Unformatted text preview: t. T 1 m = 5 kg
Refrigerant134a
T1 = 20°C
P1 = 140 kPa
∆S = ? 2 Heat
s2 s1 s FIGURE 7–12
Schematic and Ts diagram
for Example 7–3. cen54261_ch07.qxd 11/18/03 9:57 AM Page 284 284
FUNDAMENTALS OF THERMALFLUID SCIENCES EXAMPLE 7–4 Entropy Change during a ConstantPressure Process A pistoncylinder 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 We take the water in the cylinder as the system (Fig. 7–13). This
is a closed system since no mass crosses the system boundary during the
process. We note that a pistoncylinder device typically involves a moving
boundary and thus boundary work Wb. Also, heat is transferred to the system.
Assumptions 1 The tank is stationary and thus the kinetic and potential energy
changes are zero, KE
PE 0. 2 The process is quasiequilibrium. 3 The
pressure remains constant during the process and thus P2 P1.
Analysis...
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 Spring '09

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