Atmospheric air enters a100-m-long, 150-mm-diameter uninsulated heating duct at 60°C and 0.04 kg/s. The duct surface temperature is approximately constant at Ts = 15°C.

(a) What are the outlet air temperature, the heat rate q, and pressure drop ∆p for these conditions?

(b) To illustrate the tradeoff between heat transfer rate and pressure drop considerations, calculate q and ∆p for diameters in the range from 0.1 to 0.2 m. In your analysis, maintain the total surface area. As = πDL, at the value computed for part (a) Plot q, ∆p and L as a function of the duct diameter.