QUESTION 4 (PO2, CO2, C3) In an industrial facility, air is to be preheated before entering a furnace by geothermal water at 120C flowing through the tubes of a tube bank located in a duct. Air enters the duct at 20C and 1 atm with a mean velocity of 4.5 m/s and flows over the tubes in normal direction. The outer diameter of the tubes is 1.5 cm, and the tubes e arranged in line with longitudinal and transverse pitches of SL = St = 5 cm. There are 6 rows in the flow direction with 10 tubes in each row, as shown in Figure 4. The exit temperature of air, T. = 100C. By taking the mean temperature as 60C (333K), the properties of air are as listed in Table 1: Table 1 Properties of air at 1 atm and 333 K No Properties Value 1 Thermal properties, k 0.0287 W/mK Heat capacity, Ce 1.0083 kJ/kg-K 3 Dynamic viscosity. 200.176 x 107 N-s/m2 4 Density, P 1.0516 kg/m Prandtl number, Pr 0.7024 Prs at 120 C is 0.6914 a) Estimate the rate of heat transfer per unit length of the tubes. Table 2 Correction factor F to be used in Nup,N,516 = FNup N, 1 2 3 4 7 10 13 In-line 0.70 0.80 0.86 0.90 0.93 0.96 0.98 0.99 Staggered 0.64 0.76 0.84 0.89 0.93 0.96 0.98 0.99 (20 marks) 2 5 5