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Homework Help: Mass flow rate of cooling water in condenser needed

  1. Sep 8, 2008 #1
    1. The problem statement, all variables and given/known data
    Adiabatic Steam turbine produces 4000 kW
    Steam is fed at 21 bar/475 deg C
    Exhaust is saturated steam (P=0.1bar)
    Exhaust is fed into condenser where it is cooled to 30 deg C

    Problem asks for flow rate of cooling water supplied to condensener if water enters at 15 deg C and is heated to 25 deg C


    2. Relevant equations
    Turbine:
    Assume Q = 0
    H2 - H1 = -W
    m dot steam * (H2 - H1) = - Power (or W dot)
    Condenser:
    Power (in) = Power(out)
    m dot steam in (H steam in) + m dot water in (H water in) = m dot steam out (H steam out) + m dot water out (H water out)
    m2U2 - m1U1 - H'(m2 - m1) = 0

    3. The attempt at a solution

    i fond the mass flow rate of steam going into turbine using m dot * delta H = -Power
    - 4000 kW (1 kJ/s / 1 kW) = m dot steam (2584.8 kj/kg - 3411.3 kJ/kg)
    mass flow of steam = 4.84 kg/s

    then when i try to use the last equation in the above section I get stuck because I can't
    figure out U2 or the outlet states. H' = 2779.7 kj/kg, U1 = 2438.0 kJ/kg, m1 = 25000 kg, m2 =?

    please help!
     
  2. jcsd
  3. Sep 15, 2008 #2
    In order to solve this problem you have to assume a pressure for the cooling water in the condenser. I recommend 1 atm.

    I don't really understand where you are having a problem. You found the states of the steam entering the condenser and you know the states of the cooling water coming in and out of the condenser. It is a simple energy balance;

    dQsteam/dt = dQwater/dt

    dms/dt * (hi - ho)s = dmw/dt * (ho - hi)w
     
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