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

the steam in the boiler to compensate for these undesired heat losses. As a

result, cycle efficiency decreases.

Of particular importance are the irreversibilities occurring within the

pump and the turbine. A pump requires a greater work input, and a turbine

produces a smaller work output as a result of irreversibilities. Under ideal

conditions, the flow through these devices is isentropic. The deviation of

actual pumps and turbines from the isentropic ones can be accounted for by

utilizing isentropic efficiencies,defined as

(10–10)

and

(10–11)

where states 2aand 4aare the actual exit states of the pump and the turbine,

respectively, and 2sand 4sare the corresponding states for the isentropic

case (Fig. 10–4b).

Other factors also need to be considered in the analysis of actual vapor

power cycles. In actual condensers, for example, the liquid is usually sub-

cooled to prevent the onset of cavitation,the rapid vaporization and conden-

sation of the fluid at the low-pressure side of the pump impeller, which may

damage it. Additional losses occur at the bearings between the moving parts

as a result of friction. Steam that leaks out during the cycle and air that

leaks into the condenser represent two other sources of loss. Finally, the

power consumed by the auxiliary equipment such as fans that supply air to

the furnace should also be considered in evaluating the overall performance

of power plants.

The effect of irreversibilities on the thermal efficiency of a steam power

cycle is illustrated below with an example.

hT

wa

ws

h 3 h 4 a

h 3 h 4 s

hP

ws

wa

h 2 sh 1

h 2 ah 1

558 | Thermodynamics

3

IDEAL CYCLE

ACTUAL CYCLE

Pressure drop

in the condenser

Irreversibility

in the turbine

Irreversibility

in the pump Pressure drop

in the boiler

s

T

2

4

1

(a)

3

s

T

2 s

(^14) s 4 a
2 a
(b)
FIGURE 10–4
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle. (b) The effect of pump and
turbine irreversibilities on the ideal Rankine cycle.