Parallel Screw Compressor Rack Oil Fault

Hi guys,

I have a tricky problem that I can not seem to get my head around, please post some suggestion on what might be a cause to this problem.

I have a twin screw compressor rack, both compressors are Bitzer HSK7451-70 models.
My problem is that I have started the compressors up a month ago and they have both been running fine but only recently have had an oil fault issue where there is no oil in oil seperator and compressors have tripped on low oil level in oil seperator.I have checked the oil seperator as there is an upside down plate internally before the oil sep discharge pipe to catch the oil and cause it to fall back to the bottom of oil sep and this seems to be ok. Here are some operating details:

Medium Temperature,
R404A
Lead, Lag Operation/Demand
Air cooled condenser set to maintain head pressure of 260 PSI with VSD driven condenser fans,
Air cooled oil cooler with cooling fan set to cut in at 65 degrees Celsius,

Compressor 1 Cut In: 75 PSI, Cut Out: 40 PSI
" " Unloader 1 C.I: 60 PSI, C.O: 55 PSI
" " Unloader 2 C.I: 54 PSI, C.O: 41 PSI
Compressor 2 Cut In: 85 PSI, Cut Out: 50 PSI
" " Unloader 1 C.I: 60 PSI, C.O: 55 PSI
" " Unloader 2 C.I: 54 PSI, C.O: 51 PSI

The compressors are used for a couple of fast chillers, chilling freshly picked corn. Room temperature set points are +3 degrees C.

What I do know is that the compressors have a check valve in the discharge portion of the screw compressors to prevent compressor reversing in the off cycle. If one compressor is running and the other one is off, could there be a chance that if the check valve is faulty on the compressor thats off, could the oil be flowing past the faulty check valve and through the compressor and into the suction accumulator?

I have checked the oil solenoid to see if there is oil passing through it when the compressor is off but there is no flow through the sight glass which is after the oil solenoid which is a good thing I presume. If oil solenoid was leaking past when compressor is off I could imagine that the oil would flow through compressor and into low side accumulator.

Has anyone had this problem before? I am also wondering if the unloading sequence might be causing an issue but may need some technical help with that. Thanks

Ki Chemuss,
what oil are you using, ester oil, viscosity??
is there an oil cooler, if yes, what is the oil temperature.
PS:it's time to stop using R404!

chemuss,
Can you explain system a bit more.

- common or individual oil separator?
- type of evaporator, suction trap, expansion device type etc.
- oil return method.
- if 2 oil separators on common plant, how do you balance oil levels.
- does oil separator/s have non return valve on outlet?

- each compressor has inbuilt discharge check valve, if you close suction does suction pressure increase?
- does Bitzer recommend second oil separator with coalescer, as standard primary cyclonic separator not that efficient.
- oil separator sump heater working when off, what temp is it set to?

Hi,
This probably won’t be very helpful
I’ve seen this problem before
(Contact the manufacturer - they say “nope, this hasn’t ever happened before, you are the first one, it must be something unique to your application..”)
We installed a push button switch across the Separator oil level switch so anyone with a finger can get it going again

Were the oil levels checked after initial commissioning of the plant, is there sufficient oil in the system ?

Hi Chemuss

Sounds like the colder ambient temps on your air cooled condenser are causing refrigerant/oil migration. Is there a check valve installed after the oil separator?

Hi Guys,

Thanks for your replies. I will try to answer all your questions in one hit.

Yes there is an oil cooler: set point for fan to turn on is +65 degrees C. Oil is Emkarate 170 grade and viscosity last time I checked was 160 when oil test was done 12 months ago. This system only runs for 6-7 months, from May through to November due to ambient temperature affecting growing conditions in North Queensland.

Oil separator is common. There is a check valve on the discharge pipe of the oil separator. 1 x large common suction accumulator. 2 x high humidity boxes as evaporators, each high humidity box has a split evaporator coil- so 1 x TXV feeding each coil = 2 x txv's per high humidity box. TXV's are mechanical and where replaced two seasons ago when refrigerant was changed from R22 to R404A and old condensor was replaced. TXV's super heat was set to 3.5-4k at time of commissioning two seasons ago and have no reason to change, the TXV will close if the txv bulb pressure is lost and they are externally equalized. Oil is fed into compressors via discharge pressure. Oil travels from oil separator through oil solenoid valve then through oil flow switch which has a 20 second time delay and through ball valve and through sight glass and into compressor which is a common set up for most Bitzer screw compressor racks. Compressor then discharges oil after it has lubricated screws and goes through a non return valve/check valve before it is discharged through discharge service valve then through discharge line and back into oil separator and then it does the cycle over again. if there is no oil flow into compressor the oil flow switch detects no oil and switches compressor off. Oil separator sump heater set to around 55 degrees C, or around 20k above standard ambient at this time of year which is +30 to +40 degrees C. I have not shut screw compressor suction service valve as I always thought it was a bad idea as it will cause compressor to run in vacuum which Bitzer specifies not to do, I just make sure there is a normal suction and discharge pressure for the given conditions to test wether the compressor is pumping efficiently. Do you mean to shut the suction service valve with the/one compressor off and then to see wether the corresponding compressor that is running raises the suction pressure to discharge pressure to check if check valve is leaking? I will try this as it makes sense to do it with the compressor off. The cyclonic oil separator is what has always been on the rack system but it has only recently had this fault. the rack is about 10 years old, the compressors are about 4 years old, it used to have open drive bitzer screws on R22.

I had both the compressors trip on low oil level alarm this morning. there is no level in the separator. I would have thought that if the check valves were leaking and the oil had bypassed into accumulator then the oil should have come back to separator when both compressors turned on and ran.

It seems as though the oil is out in the system. I will pump system down a couple of times to get oil back and then hopefully I can get the system to settle long enough so that I can start diagnosing the problem. If the oil is out in the system it seems as though I might have a TXV issue or leaking past liquid line solenoid valve in the off cycle? I am just scratching my head with this one and chucking these ideas out there, sorry if this is confusing.

Oil separator has three sight glasses and is vertical mounted, middle sight glass is usually fill with both compressors running There is an AMOT valve that operates to bypass hot oil to oil cooler.

If it’s an older system check for (tube in tube) suction / liquid heat exchanger at the Evaporators

A potential source of flood back..

Quote:

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Originally Posted by chemuss

Hi Guys,

Thanks for your replies. I will try to answer all your questions in one hit.

Yes there is an oil cooler: set point for fan to turn on is +65 degrees C. Oil is Emkarate 170 grade and viscosity last time I checked was 160 when oil test was done 12 months ago. This system only runs for 6-7 months, from May through to November due to ambient temperature affecting growing conditions in North Queensland.

Oil separator is common. There is a check valve on the discharge pipe of the oil separator. 1 x large common suction accumulator. 2 x high humidity boxes as evaporators, each high humidity box has a split evaporator coil- so 1 x TXV feeding each coil = 2 x txv's per high humidity box. TXV's are mechanical and where replaced two seasons ago when refrigerant was changed from R22 to R404A and old condensor was replaced. TXV's super heat was set to 3.5-4k at time of commissioning two seasons ago and have no reason to change, the TXV will close if the txv bulb pressure is lost and they are externally equalized. Oil is fed into compressors via discharge pressure. Oil travels from oil separator through oil solenoid valve then through oil flow switch which has a 20 second time delay and through ball valve and through sight glass and into compressor which is a common set up for most Bitzer screw compressor racks. Compressor then discharges oil after it has lubricated screws and goes through a non return valve/check valve before it is discharged through discharge service valve then through discharge line and back into oil separator and then it does the cycle over again. if there is no oil flow into compressor the oil flow switch detects no oil and switches compressor off. Oil separator sump heater set to around 55 degrees C, or around 20k above standard ambient at this time of year which is +30 to +40 degrees C. I have not shut screw compressor suction service valve as I always thought it was a bad idea as it will cause compressor to run in vacuum which Bitzer specifies not to do, I just make sure there is a normal suction and discharge pressure for the given conditions to test wether the compressor is pumping efficiently. Do you mean to shut the suction service valve with the/one compressor off and then to see wether the corresponding compressor that is running raises the suction pressure to discharge pressure to check if check valve is leaking? I will try this as it makes sense to do it with the compressor off. The cyclonic oil separator is what has always been on the rack system but it has only recently had this fault. the rack is about 10 years old, the compressors are about 4 years old, it used to have open drive bitzer screws on R22.

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Yes, close suction when compressor off to se if pressure increases.
You might be able to check solenoids in liquid line if close one at a time, look for frost or cold pipe if leaking.
3.5 to 4 deg C super heat sounds a bit to low to me, oil in system could cause irregular superheat as well. Would have thought 6-7 deg.
check evap coils are clear 100% use torch or prove they are clean/clear, no icing or dirt build up.
If you had temp data logger consider placing on discharge for sudden temp drops while running.

Hi Ranger. I think the problem is sorted now. Changed TXV's super heats to 7k once system settled. Had to pump system down a couple of times to get oil back to oil separator, now have oil separator middle sight glass full. The cold rooms have been running with minimal load as the corn has not been harvested yet. New TXV's where installed a couple of seasons ago and I set them up to have between 3-4k super heat and they where commissioned when we had large heat loads from harvested corn and the cold rooms pulled the heat load out of the corn very quickly which was ideal. It seems that since the plant has been running for a month now with little to no load that there has been possible slow coil ice up and then liquid flood back/oil wash out occurring. I will keep an eye on it but want to thank everyone for the awesome feed back. Cheers guys.

Quote:

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Originally Posted by chemuss

Hi Ranger. I think the problem is sorted now. Changed TXV's super heats to 7k once system settled. Had to pump system down a couple of times to get oil back to oil separator, now have oil separator middle sight glass full. The cold rooms have been running with minimal load as the corn has not been harvested yet. New TXV's where installed a couple of seasons ago and I set them up to have between 3-4k super heat and they where commissioned when we had large heat loads from harvested corn and the cold rooms pulled the heat load out of the corn very quickly which was ideal. It seems that since the plant has been running for a month now with little to no load that there has been possible slow coil ice up and then liquid flood back/oil wash out occurring. I will keep an eye on it but want to thank everyone for the awesome feed back. Cheers guys.

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chemuss, hope it works out, you will know once load comes on if enough capacity.

Can you explain how loading/unloading, starts & stops.
Is there a controller, timers, pressure switches?

In your first post you asked about unloaded set points, which is fine, but forget pressures, as we are really interested in saturated suction temperatures, also PSI is ok if you like in USA, kpa or bar is more accurate anyway.
Not knocking you in any way, but the temperature ( degrees celcius) tells all straight away compared to room temps what’s going on.
Same with condensing pressure as well, as if given pressure for the million refrigerants out there, all have the one thing we have to convert to.
I will stop lecturing now, keep us updated.