Bitzer AC compressor tripping on LO diff.

[maxx21]

hi guys, im new here so im not sure if i should post my question as a comment on on of the threads or post a new thread. but anywasy, im a marine engineer, not a refrigeration engineer. im in an offshore vessel here in abu dhabi. well, few hours ago, we had a problem with our AC compressor. it keeps on tripping on LO differential pressure. it seems that the LO suction pressure pressure is going down. the oil level on the sight glass is a bit more than half full. the C/E told me that the compressor was overhauled few months ago on drydock and ever since the compressor would from time to time trip on oil diff. the gauge on LP side shows 5bars and the HP side shows 8bars but the mark says it suppose to be 8.5 bars. i fiddled with the pressure differential switch and adjusted it to 3.3bars. now it seems to be ok. but the my question is why did this happen? whats the cause? im not that familiar with refrigeration by the way. so any help is greatly appriciated. thanks.

[sedgy]

hi max , the unit is eather tripping out on the L/P switch or the oil pressure diff switch,
if we knew what gas it was on , and the desiered temp of the air coned room we might have a chance of getting a result, and I take it the unit is water cooled ? mregards sedgy ,

[aramis]

I suppose your compressor is leveled and a semi hermetic.

One unusual but hard to diagnose problem occurs if the crankcase is pressurized over suction pressure while the compressor is on.

This will prevent oil getting in from the motor to the crankcase until the oil differential trips.

In that moment all the oil logged in the motor side will flow into the crankcase and you see a normal level, but moments ago there wasn’t enough oil and the oil differential is saving your compressor.

If by “HP side” you refer to the discharge of the oil pump. 3.3 bars is too high, check the correct oil pressure differential for your compressor model.

Over pressure in the crankcase may be caused by:

- Blown gaskets
- Damaged high pressure valves
- Worn pistons/rings/cylinders
- Oil separator oil return always open
- Equalization problems in parallel compressors
- Leaking safety valves

To diagnose this you must be present with the oil differential trips OR measure your crankcase vs suction pressure (they are close but not the same).

It would be also good to bench test your oil pressure differential.

[maxx21]

hey sedgy, its tripping on oil diff switch. its to ABB HVAC unit. im not sure as to the temp it maintains but it supplies the whole accomodation from bridge to main deck. when you say its water cooled, you mean the condenser right? condenser is supplied with sea water. i have to get back to you n what gas it was. we have different gases here. R404 is used for the chilled water compressor. im not sure what gas is the AC comp using. thanks sedgy.

[maxx21]

hi aramis, yes its leveled and semi hermetic.even when the compressor is running, the oil level on sight glass is more or less below the middle line. its a solo compressor so we can rule out the equalization probs on parallel. safety valves are ok i think. i havnt done a leak test yet. ill do it tomorrow during my watch. as for blown gaskets, damaged high press valves, uhm.... they did an ovehaul few months ago. and the HP side is showing normal press. no bubling on the sight glass so i gues the piston rings and and liners are ok. ill have to check on the oil separator though. and yes, HP side before i was refering to the discharge of the oil pump. i gues the set point for the diff press swicth was set to 2bars. i checked the compressor just before i went off duty and its still ok. oil is still just below middle. thanks aramis. ill do just what you suggested tomorrow. and will report back on my findings.

[Gary]

Does it have a pump down cycle? Is the evaporator above the condenser? What is the subcooling temperature?

[aramis]

Bitzer specs for semi hermetics oil pressure differential is 0.7 bar with a time delay of 90 sec.

You may find it searching for "technical information KB-110-7" at Bitzer.com (I edited this refference though the minimum values are the same, you should check the correct for your compressor model)

Setting it higher only calls for nuisance trips.

If this doesn't solve the problem you should bench test the oil pressure diferential switch and check the mesh filter at the pump intake and the pump itself.

Good sailing!

[Gary]

Oil trips can be elusive and sometimes it's not about the oil pressure at all. Sometimes the compressor trips on internal overload and stops the compressor, but the oil control times out and trips. Then the internal overload resets itself, but the oil control does not, so it appears to be an oil problem. Check for electrical problems, loose connections, undersized wiring, burnt contactor, etc.

[NoNickName]

but the oil control times out and trips

Yeah, this used to happen until few years ago, as the electronic diff oil switch wasn't powered off when the compressor tripped for other reasons.
Not everybody knew they needed to press the red button back into its seating.
That was in the time of first release of Kriwan deltaP oil pressure control switch

[maxx21]

hi guys... im really sorry sorry for not being back for quite sometime. we had problems with our satellite/internet connection and it was just yesterday that it was fixed. anyways, status update:
refrigerant is R-417A.
@Gary, overload is not considered as per electrician report. he says everything was ok. my CE said it does not have a pump down cycle as it is running all the time. and the evaporator is on a HVAC unit right above the condenser. subcooling temp, i havnt found out what it is actually.
@aramis, havent bench test the oil diff switch and neither have we checked anything that you have suggested.
well eversince i adjusted the oil diff press to 3.3 bars, its working ok until now. only thing i noticed is that very 30mins or so, the compressor stops when it reaches the set diff press, sits there for about 30 sec, diff press levels out and then it starts again. no alarms, nothing. thats why we didint investigate it more. CE says, not to do any more stuff on it sinces its working LOL. my CE says, when we get back to port a technician will look into it. but by that time im goin home. ill jsut update you guys the report of the technician, IF he ever comes. hehe.. anyways, 2weeks ago, we added half liter oil cz eveytime it starts, the oil level is just on the lower sight glass. but when it stops its more than full. oil is Corena AP 100 i guess.. will update tomorrow. thanks agan guys. pls bear with me.

[Steve186]

First thing to check is oil diff ie oil pressure minus discharge pressure I suspect this comp has an internal oil filter which may be blocked also check diff between suction + discharge pressure as u u say there is no internal oil pump so it relies on discharge pressure to return oil to comp so as you have an oil level in the sight glass indicates it's faulty diff switch + check dp across oil filter

[wineman]

late reply blocked oil filter