No. 939 is a 2000°F gas-heated box furnace from Grieve used for preheating metals prior to forging. Workspace dimensions measure 72 inches wide x 36 inches deep x 48 inches high. Two modulating 700,000-BTU natural gas burners with a floor-mounted combustion air blower heat the unit. This furnace features 7-inch insulated walls made with 5 inches of 2300°F ceramic fiber and 2 inches of 1700°F ceramic fiber. The 7-inch floor is comprised of 4.5 inches of 2300°F firebrick and 2.5 inches of 1900°F block insulation. A firebrick plate hearth is supported by firebrick piers, and the unit has a manually operated vertical lift door. No. 939 also includes a digital indicating temperature controller, manual reset excess temperature controller with separate contactors and a fused disconnect switch.