0.9 kg/s of air is warmed from 283 K to 366 K by passing it through a bank of pipes (20 rows with 20 pipes/row). The pipes are arranged with center-to-center spacing, in both directions, equal to twice the pipe diameter. Flue gas, entering at 700 K and leaving at 366 K, with a free flow of mass velocity of 10 kg/m.s, is passed at right angles across the outside of pipes. For simplicity, the outside and inner diameters of the pipe may be taken as 12 mm. Values for thermal conductivity (k) and viscosity (u), which may be used for both air and flue gas, are given below: Temperature, T Thermal Conductivity, k Viscosity, (in K) (in W/m.K) (in mN.s/m) 250 500 800 0.022 0.0165 0.044 0.0276 0.055 0.0367 Neglecting gas radiation, pipe wall and scale resistances, calculate the length of each pipe. DATA: Specific heat capacity of air and flue gases = 1 kJ/kg.K Heat transfer arrangement factor for in-line pipes = 0.95