Kysor Parallel Rack Oil System
For me a typical Kysor Warren rack has a oil separator with removable oil float, oil reservoir & AC&R oil level controls on the side of each compressor. 3 - 5 compressors piped together.
There is a common saying with some of the guys I work with when there are multiple oil failures at a time on a rack, that "one compressor failed first and robbed, or sucked the oil out of the rest of the compressors". My question: is that even possible? How could that happen? Wouldn't the oil level regulator on the side of the compressor stop any extra oil from being fed in if the float was full and closed? Most of the time what I am finding is bad valve plates that let discharge gas into the suction header and seem to "push" the oil out of the other compressors, or stuck open EPR valves or hot gas valves, or stuck closed or open bottom oil floats. Usually seems like when a rack shuts down on oil pressure it's a bigger problem, not just one compressor robbing oil from the others.
Any input would be appreciated!
For oil to flow from one compressor to another there needs to be a definitive path, and a difference in pressure.
The path would be if the compressor crankcases are equalized together.
The pressure differential would be, as you stated, if one compressor had bad valves. The blow by would cause the crankcase pressure to be higher than the other compressors, and could possibly cause the oil to be forced through the crankcase equalizer line and into the other compressors.
If you're having oil failure problems, it's more likely that the problem is an oil return problem.
Oil problem on a rack is never simple. First, a lack of oil in all compressors shows that there is oil trap in the system. If the situation happen once in a while, it could be just before a defrost when the store is close. The TEV will move closer to the seat to maintain superheat setpoint. This reduction of mass flow can cause liquid (oil & ref) to pool in the evaporator. Reclaim condenser with in series with the outside condenser can cause an oil trap if not energize for a full cycle after a long time. Pump out solenoide will evaporator refrigerant vapor but not the oil on a low mass flow.
Finally, a weary pistons compressor will let more than 5% of the CFH volumetric flow compare to smaller percentage of other compressor on the rack. After stopping the compressor all oil will be force to other suction of other compressors that are running. That would cause a low level compressor on cycling compressor but a higher lever on running compressor.
Blow by condition on compressor can cause a low oil when running but a sudden raise oil level in sight glass after stopping. Check valve opens....
You are correct in your thinking. Ok first things first though in troubleshooting this condition.
1# if you are experiencing broken valve plates regularly, you are flooding liquid back to the compressors. Go to the systems on the rack that have the coldest stubs and adjust down the TXV. The suction stubs should be about 25 Degree F at the rack header.
2# sounds like to me the oil seperator needs to be taken apart and cleaned and or replace the float. I know not a very easy task but it needs to be done.
If the oil seperator is not working you will fill up an entire roasting pan to the top. If it is working the oil line will be hot 4 feet away from the seperator. If the oil line is cold the seperator is not working properly and service is a must.
3# Test the compressor oil float if you think they are over filling. This is easily done by removing the oil line from it and using your hand held oil pump and a 3-8 hose pump oil into the compressor thru the float. When it is full you will no longer be able to pump any more oil into the compressor. If it over fills adjust the float or replace it.
4# test the check valve on your oil reservoir. You can do this by isolating the reservoir with a gauge attached. Dump some discharge gas into it to raise its pressure above suction pressure. Then open your check valve line to the suction header. The pressure inside the reservoir should drop to 5-20 psi above the suction pressure and stop. if it falls back to suction pressure check valve is bad. Not very common though.
5# proper oil level in the reservoir should always be above the bottom sight glass of the reservoir. If its low fill it up. But be sure to clean your seperator first since this will be your problem.
You do all these things the system will run much better. Good luck. Evan
one more thing.
Suction stubs at the rack for low temp systems should be 25 Deg F.
Suction stubs at the rack for Medium temp system should be 45 Deg. F.
In my book superheat at the rack supercedes superheat at the evaporator.
If you ignore superheat at the rack stubs then the end result is broken valve plates and bad compressors and plugged up oil seperators.
Good luck Evan
I will respectfully disagree 100% with your statement above, about rack SH supersceeding evaporator SH.
Originally Posted by Fridge Repairer
The TEV serves one purpose only: To insure the proper refrigerant mass flow in the evaporator necessary for the evaporator heat load at any given time. This is done by maintaining a constant superheat at the outlet of the evaporator.
To lower the evaporator superheat in order to maintain a minimum superheat at the compressor inlet means that you are now increasing mass flow through the evaporator, refrigerating the suction line while providing no benefit to maintaing product integrity, with an increase in electrical consumption every month.
In addition, there is the possibility of causing ice buildup on the underground suction line (if indeed the piping is underground) and eventually raising the floor.
To raise the evaporator superheat in order to maintain a minimum superheat at the compressor inlet means that you are now sacrificing evaporator efficiency, again causing extra expense to maintain product temperature.
If the evaporator TEVs are set correctly, maintaining the equipment manufacturer's recommended SH at the evaporator outlet, there is NEVER any danger of compressor damage due to liquid floodback.
There might be a myriad of other reasons for floodback occuring....low airflow (dirty evaporator, wrong fan blade/motor), leaking liq/suct heat exchanger, leaking check valve on gas defrost systems, or piping issues contributing to oil slugging.
If the superheat at the evaporator outlet is set correctly, yet the suction temperature at the compressor is too high, then this problem should be addressed in another manner...either a de-superheating TEV at the compressor inlet, a Sporlan Y-1037 valve measuring compressor discharge temperature and injecting into the compressor suction when the discharge temperature becomes excessive, or Copeland's demand cooling package (which is essentially an electronically controlled version of the Y-1037 valve).
I stand corrected. You are 100% correct Bunny. But can we agree based on the information about the conditions supplied by KysorWarren that his store is suffering from severe floodback and not incidental minor floodback caused by mysterious operational conditions?
Kysor what I think you should do is to put your suction gauge onto each epr valve and measure your superheat at the header for each system and find out which stubs you measure zero superheat. I will bet you will find a stub or two with Zero superheat. Start with the medium temp stubs with all the ice on them. Probably the meat and or chicken cases.
Bunny you are correct. It very well could be fans out or dirty honeycombs and even sometimes in older Hussmann cases they have an air diffuser inside the back wall that gets covered in dirt blocking airflow. Be sure to exhaust all possibilities before adjusting TXV's.
Valve plates don't just break! Compressor manufactures recommend a MINIMUM 20 degrees of superheat measured six inches from the suction service valve to ensure no liquid is entering the compressor.
I think your customer would rather pay a little more for electricity instead of busted up compressors lost product and endless service calls.