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  1. #1
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    Chilled water Expansion tank sizing


    Hello all,

    Can you please provide me with a quick formula to size chilled water expansion tank based on the chiller capacity only?

    Many thanks,



  2. #2
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    Re: Chilled water Expansion tank sizing

    Gladiator, expansion tank size depends on system water volume and temperature fluctuation.

  3. #3
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    Re: Chilled water Expansion tank sizing

    your supplier should be able to calculate the size-let them do the work,if they want the sale they will be cheap

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    Re: Chilled water Expansion tank sizing

    The required volume of closed expansion tanks can be expressed as

    Vet = k Vw [ ( v1 / v0 ) - 1 ] / [ ( pa / p0 ) - ( pa / p1 ) ]


    Vet = required expansion tank volume (liter)

    k = safety factor (approximately 2 is common)

    Vw = water volume in the system (liter)

    v0 = specific volume of water at initial (cold) temperature (m3/kg)

    v1 = specific volume of water at operating (hot) temperature (m3/kg)

    pa = atmospheric pressure (bara)

    p0 = system initial pressure - cold pressure (bara)

    p1 = system operating pressure - hot pressure (bara)



  5. #5
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    Re: Chilled water Expansion tank sizing

    Thanks all for your time.. Happy new year!
    The problem is that I cant communicate with the supplier as it is chinese company and no body talks english!
    The contractor submitted a weired calculation. I wish you can provide me with useful quick equations to check the calculations.
    Our system consists of 2 chillers working 50% each, each chiller of 1420 kw. I do not have pressure data or piping lensh. The contractor had selected 1m3 tank. can you please provide me with useful formulas for my situation?
    Thanks again

  6. #6
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    Re: Chilled water Expansion tank sizing

    1 m3 expansion tank? It seems your system volume is about 10 m3 or more. For 3 Mw cooling capacity... Perhaps.

  7. #7
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    Re: Chilled water Expansion tank sizing

    This piece is a part of the report the author had made and is intended to share theknowledge that the author had acquired during the preparation of his EngineeringReport;
    Design, Installation, Operation and Maintenance of 700 Tons HVACSystem.
    In partial fulfillment for the requirement of taking the Professional MechanicalEngineering (PME) Board examination that he had successfully defended.He wishes to dedicate his works to his two wonderful kids (
    Amina & Amira
    ) and hisentire family that serve as his inspiration for all of his aspiration and attainment.With a hope that this will be beneficial for the reader and it may give them an input for whatever task they may find it might be useful.
    Kiram M. Solaiman. PME, AVS, MInstPM









    2
    .5.6 - Expansion Tank (Pressurization of the System)
    The expansion tanks are provided in closed hydronics systems to:1- Accept changes in water volume as water density changes withtemperature to keep system pressure below equipment and piping systemcomponent pressure rating limits.2- Maintain positive gauge pressure in all parts of the system in order toprevent air from leaking into the system.3- Maintain sufficient pressures in all parts of the system in order to preventboiling, including cavitations at control valves and similar constrictions.4- Maintain required net positive suction head (NPSH
    r
    ) at the suction of pumps.
    Step #1
    Determine the initial precharge pressure;
    P
    i
    1. Find the lowest pressure point (LPP) in the system which is the highestpoint on the return line just as it drops down to the pump.2. Determine the minimum pressurization required at the LPP to maintain apositive gauge. The commonly recommended minimum pressurization is 4psig (ASHRAE Journal March 2003)3. The tank will be the smallest and least expensive if located near the LPP.4. The static pressure rise
    P
    S,
    LPP tank
    which is simply the elevationdifference between the LPP and tank that is 36.08 ft. (Actual elevation of for this design)
















    P
    i
    =
    P
    i
    +
    P
    S,
    LPP tank
    +
    P
    f,
    tank LPP

    P
    f,
    tank LPP
    = 0 when tank is downstream
    P
    i
    = 4.0 psig + 36.08 ft/2.31 ft/psig + 0= 19.62 psig
    Step #
    2

    Determine the maximum pressure;
    P
    ma
    1. The standard pressure rating of all components in the system will be 125psig or higher. Hence,
    P
    ma
    is assumed to be at this pressure.2. Assume the pressure relief will be located near the chiller and theexpansion tank.3. The static pressure difference
    P
    s, CPPPRV
    is zero since they are at thesame elevation. (CPP-Critical Pressure Point; which is the weakest link inthe system, PRV-Pressure relief Valve).4. The relief valve is downstream of the CPP and is assumed to be equal tothe pump head; that is 108.89 ft (
    P
    f, CPPPRV
    ) = 47.14 psig (108.89 Ft isthe computed pump head for this design)5. The pressure relief valve set point is:
    P
    rv
    =
    P
    ma
    +
    P
    s,CPP PRV
    -
    P
    f,CPP

    PRV
    = 125 + 0 - 47.14= 77.86 psig














    P
    max
    =
    P
    rv
    +
    P
    s,PRV tank
    -
    P
    f,CPP

    PRV

    P
    s,PRV tank
    = 0 (they are both on same elevation)
    P
    f,CPP

    PRV
    = 0 (they are both on same elevation)= 77.86The tank minimum volume: (using Table 1, pp.5 ASHRAE Journal Mar 2003)
    V
    a
     V
    e
     V
    s
    [(v
    h
    / v
    c

    ) 1]Where:
    V
    t
    = Volume of the tank
    V
    a
    = Tank acceptance volume

    V
    e
    = Increase in volume of water as it expands
    V
    c
    = specific volume of water @ 40 F generallyUsed for chiller = 0.01602
    V
    h
    = specific volume of water @ 80 F assumedMaximum temp in the tank= 0.01608
    V
    S
    = System volume = 1810.23 gpm 1743 x [(0.01608/0.01602) 1] 6.63 gallons
    V
    t


    V
    a
    / [1- (
    P
    a
    + P
    i
    )/

    (P
    a
    + P
    max
    )
     6.63 / [1 (14.7 + 19.62) / (14.7 + 77.86)] 10.40 gallonsTherefore, the expansion tank must have an acceptable volume of greater than 6.63gallons and overall volume greater than 10.40 gallons.

  8. #8
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    Re: Chilled water Expansion tank sizing

    Thanks Alex,
    Any formulas i can use for quick calculation?
    and the two chillers are working 50% each, so the total load is 1420 kw not 2840 kw, correct?
    is it bad to select oversized tank?

  9. #9
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    Re: Chilled water Expansion tank sizing

    It doesn't depend on the capacity of the chiller. It depends only on the variation of the water specific volume between the minimum and the maximum temperatures of the water.
    For example, the specific volume of the water at +5 C is 1 ltr/Kg and at +65 C the specific volume is 1,02 ltr/Kg
    As your will have the same mass in both operating points, and you want small pressure variations in AA.CC. water pipes, you will need an expansion tank of 0,02 Ltrs per each Kg of water you have in the installation.
    You can have an installation of 1.500 kW with 2.000 ltrs of water or with 8.000 ltrs. The expansion tank is completely different in both cases and the capacity is the same.

  10. #10
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    Re: Chilled water Expansion tank sizing

    NNN has gave you correct formula.

    You NEED to know some information to correctly calculate your tank.
    Here is all explained!
    Here you could calculate it on-line:
    http://www.reflex.de/index.php?id=45&L=1
    Last edited by nike123; 01-01-2012 at 05:12 PM.

  11. #11
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    Re: Chilled water Expansion tank sizing

    nike123, that's right, you have to take into consideration the pressure change which is allowed in the system but some results with the formula (when max and min pressures are close) give you a required tank too much big for the real situation.
    Of course, the "safety factor" is something we always use to correct the theoretical calculus.
    By the other side, installing an air pressure control, you can reduce to the minimum the volume of the tank.

  12. #12
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    Re: Chilled water Expansion tank sizing

    Quote Originally Posted by AMP View Post
    By the other side, installing an air pressure control, you can reduce to the minimum the volume of the tank.
    Correct, and I would also suggest pressure maintenance with external pressure generation work system instead of expansion vessel, for such size of system.
    Last edited by nike123; 01-01-2012 at 06:59 PM.

  13. #13
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    Re: Chilled water Expansion tank sizing

    Quote Originally Posted by nike123 View Post
    Correct, and I would also suggest pressure maintenance with external pressure generation work system instead of expansion vessel, for such size of system.
    Yes, I was referring to a system using compressed air to keep stable the pressure, so the same as your solution . In big systems, a safety valve is not enough.

  14. #14
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    Re: Chilled water Expansion tank sizing

    Many thanks guys..that was very useful

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