Discharge pressure of a liquid ring compressor? 3

[chiukee]

I know that a LRC can compress a stream up to 15 bar, but is this the differential pressure or the absolute discharge pressure? I’m asking because I would like to compress a gas stream from 10 bar to 25 bar.

Would appreciate any feedback.

 

[Montemayor]

chiukee:

You cannot "know" the maximum discharge pressure on a compressor - be it liquid ring, reciprocating, or centrifugal type - unless you have the actual nameplate on the machine or you have the manufacturer's specific information. There are LRC's and then there are LRC's - just like with all other compressors. The maximum discharge pressure is determined by the manufacturer and is not "typical" of all LRC's. They are not all stamped out as in mass assembly.

A discharge pressure of 15 bar is already pretty high for an LRC, much less one of 25 psig! You've got to think as a mechanical engineer: it's the total pressure bearing on the machine (in this case, the 25 psig), not the difference between the suction and discharge that puts the stress on the machine's construction.

If you don't possess the original nameplate or documentation on the machine, then your best bet is to contact the manufacturer and request the specific rating information. Do not rely on general rules of thumb or hearsay in trying to make a sound engineering judgment about the maximum pressure rating on a compressor. It's manufacture is much too specialized and complex to allow us to make general opinions and guesses about such critical information. "Dem's that made it" are the best source.

 

[zdas04]

Whose LR will go to 15 bar? All of the references that I have limit the discharge pressure to about 2-3 bar (and up to 4 ratios depending on the seal liquid composition). The typical limit is the number of ratios to keep the heat of compression from boiling the seal liquid. Consequently, the manufacturers that I'm familiar with design their cases to a pretty low MAWP since their target market is vacuum suction and up to less than 2 bar discharge (compression ratio of 0.5 bar to 2 bar or 4 ratios)

Going from 10 bar to 25 bar is a cake walk for a single stage recip.

If you can't tolerate oil in your discharge stream, it is one of the (rare in my experience) perfect applications for a dry screw.

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

 

[chiukee]

Thanks for your advice. But maybe I didn't make myself clear enough. As I mentioned before, I want to compress a gas from about 10 bar to 25 bar. I thought maybe I can use a liquid ring compressor to achieve this, because there's an issue with regards to the leakage of the process gas. From what you guys have said, this is not possible with water as the seal liquid. How about with heavy oils or ionic liquids as the seal liquid? Any thought on this?

Best regards from Germany!

 

[Montemayor]

chiukee:

I'm old enough that I have used Mercury as the motive fluid in an LRC - it was the norm in the original Nash HyTor models and was used up until the early 1960's. That's why it was used on some applications as an air compressor, instead of just a vacuum pump.

However, as David has clearly pointed out, the performance of the basic design rests on the characteristics (particularly the vapor pressure) of the motive fluid in question. Today, with the emphasis against mercury or other potentially toxic fluids, the choice of fluid is drastically reduced on a practical basis. I have seen heavy oils, like mineral oil and even Dowtherm, used as the motive fluids. You must check out the applicability of the fluid with your manufacturer - as I keep repeating myself. You stand to affect the performance of seal and gasket materials, depending on the fluid you apply. Therefore, please follow what I recommend to ensure your success and safety. It's only common sense, and we engineers are supposed to excel in its use.

Also, please study carefully the compatibility of the selected fluid with the gases you are compressing, watching out for possible solubility - or worse, possible chemical reactions at the discharge temperatures and pressures. This part of the application probably belongs in your back yard instead of the LRC's manufacturer's.

25 bars is a hellava lot of discharge pressure to impose on the casting of an LRC. I've seen some great and beautiful German designs on compressors and the recent years may have even improved them to the point where there might not be a problem today. But, like Dave, I have to see it to believe it. That's why I recommend going to the manufacturer.

 

[MortenA]

David

Take a look at the LRC from Nash Elmo

This info is apperently not avaiable at their web page but i have performance curves for the AB2500 type with a max discharge pressure of 8 barg (two stage)

They also have a single stage with a 5 barg discharge pressure (but i dont have those data right here)

Best regards

Morten

 

[zdas04]

Morten,
I've seen LRC's with 8 barg MAWP (I saw one three stage skid advertised with 150 psig MAWP) in the advertising literature, but in real life what gets deployed tends to be closer to 3 barg MAWP.

These machines have a definate niche. I've been doing a lot of work lately with trying to take pretty big CBM wells (1-2 MMCF/d) to a moderate vacuum. The amount of gas you can get off the coal at -5 psig flowing bottom-hole pressure is amazing because the isotherm is almost vertical at those pressures. The capibility of a recip at -7 psig (0.5 bara) is truly lousy. Flooded screws are not much better. Both of them lose 5-10% of their volume-capacity for every pound of vacuum you go to. Enter the LRC. While its overall mechanical effeciency of a LRC is about half that of a single stage recip with positive suction pressure, the capacity hit that it takes going to vacuum is much less.

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

 

[Torricelli]

The two adresses for "high-pressure" liquid ring compressors are GARO (Italy) and NSB (Switzerland).

GARO on the Nash homepage:

http://www.nash-elmo.com/english/ne...MainMenu=NavNews&TierMenu=TierPress&NewsID=46

NSB Gas Compression:

http://www.nash-elmo.com/english/ne...MainMenu=NavNews&TierMenu=TierPress&NewsID=46

but 10 to 25 bar is very far away from possible for a LR-compressor as they are very sensitive to a positive inlet pressure.

Are you looking for an isothermal compression ? If not, the liquid ring is definitively not the right choice for 10 bar inlet pressure.

Hope it helps,

Torricelli

http://www.vacuum-guide.com

 

[Torricelli]

sorry, the adress for NSB Gas compression is:

http://www.nsbgas.com/prospekte PDF/SB-kompressor/PRSBE_Jan_2003_MA-WEB.pdf

Torricelli

http://www.vacuum-guide.com