Question

1. A piston – cylinder containing steam at 700 kPa and 250 deg C undergoes a constant pressure process until the quality is 70%. Determine per kilogram (a) change of internal energy and (b) change of enthalpy.

Answer 1

From steam tables at;

$P_1=700kPa$ and $T_1=250°C$

$v_1=0.336m^3/kg, u_1=2718.66kJ/kg, h_1=2954.123kJ/kg$

At 700kPa saturated condition;

$u_f=696.36kJ/kg,u_g=2971.81kJ/kg$

$h_f=697.14kJ/kg,h_g=2762.74kJ/kg$

$h_2=h_f+x(h_g-h_f)$

x=0.7

$h_2=697.14+0.7(2762.74-697.14)=2143.66kJ/kg$

$u_2=u_f+x(u_g-u_f)$

$u_2=696.36+0.7(2971.81-696.36)=2009.175kJ/kg$

a)

$∆u=u_2-u_1$

$=2009.175-2718.66$

$-709.485kJ/kg$

b)

$∆h=h_2-h_1$

=2143.66-2954.123

=-810.463kJ/kg