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

the area under the process curve on a P-Vdiagram. Note that the area under
the process curve represents heat transfer for processes that are internally
(or totally) reversible. The area has no meaning for irreversible processes.
Equations 7–14 and 7–15 can also be expressed on a unit-mass basis as

(7–16)

and

(7–17)

To perform the integrations in Eqs. 7–15 and 7–17, one needs to know the
relationship between Tand sduring a process. One special case for which
these integrations can be performed easily is the internally reversible
isothermal process. It yields

(7–18)

or

(7–19)

where T 0 is the constant temperature and
Sis the entropy change of the
system during the process.
An isentropic process on a T-sdiagram is easily recognized as a vertical-
line segment. This is expected since an isentropic process involves no
heat transfer, and therefore the area under the process path must be zero
(Fig. 7–17). The T-sdiagrams serve as valuable tools for visualizing the
second-law aspects of processes and cycles, and thus they are frequently
used in thermodynamics. The T-sdiagram of water is given in the appendix
in Fig. A–9.
Another diagram commonly used in engineering is the enthalpy-entropy
diagram, which is quite valuable in the analysis of steady-flow devices such
as turbines, compressors, and nozzles. The coordinates of an h-sdiagram
represent two properties of major interest: enthalpy, which is a primary
property in the first-law analysis of the steady-flow devices, and entropy,
which is the property that accounts for irreversibilities during adiabatic
processes. In analyzing the steady flow of steam through an adiabatic tur-
bine, for example, the vertical distance between the inlet and the exit states

his a measure of the work output of the turbine, and the horizontal dis-
tance
sis a measure of the irreversibilities associated with the process
(Fig. 7–18).
The h-sdiagram is also called a Mollier diagramafter the German scien-
tist R. Mollier (1863–1935). An h-sdiagram is given in the appendix for
steam in Fig. A–10.

qint revT 0 ¢s¬¬ 1 kJ>kg 2

Qint revT 0 ¢S¬¬ 1 kJ 2

qint rev

2

1

T¬ds¬¬ 1 kJ>kg 2

dqint revT¬ds¬¬ 1 kJ>kg 2

Chapter 7 | 345

T

s 2 = s 1 s

2

Isentropic

process

1

FIGURE 7–17

The isentropic process appears as a

verticalline segment on a T-sdiagram.

h

s

1

2

∆s

∆h

FIGURE 7–18

For adiabatic steady-flow devices, the

vertical distance 
hon an h-sdiagram

is a measure of work, and the

horizontal distance 
sis a measure of

irreversibilities.

EXAMPLE 7–6 The T-SDiagram of the Carnot Cycle

Show the Carnot cycle on a T-Sdiagram and indicate the areas that repre-

sent the heat supplied QH, heat rejected QL, and the net work output Wnet,out

on this diagram.