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Cold-starting coreless induction melting furnaces.

AFS A distributed file system for large, widely dispersed Unix and Windows networks from Transarc Corporation, now part of IBM. It is noted for its ease of administration and expandability and stems from Carnegie-Mellon's Andrew File System.

AFS - Andrew File System
 Division 8C Coreless Melting Committee, Schaumburg, Illinois Schaumburg is a village in Cook County and DuPage County, Illinois. As of the 2000 census, the village had a total population of 75,386. As of 2005, the population slightly dropped to 72,690 according to the Census Bureau.  

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 refractory

Material that is not deformed or damaged by high temperatures, used to make crucibles, incinerators, insulation, and furnaces, particularly metallurgical furnaces.
 walls. Occasionally, a metalcaster is compelled (by economic necessity or other reason) to curtail cur·tail  
tr.v. cur·tailed, cur·tail·ing, cur·tails
To cut short or reduce. See Synonyms at shorten.



[Middle English curtailen, to restrict
 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 shut·down  
n.
A cessation of operations or activity, as at a factory.


shutdown
Noun

the closing of a factory, shop, or other business

Verb

shut down
 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 An induction furnace is an electrical furnace in which the heat is applied by induction heating of a conductive medium (usually a metal) in a crucible around which water-cooled magnetic coils are wound.  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 See inductor.
Induction coil

A device for producing a high-voltage alternating current or high-voltage pulses from a low-voltage direct current. The largest modern use of the induction coil is in the ignition system of internal combustion engines, such as
 under varying conditions.

Line frequency systems require starter blocks when cold-starting. When power is applied and the temperature of the starter blocks is raised, their magnetic and electrical properties are affected. As a consequence of line frequency design characteristics, the furnace coil is incapable of drawing full power from the power supply, and a dramatic and pronounced "power slump" can occur. Until the furnace is at least half to two thirds full of completely liquefied metal, furnace power can be increased only incrementally over an extended period of time (the duration is dependent upon the furnace capacity and metal type). The result is an extended cold-start cycle that demands constant attention from the melt operator (power tuning, tap changing, etc.). As a rule of thumb, it is rarely advantageous to shut a line frequency heel melting furnace down and subject it to a cold-start unless it is scheduled to be non-productive for at least four consecutive days. Since circumstances vary from facility to facility, the determination should be made after carefully evaluating the economic impacts of a shutdown, including kilowatt hours Kil´o`watt` hour

1. (Elec.) A unit of work or energy equal to that done by one kilowatt acting for one hour; - approximately equal to 1.34 horse-power hour.

Noun 1.
 required to hold the furnaces at low power over an extended period of time versus the electrical, manpower and other costs associated with an extended cold start.

[FIGURE 1 OMITTED]

The more modern medium and high frequency batch melt systems are designed with solid state inverter (1) A logic gate that converts the input to the opposite state for output. If the input is true, the output is false, and vice versa. An inverter performs the Boolean logic NOT operation.

(2) A circuit that converts DC current into AC current. Contrast with rectifier.
 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 the refractory be completely re-sintered. However, it does require that the refractory be re-heated at an initial rate sufficient to thermally close and seal all surface cracks. It is not wise to apply plastic or refractory patch to cracks and consider them closed. Cracks must be closed and sealed by thermal expansion thermal expansion

Increase in volume of a material as its temperature is increased, usually expressed as a fractional change in dimensions per unit temperature change.
 prior to the creation of or introduction of molten metal. In general, the same procedures should be used on a cold start as those employed in a full sinter sinter

Mineral deposit with a porous or vesicular texture (having small cavities). Siliceous sinter is a deposit of opaline or amorphous silica that occurs as an incrustation around hot springs and geysers and sometimes forms conical mounds (geyser cones) or terraces.
 up to the point that the furnace temperature reaches 1,600F and is held there for a one to two hour soak. Thermocouples must be used for accurate measurement of temperature. After a cold-start, it also is recommended that the lining be treated as if it were a newly-sintered lining (i.e. keep carbon in the bath as low as possible and silicon as high as possible at least during the first few heats). This can be accomplished by using gray iron returns as much as possible in the initial heats. Additional information on good refractory practices under initial sintering sintering, process of forming objects from a metal powder by heating the powder at a temperature below its melting point. In the production of small metal objects it is often not practical to cast them.  and cold-start conditions can be obtained through the refractory suppliers and manufacturers.

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 a. 1. In devices generating heat, such as gasoline-engine motor vehicles, the cooling of the device by increasing its radiating surface by means of ribs or radiators, and placing it so that it is exposed to a current of air. Cf. Water cooling. : It is good practice to force-cool the furnace to ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade.  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 reseal
Verb

to close or secure tightly again

Verb 1. reseal - seal again; "reseal the bottle after using the medicine"
seal, seal off - make tight; secure against leakage; "seal the windows"
 at restart. Force cooling should be done through the use of dedicated high pressure, high volume, centrifugal centrifugal /cen·trif·u·gal/ (sen-trif´ah-gal) efferent (1).

cen·trif·u·gal
adj.
1. Moving or directed away from a center or axis.

2.
 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 temperature gradient
n.
The rate of change of temperature with displacement in a given direction from a given reference point.



temperature gradient 
 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 frit (frit) imperfectly fused material used as a basis for making glass and in the formation of porcelain teeth.
frit (frit),
n
 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 inlet /in·let/ (-let) a means or route of entrance.

pelvic inlet  the upper limit of the pelvic cavity.

thoracic inlet  the elliptical opening at the summit of the thorax.
 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 dew point: see dew.  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 preset Cardiac pacing A parameter of a pacemaker that is programmed permanently when manufactured  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 Editor's Note (foaled in 1993 in Kentucky) is an American thoroughbred Stallion racehorse. He was sired by 1992 U.S. Champion 2 YO Colt Forty Niner, who in turn was a son of Champion sire Mr. Prospector and out of the mare, Beware Of The Cat.

Trained by D.
: 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.
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Title Annotation:CAST TIP--Technical Information and Practice
Publication:Modern Casting
Date:Aug 1, 2009
Words:1501
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