Air is heated from 20ºC to 827ºC. The error in the enthalpy change assuming a constant specific heat Cp:
A) 4.5%
B) 5.5%
C) 6.5%
D) 7.5%
D) 7.5%
C p h = C p (T2 ? T1) = 1.003× (823 ? 20) = 805 kJ/kg
h = h2 ? h1 = 1161? 293 = 868 kJ/kg
The error is quite large but the constant specific-heat assumption is often used especially for quick comparison between proposed design changes. If Cp at the average temperature of 400ºC, a value of
h = C p (T2 ? T1) = 1.075× (823 ? 20) = 863 kJ/kg
would result (the error is less than 1%).
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