Tandem Danfoss Scroll Compressors _ Oil Level Problems

[Refrigerologist]

We have a water chiller with 2 separate refrigeration circuits into a shell and tube evaporator. These are equipped with electronic expansion valves and the superheat has been checked on the controller and via electronic manifold and temperature sensors. Super heat is 8.0K on both circuits.

Subcooling is around 10K, discharge temperatures are around 73°C. The refrigerant is R454B.

The problem we are seeing is that on the off cycle the oil level in both compressors on a circuit is at the top of both sight glasses. If we operate on compressor on its own the oil levels remain stable. When we switch on the second compressor, with a few minutes the oil level drops in the second compressor, this is actually always the right hand compressor in the pair.

After about 15 to 20 minutes to oil level in the right side compressor drops to the very bottom of the sight glass. If we continue to operate the compressors the oil level in the right side compressor drops further. The left side compressor oil level is above the sight glass.

Danfoss scrolls have an 'organ pipe' installed in the oil equalisation ports of each compressor. This is actually to prevent oil being pushed out of an idle compressor into the discharge pipe. There is a video on You Tube that explains this. We have the exact opposite with oil leaving the right side compressor only when both compressor are operating.

The chiiler manufacturer has received video evidence of what is happening. They have responded by stating that it is impossible. The size of compressor that is installed has the 'organ pipe' fitted as standard, so we know this is not missing. With a superheat of 8.0K we are sure it is not saturated vapour or liquid entering the compressor.

The manufacturer has discounted the idea of pressure equalising lines between the sumps as the 'organ pipe' design should take care of this.

Has anyone come across this phenomena before? If so what was the cure?

[seanf]

Could the organ pipe have fallen out or turned during operation or assembly?

Is there an oil seperator?

What happens to the oil levels when both compressors are switched off, does it quickly return to both comps with oil filling the sight glasses?

Are both comps running at full capacity?

[Preethi]

Issues with oil levels in tandem Danfoss scroll compressors can arise due to various factors such as inadequate lubrication, oil leakage, or improper installation. Regular maintenance and proper monitoring are crucial to prevent such problems and ensure compressor efficiency.

[Refrigerologist]

Could the organ pipe have fallen out or turned during operation or assembly?

As the organ pipe is factory fitted on these compressors by Danfoss and the compressors are installed by the OEM, we have no idea. I think it is unlikely. Also the organ pipe is to prevent oil loss from an idle comrpessor in a tandem or triple circuit. This is happening with both compressors in operation, oil levels in both compressors remian good with only one compressor in operation.

Is there an oil seperator?

Oil Separators are installed

What happens to the oil levels when both compressors are switched off, does it quickly return to both comps with oil filling the sight glasses?

Oil level returns to top of the sight glass within a couple of minutes, it is only the right side compressor that drops when both are in operation.

Are both comps running at full capacity?

The compressors are at full capacity as these are scrolls with no unloading capability.

[Refrigerologist]

Issues with oil levels in tandem Danfoss scroll compressors can arise due to various factors such as inadequate lubrication, oil leakage, or improper installation. Regular maintenance and proper monitoring are crucial to prevent such problems and ensure compressor efficiency.

This is happening on a pair of brand new chillers. The commissioning has been double checked by the distributors own senior engineer.

[seanf]

As the organ pipe is factory fitted on these compressors by Danfoss and the compressors are installed by the OEM, we have no idea. I think it is unlikely. Also the organ pipe is to prevent oil loss from an idle comrpessor in a tandem or triple circuit. This is happening with both compressors in operation, oil levels in both compressors remian good with only one compressor in operation.

Are there any components in the suction line that might cause a pressure drop between the shared suction and the left hand compressor?
Does the suction pipework return and feed the left hand compressor first? So that maybe the right hand comp when off sits in stagnant gas.

Oil Separators are installed

Do the compressors shared one oil seperator or does each have its own?
Could oil return to the right hand comp be restricted, or be being diverted to the left hand comp?

Oil level returns to top of the sight glass within a couple of minutes, it is only the right side compressor that drops when both are in operation.

The compressors are at full capacity as these are scrolls with no unloading capability.

Does the right hand comp perform as well as the left when ran by itself? Oil level, amps, pressure, temps and all that.


See blue text

[monkey spanners]

Probably not much help but we had issues with oil return many years ago on a five compressor pack/rack, turns out the manufacturer of the oil shut off taps forgot to drill the hole through one of them.

[Refrigerologist]

Are there any components in the suction line that might cause a pressure drop between the shared suction and the left hand compressor?
Does the suction pipework return and feed the left hand compressor first? So that maybe the right hand comp when off sits in stagnant gas.

There are on components in the suction line. The oil return line from the separator is into the suction line and is equidistant from each compressor.

As I stated, the oil loss is not happening when the right side compressor is idle.

Do the compressors shared one oil seperator or does each have its own?
Could oil return to the right hand comp be restricted, or be being diverted to the left hand comp?

Yes it is a shared separator. It is a single return line to the suction pipe. The left side compressor oil level is high. I would say too high as it could be above the level of the sight glass. Oil balance pipe is 13/8" so I doubt it is restricted.

Does the right hand comp perform as well as the left when ran by itself? Oil level, amps, pressure, temps and all that.

Both compressors perform perfectly by themselve. Almost identical amps, temps and pressures.


The distributor has engaged the services of an outside expert, who is well know in the industry. He has taken a look at a similar chiller with the same problem.

His verdict, the OEM has added too much oil to the system. This is causing the level in one to rise too high and it is effectively blocking the gas flow path to equalise the compressor sumps. I must admit it is not something that I had considered, but it may well be correct.

Hopefully if this wet and windy weather goes away, we will go back to site and will remove oil 1 litre at a time to see what happens. I am very hopeful that this is the answer.

What do you guys think? Obviously I'll report back. I will be taking videos to present to the manufacturer. Maybe I will post it on I & I Tube, sorry, I have just seen the Bob Marley biopic.

[Refrigerologist]

Probably not much help but we had issues with oil return many years ago on a five compressor pack/rack, turns out the manufacturer of the oil shut off taps forgot to drill the hole through one of them.

Yes, I used to work on supermarket multi pack systems.

I had a service manager that added 50 litres of oil to a triple 30hp Bitzer compressor pack. He didn't realise or understand that the strainer socks in the oil separator had burst open due to a build up of crap. Next he sent a guy that had done a 2 week refrigeration training course to site. He figured that the oil float was cream crackered, so he changed the float and added yet more oil.

After we changed the oil separator it took about 2 weeks of return visits to get the excess oil out of the system. It did slightly cause one compresor to smash itself to bits. The service manager was a panel beater by trade, trained by an electrician.

[seanf]

It'll be interesting to hear how things go.

[Refrigerologist]

Attended site this morning and removed 2 litres of oil. The one litre that we put is at the behest of the distributors commissioning engineer and another one for luck.

Monitored and observed the results. No change, still getting very low oil level on the rights side compressorand oil level still very high on left compressor.

Dropped out another litre of oil. This time the left side compressor oil level dropped so that to just over ¾ full, but the right side stayed low. I should point out that we did allow the oil levels to equalise each time with the compressors both off prior to restarting them.

What I did notice was that there seemed to be a lot refrigerant in the oil, even though we have an 8K superheat on the electronic expansion valve.

We also noticed there seem to be a lot of vapour in the oi return line from the oil separator. I think it is a problem with R454B and we will now be getting a visit from the expert to give us his opinion.

[seanf]

Is the oil return only controlled by the separators float, or is there a solenoid?

Is there a check valve after the oil separator?

Do the compressors have liquid injection?

Do the oil separator and compressors have sump heaters?

[Refrigerologist]

Is the oil return only controlled by the separators float, or is there a solenoid?

Oil return in control by the float. There is no solenoid valve.

Is there a check valve after the oil separator?

There are no check valves. I am not sure what purpose a chek valve in the oil return pipe would serve. The oil return is into the suction pipe. Any pressure in the discharge pipe would overcome any pressure in the suction side of the system. To my mind a check valve would be pointless.

Likewise a check valve in the discharge pipe after the separator would serve no useful function. This pipe will be heading to the condenser and there is probably a very small pressure drop through the separator, so the pressure from the compressors will be very slightly higher than that leaving the separator. Flow will be one way. There is no problem when both compressors are idle, or with one compressor operating.

Both compressors have inbuilt non return valves in the discharge side. I have posed the question that if one of these had failed open would it cause an oil problem. I have received no reply to that yet.

Do the compressors have liquid injection?

No, there is no liquid injection.

Do the oil separator and compressors have sump heaters?

Yes, there are sump heaters, these are all working. The compressor base plates are extremely warm to the touch.



This is not going to be a simple fix. Most of what you are intimating is all very basic stuff. We think this is something that is unique to R454B. Apparently other chillers that have been having oil problems have been converted from R454B to R410a and this has stopped the problems.

We are getting a visit from an expert in the field. If no one on here comes up with a probable cause, then we will have to wait for his verdict. That will be in a couple of weeks time. I will keep you all updated if anything changes.

All help and ideas is appreciated.

[seanf]

There are no check valves. I am not sure what purpose a chek valve in the oil return pipe would serve. The oil return is into the suction pipe. Any pressure in the discharge pipe would overcome any pressure in the suction side of the system. To my mind a check valve would be pointless.

Sorry, I should have said on the discharge pipe after the separator.

Likewise a check valve in the discharge pipe after the separator would serve no useful function. This pipe will be heading to the condenser and there is probably a very small pressure drop through the separator, so the pressure from the compressors will be very slightly higher than that leaving the separator. Flow will be one way. There is no problem when both compressors are idle, or with one compressor operating.

If I remember they put a check valve on the discharge after the separator to stop any liquid forming or returning on the off cycle.

Seen a few systems where a check valve has been retro fitted because of liquid being returned via the oil return line.
And one where it didnt have a check valve, and a problem with the TEV gave a nice frosty oil return line while the system was runnning.

Could liquid be condensing in, or running back to the oil separator on the off cycle, and being drained to the comp by the float?


Could poor condensor fan control and/or excessive refrigerant charge cause liquid to appear at the oil sep?

Sorry, it is just the very basic stuff I'm thinking of. But sometimes it is the daft stuff that gets forgotten about.

[knighty]

could check compressor amps.... shouldn't tell you anything... but if it does....

[RANGER1]

https://assets.danfoss.com/documents/latest/362276/AB230286438491en-001501.pdf

https://assets.danfoss.com/documents/latest/231072/AB230086439211en-000901.pdf

Danfoss have gone to a bit of trouble to explain things here.
Someone might get something out of it.

[Refrigerologist]

could check compressor amps.... shouldn't tell you anything... but if it does....

Not sure what compressor amps has to do with oil leaving a compressor, but the amps have been checked. 8No. compressors on 2No. chillers, all pretty much the same across all 3 phases given similar conditions.

[Refrigerologist]

https://assets.danfoss.com/documents/latest/362276/AB230286438491en-001501.pdf

https://assets.danfoss.com/documents/latest/231072/AB230086439211en-000901.pdf

Danfoss have gone to a bit of trouble to explain things here.
Someone might get something out of it.

Yes, I have read those, there is also a video on you tube expaining the purpose of the organ pipe in the oil return ports of the compressors. Sadly, what we are seeing is not something explained in the video or the documentation.

It pretty much looks like it is going to be a conversion to R410A as recommended by the distributor who has witnessed oil problems on triple compressor circuits, but this one on a tandem set up is not one they have seen before.

As I stated, the manufacturer has said it's impossible, but we have videos of the compressors operating singly or in pairs and when operating together it is the only time the right side compressor empties. The left compressor never does. Suction pipe from the manifold tee is equal. They have used the Danfoss tandem pipe kit. It has been installed as per Danfoss instructions.

Removing the oil sight glasses, installing oil balance floats and rerouting the oil return from the suction pipe to the oil floats might cure the problem. Even with that I have my doubts due to the speed at which the oil leaves the sump. Unfortunately there is no room to install the floats as the compressors are in accoustic housings and they are a tight fit, so it is not feasible in any case.

It is a very weird one to say the least.

[Refrigerologist]

On reflection the manifold distance to the compressor is not equal. This may actually be done on purpose.

We have had it explained to us that Copeland, when using their compressor in a tandem or triple set up, purposely use a smaller diameter suction pipe to one or two of the compressors. This is done so that the compressor with the larger size suction pipe acts as an oil reservoir. The higher than normal pressure drop in the other two comprssesors induces oil to be pushed / pulled from the 'reservoir' compressor via the oil equalisation pipe.

I am wondering if danfoos purposely want a slightly longer suction pipe to the right side compressor to induce such a pressure drop and the first compressor is meant to act as an oil reservoir.

We may also be mistaken in regard to the organ pipe being installed at the factory. Having reread the manual for the compressors, it states that the organ pipe is an accessory. As one of the compressors has been changed, this may be missing as the engineer that changed the compressor, like us, was not aware of the pipe.

This is also happening on an identical chiller on the same site. It would be a massive coincidence if the factory has forgotten to install the organ pipe on the No. 2 compressor. Both other tandem circuits balance the oil correctly.

We are stumped and are awaiting the report from the company that have checked over the chillers using a Clima Check device.

The good news is that the guys that came over did say the operating conditions of the chiller is amongst the very best they have witnessed. There is very little devience is pressures and the pull down rate is extremely steady with an almost straight sloping line. The chiiler output is also as we expected at the -7°C flow temperature and 33% ethylene glycol solution. So we know the chilled water flow rate is within spec. The measured COP was around 2.8 so that would appear to be pretty good to me.

It will be interesting to see what the factory response is.

[seanf]

Be interesting to hear whats causing it.

Cheers

[seanf]

Hi Refrigerologist,

Have you had anything back from the expert? or found the problem yourselves?

Cheers