state and federal governments are tightening the restrictions on outgassing from tanks, people are starting to get serious about stabalizing liquid hydrocarbons in a place where the vapors can be recovered.

I get questions about VRU's every day.  The solutions range from amazingly ineffectual, to ok.

David
bcs5274 (Industrial)
20 Jul 11 23:23
For longevity at 125 psi the oil flooded screw would be my choice. In a low volume and low discharge pressure I.e. less than 50 psi the rotary vane is ok but efficiency over time is not one of its best atributes  
25362 (Chemical)
23 Jul 11 6:56

Sliding vane compressors are noisy.
dcasto (Chemical)
1 Aug 11 18:06
My experience with rotary vane are that the vanes break and the unit is down all the time.  That was in nat gas service. The large differential pressures cause lots of problems with the vanes too.

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