Bitzer chiller problem

[rasscal]

Hi
I'm new to this forum, so excuse me if I ask a stupid question that's covered somewhere else.
We have a chiller plant onboard, 2 circuits.
2 Bitzer semi-hermetic screw compressors parallel on 1 circuit. Oil separator, (oil injection) SW cooled tube condenser and plate evaporator.
By a mistake the chilled water circulation pump was switched of.
After this incident the liquid line is full of bubbles. Oil migrates from oil separator, where it's really foamy.
I've replaced the filter dryer, even ran it shortly without any insert to check.
LP and HP are normal. Compressor seems fine, but housing seems much colder than normal running compressor.
I'm guessing what happend when the evaporator lost it's flow superheating didn't happen and maybe liquid got into the compressor. But I still don't understand why the oil migrates and it makes fault finding difficult as I can only run the compressor for a short time...
Obvious is low refrigant level, but I cant see where it's gone, we have sniffers below the unit and in safety valve outlet piping. And pressure would drop as well?
So maybe someone out there had similar problems...
Cheers!!

[savoc]

i may be wrong dont understand the question. but it sounds like you haved flooded your evaporator with liquid. this would cause liquid to come back down your suction line. then the oil in your comp will foam up then your oil separtor may have liquid in it and fail on oil level. i had plant which the liquid line solenoid were wired up in correctly. they would never shut off. when the comp shut down on temp liquid would still be filling up the evaportors once the comp started again it would draw back liquid and the oil would foam up again. pump the unit down a few times and check your sight glass on your separator it may have some liquid in it.

[Sandro Baptista]

"By a mistake the chilled water circulation pump was switched of."

After the pump was turning off what things happen then so you notice the pump was switched off? » LP tripping?, Low chilled water temperature?, Flow switch off?

[rasscal]

Hi Sandro and savoc
Thank you for your reply
I didn't witness it unfortunately! But now it doesn't trip on LP or HP, it trips on some internal alarm that I can't find any info on!! The automation is quite bad on this setup, so all I know is that it trips. And if I start the compressor again the oil separator is in low level...
I tried shutting the liquid line and the compressor pumps down really quick and trips on LP, so I don't know if the evaporator is filled with liquid. The solenoid seems to open and close properly...

[rasscal]

Hi Sandro and savoc
Thank you for your reply
I didn't witness it unfortunately! But now it doesn't trip on LP or HP, it trips on some internal alarm that I can't find any info on!! The automation is quite bad on this setup, so all I know is that it trips. And if I start the compressor again the oil separator is in low level...
I tried shutting the liquid line and the compressor pumps down really quick and trips on LP, so I don't know if the evaporator is filled with liquid. The solenoid seems to open and close properly...

[rasscal]

Sorry, forgot. The flow sensor is not working, that's why it didn't shut down the compressors!

[NoNickName]

Sorry, forgot. The flow sensor is not working, that's why it didn't shut down the compressors!

Bad luck. It's the most important device of the whole thing.

[Sandro Baptista]

The oil level could be low but nevertheless still exist oil flow circulating.

Have you try charge the oil separator? If the oil disappear it must be somewhere...part on the evaporator other part on the condenser.

As savoc has said it can be liquid slug to the compressor. Does the discharge temperature is much lower than is usually.

[savoc]

could also be the oil flow switch. i found that little sensor that sits in the oil flow switch can be adjusted with tiny allen key. whats your condensing water temp if its too cold that may cause problems to. if the unit pumps down quickly is the tx frozen? open your liquid line and see if your pressures balance out. check the motor thermistor if there is no motor cooling they will cut out. make sure you have full flow of water through your evap before starting may still be frozen. ive had plate heat exchangers that have taken hours to defrost. run pumps on manual with no refrigeration.

[rasscal]

Hi again and thank you for the suggestions - something to work on.
I agree the flow sensor is important, but sailing far away from supply lines, it's not always easy to get a specific sensor that is required :-)
There is oil flow, as I have a flow sensor and sightglass on the oil injection line. The separator has sightglass too and there's definitely only oil in there (no liquid refrigerant). I agree that there's probably been liquid slugging when the cw pump was turned off, but if the compressor is damaged I would think it wouldn't be able to pump up to normal pressure?
Thermistor doens't cut out, on the contrary the compressor seems much colder. I'll try and measure the discharge temp, but that's probably colder...
I agree that the oil must be somewhere, and I guess condenser or evaporator are the only places it can go!
I've left the cw pump run all night so I don't think there's frost in there. TX valve isn't really that cold, but when running it's colder on outlet than inlet, so it's not completely blocked.
When compr. is stopped the pressure is equalized. But there is an off cycle bypass line and the solenoid is burning hot, what if the off cycle bypass doesn't close? I guess if the compr is destroyed the pressure will also equalize :-(

[savoc]

what is the plant doing? and the fault you have at the moment? if you dont have oil in a screw compressor the comp will cut out on high discharge. the oil is what gives the screw seal. hot gas will by pass causing comp to over heat. i have found bitzer screws to be very strong compressor unlikly the comp is damaged. The by-solenoid do you mean unloader solenoid (not a capacity unloader on the side of the comp). some plants have a valve connected from the dichagre to suction line. it is only energised for around 10 sec so the comp starts up easy. a srcrew comp will equalize you normal have a suction line check valve and stop check valve in the dicharge.

[rasscal]

Hi savoc
Fault is that it empties the oil separator and trips on low oil level...
I mean the by-solenoid from suction to discharge. On my plant it's open when the plant is stopped, so I was thinking maybe it doesn't shut when starting. I can try and close the valve manually.
I'll take the TX valve apart and see if anything is wrong there. Other than that I'm kind of getting at the end of my line...
Cheers,

[savoc]

i dont think your tx valve is faulty. that valve (by-solenoid from suction to discharge) should only be open during start up. if the valve is opened on the off cycle your pressure will equalize and possible the oil may be forced out of your seporator. that is how i have seen the by pass by valve used. we have wired them in to the same circiut on part start winding compressor to min the start up current. i cant see why the valve would be on while the compressor is off. if the oil has left the seporaot it must be in the system and it will return. disconect that valve and try running the comp a few times. other option is pump some oil into the system.

[rasscal]

Hi Savoc
Closed the by-solenoid and stopped the SW cooling for the condenser. Oil returned immediately. I think the oil heater on the separator is faulty, so the oil is too cold.
Thinking back, then when the ch.water pump was stopped the oil was probably passed through to the condenser, because if the missing superheat. Then being in the condenser it's way too cold and wont be separated, and will foam.
I'll leave the oil to heat up and hopefully everything will be back to normal....
Cheers,

[rasscal]

Hi savoc
Turned out to be the oil being too cold!? well, I've learned something then. HAd to run it 10 minutes with the SW turned on/off until the oil reached 60 C....
But I checked the electrical drawings and that solenoid bypass is opened when the circuit is shut down...
But thanks for all the great input, this is a good forum!
Thanks!

[baycuclaudyu]

I sugest you to call an authoryzed company. It is clear that all these faillures demonstrate that this chiller wasn't checked before. To avoid big problems then that and you will be covered by the extern company, also costs to repair 2 compressor will be comparable .

[savoc]

good to hear that the plant is up and running. i am looking through my Bitzer techincal specs i cant find that vavle that is energised while the comp is cycled off. maybe take photo and ask on another forum . may i ask what and where is this chiller. iam from Australia Adelaide.