Air is throttled from 100°C and 900 kPa to a pressure of 200 kPa at a rate of 1.2 kg/s in an environment at 25°C. The change in kinetic energy is negligible, and no heat transfer occurs during the process. The power potential wasted during this process is
A)0
B)129 kW
C)13 kW
D)154 kW
E)840 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. 
R=0.287 "kJ/kg.K"
Cp=1.005 "kJ/kg.K"
m=1.2 "kg/s"
T1=100+273 "K"
P1=900 "kPa"
To=25 "C"
P2=200 "kPa"
"Temperature of an ideal gas remains constant during throttling since h=const and h=h(T)"
T2=T1
ds=Cp*ln(T2/T1)-R*ln(P2/P1)
X_dest=(To+273)*m*ds "kW"
"Some Wrong Solutions with Common Mistakes:"
W1_dest=0 "Assuming no loss"
W2_dest=(To+273)*ds "Not using mass flow rate"
W3_dest=To*m*ds "Using C for To instead of K"
W4_dest=m*(P1-P2) "Using wrong relations"

Air Is Throttled from 100°C and 900 KPa to a Pressure