Compressor change

[HeathT]

Hello,
I am due to do a compressor change on a Lennox A.H.U. its operates on R407C and has a copeland scroll hermetic compressor. It is the 2nd comp[previous one installed by another engineer] in about 6 months. I susupect that there is liquid getting back to the compressor and want to check the superheat once installed the new comp. What would an acceptable superheat setting read for R407 as well as suitable subcooling reading as I don't want the same thing to happen 6 months down the line. I appreciate any comments.

[Brian_UK]

Do you know why the first compressor failed ?

Was it a burn out or similar?

Is the oil still contaminated from the first in which case it will have killed the second.

[GHAZ]

hi heatht try to get the full gas charge in first with out using the compressor,because they get very hot otherwise, and do oil acid test,also the r407c has glide of about 8c ,so set the superheat of about 14c or liquid will be coming back,most important bit getting the right running direction of the compressor

[HeathT]

Hi Brian,
I am not sure f the cause of the first comp, this one that has gone down now is mechanical failure probably bearings. The comp starts but is not pumping. In heat mode low amp draw and in cool mode it tripped the main circuit breaker in the dist board set o 80A! Also ther was no sign of any oil in the sight glass so may be logged in the evap or filter dryer possibly blocked. Am going to change dryer as well but just want to be thorough with my checks whn commissioning ti back up.

[HeathT]

Hi Ghaz,
Will check that all out when I change it. If I set the subcool to 14c. What ort of subcooling should I expect to find? thanks for the input, much appreciated.

[HeathT]

sorry meant if i set the superheat to 14c what sort of subcooling should I expect!

[Brian_UK]

If no oil in the compressor then perhaps investigate the number and frequency of starts.
Too many with a short run time will empty the compressor oil sump.

[HeathT]

Thanks for the advice will do aall those checks once it is changed, cheers Brian

[rude]

I would also recommend a suction dryer just in case.

[HeathT]

Did acid test on oil today but proved inconclusive. 2 tests carried out one definately had a yellow tinge but not conclusively yellow[acid] and the other showed safe. Those were the last 2 test kits at the supplier so will have to wait till monday to do a couple more. There was no sign of any oil in damaged comp but no sign of it anywhere else in the system as blasted nitro through circuit when comp was out and no oil was pushed out, took filter dryer out which was not blocked and again no sign of any oil. Would anyone recommend a burnout dryer even if the next tests prove safe? Or would that be unneccessary as i understand that is a costly process

[Brian_UK]

Question comes to mind....

Are the new compressors supplied with oil ? I know they normally are but you never know.

[Gary]

The oil is out there in the system somewhere... and you need to find out where and why.

The nitro failed to push it through so there must be a low spot where the nitro can flow over and past the oil.

[Slatts]

what Gary said.
And a burnout drier is a lot cheaper than yet another compressor. if your oil test came back as possibly positive, I think it would be false economy not to fit one.
A bit like changing a compressor and using the start gear from the failed one.
As the compressor has an oil sight glass, when you start it up, monitor the oil level for the first few hours and add more if it needs it.
I'd return after it's been running for a day or so and check it again.

EDIT: It's good practice to remove the burnout drier after you get a clear oil test or replace it if you don't, until you do. So a way to pump down or isolate and vac out the burnout drier would be preferable to having to decant the whole system.