Many of the impracticalities associated with the Carnot cycle can be elimi-nated by superheating the steam in the boiler and condensing it completely in

the condenser, as shown schematically on a T-sdiagram in Fig. 8–39. The cy-cle that results is the Rankine cycle,which is the ideal cycle for vapor power

plants. The ideal Rankine cycle does not involve any internal irreversibilities

and consists of the following four processes:

1-2 Isentropic compression in a pump

2-3 Constant pressure heat addition in a boiler

3-4 Isentropic expansion in a turbine

4-1 Constant pressure heat rejection in a condenser

Water enters the pumpat state 1 as saturated liquid and is compressed isen-tropically to the operating pressure of the boiler. The water temperature in-creases somewhat during this isentropic compression process due to a slight

decrease in the specific volume of the water. The vertical distance between

states 1 and 2 on the T-sdiagram is greatly exaggerated for clarity. (If water

were truly incompressible, would there be a temperature change at all during

this process?)

Water enters the boileras a compressed liquid at state 2 and leaves as a su-perheated vapor at state 3. The boiler is basically a large heat exchanger where

the heat originating from combustion gases, nuclear reactors, or other sources

is transferred to the water essentially at constant pressure. The boiler, together

with the section where the steam is superheated (the superheater), is often

called the steam generator.

The superheated vapor at state 3 enters the turbine,where it expands isen-tropically and produces work by rotating the shaft connected to an electric

generator. The pressure and the temperature of the steam drop during this

process to the values at state 4, where steam enters the condenser.At this state,

steam is usually a saturated liquid–vapor mixture with a high quality. Steam is

condensed at constant pressure in the condenser, which is basically a large

heat exchanger, by rejecting heat to a cooling medium such as a lake, a river,

or the atmosphere. Steam leaves the condenser as saturated liquid and enters

the pump, completing the cycle. In areas where water is precious, the power

plants are cooled by air instead of water. This method of cooling, which is also

used in car engines, is called dry cooling.Several power plants in the world,

including some in the United States, use dry cooling to conserve water.

Remembering that the area under the process curve on a T-sdiagram repre-sents the heat transfer for internally reversible processes, we see that the area

under process curve 2-3 represents the heat transferred to the water in the

boiler and the area under the process curve 4-1 represents the heat rejected in

the condenser. The difference between these two (the area enclosed by the

cycle curve) is the net work produced during the cycle.