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

566 | Thermodynamics

Tavg,reheat

s

T

FIGURE 10–12

The average temperature at which heat
is transferred during reheating
increases as the number of reheat
stages is increased.

The reheat cycle was introduced in the mid-1920s, but it was abandoned

in the 1930s because of the operational difficulties. The steady increase in

boiler pressures over the years made it necessary to reintroduce single

reheat in the late 1940s and double reheat in the early 1950s.

The reheat temperatures are very close or equal to the turbine inlet tem-

perature. The optimum reheat pressure is about one-fourth of the maximum

cycle pressure. For example, the optimum reheat pressure for a cycle with a

boiler pressure of 12 MPa is about 3 MPa.

Remember that the sole purpose of the reheat cycle is to reduce the mois-

ture content of the steam at the final stages of the expansion process. If we

had materials that could withstand sufficiently high temperatures, there

would be no need for the reheat cycle.

EXAMPLE 10–4 The Ideal Reheat Rankine Cycle

Consider a steam power plant operating on the ideal reheat Rankine cycle.

Steam enters the high-pressure turbine at 15 MPa and 600°C and is con-

densed in the condenser at a pressure of 10 kPa. If the moisture content of

the steam at the exit of the low-pressure turbine is not to exceed 10.4 per-

cent, determine (a) the pressure at which the steam should be reheated and

(b) the thermal efficiency of the cycle. Assume the steam is reheated to the

inlet temperature of the high-pressure turbine.

Solution A steam power plant operating on the ideal reheat Rankine cycle

is considered. For a specified moisture content at the turbine exit, the reheat

pressure and the thermal efficiency are to be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential

energy changes are negligible.

Analysis The schematic of the power plant and the T- sdiagram of the cycle

are shown in Fig. 10–13. We note that the power plant operates on the ideal

reheat Rankine cycle. Therefore, the pump and the turbines are isentropic,

there are no pressure drops in the boiler and condenser, and steam leaves

the condenser and enters the pump as saturated liquid at the condenser

pressure.

(a) The reheat pressure is determined from the requirement that the

entropies at states 5 and 6 be the same:

State 6:

Also,

Thus,

State 5:

Therefore, steam should be reheated at a pressure of 4 MPa or lower to pre-

vent a moisture content above 10.4 percent.

T 5 
600°C

s 5 
s 6

f¬

P 5 
4.0 MPa

h 5 
3674.9 kJ>kg

h 6 
hfx 6 hfg
191.810.896 1 2392.1 2 
2335.1 kJ>kg

s 6
sfx 6 sfg
0.64920.896 1 7.4996 2
7.3688 kJ>kg#K

x 6
0.896¬ 1 sat. mixture 2

P 6 
10 kPa