Chapter 10 | 593The Reheat Rankine Cycle
10–29C How do the following quantities change when a
simple ideal Rankine cycle is modified with reheating?
Assume the mass flow rate is maintained the same.
Pump work input: (a) increases, (b) decreases,
(c) remains the same
Turbine work (a) increases, (b) decreases,
output: (c) remains the same
Heat supplied: (a) increases, (b) decreases,
(c) remains the same
Heat rejected: (a) increases, (b) decreases,
(c) remains the same
Moisture content (a) increases, (b) decreases,
at turbine exit: (c) remains the same
10–30C Show the ideal Rankine cycle with three stages of
reheating on a T-sdiagram. Assume the turbine inlet tempera-
ture is the same for all stages. How does the cycle efficiency
vary with the number of reheat stages?
10–31C Consider a simple Rankine cycle and an ideal
Rankine cycle with three reheat stages. Both cycles operate
between the same pressure limits. The maximum temperature
is 700°C in the simple cycle and 450°C in the reheat cycle.
Which cycle do you think will have a higher thermal
efficiency?
10–32 A steam power plant operates on the ideal
reheat Rankine cycle. Steam enters the high-
pressure turbine at 8 MPa and 500°C and leaves at 3 MPa.
Steam is then reheated at constant pressure to 500°C before it
expands to 20 kPa in the low-pressure turbine. Determine the
turbine work output, in kJ/kg, and the thermal efficiency of
the cycle. Also, show the cycle on a T-sdiagram with respect
to saturation lines.
10–33 Reconsider Prob. 10–32. Using EES (or other)
software, solve this problem by the diagram
window data entry feature of EES. Include the effects of the
turbine and pump efficiencies and also show the effects of
reheat on the steam quality at the low-pressure turbine exit.
Plot the cycle on a T-sdiagram with respect to the saturation
lines. Discuss the results of your parametric studies.
10–34 Consider a steam power plant that operates on a
reheat Rankine cycle and has a net power output of 80 MW.
Steam enters the high-pressure turbine at 10 MPa and 500°C
and the low-pressure turbine at 1 MPa and 500°C. Steam
leaves the condenser as a saturated liquid at a pressure of
10 kPa. The isentropic efficiency of the turbine is 80 percent,
and that of the pump is 95 percent. Show the cycle on a T-s
diagram with respect to saturation lines, and determine
(a) the quality (or temperature, if superheated) of the steam at
the turbine exit, (b) the thermal efficiency of the cycle, and
(c) the mass flow rate of the steam. Answers:(a) 88.1°C,
(b) 34.1 percent, (c) 62.7 kg/s
10–35 Repeat Prob. 10–34 assuming both the pump and the
turbine are isentropic. Answers:(a) 0.949, (b) 41.3 percent,
(c) 50.0 kg/s
10–36E Steam enters the high-pressure turbine of a steam
power plant that operates on the ideal reheat Rankine cycle at
800 psia and 900°F and leaves as saturated vapor. Steam is
then reheated to 800°F before it expands to a pressure of
1 psia. Heat is transferred to the steam in the boiler at a rate of
6 104 Btu/s. Steam is cooled in the condenser by the cooling
water from a nearby river, which enters the condenser at 45°F.
Show the cycle on a T-s diagram with respect to saturation
lines, and determine (a) the pressure at which reheating takes
place, (b) the net power output and thermal efficiency, and
(c) the minimum mass flow rate of the cooling water required.
10–37 A steam power plant operates on an ideal reheat Rank-
ine cycle between the pressure limits of 15 MPa and 10 kPa.
The mass flow rate of steam through the cycle is 12 kg/s. Steam
enters both stages of the turbine at 500°C. If the moisture con-
tent of the steam at the exit of the low-pressure turbine is not to
exceed 10 percent, determine (a) the pressure at which reheat-
ing takes place, (b) the total rate of heat input in the boiler, and
(c) the thermal efficiency of the cycle. Also, show the cycle on
a T-sdiagram with respect to saturation lines.
10–38 A steam power plant operates on the reheat Rankine
cycle. Steam enters the high-pressure turbine at 12.5 MPa
and 550°C at a rate of 7.7 kg/s and leaves at 2 MPa. Steam is
then reheated at constant pressure to 450°C before it expands
in the low-pressure turbine. The isentropic efficiencies of the
turbine and the pump are 85 percent and 90 percent, respec-
tively. Steam leaves the condenser as a saturated liquid. If the
moisture content of the steam at the exit of the turbine is not
to exceed 5 percent, determine (a) the condenser pressure,
(b) the net power output, and (c) the thermal efficiency.
Answers:(a) 9.73 kPa, (b) 10.2 MW, (c) 36.9 percentBoiler3451Turbine6CondenserPump2FIGURE P10–38