Question 6.CSGP.85: A heat exchanger, shown in Fig. P6.85, is used to cool an ai......
A heat exchanger, shown in Fig. P6.85, is used to cool an air flow from 800 K to 360 K, both states at 1 MPa. The coolant is a water flow at 15°C, 0.1 MPa. If the water leaves as saturated vapor, find the ratio of the flow rates \dot{ m }_{ H _2 O } / \dot{ m }_{ air }
C.V. Heat exchanger, steady flow 1 inlet and 1 exit for air and water each. The two flows exchange energy with no heat transfer to/from the outside.
Continuity Eqs.: Each line has a constant flow rate through it.
Energy Eq.6.10: \dot{ m }_{ air } h _1+\dot{ m }_{ H _2 O } h _3=\dot{ m }_{ air } h _2+\dot{ m }_{ H _2 O } h _4
Process: Each line has a constant pressure.
Air states, Table A.7.1: h _1=822.20 \,kJ / kg , \quad h _2=360.86 \,kJ / kg
Water states, Table B.1.1: h _3=62.98\, kJ / kg \left(\text { at } 15^{\circ} C \right)\text {, }
Table B.1.2: \left. h _4=2675.5 \,kJ / kg \text { (at } 100 \,kPa \right)
\dot{ m }_{ H _2 O } /\dot{ m }_{\text {air }}=\frac{ h _1- h _2}{ h _4- h _3}=\frac{822.20-360.86}{2675.5-62.99}= 0 . 1 7 6 6
