| OCR Text |
Show Continuous heat treating furnaces are commonly used to process high part volumes, to maintain a constant material flow (which facilitates automation and integration with other process equipment), and to consistently heat treat parts to a known quality. The major types of continuous heat treating furnaces are: • Rotary hearth • Pusher • Roller hearth • Conveyor • Continuous strand. In general, continuous furnaces considerably reduce the labor requirements for handling workpieces as compared with batch furnaces. However, they are typically not as flexible as the batch design. Table 1 shows the number of ferrous heat treating furnaces in operation in the United States in 1985, segmented by heat treating process and furnace type. This table shows a total of roughly 52,590 ferrous heat treating furnaces, comprised of 9,965 continuous furnaces and 42,625 batch furnaces. Almost 75 percent of these heat treating furnaces are employed in the machinery indus t ry (including automotive products). The most common heat treating operations are annealing and normal-izing, followed by through hardening and tempering/ stress relieving. Table 2 breaks down the heat treating equipment market by energy type. Natural gas is the predominant energy source, accounting for approximately 95.6 trillion Btu or 80 percent of the total ferrous heat treating energy consumption of 120.6 trillion Btu. Electricity, mainly employed for induction hardening in the machinery industry and for annealing and normalizing in the foundry industry, accounts for 20.5 trillion Btu, or roughly 16 percent of the total ferrous heat treating energy consumption. In 1985, fuel oil accounted for a relatively minor portion of the ferrous metal heat treating market, with only 4.5 trillion Btu or 4 percent of the total energy consumption. Fossil fuel-fired furnaces, and more specifically natural gas-fired furnaces, offer several advantages over electric-based technologies. The capital cost of a gas-fired furnace is usually much less than that of a comparably-sized electric induc t ion or res is tance furnace. For example, an induction furnace for through heating applications will cost 2-3 times as much as a comparable gasfired furnace without a recuperator. With recuperator costs included, a gas-fired furnace would still be roughly half the cost of an induction furnace. The capital costs for such an induction Table 1 - Heat Treating Equipment Population Annea ling, Through hardening, Tempering, drawing, noma liz ing, Surface Case Patenting, Industr~ Furnace type heat and quench stress re lieving spheroidizing hardening hardening wire treatment Steel Box 40 20 65 Car bottom 40 10 345 Bell, hood, tip-up 1,990 Vertical pit 45 20 Vacuum 10 Lead pot (cont i nuous ) 55 Roller hearth 40 30 160 Barrel-roller 15 15 20 Continuous s trip line 130 Cont inuous induction Conveyor 25 Forging/ ext rus ion Car bottom 2,200 Hood, tip-up 500 Induct ion 360 Pusher 500 Conveyor 900 Povder metals Box 500 Conveyor 260 Foundry Box 1,300 Car bottom 1,700 pit 700 Pusher 1,500 Machinery Box 4,000 4,000 1,000 2,500 Car bottom 100 Vert ical pit 1,000 3,000 2,400 Vacuum 2,500 Fluidized bed 200 Salt-bath 3,000 4,700 Roller hearth 100 Pusher 2,800 500 700 Conveyor 500 1,000 500 Rotary hearth, shaker hearth 100 100 Ion nitriding 60 Induction 4,200 Flame heads (hardening) 120 Total furnaces: 52,590 255 |
