how to put compressors in parallel

[Lc_shi]

What should be considerd by connecting two or three compressors in parallel? install a oil separator in discharge line and return the oil to suction side at times. Is it correct? welcome your input. thanks in advance!


best regards
LC

[US Iceman]

LC,

Oil recovery is important as you suggested. The oil could be recovered by an oil separator and the oil levels equalized between the compressors.

I have seen this done two ways.

In one application the the oil level was equalized by a common line between two crankcases. In this system no oil separator was used.

Peter_1 also posted an installation bulletin about this sometime ago. This was another method of piping multiple compressors in parallel without the oil separators.

The more common method is to use an oil separator and the oil level regulators to maintain a constant oil level. You also need an oil reservoir for this system. The oil is returned from the separator to the reservoir. The reservoir then supplies oil to the individual oil level regulators.

Best Regards,

[Rob S]

The other issue is proper discharge and suction piping.
Copeland has some great service bulletins on this.

Basicly all the discharge lines will flow in one direction. To prevent head locking.
All the suction lines will be Tee'd and Tree'd together or use of a header.

[Carlo Hansen]

Hi Ic_Shi

Do not forget the one way valve in the high pressure line
from the compressors to the condenser.

Best regards

Carlo Hansen

[Lc_shi]

thanks for all your input.
I'll check with compressor suppliers.

[Lc_shi]

I've got a sketch.pls have a review and make comment.

thanks

rgds
LC

[Argus]

Generally it is common to parallel 2 or more compressors in air conditioning chiller applications fitted to single evaporators and condensers - these are usually semi hermetic reciprocating compressors and the systems are not fitted with oil separators as a rule.

The method is simple and in my experience trouble-free.

1. All compressors must be level.
   
2. Discharge and suction pipes are connected together at the compressors with equal pipe and fittings.
   
3. Oil lines are interconnected between each sump.
   
4. The sump space above the oil levels are interconnected. (Carrier, for example, in their large packaged chillers, use a single 28mm pipe between the sumps connected to the spare sight glass flange; as this is at the optimum oil level, it maintains the levels of the oil in all the compressors and equalises the sump pressures at the same time in one pipe).

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Toyota JZ engine history

[Josip]

I've got a sketch.pls have a review and make comment.

thanks

rgds
LC

Hi, LC

Sketch is more or less ok. Speaking about there should be NRV on discharge pipes from each compressor. Also all other needed stop valves not show.

Oil separator to be or not to be installed?

For that purpose you can use only oil reservoir, but

definitely it is better (not a must) to install an oil separator to obtain better COP at condenser and keep oil in the shortest circuit: compressor - oil separator- oil reservoir - compressor, where oil must be No need to run oil through condenser, receiver, evaporator back to compressor becuse each drop of oil in refrigerant line reduce our COP and cause some other problems

NRV at oil reservoir is ok (it must be there) becuse it is closed all the time when compressors are working. There we have enough high pressure to push oil back to crank case when come to low oil level. NRV is there to equilize the suction and discharge pressure at compressor stop. You can push all oil from oil reservoir into compressor crank case due to pressure equilizing and we do not want and do not need that. For that reason that line is as you can se "vapour" line.

Hope this is of little help

[Servicefrigo]

I think is not the same things in low applications were is apsolutly necesary to have an oil separator
Regards

[Argus]

I think is not the same in things in low applications were is apsolutly necesary to have an oil separator
Regards

Never said it was low temperature - in fact the oposite.

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Yamaha FZ1 history

[Peter_1]

Lichuan,
Your sketch is +/- right just for 1 thing: you may not use an NRV between oil reservoir and suction but yoy have to use a NRD which keeps the pressure of the reservoir +/- 1,5 bar higher then the suction pressure.
Otherwise, there's no possibility (no pressure differential) that the oil can flow to the sumps.

[Servicefrigo]

Sorry the message wos't for you Argus.
Regards

[Peter_1]

I think is not the same things in low applications were is apsolutly necesary to have an oil separator
Regards

If the compressors are connected together like argus specified - crankcases connected together - then I should never install an oil separator, even in a freezer application.
Give me some, or even better...one good reason why you should need this with 2 compressors in parallel in the case Argus explained.

I never installed one on a freezer.
But we discussed this already many times, porely piping is the most important reason why you should need an oil separator.

[chillin out]

Pipework looks ok to me, Do you know about all the electrical componants?

S/V in oil lines, Might need oil sep heater.

What size of system is this?

If its only small you can just fit oil sep feeding straight into common oil equalizer. No need for oil reg devices. (this way must have oil sep heater).


Chillin

[Peter_1]

This is a prallel setup with 3 compressors where you don't need an oil separator.

[chillin out]

Peter, I would hate to change that middle comp!

What would happen if 1 comp was leaking across the head? would it fail the whole system? Theres no place to isolate an indivdual comp, is there?

Chillin

[liverpool Eng]

The oil floats may take the high pressure but not the diff.
for aplications above a 2 bar diff i would use a electronic version like oiltrax that has its own solinoid and pulses in the oil.
This stops the foaming and passing that give problems with the mech versions.
otherwise the NRD at 1.4 bar is correct for most aplications.
And if we are going to use a high pressure oil syestem then i would recomend the combind oil sep and res avaleble from the likes of Tempright.

[zaheer]

What should be considerd by connecting two or three compressors in parallel? install a oil separator in discharge line and return the oil to suction side at times. Is it correct? welcome your input. thanks in advance!


best regards
LC

Yh.its sure. thi same system in daikin vrv11,vrv111.you can see.

[Tom C]

We manufacture 2 and 3 compressor parallel systems for the foodservice industry.
On a current system (medium temp only) we have 3 equal scroll, 3 hp compressors. We have an oil separator, reservoir, and 3 level regulators (S-9010). We, as in the past, did not install a pressure valve between the vent line and the suction line. Up until now, we have not had any problems, except with the 9010's, which we are going to scrap in lieu of the OP-02.
The difference here is that this is in a low ambient condition on a rooftop in NYC (instead of an indoor wc system, which is normal for us). The installation tech is telling me that he can't get flow to the level regulators. Although, when the feed line connected to the level regulator is loosened, oil will flow.
We have lost (burnt windings) 1 compressor, so far.

The obvious, would be to install a pressure valve in the vent line to maintain the oil reservoir pressure slightly above the cc pressure. However, on the 2 compressor parallel, we use an equalization line to maintain levels. We have never had a problem with this, in any ambient condition, even though the cc pressure in both compressors is the same. On all other 3 compressor parallel systems, oil return has just not been an issue. Only this one.

The installation tech said that he had to adjust all of the level regulators. When the system left the shop, they were all working and set correctly. Of course, 3k miles on a flat bed tends to screw up any setting.

So, install the pressure valve and eat the compressor,
or, as I suspect, the oil lever regulators are out, and maybe stripped?

Comments would be appreciated.
Thanks.

[Peter_1]

Insulating th oil separator or heating it slightly with a drain heater will perhaps help. Temperature is perhaps that low and load on pack so small that vessel cools down so deep that you loose pressure.