Chapter 7, Problem 7.14CU

Apply exergy balance to closed systems and control volumes.

Determine the change in exergy in kJ for each of the following processes in the system with 1 kg of steam at 20 bar and 240 °C initially. a) In case the system is heated to double its volume at constant pressure. b) In case of expansion by doubling the system volume isothermally. dead state; T0=20 °C, P0=1 bar

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.

1. Water and air are used as working fluids in a counter-flow heat exchanger operating at steady state. Water enters as a saturated vapor at 300 kPa with a mass flow rate of 10 kg/s and exiting as saturated liquid. Air enters in a separate stream at 0°C, 100 kPa and exits at 37°C. Pressure changes and the heat transfer between the heat exchanger and its surroundings are negligible. Determine the rate of exergy destruction in the heat exchanger.

Using image below Evaluate the exergy X1 of the initial state 1 and answer the following question: • Is the useful work in the process 1 → 2 → DS smaller, equal, or greater than exergy X1? • Discuss your result

Exergy flow associated with a fluid stream when the fluid properties are variable can be determined by.

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.

Figure PZ55 and the accompanying table provide the schematic and steady-state operating data for a flash 7.55 chamber fitted with an inlet valve that produces saturated vapor and saturated liquid streams from a single entering stream of liquid water. Stray heat transfer and the effects of motion and gravity are negligible. Determine (a) the mass flow rate, in Ib/s, for each of the streams exiting the flash chamber and (b) the total rate of exergy destruction, in Btu/s. Let To = 77°F, Po =1 atm State Condition T(°F) p(lbf/in.°) h(Btu/lb) s(Btu/lb R) liquid 300 80 269.7 1 0.4372 1.6996 30 1164.3 2 sat. vapor 3 sat. liquid 218.9 0.3682 30 2 Saturated vapor P2=30 lbf/in.2 Flash chamber Valve =100 lb/s T 300°F P=80 lbf/in.2 Saturated liquid,A+ P3=30 lbf/in.2 3 FIGURE P7.55

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