Cold-starting coreless induction melting furnaces.
Induction melting furnaces are intended for continuous and consistent use. The retained heat from continued use improves the melting time for each heat and improves the life of the refractory walls. Occasionally, a metalcaster is compelled (by economic necessity or other reason) to curtail production operations to such a degree that melting and pouring are inconsistent, such as three and four day per week melt schedules. Under these circumstances, metalcasting personnel will need to determine whether it makes economic sense to leave the furnaces filled with molten metal (with power applied) during the extended non-productive periods, or to empty the furnace and turn the power off. The principal factors that must enter into any decision to idle furnaces and then restart them from a cold condition include the type of furnace technology employed, the effect on furnace refractory and the establishment of shutdown procedures. Any decision to idle a furnace and restart from the cold condition should be made after consulting the appropriate furnace manufacturer and refractory supplier.
Technical Information and Practice
Furnace Technology. When considering whether or not to shut a coreless induction furnace down for any length of time, consideration first should be given to the age and vintage of the melt system technology. Two distinct types of furnace technologies are in wide use today: line frequency (heel melt) applications and medium to high frequency (batch melt) applications. With rare exceptions, any furnace operating at 200 Hz or below is considered to be a line frequency application. Those operating above 200 Hz are considered medium and high frequency. The fundamental operational difference between the two is the rate at which each applies power to the induction coil under varying conditions.
Line frequency systems require starter blocks when cold-starting.When power is applied and the temperature of the starter blocks israised, their magnetic and electrical properties are affected. As aconsequence of line frequency design characteristics, the furnace coilis incapable of drawing full power from the power supply, and a dramaticand pronounced "power slump" can occur. Until the furnace isat least half to two thirds full of completely liquefied metal, furnacepower can be increased only incrementally over an extended period oftime (the duration is dependent upon the furnace capacity and metaltype). The result is an extended cold-start cycle that demands constantattention from the melt operator (power tuning, tap changing, etc.). Asa rule of thumb, it is rarely advantageous to shut a line frequency heelmelting furnace down and subject it to a cold-start unless it isscheduled to be non-productive for at least four consecutive days. Sincecircumstances vary from facility to facility, the determination shouldbe made after carefully evaluating the economic impacts of a shutdown,including kilowatt hours required to hold the furnaces at low power overan extended period of time versus the electrical, manpower and othercosts associated with an extended cold start.
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The more modern medium and high frequency batch melt systems are designed with solid state inverter technology that allows the coil to draw full power from the power supply from the moment it is turned on until the bath is completely liquefied. (Due to refractory considerations, full power should never be applied to a furnace that has a cold lining). These systems do not require the use of starter blocks and can be restarted from a cold condition using high density returns or a standard charge makeup. Alternately, a controlled gas-fired torch can be used to raise furnace and refractory temperature prior to the introduction of either a cold or molten charge, after which full furnace power can be applied. Medium and high frequency furnaces are good candidates to be completely shut down and restarted almost without regard to the length of time they are expected to be down.
Furnace Refractory. The central issue in restarting a furnace from a cold-start condition is the preservation of the existing refractory lining. Furnace refractories prefer a steady state environment and do not react well to the stresses of thermal cycling. An improperly conducted cold-start can easily result in damage to the existing lining such that any savings generated by shutting a furnace down will be more than offset by the collateral costs of the subsequent restart.
Cold-starting with an existing lining does not require that therefractory be completely re-sintered. However, it does require that therefractory be re-heated at an initial rate sufficient to thermally closeand seal all surface cracks. It is not wise to apply plastic orrefractory patch to cracks and consider them closed. Cracks must beclosed and sealed by thermal expansion prior to the creation of orintroduction of molten metal. In general, the same procedures should beused on a cold start as those employed in a full sinter up to the pointthat the furnace temperature reaches 1,600F and is held there for a oneto two hour soak. Thermocouples must be used for accurate measurement oftemperature. After a cold-start, it also is recommended that the liningbe treated as if it were a newly-sintered lining (i.e. keep carbon inthe bath as low as possible and silicon as high as possible at leastduring the first few heats). This can be accomplished by using gray ironreturns as much as possible in the initial heats. Additional informationon good refractory practices under initial sintering and cold-startconditions can be obtained through the refractory suppliers andmanufacturers.
Shutdown Procedures. Regardless of whether a furnace is operating as a batch melt or heel melt application, two steps need to be taken when metal production ceases and the furnace is emptied:
1. Slag Removal: All slag adhering to the sidewalls of the refractory should be dislodged and removed prior to allowing the furnace to cool. During the restart, this will reduce the chance of re-melted slag coming into contact with the refractory hot face before it seals the surface cracks and preventing the formation of metal fins. If wall cleaning is impractical, it may be worthwhile to use a de-slagging flux in the final heats prior to shutdown.
2. Forced Air Cooling: It is good practice to force-cool the furnace to ambient temperature rather than allow it to cool naturally. Force cooling may result in a series of random cracks as the lining contracts, but these will be smaller and relatively easy to reseal at restart. Force cooling should be done through the use of dedicated high pressure, high volume, centrifugal blowers aimed directly into the furnace cavity. The free-standing circulating fans typically seen on melt decks and used primarily to provide cooling air for employees will not be sufficient (Fig. 1).
Other Considerations. Remember the following three things when performing a cold-start:
1. Slip Plane Material: Allowances must be made for the thermal expansion and contraction of the lining under the extreme temperature gradients that cold-starting creates. The use of fiber or other textile-based slip plane materials should be avoided, since they tend to retain the lining in place and act to bind the lining rather than facilitate expansion. The slip plane is not intended as an insulating medium but to permit the retention of refractory frit at the cold face of the lining and encourage free movement of the lining along the slip plane interface. This requires the use of a dedicated (mica-based) slip plane material, which should be readily available from a multitude of sources.
2. Cooling Water: The temperature of the cooling water at the inlet should not be allowed to drop too low even when the furnace is empty and cold. If the cooling water temperature drops below the ambient dew point at any time during which the furnace is empty or at any time during the cold-start, condensation will form on the coil tubing and introduce moisture into the coil refractory, resulting in a high ground fault potential, which will delaying the restart or create electrical arcs between the coil turns.
3. Extended Low Power Hold: An alternative to cold-starting a furnace is an extended hold at low power. When the final heat has been emptied from the furnace, a cold charge is introduced into the furnace to its fill point. At the same time, furnace power is reduced to a point sufficient to maintain and hold the furnace temperature at 1,200F indefinitely. At restart, a PLC-controlled interface is used to increase the temperature and reduce the charge to the molten bath over a five to six hour preset schedule. Although this technique depends heavily on the PLC logic employed, in addition to well-trained and reliable melt operators, it may be a viable option for users of medium frequency batch melting equipment.
Editor's Note: The following is the first in a new series of technical columns. The "Cast TIP" (Technical Information and Practice) columns will offer step by step instructions on how to perform tasks around the metalcasting facility.