In a heat treatment process, a 30 mm diameter stainless steel (aisi 304) ball (ρ= 7900 kg/m3 , cp = 477 j/kg·k, k = 14.9 w/m·k) is removed from the oven at a uniform temperature of 500ºc. the ball is then subjected to a flow of air at 27ºc which provides a convection coefficient of 500 w/m2 ·k. determine: a. how long does it take for the ball to reach a minimum temperature (t) of 300ºc and the rate of heat removal (q) by each ball during this process.

Along 8-cm diameter steam pipe whose external surface temperature is 900c connects two buildings. the pipe is exposed to ambient air at 70c with a wind speed of 50 km/hr blowing across the pipe. determine the heat loss from the pipe per unit length. (b) air at 500c enters a section of a rectangular duct (15 cm x 20 cm) at an average velocity of 7 m/s. if the walls of the duct are maintained at 100c. a) the length of the tube for an exit temperature of the air to be 40 0c. b)the rate of heat transfer from the air. c) the fan power needed to overcome the pressure drop in this section of the duct.