Liquid water enters a heat exchanger operating at steady state at T1 5 60°F, p1 5 1 atm and exits at T2 5 160°F with a negligible change in pressure. In a separate stream, steam enters at T3 5 20 lbf/in.2, x3 5 92% and exits at T4 5 140°F, p4 5 18 lbf/in.2 Stray heat transfer and the effects of motion and gravity are negligible. Let T0 5 60°F, p0 5 1 atm. Determine (a) the ratio of the mass flow rates of the two streams and (b) the rate of exergy destruction, in Btu per lb of steam entering the heat exchanger.