Parallel compressor problem

[dsltung]

I have a problem with a parallel compressor system, hope that someone can help.

The system contains three compressors connected to common suction header and discharge header. Immediately after the discharge header, a oil separator is installed and the oil return pipe is connected to the common suction header. The compressors have features that pump out excess oil to the discharge header if oil level inside it is more than required. Compressor suction pipes are tee-off at the side (not bottom) of suction header to oil accumulation in idle compressor.

When three compressors are running, the one with longest distance from common header has higher running current and higher discharged temperature as compared to the other two. If only one compressor is running alone (ie. the other two are in idle), they all give similar current reading and pressure reading no matter their distance from the common header. I have experience with similar system but didn't face this problem.

Can any one give me advice?

Is there any article or book that I can learn practical design on parallel compressor refrigeration system?

[nike123]

Maybe this (http://www.hvacrinfo.com/cope_ae_bulletins/TAE1235.PDF) bulletin helps!

[dsltung]

Thanks Nike, it is a very useful information.

Can I ask why the running current of the compressors in a parallel system differs? Is it caused by the difference in distance from the common header?

[Tesla]

Likely to be the cause - a differance in piping pressuredrop, it is differcult to ballance a dynamic system. If adverse opperating conditions exist a cpr or bpr valve could be used (crankcase/back pressure regulating)

[dsltung]

Thanks Tesla,

One important thing I forgot to mention is that the system operated normally at start-up, the problem occured two months after start-up. What will be the cause?

[Peter_1]

You will have to measure suction temperature and discharge temperature and LP and Hp for each compressor separately, measured on the compressor itself. Pressure measuring not on a distant connection (preferable on the cylinder head)

Also the main suction temperature and LP on the collector, Then we can help you perhaps further.

All compressors are the same?

[dsltung]

Thanks Peter_1

All the compressors are the same.

The compressor with problem had discharge temperature measured (just outlet)about 18 degree C higher than the other two, why?

The suction temperature measured were more or less the same. However I am not quite sure my measurement are correct as they were affected by the ambient temperature.

I couldn't measure the individal HP & LP pressure because no service valve are there.

There is a check valve install at outlet of each compressor, at first I though it could be failure of check valve cause the problem, but while each compressor was running alone, the HP & LP were almost the same.I guest check valve is simple that may not be the cause.

If it is oil return problem, the increase in current will not be so fast since it take time to pumped out all oil, then what cause the increase of current when one more compressor is switched on?

I need help immediately, your advice is highly appreciated.

[Gary]

The compressor with problem had discharge temperature measured (just outlet)about 18 degree C higher than the other two, why?


If the compressor inlet superheat is higher, then the discharge temperature will be higher.

[dsltung]

Hi Gary,

Appreciate your help.

But why only one compressor in the circuit get higher inlet superheat? They are all comming from the common suction header.

[nike123]

Do you have any photo of that setup? Could you make few and post here?
Does other two compressor have high discharge superheat when works alone?

[dsltung]

Here is the photo.

The compressor on the left, close to the end of the suction and discharge header, had problem of overload when all three compressor were running. When only this compressor was running, the current was normal.

check valves are located at discharge pipe of compressors.

I need to solve this problem urgently, your help is highly appreciated.

[nike123]

Does other two compressor have high discharge superheat when works alone?

And how much difference is that!

Please, answer these questions!

[dsltung]

When compressor worked alone (i.e. only one was ON and the other two were OFF), all three compressors had similar discharge superheat temperature. When all three compressors worked simultaneously, the one on the left (close to end of common header) had about 18 degree C higher than the other two compressors.

[nike123]

If all three have same suction superheat, then It looks like compressor with high discharge superheat works against high pressure from other two compressors. I would change non-return valve of that compressor and install separate suction and discharge measuring port on each compressor.

[dsltung]

Thanks Nike,

If non-return valve create problem, what about take away all non-return valves? Is it OK?

[nike123]

Thanks Nike,

If non-return valve create problem, what about take away all non-return valves? Is it OK?

If these compressors are scroll (to me, it looks like that), then I would not.

[dsltung]

Dear Nike,

Do you mean keep the non return valves?

[nike123]

Dear Nike,

Do you mean keep the non return valves?

Yes! I mean, that you need to change non return valve (replace with new one) on that compressor who has high discharge superheat, if his suction superheat is same as on other compressors.

[dsltung]

Dear Nike,

Thanks very much.

[nike123]

Dear Nike,

Thanks very much.

Ok, but be aware that my advice is based only on limited data you suplied to this thread.

[K.R.Iyer]

Hold on! The problem may not be the non-return valve. Because when individual compressor is run, it behaves fine.

[nike123]

Hold on! The problem may not be the non-return valve. Because when individual compressor is run, it behaves fine.

In one direction, (open) yes. But, what about in closed direction (gases from other two compressors could reach in that one if his check valve is not closing) ?