We have mentioned repeatedly that the Carnot cycle is the most efficient cycle

operating between two specified temperature limits. Thus it is natural to look

at the Carnot cycle first as a prospective ideal cycle for vapor power plants. If

we could, we would certainly adopt it as the ideal cycle. As explained here,

however, the Carnot cycle is not a suitable model for power cycles. Through-out the discussions, we assume steamto be the working fluid since it is the

working fluid predominantly used in vapor power cycles.

Consider a steady-flow Carnot cycleexecuted within the saturation dome of

a pure substance, as shown in Fig. 8–38a. The fluid is heated reversibly and

isothermally in a boiler (process 1-2), expanded isentropically in a turbine

(process 2-3), condensed reversibly and isothermally in a condenser (process

3-4), and compressed isentropically by a compressor to the initial state

(process 4-1).

Several impracticalities are associated with this cycle:

1.Isothermal heat transfer to or from a two-phase system is not difficult

to achieve in practice since maintaining a constant pressure in the device will

automatically fix the temperature at the saturation value. Therefore, processes

1-2 and 3-4 can be approached closely in actual boilers and condensers. Lim-iting the heat transfer processes to two-phase systems, however, severely lim-its the maximum temperature that can be used in the cycle (it has to remain

under the critical-point value, which is 374˚C for water). Limiting the maxi-mum temperature in the cycle also limits the thermal efficiency. Any attempt

to raise the maximum temperature in the cycle will involve heat transfer to the

working fluid in a single phase, which is not easy to accomplish isothermally

2. The isentropic expansion process (process 2-3) can be approximated

closely by a well-designed turbine. However, the quality of the steam de-creases during this process, as shown on the T-sdiagram in Fig. 8–38a. Thus

the turbine will have to handle steam with low quality, that is, steam with a

high moisture content. The impingement of liquid droplets on the turbine

blades causes erosion and is a major source of wear. Thus steam with qualities

less than about 90 percent cannot be tolerated in the operation of power plants.

This problem could be eliminated by using a working fluid with a very steep

saturated vapor line.

3.The isentropic compression process (process 4-1) involves the com-pression of a liquid–vapor mixture to a saturated liquid. There are two difficul-ties associated with this process. First, it is not easy to control the condensation

process so precisely as to end up with the desired quality at state 4. Second, it

is not practical to design a compressor that will handle two phases.

Some of these problems could be eliminated by executing the Carnot cycle

in a different way, as shown in Fig. 8–38b. This cycle, however, presents other

problems such as isentropic compression to extremely high pressures and

isothermal heat transfer at variable pressures. Thus we conclude that the

Carnot cycle cannot be approximated in actual devices and is not a realistic

model for vapor power cycles.