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Show The furnace shell is made up of a steel framework and panels. The hearth or the product support rails, or both, can be made of refractory or metallic material. When metallic materials are used, a water-cooled system must be employed to minimize thermal collapse of the skid rail and support sys tem. Reheat furnaces may be fired by fossil fuels or powered with electricity. Within the fossilfueled category, which are typically gas-fired, there are two major subcategories of furnaces: batch and continuous. In batch furnaces, the steel workpieces remain in a fixed position on the hearth during the entire reheating process. Batch furnaces are typically found in old steel plants, or in structural mills or specialty applications where odd shapes or small workloads are reheated. In continuous furnaces, the steel workpieces move through the furnace as they are heated. Continuous furnaces include pushers, walking beams, rotary hearths, walking hearths, and roller hearths, most of which are pushers and walking beams. Electric-powered reheat furnaces can be subdivided into batch and continuous furnaces. The steel industry, however, generally reserves the terminology "cont inuous furnace" for foss i l-fue led cont inuous furnaces. Electric-powered batch furnaces are only used in metal heat treating applications, and are thus not covered in this study. Electricpowered continuous furnaces are represented by electric induction furnaces, which reheat steel by inducing a current with an oscillating magnetic field. Electric induction technologies hold only a small portion of the u.s. reheat market, but they have the potential to become a major competitor of gas reheat technologies in the future. The major reheat technologies by production capacity, capital cost, and operating efficiency are shown in Table 1. Batch furnaces are generally used to heat small and non-uniform charges as opposed to continuous and induction furnaces, which are designed to heat large, more uniform charges. Batch furnaces typically heat 5-30 tph cold charged. The capacity of continuous furnaces ranges between 25-450 tph, while induction furnaces range from 5-150 tph for single units, and 30-600 tph for mUltiple units in one process line. Fossil-fired continuous furnaces tend to have the lowest capital cost per ton of design capacity, followed by batch furnaces and induction furnaces. However, induction furnaces generally have the lowest material loss from scaling and the highest operating efficiency since there is no heat lost through exhaust gas as in fossil-fueled furnaces. This efficiency drops off sharply, though, if offdesign shapes are heated. The range of efficiencies for continuous, batch, and induction furnaces can vary as much as 60 percent, depending on factors such as application, size, operating schedules, and amount of heat recovery employed. Overall, batch furnaces typically have the lowest operating efficiency because of intermittent operation, low production rates, and high exhaust temperatures. CURRENT STEEL REHEAT MARKET Estimated steel reheat market shares by furnace type, based on furnace population, tons of steel reheated, and energy consumption, are shown in Table 2. These estimates are derived from the total number of furnaces installed and in operation and from data supplied by users of steel reheat furnaces. Fossil-fueled batch furnaces are the most prevalent market group based on furnace population, accounting for approximately 475 units. The number of fossil fuel-fired continuous furnaces is estimated at roughly 340 units, with an electric induction furnace count of approximately 15 units. Table 1 - Comparison of Current Steel Reheat Technologies Production capacity (tons/hour) Furnace type Typical Fossil-fueled: Walking beam 350 Pusher 350 Batch 20 Electric induction 125 1Des ign production range. 2Includes heat recovery. 3Single-unit. 4Mu l t iple-unit. Rangel 50-450 25-400 5-30 5-1503 30-6004 Capital cost ($OOO/ton design caEacity) Typical Range 35 30-75 29 25-60 65 40-150 42 25-65 296 Operating Energy use efficiency (MMBtu/ton) (percent) Typical Typical Typical Typical design operation design2 operation 1.8 1. 6-2.8 40 25-45 1.7 1.6-2.8 41 25-45 2.7 2.0-4.7 26 15-35 1.1 1.0-2.0 64 10-75 |
