Moran M.J., Shapiro H.N. Fundamentals of Engineering Thermodynamics

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Engineering Model:

The turbine and the pump can each be analyzed as a control volume at steady state.

2. The turbine and pump operate adiabatically and each has an isentropic efficiency of 85%.

3. Kinetic and potential energy effects are negligible.

4. Only 69% of the exergy entering the plant with the fuel remains after accounting for the stack loss and

combustion exergy destruction.

5. T

5 228C, p

5 1 atm.

Analysis: The rate of exergy destruction can be found by reducing the exergy rate balance or by use of the

relationship E

5 T

, where s

is the rate of entropy production from an entropy rate balance. With either

approach, the rate of exergy destruction for the turbine can be expressed as

5 m

2 s

From Table A-3, s

5 5.7432 kJ/kg ? K. Using h

5 1939.3 kJ/kg from the solution to Example 8.2, the value of

can be determined from Table A-3 as s

5 6.2021 kJ/kg ? K. Substituting values

5 14.449 3 10

h21295 K216.2021 2 5.743221kJ

kg ? K2

5 a0.602 3 10

1 h

3600 s

1 MW

`5 16.72 MW

From the solution to Example 8.7, the net rate at which exergy is supplied by the cooling combustion gases

is 231.28 MW. The turbine rate of exergy destruction expressed as a percentage of this is (16.72/231.28)(100%) 5

7.23%. However, since only 69% of the entering fuel exergy remains after the stack loss and combustion exergy

destruction are accounted for, it can be concluded that 0.69 3 7.23% 5 5% of the exergy entering the plant with

the fuel is destroyed within the turbine. This is the value listed in Table 8.4.

Similarly, the exergy destruction rate for the pump is

5 m

2 s

With s

from Table A-3 and s

from the solution to Example 8.7

5 14.449 3 10

h21295 K210.5957 2 0.592621kJ

kg ? K2

5 a4.07 3 10

b `

1 h

3600 s

1 MW

`5 0.11 MW

Expressing this as a percentage of the exergy entering the plant as calculated above,

we have (0.11/231.28)(69%) 5 0.03%. This value is rounded to zero in Table 8.4.

The net power output of the vapor power plant of Example 8.2 is 100 MW.

Expressing this as a percentage of the rate at which exergy is carried into the

plant with the fuel, (100/231.28)(69%) 5 30%, as shown in Table 8.4.

Ability to…

❑

perform exergy analysis of a

power plant turbine and pump.

✓

Skills Developed

What is the exergetic efficiency of the power plant? Ans. 30%.

The following example illustrates the exergy analysis of the condenser of the case

study vapor power plant.

Vapor Cycle Exergy Analysis—Condenser

c c c c EXAMPLE 8.9 c

The condenser of Example 8.2 involves two separate water streams. In one stream a two-phase liquid–vapor

mixture enters at 0.008 MPa and exits as a saturated liquid at 0.008 MPa. In the other stream, cooling water

enters at 158C and exits at 358C. (a) Determine the net rate at which exergy is carried from the condenser by

the cooling water, in MW. Express this result as a percentage of the exergy entering the plant with the fuel.

(b) Determine for the condenser the rate of exergy destruction, in MW. Express this result as a percentage of

the exergy entering the plant with the fuel. Let T

5 228C and p

5 1 atm.

8.6 Case Study: Exergy Accounting of a Vapor Power Plant 473

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474 Chapter 8 Vapor Power Systems

Saturated

liquid,

0.008 MPa

Liquid,

15°C

Two-phase

liquid-vapor mixture,

0.008 MPa

Liquid,

35°

Fig. E8.9

Engineering Model:

The control volume shown on the accompanying figure operates at steady state with

5 W

5 0.

2. Kinetic and potential energy effects can be ignored.

3. Only 69% of the fuel exergy remains after accounting for the stack loss and combustion exergy destruction.

4. T

5 228C, p

5 1 atm.

Analysis:

(a)

The net rate at which exergy is carried out of the condenser can be evaluated by using Eq. 7.18:

net rate at which exergy

is carried out by the

coolin

water

§5 m

2 e

5 m

2 h

2 T

2 s

where m

is the mass flow rate of the cooling water from the solution to Example 8.2. With saturated liquid values

for specific enthalpy and entropy from Table A-2 at the specified inlet and exit temperatures of the cooling water

2 e

9.39 3 10

231

146.68 2 62.99

kg 2

295 K

0.5053 2 0.2245

kg ? K

8.019 3 10

3600 s

1 MW

`5 2.23 MW

Expressing this as a percentage of the exergy entering the plant with the fuel, we get (2.23/231.28)(69%) 5 1%.

This is the value listed in Table 8.4.

(b) The rate of exergy destruction for the condenser can be evaluated by reducing the exergy rate balance. Alterna-

tively, the relationship E

5 T

can be employed, where s

is the time rate of entropy production for the condenser

determined from an entropy rate balance. With either approach, the rate of exergy destruction can be expressed as

5 T

2 s

1 m

2 s

Substituting values

5 295

4.449 3 10

0.5926 2 6.2021

9.39 3 10

0.5053 2 0.2245

416.1 3 10

3600 s

1 MW

`5 11.56 MW

Expressing this as a percentage of the exergy entering the plant with the fuel,

we get (11.56/231.28)(69%) 5 3%. This is the value listed in Table 8.4.

Referring to data from Example 8.2, what percent of the energy

supplied to the steam passing through the steam generator is carried out

by the cooling water? Ans. 68.6%.

Ability to…

❑

perform exergy analysis of a

power plant condenser.

✓

Skills Developed

SOLUTION

Known:

A condenser at steady state has two streams: (1) a two-phase liquid–vapor mixture entering and condensate

exiting at known states and (2) a separate cooling water stream entering and exiting at known temperatures.

Find: Determine the net rate at which exergy is carried from the condenser by the cooling water stream and the

rate of exergy destruction for the condenser. Express both quantities in MW and as percentages of the exergy

entering the plant with the fuel.

Schematic and Given Data:

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This chapter begins with an introduction to power generation

that surveys current U.S. power generation by source and looks

ahead to power generation needs in decades to come. These

discussions provide context for the study of vapor power plants

in this chapter and gas power plants in Chap. 9.

In Chap. 8 we have considered practical arrangements for

vapor power plants, illustrated how vapor power plants are mod-

eled thermodynamically, and considered the principal irreversi-

bilities and losses associated with such plants. The main compo-

nents of simple vapor power plants are modeled by the Rankine

cycle.

In this chapter, we also have introduced modifications to the

simple vapor power cycle aimed at improving overall perfor-

mance. These include superheat, reheat, regeneration, super-

critical operation, cogeneration, and binary cycles. We have also

included a case study to illustrate the application of exergy anal-

ysis to vapor power plants.

The following checklist provides a study guide for this chap-

ter. When your study of the text and end-of-chapter exercises has

been completed you should be able to

write out the meanings of the terms listed in the margin

throughout the chapter and understand each of the related

concepts. The subset of key concepts listed below is particu-

larly important.

sketch schematic diagrams and accompanying T–s diagrams

of Rankine, reheat, and regenerative vapor power cycles.

apply conservation of mass and energy, the second law, and

property data to determine power cycle performance, including

thermal efficiency, net power output, and mass flow rates.

discuss the effects on Rankine cycle performance of varying

steam generator pressure, condenser pressure, and turbine

inlet temperature.

discuss the principal sources of exergy destruction and loss

in vapor power plants.

c CHAPTER SUMMARY AND STUDY GUIDE

c KEY ENGINEERING CONCEPTS

Rankine cycle, p. 433

thermal efficiency, p. 435

back work ratio, p. 435

ideal Rankine cycle, p. 437

superheat, p. 447

reheat, p. 447

supercritical, p. 448

regeneration, p. 453

open feedwater heater, p. 453

closed feedwater heater, p. 458

organic cycle, p. 464

binary vapor cycle, p. 464

cogeneration, p. 465

district heating, p. 465

exergy accounting, p. 468

c KEY EQUATIONS

h 5

2 W

2 h

22 1h

2 h

(8.5a) p. 435 Thermal efficiency for the Rankine cycle of Fig. 8.2

bwr 5

2 h

(8.6) p. 435 Back work ratio for the Rankine cycle of Fig. 8.2

< y

2 p

(8.7b) p. 437 Approximation for the pump work of the ideal

Rankine cycle of Fig. 8.3

c EXERCISES: THINGS ENGINEERS THINK ABOUT

1. Many utility companies offer special rates for “green power.”

What does that mean?

2. Brainstorm some ways to use the cooling water exiting the

condenser of a large power plant.

3. What effects on a river’s ecology might result from a power

plant’s use of river water for condenser cooling?

4. Referring to Fig. 8.1a, what environmental impacts might

result from the two plumes shown on the figure?

5. What is a baseload power plant?

6. If Iceland completes its planned transition to using only renew-

able energy throughout its society by mid-century, what sig-

nificant changes in lifestyle will Icelanders have to tolerate?

7. What type of power plant produces the electricity used in

your residence?

8. What is the relationship between global climate change

and the hole in Earth’s ozone layer?

9. Why is it important for power plant operators to keep pipes

circulating water through plant components free from fouling?

Exercises: Things Engineers Think About 475

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476 Chapter 8

Vapor Power Systems

10. What is the difference between solar-concentrating and

solar-photovoltaic electricity generation?

11. Decades of coal mining have left piles of waste coal, or culm,

in many locations through the United States. What effects

does culm have on human health and the environment?

12. How do operators of electricity-generating plants detect

and respond to changes in consumer demand throughout

the day?

12. What is an energy orb?

 PROBLEMS: DEVELOPING ENGINEERING SKILLS

Analyzing Rankine Cycles

8.1 Water is the working fluid in an ideal Rankine cycle. The

condenser pressure is 6 kPa, and saturated vapor enters the

turbine at 10 MPa. Determine the heat transfer rates, in kJ

per kg of steam flowing, for the working fluid passing

through the boiler and condenser and calculate the thermal

efficiency.

8.2 Water is the working fluid in an ideal Rankine cycle.

Superheated vapor enters the turbine at 10 MPa, 4808C, and

the condenser pressure is 6 kPa. Determine for the cycle

(a) the rate of heat transfer to the working fluid passing

through the steam generator, in kJ per kg of steam flowing.

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam flowing.

8.3 Water is the working fluid in a Carnot vapor power cycle.

Saturated liquid enters the boiler at a pressure of 10 MPa,

and saturated vapor enters the turbine. The condenser

pressure is 6 kPa. Determine

(a) the thermal efficiency.

(b) the back work ratio.

passing through the boiler, in kJ/kg.

(d) the heat transfer from the working fluid per unit mass

passing through the condenser, in kJ/kg.

8.4 Plot each of the quantities calculated in Problem 8.2

versus condenser pressure ranging from 6 kPa to 0.1 MPa.

Discuss.

8.5 Plot each of the quantities calculated in Problem 8.2

versus steam generator pressure ranging from 4 MPa to 20

MPa. Maintain the turbine inlet temperature at 4808C.

Discuss.

8.6 A Carnot vapor power cycle operates with water as the

working fluid. Saturated liquid enters the boiler at 1800 lbf/in.

and saturated vapor enters the turbine (state 1). The condenser

pressure is 1.2 lbf/in.

The mass flow rate of steam is 1 3 10

lb/h. Data at key points in the cycle are provided in the

accompanying table. Determine

(a) the thermal efficiency.

(b) the back work ratio.

(d) the rate of heat transfer to the working fluid passing

through the boiler.

State p(lbf/in.

) h(Btu/lb)

1 1800 1150.4

2 1.2 735.7

3 1.2 472.0

4 1800 648.3

8.7 Water is the working fluid in an ideal Rankine cycle.

Saturated vapor enters the turbine at 16 MPa, and the

condenser pressure is 8 kPa. The mass flow rate of steam

entering the turbine is 120 kg/s. Determine

(a) the net power developed, in kW.

(b) the rate of heat transfer to the steam passing through

the boiler, in kW.

(d) the mass flow rate of condenser cooling water, in kg/s,

if the cooling water undergoes a temperature increase of

188C with negligible pressure change in passing through the

condenser.

8.8 Water is the working fluid in a Carnot vapor power cycle.

Saturated liquid enters the boiler at 16 MPa, and saturated

vapor enters the turbine. The condenser pressure is 8 kPa.

The mass flow rate of steam entering the turbine is 120 kg/s.

Determine

(a) the thermal efficiency.

(b) the back work ratio.

(d) the rate of heat transfer from the working fluid passing

through the condenser, in kW.

8.9 Plot each of the quantities calculated in Problem 8.7

versus turbine inlet temperature ranging from the saturation

temperature at 16 MPa to 5608C Discuss.

8.10 Water is the working fluid in an ideal Rankine cycle. Steam

enters the turbine at 1400 lbf/in.

and 10008F. The condenser

pressure is 2 lbf/in.

The net power output of the cycle is

1 3 10

Btu/h. Cooling water experiences a temperature

increase from 608F to 768F, with negligible pressure drop, as

it passes through the condenser. Determine for the cycle

(a) the mass flow rate of steam, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

(d) the mass flow rate of cooling water, in lb/h.

8.11 Plot each of the quantities calculated in Problem 8.10 versus

condenser pressure ranging from 0.3 lbf/in.

to 14.7 lbf/in.

Maintain constant net power output. Discuss.

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Problems: Developing Engineering Skills 477

Solar

collectors

Turbine

Condenser

out

= 0.3 kW/m

State

276.83

261.01

86.78

87.93

0.9952

—

h (kJ/kg) xp (bar)

Pump

Fig. P8.13

8.12 A power plant based on the Rankine cycle is under

development to provide a net power output of 10 MW. Solar

collectors are to be used to generate Refrigerant 22 vapor at

1.6 MPa, 508C, for expansion through the turbine. Cooling

water is available at 208C. Specify the preliminary design of

the cycle and estimate the thermal efficiency and the

refrigerant and cooling water flow rates, in kg/h.

8.13 Figure P8.13 provides steady-state operating data for a

solar power plant that operates on a Rankine cycle with

Refrigerant 134a as its working fluid. The turbine and pump

operate adiabatically. The rate of energy input to the

collectors from solar radiation is 0.3 kW per m

of collector

surface area, with 60% of the solar input to the collectors

absorbed by the refrigerant as it passes through the collectors.

Determine the solar collector surface area, in m

per kW of

power developed by the plant. Discuss possible operational

improvements that could reduce the required collector

surface area.

8.14 On the south coast of the island of Hawaii, lava flows

continuously into the ocean. It is proposed to anchor a

floating power plant offshore of the lava flow that uses

ammonia as the working fluid. The plant would exploit the

temperature variation between the warm water near the

surface at 1308F and seawater at 508F from a depth of 500 ft

to produce power. Figure P8.14 shows the configuration and

provides additional data. Using the properties of pure water

for the seawater and modeling the power plant as a Rankine

cycle, determine

(a) the thermal efficiency.

(b) the mass flow rate of ammonia, in lb/min, for a net power

output of 300 hp.

8.15 The cycle of Problem 8.3 is modified to include the effects

of irreversibilities in the adiabatic expansion and compression

processes. If the states at the turbine and pump inlets remain

unchanged, repeat parts (a)–(d) of Problem 8.3 for the

modified Carnot cycle with h

5 0.80 and h

5 0.70.

8.16 Steam enters the turbine of a simple vapor power plant

with a pressure of 10 MPa and temperature T, and expands

adiabatically to 6 kPa. The isentropic turbine efficiency is

85%. Saturated liquid exits the condenser at 6 kPa and the

isentropic pump efficiency is 82%.

(a) For T 5 5808C, determine the turbine exit quality and

the cycle thermal efficiency.

(b) Plot the quantities of part (a) versus T ranging from 580

to 7008C.

8.17 Water is the working fluid in a Rankine cycle. Superheated

vapor enters the turbine at 10 MPa, 4808C, and the condenser

pressure is 6 kPa. The turbine and pump have isentropic

efficiencies of 80 and 70%, respectively. Determine for the cycle

(a) the rate of heat transfer to the working fluid passing

through the steam generator, in kJ per kg of steam flowing.

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478 Chapter 8

Vapor Power Systems

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam flowing.

8.18 Steam enters the turbine of a Rankine cycle at 16 MPa,

5608C. The condenser pressure is 8 kPa. The turbine and pump

each have isentropic efficiencies of 85%, and the mass flow

rate of steam entering the turbine is 120 kg/s. Determine

(a) the net power developed, in kW.

(b) the rate of heat transfer to the steam passing through the

boiler, in kW.

Plot each of the quantities in parts (a)–(c) if the turbine and

pump isentropic efficiencies remain equal to each other but

vary from 80 to 100%.

8.19 Water is the working fluid in a Rankine cycle. Steam enters

the turbine at 1400 lbf/in.

and 10008F. The condenser pressure

is 2 lbf/in.

Both the turbine and pump have isentropic

efficiencies of 85%. The working fluid has negligible pressure

drop in passing through the steam generator. The net power

output of the cycle is 1 3 10

Btu/h. Cooling water experiences

a temperature increase from 608F to 768F, with negligible

pressure drop, as it passes through the condenser. Determine

for the cycle

(a) the mass flow rate of steam, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

(d) the mass flow rate of cooling water, in lb/h.

8.20 Water is the working fluid in a Rankine cycle. Superheated

vapor enters the turbine at 8 MPa, 5608C with a mass flow

rate of 7.8 kg/s and exits at 8 kPa. Saturated liquid enters the

pump at 8 kPa. The isentropic turbine efficiency is 88%, and

the isentropic pump efficiency is 82%. Cooling water enters

the condenser at 188C and exits at 368C with no significant

change in pressure. Determine

(a) the net power developed, in kW.

(b) the thermal efficiency.

8.21 Figure P8.21 provides steady-state operating data for a

vapor power plant using water as the working fluid. The

Turbine

Boiler

Condenser

Cold sea water

Warm sea water

Pump

50°

130°F

125°F

55°

= 60°F

= 120°F

Ammonia

sat. vapor

= p

= 85%

sat. liquid

= 0.8

Fig. P8.14

Steam generator

Turbine

Condenser

= 12 kg/s

Pump

State

6 MPa

10 kPa

7.5 MPa

7 MPa

6 MPa

500

- - -

Sat.

- - -

550

3422.2

1633.3

191.83

199.4

167.57

3545.3

h (kJ/kg)T (°C)p

out

Fig. P8.21

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Problems: Developing Engineering Skills 479

mass flow rate of water is 12 kg/s. The turbine and pump

operate adiabatically but not reversibly. Determine

(a) the thermal efficiency.

(b) the rates of heat transfer Q

and Q

out

, each in kW.

8.22 Superheated steam at 8 MPa and 4808C leaves the steam

generator of a vapor power plant. Heat transfer and frictional

effects in the line connecting the steam generator and the

turbine reduce the pressure and temperature at the turbine

inlet to 7.6 MPa and 4408C, respectively. The pressure at the exit

of the turbine is 10 kPa, and the turbine operates adiabatically.

Liquid leaves the condenser at 8 kPa, 368C. The pressure is

increased to 8.6 MPa across the pump. The turbine and pump

isentropic efficiencies are 88%. The mass flow rate of steam

is 79.53 kg/s. Determine

(a) the net power output, in kW.

(b) the thermal efficiency.

steam generator and the turbine, in kW.

(d) the mass flow rate of condenser cooling water, in kg/s, if

the cooling water enters at 158C and exits at 358C with

negligible pressure change.

8.23 Water is the working fluid in a Rankine cycle. Steam exits

the steam generator at 1500 lbf/in.

and 11008F. Due to heat

transfer and frictional effects in the line connecting the

steam generator and turbine, the pressure and temperature

at the turbine inlet are reduced to 1400 lbf/in.

and 10008F,

respectively. Both the turbine and pump have isentropic

efficiencies of 85%. Pressure at the condenser inlet is 2 lbf/in.

but due to frictional effects the condensate exits the condenser

at a pressure of 1.5 lbf/in.

and a temperature of 1108F. The

condensate is pumped to 1600 lbf/in.

before entering the steam

generator. The net power output of the cycle is 1 3 10

Btu/h.

Cooling water experiences a temperature increase from 608F

to 768F, with negligible pressure drop, as it passes through the

condenser. Determine for the cycle

(a) the mass flow rate of steam, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

(d) the mass flow rate of cooling water, in lb/h.

8.24 Steam enters the turbine of a vapor power plant at 600 lbf/

in.

, 10008F and exits as a two-phase liquid–vapor mixture at

temperature T. Condensate exits the condenser at a

temperature 58F lower than T and is pumped to 600 lbf/in.

The turbine and pump isentropic efficiencies are 90 and

80%, respectively. The net power developed is 1 MW.

(a) For T 5 808F, determine the steam quality at the turbine

exit, the steam mass flow rate, in lb/h, and the thermal

efficiency.

(b) Plot the quantities of part (a) versus T ranging from 80

to 1058F.

8.25 Superheated steam at 18 MPa, 5608C, enters the turbine

of a vapor power plant. The pressure at the exit of the

turbine is 0.06 bar, and liquid leaves the condenser at 0.045

bar, 268C. The pressure is increased to 18.2 MPa across the

pump. The turbine and pump have isentropic efficiencies of

82 and 77%, respectively. For the cycle, determine

(a) the net work per unit mass of steam flow, in kJ/kg.

(b) the heat transfer to steam passing through the boiler, in

kJ per kg of steam flowing.

(d) the heat transfer to cooling water passing through the

condenser, in kJ per kg of steam condensed.

8.26 In the preliminary design of a power plant, water is

chosen as the working fluid and it is determined that the

turbine inlet temperature may not exceed 5208C. Based on

expected cooling water temperatures, the condenser is to

operate at a pressure of 0.06 bar. Determine the steam

generator pressure required if the isentropic turbine

efficiency is 80% and the quality of steam at the turbine exit

must be at least 90%.

Considering Reheat and Supercritical Cycles

8.27 Steam at 10 MPa, 6008C enters the first-stage turbine of an

ideal Rankine cycle with reheat. The steam leaving the reheat

section of the steam generator is at 5008C, and the condenser

pressure is 6 kPa. If the quality at the exit of the second-stage

turbine is 90%, determine the cycle thermal efficiency.

8.28 Water is the working fluid in an ideal Rankine cycle with

superheat and reheat. Steam enters the first-stage turbine

at 1400 lbf/in.

and 10008F, expands to a pressure of 350 lbf/in.

and is reheated to 9008F before entering the second-stage

turbine. The condenser pressure is 2 lbf/in.

The net power

output of the cycle is 1 3 10

Btu/h. Determine for the

cycle

(a) the mass flow rate of steam, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

passing through the reheater.

(d) the thermal efficiency.

8.29 Water is the working fluid in an ideal Rankine cycle with

reheat. Superheated vapor enters the turbine at 10 MPa,

4808C, and the condenser pressure is 6 kPa. Steam expands

through the first-stage turbine to 0.7 MPa and then is

reheated to 4808C. Determine for the cycle

(a) the rate of heat addition, in kJ per kg of steam entering

the first-stage turbine.

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam entering the first-stage turbine.

8.30 For the cycle of Problem 8.29, reconsider the analysis

assuming the pump and each turbine stage has an isentropic

efficiency of 80%. Answer the same questions as in Problem

8.29 for the modified cycle.

8.31 Investigate the effects on cycle performance as the reheat

pressure and final reheat temperature take on other values.

Construct suitable plots and discuss for the cycle of

(a) Problem 8.29.

(b) Problem 8.30.

8.32 An ideal Rankine cycle with reheat uses water as the

working fluid. The conditions at the inlet to the first-stage

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480 Chapter 8

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turbine are p

5 2500 lbf/in.

, T

5 10008F. The steam is

reheated at constant pressure p between the turbine stages

to 10008F. The condenser pressure is 1 lbf/in.

(a) If p/p

5 0.2, determine the cycle thermal efficiency and

the steam quality at the exit of the second-stage turbine.

(b) Plot the quantities of part (a) versus the pressure ratio

p/p

ranging from 0.05 to 1.0.

8.33 Steam at 32 MPa, 5208C enters the first stage of a

supercritical reheat cycle including three turbine stages.

Steam exiting the first-stage turbine at pressure p is

reheated at constant pressure to 4408C, and steam exiting

the second-stage turbine at 0.5 MPa is reheated at constant

pressure to 3608C. Each turbine stage and the pump has

an isentropic efficiency of 85%. The condenser pressure is

8 kPa.

(a) For p 5 4 MPa, determine the net work per unit mass

of steam flowing, in kJ/kg, and the thermal efficiency.

(b) Plot the quantities of part (a) versus p ranging from 0.5

to 10 MPa.

8.34 Steam at 4800 lbf/in.

, 10008F enters the first stage of a

supercritical reheat cycle including two turbine stages. The

steam exiting the first-stage turbine at 600 lbf/in.

is reheated

at constant pressure to 10008F. Each turbine stage and the

pump has an isentropic efficiency of 85%. The condenser

pressure is 1 lbf/in.

If the net power output of the cycle is

100 MW, determine

(a) the rate of heat transfer to the working fluid passing

through the steam generator, in MW.

(b) the rate of heat transfer from the working fluid passing

through the condenser, in MW.

8.35 An ideal Rankine cycle with reheat uses water as the

working fluid. The conditions at the inlet to the first-stage

turbine are 14 MPa, 6008C and the steam is reheated between

the turbine stages to 6008C. For a condenser pressure of 6 kPa,

plot the cycle thermal efficiency versus reheat pressure for

pressures ranging from 2 to 12 MPa.

8.36 An ideal Rankine cycle with reheat uses water as the

working fluid. The conditions at the inlet to the first turbine

stage are 1600 lbf/in.

, 12008F and the steam is reheated

between the turbine stages to 12008F. For a condenser pressure

of 1 lbf/in.

, plot the cycle thermal efficiency versus reheat

pressure for pressures ranging from 60 to 1200 lbf/in.

Analyzing Regenerative Cycles

8.37 Water is the working fluid in an ideal regenerative

Rankine cycle. Superheated vapor enters the turbine at 10 MPa,

4808C, and the condenser pressure is 6 kPa. Steam expands

through the first-stage turbine to 0.7 MPa where some of the

steam is extracted and diverted to an open feedwater heater

operating at 0.7 MPa. The remaining steam expands through

the second-stage turbine to the condenser pressure of 6 kPa.

Saturated liquid exits the feedwater heater at 0.7 MPa.

Determine for the cycle

(a) the rate of heat addition, in kJ per kg of steam entering

the first-stage turbine.

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam entering the first-stage turbine.

8.38 For the cycle of Problem 8.37, reconsider the analysis

assuming the pump and each turbine stage has an isentropic

efficiency of 80%. Answer the same questions as in Problem

8.37 for the modified cycle.

8.39 Investigate the effects on cycle performance as the

feedwater heater pressure takes on other values. Construct

suitable plots and discuss for the cycle of

(a) Problem 8.37.

(b) Problem 8.38.

8.40 A power plant operates on a regenerative vapor power

cycle with one open feedwater heater. Steam enters the first

turbine stage at 12 MPa, 5208C and expands to 1 MPa, where

some of the steam is extracted and diverted to the open

feedwater heater operating at 1 MPa. The remaining steam

expands through the second turbine stage to the condenser

pressure of 6 kPa. Saturated liquid exits the open feedwater

heater at 1 MPa. For isentropic processes in the turbines and

pumps, determine for the cycle (a) the thermal efficiency

and (b) the mass flow rate into the first turbine stage, in

kg/h, for a net power output of 330 MW.

8.41 Reconsider the cycle of Problem 8.40 as the feedwater

heater pressure takes on other values. Plot the thermal

efficiency and the rate of exergy destruction within the

feedwater heater, in kW, versus the feedwater heater pressure

ranging from 0.5 to 10 MPa. Let T

5 293 K.

8.42 Compare the results of Problem 8.40 with those for an

ideal Rankine cycle having the same turbine inlet conditions

and condenser pressure, but no regenerator.

8.43 For the cycle of Problem 8.40, investigate the effects on

cycle performance as the feedwater heater pressure takes on

other values. Construct suitable plots and discuss. Assume

that each turbine stage and each pump has an isentropic

efficiency of 80%.

8.44 Water is the working fluid in an ideal regenerative Rankine

cycle with one open feedwater heater. Steam enters the turbine

at 1400 lbf/in.

and 10008F and expands it to 120 lbf/in.

, where

some of the steam is extracted and diverted to the open

feedwater heater operating at 120 lbf/in.

The remaining

steam expands through the second-stage turbine to the

condenser pressure of 2 lbf/in.

Saturated liquid exits the open

feedwater heater at 120 lbf/in.

The net power output of the

cycle is 1 3 10

Btu/h. Determine for the cycle

(a) the mass flow rate of steam entering the first stage of the

turbine, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

8.45 Water is the working fluid in a regenerative Rankine cycle

with one open feedwater heater. Steam enters the turbine at

1400 lbf/in.

and 10008F and expands to 120 lbf/in.

, where

some of the steam is extracted and diverted to the open

feedwater heater operating at 120 lbf/in.

The remaining steam

expands through the second-stage turbine to the condenser

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Problems: Developing Engineering Skills 481

pressure of 2 lbf/in.

Each turbine stage and both pumps have

isentropic efficiencies of 85%. Flow through the condenser,

open feedwater heater, and steam generator is at constant

pressure. Saturated liquid exits the open feedwater heater at

120 lbf/in.

The net power output of the cycle is 1 3 10

Btu/h.

Determine for the cycle

(a) the mass flow rate of steam entering the first stage of the

turbine, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

8.46 Water is the working fluid in an ideal regenerative Rankine

cycle with one open feedwater heater. Superheated vapor

enters the first-stage turbine at 16 MPa, 5608C, and the

condenser pressure is 8 kPa. The mass flow rate of steam

entering the first-stage turbine is 120 kg/s. Steam expands

through the first-stage turbine to 1 MPa where some of the

steam is extracted and diverted to an open feedwater heater

operating at 1 MPa. The remainder expands through the second-

stage turbine to the condenser pressure of 8 kPa. Saturated

liquid exits the feedwater heater at 1 MPa. Determine

(a) the net power developed, in kW.

(b) the rate of heat transfer to the steam passing through the

boiler, in kW.

(d) the mass flow rate of condenser cooling water, in kg/s, if the

cooling water undergoes a temperature increase of 188C with

negligible pressure change in passing through the condenser.

8.47 Reconsider the cycle of Problem 8.46, but include in the

analysis that each turbine stage and the pump has an isentropic

efficiency of 85%.

8.48 For the cycle of Problem 8.47, investigate the effects on

cycle performance as the feedwater heater pressure takes on

other values. Construct suitable plots and discuss.

8.49 Water is the working fluid in an ideal regenerative

Rankine cycle with one closed feedwater heater. Superheated

vapor enters the turbine at 10 MPa, 4808C, and the condenser

pressure is 6 kPa. Steam expands through the first-stage

turbine where some is extracted and diverted to a closed

feedwater heater at 0.7 MPa. Condensate drains from the

feedwater heater as saturated liquid at 0.7 MPa and is

trapped into the condenser. The feedwater leaves the heater

at 10 MPa and a temperature equal to the saturation

temperature at 0.7 MPa. Determine for the cycle

(a) the rate of heat transfer to the working fluid passing

through the steam generator, in kJ per kg of steam entering

the first-stage turbine.

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam entering the first-stage turbine.

8.50 For the cycle of Problem 8.49, reconsider the analysis

assuming the pump and each turbine stage have isentropic

efficiencies of 80%. Answer the same questions as in Problem

8.49 for the modified cycle.

8.51 For the cycle of Problem 8.50, investigate the effects on cycle

performance as the extraction pressure takes on other values.

Assume that condensate drains from the closed feedwater

heater as saturated liquid at the extraction pressure. Also,

feedwater leaves the heater at 10 MPa and a temperature

equal to the saturation temperature at the extraction

pressure. Construct suitable plots and discuss.

8.52 A power plant operates on a regenerative vapor power

cycle with one closed feedwater heater. Steam enters the

first turbine stage at 120 bar, 5208C and expands to 10 bar,

where some of the steam is extracted and diverted to a

closed feedwater heater. Condensate exiting the feedwater

heater as saturated liquid at 10 bar passes through a trap

into the condenser. The feedwater exits the heater at 120 bar

with a temperature of 1708C. The condenser pressure is

0.06 bar. For isentropic processes in each turbine stage and

the pump, determine for the cycle (a) the thermal efficiency

and (b) the mass flow rate into the first-stage turbine, in

kg/h, if the net power developed is 320 MW.

8.53 Reconsider the cycle of Problem 8.52, but include in the

analysis that each turbine stage has an isentropic efficiency

of 82%. The pump efficiency remains 100%.

8.54 Modify the cycle of Problem 8.49 such that the saturated

liquid condensate from the feedwater heater at 0.7 MPa is

pumped into the feedwater line rather than being trapped

into the condenser. Answer the same questions about the

modified cycle as in Problem 8.49. List advantages and

disadvantages of each scheme for removing condensate from

the closed feedwater heater.

8.55 Water is the working fluid in an ideal regenerative

Rankine cycle with one closed feedwater heater. Steam

enters the turbine at 1400 lbf/in.

and 10008F and expands

to 120 lbf/in.

, where some of the steam is extracted and

diverted to the closed feedwater heater. The remaining

steam expands through the second-stage turbine to the

condenser pressure of 2 lbf/in.

Condensate exiting the

feedwater heater as saturated liquid at 120 lbf/in.

undergoes

a throttling process as it passes through a trap into the

condenser. The feedwater leaves the heater at 1400 lbf/in.

and a temperature equal to the saturation temperature at

120 lbf/in.

The net power output of the cycle is 1 3 10

Btu/h.

Determine for the cycle

(a) the mass flow rate of steam entering the first stage of the

turbine, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

8.56 Water is the working fluid in a regenerative Rankine

cycle with one closed feedwater heater. Steam enters the

turbine at 1400 lbf/in.

and 10008F and expands to 120 lbf/

in.

, where some of the steam is extracted and diverted to

the closed feedwater heater. The remaining steam expands

through the second-stage turbine to the condenser pressure

of 2 lbf/in.

Each turbine stage and the pump have isentropic

efficiencies of 85%. Flow through the condenser, closed

feedwater heater, and steam generator is at constant pressure.

Condensate exiting the feedwater heater as saturated liquid at

120 lbf/in.

undergoes a throttling process as it passes through

a trap into the condenser. The feedwater leaves the heater at

1400 lbf/in.

and a temperature equal to the saturation

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482 Chapter 8

Vapor Power Systems

temperature at 120 lbf/in.

The net power output of the cycle

is 1 3 10

Btu/h. Determine for the cycle

(a) the mass flow rate of steam entering the first stage of the

turbine, in lb/h.

(b) the rate of heat transfer, in Btu/h, to the working fluid

passing through the steam generator.

8.57 Water is the working fluid in an ideal regenerative

Rankine cycle with one closed feedwater heater. Superheated

vapor enters the turbine at 16 MPa, 5608C, and the condenser

pressure is 8 kPa. The cycle has a closed feedwater heater

using extracted steam at 1 MPa. Condensate drains from the

feedwater heater as saturated liquid at 1 MPa and is trapped

into the condenser. The feedwater leaves the heater at 16 MPa

and a temperature equal to the saturation temperature at

1 MPa. The mass flow rate of steam entering the first-stage

turbine is 120 kg/s. Determine

(a) the net power developed, in kW.

(b) the rate of heat transfer to the steam passing through the

boiler, in kW.

(d) the mass flow rate of condenser cooling water, in kg/s, if the

cooling water undergoes a temperature increase of 188C with

negligible pressure change in passing through the condenser.

8.58 Reconsider the cycle of Problem 8.57, but include in the

analysis that the isentropic efficiencies of the turbine stages

and the pump are 85%.

8.59 Referring to Fig. 8.12, if the fractions of the total flow

entering the first turbine stage (state 1) extracted at states

2, 3, 6, and 7 are y

, y

, and y

, respectively, what are the

fractions of the total flow at states 8, 11, and 17?

8.60 Consider a regenerative vapor power cycle with two

feedwater heaters, a closed one and an open one, as shown

in Figure P8.60. Steam enters the first turbine stage at 12 MPa,

4808C, and expands to 2 MPa. Some steam is extracted at 2 MPa

and fed to the closed feedwater heater. The remainder

expands through the second-stage turbine to 0.3 MPa, where

an additional amount is extracted and fed into the open

feedwater heater operating at 0.3 MPa. The steam expanding

through the third-stage turbine exits at the condenser

pressure of 6 kPa.

Feedwater leaves the closed heater at 2108C, 12 MPa, and

condensate exiting as saturated liquid at 2 MPa is trapped into

the open feedwater heater. Saturated liquid at 0.3 MPa leaves

the open feedwater heater. Assume all pumps and turbine

stages operate isentropically. Determine for the cycle

(a) the rate of heat transfer to the working fluid passing

through the steam generator, in kJ per kg of steam entering

the first-stage turbine.

(b) the thermal efficiency.

through the condenser to the cooling water, in kJ per kg of

steam entering the first-stage turbine.

8.61 For the cycle of Problem 8.60, reconsider the analysis

assuming the pump and each turbine stage has an isentropic

efficiency of 80%. Answer the same questions as in Problem

8.60 for the modified cycle.

8.62 For the cycle of Problem 8.60, investigate the effects on

cycle performance as the higher extraction pressure takes on

other values. The operating conditions for the open feedwater

heater are unchanged from those in Problem 8.60. Assume

that condensate drains from the closed feedwater heater as

saturated liquid at the higher extraction pressure. Also,

Steam

generator

Condenser

6 kPa

= 1.5 × 10

kg/s

= 480°C

= 12 MPa

= 210°C

10 11

Pump 2 Pump 1

Closed

heater

Open

heater

0.3 MPa

2.0 MPa

Trap

Sat.

liquid

Sat.

liquid

= 12 MPa

Fig. P8.60

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