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

386 | Thermodynamics

(c) The entropy generated during this process can be determined by applying

an entropy balance on an extended systemthat includes the iron block and

its immediate surroundings so that the boundary temperature of the extended

system is at 285 K at all times:

or

Discussion The entropy generated can also be determined by taking the iron

block and the entire lake as the system, which is an isolated system, and

applying an entropy balance. An isolated system involves no heat or entropy

transfer, and thus the entropy generation in this case becomes equal to the

total entropy change,

which is the same result obtained above.

Sgen¢Stotal¢Ssystem
¢Slake12.65
16.974.32 kJ>K

Sgen

Qout

Tb

¢Ssystem

4838 kJ

285 K

 1 12.65 kJ>K 2 4.32 kJ/K



Qout

Tb

Sgen¢Ssystem

SinSout¬
¬ Sgen¬¬¢Ssystem

EXAMPLE 7–20 Entropy Generation in a Mixing Chamber

Water at 20 psia and 50F enters a mixing chamber at a rate of 300 lbm/min

where it is mixed steadily with steam entering at 20 psia and 240F. The

mixture leaves the chamber at 20 psia and 130F, and heat is lost to the

surrounding air at 70F at a rate of 180 Btu/min. Neglecting the changes in

kinetic and potential energies, determine the rate of entropy generation

during this process.

Solution Water and steam are mixed in a chamber that is losing heat at a

specified rate. The rate of entropy generation during this process is to be

determined.

Assumptions 1 This is a steady-flow process since there is no change with

time at any point and thus mCV0, ECV0, and SCV0. 2 There are

no work interactions involved. 3 The kinetic and potential energies are

negligible, ke pe 0.

Analysis We take the mixing chamberas the system (Fig. 7–68). This is a

control volumesince mass crosses the system boundary during the process.

We note that there are two inlets and one exit.

T 3 = 130°F

T 2 = 240°F

T 1 = 50°F

300 lbm/min Mixing

chamber

P = 20 psia

180 Btu/min

FIGURE 7–68

Schematic for Example 7–20.

123

Net entropy transfer

by heat and mass

123

Entropy

generation

123

Change

in entropy