Help! LP cut out is driving me mad!

[Mish]

Hi im having a persistant problem with one system tripping on LP. This system has a large pipe run and uses a Carlyle reciprocating compressor with R22. The room set point is 2.5C and the LP cut out is 1b. When it regains suction pressure to kick in and pulls 4b the pressure immediately drops to the cut out point. Regains prerssure, runs and cuts out. It is like the system is in pumpdown mode. This invariabley leads to the system then tripping on oil diff.

[PaulZ]

Hi Mish
From the limited info it's a bit hard to say what the problem could be. Have you checked all the basic things.
Is the evap iced up?
Are the evap fans running?
Is the liquid solenoid coil burnt out?
Is the liquid solenoid opening?
Is the drier blocked?
Is the TX valve operating properly?
Has the electrics been checked?
If you provide some more info I am sure someone will point you in the right direction.
Paul

[GXMPLX]

Tripping the oil pressure differential has little to do with tripping on LP unless your compressor is short cycling. I already said you should measure differential pressure of the oil pump.

1 bar LP SHOULD NOT be a correct setting for keeping 2.5C unless you have high pressure drop in suction or you evap is terribly small.

It sounds you need to install capacity control, or hire more help!

[Brian_UK]

If you override the LP switch during start up does the unit stabilise at a more suitable pressure/temperature ?

We are all assuming that the system used to run correctly before ???? something happened, what was it ?

[Tesla]

I agree with Brian - check the history, then all the other things mentioned and also the suction strainer could be restricted (rare). Are you sure it's LP cut out - ive been fooled before.

[Mish]

Hey
The system had been running fine until it started suffering liquid carry over. One moring the whole compressor, Carlyle, was badly iced. The worst iv seen. Even the cylinder head and oil diff switch had ice on it. Checked the entire system out and everything worked fine, TEV, Solinoid, i changed the drier too. When started it would fllod back initially then run properly. Set the 2 TEVs, system has 2 seperate Evaporators. System ran fine for three days then started to trip on oil diff. Crankcase heater was fine. Then the compressor started to run as described above. Evaporators are clear of ice and all fans running in the correct direction. Adjusted LP and system ran but continues to trip on oil diff every day.
I have just finished my apprenticeship, maintenance engineer in a large food processing factory. Im electrical bias and done City and Guilds refrige coes a couple of years then didnt touch a system uintil a few months ago. Im now in at the deep end and rapidly trying to increase my knowledge.

[PaulZ]

Hi Mish
You have said there was a lot of liquid flood back. Check the two evaps to see if they have a liquid sub cooler on the suction line, these are generally in behind the fans. If they are fitted the liquid line could have split inside the suction line. This will allow liquid to enter the suction line after the evap and flood back to the compressor.
If this has occurred pump out the evap and bypass the sub cooler and seal it off.
Paul