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coolhandlew
21-12-2016, 01:26 PM
Hello all,

I work in a data centre, I have been in the trade for a long while but my job now is to oversee contractors work on the cooling plant.

This week we replaced a compressor in a data hall CRAC unit. The unit is a York YC-UPA 932. Twin circuit, DX, downflow, approx 90kW. The compressor is a Panasonic fixed speed scroll C-SCP510H38B.

I didn't witness the actual replacement process but I did witness the folowing;

-Three stage evacuation, each time achieving close to absolute zero with OFN added between vacuums to around atmospheric.
-Refrigerant charged into liquid line R410a to holding pressure.
-Unit started up, further refrigerant charged into suction line (I know what you're going to say but this was done with care).
-All the time system pressures were being monitored, Head pressure was in the region of 24bar, suction pressure was around 8bar, superheat was around 16K. The sightglass was flashing intermittently. Running current was checked on three phases - 17A.
-The compressor sounded fine, no vibration, temperature at the top around 65 degrees C.
-We decided to take a break and let the unit run for half an hour to settle down.
-On our return to the room the compressor was not running, we assumed due to achieving setpoint. We then observed the compressor start.
-It sounded like it had spanners in it! Very noisy, machine gun like.
-We immediately turned it off, I'd estimate that it ran for around 5 seconds, the motor breaker did not trip.
-The temperature on the compressor body was fairly even top and bottom and around 60 degrees C.
-The oil visible in the compressor sightglass was noticeably black, looks like your cars oil after 10000 miles. The level of the oil is good.
-I'm told that prior to the replacement the system pipework was purged (blasted through) with OFN.

Obviously we checked three phases present, and rotation.
It had ran fairly sweetly for an hour.

Is it possible a bearing could break up in an hour and allow the scroll to clang about but not trip the MCB?
I wondered whether the contactor may have only closed on two phases intermittently, I would have expected more heat though, and probably clean oil.

The filter drier was changed also at the same time.

This was a brand new compressor, failed after one hour.

Any ideas/suggestions greatly appreciated.

cadwaladr
21-12-2016, 08:05 PM
The first question I would ask is why was it replaced and was an acid test carried out,I always changed the contactor one overload on any compressor change along with the expansion valve,if I spot any contamination at the expansion valve screen the a thorough flush of the system is carried out,I have heard of many replacement compressors failing prematurely because of a faulty contactor some people cut corners for numerous reasons and can get lucky this failure sounds dramatic to say the least.

Rob White
22-12-2016, 12:31 AM
.

It is possible for a brand new comp to fail.
I have had a new scroll fail within one day.
After running fine for about four hours it shut
down as it was one of six and by the next
morning it was seized solid. It was replaced
and we later found out the top part of the crank
that goes into the moving scroll had failed and
collapsed, this caused it to seize solid.

So no help to you other than to confirm Sh*t happens.

Rob

.

frank
22-12-2016, 12:33 AM
Liquid floodback?

Glenn Moore
22-12-2016, 02:55 AM
It doesn't take long to destroy a scroll compressor. I was called to a new, York chiller which had wrecked 6 Maneurop scrolls in less than an hour during commissioning. The unit was mounted outside with no crankcase heaters fitted. Liquid migration had caused the scroll set smash ups.
But in your case I would suspect an LP problem may have occurred.Scrolls must never be allowed to run down into a vacuum as the vacuum created by the scrolls pulls the 2 scrolls together (fixed scroll & orbiting scroll) this causes the scrolls to run dry of lubrication and hence the scrolls start to shed metallic particles as the scrolls start to break up . This often results in the oil turning black due to the metallic debris in the oil. So check that the LP switch stops the machine at a suction pressure of at least1.5 Bar gauge.
The cause you will find is always inside the compressor.
This sounds like a system set up problem but getting the compressor analysed to determine the cause of failure is a worthwhile exercise to prevent further failures. I am working on a system in London which has experienced several scroll failures, the compressors will be cut and stripped before any new compressors are fitted.

Greek_engineer
22-12-2016, 11:22 AM
the best way is to cut the old compressor in an effort to detect the problem....I agree with Glenn.

norseman
23-12-2016, 01:25 PM
Scroll compressors and suction pressure
I am now and then into shock blood plasma freezers using scrolls and find that they are not that sensitive to a low suction pressure. The systems i am into use a 4 hp. Bristol running with R-404A
The units cool down some pockets to hold the blood bags to the last of them all reach -55 C
Some of the pockets reach -57 C before kicking off. We sit with a suction around -60 C at the end.
At least for 8 hours a day 24/7 the unit is running at -55 C and at idle over night at -45 C.
We are into a suction pressure as low as 0,5 bar gauge pressure and those units stay alive for at least 10 Years.
Sure not an ideal condition, but just to verify that some brands are capable to take a lot of stress.
The liquid return is also a big concern, but the ones here can take whatever it looks like
The systems use an electronic expansion valve with a suction tube sensor for the overheating calculation.
On some of those units the sensor get moisture inside due to a quality issue.
The result is that the smart controller believe it is an extreme high overheating.
It cracks open the valve almost fully so 1,8 kg rush through the small system.
The customer calls us and tell that they only reach like -40 C in the pockets and what we often find is that
the scroll is heavily iced up from the bottom to at least half the height of the outer shell.
Still with this "fatal" situation, they can run like this over days and still behave nicely with a new sensor.

Just mention those experiences with scrolls since it is so much talk about you have to handle scrolls like a new born baby :-)
Wish you all a Merry Christmas and a Happy New Year