con st t 1 m 5 kg refrigerant 134a t1 20c p1

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Unformatted text preview: t. T 1 m = 5 kg Refrigerant-134a T1 = 20°C P1 = 140 kPa ∆S = ? 2 Heat s2 s1 s FIGURE 7–12 Schematic and T-s diagram for Example 7–3. cen54261_ch07.qxd 11/18/03 9:57 AM Page 284 284 FUNDAMENTALS OF THERMAL-FLUID SCIENCES 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 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 piston-cylinder 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 quasi-equilibrium. 3 The pressure remains constant during the process and thus P2 P1. Analysis...
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This document was uploaded on 11/28/2012.

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