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Find the internal energy of 1 kg of superheated steam at a pressure of 10 bar and 280ºC. If this steam is expanded to a pressure of 1.6 bar and 0.8 dry, determine the change in internal energy. Assume specific heat of superheated steam as 2.1 kJ/kg-K.
Find the internal energy of 1 kg of superheated steam at a pressure of 10 bar and 280ºC. If this steam is expanded to a pressure of 1.6 bar and 0.8 dry, determine the change in internal energy. Assume specific heat of superheated steam as 2.1 kJ/kg-K.
Given that
State 1 : 10 bar 280°C
State 2 : 1.6 bar, 0.8 dry
Specific heat of superheated steam = 2.1 kJ/kg K
Internal energy at state 1 is:
u1 = ug + m.c.(T1 – Tsat) = (hg – pvg) + m.c. (T1 – Tsat)
= (2776.2 – 1000 × 0. 19429) + 2.1(280 – 179.88) = 2792.16 kJ/kg
Internal energy at state 2;
u2 = uf2 + xuf g2
= (hf – Pvf )2 + x [hfg – P (vfg)]2
= (hf – Pvf ) + x (hg – hf) – P (vg – vf )]
= (hf – Pvf ) + x ((hg – Pvg) – (hf – Pvf))
= [475.38 + 160 (0.0010547)] + [(2696.2 – 160 * (1.0911))
– (475.38 – 160 (0.0010547))]
= 475.21 + 0.8 (2521.62 – 475.21)
= 2112.34 kJ/kg
Change in internal energy = 211234 – 2792.16 = – 679.82 kJ/kg
-ve sign shows the reduction in internal energy