Frick RXF oil pressure drop on startup

[PadenCold]

I have a NH3 Frick screw that drops oil pressure ONLY on startup... I have replaced the filter (thinking it was blocked) and the amot valve (thinking it might be stuck open). All the transducers have been calibrated and guages added to verify readings. If I apply pressure to the oil pressure sensor giving it a false reading until it warms up, (for about 10 minutes) the pressure runs about 20psid and stays running. But, on startup without assistance, oil pressure runs just a few psi over suction causing shutdown for low oil pressure. I usually change the oil filter when psid reaches 15,(running normally around 5-10 psid) but now it is 20 psid with a new filter. I even tried shutting the needle valve for the slide, knowing it uses oil pressure to adjust.

I added liquid injection late last year due to unsolveable issues with the thermosiphon cooling system, the oil started foaming a bit after the install, but has never been a problem. I have tried shutting off the LI line seeing if it made a difference, it did not.

The screw does not use an oil pump, it relys on discharge pressure.

Maybe the transducers have been adjusted too far to compensate for wear? They seem quite accurate... I have been told about a fixed orifice that controls the oil flow inside the piping, but I have never heard of one wearing out or coming loose...

I have 2 new transducers as a last resort,(since the offset is begining to become large) but am tired of throwing parts at the compressor, leaving me with the same problem. Any ideas on where my oil pressure is going?

[Segei]

It is not clear what issue do you have.
Usually, oil pressure should be 55 psig above suction pressure to keep compressor running. If it is lower than 55 psig compressor will cut off. This is one issue.
Second. Pressure drop in the filter can be high and compressor will cut off as well.
What issue do you have?
Do your oil heaters work when compressor off? It looks like it work OK when oil warm. May be issue are the heaters.

[HVACRsaurus]

Hi Buddy,

I suggest fitting regular gauges to compressor discharge & oil delivery, suction too if you can. Run it up with gauges on & check against your transmitter readings.

Also, is the compressor reaching a suitable discharge pressure quickly? As mentioned, the compressor relies on discharge / suction differential to push the oil through, but problems arise if the head pressure is too low. Compressors often use a "cold start valve" or similar in discharge line for this purpose.

[PaulZ]

Hi PadenCold
With differential pressure oil feed you will need to get the discharge pressure up fairly quickly.
Are there other compressors operating on this system?
How quickly does the compressor load up?
If there are no other compressors on the system do you have an inlet pressure regulator on the discharge line directly after the separator to get the discharge pressure in the oil separator up quickly?
Paul