Refrigerant-134a expands in an adiabatic turbine from 1 MPa and 120°C to 0.10 MPa and 50°C at a rate of 0.8 kg/s. The power output of the turbine is
A)72.5 kW
B)58.0 kW
C)43.5 kW
D)46.4 kW
E)54.4 kW

Question

Quizplus · Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
P1=1000 "kPa"
T1=120 "C"
P2=100 "kPa"
T2=50 "C"
m_dot=0.8 "kg/s"
Q_dot_loss=0 "kJ/s"
h1=ENTHALPY(R134a,T=T1,P=P1)
h2=ENTHALPY(R134a,T=T2,P=P2)
"The rate form of energy balance for a steady-flow system is E_dot_in = E_dot_out"
-W_dot_out-Q_dot_loss=m_dot*(h2-h1) 
"Checking using properties from tables:"
h11=332.47
h22=295.45
-Wtable-Q_dot_loss=m_dot*(h22-h11) 
"Some Wrong Solutions with Common Mistakes:"
-W1_Wout-Q_dot_loss=(h2-h1)/m_dot "Dividing by mass flow rate instead of multiplying"
-W2_Wout-Q_dot_loss=h2-h1 "Not considering mass flow rate"
u1=INTENERGY(R134a,T=T1,P=P1)
u2=INTENERGY(R134a,T=T2,P=P2)
-W3_Wout-Q_dot_loss=m_dot*(u2-u1) "Using internal energy instead of enthalpy"
-W4_Wout-Q_dot_loss=u2-u1 "Using internal energy and ignoring mass flow rate"

Refrigerant-134a Expands in an Adiabatic Turbine from 1 MPa and 120°C