Chapter 7, Problem 7.1P

3.1 For discussion: (a) Is it possible for exergy to be negative? Discuss. (b) Consider an evacuated space with volume V as the system. Eval- uate its exergy and discuss. PH associated with (c) Is it possible for the specific physical exergy e' a stream of matter to be negative? Discuss.

Determine the specific exergy of saturated water vapor at 137 °C, where To = 313K, Po = 101.3kPa. Assume the velocity and elevation is zero with reference to the environment. You must use following tables to solve this problem. (answer to 2 decimal) Saturated water temperature table Sat Liq. Temp., Sat Liq. Sat Liq. Sat Liq. vf uf hf sf °C m3/kg kJ/kg kJ/kg kJ/kg.K 30 0.001004 125.73 125.74 0.4368 35 0.001006 146.63 146.64 0.5051 40 0.001008 167.53 167.53 0.5724 45 0.00101 188.43 188.44 0.6386 Saturated water temperature table Temp., Sat. Vap. Sat. Vap. Sat. Vap. Sat. Vap. hg kJ/kg vg ug sg °C m3/kg kJ/kg kJ/kg.K 125 0.7508 2534.5 2713.5 7.0745 126 0.7358 2535.5 2714.8 7.0649 127 0.7208 2536.5 2716.1 7.0553 128 0.7058 2537.5 2717.4 7.0457 129 0.6908 2538.5 2718.7 7.0361 130 0.6758 2539.5 2720.0 7.0265 131 0.6608 2540.5 2721.4 7.0169 132 0.6458 2541.4 2722.7 7.0073 133 0.6308 2542.4 2724.0 6.9977 134 0.6158 2543.4 2725.3 6.9881 6.9785 135 0.6008 2544.4 2726.6 136 0.5858 2545.4 2727.9…

Steady-state operating data are shown in the figure below for an open feedwater heater. Heat transfer from the feedwater heater to its surroundings occurs at an average outer surface temperature of 50°C at a rate of 100 kW. Ignore the effects of motion and gravity and let To = 25°C, po = 1 bar. Determine (a) the ratio of the incoming mass flow rates, m/ṁ2. (b) the rate of exergy destruction, in kW. P2 = 1 bar Tz = 400°C 1 ṁy = 0.7 kg/s Pi = 1 bar T, = 40°C Feedwater heater X3 = 25% P3 = 1 bar Tp = 50°C %3D 2)

7.58 Figure PZ.58 shows a gas turbine power plant using air as the working fluid. The accompanying table gives steady-state operating data. Air can be modeled as an ideal gas. Stray heat transfer and the effects of motion and gravity can be ignored Let To 290 K, po = 100 kPa. Determine, each in kJ per kg of air flowing, (a) the net power developed, (b) the net exergy increase of the air passing through the heat exchanger, (eg- e), and (c) a full exergy accounting based on the exergy supplied to the plant found in part (b). Comment. State p(kPa) T(K) h(kJ/kg) s° (kJ/kg K) 1100 290 290.16 1.6680 500 505 508.17 2 2.2297 3 500 875 904.99 2.8170 4 100 635 643.93 2.4688 a o is the variable appearing in Eq. 6.20a and Table A-22. Heat exchanger Compressor Turbine FIGURE P7.58

EXERGY TRANSFER BY HEAT, WORK, AND MASS

7.29 A gearbox operating at steady state receives 4 hp along the input shaft and delivers 3 hp along the output shaft. The outer surface of the gearbox is at 130°F. For the gearbox, (a) determine, in Btu/s, the rate of heat transfer and (b) perform a full exergy accounting, in Btu/s, of the input power. Let To 70°F.

At steady state, an electric pump motor develops power along its output shaft of 0.7 hp whiledrawing 6 amps at 100 V. The outer surface of the motor is at 150°F. Let T = 40°F.Determine:(b) the exergy flow with input power, exergy flow with output power, magnitude of exergy flowwith heat transfer leaving the motor, and exergy destruction, all in Btu/h.

7.66 Referring to the discussion of Sec. Z.6.2 as required, evaluate the exergetic efficiency for each of the following cases, assuming steady-state operation with negligible effects of heat transfer with the surroundings: a. Turbine: Wer 1200 hp, e 250 Btu//lb, eg = 15 Btu/lb, m 240 lb/min. b. Compressor: Wev/m=-105 kJ /kg, e = 5 kJ/kg, eg = 90 kJ/kg, m 2 kg /s. c. Counterflow heat exchanger: mh = 3 kg/s, me 10 kg /s, ef = 2100 kJ/kg, e = 300 kJ/kg, É = 3.4 MW 10 lb /s, m3 15 b /s, en = 1000 Btu/Ib, eg = 50 Btu/Ib, eg = 400 Btu/lb d. Direct contact heat exchanger: m1

At steady state, an electric pump motor develops power along its output shaft of 0.7 hp whiledrawing 6 amps at 100 V. The outer surface of the motor is at 150°F. Let T = 40°F.Determine: (a) the magnitude of the rate of heat transfer leaving the motor, in Btu/h.(b) the exergy flow with input power, exergy flow with output power, magnitude of exergy flowwith heat transfer leaving the motor, and exergy destruction, all in Btu/h.