Liquid Ring Compressors 1

[zdas04]

I've been asked to describe the region where Liquid Ring Compressors would be effective. After a couple of hours in Google, it looks like all the companies are really cagey about reporting max discharge pressure. I was able to dig out a couple that had a max discharge slightly above atmospheric. Anyone out there with experience in this equipment with a discharge pressure above 1 atm (gauge)?

Also, comparing performance curves to recips, it looks like the adiabatic effeciencies are around 50% (single-stage recips work out to over 85%). Does anyone have experiences that would indicate higher values?

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.

 

[Scipio]

I've seen liquid ring compressors used in Vapour Recovery Units, drawing overhead vapours off stock tanks slightly above atmospheric pressure and discharging in the neighborhood of 400 - 500 kPag. Haven't had any hands on in their design or performance though, so don't have anything useful as far as efficiencies go.

 

[Montemayor]

David:

Before you can discuss max. discharge pressures achievable with a liquid ring compressor, I believe you have to specify the liquid fluid you plan to employ as the liquid sealant-piston medium. As you know, the higher pressures (especially in a single-stage) will generate a higher discharge temperature and that higher temperature will affect the corresponding fluid medium's vapor pressure.

As an example, I've used mercury in a Nash Hytor (back in the early 1960's) with 135 psig discharge (but not consistantly at that pressure level. This, of course, has fallen into disfavor today because of environmental alarms. I've also used heavy lube oils at lower discharge pressures - 35 to to 50 psig with heavy fluid cooling externally.

I don't know what you mean by "adiabatic" efficiency but I would expect the machines to be far superior to the recip on a volumetric efficiency level but inferior in the overall mechanical efficiency due to the pumping of heavier fluid medium being necessitated. I believe you will find that the machine will approach pure adiabatic operation much less than the recip due to the cooling effect of the liquid fluid employed - especially one with relatively high vapor pressure - such as water. Once the compression ratio is raised, the need for cooling the liquid fluid medium will increase. This is a marked and notable effect when the machine is used as a vacuum "pump"; if the liquid medium's vapor pressure is approached, the machine stalls and vacuum "pulling" ceases. This is a situation where the correct and logical selection of the liquid medium is very important in order for the machine to impart its "liquid piston" effect.

While the vapor pressure of the liquid medium is the machine's "Achille's tendon", its ability to cool while compressing and to handle dust, liquids, and solids in the gaseous compressed medium with ease while effecting a "scrubbing" action is next to none. I never had the opportunity to employ these machines in such critical and unique compression applications such as Chlorine, HCL, SO3, etc and such bad actors, but I do know that Nash had a wide list of these working, empirical applications in the industrial world when they had a monopoly on their patent and license over the machine. Now that the original patent and license has surely expired, many top machinery fabricators have come forward with their version of the original Nash model. Siemens (SiHi), I believe, is one of the more reknowned fabricators today.

These machines are a unique, ingenious pure American invention born of Yankee ingenuity - something hard for a good ole Texas boy to admit, but true. They have a niche and an application window that will not, in my lifetime, be replaced by any other type. The concept is so pure and flawless when applied according to the basic scope of the machine, that little or nothing can go wrong. However, as I stated previously, the underpinning of the whole successful operation is the selection and employment of the correct liquid piston fluid medium.

I'm curious to learn about other forum members experiences with this machine type and their comments on its reliability, efficiencies, and engineering input. I hope these key learnings are of some use.

Art Montemayor
Spring, TX

 

[zdas04]

Art,
"Adiabatic Effeciency" is short-hand for "Mechanical Effeciency at Adiabatic Conditions". I know that any machine will lose some amount of heat to the environment, but the question is "if the heat loss to the environment is assumed to be negligible, what is the oveall mechanical effeciency?" I guess the term isn't as widely used as I had expected.

I didn't realize how sensitive to vapor pressure the discharge pressure was in these machines. I don't know of anyone in gas fields that is willing to use mercury for a seal-liquid. Thanks for bringing that up. Mostly the seal is water, some use synthetic oil.

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.

 

[Torricelli]

David,

As Art said the overall mechanical efficiency of a liquid ring compressor is not really attractive:

The total power absorbed (HP motors to pay) consist of
- isothermal work of compression (very good) plus
- mechanical looses (very high - as you can read on the performance curve at delta p =0)

LR-compressors are not used because of their efficiency but they provide a safe and nearly isothermal compression.

PST

http://www.vacuum-guide.com

 

[Torricelli]

David,

LR-compressors are available up to 15 barA (1500m³/h).
A typical application is the ozone compression for pulp bleaching in the paper industry (The ozone would dissociate at T° higher than 40°C).

PST

http://www.vacuum-guide.com