7.128          Saturated water vapor at 1 bar enters a direct-contact heat exchanger operating at steady state and mixes with a stream of liquid water entering at 25°C, 1 bar. A two-phase liquid–vapor mixture exits at 1 bar. The entering streams have equal mass flow rates. Neglecting heat transfer with the surroundings and the effects of motion and gravity, determine for the heat exchanger

(a)   the rate of exergy destruction, in kJ per kg of mixture exiting.

(b)   the exergetic efficiency given by Eq. 7.29. Let T0 5 20°C, p0 5 1 bar.

 

 

 

 

T4 = 720°R

Insulation

 

 

4                                                                                    3

 

m·   air = 1 lb/s

T3 = 500°R

2

 

p4 = 40 lbf/in.2

 

·

Air

1                                                                                              2

CO2

p3 = 50 lbf/in.

mCO2  = 2 lb/s

T1 = 1200°R

p1 = 18 lbf/in.2

T2 = 1100°R

p2 = 14.7 lbf/in.2

 

Fig. P7.123

 

Combustion products

 

 

 

 

 

 

Fuel

 

 

Turbine

1

 

Power out

 

 

Air

 

 

Steam generator

 

 

 

 

 

4

 

 

 

 

 

 

Condenser

 

2

 

6              Process steam

m·      = 50,000 lb/h

5

Feedwater

 

 

Pump

 

 

 

 

Fig. P7.131

 

3

Power

in