Standards for firewalls may be based on their thermal response to a prescribed radiant heat flux. consider a 0.25-m-thick concrete wall (rho= 2300 kg/m3, c= 880 j/kg·k, k= 1.4 w/m·k), which is at an initial temperature of ti= 25°c and irradiated at one surface by lamps that provide a uniform heat flux of qs′′= 0.85·104 w/m2. the absorptivity of the surface to the irradiation is αs= 1.0. if building code requirements dictate that the temperatures of the irradiated and back surfaces must not exceed 325°c and 25°c, respectively, after 30 min of heating, will the requirements b

A3-mm-thick panel of aluminum alloy (k 177 w/m-k, c 875 j/kg-k and ? = 2770 kg/m) is finished on both sides with an epoxy coating that must be cured at or above t,-150°c for at least 5 min. the production line for the curing operation involves two steps: (1) heating in a large oven with air at ts,0-175°c and a convection coefficient of h, 40 w/m2. k, and (2) cooling in a large chamber with air at 25°c and a con- vection coefficient of he 10 w/m2.k. the heating portion of the process is conducted over a time interval te which exceeds the ime required to reach 150°c by 5 min (h = r + 300 s). the coating has an emissivity of ? = 0.8, and the temperatures of the oven and chamber walls are 175 and 25°c, respectively. if the panel is placed in the oven at an initial temperature of 25°c and removed from the chamber at a safe-to-touch tempera ture of 37°c, what is the total elapsed time for the two-step curing operation?