Parellel Condensing Units

Dear All

I have 2 units of 4 hp hermetic condensing units (CU) that I want to use for a supermarket display case application ( 3 cabinets of different sizes with common liquid and suction line). The ideal way would be to have 1 bigger CU or a common rack. In this case , it was a matter of having available cheaper CU's of that capacity.

The plan is to have a common suction header and a common liquid header, oil return from Oil Separator to individual suction lines ( after common suction header) , oil and gas equalization between crankcases without capacity control.

Questions :Is there an issues with having 2 condensors and 2 receivers on a common system like this? How can I effect capacity control and pumpdown? Apart from the check valves in the discharge line, do I also need check valves in the individual suction to the compressors ?

Any other foreseeable issues ?

Hi Bersaga,
I feel that two condensers and two liquid receivers would be more trouble than they are worth. I would at least remove the liquid receiver and fit one larger receiver designed to hold the conplete system charge. You could either replace the condensers or trap the liquid condensate from them before entering the main condensate line to the receiver. Remember if runing two condensers to cycle the fans in groups, ie the fan next the header in one condenser cycled with the same fan on the other condenser and so on.
Regards. Andy:)

Thanks Marc , points all taken.

Andy, Having one common condensor is what ASHRAE also recommends. Your cycling the fans in that particular order was valuable - didn't think about that. Thanks.

Is there any advantage to a suction-stop solenoid valve in such applications (as opposed to a liquid line solenoid valve) for pump-down?

If you were serving freezer merchandisers, would one NOT want to use suction-stop, due to the heating of the evaporators during defrost?

Suction Stop Solenoid Valve - that's interesting. Never thought about it. Is this application very common for freezers ?

Been always using Liquid Line SV's for all pumpdown applications.

Dear All

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I have 2 units of 4 hp hermetic condensing units (CU) that I want to use for a supermarket display case application ( 3 cabinets of different sizes with common liquid and suction line). The ideal way would be to have 1 bigger CU or a common rack. In this case , it was a matter of having available cheaper CU's of that capacity.

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I wonder if the refrigerators are accomplishing similar duty or not. I will assume that you have worked these together and found common suctions and defrost needs, or otherwise you would not say that it would be a better application to use a single larger unit.

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The plan is to have a common suction header and a common liquid header, oil return from Oil Separator to individual suction lines ( after common suction header) , oil and gas equalization between crankcases without capacity control.

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Oil return will be a problem for you. Make sure the oil level is at its required ideal level somewhere in the middle of the oil equalization piping and that the piping is perfectly level. The oil levels will do strange things. This is why positive oil management systems exist. Manufacturers have all but abandoned oil equalization piping for this reason.

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Questions :Is there an issues with having 2 condensors and 2 receivers on a common system like this?

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Andy and Marc have addressed this, and I agree there are issues.

My thoughts are that you will have more problems with two receivers than you will with the two condensers. You are stuck with the condensers, but perhaps not so stuck with the two receivers, depending upon pumpdown capacity required, or the importance of pumpdown capacity in your application.

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How can I effect capacity control and pumpdown? Apart from the check valves in the discharge line, do I also need check valves in the individual suction to the compressors ?

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If your cases are actually different from each other, for example, a single deck meat case, a dairy case and a service deli case, you can install evaporator pressure regulators as primary temperature control. They will provide the advantage of averaging and smoothing out the capacity demand on the compressors, as compared to strict liquid line and thermostat control, which may repeatedly upset the compressor operation and lead to short cycling, depending on the load mix.

I don't see check valves in the suction line as helping the problems you will be facing.

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Any other foreseeable issues ?

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Hahaha. In a word, yes. You haven't asked about defrost frequency and durations so I have to think it is not an issue.

Either you or your customer will arrive at the conclusion within a year or two that this was not a good economical idea. Especially if your customer thinks he can use somebody else to service his equipment. The largest issue is:

You have taken on the role as Original Equipment Manufacturer (OEM) and there will be reluctant warranty assistance from the providers of your equipment installation.

But if you and your customer tough it out together... you will rise above people such as I. I think it is worth trying.:)

Speaking of other issues, and furthermore speaking from experience... Are the three cabinets to act independantly? If you have all cases calling, the comrpessor should be rather happy with the mass flow and return temperature. However you need to consider what the load is of the samllest case.

In an application that I corrected (a fresh meat case, a deli case and a fish case), when the (fish case) was the only load calling, my suction dropped to below the minimum evaporating temperature rating of the newly applied CU, and my suction temperature rose to 80F. I had to apply a hot gas by-pass valve (to increase suction pressure) and a desuperheating expansion valve (to control return gas temperature).

In reference to even specific threads on this forumn, the application of the desuperheating expansion valve would be best applied with a CU that is equipped with an accumulator.

:)

How is a desuperheating TEV sized?

If you have to go to the trouble of making up headers, why dont you go all the way and make a rack, both comps on one frame, common receiver and separator, you could probably get away with the two existing condensers in parallel. Then penny pinching customer only has to spring for a new frame, receiver and separator.

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Originally posted by angryk
How is a desuperheating TEV sized?

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This is likely best answered by the Prof :)