Unformatted text preview: ture region. Also, the constant-pressure lines almost coincide
with the saturated liquid line in the compressed liquid region. T, °C P=1 0 MP a 500 P=1M Pa Critical
state 400 300 Saturated
3 /kg 200 υ= 0.1 m Saturated
vapor line 3
υ = 0.5 100 FIGURE 7–11
Schematic of the T-s diagram
for water. 0 1 2 3 4 5 6 7 8 s, kJ/kg K cen54261_ch07.qxd 11/18/03 9:57 AM Page 283 283
CHAPTER 7 EXAMPLE 7–3 Entropy Change of a Substance in a Tank A rigid tank contains 5 kg of refrigerant-134a initially at 20°C and 140 kPa. The
refrigerant is now cooled while being stirred until its pressure drops to 100 kPa.
Determine the entropy change of the refrigerant during this process. SOLUTION We take the refrigerant in the tank as the system (Fig. 7–12). This
is a closed system since no mass crosses the system boundary during the
process. We note that the change in entropy of a substance during a process is
simply the difference between the entropy values at the final and initial states.
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