Answer
$w_{1-3}=61.1\ Btu/lbm$
Work Step by Step
$w=\int vdP$
1-2: $w_{1-2}=\dfrac{v_1+v_2}2(P_2-P_1)$
With $v_1=1\ ft^3/lbm,\ P_1=15\ psia,\ v_2=3\ ft^3/lbm,\ P_2=180\ psia$
$w_{1-2}=61.1\ Btu/lbm$
2-3: $w_{2-3}=0$ Constant pressure.
$w_{1-3}=w_{1-2}+w_{2-3}=61.1\ Btu/lbm$
