Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 10 | 595

water leaves the heater at the condensation temperature of the
extracted steam and that the extracted steam leaves the heater
as a saturated liquid and is pumped to the line carrying the
feedwater.

10–52 A steam power plant operates on the reheat-

regenerative Rankine cycle with a closed feedwater heater.

Steam enters the turbine at 12.5 MPa and 550°C at a rate of

24 kg/s and is condensed in the condenser at a pressure of

20 kPa. Steam is reheated at 5 MPa to 550°C. Some steam is

extracted from the low-pressure turbine at 1.0 MPa, is com-

pletely condensed in the closed feedwater heater, and pumped

to 12.5 MPa before it mixes with the feedwater at the same

pressure. Assuming an isentropic efficiency of 88 percent for

both the turbine and the pump, determine (a) the temperature

of the steam at the inlet of the closed feedwater heater,

(b) the mass flow rate of the steam extracted from the turbine

for the closed feedwater heater, (c) the net power output, and

(d) the thermal efficiency. Answers:(a) 328°C, (b) 4.29 kg/s,

(c) 28.6 MW, (d) 39.3 percent

10–51E A steam power plant operates on an ideal
reheat–regenerative Rankine cycle with one reheater and two
open feedwater heaters. Steam enters the high-pressure tur-
bine at 1500 psia and 1100°F and leaves the low-pressure tur-
bine at 1 psia. Steam is extracted from the turbine at 250 and
40 psia, and it is reheated to 1000°F at a pressure of 140 psia.
Water leaves both feedwater heaters as a saturated liquid.
Heat is transferred to the steam in the boiler at a rate of 4 
105 Btu/s. Show the cycle on a T-sdiagram with respect to
saturation lines, and determine (a) the mass flow rate of
steam through the boiler, (b) the net power output of the
plant, and (c) the thermal efficiency of the cycle.

Condenser

1 – y

Boiler

Closed

FWH

P II

4 9

5

10

2

7

1

Mixing y 8

chamber

P I

High-P

turbine

Low-P

turbine

3

6

FIGURE P10–50

Condenser

1 – y

Boiler

Open

FWH

II

4

7

3

2

z

y

12

8

P III

1 – y – z

P II P I

9

y

z

10

5 1

6

Reheater

Low-P

turbine

Open

FWH

I

High-P

turbine

11

FIGURE P10–51E

Boiler

Mixing

chamber

Closed

FWH

5

6

7

8

1

Low-P

turbine

High-P

turbine

9

1 – y

y

Condenser

P II P I

4

(^102)
(^113)
FIGURE P10–52
Second-Law Analysis of Vapor Power Cycles
10–53C How can the second-law efficiency of a simple
ideal Rankine cycle be improved?
10–54 Determine the exergy destruction associated with
each of the processes of the Rankine cycle described in Prob.
10–15, assuming a source temperature of 1500 K and a sink
temperature of 290 K.
10–55 Determine the exergy destruction associated with
each of the processes of the Rankine cycle described in Prob.
10–16, assuming a source temperature of 1500 K and a sink
temperature of 290 K. Answers:0, 1112 kJ/kg, 0, 172.3 kJ/kg
10–56 Determine the exergy destruction associated with the
heat rejection process in Prob. 10–22. Assume a source tem-
perature of 1500 K and a sink temperature of 290 K. Also,
determine the exergy of the steam at the boiler exit. Take P 0

100 kPa.
10–57 Determine the exergy destruction associated with
each of the processes of the reheat Rankine cycle described
in Prob. 10–32. Assume a source temperature of 1800 K and
a sink temperature of 300 K.