Scroll internal bypass

[Larry2]

On shut-off, a new scroll compressor momentarily makes a unique sound while the orbiting scroll spins backwards and pressures equalize.

If this sound stops and is replaced with the sound of refrigerant equalizing, does this mean that the compressor has unseated the internal bypass, and is leaking internally?

[NoNickName]

Sanyo and LG do not have check valve on the discharge, so they whistle when stopped. They are intended to whistle so in case of quick restart, the suction is at the same pressure than discharge.

[LRAC]

Hi Larry2

Don't worry about it their is nothing wrong with the new compressor you've fitted, this is quite a normal occurance.

Regards
Lrac

[Larry2]

A new scroll unit (thinking back a few years) would go burrrrrrrrrppppt for a second or two when power shut off. I read that the scroll spins backwards during this and the pressure equalizes very quickly so that it should have no trouble starting against pressure. It is a distinctive sound that I'm sure some here are familiar with. I've also read that the scroll unseats once it stops, making it forgiving about starting against an expansion valve.

I read there is an internal pressure bypass valve. When head pressure is excessive, the valve directs refrigerant from the head back into the suction side. This hot gas is directed past the klixon on top to shut the compressor down.

What I am wondering is if this internal bypass can fail in such a way that there is leakage from the head back to the suction side. Has anyone seen this sort of failure? Would one hear refrigerant equalize through the compressor if this internal valve failed? What means are there for proving this internal problem?

[LRAC]

Larry2

is this an air conditioning application or refrigeration condensing unit please advise.

Regards
Lrac

[Larry2]

Hi Larry2

Don't worry about it their is nothing wrong with the new compressor you've fitted, this is quite a normal occurance.

Regards
Lrac

I was typing while you replied. I have not fitted a new compressor yet. The original compressor and the other two units produced the charactoristic scroll shut down sound.

On the system that performs poorly, the shut down sound is missing. In place of that sound, I hear refrigerant equalize in the compressor. I am getting somewhat low high side, high low side and poor production. I'm thinking the pressure relief is leaking and I'm looking for other signs the confirm that notion.

[LRAC]

I am getting somewhat low high side, high low side and poor production.

Hi Larry2

I think you've answered this yourself if you can carry out a pump down test by doing the usual front seating of valves etc, if the suction rises then yes you have a problem with the compressor.

Regards
Lrac

[Larry2]

Copeland warns against active pumping tests and specifically pumping the scroll units to zero. They talk of arcing on the fusetite. For that reason, I have shyed away from closing valves and watching the gauges. Are my concerns unfounded and I should just watch the gauges and shut down as it nears zero? I guess at this point, there's not much at risk anyway.

[taz24]

Are my concerns unfounded and I should just watch the gauges and shut down as it nears zero? I guess at this point, there's not much at risk anyway.

i think you have found the problem. I have not come across this problem personaly but it could happen and it seems to have with you.

I would be tempted to pump it down. The thing sounds buggered anyway.
Looks like a new comp is required.

Cheers taz.

[monkey spanners]

Hi Larry2
I think the fusite compressor terminals on the copeland scrolls are the same as the one one reciprocating hermetics, i think you'd be ok with a pump test after all you can't get more broken than broken (having said that has anybody noticed the explosion warning on L'unite compressors lately about fissing/poping noises coming from the terminal area, basicaly saying run and hide ). If its pumping ok i'd expect it to pull 5/7in HG then the scrolls would seperate, pressures equalize a bit then start pumping again.
I'd pump it down and with my gauges on the suction and discharge valves and the valves shut off to the system i'd put some nitrogen in the high side and see if it bleeds across to the low side. If it does its either the discharge check valve or the high pressure bypass thing you mentioned at fault. If it was only the check valve and it was pumping ok you could fit an external one like on the York/Bristol srcolls.

Some other scroll faults ive seen. On a single phase unit on a DX milk tank was called to unit not switching off. Found unit running backwards due to a power cut when running and the power coming back on as it was spinning backwards, fitted delay timer to start circuit.
On a three phase compressor (one of two) found to be running backwards, swap phases over, ran ok, just packing up tools when it made an odd noise, fitted gauges and found to be not pumping. Swaped phases again only for it to pump ok for 5/10 minutes and the appear to be running backwards again. New compressor.

Cheers Jon

[Larry2]

Thanks Jon. I had read about the scroll spinning backwards after a power glitch. Part of normal shut down, the scroll spins backwards a moment and if power comes again this, the motor spins backwards until the overtemp decideds it hot in there with no refrigerant flowing.

The other observation I have with this compressor is it is mechanically very noisy. It makes a loud mechanical buzz and grinding noise as it runs. This noise goes through quite some changes when the unit first comes on. It sounds as though it's dealing with liquid but this can't be with subcooling set by the spec. Its as though the bypass is leaking liquid into the suction and when the amount is steady, the noise become steady.

One fellow I've been in contact with recommends disabling the condensor momentarily and watch if it will pump to 350 psig and then fall off after running a few minutes. I'll think more on this. I don't want to be hasty.

[taz24]

The other observation I have with this compressor is it is mechanically very noisy. It makes a loud mechanical buzz and grinding noise as it runs. .

I have had experience of scroll comps failing on the main bearing. In effect the bearing seizes and the comp fails. The noise and poor performance from your comp may be an indication of this fault.
Check the comp running amps and compare to the other good ones.

Cheers taz.

[Larry2]

I have had experience of scroll comps failing on the main bearing. In effect the bearing seizes and the comp fails. The noise and poor performance from your comp may be an indication of this fault.
Check the comp running amps and compare to the other good ones.

If you get bearing failure but not winding burn-out, how much debris goes into the system? Does this become a big clean-up problem or will a fresh filter/drier attend to it?

[monkey spanners]

Changed a 6Hp copeland scroll a couple of months back due to it making an odd noise every few minutes. Sounded like it had been hit with a lump hammer, the went quiet with a small drop in the run current and apeared to have stopped pumping and then would carry on as if normal too soon to see if the pressure was going to rise. Due to the system history i suspected it may have had too much oil in (my previous employers work ) drained oil and found quantity ok but to be the colour of used engine oil
Filled with fresh oil but the noise persisted and got worse over time so it was replaced. I fitted a 30cu in suction drier and 16cu in liquid line drier to help clean up the system.

Cheers Jon

[taz24]

If you get bearing failure but not winding burn-out, how much debris goes into the system? Does this become a big clean-up problem or will a fresh filter/drier attend to it?

I have done this replacement about half a dozen times over the last few years. All the comps were just one of many on a pack. The comp was replaced along with the usual oil filters. The liquid dryers are changed as a matter of course and the suction has felts in to stop solids coming back (which you can check / change) to the comps. Oil acid tests are done yearly as with liquid cores and after the comp change.

Is the comp on rota-locks or brazed in.
Is it high or low temp.

No real difference in time taken to change but soooooo much easier with rota-locks on.

Cheers taz.

[Larry2]

The new compressor is installed and what a difference it made for heating and cooling. It did not have rotolock fittings, just braze connections.

When I get a chance, I plan to cut the old unit open for a look. I'm keen to see the scroll assembly first hand and I've always been interested in analyzing failures. If anyone has cut up a scroll before, let me know the best place to cut.

I did some more reading on the topic and found that acid in the system causes bearing failure due to electrical metal migration to the bearings. Acid conducts electricity and it carries metals over to the bearings which are grounded. I have a hunch this system was not pumped down nicely when first installed. The moisture indicator was not yellow, but it was not bright green either. It's also possible the internal bypass never reseated after the condensor fan caused high head.

It is amazing how quiet the new compressor is. Its hard to tell it's even running over the fan motor sounds. The old one sent vibrations through the pipework into the house.

[taz24]

It is amazing how quiet the new compressor is. Its hard to tell it's even running over the fan motor sounds. The old one sent vibrations through the pipework into the house.

Good news with the replacement. I have cut open a few compressors and including a scroll. The best place to cut is where the welded joint is. It is there that they put it together. You will be impressed by how the scroll works.
Cheers taz.

[jomama]

i have a copland scroll when over heated the pressures almost equlize after a few hours shut off the compressor resets

[taz24]

i have a copland scroll when over heated the pressures almost equlize after a few hours shut off the compressor resets

Hello.

Sounds like you have a fault with the valve that equlises the pressure from discharge to suction.
Your problem sounds like a common fault with these comps.

Cheers taz

[frank]

Which leads to another question - why does the compressor overheat?

[taz24]

Which leads to another question - why does the compressor overheat?

Ah!!!! now then the $64000 question.
Which come first the overheating compressor causing the by pass to opperate or the by pass opperating causeing the comp to overheat.

When I first read the post I read it as comp overheating due to the relief but looking from your point of view there could be a fault that has caused the comp to overheat.

Cheers taz.

[frank]

Ah!!!! now then the $64000 question.
Which come first the overheating compressor causing the by pass to opperate or the by pass opperating causeing the comp to overheat.

When I first read the post I read it as comp overheating due to the relief but looking from your point of view there could be a fault that has caused the comp to overheat.

Cheers taz.

I think they call it "thinking outside the box" Taz

[sparrow]

Taz/Larry2

This was an interesting post for me to read, i was just wondering if it were possible to take a couple of photo,s of the inside of a scroll comp.

Sparrow

[monkey spanners]

Check out

http://www.copelandscroll.com/index2.html

For scroll internal workings

Jon

[sparrow]

Thanks Jon, would the high side be the top of this type of compressor?

cheers

[taz24]

Thanks Jon, would the high side be the top of this type of compressor?

cheers

Yes

taz

[joel]

possible reasons for over heating of scroll compressors

1. high discharge pressure.( over charging,defective condenser fan )

2.undercharged system.( system leak?)...air in the system.

3. contaminated compressor oil.( oil migration at evap/cond)

i replaced copeland compressor too.reason is leak in the system.low system pressure( not enough to cool the compressor) but above the low pressure cut out set point.compressor is running continously with high compressor temp.

more over, i experienced the compressor doesnt shut off during pump down. ( pressure switch failed ) and compressor runs continously.its very hot already, but the current is still low, ( already pumped down,( solenoid is closed).

[ateeq.baig]

the internal by pass of the scroll comp will active when the comp will run under or over capacity
its suctin and discharge pressure should not be high or low then limits