View Full Version : Scroll compressor noise

02-09-2015, 07:17 PM
Hello people, it's my first thread and I have a major problem with a scroll compressor which I haven't found a solution. Please help me out with your experience.

I have a digiscroll compressor type ZRD125KCE-TFD-425. It makes bad noise and vibration whenever the compressor goes on unloaded state because of capacity control. Also it makes similar but a little bit harder noise when the compressor shuts down. From what I have researched on the internet eveyrone says it's the scroll running on reverse because of the high pressure refrigerant flowing back to the scroll and the internal check valve not working correctly.

Emerson actually told me this also and suggested to me to install an external check valve. Well I did install a check valve type Danfoss NRVH 19s but still got same problem..

For your information, subcooling was about 12K, overheating kept constant at 8-10K except form when it goes to unloaded state and by-passes the hot gas back to suction, overheating goes up to 25K for that moment, and immediately drops when it goes back to loaded state.
Low pressure at 4-5bar and high pressure at 16-18bar, after 18bar external condenser fan unit goes on and drops the pressure. Amps from another time I measured was less than Imax of compressor it was fine.

Any idea guys about this problem? maybe check valve still can't hold the pressure? maybe something else? maybe only solution is to replace the whole compressor?

Please help me out thanks

03-09-2015, 01:19 PM
could be bottom bearing damaged from liquid.
check currant on three phase while running to see if there is any difference.

condenser fan should come in at lower pressure. are you using r-22?

03-09-2015, 04:10 PM
I'll check next time I go there the 3phases current..however last time we was there current values were fine, much lower than the Imax of the compressor. However, if the compressor goes to unloaded state (capacity control), it's still running but not compressing, so the current should drop normally correct?..most posts on itnernet I've seen they say scroll is running backwards..maybe the check valve I chose is not good enough? if there is any guide of how to choose correct check valve for the system..

I can change the condenser fan to start at 16bar no problem..however, I prefer 18bar so it doesn't work all the time..I mean it's normal to have 16-17bar at summer conditions in Greece I believe..also I'm using R407C

Any idea is good guys, the more ideas you give me the more I hhave to try when i go on site again, to find solution atlast.

Glenn Moore
03-09-2015, 05:37 PM
Hi Alex
from your comments it sounds that the check valve is not functioning correctly . Scroll compressors are similar to screw compressors , ie when the motor stops the high discharge pressure will naturally blow back through the scroll set and turn the motor in reverse, to prevent this happening a Non return valve is inserted in the discharge of the compressor. So when the motor stops (Electrically) the NRV closes and should stop the discharge gas getting back through the Scroll set. If the NRV fails then the discharge gas flows back through the scroll set turning the scroll and motor until the suction and discharge pressure equalises. If the NRV has failed then when the compressor stops the gas rushes back and forces the scroll set apart to allow the gas to equalise to the suction, the noise from the compressor sounds like a racing motorbike accelerating away for a few seconds and stops when the pressure has been equalised. Fitting an NRV close to the compressors discharge outlet normally cures the problem.
In the case of a digital scroll the same issues would occur and fitting the NRV should have solved the problem, but when the compressor is running in an unloaded state if the NRV fails to close then the gas will try to equalise even though the compressor is running and would make the scroll set vibrate and be very noisy. The compressor would at this point be running electrically in the correct rotation but with the discharge gas trying to flow back to the suction.
. Put a guage on the suction connection and a guage on the discharge connection and then run the machine. When the compressor is running with its normal pressures , turn the compressor OFF and watch the gauges. IF they equalise then the NRV valve is not working , if they stay similar to there running condition then the NRV is OK
The compressor must be mechanically OK if the discharge and suction pressures you mentioned are correct. Regarding the compressor bearings Chemi mentioned I would think it unlikely as the Top bearing is the bearing that fails because of liquid as its further away from the oil in the sump, and when the bottom bearing wears often this causes rotor / stator contact with the inevitable Burn Out, but also I would fit a fan speed controller and run the condenser fan, or fans continually with a lower discharge pressure that the condenser will allow , this will help the compressor and reduce your current consumption with a more stable cooling effect. Hope that's give you a few ideas

03-09-2015, 06:44 PM
Mr. moore,
I liked what you have written but as Alex said, he added an NRV on the discharge line so i believe that one of them should work.
Regarding what I have written about bottom bearing, I have lost a few scrolls from liquid entering the sump and washed the oil up the discharge. this action damaged the bottom bearing as it was running dry and the top bearing was ok. If oil returning fast to the sump [short lines], the damage is minimal but the shaft is free and the currant in the three phases is not equal anymore. Compressor becomes noisy and its life span gets very short.
I have cut them after replacing with new compressors and saw exactly what went wrong.

if I could listen to sound of it and feel it with my hands, I would be wiser....

03-09-2015, 09:09 PM
thanks mr moore for analyzing it..ill also do that with the gauges also to see if NRV is working.

Now I'm just thinking about the bearings that could have failed, wouldn't that mean that the compressor would also make that bad noise when it works normally? I mean when the compressor is on loaded state, it works doesnt sound that bad. The noise occurs only when it goes to unloaded state or shuts off.

I write below 2 videos on youtube which have same noise. the 1st video I also had that loud operating sound, and also the really bad noise when it goes to unloaded state. (last time I went there to install check valve, I also reduced the amount of refrigerant and the normal operation noise reduced..but the unloaded state noise was still there)


the 2nd video just shows the sound I have when it shuts down


Also keep in mind that whenever it goes to unloaded state or it shuts down, the compressor produces lot of vibrations. Also, if the NRV isn't working, do you know how to choose one that will actually work? because I just chose it according to the pipe size, but maybe it's not powerful enough to hold the hot gas from flowing back.

Another info is that for some reason the manufacturer of the AHU who also programmed the refrigeration control system, for some reason, every time the compressor goes on start-up phase, the compressor starts running 100%, but the expansion valve stays closed 10%, and it's not really compressing anything, the high pressure goes very slowly up, the low pressure goes up, and then suddenly expansion valve opens and the whole system starts functioning..I have seen other systems and this doesn't look normal, I don't know if this could help you to understand if this could have damaged something in the compressor

thanks for helping

Glenn Moore
04-09-2015, 11:35 AM
Scroll compressors must never be run down to very low suction pressure or into a vacuum . On start up if the expansion valve is not allowed to open fully this could cause the compressor suction to run into a vacuum which can damage the scroll set as the scroll set will suck the fixed scroll and the oscillating scroll into each other which will cause the scrolls to grind against each other quickly destroying the scrolls as metallic particles start to fly off the scrolls , these metallic particles could be the reason the nrv's failed. NEVER RUN A SCROLL IN TO VACUUM AS THE SCROLL SET WILL BE irrepairably damaged. The LP switch should be set to cut out at 1. 5 Bar this will keep a gas and oil seal between the scrolls to prevent metallic contact when the machine is running

04-09-2015, 06:32 PM
I will tell the manufacturer to program the expansion valve to open, no problem. However, I guess the compressor still works, I mean when the expansion valve opens it works normally, the only problem occuring with noise whenever capacity goes on.

.is there any way to actually select a check valve that might work? I mean from what I see there are capacity tables for check valves and as I want to install it on the discharge I would have to look at the Hot Gas Capacity tables. If I'm thinking right, I should know what hot gas capacity my system is and I should pick the check valve according to that. However, how do I calculate the hot gas capacity on my system? do I get this from the one who designed the refrigeration system?

Glenn Moore
04-09-2015, 11:14 PM
Speak to Copeland , give them the system working conditions and they should be able to give you the information you need to select the NRV using the Danfoss Coolselector programme

05-09-2015, 10:48 PM
Hi Alex !
A digital scroll compressor rotate all the time when is working and only internalparts are splitting and joining at regular intervals to achive the variable duty .For example joined for 2 sec and split for 8 sec than capacity 20 % . So it is not possible to rotate backwards .

05-09-2015, 10:50 PM
And if rotate in reverse does not compress .

Glenn Moore
06-09-2015, 10:38 AM
Scroll compressors can run in reverse when the power to the motor is removed if the check valve in the compressor discharge is damaged , as the discharge gas equalises back to suction and spins the compressor in reverse as the gas blows back through the scrolls. When this happens the compressor can be very noisy as the scrolls are thrown apart and can last for a few seconds

monkey spanners
06-09-2015, 11:03 AM
They are noisy compressor, especially when they get old, here are some running on a roof i worked on.


06-09-2015, 04:12 PM
I have seen other digi compressors who work just fine..they make a sound when unloading but nothing compared to this noise I have..I mean if you just hear it you know something is definetly wrong with it..I'll try to contact some office of Copeland/Danfoss to see if they can help me because the representatives here in Greece have no idea they just wanna sell a check valve and they don't know how to choose one..anyway I hope a correct check valve fixes it becuase this compressor cost 2000 euros

06-09-2015, 04:56 PM
Hi Glenn ! Of course scroll can rotate in reverse when power is cut and the check valve is defective but the Alex's problem is the noise when the the compressor unloads .

06-09-2015, 06:59 PM

Glenn Moore
06-09-2015, 07:22 PM
Hi keapog
Alex's problem is the compressor is noisy when it unloads and when it stops

06-09-2015, 08:53 PM
keapog: yah, it's like same noise..just a bit worse when it stops..if the capacity is at 90% for example it will go unloaded state just for a while so the noise is just for 2-3 secs..if capacity is at 50% it goes unloaded for a while and noise lasts more..but when compressor full the noise is more intense..like the 2nd video i posted

nike123: if you check the 1st of the youtube video I posted, check 0:20-0:22..your video is normal sound I agree it's a bit noisy, but that noise on 0:21 is really bad...and when it goes unloaded for 10secs for example that awful noise lasts longer

07-09-2015, 09:08 AM
What I see on display at that video is that your unit is running at condensation temperature at 57C!
If it is not sanitary water heating unit, than this is to high for any outdoor ambient temperature below 36-37C.

Prolonged working on such high condensation temperature could made some permanent damage to compressor. Maybe there is issue with unloader!

07-09-2015, 06:24 PM
yeh but video isn't from my compressor, just similar noise to my compressor that's why I show video

I'll try to contact tommorow with danfoss/copeland to see wha they got to say.


07-09-2015, 10:31 PM
As chemi -cool mentioned i think that bearing is damaged . I noticed that none of the videos is from the actual system so if possible record a short because i am curious to hear that noise when motor is running . Also other info about actual system will clarify some things . Anyway the right answer is inside the compressor . Usually i cut the damaged compressor to see what is broken and why . I bet on the upper bearing damage . In this case when compressor loads thegas helps the motor a bit to be in center and when unloads the increased clearance make the scrolls to hit each other making more noise (dry metalic noise) .

monkey spanners
07-09-2015, 10:50 PM
Heres a failed digital scroll i cut open to see what had failed,


08-09-2015, 12:10 AM
I enjoyed your
detailed video monkey spanners ! Thank you !

11-09-2015, 04:42 PM
OK guys, I upload a video on youtube with my compressor. we removed a lot of refrigerant and it sounded better than before, still some weird noise and also bad noise when shut-down although we installed check valve as you can see. The compressor power is 10kW for your info.


Another weird thing I saw is that expansion valve regulated overheating quite well 8-10K almost all the time, and then suddenly it's like it can't keep the overheating down. It starts increasing going up 15K..20..goes up to 25-30K..then suddenly it drops down the overheating and goes down to 1K, 0K..then goes up again to 8-10K...I don't understand, it's controlling it fine and it's like it's suddenly losing control..