Oil return

[Peter_1]

Setup freezer application: Bitzer OSNA7471 + SSNA7471 on a main suction collector.
Collector connected to the gas side of a LP receiver .
A Bitzer primary OA-type oil separator feeding direct the screws with recovered oil.
After the primary oil separator, 2 parallel connected secundary Bitzer OAS-oil separators with inbuild filters feeding the oil to the suction header, controlled by one common PLC controlled SV.
Air cooled Helpman oilcooler (large distance) with a 3-way after the cooler valve to mix hot with cooled oil to deliver oil at right temperature.
Gunter air cooled condensor. Flash gas separator after condensor.
Under the LP separatorn the 'belly' where the 2 pumps under the LP separator are connected, feeding 3 plate spiral freezers.
Flash gas separator after the condensor feeding direct an ice cream generator machine (no LP pumps) Have to go on site to verify how they accomplish low pressure regulating control in this machine.
No provisions for oil separation after the belly of the LP separator.
Oil separators are loosing their oil within some weeks and oil must then be drained after a night standstill out of the belly of the LP receiver.
In how far can you rely on the primary and the secundary oil separators to be sure they will remove the oil for let's say 99%?
Any ideas what to search for?

[RANGER1]

Peter, some suggestions. Check oil return float secondary oil separator, check it filter & consider replacing if a few years old or pressure drop across it, if not check its condition & is it tight in its correct position. Maybe also check its oil return line if it returns on back seat of suction service valve (maybe closed). Check if Bitzer oil injection solenoid & discharge check are holding so if turned off oil cannot migrate into compressor & suction line.

[Peter_1]

Thanks Ranger1, will do.
Why I see on so many NH3 installations needs one to drain the oil manually out of the belly of the LP receiver instead of pumping this back to the oil receiver?
In this partical setup, the compressors are stopping each night and after each night standstill, they easily can drain the oil out. They do this every 2 weeks or so I guess.

[RANGER1]

Peter, depending on design etc you will have to drain oil periodically but depends, each plant can be different. We have some plants that may require top up after 1 year of continuous running, but of course not all plants are perfect. If all is correctly sized & designed I would have thought once a month or two. Some systems can return back to oil receiver through filter, but would want it to be very good clean plant to do this.

[Josip]

Hi, Peter_1

Thanks Ranger1, will do.
Why I see on so many NH3 installations needs one to drain the oil manually out of the belly of the LP receiver instead of pumping this back to the oil receiver?
In this partical setup, the compressors are stopping each night and after each night standstill, they easily can drain the oil out. They do this every 2 weeks or so I guess.

Oil return system within oil separator just after compressor i.e. coalescer filter is capable to remove almost all of oil passing coalescer filters and return it to compressor suction port if complete oil return system is in good working condition ...

It is possible to install full automatic oil recovery system, but it is very expensive ... also, there we have operators and they have to monitor the system ...

The reason to drain the oil from the bottom of LP separator (and some other places) is very simple .... NH3 systems are build with steel vessels and pipes prone to corrosion ... to avoid to inject dirty oil and small corroded particles sometimes even a water back to compressor suction port, it is better to drain it from the whole system ..... if everything is OK within the refrigeration plant and oil return system the loss of drained oil is acceptable .... drained oil we can control to see the status of our complete plant ...

Best regards, Josip

[Magoo]

Check amot 3 way oil temperature regulator is controlling oil temp at about +40>45' C entering compressor, high oil temps equal high discharge temp and high oil carryover to system. The elements in amot vav are replaceable and have varying temperature options. Check discharge gas temperature from compressor, should be around 70 > 75' C on ammonia. Another issue that can cause oil carryover is that the compressor loading rate is too fast, and gas velocities through separators exceeds design oil separation capability during rapid pull down from start up.