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Modern Engineering Thermodynamics

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efficiency increase if the condenser pressure is lowered from<br />

0.100 to 1.00 × 10 –3 MPa?<br />

24. Steam enters the turbine of a Rankine cycle power plant at<br />

200. psia and 500.°F. How much does the isentropic thermal<br />

efficiency increase if the condenser pressure is lowered from<br />

14.7 to 1.00 psia?<br />

25. Steam leaves the boiler of a Rankine cycle power plant at<br />

3000. psia and 1000.°F. It is isenthalpically throttled to 600.<br />

psia before it enters the turbine. It then exits the turbine at 1.00<br />

psia. Neglecting the pump work, determine<br />

a. The maximum thermal efficiency of this plant.<br />

b. Its maximum thermal efficiency if the boiler is operated at<br />

600. psia and 1000.°F and no throttling occurs at the<br />

entrance to the turbine.<br />

26. Steam exits a boiler and enters a turbine at 200. psia, 600.°F<br />

with a mass flow rate of 30.0 × 10 3 lbm/h. It exits the turbine at<br />

1.00 psia and is condensed. The condensate then reenters the<br />

boiler as a saturated liquid at 90.0°F. The turbine drives an<br />

electrical generator that delivers a net 3000. kW of power.<br />

Determine<br />

a. The Rankine cycle actual thermal efficiency of the entire<br />

system.<br />

b. The isentropic efficiency of the combined turbine–generator<br />

unit.<br />

27. An isentropic Rankine cycle using steam is shown in Figure 13.68.<br />

For the given data, determine<br />

a. The quality of the turbine exhaust steam.<br />

b. The cycle thermal efficiency.<br />

c. The mass flow rate required to produce 10,000 Btu/s net<br />

output power.<br />

W P<br />

FIGURE 13.68<br />

Problem 27.<br />

p 2 = 3:00 psia p 3 = 3:00 psia p 4 = 1000: psia<br />

s at 1.00 MPa.<br />

2 = s 1 x 3 = 0:00 s 4 = s 3<br />

35. Steam at 600. psia and 800.°F enters the high-pressure stage of<br />

Problems 529<br />

1.00 to 20.0 MPa. Ignore the power required by the boiler<br />

feed pump.<br />

29.* Steam is supplied to a turbine at a rate of 1.00 × 10 6 kg/h at<br />

500.°C and 10.0 MPa, and it exhausts to a condenser at 2.00<br />

kPa. A single open loop regenerator is used to heat the boiler<br />

feedwater with steam extracted from the turbine at 6.00 MPa.<br />

The condensate exits the regenerator at 6.00 MPa as a saturated<br />

liquid. Neglecting the pump power, determine<br />

a. The mass flow rate of steam extracted from the turbine.<br />

b. The system’s isentropic Rankine cycle thermal efficiency.<br />

c. The isentropic Rankine cycle thermal efficiency of the same<br />

system without a regenerator.<br />

d. The percent increase in thermal efficiency due to the<br />

regenerator.<br />

30. Steam enters a turbine with an isentropic efficiency of 83.0%<br />

at 300. psia, 800.°F and exhausts to a condenser at 0.250 psia.<br />

The boiler feedwater is heated in a single, open loop regenerator<br />

with steam extracted from the turbine at 100. psia. Saturated<br />

liquid leaves the regenerator at the pressure of the extraction<br />

steam. Neglecting all pump work, determine<br />

a. The percent mass flow of steam extracted from the turbine.<br />

b. The turbine power output per unit mass flow of steam.<br />

c. The system thermal efficiency.<br />

d. The turbine power output per unit mass flow of steam when<br />

the regenerator is not in use.<br />

e. The system thermal efficiency when the regenerator is not<br />

in use.<br />

f. The percent increase in system thermal efficiency produced<br />

by the regenerator.<br />

31. Repeat items a, b, and c of Problem 30 for steam extraction<br />

pressures of 75.0, 50.0, 25.0, and 10.0 psia and plot these<br />

Q H<br />

results vs. the extraction pressure.<br />

32. A turbine having an isentropic efficiency of 86.0% receives<br />

steam at 4.00 × 10 6 lbm/h, 300. psia, 1000.°F and exhausts it to<br />

4<br />

1<br />

a condenser at 1.00 psia. Steam is extracted from the turbine at<br />

Boiler<br />

788,000 lbm/h and 200. psia to heat the boiler feedwater in a<br />

single, closed loop regenerator. The extract steam then exits the<br />

Pump<br />

Turbine W regenerator as a saturated liquid. Neglecting the pump power,<br />

T<br />

determine<br />

a. The pressure and temperature of the extract steam as it leaves<br />

3<br />

Condenser<br />

the regenerator.<br />

2<br />

b. The Rankine cycle thermal efficiency of this system.<br />

Q L<br />

c. The Rankine cycle thermal efficiency of this system without<br />

regeneration.<br />

d. The percent increase in thermal efficiency due to the<br />

regenerator.<br />

33. Determine the blade tip velocity of DeLaval’s first steam turbine.<br />

It had a rotor diameter of 3.00 inches and ran at 40.0 × 10 3 rpm.<br />

34.* Determine the isentropic exit velocity from a reaction turbine<br />

nozzle if steam enters the nozzle at 30.0 MPa, 500.°C and exits<br />

an isentropic turbine and is reheated to 60.0 psia and 700.°F<br />

before entering the low-pressure stage. The steam then exhausts<br />

to a condenser at 1.00 psia. Neglecting pump power, determine<br />

a. The isentropic Rankine cycle thermal efficiency.<br />

b. The isentropic Rankine cycle thermal efficiency that would<br />

occur if the steam were not reheated.<br />

c. The percent increase in thermal efficiency due to the<br />

reheating operation.<br />

Station 1 Station 2 Station 3 Station 4<br />

p 1 = 1000: psia<br />

T 1 = 1000:°F<br />

The process path from station 1 to 2 is an isentropic expansion<br />

and that from station 3 to 4 is an isentropic compression.<br />

28.* For a constant steam boiler temperature of 400.°C, a constant<br />

turbine exhaust moisture content of 5.00%, and a constant<br />

condenser pressure of 3.00 kPa, plot the Rankine cycle thermal<br />

efficiency vs. boiler pressure over the boiler pressure range from

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