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  1. #1

    Sizing the Supply and return tanks for chilled water system

    Well i am currently working as a project engineer for utilities at my factory .This is my first installation of chilled water system.So would be requiring some help in sizing the Chilled water supply and return tanks.I am attaching the picture along so that the system is easily understandable.

    Water in Chilled water tank is at temperature of 5-6 C
    Temperature of Water in Return Tank is around 18 C in Summer and around 14-15 C in winter

    Chilled water generated is at 1228 m3/h i.e. 307m3/h by each chiller.

    System requirement of chilled water is 856m3/h

    I designed the Forward tank(140m3/h) with 10 minutes of buffer time that is the case if chillers go on shutdown due to power failure we would have 10 minutes to get system back online.

    I calculated that all the time 131 m3 of water is inside the lines.

    Now my concern is the Return Tank .How do i size it and on what basis.SHoudl i jsut keep it 140m3/h or need to size it and on what basis .

  2. #2
    Sorry for this post i tried attaching the picture in main thread but could not so am posting it again .

  3. #3
    Join Date
    Jun 2006
    New Jersey
    I'm no engineer, but I would think minimum operating capacity of the chiller plant and minimum applied load is going to dictate system(tank capacity)

    Also what kinda of chillers you got there? I would think staging would also need to be factored in.

  4. #4
    Join Date
    Jul 2000
    I think you are over thinking this project.
    Chillers for comfort cooling typically do not use buffer tanks.
    If this is primarly for process application and you feel the need for buffer, use one large tank, chill the tank and pump your processes from the tank.

    Supply and return pumps and tanks are double the expense and will be an ongoing headache to coordinate.

  5. #5
    yes i will discuss this one tank senario with my project manager but is there anyway we cancalculate the retrn tank because i jave seen tjree factories with same setup already asentioned in picture so justifocation that why they ise forward and return tank i can provide by end of today. but still i would like to size the return tank . help would be highly appreciated.

  6. #6
    sorry for the typos
    asentioned=as mentiomed

  7. #7
    Join Date
    Nov 2011
    This is cool... from what your diagram shows me is that you have a primary chilled loop going through your chillers and secondary feeding your storage tank and then pumped to your loads AHUs etc. From your load you have return chilled water going to a storage tank back into your chiller. thats the first i have seen something like that. Does your system have a cooling tower? From what i can see your return water temp is going to pick up the load so what i would try and calaculate is the BTUs needed to remove the the temp in your return tank. what i would advise you as well is to run a bypass from the chillers to return tank. I would also like to know the heat load calaculation and formula ???

  8. #8
    Join Date
    Dec 2008
    Dixiana, AL
    Buffer tanks are for nothing more than to give the overall system sufficient fluid volume to keep the chillers from short cycling. There are different opinions on amounts needed, but generally they fall in the 5 to 11 gallons per ton range. Most buffer tanks are installed on the pump return, but don't necessarily have to be there.

    I agree with the statement that you have a very complicated system here and some rethinking may be in order. At present, you have a booster pump system (straight primary loop with supply and return pumps). Why not install the system as a primary/secondary? You'll use the same number of pumps that you have now and if the system volumes calculate out to be small enough that you need a buffer tank, use the tank as your decoupler? You'll have far fewer problems with flow balancing, and I think you'll be much happier with the system performance.

    Also, your tank temperatures are showing the same at 18* C. What's up with that? And what's that manifolded line coming back into the leaving side of the chillers? It just goes over to the left of the chillers and stops.

  9. #9
    Join Date
    Dec 2010
    Central US
    Agree with KLOVE and hope you have a generator in the design to power pumps and AHU's on power fail or the chilled water storage investment really isnt gaining you anything for 10 minutes of down time. It will take that long for chiller to recover depending on chiller type.

  10. #10
    Thanx for the advice rocket60 i would add the bypass into my system secondly my chillers are made by GEA and use evaporative condensers to cool the oil in compressors as well as the refrigerant that is ammonia.And regarding "heat load calculation and formula" Well i will copy paste them for you today after am back form office .

  11. #11
    rocket 60 : Thanx for the advice rocket60 i would add the bypass into my system secondly my chillers are made by GEA and use evaporative condensers to cool the oil in compressors as well as the refrigerant that is ammonia.And regarding "heat load calculation and formula" Well i will copy paste them for you today after am back form office .

    klove,emcontrols: Firstly sorry for the typo over the diagram its 18 C in Summer and around 14-15 C in winter for return tank and 4-5 *C for forward tank.Secondly why i need return tanks.I work in food industry and here we cant even afford to have our Ahu's down for a minute so 10 minutes buffer is must for us and why the return tank i can describe it with an example.For example its Monthly maintenance day and we need to shutdown the whole system .When we restart the forward pumps the water inside the forward tank would be warm so cant feed it to the system.Therefore we need a return tank.I hope it answers why i need such complicated system . About generators we already run the whole factory over the Generators and we have standby generators in case of power failure i.e. IF the main generators FAILS we go on backup generators and backup generators cant take full load of factory therefore we only can run the pumps .I hope it clarifies the issue.

  12. #12
    Join Date
    Sep 2007
    Boise, ID
    This is interesting. A few years ago I had a similar project. The calculations were fairly complex, but I did not have a return tank. One of the things you need to pay close attention to is the thermal cline in the tank. If you introduce too much into the tank it will create turbulence and affect the supply temperature. I will see ff I can dig up some of my old calculations next week.
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  13. #13
    Join Date
    Mar 2008
    The Great State of Texas
    Tanks are used to maintain the chill water loop volume. This is important for proper temperature control. It allows the chiller time to process the inputs of the variables and load and unload the machine as needed. The tank should be on the return side of the piping before entering the chiller. It should be baffled. The total water volume of the loop that circulates through the chiller needs to be a minimum of 3 gallons per ton for air conditioning and a minimum of 6 gallons per ton for process cooling. The total volume goes up from there when the loop contains brine.

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