A plywood-manufacturing concern is using a binder that requires a temeprature of 180°F in order to obtain adequate holding strength. Large slabs of the plywood at an initial uniform temperature of 70°F are placed in a heater, and the heating unit is maintained at a constant temperature. The manufacturer wishes to pass the slabs through the heater continuously at such a rate that each slab remains in the heater for 15 min. If the slabs are 1 in. thick, determine the minimum constant temperature (i.e., with negligible surface resistance) required for the heater if the temperature at the center of each slab is to reach 180°F before leaving the heater. The following average data apply to the plywood: density = 35 Ib/ft3; thermal conductivity = 0.10 Btu/(h)(ft2)(°F/ft); heat capacity = 0.50 Btu/(lb)(°F). Consider the plywood as homogeneous and isotropic.