oil carryover when pumping down

[Tung]

Will pumpdown cause oil carryover?
When working on a FRICK refrigeration plant equipped with a twin screw compressor, it seems no problem, the oil level in the separator remains almost the same after pumping down.
But for YORK twin screw compressor chiller, oil carryover problem seems much more serious, sometimes, the separator is empty(no oil level) after pumping down;
As I know the screw compressor used on a YORK YS chiller is actually derived from FRICK TDSH compressor, they just put it upside down, then why there is such difference?
I realize if the compressor is running at low “head, it would have oil carryover problem, but why?
Would anybody here know this can help me on answering the question?
It is the first time I present questions on this forum, I could make mistakes, then please advise as much as you can.

Tung

[NH3LVR]

Welcome to the Forum Tung!

A few questions to get the ball rolling.

When you say pumpdown, are you referring to pumping all the Refrigerant back to the receiver?
Does the oil come back when you restart the Compressor?
What Refrigerant and what type of system (Flooded with accumulators, pump recirculated, Direct Expansion)?
Also the conditions you operate at during the Pumpdown would be helpful. You say you have low Head, how low and why is that?

[TXiceman]

When a compressor is running low head (discharge pressure), the specific volume of the gas is up (or there are more cubic volume per cubic unit of measure). The mas flow is the same of higher which results in a higher velocity in the oil separator. This will usually result in a higher oil carry over unless the oil separator is design for the higher volume flow.

The YS is an air conditioning application piece of equipment does not have the operating range and design or a more industrial compressor. As a result, the oil separator in pretty minimal on a YS and you will see more oil carry over, especially at any off design condition.

The oil level in the separator will vary during shut down depending on if the compressor was loaded or unloaded at shutdown.

Ken

[Tung]

Dear Mr.NH3LVR
Yes, the refrigerant is pumped to the receiver for fixing a breakdown part, but for a YS chiller, the refrigerant is pumped to condenser since ther is no receiver.
When restart the chiller, sometimes the oil floods back to oil separator, but in most cases, the oil separator remains empty or only a little oil comes back.
R22 used to used and R134a for updated design chiller, the chiller is equipped with a flooded evaporator.
When pumping down, the discharge pressure(not condensing pressure) of R22 chiller could go below 170Psig, and for 134a chiller, 95Psig, sorry I can not recall exactly.
Also the oil carryover happens when the chiller run at low load condition during winter time.
I think Mr.TXiceman has offered a clue to that condition, but I am not quite understand.
Thanks a lot for your reply.
Best Regards

Tung

[NH3LVR]

Tung;
I am not an Air-Con guy and have little experience in the field, except for one plant, Therefore I have no experience with that particular Chiller.

What TX iceman is saying is that when the Head Pressure is lower than design, the Gas is less dense, and must flow at a greater velocity for the same mass to pass the the Separator. He explains it very well. Just takes a bit to grab ahold of the concept.

Do you have a means to keep the head pressure up to design conditions?

Although I am not qualified in this application, I would have concerns about losing Oil into the Evaporator and not getting it back.

[Tung]

Dear Mr.NH3LVR:
Thanks for explaining TX iceman's post to me,
To tell the truth, I've been thinking of it all this afternoon trying to get the concept clear, but I still feel puzzled;
What I don't understand is will the pumped oil and refrigerant passing oil separator in a higher mass flow and velocity when the compressor is running at low load?
For keeping the head pressure, the only way I know is to regulate the flow of cooling water, but hrer In Taiwan, since the wet bulb temp is not much different through the whole year, we seldom apply it
to an air con system or refrigeration plant.
For YS screw chiller,We have an eductor to force oil back from evaporator to compressor, and the FRICK refrigeration plant is equipped with an oil distiller, both of them work fine.
Thanks again for yor help!

Best Regards

Tung

[NH3LVR]

Dear Mr.NH3LVR:
What I don't understand is will the pumped oil and refrigerant passing oil separator in a higher mass flow and velocity when the compressor is running at low load? Tung

I believe that a a low load with a lower slide valve position the mass flow would be less, and the problem should not exist. But I do not have the expertise to comment on this in a definitive way, so I best leave that to others, such as TXiceman. I wish I could be of more assistance.
If your oil return is working do you eventually get all the oil back?
It would seem that pumpdown for service would not take long enough that the oil would disappear from the compressor. But again I am a bit out of my element here.

[Tung]

Dear Mr. TXiceman:
Thanks for yor reply,
Yes, that's true, the YS chiller would only have a minimal oil separator because of the cost consideration, and not only for YS chiller, I've found some locally manufactured air conditioning chillers also have the same problem;
The concept you state in the post puzzles my head,
I am still trying hard to understand it.
Best Regards

Tung

[Tung]

Dear Mr. NH3LVR:
When I have a YS chiller that loses oil during pumpdown, I will charge some oil to reset the oil level switch then restart chiller, and I'll throttle cooling water to get a higher head, then get the oil back through oil return device(eductor), when the oil level exceed the higher sight glass, I drain oil to a normal level.
I really appreciate your kindness!

All the best wishes

Tung

[US Iceman]

Tung,

I'll try to answer some of the questions about the oil separator that TXiceman posted earlier.

Here is the basic issue...

The oil separators and the coalescing elements inside the separator are selected on velocity. This is due to the need to control the refrigerant vapor velocity to some maximum value to achieve reasonable oil separation.

If the velocity is too high, then oil particles are carried out of the separator.

Volume flow = suction mass flow X specific volume of the vapor

Velocity = Volume flow / area of separator

From this you will find the following:
If the suction mass flow increases, then volume flow also increases. This means the velocity also increases.

If the mass flow remains constant and the specific volume increases, then the volume flow also increases as does the velocity.

So... For low discharge pressure operation the specific volume of the refrigerant vapor increases as the pressure decreases, so you can see what happens based on the above explanation.

Any time you exceed the velocity limits on the separator or coalescing elements you have a greater tendency to push more oil out of the separator and into the system.

Another problem is if the suction pressure increases. This increases the suction mass flow, which affects the separator velocity/

Does that help?

[Tung]

Dear Mr.USiceman:
The fomula: Volume Flow=Suction Mass Flow x Specific Volume of Vapor,
Does it mean the discharge Volume Flow?

For this issue, I'll emphasize the download status of a compressor since the carryover problem occurs most of the time when compressor is running at low load, as I realize, when a compressor is running at partial load, only part of the suction gas will be compressed into discharge port then the oil separator, the mass flow will be less than when it is running at full load or a relatively higher load;
and with a less mass flow times an increasing specific volume, I just can't make sure the Volume Flow will increase.

I really have had a confused train of though, and I don't even know how to raise "suitable" questions,

would you please kindly re-explain the theory more detailed?

Best Regards

Tung

[NH3LVR]

US&TX iceman;
I am getting a bit confused here myself.
If he is at low load, I would think he would have low velocity, and good oil separation.
If the pumpdown is causing the trouble as well, could his Educter be at fault? Does York use a Educter operated off the oil pump on Air-Con Compressors?

[Lowrider]

My understanding is that a screw compressor will unload to it's lowest load and then shut down. This should normally take less than 2 minutes on normal operation.

If it does a pump down, like on a reciprocating compressor, it's possible the setting of the pump down pressostat is too low.

If the screw is left running at minimal load, for too long it will push all the oil out and if the discharge temp is too high will trip and shut down at once, taking away the rest off the oil.

Futhermore an oil separator will only remove 90-95% off the oil. The rest will have to go back by means of a "gas pump" or oil return line from the bottom off the bath evaporator.

For shell and tube, just running at high load will get back all the oil.

[US Iceman]

The formula: Volume Flow=Suction Mass Flow x Specific Volume of Vapor,

Does it mean the discharge Volume Flow?

Yes. The specific volume in this example is at discharge pressure and temperature, so we are discussing discharge volume flow only as this is what affects the oil separator.

At low capacity the discharge volume flow does decreases because the suction mass flow is reduced.

However, there could be an instance of low discharge pressure at these unloaded conditions. If the discharge pressure is lower, the specific volume of the discharge gas increases. In addition, if during this time the suction pressure is allowed to increase, the suction mass flow could increase.

In this example an increase in suction pressure (higher mass flow) and a decrease in discharge pressure (higher specific vapor volume) the discharge volume could see an increase over full load normal conditions.

I'm not saying this is what the problem is, only that this is how the oil separator capacity can be overcome.

If the separator velocity is too high, additional oil is carried over to the coalescing elements which cause them to flood (oil logged). The the higher gas velocity pushes the extra oil off of the elements and back into the discharge gas leaving the separator.

If the oil return rate is lower than the oil leaving the separator then the oil level in the separator will continue to decrease.

[NH3LVR]

Sanderh;
I do disagree with a couple of statements. Bear in mind I am an Industrial guy-not Air-Con!

My understanding is that a screw compressor will unload to it's lowest load and then shut down.

Many times a Screw Compressor is not set up to cycle. It causes major problems in some systems.

Futhermore an oil separator will only remove 90-95% off the oil.

Oil Separators on modern screw compressors are very efficient. I have seen R-22 and NH3 Screws that pumped almost no oil at all, with no oil return at all.
I take care of a plant located 1500 miles away. The FES Screws only require oil to be added twice a year at most, and then only a minimal amount

[US Iceman]

The industrial refrigeration oil separators perform on the order of 3-5 ppm carryover if they are properly sized and operate within their design specifications.

That is not much oil that could be lost to the system.

[TXiceman]

It is very possible that once the screw compressor unloads, the oil cooling is not sufficient and the discharge increases. At higher oil temperatures, more oil is vaporized and oil separators can not stop vapor state oil. It is condenser in the condenser and winds up in the evaporator.

Most screw compressor have an increase in oil cooling load and an increase in discharge temperature as the capacity reduces. This is because part of the heat of compression is carried out of the compressor with the oil and most is carried out with the refrigerant. The mass flow is less at reduced loads, so the oil has to handle more of the cooling requirement. The oil cooling system on an industrial unit is designed to keep the oil within design parameters under part load as well as full load conditions.

Have you ever changed the oil separator coalescing elements? They may be allowing oil carry over and it is just aggravated at part load and pump down conditions.

Also, are you using the vendor recommended oil in the unit?

Another thing that can cause sudden oil loss is the ingestion of liquid refrigerant in the compressor. The liquid will cause a sudden increase in volume flow at the discharge and over load the separator elements. This can be so bad, that it will "blow-out" the coalescing elements.

Sorry for the rambling, but I tend to solve problems by tossing out ideas and see what the answers are. As you know, a refrigeration system is comprised on many components that all have to interact. One thing effects another and so on until you finally find the source of the problem. Some problems are readily apparent and others take some investigation.

Ken

[Mike W]

Tung
Did you solve your problem with the oil disppearing???

[Tung]

Hi, Mike W:
How are you going there,
one question before I response to your post,
do you work for York? and have you ever been to Shanhai China?

For the old York YS style A chillers, I throttle the cooling water flow, it raises the head and make oil carryover problem not so serious, but it seems it doesn't work in winter time-the temp of inlet cooling water could go down to 24 or lower.
now the YS chiller has been revised to style F, and I noticed that the oil separator has also been changed,
as my own experience, the new style YS chillers are better.
If you are working for York, then I think we can talk more about the York chiller, I also have experienced oil loss problem on YK-Centrifugal chiller in winter time even though the chiller is idle.

Best Regards

Tung

[Mike W]

[quote=Tung;77275]Hi, Mike W:
How are you going there,
one question before I response to your post,
do you work for York? and have you ever been to Shanhai China?


Tung, No I do not work for York, I work for a privately own company in New Zealand and I have never being to China.