Mass flow rate of cooling water in condenser needed

[staceybiomed]

Homework Statement

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

Homework 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

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!

 

[Topher925]

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

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;

dQ_steam/dt = dQ_water/dt

dm_s/dt * (hi - ho)_s = dm_w/dt * (ho - hi)_w