7.87 An air duct heater consists of an aligned array of electri- cal heating elements in which the longitudinal and trans- verse pitches are SL -ST-24 mm. There are 3 rows of elements in the flow direction (NL-3) and 4 elements per row (Nr 4). Atmospheric air with an upstream velocity of 12 m/s and a temperature of 25°C moves in cross flow over the elements, which have a diameter of 12 mm, a length of 250 mm, and are maintained at a sur- face temperature of 350°C. Heating elements, 7, 350°C- D= 12 mm N=3 250 mm N=4 Air T₁ = 25°C V=12 m/s -Air duct -Terminal panel S₁ = 24 mm S₁ = 24 mm (a) Determine the total heat transfer to the air and the temperature of the air leaving the duct heater. (b) Determine the pressure drop across the element bank and the fan power requirement. (c) Compare the average convection coefficient obtained in your analysis with the value for an isolated (single) element. Explain the difference between the results. (d) What effect would increasing the longitudinal and transverse pitches to 30 mm have on the exit tem- perature of the air, the total heat rate, and the pres- sure drop?