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Calculating Vapor Pressure of Water with Desolved Solids

  1. Jul 16, 2012 #1
    Hi i'm currently writing a some software for calculating the effects of
    differant grades of water on Centrifical Pump Purformance

    i have be able to calculate all the apropiate Pressures, flows, viscositys, density, and pressure loss effects on the pump purformance

    where i'm having trouble is in calculation of NPSH(a) (net positive suction head),
    however to calculat this correctly i need to know the vapor pressure of the water that if feeding the pump
    using the following formula
    NPSHa = Fluid Surface Pressure (atompheric pressure as fluid surface)
    + Suction Head
    + Guage Pressure at Fuild Surace (almost always Zero)
    - Suction Pipe friction Losses
    - Fluid Vapour Pressure

    however its simple enuf to fin the vapour pressure of water at a given temp just using the
    Antoine equation
    (with Temp Range of 0 - 100c)
    vP = 10 ^ (A -(B/C+T))
    where
    A = 8.07131
    B = 1730.63
    C = 233.426
    T = Temp C


    however i now want to adjust the Vapour pressure to match the not so pure water

    i wish to addin the effect of the salinity to calculate the correct vapour pressure
    i can enter the salinity as EC, PPT, PPM or PSU (and convert between then accuratcy or +- 1%, with my own made up equation as i couldn't find one to do it)

    So after all that background
    First thing i need to know is that my EC to PSU, PPT calculator only works when messuring at 25c
    if i rase the temp does the EC change... if so is it enough to worry about?
    and is there a formula to help calculate this
    And second and most important is there an equation to correct the vapour pressure based on the EC, PPT, or PSU?
    or denisty of the water

    Hope the questions make sense, and thanks up front for the help
     
    Last edited: Jul 17, 2012
  2. jcsd
  3. Jul 17, 2012 #2
    Seems my question is a little above some of you, or i just asked it in a bad way

    i''ll try simplify it,

    i'm trying to find out IF/What is the relationship of vapour pressure vs desolved solids, however i don't know the exact solids that are disolved, just the total (from the EC meter), and of course if and how it can be calculated

    finding the vapour pressure of pure water is easy, however i never use pure water

    IE
    oi have water at 40c, with an EC reading as 1400 or 699 mg/l (ppm)
    and want to calc the vapour pressure of this solultion

    it doesn't have to be exact just within 10% would be sufficent
     
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