oil separator inefficient

[jackcn]

oil separator for screw compressor inefficient when the condensing temperature is low.
why?

thks

[PobodysNerfect]

The volume that pass through the oil separator will increase. Normally I would expect that the oil separator has been dimensioned such that it would have sufficient capacity under all normal conditions.
Anyway, you can not take a compressor which is dimensioned for -30 / +35 and start using it as booster (-30 / -10), as the oil separator would be too small.
Steen

[jackcn]

Thanks to Pobodysnerfect

I think maybe the velocity will increase,

also the mutual solubility between the refrigerant and the oil will have influence on it

which one is most importmant?

and any other reason?

[Dan]

Under normal circumstances, an oil separator's efficiency increases as entering velocity decreases, as would be the case with the single variable of condensing pressure lowering. (I expect arguments here )

I think something else is changing that you are missing. Possible starvation of the evaporators which could mitigate the oil return from the suction lines camouflaging itself as an inefficient oil separator?

Okay, if that doesn't pan out, how about liguid overfeed!? That could dilute the oil in the separator, perhaps providing the same misindication of the loss of oil separator efficiency? Hmmm. .... possibly caused by ambient subcooling?

Good question.

[jackcn]

Thanks Dan

I wondered if the percent of the oil in the refrigerant gas will increase as the condensing pressure lowering?Can this influence the efficient of oil separator? or if we use different refrigerant and oil (E.G NH3 and R22 with their oil),the condensing temperature will have differant impact?

yes,for a reciprocal compressor this is a very important reason,but for an oil injection screw compressor,most of the oil is not from the evaporator.It's from the oil separator.so maybe the oil return from the evaporator will have less impact on it?

thanks to all reply

[Andy]

Hi jackcn
why do you state that the oil separator is inefficient at low condensing Are you having oil starvation to the screw. This may be due to the low differential pressure, not an inefficency in the oil separator, with the condensing pressure being too close to the suction pressure to create good oil flow.
Regards. Andy.

[PobodysNerfect]

Dan, maybe we have different assumptions. As I see it, if we have the same suction pressure and temperature, and we lower the condensing pressure then the volume (and velocity) going through the oilseparator will increase.
Jackcn, which refrigerant are you using? (or is it a theoretical question?)
Andy, a comment about the volume/velocity, please!
Saludos
Jan

[Andy]

Hi jackcn
If your discharge pressure increases, the mass flow decreases, but the velocity in the discharge line increases.
Reduced discharge pressure means increased volume, but decreased velocity.
If your separator is over sized the lower pressure will decrease the velocity to a point where the oil does not fall out of the gas and is carried over, remember oil separators for the most work by inpingement, the oil is knocked out of the gas by hitting the knitmess at speed, no speed, no knocking effect.
Hope this helps. Regards. Andy

[lin]

How discharge superheat work on oil seperating efficiency?
On some manufacturers manual(like york), efficiency decrease with superheat. so they control the expand device maintain higher superheat when oil level low.

[Andy]

Marc,
can you elaborate. As I was look for a little info on this area, but I have yet to find any.
Kind Regards. Andy

[Andy]

Hi
surely if the discharge pressure drop so does the evaporation. will this not cause the mass flow to change, or am I missing something.
Regards. Andy

[Dan]

As I see it, if we have the same suction pressure and temperature, and we lower the condensing pressure then the volume (and velocity) going through the oilseparator will increase.

Yep. You are right, Pobody. I was wrong. Thanks. Nice explanation, Marc.

I still don't think the change in the capacity of the separator would be noticeable simply because of a lowered discharge pressure.

How discharge superheat work on oil seperating efficiency?
On some manufacturers manual(like york), efficiency decrease with superheat. so they control the expand device maintain higher superheat when oil level low.

Lin, are you saying that York increases suction or evaporator superheat to collect more oil in the separator? We are talking about screw compressors, correct?

[lin]

Yes. it's york YS "E" screw chiller. digested from manual:

Excess oil mixed with the refrigerant in the
evaporator will cause severe foaming when the chiller
is started. This foaming carries liquid refrigerant into
the compressor, lowering the compressor discharge temperature.
As the compressor discharge temperature
approaches the condenser saturation temperature (low
discharge superheat), the refrigerant gas has a greater
affinity to absorb more oil. When the discharge superheat
is reduced, oil cannot be separated from the refrigerant
gas in the oil separator.

[lin]

And in carrier 30HXC screw chiller with low oil level, some service men bypass oil level switch, start chiller with nearly closed liquid supply valve, the ensure high superheat. before chiller shutdown of low evap. pressure, most of oil is reclaimed to oil sump.

I wonder if superheat just help oil returnning from flooded evap. or it increase oil seperator efficiency.
But many screw chillers have oil problem under low condensing temp.

[jackcn]

Sorry I have not described my question clearly. There are 2 problems accompanied with the condensing pressure decreasing in the system. The first is oil starvation to the screw compressor, the reason is the pressure differential is not enough; the second is when the compressor¡¯s discharge temperature(super heat) decrease with condensing pressure decreasing, the oil will go to the system and cause other problem.

My question is about the second one.

Marc said,
Aerosol sized oil droplets. Those that are too small to be a mist, and so behave more like a gas, meaning they cannot be impinged. Instead, true coalesers or chemical type eliminators need to be used.

As I know, for all high efficiency separator(nearly 5-10 ppm), the gas type oil cann¡¯t be separated, and will go to the evaporator. I wondered if the oil¡¯s ppm (percent of oil gas in the refrigerant gas), will increase with the discharge temperature decrease. And If it will influence the efficiency of the separator greatly? (Maybe the influence on the refrigerant R22 and NH3 is different?)

Or some other reasons cause the migration of the oil?

Surely, It¡¯s a theoretical question.

from Lin's manual:
When the discharge superheat
is reduced, oil cannot be separated from the refrigerant
gas in the oil separator.

that's just my question.

thanks all reply.

I think this is really a good forum for me.

[lin]

but i still doubt "the refrigerant gas has a greater
affinity to absorb more oil." on the manual.

[Bones]

well... from corys manual hehe that makes me sound like i'm smart, i only hope my explaination is close to making sense in my world.

with a higher superheat you should get more oil returning with your refrigerant then you would with a low superheat setting because more liquid would be coming back giving you a lower pressure (as andy said closer to suction pressure) therefore a lower temp and less ability to carry oil back because it would be cold and less likely to move... and could even start to log in the evap coil.

it would also depend on correct pipe sizing, allowing enough velocity for the refrigerant to carry the oil back to the compressor(and correct temp/pressure to do so). also correctly sized oil separator and care so no liquid condensation at low loads to cause liquid slugging and associated problems (insulated separator?).

gota fly dinners here! hope this makes sense...

[Andy]

Hi
most screw manfacturers recomend a discharge temperature that is a minimum of 15K higher than the condensing temperature. This is simply to stop the refrigerant from condensing in the oil separator, replacing the oil returning to the oil accumulator section with liquid refrigerant. This refrigerant would boil off and foam the oil, reducing it's lubrication properties.
I was wrong about the velocity. Velocity will increase with volume flow, for a given pipe size. If you reduce the discharge pressure the volume flow will increase (the compressor pumps more gas due to the lower thermal lift.
Hope this helps. Kind Regards. Andy.

[Andy]

Hi Marc
another thought I had was the screw compressors internal VI (VR) outside design conditions the Isentropic effeciency will drop due to over compression, in this case where the discharge pressure is lowered.
Take a look at the Vilter cool compression design, they say the effeciency of their system is similar to thermosyphon, but their system is basically liquid injection, all be it close to the discharge point. Would not the liquid be carried over will the oil to a certain extent, regardless that it is suposed to sit on the oil. Would this not effect the volumetric effeciency.
Regards. Andy.

[Dan]

First things first, merry Christmas to all.

Marc, you are bringing much to the fore regarding oil separation. I wonder (for my sake, although I would like to think for others' as well) if we could put into a sorted listed, the elements of oil separation, and perhaps see if different aspects require differing focus.

1. Reduction in velocity
2. Change in direction
3. Precipitation

I want to add a fourth, which would be descriptive of the coalescent style of separator, but I think the word "precipitation" covers it for now.

Would it be fair to say that the 3 points listed above are key elements of oil separation?