Oil carry-over

[Segei]

Hello, everyone.
I'd like to hear your opinions about oil carry-over for screw compressors. A compressor manufacturer can recommend certain minimum condensing pressure for certain compressor to prevent oil carry-over. Assume that this pressure is 110 psig. What will happen at condensing pressure of 105 psig? Will oil carry-over double, triple or increase gradually? Can excessive oil carry-over damage the coalescent filter?

[Grizzly]

Hi Segei.

I am a little confused with your post.
Only because most of the screw comps I work on require Discharge pressure to push the oil around the compressor.
Oil is then recovered from the oil separator or coalescer in the discharge line.
Secondly and I may be wrong but I was always taught that a certain amount of oil carryover is inevitable!

So I will be interested to see what the rest of the guys have to say.
You may well be teaching me here!
Cheers Grizzly

[RANGER1]

The smaller differential pressure the compressor runs, the more mass flow rate of gas it will pump .
A booster duty compressor will pump more gas than a high side duty machine if they are same size , so booster needs more coalescers .

By dropping head pressure by 5 psi can't see any real reason to throw over any more oil .

If coalescers were for some reason were heavily logged with to much oil filters could be damaged
ie if coalescer compartment was holding oil level and not returning excess oil back into suction of comp via oil return for whatever reason .

Its pretty common in this situation that filters have become loose and gasket has shifted .
Check filters for this first and see whats going on .

[US Iceman]

Hi Segei.

Looks like all the old timers beat me to this thread, he he.:D

Generally speaking, if the oil separator is properly sized for operation at 110 psig discharge pressure & the suction pressure does not increase, the oil separator should be OK.

You have two factors that affect oil separator performance (assuming the coalescers are correctly sized and in the proper quantity). An increase in suction pressure increases the compressor mass flow due to an increase in vapor density. Higher mass flows equates to higher velocities in the separator.

If the discharge pressure reduces, the specific volume of the discharge gas increases, which also increases the separator velocity.

If the suction pressure rises and the discharge pressure is reduced simultaneously you can have some increases in oil carryover.

If the separator velocity increases too much the velocity through the element increases and strips oil off of the element and pushes the oil back into the discharge gas. Also, if the elements flood out with oil, the same thing occurs.

Normally, you have more problems with elements when you flood out a screw with liquid refrigerant!

[Magoo]

Sounds like one of those power saving gurus has been to site.
As per previous, compresson ratios, gas velocities, separator design, etc, etc,.
You save power but system is out of balance.
How often do we all hear it. I want a job as a consultant power saver. Turn everthing off if you want save power, screw quality.

[Segei]

Hi Segei.

Looks like all the old timers beat me to this thread, he he.:D

Generally speaking, if the oil separator is properly sized for operation at 110 psig discharge pressure & the suction pressure does not increase, the oil separator should be OK.

You have two factors that affect oil separator performance (assuming the coalescers are correctly sized and in the proper quantity). An increase in suction pressure increases the compressor mass flow due to an increase in vapor density. Higher mass flows equates to higher velocities in the separator.

If the discharge pressure reduces, the specific volume of the discharge gas increases, which also increases the separator velocity.

If the suction pressure rises and the discharge pressure is reduced simultaneously you can have some increases in oil carryover.

If the separator velocity increases too much the velocity through the element increases and strips oil off of the element and pushes the oil back into the discharge gas. Also, if the elements flood out with oil, the same thing occurs.

Normally, you have more problems with elements when you flood out a screw with liquid refrigerant!
Hi, Mike.
I agree with you. This is the reason that I asked this question.
Recently I've read. "There are specific recommendations from compressor manufacturers based upon extensive testing. These recommendations should be followed without field experimentation by the operators." This is about lowering head pressure below 110 psig. It sounds like demands but not recommendations.
Example. End user built a refrigeration plant. Compressor manufacturer recommended(demanded) to keep head pressure 130 psig or higher. This recommendation was followed for 5 years. After that end user lowered minimum head pressure to 120 psig. No additional oil carry over, but he saved $20,000 on energy costs. Should compressor manufacturer compensate $100,000 to end user? Every demand should come with responsibility.

[Segei]

Sounds like one of those power saving gurus has been to site.
As per previous, compresson ratios, gas velocities, separator design, etc, etc,.
You save power but system is out of balance.
How often do we all hear it. I want a job as a consultant power saver. Turn everthing off if you want save power, screw quality.
Magoo,
Thank you for calling me guru. I'm not a guru right now, but hopefully I will be in future;)
Actually I have been on the sites for the last 30 years. Unfortunately, I'm getting old.:( I understand your frustration about so called "experts" but these people do not participate in our forum. Sometimes they do one post and disappeared.
By the way. I saw more than 100 refrigeration plants and every plant has a balance even one that operated at 60psig(4 bars) head pressure.:)

[Magoo]

Hi Serg.
I have seen several plants operating real low discharges, but compression ratios were controlled to maintain velocities through compressor and oil separators so as not to carry-over excessive oil, generally done by compressor load rates and maximum loading positions reduced, or alternatively using smaller compressor and swept volume, there by maintaining system balance.
magoo.

[sterl]

Screw compressor packages use coalescent elements (largely) and they act like a filter IN THE SENSE that they loose effectiveness when subject to more than a certain gas speed. Did you know: Some of those same folks put (4) coalescing elements in a separator set up for (6) and blank off the extras?

So one method that does work:

Put the extra coalescers in and run the oil level a little low....

Other than that: There is a limit at which the oil will get too cold for decent distribution but I only see anything even close on a 2-stage R-507 plant and the fun with that one could fill a novel.

[sterl]

Oh there is another hard line limitation: Some of those older liquid injection control arrangements just don't function worth ...it at 110 Psi on R-717.

[US Iceman]

Oh there is another hard line limitation: Some of those older liquid injection control arrangements just don't function worth ...it at 110 Psi on R-717.

That's a secondary issue to low discharge pressures. If you use liquid from the high pressure receiver (it's not so high at 80 psig:D) you do not have sufficient pressure to develop the rated valve capacity. Same thing happens with other liquid feed valves also.

[US Iceman]

Unless it has changed Vilter used to state the minimum design discharge pressure was 140 psig. That's not that low. However, we did build some screw packages that were designed for 90 psig or so. It's possible if you carefully evaluate the separator operation.

One thing people have to understand: when a manufacturer provides a piece of equipment, you either:

A) tell them how you intend to operate the equipment, so that they can design it for those operating conditions, or

B) accept what the standard design is from a manufacturer. Then if you operate it out side of the design envelope you own it. People should not expect the manufacturer to cover their lack of insight as a warranty issue (in all fairness to the equipment manufacturers) or,

C) ask what the minimum and maximum design criteria are so that allowances can be made or controlled.

60 psig is possible. Probably lower for normal cooling, but you might have to periodically raise the discharge pressure during defrosts or use a dedicated recip. compressor to generate the discharge gas for defrosts, while allowing the majority of the compressors to operate at low discharge pressures.

There are a lot of ways to get this done. But not very many are used because it requires thinking outside of the box!

[Segei]

A lot of thinks can be done during design stage, but my concerns about existing refrigeration plants. Several times I heard that compressor manufacturers require unnecessary high minimum condensing pressure. My guess. They have done extensive testing of the compressor and they know that border minimum pressure is 110psig. However, to be safe they recommend 130psig. They don't pay for the energy. Sometimes contractors recommend 4 defrost per day and later operator determine that 1 defrost per day more than enough.

[US Iceman]

Hi Segei,

Yeah, you're right.. I think a lot of manufacturers have based their package designs on higher discharge pressures for oil circulation, etc. The design premise they used was probably based on the days when everyone ran a minimum of 150 psig discharge pressure. Now that people want to run at lower discharge pressures they have to stipulate the minimum allowable pressures to keep from having problems. The issues they consider may determine what those allowances are.

Defrosts is another subject! I've seen the same think. Four a day, no matter what happens. The one thing I don't think a lot of people realize is: when you reduce the number of defrosts you pick up a lot of cooling capacity, since more evaporators are cooling, instead of defrosting.

[Segei]

Hi Segei,

Yeah, you're right.. I think a lot of manufacturers have based their package designs on higher discharge pressures for oil circulation, etc. The design premise they used was probably based on the days when everyone ran a minimum of 150 psig discharge pressure. Now that people want to run at lower discharge pressures they have to stipulate the minimum allowable pressures to keep from having problems. The issues they consider may determine what those allowances are.

Defrosts is another subject! I've seen the same think. Four a day, no matter what happens. The one thing I don't think a lot of people realize is: when you reduce the number of defrosts you pick up a lot of cooling capacity, since more evaporators are cooling, instead of defrosting.
Hi, Mike.
About defrosting. My people don't realize that this is very inefficient process. Typically, less that 20% of the heat goes to melt the frost and 80% heating the cold room. This is if you have a frost. At four defrost per day efficiency is in the range of 1-2% because almost no frost. It means that every 6 hours we just heating the cold room. That isn't all. Typically, after 20 min of defrosting next 30 min we should run this coil and part of refrigeration plant to remove the heat of defrosting! This is huge energy wasting.

[US Iceman]

Not to mention the amount of hot gas blown back into the suction line because of defrost relief valves!

I like drainers.;)