Oil Lifting Problem in NH3 screw compressor

Dear friends

We are facing oil lifinting problem in Bitzer NH3 screw compressor model- OSKA 5361K,Opoeration at -6 deg. evaporation, 30 deg condensing, 150 psi discharge, We drain the 10 litre oil for evry 24 hrs running from PHE chiller surge drum. Compressor having inbuilt NRV at discharge port.

We observe that there is no oil collection neither from discharge line oil seperator nor from receiver bottom.

Please suggest to resolve the problem.

If any information required contact us.

Regards,

Suresh & Murali

Hi... everybody please help me out from this problem. If you require any information for solving and understanding the problem kindly let me know.....

Does it have a secondary oil seperator with coalescer filter ?
Usually first sep with oil resevoir is primary and second is final filtration ( inline type with oil return ) .
Believe it should have and if it does it may need changing (they are very expensive for what they are ) .

it have both primary & secondary oil separator. Both are working ok.

I am a bit confused, maybe its the terminology but "Oil lifting problem" is this oil loss or something else?

probebly you have a problem with a checkvalve ,if you have it in the system

Quote:

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Originally Posted by SURESH YADAV

We drain the 10 liter oil for every 24 hrs running from PHE chiller surge drum.

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If you have to drain this much oil every 24 hours, then the compressors/oil separators are pushing too much oil into the system.

And, your problem may be caused by this...

Quote:

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Originally Posted by SURESH YADAV

Opoeration at -6 deg. evaporation, 30 deg condensing, 150 psi discharge

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Since you provided the discharge pressure in psi, are the temperatures above in °C or °F?

Oil is lifted by the compressor and it is collected from the evaporaror...

yes there is

yes there is check valve in the Discharge line as well as in the suction side...

Quote:

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Originally Posted by US Iceman

If you have to drain this much oil every 24 hours, then the compressors/oil separators are pushing too much oil into the system.

And, your problem may be caused by this...



Since you provided the discharge pressure in psi, are the temperatures above in °C or °F?

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Discharge temparature is 100 °C

If the condensing temperature is 30°C (86°F) and you have 100°C (212°F) discharge temperatures the discharge superheat of the compressor is 70° (or 126° on the F Scale). That is very high!

At the same time you say the discharge pressure is 150 psi, which is fairly close to the saturation temperature of your condensing temperature.

You either have extremely high suction superheat or no oil cooling based on what this seems to be.

Another possible cause for high volumes of oil loss is if the discharge pressure suddenly decreases. Do you have hot gas defrost on the evaporators? When the coils go into defrost, what happens to the discharge pressure?

Another thing to look for is a bad check valve in another machine that may be off.. If you have another machine on the same system and it is off close the suction valve see if this solves your problem. (had similar issues here) Not sure how its piped in. But if you have a machine that is losing allot of oil i would start there. Are you draining the oil from the compressor or the surge drum? Is the sight glass on the oil separator side full? Is the pump in working order for oil return? Just some other ideas

The subject machine used to work on the same suction pressures and discharge pressures as it operates on currently, right?

Considering your operating discharge temperature for the listed Evaps:the coalescers in the oil separtor are being challenged, but I have and idea thats a secondary issue. Could be the temperature has compromised one of the seal rings, or O-Rings at the coalescer's flange??


What method is used for oil cooling? Thermosyphone, secondary coolant or liquid injection or ?? What sort of oil? Usiing a low viscosity mineral oil at these temperatures is going to promote oil carryover.

Shut the subject machine off, and watch its shaft or motor as it stops...Watch the separatorr pressure as well.

If it stops and turns about 60 deg in reverse, and the separator pressure and the suction housing pressure head for the same pressure pretty quickly, a pressure well above the the nominal suction, it doesn''t mean anything....

If the machine rotors backwards any more than about 3-turns: Your suction check valve is probably not holding...

If it continues to back rotor for quite some time: Both check valves have faulted, S & D, or if so equipped: Suction only, the path for entry gas is via the economizer port; though I doubt this machine is economized,..

Depending on the machine and the package builder: you may observe the vessel and suction pressure stay around 90 psig; both pressures drop slowly, over 5 to 6 minutes, to about 3-psi over suction; or both go to near condensing pressure and stay there.

If both drop quickly: The suction check valve or the suction re-start bypass or the oil return from the secondary side of the separator is not right....

Also ensure that the machine has unloaded prior to the main motor engaging on a restart.

If the machine restarts with the Oil Separator pressure low and the slide valve obove a minimum percentage, approximately 10%: Good chance it will blow its oil out the discharge in the first few minutes before discharge pressure is re-established. Thsi simply because the volume flow through separator is very high when its discharge is low....

This just a start but tell us, is there a sight glass on the secondary side of the separator? Is there one on the oil return line from the secondary side to the suction housing? Or does the return go somewhere else?






On a restart under these conditions: The compressor makes so much volume flow the oil w

Quote:

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Originally Posted by US Iceman

If you have to drain this much oil every 24 hours, then the compressors/oil separators are pushing too much oil into the system.

And, your problem may be caused by this...



Since you provided the discharge pressure in psi, are the temperatures above in °C or °F?

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Have you used the Bitzer oil seperator , or you have made its own

Quote:

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Originally Posted by SURESH YADAV

Discharge temparature is 100 °C

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Have you installled secondary filter with what mesh size ?
& coalsecent filter with what size ?

Suresh Yadav,

Firts try to lower the oil/discharge temperature. For the same pressure you will have more oil carrier to the system with higger temperatures. Ensure that the oil cooler is well fedeed with refrigerant (termosyfon) or water (water cooled).

your oil return to comperessor . why it is not coming back?
if your pipe work in system does not right - slope of pipes .it stay in evap.

Oil is escaping from separator in probably (in part ) vapor form . Coalescing filter will arrest minutest of the oil in liquid form . Address oil vaporization first . Drain system of oil separator clogging can also carry forward oil . What are the design parametrs ?

Dear oil lifting means oil level continously going down in oil sepaator, it is not coming back in oil separator

Are you using the Bitzer recommended oil in the mini-screw. From Memory Clauvis 68 or Mobil SHC 224 synthetic, the discharge is the real problem, oil at that temp will not separate in oil separator, either high suction super heat or oil cooler not working.