Using the steam tables: a) Calculate Cp for superheated steam at 200 bar and 450

Question

Using the steam tables:

a) Calculate Cp for superheated steam at 200 bar and 450 C. Compare this value to Cp for superheated steam at 450 C and lower pressures. At what point does the assumption Cp=Cpig become valid, so that the assumption (H2-H1)=Cpig(T2-T1) is ok to use (assume 5% error is acceptable)?

b) Use your best estimate of Cpig for 450 C to estimate H for superheated steam at 5 bar and 500 C, and compare your result to that shown in the steam table. What do you conclude from this result?

Explanation / Answer

At 200 bar and 450 C, the value of Cp is 4.0073 kJ/kg K

at 450 C,

150 bar --3.268 kJ/kg K

100 bar - 2.747 kJ/kg K

50 bar-2.37 kJ/kg K

1 bar - 2.101

0.5 bar- 2.099

both are almost equal .so , 450 C and 1 bar the assumption Cp=Cpig become valid.

b)

from steam tables , steam at 5 bar and 500 C . enthalpy = 3484.41 kJ/kg

now enthalpy at 5 bar and 450 = 3377.669 kJ/kg

enthalpy at 500 = enthalpy at 450 + cp(500-450)

= 3377.669 kJ/kg + 2.10*(50)

= 3482.669 kJ/kg

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