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Homework Help: Thermodynamics-psychrometry question, mixing two streams of warm moist air

  1. Jul 17, 2011 #1
    1. The problem statement, all variables and given/known data

    I'm trying to determine how to go about attacking this problem that I'm doing to

    I have 2 streams of air mixing, and I'm trying to find the temperature and relative humidity of the third stream.

    Stream 1: 2bar, 20C, 40%rh, and 1kg/s
    Stream 2: 1bar, 20C, 4kg/s

    So I know I can find all my values for the first stream, but the second stream doesn't give me enough to find it on the psychometric charts.

    2. Relevant equations

    m=mass of mixture
    ma=mass air
    mv=mass vapour
    h=enthalpy of mixture
    ha= enthalpy dry air
    hv=enthalpy of water

    Mass Balance

    Energy balance
    m1h1+m2h2=m3h3 and all variations of this with the contents of the air


    where phi is the relative humity
    Pv1 is the partial pressure of water
    Pg1 is the saturation pressure of water at the given temperature

    w=mv/ma for all values.


    where c is the constant used to determine this (name escaping me right now) (units of kj/kg.C)

    3. The attempt at a solution

    I got my values for state one, I've tried energy balance and mass balance, but I end up with more variables than equations and I've been confusing myself all day trying to do this. I can't find ANY question in over 3 books that excludes all data about state three other than the mass flow rate, and excludes the data required from the second stream of air.
  2. jcsd
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