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Thread: Tandem Circuits

  1. #1
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    Tandem Circuits

    Hi , i need some view on pros and cons of two separate refrigerant circuits in a chiller.

    Redundancy is one point but need more clarification

  2. #2
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    There are too many variables that can come into play here without knowing more details on the scenario of your project such as tonnage, electrical details, comfort cooling vs process cooling, etc... However, here are some opinions. Pro's- If one circuit should happen to fail, you still have another circuit that may be able to handle some of the load. Depending on what machines you are talking about, if you were to only need to run circuit you may be looking at lower energy consumption versus running a larger circuit on lighter load days.
    Cons- More parts that can fail. Depending on tonnage, design may be better with a single circuit larger machine. We could probably go on and on but we need more information to give a better answer.

  3. #3
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    don't forget that you will only have 1 flow switch and usually only 1 control board, 1 control transformer, 1 entering chilled water temperature sensor and 1 leaving chilled water temperature sensor....1 failure of any of these and your redundant chiller is completely down.
    When I am late for work, I usually make up for it by leaving early.

  4. #4
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    Thanks guys . here are furthur details .

    it is 200 # 2 water cooled screw with single point 415/3 supply. Water quality is not an issue as it is industrial application.

    Trane has option of two circuits. They offer redundancy . But does it mean that at say 50 TR load one circuit can be shut off and only one will work ? Is such selective compressor working possible?

    Also the point of only one transducer , flow switch , etc is something tht i never thought of . Pls explain furthur.

    If i go with VSD for both starter and slide valve, then using two circuits actually looks like a risk ( more chances of breakdown ) , right?


    Detailed clarification will be a lotta help.

  5. #5
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    Water quality is not an issue? I hope you mean something like this is a chiller to be added to an existing system which already has water chemistry control. Especially if this industrial application has an open sump chilled water system. If water is used, then water quality is always something to be considered.

  6. #6
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    Now I think we are in a different situation. At 50 ton and 2.4 gom's you have 120 gpm's. Now with a 200 ton chiller you need 480 gpm's . You are probably would be short cycling compressors if you can get your flow switch to make. True redundancy is another chiller.
    UA 100

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  7. #7
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    Quote Originally Posted by thegoodlistener View Post
    ...True redundancy is another chiller.
    AMEN!
    When I am late for work, I usually make up for it by leaving early.

  8. #8
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    Quote Originally Posted by Nuclrchiller View Post
    Water quality is not an issue? I hope you mean something like this is a chiller to be added to an existing system which already has water chemistry control. Especially if this industrial application has an open sump chilled water system. If water is used, then water quality is always something to be considered.
    i mean , there will be a water treatment setup

  9. #9
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    Quote Originally Posted by thegoodlistener View Post
    Now I think we are in a different situation. At 50 ton and 2.4 gom's you have 120 gpm's. Now with a 200 ton chiller you need 480 gpm's . You are probably would be short cycling compressors if you can get your flow switch to make. True redundancy is another chiller.


    did not get you .. the short cycling part ..

    You say true redundancy is another chiller , but trane is convincingly pushing for their dual circuit saying that it has better part load efficiency.

  10. #10
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    Quote Originally Posted by jcistc View Post
    did not get you .. the short cycling part ..

    You say true redundancy is another chiller , but trane is convincingly pushing for their dual circuit saying that it has better part load efficiency.
    What is your true load? A chiller running at part load will die a lot quicker than one running more loaded. Does Trane offer VSD's on their compressors? Chillers should not be oversized by 15% I think if I remember correctly. Sales pitches usually lean toward going against the end users specs.
    UA 100

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  11. #11
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    if you have a 200 ton-rated load, then you could have 2, 200 TR compressors on 1 chiller barrel. this is 2 different refrigerant circuits. this means that you have a 400 ton chiller with redundant refrigerant circuits. either circuit could carry the load (minus the obvious problems of a single flow switch, single controller, single leaving water temperature sensor, etc) but you could only go down to 40 tons reliably (20% of any 1 compressor)

    or

    you could get 2 chillers, each rated at 200 tons per chiller which would be a total of 4, 100 TR compressors on 2 different chiller barrels. you would still have the tonnage redundancy but you would then have 2 flow switches, 2 controllers, etc. you would also be able to cycle down to 20 tons of load (20% of any single compressor) and reliably maintain that load with out too much trouble.

    all of the literature will state that they can unload down to some goofy-ass number like 5-10% but i said RELIABLY. when you get to very low loads, the controls are not responsive enough to maintain very reliably. you can also have issues of not enough oil return, problems with head pressure control, etc. with 2 smaller chillers, you can also control your water flows more precisely. the salesman will tell you that it can maintain these numbers (see, it is in the literature) however, the salesman does not have to work on the chillers. if the chillers don't do what the liturature says, then it is not their problem...you have to call the service department and make them 'fix' something that is not broken.
    When I am late for work, I usually make up for it by leaving early.

  12. #12
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    Trane has no VSD , but there are two venders offering the same. My least load was calculated as 95-100 TR.

    So again the same problem . 200 * 2 dual circuit or no ?

    trane dual circuit offering no VSD , york offering 200 * 2 single circuit VSD for starter and slide valve and a local vender offering 200 * 2 single circuit and VSD only for starter .

    Now, for base load of 100 Tr - trane looks best , but VSD option is not bad . So is going for single circuit that bad as trane says ? How would you rate VSD vs dual ref circuits ?

  13. #13
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    Quote Originally Posted by jcistc View Post
    Trane has no VSD , but there are two venders offering the same. My least load was calculated as 95-100 TR.

    So again the same problem . 200 * 2 dual circuit or no ?

    trane dual circuit offering no VSD , york offering 200 * 2 single circuit VSD for starter and slide valve and a local vender offering 200 * 2 single circuit and VSD only for starter .

    Now, for base load of 100 Tr - trane looks best , but VSD option is not bad . So is going for single circuit that bad as trane says ? How would you rate VSD vs dual ref circuits ?
    York I believe is only a VSD'S no slide valves. If you are running that VSD near full load then there is not much energy saving like running across the line start. Between York and Trane then Trane it is. Another thing about the York at least on the air cooled version it is one drive running multiple compressors. So I know you are looking in redundancy aspect.
    UA 100

    It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.

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