Continuous Furnace Heating Capacity Practice

Capacities for steel heating furnaces are based on uninterrupted operation throughout the work week. (Delays in the mill or forge shop reduce the weight of steel heated in the furnace, but do not reduce the heating capacity of the furnace.) 

Example 4.1 Determine the size needed for a three-zone 1200 C, top-fired-only walking hearth furnace with half the furnace using enhanced heating for 100 tph of 127 mm x 127 mm x 6.71 m (5 x 5 x 22 ) steel billets. 

Heat Transfer by Hot Gas Movement. (See also chap. 7.) An axiomatic thought that must be reviewed when calculating heat transfer in furnaces is High-temperature areas must be provided with constant source of a high-temperature gas or solids radiation from refractories for equilibrium conditions to be maintained. For example, for hot walls, roof, and hearth to sustain heat transfer between themselves and the load pieces, hot gases must provide a constant supply of gas radiation or convection to the hot refractory otherwise, their temperature will fall to some lesser temperature and the heat transfer rate to the loads will be reduced. 

Another case is the gas movement or lack of movement of hot gases between product. With the movement of hot gases between product (e.g., rounds on a rotary hearth on 1.6 to 2.0 space [centerline of product to the adjacent centerline of product 

The gases flowing between and around the product can be at much higher momentum than furnace chamber gases on the top furnace, thereby increasing convection transfer from 5 to 7% of the total heat transfer at that position in the furnace. 

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A continuous furnace may be heated so that the temperature of its zones is practically the same across the furnace. This temperature uniformity can be obtained by lengthwise firing in several zones (as illustrated by fig. 4.2), or by roof firing or side firing in several zones (as shown in fig. 4.3). In such furnaces, the heating capacity of a continuous furnace will equal or exceed the capacity of a same-size batch furnace. 

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