About Copeland ZB scroll compressor

[ernestlin]

Hi, guys.is anyone fammiliar with Copeland ZB scroll compressor which is designed for commercial refrigearion? Compared with reciprocating compressor, what's about it's reliability? Some guys said it's cost less pull down time than reciprocating compressor even MT..is that really?Any input is appreciated.
regards
ernest

[NoNickName]

Never used ZB, but lots of ZR and ZH.
Reliable, but prone to failure by liquid hits.

[US Iceman]

I have only looked at the ZB compressors for heat pump duty on a new application, so I can't say much about life expectancy.

I'm not to sure why their pull down time would be much greater over any other compressor. Pull down time is simply a matter of load versus capacity. I believe the scroll compressors have a better volumetric efficiency than recip compressors, however, I would not expect a large difference.

[Cofreth]

Scroll have high swift volume like rotary comp but build-up pressure slower than recipro.

[Andy]

Never used ZB, but lots of ZR and ZH.
Reliable, but prone to failure by liquid hits.

Copeland market them as a compliant scroll, scrolls deflect to allow for liquid

I think not

Scroll are a high temp compressor, low temp use recips or screws, not saying they don't work on LT just not as well as recips

Kind Regards Andy

[wambat]

The ZB is a medium temperature complient compressor. Compliance is the ability of scrolls to separate under abnormal conditions. Copeland's scroll design uses both axial and radial compliance to deal effectively with liquid and because it has only two moving parts per compressor as opposed to nine parts per cylinder for a reciprocating compressor the reliability is much improved.

[NoNickName]

Copeland market them as a compliant scroll, scrolls deflect to allow for liquid

I think not

Compliant and tolerant are not necessarily synonims.
I've got a refused warranty coverage from Copeland for a ZR, and the explanation is "suspect low superheating".
I think this forum can be also useful for advocacy and motions to producers... I think LG will not sell a single split between us, considered the recent threads, and if we push hard to Copeland, then their attitude can change.

[US Iceman]

I've got a refused warranty coverage from Copeland for a ZR, and the explanation is "suspect low superheating".

That raises a very interesting question doesn't it? I seem to remember some Copeland literature that states the scroll compressors are liquid tolerant?

The scrolls could unload (separate) during an attempt to compress liquid, so no damage would occur (maybe). You would still have a problem with oil dilution due to the liquid entering the compressor, right?

I have also had a conversation with someone who said you need to watch the oil dilution on the scroll compressors.

Did Copeland state if the "low superheat" was suspected on the suction side or discharge side?

It could be hard to distinguish, since in all probability the compressor failed due to oil dilution problems.

[NoNickName]

Low superheat on the suction is the alleged cause by Copeland, though I'm pretty concerned it has nothing to do with it.

[wambat]

We all know that for every one who has an opinion in the negative there are those who have the opposite opinion so it always comes down to individual choice.. Kind of like your choice of women :>) opps my bad, sorry ladies

[NoNickName]

Don't get me wrong: I have a fair opinion of their compressors, and I'm using dozens daily. But customers do not deserve to be get down that way.

[Brian]

What is the diference between low side superheat and high superheat. Are you suggesting valve damage?

Brian

[US Iceman]

Hi Brian,

What is the difference between low side superheat and high superheat. Are you suggesting valve damage?

You actually have superheat on both sides of the compressor. Suction superheat is pretty straightforward. It's the evaporator superheat plus any additional heat gain in the suction gas (basically).

Discharge superheat is calculated the same way as evaporator or suction superheat (both are different items)

With different types of compressors you have to pay attention to where the oil is. On screw compressors, the oil is at discharge pressure. On recip compressors, the oil is at suction pressure.

For a screw compressor if the oil temperature is close to the condensing temperature of the refrigerant, the refrigerant dilution in the oil increases. This reduces the viscosity of the oil.

If the discharge superheat is sufficiently high enough, the actual discharge temperature of the gas is much higher than the condensing temperature of the refrigerant at that pressure. You will also find the oil temperature is normally higher than the condensing temperature.

This makes it highly unlikely the oil viscosity will be diluted (i.e., lower viscosity).

On recip compressors, the oil temperature is almost always higher than the return gas temperature and certainly higher than the saturation temperature of the suction pressure. So oil dilution is pretty hard to develop.

On scrolls, I'm not sure if the oil is at discharge pressure or suction pressure. One person who is pretty knowledgeable about this subject told me discharge.

So, if that's the case... A scroll compressor would be subject to the same discussion as screws.

I made a guess at the statement NoNickName brought up. Low suction superheat seems to me they are saying there were some indications of oil dilution wiping the bearings clean.

Depending on the situation of the oil "at discharge or suction pressure", we might have to look for various problems.

[ernestlin]

Thanks! buddies, forgive my late reply. there is a brilliant discussion from which I can learn many things.
Iceman, as I know there are two types of scroll compressor which are Low Pressure shell and High Pressure shell, and most is Low Pressure shell, so the oil is mainly in suction pressure.
Aming at the "Low Suction Superheat" points which Nonikename brought up, I regard the reason is the volumetric efficiency of scroll is too high (nearly 100%), and the discharge pressure(temperature) is easily too high to ruin the compressor if the suction pressure is higher.
welcome attend to this topic.
regards
ernest

[mad fridgie]

I have just been to a Copeland seminar, they pushed the virtues of the scroll compressor pumping liquid, even to the point that they stated that they filled the compressor full of liquid then started the machine, no problems!
I decided to test for my self, but with a bit more reality? I flooded the compressor when it was running!
Bang Crash, pressure pulses, finally compressor internal pressure relief valve gave way. (only reset when pressure totally equalised)
My recommendation treat as you would any other compressor, do your best to ensure no liquid floodback, install a suction pot (accum.)

[desA]

@ mad_fridgie:

What refrigerant mass charge did you fill the system to?

Copeland have an upper limit below which they claim that the lower shell acts as an accumulator - 10 lb (~ 4.5 kg).

By flooding the compressor while running, you surely would have had liquid carry-over directly into the scroll. I doubt Copeland would have meant this to be the case.

[NoNickName]

I have just been to a Copeland seminar, they pushed the virtues of the scroll compressor pumping liquid, even to the point that they stated that they filled the compressor full of liquid then started the machine, no problems!
I decided to test for my self, but with a bit more reality? I flooded the compressor when it was running!
Bang Crash, pressure pulses, finally compressor internal pressure relief valve gave way. (only reset when pressure totally equalised)
My recommendation treat as you would any other compressor, do your best to ensure no liquid floodback, install a suction pot (accum.)

Thanks for your enlighting contribution.
One lesson learned: Beware Snake Oil Salesmen

[nike123]

I have just been to a Copeland seminar, they pushed the virtues of the scroll compressor pumping liquid, even to the point that they stated that they filled the compressor full of liquid then started the machine, no problems!

Accumulators
Due to the inherent ability of scroll compressors to
handle liquid refrigerant in flooded start and defrost
cycle operation conditions, accumulators may not
be required. An accumulator is required on single
compressor systems when the charge limitations
exceed those values listed in Table 2. On systems
with defrost schemes or transient operations that
allow prolonged uncontrolled liquid return to the
compressor, an accumulator is required unless a
suction header of sufficient volume to prevent liquid
migration to the compressors is used. Excessive liquid
floodback or repeated flooded starts will dilute the oil
in the compressor causing inadequate lubrication and
bearing wear. Proper system design will minimize liquid
floodback, thereby ensuring maximum compressor life.

http://www.hvacrinfo.com/cope_ae_bulletins/TAE1318.PDF

I flooded the compressor when it was running!

[mad fridgie]

Let me make it clear that the forced liquid floodback was undertaken on a test rig! The reason for the test was to see i could reduce my unit cost by removing the suction pot! Simulation based upon a faulty fully open TEV.
Therory indicated that I do not need the pot (not a large charge 2.5Kg), in practice I believe we do, so for the sake of $100 I will be installing the pot