Chapter 7, Problem 7.35P

If heating from saturated liquid to saturated vapor would occur at 100°C (373.15 K), evaluate the exergy transfers accompanying heat transfer and work, each in kJ/kg. Ans. 484, 0.

7.36 At steady state, hot gaseous products of combustion from a gas turbine cool from 3000°F to 250°F as they flow through a pipe. Owing to negligible fluid friction, the flow occurs at nearly constant pressure. Applying the ideal gas model with ₂ = 0.3 Btu/lb/ºR, determine the exergy transfer accompanying heat transfer from the gas, in Btu per lb of gas flowing. Let T. = 80°F and ignore the effects of motion and gravity. -568.43

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

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

Determine the exergy in kJ at 1 kg, 0.7 bar and 90 °C. a) In case of Water (H2O) as fluid type, b) Refrigerant R-134a, c) With air condition as ideal gas for constant cp value For T0=20 °C, P0=1 bar, evaluate with reference to the exergy reference medium, in any case, assuming the mass at rest.

At a pressure of 1 bar, a temperature of 17 °C and a mass flow of 0.3 kg/s, air enters a stable insulated compressor and exits at 3 bar, 147 °C. Determine the power required by the compressor and the exergy destruction in kW. Express the exergy disappearance as a percentage according to the power required by the compressor. Changes in kinetic and potential energy will be neglected. dead state; T0=17 °C, P0=1 bar

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)

Determin the exergy, in kJ, of the contents of a 1.5 m3 storage tank, if the tank is filled with: a) air as an ideal gas at 440°C and 0.70 bar b) water vapor at 440°C and 0.70 bar Ignore the effects of motion and gravity and let To = 22°C and Po=1 bar.

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.