There are two basic types of induction melting furnaces that have been in use since the early s. First came channel furnaces, operating with mains frequency, and then many foundries switched to smaller coreless furnaces, for which a range of frequencies (from medium to high) were used. Channel furnaces were originally developed to melt brass but now are commonly used for copper, bronzes, zinc, aluminum and iron. Coreless furnaces are commonly used for virtually all metals.
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Channel furnaces produce heat for melting by utilizing the principle of the short-circuited iron core transformer. Alternating current is passed through a multiple-turn primary coil surrounding an iron core. The secondary is a loop of molten metal surrounding the primary and the core. Current flowing through the primary induces a much larger current in the metal loop, whose resistance creates heat for melting.
Core-Type Channel ' Channel furnaces are the most efficient type of induction furnace because their iron core concentrates magnetic flux in the molten loop, ensuring maximum power transfer from primary to secondary. Efficiency in the use of power can be as high as 95 to 98%.
Channel furnaces are most effective for high-production operations that need: a large throughput of metal; continuous and multi-shift casting schedules; few alloy changes; holding capacity for shift melting operations to take advantage of off-peak energy savings; and power demand limiting. Channel furnaces are an economical way to hold a large quantity of molten metal for an extended period of time to provide a continuous supply of ready-to-pour metal, seven days a week. Melting can take place during off-peak energy demand periods in a coreless furnace and then the molten metal can be stored in the channel furnace and tapped for production during on-peak hours.
However, an essential loop of metal must be maintained continuously in a channel furnace. If allowed to freeze, extreme care is necessary in remelting because the loop may rupture and disrupt the circuit. This could require extensive work to restore the loop, or the furnace may need to be disassembled completely and relined. Consequently, channel furnaces rarely are permitted to cool, which makes alloy changes difficult because a heel of molten metal always is required.
Another consideration is the need to keep the relatively narrow melting channels as clean as possible. Slag may build up on the channel walls, restricting the free flow of metal and cause overheating.
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Operating characteristics of this kind of furnace are constant, and refractory lining life is good because of the constant temperature. Depending on the metal being melted, uppercase lining life may extend several years and for millions of pounds of molten metal, with proper maintenance. Channel furnaces are ideally suited to melting a single alloy as a holding vessel or as a pressure-pour furnace.
Coreless-Type ' A coreless induction furnace operates at low, intermediate and high frequencies ' from 60 to 10,000 cycles/second ' and is unmatched for flexibility. It also operates by transformer action, but in this case it is similar to an air-core transformer. The primary coil induces current directly in the metal charge, which is contained in an unobstructed crucible or refractory lining. There is no internal melting loop or channel. The primary winding is a water-cooled copper coil surrounding the outside of the crucible or refractory lining.
The coreless furnace can be started cold and usually is poured empty. This greatly simplifies alloy changes and makes it possible to shut down as desired.
Linings of coreless furnaces may be as durable as core-type furnace linings in applications where a molten charge is maintained 24 hours a day. Because of alloy changes or analysis control, however, batch melting is the usual practice and the recurring heating and cooling results in less lining life. On the other hand, these linings are considerably less expensive and can be changed quickly.
Because there is no flux-concentrating core as on core-type furnaces, the efficiency of the coreless furnace usually is lower than that of core-type furnaces. Normal efficiency is approximately 75%. Smaller coreless furnaces are designed to operate at higher frequencies in order to maintain good efficiency and proper stirring action.
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