low super heat

[mgtet]

i have a problem with a climaveneta chiller. Found the compressor running with high discharge super heat, carried out head inspection (copeland d8) and found compressor beyond repair. Replaced compressor, expansion valve powerhead and cage and the drier core. System charged with nameplate charge virgin R407c. System ran at 3.6 bar with chilled water return at 18deg flow at 16deg (2 circuit machine, 1 circuit running) as chilled water temperature dropped so did the suction pressure, down to 3bar at 10deg return chilled water temp. Superheat constant at around 2deg, if the valve is shut down the suction pressure drops but the superheat remains constant. If the valve is opened up the superheat reduces further. Discharge pressure is a constant 17bar 7deg subcooling.this is obviously what caused the compressor to fail in the first place. Chiller has a shell and tube evaporator, I told my boss that the problem is obviously to do with the evaporator, with the refrigerant not gaing heat from the chilled water, ie water bypassing or refrigerant bypassing, but he's not convinced. I think that the end gasket could have split on the evaporator causing the suction line to be flooded with liquid after by-passing the cooler, if you see what i mean! Other circuit operates correctly. Any ideas greatfully recieved.

[ptsac]

I have experienced a similiar situation ( not on a Climavenita ) and discovered that the liquid distribution plate had come away. This is located on the evaporator inlet from the expansion valve outlet. The liquid takes the most direct route and does not disperse to all the tubes. Result low evap pressure and low superheat. Worth a check.

Regards

Phil

[NoNickName]

I have experienced a similiar situation ( not on a Climavenita ) and discovered that the liquid distribution plate had come away. This is located on the evaporator inlet from the expansion valve outlet. The liquid takes the most direct route and does not disperse to all the tubes. Result low evap pressure and low superheat. Worth a check.



Yep, it also happened to me. Especially a specific brand of italian s&t evaporators.

[mgtet]

looks like the evaporator end plate will have to come off! I have had a problem with end gaskets before but passing between circuits!! Didn't give the distribution plates a thought, has to be one or the other i think. Cheers guys.

[mgtet]

hooray have the go ahead to remove end plate, has to be done over the the bank holiday weekend though!!
Will let you know the result.

[mgtet]

ok, removed the end plate, distributor plates all ok, changed gasket and reaasembled. Getting same problems!!
Had a brain wave and decided to carry out a test on the oil, to my horror found oil was mineral on r407c. So waxy residue starting to form on tubes inside the cooler and so reducing heat exchange and giving me all my problems. Carried out 5 oil changes and the circuit now runs in an accceptable manner, ie decent suction pressure and super heat of 7 deg. Will return in 3 weeks to carry out further oil changes. Theres one for your note book!!

[taz24]

ok, removed the end plate, distributor plates all ok, changed gasket and reaasembled. Getting same problems!!
Had a brain wave and decided to carry out a test on the oil, to my horror found oil was mineral on r407c. So waxy residue starting to form on tubes inside the cooler and so reducing heat exchange and giving me all my problems. Carried out 5 oil changes and the circuit now runs in an accceptable manner, ie decent suction pressure and super heat of 7 deg. Will return in 3 weeks to carry out further oil changes. Theres one for your note book!!

Good call well done with the diagnosis.

Cheers taz.

.

[mgtet]

thanks, i was just relieved to get to the bottom of the problem and get the damn thing up and running after a lot of head scratching and hard work. In future i will tell the compressor recon company to send compressor oil less and charge them myself!!

[ptsac]

Well done! I shall definitrly put that one in the book.

[Gary]

Good call.

Assuming the evap is completely flooded (normal to low superheat), then approach temperature will indicate oil logging.

Approach temperature is the difference between the saturated suction temp (SST) and the water leaving temp.

Given good heat transfer, the refrigerant warms as it goes through the evap and the water cools as it goes through the evap. The two temperatures "approach" each other.

The difference between them will increase if the heat transfer is poor (insulative oil coating).