Hearths, Skid Pipes, Hangers, Anchors

Warning When using burners for dryout or warm-up operations, do not skimp on adequate flame safety and programming just because you think this is a temporary operation. Too many new furnaces have never produced a nickel because of start-up explosions. The most dangerous times for furnaces are, as with airplanes, (pardon the expression) during takeoff and landing. 

Warm-up times can be considerably less than dryout times, if no moisture needs to be driven off. Some warm-up time is important even for previously dried-out furnaces to minimize refractory spalling because of too-rapid or uneven thermal expansion of the dry, solid refractory. 

In continuous furnaces, cast or wrought heat-resisting alloys are used for skids, hearth plates, walking beam structures, roller, and chain conveyors. In most furnaces, the loads to be heated rest on the hearth, on piers to space them above the hearth, or on skids or a conveyor to enable movement through the furnace. The furnace interior 

To protect the foundation and to prevent softening of the hearth, open spaces are frequently provided under the hearth for air circulation—a ventilated hearth. Natural convection cooling of these spaces under a furnace is really not very effective— unless some forced flow cooling air is provided. Actually, a solid contact between furnace bottom and the earth may be better than still air cooling. If, however, the hearth is so hot that conducted heat might damage the furnace foundation, forced underside ventilation is necessary. 

In modern practice, hearth life is often extended by burying stainless-steel rails up to the ball of the rail to support the loads. The rail transmits the weight of the load 3 to 5 in. (0.07 to 0.13 m) into the hearth refractories. At that depth, the refractories are not subjected to the hot furnace gases that, over time, soften the hearth surface refractories. The grades of stainless rail used for this service usually contain 22 to 24% chromium and 20% nickel for near-maximum strength and low corrosion rates at hearth temperatures. With stainless-steel rails imbedded in a hearth, the hearth life can be extended by a factor of 1.5 to 3 times. Attempts to use other imbedding material have not been successful. 

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