Question 6.CSGP.88: A superheater brings 2.5 kg/s saturated water vapor at 2 MPa......
A superheater brings 2.5 kg/s saturated water vapor at 2 MPa to 450°C. The energy is provided by hot air at 1200 K flowing outside the steam tube in the opposite direction as the water, which is a counter flowing heat exchanger. Find the smallest possible mass flow rate of the air so the air exit temperature is 20°C larger than the incoming water temperature (so it can heat it).
C.V. Superheater. Steady state with no external \dot{ Q } \text { or any } \dot{ W } the two flows exchanges energy inside the box. Neglect kinetic and potential energies at all states.
Energy Eq.6.10: \dot{ m }_{ H _2 O } h _3+\dot{ m }_{\text {air }} h _1=\dot{ m }_{ H _2 O } h _4+\dot{ m }_{\text {air }} h _2
Process: Constant pressure in each line.
State 1: Table B.1.2 T _3=212.42^{\circ} C , h _3=2799.51 \,kJ / kg
State 2: Table B.1.3 h _4=3357.48 \,kJ / kg
State 3: Table A.7 h _1=1277.81 \,kJ / kg
State 4: T _2= T _3+20=232.42^{\circ} C =505.57 \,K
A.7 : h _2=503.36+\frac{5.57}{20}(523.98-503.36)=509.1 \,kJ / kg
From the energy equation we get
