Oil Logged evap?

[Temprite]

Hi All.

Was hoping to get some advice on an issue I am having at present.

Recently changed a condensing unit attached to a reach in display case and are having some problems.

Original problem was the compressor had bad valves. Condensing unit was replaced and system evacuatted and re charged. System operates but seems to have a lack of capacity.

Checked superheat and found erratic reading so to be on the safe side replaced valve with new orifice.

System readings as follows with case at approx 3 deg c
Suction pressure at TX valve fluctuates between 360kPa-430kPa
Superheat fluctuates between about 2-8 degrees c
Discharge pressure with fan speed control 1830kPa-1635kPa
Sight glass is clear with very slight bubbles cycling with condenser fan.
TX valve is danfoss ext equalised r404a with No 5 orifice (This is manufacturers specs)
Evaporator is rated at 6343w@-8 degc
condensing unit is rated at 7243w@-10 degc 32 Ambient.
Liquid line is 1/2"
Suction line I think is 7/8" (will have to check it may be bigger). I believe it should be 3/4" has small oil trap
Pipe run is about 5 metres between evap and condensing unit.
Condensing unit is about 3.5 metres above case on roof.

Any advice would be greatly appreciated.

[aramis]

The first thing you should have done was to investigate why the previous compressor failed.

Just changing the unit won’t necessarily make the original problem go away.

You have to do a tear down analysis and find the causes of failure.

Superheat should be not lower than 5K, so it seems you are getting liquid refrigerant out of the evaporator, this could be TEV fault or evaporator fault or bad (or no) load problems and the system is hunting.

[Temprite]

The first thing you should have done was to investigate why the previous compressor failed.

Just changing the unit won’t necessarily make the original problem go away.

I would never just change a compressor and hope for the best. That is why I am posting this thread. The previous compressor failed from faulty valves probably caused by liquid flood back by the look of the measurements taken.


Superheat should be not lower than 5K, so it seems you are getting liquid refrigerant out of the evaporator, this could be TEV fault or evaporator fault or bad (or no) load problems and the system is hunting.

Its probably unlikely to be a TEV fault because the valve is brand new and factory set for 6K with the correct orifice. Given the hunting of the valve it seems we may have an oil logged evap. I have never had this fault before so I thought someone here may have experienced this and may give me some advice.

[mad fridgie]

Firstly, would you say that your erratic behavior, is directly following your cycling fan. This is normal.
The figures do seem to match your system, slightly low SST, but slightly lower SCT, sort of in balance.
Suction line maybe slightly over sized, but not excessive.
I would perhaps look at the cabinets fans or coil cleanliness

[Temprite]

Firstly, would you say that your erratic behavior, is directly following your cycling fan. This is normal.
The figures do seem to match your system, slightly low SST, but slightly lower SCT, sort of in balance.
Suction line maybe slightly over sized, but not excessive.
I would perhaps look at the cabinets fans or coil cleanliness

Hi Mad fridgie
Coil is not dirty and all the fan motors have been changed because due to the age of the motors I suspected that one might have been stopping intermittently and icing the coil. All fans are brand new and spinning with correct rotation.

[Tesla]

Hi Temprite
The main causes of oil logging in the evap is low refrigerant flow, excess oil passing compressor & poor oil return from evap or insufficient seperation when required. I think it could be a combination of the above - oversized suction line would reduce velocity of refrigerant to return sufficiant amounts of oil from evap, as would under sized condensing unit or an under charge of refrigerant - oil trap could be too long or too deep. A slightly higher head pressure could help too depending on ambient and if cond unit is designed for heigher ambients. Are there extended periods of low loads where cond unit runs for only a short period of time. Just a few sugestions here to check out if not already considered.

[aramis]

The previous compressor failed from faulty valves probably caused by liquid flood back by the look of the measurements taken.

OK. Then you had liquid floodback. I suppose you saw evidence in the compressor to support that, for example increasing wear along the crankshaft.

And you still have it!

More information on the evaporator would be helpful. A picture perhaps?

Is the evaporator frosted evenly?

How many circuits does it have?

Is it in good conditions? You said it is clean but mechanically how is it?

Possibility of cold air short circuit? Study the air currents within the case.

Do you have thermal load?




Its probably unlikely to be a TEV fault because the valve is brand new and factory set for 6K with the correct orifice. Given the hunting of the valve it seems we may have an oil logged evap. I have never had this fault before so I thought someone here may have experienced this and may give me some advice.

What makes you think it is trapping oil, is compressor´s amp reading going up?, strange sound?

Generally hunting traps oil in the suction line, because of low average refrigerant flow, not in the evaporator for intermittent waves of liquid washes it away.

Hunting is caused by a mismatch between the evaporator´s effective heat transfer and the system’s capacity so you should check for any reason your evaporator is not working properly.

[Grizzly]

Hi temprite.
Thanks for an interesting post!
These may seem stupid questions but!
What is the subcooling and what's the difference across the liquid line filter?
Only I have had systems with similar before where the issue ended up being the liquid line filter was blocked.
Also could the system be overcharged?
Cheers grizzly

[Gary]

Generally, if the evap airflow is insufficient, the air is in contact with the coil for a longer time period, resulting in lower air off temp and greater difference between air on and air off (high dT).

When a coil is oil logged, there is an insulative coating between the refrigerant and the air, resulting in the refrigerant running colder and the air off running warmer, thus a greater difference between air off and saturated suction temp (high approach temp), as well as less difference between air on and air off (lower dT).

And all of this assumes sufficient refrigerant feed (normal to low SH) and refrigerated space at or near design temp.

New fans doesn't necessarily mean the right fans. If the problem turns out to be insufficient airflow, as confirmed by high dT, the fan motors may not be correctly sized and/or the fan blades may not have the correct pitch... or many other causes of insufficient airflow.

[Gary]

Is the evap air ducted? If so, there could be blockage in the ducts.

Another problem, depending on the layout, could be recirculating part of the evap air, instead of throwing it out into the refrigerated space.

[Magoo]

Hi Temprite,
does the evaporator have a factory fitted suction/ liquid heat exchanger. I have had one that leaked between, similar symptoms as your problem.

[mad fridgie]

I would look at getting some stability in your system during the test process, block some of the cond, so cond does not cycle. Then see what happens. the numbers then may start to make a little more sense.