A counterflow heat exchanger operating at steady state has oil and liquid water flowing in separate streams. The oil is cooled from 700 to 580°R, while the water temperature increases from 530 to 560°R. Neither stream experiences a significant pressure change. The mass flow rate of the water is 3 lb/s. The oil and water can be regarded as incompressible with constant specific heats of 0.51 and 1.00 Btu/lb ? °R, respectively. Heat transfer between the heat exchanger and its surroundings can be ignored, as can the effects of motion and gravity.
Determine
(a) the mass flow rate of the oil, in lb/s.
(b) the exergetic efficiency given by Eq. 7.27.
(c) the hourly cost of exergy destruction if exergy is valued at 8.5 cents per kW ? h.
Let T0 5 50°F, p0 5 1 atm.