Unequal parallel compressors

I maintain a condenser pack with 3 Bock HGX5/725-4S piston compressors.

Since first installed the pack has seemed oversized for the load - 1 Freezer & 1 Blast freezer

For the last few years the blast freezer has been out of use.

Recently one of the 3 compressors failed & on dismantling we found a snapped con-rod & consequential collateral damage. The other 2 compressors seem un-damaged.

My concern is that if light load & compressor short cycling has caused the compressor failure, the best solution may not be to repair/remanufacture the current compressor but replace it with a smaller compressor specified to cope with the minimum load.

Please could you let me know if it is possible to have a refrigerant pack with different sizes of compressors or if they should all be the same size?

I am confident with the controls side of it & re-programming the controller. I am also confident doing the mechanical work of changing the pipe-work & oil scavenging system etc. I just have no experience of packs with unequal parallel compressors and I am unsure if there are factors to take into account that I am unaware of.

Any comments would be appreciated . . .

Leave them as 3 of the same size.
Set the controller to rotate the lead around run hours or manually rotate then over any given period.

It is only more practical to have differing size compressors when large variations of load are expected.

As we say in Industrial.
It's like being up to your chin in muck and saying "Don't Make waves"!

Light load I suspect is not your problem. Then again I don't have all the facts like you.

I would more likely suspect liquid carry over.
That usually results in large physical damage.

Could it not be "pot luck as to which compressor was running at the time of failure.

Then again what sort of service regime do you operate.
IE How many hours since the last service?
The questions are endless. But based on the little given.. leave well alone.

Its highly possible when on light loads you would only need 1 or 2 comps..
Which pair do you choose if of different sizes?
Grizzly

Thank you for your interest.

The compressors are on a ridiculously light load.

It only has 2 evaporators currently working on a –20ºC freezer. Each compressor has an un-loader on it (an early mod to help the system cope with the light load).

The condensing unit is over 6m above the evaporating units & 10 – 15m horizontal distance. The system in on R404A with a suction pressure of 0.8 bar (~-51ºC). So liquid carry over seems unlikely.

I am in the process of checking the superheat on the evaporators but due to a heavy cold/flue I don’t want to spend more time in the freezer than necessary.

The compressors do pull the suction pressure down very quickly, even when on half load, but due to the nature of the controller I am unable to adjust the pressure delta to increase the run time.

A change of controller might help but since the compressor was in need of repair/replacement I wanted to consider the smaller compressor option, which would reduce the short cycling situation.

The controller has the facility of programming the size of compressor & then it will select the most suitable compressor dependant on the load.

Now your suggestion is starting to make sense from the load point of view.
This however does not!

Quote:

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The condensing unit is over 6m above the evaporating units & 10 – 15m horizontal distance. The system in on R404A with a suction pressure of 0.8 bar (~-51ºC). So liquid carry over seems unlikely.

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Grizzly

Quote:

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Originally Posted by Grizzly

Now your suggestion is starting to make sense from the load point of view.
This however does not!

Grizzly

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Have I missed something - can you explain

I am assuming yours are std DX evaps,
which are themselves above the comp.
Which makes it easier for the charge to migrate to the coldest point and flow back to the comps on start up.
Assuming (again) the liquid line solenoid is passing on one of the evaps.
The suction line would become a wet return.
Unless you have slop pots or similar.
In hind sight I am assuming rather a lot.
So put another way, what's between the evap and comp suction.
Grizzly

why not run just the one or two compressors and unload as needed.have found that faulty compressor has been nearest to the system suction line of which could be giving you grief.

Just to clarify a few points. - I have found this forum very useful in the past and would very much appreciate benefiting from the very considerable combined experience on this forum.

To answer some earlier questions

The condensing unit is over 6m ABOVE the dx evaporators.

I understand that liquid return can occur for many reasons under FAULT conditions & they are being investigated – we don’t want to replace/repair one compressor & for it to fail again especially as it is a crane job to remove it.

The current system controller WILL run only one or two compressors & unload them as required, they are just oversized for the current load

The original question was for experienced input on modifying a system designed for equal sized compressors to a system with unequal compressors i.e. replacing the faulty compressor with one more suited to the current load.

I would very much appreciate answers on this subject

Thank you all for an excellent informative & well-mannered forum

Mark

Any comments on my question

Mark

Hi,

I have been going through this thread and felt compeled to add to the discussion. Here is a philosophy that I have seen working without fail. I do not think it is wise to change the size of one of the compressors. Here is what you can do. Repair the brocken one and program your controller to run one compressor or the number of compressors as per your load requirements. Follow Grizzly's theory of rotating your compressors on running hours. On the pther hand I think it would be much economical to invest in repairing the brocken compressor than buying a new one. From an ernergy saving aspect Grizzly's proporsal makes sense as well. Good luck.