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Dan
03-03-2001, 04:10 AM
Hussmann racks in a Costco. The compressors have discharge check valves mounted about 3 feet from the discharge of the compressor. I have 2 racks, and each has a compressor that makes a god-awful noise after it cycles off.

I assume it is the reverse rotation induced by the captured discharge gas in the 3-foot piping. Why it only happens to one compressor on a ten-compressor rack is puzzling.

I asked engineering if there is anything to concern me and was told not to be concerned.

These compressors are equipped with Trax-oil controls which use 3 small magnets on a tiny float to induce a field that energizes or de-energizes the oil feed. These magnets were fouled with iron shavings on the compressors that made the noise. Further, at least one of them was covered with iron fur on the magnet at the outlet. I don't know why there is a magnet on the outlet.

Visual inspection of the other Trax-oils showed no filings.

Do you think it is possible that the short reverse-burps are delivering shavings or filings backward through the Trax-oil controls? Is anybody familiar with Trax-oil controls?

subzero*psia
03-03-2001, 04:57 AM
I am not familiar with the situation you are describing but I am wondering about the condition of the check valve, is it operating correctly? That burp may be the check valve having a flutter and not sealing properly, possibly back gassing the compressor. I would also be suspicious of the iron filings... where are they coming from?

This may sound stupid, but check the check valve for position and for blow-bye if there are access fittings that will allow. Check your amperage against the running load amp rating this may offer another clue. Have you observed or felt the suction line during the event and are there any other flow control devices in the system anywhere... perhaps even between the compressor and the check valve?

This sounds like an interesting occurance and I would be concerned with the iron filings also.

Dan
03-03-2001, 05:35 AM
Sorry for the shorthand description, but there are some things we pretty much made our mind up on. However, your thought about the check valve has me thinking. Just how to check on a check valve.

Maybe it doesn't seat as quickly as the others. There is something there. Maybe the check valve is experiencing the same problem that shows up at the Trax-oil: metal debris. Why do I only have one compressor on each rack doing this? The check valve has to be suspect. Good reasoning.

Just to make sure I didn't over-shorthand my question, a scroll compressor is supposed to be able to turn backwards with no damage. But that is a lie, because if you run a scroll backwards long enough you will seize the bearings from lack of lubrication; that shouldn't come into play with a 3 second burp. I guess I was wondering if the scrolls themselve scrape differently when they reverse direction.

Is it possible that we had iron filings floating around the system, equally shared from all compressors. And after several hundred cycles, collected on the compressor that burps. The burping compressor simply provides an unfiltered path for the metal to attach to the magnets, from the discharge header, no matter how briefly. The oil inlet is filtered. There is something backward happening.

As unsure as I am about what is happening, I still think it is volute residue collecting on these magnets. There ain't that much iron anywhere else. Perhaps it is just "run-in" offal. I suppose the only thing to do is wait for a pattern.

The fact that the trax-oil has magnets should be the biggest clue. Why does Trax-oil choose to have a magnet on its outlet?

Dan

subzero*psia
05-03-2001, 06:18 PM
On most rack systems your compressors are varying in capacity depending on the demand. You have common suction and discharge headers so as the load drops off the compressors stage themselves down to match the demand and prevent such things as liquid floodback or starving the compressor etc. On very large systems you might see compressors with cylinder unloader etc.

That would explain the iron filings coming from all units possibly, again that would depend on the configuration of your flow controls. That also brings another question to mind, is it possible this system is seeing liquid slugging, floodback or migration occuring? If so it could easily cause your oil to flash in the compressor crankcase and starve your compressor(s) of oil until it is carried back through the system. You might want to look for signs of an oil logged evaporator too.

subzero*psia
05-03-2001, 06:21 PM
I am not familiar with the oil system you mentioned... I am going to see if I can find a site or company info to help me understand it better. If you have a link, post it and I will go there if I haven't found it by then.

subzero*psia
08-03-2001, 04:29 AM
No luck finding much about the Trax Oil. I did read about the magnetic field switch which they refer to as the hall effect switch. It was vague at best, someone here on the board probably has way too much experience with it!! LOL!!

If nobody does, then it looks like you are going to be our resident expert on this subject, Dan!!

Dan
09-03-2001, 12:39 AM
http://www.alcocontrols.com/aw200001/web/oilcntrl.pdf

As I understand the use of the 3 magnets, it is to induce a snubbing action for unnecessary cyclying of the feed solenoid with a turbulent oil level. If metal debris gets past the oil filter, it will adhere to these hall effect magnets and either jam the float or influence the magnetic field - causing problems, the worst of which is to hang in the satisfied position, depriving the compressor of all lubrication and lunching the compressor.

My thought (just speculation) is the reason for the magnet on the outlet of the Trax-oil is to catch filings that may feed backwards during the reverse rotation of the compressor when it off-cycles and the high pressure equalizes to the suction pressure. I think there is another particle-catching magnet on the inlet, too.

I am not crazy about the Trax-oil just yet, it has an extremely fragile mechanical float apparatus and no manual reset capability of its own. and I still am concerned about the filings found on the magnets. Maybe it is just "run-in" debris.

Jack Lester
27-05-2001, 12:34 PM
I have run into a Scroll Compressor that runs backwards due to a bad presure switch. Unit shuts down, compressor runs backwards briefly, switch makes contact and unit is off and runing for a periode. Perhaps this will help

stevec
22-04-2002, 05:11 AM
Hi guys. I've just joined this forum.So it's greetings from samoa.

We have many scrolls running in A.C, DX systems, though single circuits. I have noticed this noise as the compressor stops.The systems work fine so hadn't given it to much thought. My initial idea was that it may have been something to do with the sudden stoppage from full load creating a harmonic/vibration or something.Can't see it being a bad mechaical defect as it appears on to many scrolls that are working fine. Unless a longterm problem. Assuming though it is the same noise

Preston Roy Powell
04-06-2002, 04:23 AM
OK. I have work on some rack system with Scroll but not with the controls your talking about. The units we had had a problem with the scrolls becoming noise, also noisy after shutting off. What i found was the scrolls where turning backward. And yes the did had ckeck valves in them. I had to recover all the refrigerant, or and seal off that compressor, changed the ckeck valve, then evacuated the system. THen Ckeck the pressure control settings. Most of mine i found some one decided it was a good idea to pump them down into a vacuum. Scrolls make a funny sound when they go into a vacuum. SO thats my thought

frank
05-06-2002, 08:53 PM
I've been called out to a split with a Copeland scroll that wasn't cooling. When the gauges were attached the pressures were equal with compressor running. changed over 2 of the phases and the baby ran perfect!!.

Apparently the unit had been like this since it was fitted some 14 months before so the theroy about scrolls running backward have certainly been dispelled in my book!!

Unit is still operating now some 11 months later. Seems like some of the cowboys out there still don't know how to commission properly.

Dan
05-06-2002, 11:50 PM
Frank, that's nothing short of a miracle! How did the upper bearings get lubricated all this time? Why would somebody tolerate no cooling for 14 months? Perhaps a phase shift from the power company recently happened?

Jack Lester
06-06-2002, 03:24 PM
I have to agree with Dan. There is no refrigeration when these units run backwards and they make a very loud scream under this situation. Perhaps the company where there located had some electrical work done. If not I am scratching my head.

frank
06-06-2002, 09:18 PM
The unit was fitted to serve a projection room in one of the local University's and after the projector overheated (the repair bill was £9000) we were called in to provide some temporary cooling while the main split system was investigated. Well, I only had to put my gauges on to find the fault which was quickly rectified and ever since the client just keeps ringing with more work!!

He says that it had never worked since the building was built (14 months). I suppose the compressor has suffered damage and premature wear but while it is running - hey - why fix what ain't broke?.

Client knows it may go at any time but it's been going now for over a year.

Keeping an eye on it. The Copeland scroll had a small tag attached which advised the installer to check for excessive noise indicating a wrong direction but I find it incredible that the "engineer"/installer could actually switch the thing on without gauges attached and an amp probe and walk away saying that it was commissioned and working fine!

Dan
29-11-2002, 05:06 AM
Just discovered another interesting thing about scrolls. It is possible to feed liquid refrigerant out of the discharge when you feed subcooled liquid through the liquid injection port. This says a lot about mass flow through an orifice. The way this was brought to my attention was because of oil failure trips with the Trax-oil.

I don't have much detail at this point, but it is interesting.