Explain how the equation circled in red is derived.

Example 12.4. Air at 1 atm pressure is passed through a horizontal 2 -in (51mm) Schdule. 40 steam-jacketed steel pipe at a velocity of 1.5ft/s(0.457m/s) and an inlet temperature of 68F(20C). The pipe-wall temperature is 220F(104.4C). If the outlet air temperature is to be 188F(86.7C), how long must the heated section be? Solution To establish the flow regime, the Reynolds number based on the average temperature is calculated. The quantities needed are T=268+188=128FD=122.067=0.1723ft(Appendix5)(at128F)=0.019cP(Appendix8)(at68F)=3592968+460492=0.0753Ib/ft3