the exhaust pressure, is held constant. The heat rejected decreases by area b–4–4–b–b. The
net work increases by the amount of the single cross-hatching and decreases by the amount
of the double cross-hatching. Therefore, the net work tends to remain the same but the heat
rejected decreases, and hence the Rankine-cycle efficiency increases with an increase in
maximum pressure. Note that in this instance too the average temperature at which heat is
supplied increases with an increase in pressure. The quality of the steam leaving the turbine
decreases as the maximum pressure increases.
To summarize this section, we can say that the net work and the efficiency of the
Rankine cycle can be increased by lowering the condenser pressure, by increasing the
pressure during heat addition, and by superheating the steam. The quality of the steam
leaving the turbine is increased by superheating the steam and decreased by lowering the
exhaust pressure and by increasing the pressure during heat addition. These effects are
shown in Figs. 9.7 and 9.8.
In connection with these considerations, we note that the cycle is modeled with four
known processes (two isobaric and two isentropic) between the four states with a total of
eight properties. Assuming state 1 is saturated liquid (x1 = 0), we have three (8 − 4 − 1)
parameters to determine. The operating conditions are physically controlled by the high
pressure generated by the pump, P2 = P3, the superheat to T3 (or x3 = 1 if none), and the
condenser temperature T1, which is a result of the amount of heat transfer that takes place.