FUND OF ENG THERMODYN(LLF)+WILEYPLUS
9th Edition
ISBN: 9781119391777
Author: MORAN
Publisher: WILEY
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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
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
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