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

342 | Thermodynamics

EXAMPLE 7–4 Entropy Change during a Constant-Pressure

Process

A piston–cylinder device initially contains 3 lbm of liquid water at 20 psia

and 70°F. The water is now heated at constant pressure by the addition of

3450 Btu of heat. Determine the entropy change of the water during this

process.

Solution Liquid water in a piston–cylinder device is heated at constant

pressure. The entropy change of water is to be determined.

Assumptions 1 The tank is stationary and thus the kinetic and potential

energy changes are zero, KE PE 0. 2 The process is quasi-equilibrium.

3 The pressure remains constant during the process and thus P 2 P 1.

Analysis We take the water in the cylinder as the system(Fig. 7–13). This is

a closed systemsince no mass crosses the system boundary during the

process. We note that a piston–cylinder device typically involves a moving

boundary and thus boundary work Wb. Also, heat is transferred to the system.

Water exists as a compressed liquid at the initial state since its pressure is

greater than the saturation pressure of 0.3632 psia at 70°F. By approximat-

ing the compressed liquid as a saturated liquid at the given temperature, the

properties at the initial state are

State 1:

At the final state, the pressure is still 20 psia, but we need one more prop-

erty to fix the state. This property is determined from the energy balance,

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

since UWbHfor a constant-pressure quasi-equilibrium process. Then,

State 2:

P 2 20 psia

h 2 1188.1 Btu>lbm

f¬

s 2 1.7761 Btu>lbm#R

1 Table A-6E, interpolation 2

h 2 1188.1 Btu>lbm

3450 Btu 1 3 lbm 21 h 2 
38.08 Btu>lbm 2

Qin¢Hm 1 h 2 
h 12

Qin
Wb¢U

Ein
Eout¬

¬

¢Esystem

P 1 20 psia

T 1 70°F

f¬

s 1 
sf @ 70°F0.07459 Btu>lbm#R

h 1 
hf @ 70°F38.08 Btu>lbm

T

s 1 s

1

2

s 2

P = const.

Qin P 1 = 20 psia

H 2 O

T 1 = 70°F

FIGURE 7–13

Schematic and T-sdiagram for

Example 7–4.

⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫

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