Unformatted text preview: mmonly used in various cryogenic applications. The critical temperature of methane is 191 K (or 82°C), and thus methane must be
maintained below 191 K to keep it in liquid phase. The properties of liquid
methane at various temperatures and pressures are given in Table 7–1. Determine the entropy change of liquid methane as it undergoes a process from 110
K and 1 MPa to 120 K and 5 MPa (a) using actual data for methane and (b) approximating liquid methane as an incompressible substance. What is the error
involved in the latter case? P2 = 5 MPa
T2 = 120 K
Heat SOLUTION The entropy change of methane during a process is to be determined using actual data and assuming it to be incompressible.
Analysis (a) We consider a unit mass of liquid methane (Fig. 7–29). The entropies of the methane at the initial and final states are
State 1: P1
T1 1 MPa
110 K s1
Cp1 4.875 kJ/kg · K
3.471 kJ/kg · K State 2: Methane
pump P2
T2 5 MPa
120 K s2
Cp2 5.145 kJ/kg · K
3.486 kJ/kg · K P1 = 1 MPa
T1 = 110 K FIGURE 7–29
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 Spring '09
 Thermodynamics, Energy, Entropy, entropy change

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