Answer
$\Delta v = -8.50\ ft³/lbm$
Work Step by Step
Assuming the air retains ideal gas behaviour throughout the process:
$v=\frac{RT}{P}$
With $P_1=20\ psia, P_2=150\ psia, T_1=T_2=70°F=529.67°R, R=0.3704\ psia.ft³/lbm.°R$
$v_1 = 9.81\ ft³/lbm$
$v_2=1.31\ ft³/lbm$
$\Delta v = v_2-v_1 = -8.50\ ft³/lbm$
