Oil refineries use large fired heaters to heat oil and other products for processing. These heaters consist of a firebox where a fuel gas (typically natural gas or a refinery fuel gas) is burned, and the radiation from the fire heats the fluid which flows through the tubes in the firebox. The flue gas (combustion products + excess air) still has a relatively high temperature (typically = 1500 F to = 1700 F). Therefore, refineries place "convection sections' atop the fired heater which are essentially staggered tube banks to absorb more heat from the flue gas. Consider a convection section, consisting of 10 rows of 6 tubes each. The tubes have a diameter of 4 in. The longitudinal and transverse pitches are both 6 in. The flue gas is at 1500 F and has a velocity of 30 ft/s. The tube walls are maintained at 700 F. Assuming that the flue gas has the same properties as air and that air's properties do not change significantly over the length of the tube bank, determine the following: (a) The average heat transfer coefficient for the tube bank (b) The heat transferred per unit length of tube