Air enters a two-stage compressor operating at steady state at 520ºR, 14 lbffin 2 The overall pressure ratio across the stages is 12 and each stage operates isentropically. Intercooling occurs at constant pressure at the value that minimizes compressor work input as determined in Example 9.10, with air exiting the intercooler at 520°R. Assuming ideal gas behavior, with k = 1.4. Determine the work per unit mass of air flowing for the two-stage compressor, in Btu per lb of air flowing.