If a rectangular block of rubber (see Problem 18-12 for properties) is set out in air at 297 K to cool after being heated to a uniform temperature of 420 K, how long will it take for the rubber surface to reach 320 K? The dimensions of the block are 0.6 m high by 0.3 m long by 0.45 m wide. The block sits on one of the 0.3-m by 0.45-m bases; the adjacent surface may be considered an insulator. The effective heat-transfer coefficient at all exposed surface is 6.0 W/m² · K. What will the maximum temperature within the rubber block be at this time?

Data From Problem 18-12

A type-304 stainless steel billet, 6 in. in diameter, is passing through a 20-ft-long heat-treating furnace. The initial billet temperature is 200°F, and it must be raised to a minimum temperature of 1500°F before working. The heat-transfer coefficient between the furnace gases and the billet surface is 15 Btu/h ft² °F, and the furnace gases are at 2300°F. At what minimum velocity must the billet travel through the furnace to satisfy these conditions?