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Batch Induction MeltingThe Science and Technology
By John H. Mortimer, RE.
InductothermRancocas, New Jersey, USA
A compilation and update of Mr. Mortimer's papers andarticles prepared for the American Foundry Society,The Centre Francais de 1'Electricite the Australasian
Foundry Institute, Foundry Management & Technologymagazine and Modern Casting magazine
INDUCTOTHERMInductotherm Corporation
Batch Induction MeltingThe Science and Technology
By John H. Mortimer, P.E.Inductotherm, Rancocas, NJ
Introduction
The last quarter of the past century saw the develop-ment of high reliability, solid state power supplies forcoreless induction melters, with inverters capable ofdeveloping and maintaining maximum power through-out the melt cycle. The efficiency of these units in-creased from the 85 percent level of the 1970s to 97percent today. Now, medium frequency power sup-plies of up to 16500 kW at 200 Hz are in operation.
The development of flexible, constant power track-ing, medium frequency induction power supplies hasresulted in the widespread use of the batch meltingmethod in modern foundries. Research has shown thatthis method substantially increases overall melting ef-ficiency and furnace production at loweroperating and fixed costs.
Batch melting
First, batch melting is defined as a pro-cess where the furnace volume is pouredempty after the melt has reached theproper temperature and successive meltsare started using ordinary unheated orpreheated solid charge materials.
Although this type of operation was typi-cal of the small to medium-sized systems,either their inherent design characteris-tics or the lack of reliable power compo-nents prohibited their manufacture at highpower levels until recent years. As a re-sult, virtually all of the large productionrequirements were handled by the lowfrequency 50 or 60 Hz heel melting units.
Heel melting is a process where moltenmetal is held in the furnace after tapping.Typically, this heel of metal is between60-80 percent of the furnace capacity.
Although the best choice of equipment available atthe time of their manufacture, these low frequencysystems imposed several significant operating limita-tions on the foundry, such as:
• The necessity of keeping a molten heel in thefurnace between taps which meant that, in mostcases, charge drying or preheating equipment hadto be employed to reduce the risk of wet materi-als becoming submerged below the melt line.
• The need to accept a larger-than-necessary fur-nace melt volume just to reduce the turbulent, lowfrequency molten metal stirring activity rather thanselecting the melt volume best suited for thefoundry requirement.
• The extra cost for electric energy needed to notonly keep the molten heel at liquidous tempera-tures during the off-shift time, but also tocompensate for the greater heat losses of the largerfurnace area.
• The higher maintenance and component costs as-sociated with and required for the larger furnace.
• The extra cost to cast starter blocks to help re-duce the long, cold lining melting time which ischaracteristic of the heel melting design.
FIGURE I. Low frequency induction melting systems, typicallyoperating at the mains frequency of 60 Hz, required a molten heeloccupying 60 to 80 percent of the furnace capacity. When emptiedfor a lining change or maintanence, restarting required either amolten charge or cast starter blocks.
The first technological advance necessary to makehigh-volume batch melting a practical reality was thedevelopment of powerful, reliable, solid state invert-ers for coreless induction furnaces. Line frequency(60 Hz) induction power systems that required a mol-ten heel for melting could not be used for the batchmelting process. The early spark gap and motor gen-erator units, first used to achieve frequencies above60 Hz, were less than 75 percent efficient and couldnot achieve the high power levels needed for high-volume batch melting. Later systems, which multi-plied line voltage magnetically, and the first solidstate power supplies, were much more efficient, butwere still too small to make high-volume batch melt-ing practical.
Early coreless induction batch melting equipmentwas best suited to small steel and investment castingfoundries. Furnace size wasn't the obstacle to greaterproduction, applying power to the furnace was. Inearly solid state induction power supplies, power den-sities approaching 400 kW per metric ton were avail-able in units able to track furnace loads and supplyfull power throughout the melting process. Powersupplies of 500 kW were considered quite large.
The subsequent development of powerful, reliable,solid state inverters incorporating "hockey puck" de-sign SCR devices made high-volume batch meltingnot only practical, but the most efficient way for manyfoundries to produce metal. (Fig. 2) Now, mediumfrequency power supplies providing 700 k W per met-ric ton are common. Units able to apply 1000 kWper metric ton are achievable with existing technol-ogy. For Inductotherm alone there are well in excessof 500 induction batch melting installations world-wide boasting power units of 2000 kW or larger. Thereis even a solid state unit providing 42000 kW of in-duction power. It is used at Geneva Steel in Utah forheating massive steel slabs.
Modern induction power units achieve electrical ef-ficiency levels exceeding 97 percent. It is not likelythat tomorrow's equipment will show further signifi-cant increases in electrical efficiency. What will provemeaningful to foundrymen, however, will be new lev-els of equipment reliability.'
New power supplies shrug off faults
While the reliability of even early solid state powersupplies was high, particularly in units engineered witha substantial safety margin in component ratings, ab-normally large voltage spikes on the incoming powerlines, shorts caused by accidental shorting of the busbars or furnace problems could cause components tofail, fhis interrupted production and often requiredcostly repairs.
Now envision a highly advanced induction power sup-ply with the ability to absorb faults and keep on run-ning. Such units are available now from Inductotherm.
These new units use special energy absorbing tech-nology to protect an induction power unit's electroniccomponents. These extremely reliable units simplyshut down in response to electrical faults, without dam-age to any component parts. Restarting these units isas simple as resetting the system (in the absence ofthe condition that caused the fault in the first place, ofcourse). With power fully restored, production canresume immediately. (Fig. 3)
FIGURE 2. Silicon controlled rectifier (SCR)
FIGURE 3. Before leaving the factory, Inductothermtests every power supply by creating a dead shortacross the bus bars while running the unit at fullpower. To pass this rigorous test, the unit has to ab-sorb this short without damage and be able to re-sume full operation when reset.
Stirring
In the early thinking about the viability of high vol-ume batch melting, concerns about furnace stirringled many to believe that the high power densities re-quired would produce too much stirring in the fur-nace.
Furnace stirring was one of the least understood ar-eas in the coreless induction furnace. In the mid-1970sresearch2 was undertaken at Inductotherm regardingcoreless furnace stirring. Up until that time it had beenthought that inductive stirring in an induction furnacewas related linearly to the meniscus height in the fur-nace. This relationship is described in the followingformula and illustration (Fig. 4).
MR =7050 xkW
DxHxSGxJ(pxf)
Where MH — meniscus height (inches)
D = diameter of the melt (inches)
H = furnace metal height (inches)
SG = specific gravity of metal
p = metal resistivity (microhms-cm)
f frequency in hertz
Meniscus height varies directly with kilowatts andinversely with the square root of frequency. How-ever, it became apparent from actual melting opera-tions that this simplistic approach was not an accu-
rate measure of stirring. Furnaces containing the sameamount of the same metal, but running at differentfrequencies, did not stir the same, even though themeniscus heights were identical. It was found thatmeniscus height is caused by the interaction of themagnetic field from the induction coil and the cur-rent that is flowing in the molten metal. This force isequal to the vector product of the magnetic flux den-sity multiplied by the current density of the melt (Bx J). Current flows in the surface of the melt to adepth determined by the frequency of the currentflowing in the induction coil and the metal type. Thisis called the "depth of penetration" and is describedby the following equation:
d = 2 inches
Where d = depth of current penetration (inches)
p = metal resistivity (microhms-cm)
f = frequency in hertz
ц — permeability (magnetic property)
This force acting on the surface of the metal at thetop of the melt opposes gravity and causes the for-mation of the meniscus. As both В and J are propor-tional to the current flowing through the coil, themeniscus height is proportional to the current flow-ing through the coil squared. As kW = PR where Ris the resistance of the coil and the melt, the menis-cus height is proportional to the kilowatts applied tothe furnace and inversely proportional to the resis-tance of the furnace coil and the melt. Meniscusheight (MH) represents the potential energy of themelt in the same way as the height of water in a res-ervoir (WH) represents the potential water energypressure in that reservoir. (Fig. 5)
FIGURE 4. Meniscus height to furnace diameter.FIGURE 5. Meniscus height compared to waterheight.
Figure 6. Depth ofcurrent penetration.
In the furnace, the flow of metal is accelerated onlywhen current is flowing in the melt. Thus, the accel-erated flow only occurs in the region defined as thedepth of current penetration. This depth of penetra-tion is equated to the size of a pipe connected to areservoir. A large depth of current penetration wouldbe a large pipe and a very small depth of current pen-etration, a very small pipe. (Fig. 6)
Obviously, for the same meniscus height (pressure ofwater available), the larger the depth of current pen-etration (the larger the diameter of pipe), the greaterthe flow (of water).
To carry this analogy further, if you considered thesepipes as hoses feeding into a swimming pool, the sizeof the swimming pool would be related to the size ofthe furnace. (Fig. 7) Thus, a very small hose beingplaced in the swimming pool, like a small depth ofpenetration with a given size furnace, would result invery light stirring. However, a large fire hose beingplaced inside a swimming pool, like a large depth ofpenetration for a given meniscus and furnace size,would obviously result in very high stirring.
When you do the math on this process, you find that
stirring is not linearly proportional to the meniscusheight, but is much more dependent on the frequencyitself. The following formula predicts the level of stir-ring in a given system using factors that include power,
FIGURE 7. Light and heavy stirring.
STIRRING INDEX CHART
Stirring Index = 80 to 120 Heavy to violent stirring
Stirring Index = 55 to 80 Heavy stirring
Stirring Index = 40 to 55 Medium stirring
Stirring Index = 20 to 40 Light stirring
Stirring Index = 0 to 20 Very light stirring
A stirring index of 40 to 55 is ideal for iron (gray, ductile iron and borings).A stirring index of 55 to 80 is preferred for aluminum (UBC, scalpings andchips).
FIGURE 7a. TheStirring Index num-ber provides a goodindication of the de-gree of stirring thatcan be expected in agiven system.
frequency, furnace size and the alloy being melted3:
60,000 xSI = \
kW x DSG x p x f
Where
SI = stirring index (see Fig. 7a)
kW = kilowatts
D = furnace diameter in inches
SG = specific gravity of the bath
p metal resistivity (microhms-cm)
A = (71 D2) / 4
/= frequency
Stirring examples
Iron
Iron foundries normally want a medium amount ofbath stirring to properly mix in additives and providea homogeneous alloy.
One foundry melting ductile base iron achieved its
desired level of stirring with a 9000 kW inductionmelter powering a 12.5 metric ton furnace. This sys-tem operated at 210 Hz with a medium stirring indexof 42.3.
A larger foundry melting gray iron ran its 16500 kW,20 metric ton induction melting system at 180 Hz toachieve a medium stirring index of 47.9.
Aluminum (stirrers and melters)
Aluminum melting demands a higher level of stirring.One aluminum alloy producer achieved its requiredstirring with a 300 kW induction power supply run-ning a .8 metric ton furnace. Operating at 60 Hz, itproduced a very heavy stirring index of 117.27.
An aluminum alloy producer melting UBCs ran its1500 kW, 7 metric ton aluminum melting system at60 Hz to achieve a heavy stirring index of 75.7.
Steel
Steel foundries typically run their alloys at high tem-peratures and want a low level of stirring to maxi-mize lining life. A steel abrasive plant operated a 1500kW induction power supply and 2.2 metric ton fur-nace at 590 Hz to achieve a light stirring index of22.6.
A steel investment plant melting various steel alloysran its 175 kW, 75 Kg induction melting system at2800 Hz to achieve a light stirring index of 25.7.
7
10,000
FURNACE SIZE vs FREQUENCY FOR CORELESS INDUCTION FURNACES
FURNACE SIZE (kg.)
2.5 5 25 50 250 500 2500 5,000 25,000 50,000Т 'W
T~ I > I 1—I—ГТТ «10,000
50
2.5 5A.
250 500FURNACE SIZE (kg.)
50
2500 5,000 25,000 50,000
FIGURE 8. Frequency selection chart.
Of course, an easier way to determine stirring is touse the chart shown in Fig. 8.4 The research on stir-ring showed that much higher power densities couldbe placed on induction melting furnaces as the fre-quency was increased than had otherwise beenthought. For instance, the stirring in a 6 ton furnacepowered by 1500 kW at 60 Hz would have beenthought to have the same stirring as it would at 3000kW at 250 Hz. In fact, the equivalent stirring occursat 6000 kW at 250 Hz. This means a power density of1000 kW per ton at 250 Hz provides the same stir-ring as 250 kW per ton at 60 Hz. The knowledge ofstirring gained by the 1975 studies was pivotal in thedevelopment of batch melting as it allowed for thehigh power densities required for this process.
Furnace efficiency
In 1980 batch melting was common in small furnaceswith low kilowatt power supplies. Earlier, in 1976,Inductotherm had developed a line of equipment withconstant power draw characteristics throughout themelt. It had been determined that these particular unitswere melting at about a 10% higher melt rate for ironand steel than had been calculated, even allowing forthe unit's full power from start to finish operation.
A research project5 was started in 1980 to determinewhy this was so. The initial thought had been concernover accuracies in the metering system. Furnace effi-ciency studies over a wide variety of metals andfurnace sizes found that the reason for this was not
faulty metering, but was the result of change in theresistance of the melt related to the magnetic proper-ties of iron and most steels.
In its simplest form, and ignoring many significantengineering considerations, the electric efficiency ofa coreless furnace can be represented by the formula:
Melt Resistance-Efficiency^
Melt Resistance + Induction Coil Resistance
If we will assume a resistance of 80 microhms* for afurnace having a molten heel and an effective resis-tance of 20 microhms for the coil, the efficiency wouldbe:
- Efficiency^ 80 80
80 + 20 100
When batch melting, however, the furnace is chargedwith materials which could easily increase the con-tact resistance between the charge pieces to about 200microhms*, raising the batch efficiency as shown inthe above formula to:
_ Efficiency^200 + 20 220
= — = 90% (A 10% increase.)
If magnetic materials are melted, the coil efficiency atthe beginning of the cycle could be as high as 95 per-
*Thc values for resistance used in the formulas were selected to illustrate thecomparative efficiencies and do not represent actual values.
cent because of the shallower depth ofcurrent penetration, the increased mag-netic coupling, as well as the additionof hysteresis losses which are inducedin the charge pieces. As melt tempera-tures rise above the Curie point, themagnetic effect disappears but the con-tact resistance still keeps the efficiencyin the 90 percent range until molten con-ditions are reached.
Whereas a furnace with a fully moltencharge may have an efficiency of around80 percent, the same furnace with amagnetic charge of solid material mayhave an efficiency of 95 percent or moreat the start of the melt. Since batch melt-ing always starts with solid chargematerial, this results in an overall fur-nace efficiency of 88 percent for thebatch melter through a melt vs. 80 per-cent for a heel melter using an 80 percent heel.However, only the system with a constant power drawcould take advantage ofIhese higher efficiencies sinceit alone could deliver full power into the charge fromthe start of the melt, when the furnace was at its high-est efficiency level. This work was finally published
90'
70
Batch Welter
Heel Welter
Time
FIGURE 9. A comparison of the batch melter versus heel meltershows that efficiencies for the batch melter are much higher untilmolten conditions are reached. After that time, efficiencies areessentially the same.
inl9856andl987.7(Fig. 9)
Production increases
The increased production possible from a batch melt-ing system is shown in Fig. 10. These early systemsdemonstrated the practicality of the theoretical work.
FIGURE 10.Comparison of1987 systems. I HOT
1000 kW
Нм1
4 Urn.mm Hot fti§
амнхимтнмимиг
4 Hi». 45 Win, 1 Him. 16 &
1 Hr. 15 Min.
1000 kW508 Hi 1.4 »T
52 Min,
A COMPARISON OF kW FURNACESMELTING GRAY IRON (Pour Temp. • 1SQQ"C)
Tap SizeChargeMethod
Melt TimeIdle Time
CycleUtilization
ProductionComparison*
(kWH/MT)
2500 kW60 Hz 9 MTHeel l/Ieiter
3400 Kg.2x1700 Kg.
Bucket47 Min.8Min.
55 Min.85%
3700 Kg/Hr585
2500 kW200 Hz 6 MTHeel Meiter
3400 Kg.4 x 850 Kg.
Bucket43 Min.10 Min.53 Min.
81%3850 Kg/Hr
540
2500 kW500 Hz 3.4 MTBatch Meiter
3400 Kg.3400 Kg.Feeder39 Min.7 Min.
46 Min.85%
4430 Kg/Hr500
*Assumes 3-shift operation and does not account for auxiliaries such ashydraulic and water cooling/recirculating systems, etc.
FIGURE 11.Comparison of1987 systems.
Two heel melting systems - 1000 kW, 60 Hz 3.6 met-ric ton and 1000 kW, 200 Hz 1.8 metric ton - arecompared with a 1000 kW, 500 Hz, 1.4 metric ton batchmelting furnace. The major advantages of the 500 Hzbatch equipment include substantially faster melt timesand improved production.
Systems of 2500 kW are also compared. (Fig. 11) The50/60 Hz system is a heel melter with a 9 metric tonfurnace. The tap size is 3400 Kg and there are two bucketcharges of 1700 Kg producing 3700 Kg/Hr with a pour-ing temperature of 1500 degrees C. The mediumfrequency heel melter with a 5 metric ton furnace alsois bucket charged but, because of the smaller furnace,smaller buckets and more charges are required. The re-sultant production rate of iron is 3850 Kg/Hr.
The batch melter is continuously charged using a feederwith the charge being preheated by the previous chargebefore melting. The production rate has increased to
4430 Kg/Hr. The energy usage has been reduced bynearly 15 percent due to the increased efficiency andreduced thermal losses of this method.
All three systems described require a large tap size forgood utilization and perfect timing of the ladle. In addi-tion, the heel melters require that the charge bucketalways be ready as there is no charge storage on the meltdeck as with the batch method.
Further production increases can be obtained by addinganother furnace to the same power supply. After the firstfurnace is charged, but still melting, a second charge isprepared on the feeder. When the first furnace is at tem-perature, the melting process starts on the second furnaceby switching over the power supply and continuouslycharging. The first furnace may now be emptied intosmaller ladles over a 15-minute period. This process isnow repeated by charging and melting in the first fur-nace while the second is poured.
10
First large application
The first high-powered batch melter had a targetproduction of 3200 metric tons per month of gray iron,which corresponds to a 10 metric ton per hour pourrate. In the past, this would have been handledexclusively by a low frequency system, but Fig. 12
shows the many advantages of the 300 Hz batchmelter.
The 5000 kW Inductotherm batch melting systemwas selected and put into operation in April 1987.Included were many features that would be standardin melters to come.
A COMPARISON OF HEEL AND BATCH MELTERS(Production Target = 3200 MT/Mo. 10 MT/Hr. - 16 Hr.
60 Hz 200 HzHeel Melter Heel Melter
|pp ^imp
'[.fTT.j ||. I :,.
13 MJ\
i | • ! Ir____J I :__-_! J I J
i]
Hi_ **"
7ШТType Coreless Coreiess
Frequency 60 Hz, Fixed 200 Hz, VariablePower Rating 3500 kW x 2 3500 kW x 2
Power Control Off-Load, 10 Taps StepiessInfinitely Variable
Furnace Capacity 12,700 Kg x 2 12,700 Kg x 2Tap Size 1800 Kg 1800 Kg
Charge Method Bucket BucketQuantity 1800 Kg 2x900 Kg
Melt Rate 6350Kg/Hr X 2 6680 Kg/Hr X 2Melt Time 17.0 Minutes 16.2 Minutes
Cycle Time 22.0 Minutes 22.0 MinutesUtilization 77% 74%
Production Rate 9.8 MT/Hr 9.8 MT/HrHold Overnight YesStart Up (Cold) 4:45 Hrs
Starter Blocks Required Yes
Optional1:45 Hrs.
NoElectrical Demand 7400 kW 7500 kW
Energy Used to 1510" С 610 kWH/MT 580 kWH/MT(TwO'Shift operation)
X 20 Days)
300 HzBatch Melter
EIZI]i — — i•y I у
5.51ЛТCoreless
300 Hz, Variable5000 kW x 1
StepiessInfinitely Variable
5500 Kg x 2Up to 5500 Kg
Conveyor/Feeder5500 Kg
10,500 Kg/Hr X 231.0 Minutes32.0 Minutes
97%10,3 MT/Hr
No1:00 Hrs.
No5200 kW
510 kWH/MT
Natural Gas for Dryer 975,000 CF 975,000 CFFfoor Space, Two Units 40' by 24' 34' by 32'
Shipping Weight «6 MT 87 MTBudgetary Pricing $1,000,000 $965,000
(Comparative)
32' by 21'73 MT
$895,000
FIGURE 12: A major advantage of the smaller furnace-higher frequency concept is that much faster melttimes for both cold and hot lining conditions are achieved with the batch melter as shown on this 1987 table.
11
FIGURE 13 (left). In1987, this foundry inCanada was the firstto melt with a high-powered batch melt-ing system.
Figure 13a (above). Europe's firsthigh-powered batch melting systemwas this foundry in the Netherlands.It operated with a 7000 kWpowersupply. It began operations in 1989.
A weighing system was used toprepare the charge that was dis-charged from a conveyor onto aswivel vibrating chute that fed ei-ther of two furnaces. (Fig. 13) Themovement of the charge materialfrom the weigh feeder to the fur-naces was under the control of themelt control computer.
The following year, Inductotherminstalled a similar 7000 kW systemin Europe. (Fig. 13a)
The furnaces were equipped withback slagging. (Fig. 14) This wasdone at the end of each melt withpower on the other furnace at thestart of its melt cycle.
Silica lining material with a backupof alumina brick against the coilwas used. Expected lining life was
FIGURE 14. Furnace back-slagging makes slag removal easier.
12
FIGURE 15. This pioneering computer system was used for operator in-formation and provided control of the entire melting process during nor-mal operations.
Setup of the system was accom-plished easily by foundrysupervision using a setup screen.
Initial cold melts were made at thestart of each operating shift onboth furnaces using theMeltminder® computer controlsystem. Both furnaces were pre-heated ready for use at the start ofeach operating shift. Lining lifewas further enhanced by the sin-ter controls on the melt computer.The computer controlled the fur-nace power to follow a presettemperature time curve, alwaysgiving the ideal sinter.
Batch melting schemes
1400 metric tons per furnace. A push-out lining re-moval system was used for safe, quick liningturnaround.
Another feature of this advanced melting system wascomputer control. A plasma screen (Fig. 15) was usedfor operation information so that the controls could beplaced on the melt deck. A series of menu drivenscreens was available for operation of the system. Themelt control screen showed control of the melting pro-cess during normal operation. The computer readweigh cells on each furnace to determine the chargein the furnace.
An initial charge of 1500 Kg was added by the feederunder computer control. At the start of the melt se-quence, maximum power was applied to the furnace(5000 kW) and the melt condition estimated from theenergy (kWH) and weight readings. When the com-puter determined that melting was taking place, itautomatically added charge from the weigh feeder tomaintain furnace conditions. This ensured that coldcharge was not added directly to molten metal andincreased operator safety.
As the temperature approached the set level, the op-erator was alerted to take a test dip temperaturereading which was read by the computer. If acceptedby the operator, the reading was used as true tem-perature and the melt was completed to the desiredtemperature automatically. Diagnostics for faults andindications of system parameters also were included.
The understanding of batchmelting efficiency and furnace
stirring, coupled with the development of large solid-state power supplies, led to the rapid development ofhigh production batch melting. Various operational ap-proaches to batch melting have been developed overtime. These use a variety of equipment types and op-erating cycles.
Single power supply on a single furnace
This system works extremely well where the com-plete furnace can be rapidly emptied. This type ofsystem is one with a very small, up to say a 3 tonfurnace, and a power supply no larger than 2500 kW.(Fig. 16) However,when the furnace sizebecomes much largerthan 3 tons, there maybe too much metal tobe taken out all atonce. Therefore, thefurnace must either beemptied into a holdingfurnace or the powersupply must be turnedto holding powerwhile the furnace isemptied progressivelywith resulting loss ofutilization.
FIGURE 16.
SinglePower Unit
13
Butterfly batch melter
To overcome the need for a holding furnace and al-low small taps to be taken to the pouring line, butter-fly batch melting has been used. In a typical butter-fly batch melting operation, a pair of induction fur-naces is used to produce a continuous supply of metal.
SinglePower Unitw/Switches
ence report8 on a 7 MW power supply with a 7.5metric ton furnace. This system works well as de-scribed in the article with the only reported problembeing maintaining temperature during the 30-minutetap cycle. (Fig. 18)
Melter and holder
To overcome the temperature loss as a furnace is be-ing poured over time, a holding power supply is pro-vided. (Fig. 19) This provides the power needed to
MeltingPower Unit • Holding
• Power
Temperature (°C)
Figure 17.
One furnace melts while the other pours, with thefurnaces alternating in their melting and pouringroles. In standard but-terfly systems, the twofurnaces share a singlepower unit that sup-plies current to onefurnace at a time formelting or reheating.This involves the me-chanical switching ofpower between the fur-naces. (Fig. 17) Typi-cally, a swivel pivotconveyor or someother mechanismcharges each furnace.When the charge inone furnace is fullymelted, the power sup-ply switches to theother furnace thatstarts its melt cycle.The first furnace isthen emptied. Thissystem is well de-scribed in the BCIRAInternational Confer-
FIGURE 19.
1600 r
1590 -
1580 . r
1570 -
1560 -
1550 -
1540 -
1530 -
1520
12 15 18 21 24 27
UCL
5th tap
LCL
reheating
30
time (minutes)
FIGURE 18. Temperature drop during pouring cycle.
14
Ihold the temperature of the pouring furnace whilethe other power supply provides melting power. Aweakness in this system is that sometimes superheat-ing is needed and the holding power unit may not besuitably rated for this. Therefore, it may still be oc-casionally necessary for the melting power supplyto be switched back to the pouring furnace. Also,mechanical switches are still involved. The holdingunit must operate at a similar or higher frequencythan the melting furnace to match the same coil. Thispower supply, therefore, produces inadequate stir-ring for any late alloy additions, necessitating switch-ing the main power supply back and forth.
Half and half
In this scheme there are two equal power suppliesand two equal furnaces. A switching setup is uti-lized so that both power supplies can be connectedtogether to one of the furnaces. In a system with two3000 kW power supplies and two 6 ton furnaces, forexample, the two 3000 kW power supplies would beconnected to one of the furnaces for its melting cycle.Once that furnace charge was molten, the two unitswould be re-configured to start melting on the otherfurnace. When additional heating was required onthe first furnace, one of the 3000 kW units would beswitched back to that furnace so that holding powercould be supplied and some melting power wouldstill be maintained on the melting furnace. (Fig. 20)This system proved to be unsuccessful due to the
Single 1 SinglePower Unit I I Power Unitw/Switches I I w/Switches
FIGURE 20.
amount of switching back and forth, the variabilityof electrical demand and the fact that the frequencyof the two power supplies together was inherentlylower than the frequency of each power supply whenconnected to a single furnace, resulting in incorrectstirring patterns.
Dual-output power via switches
The first attempt to design a power unit engineeredspecifically for butterfly batch melting was in 1986when Inductotherm9 developed a single power sup-ply which was connected to two furnaces with thecoils tapped in such a way that most of the power wasapplied to the furnace during the melting and a smallpart of the power was tapped off to the pouring fur-nace to hold at temperature. Thus, a single 750 kWunit was able to apply about 650 kW for melting and100 kW for holding to each of two furnaces. Thisworked extremely well in the low kilowatt levels, butwas severely limited by switch ratings and switch re-liability from going to any higher levels.
Solid-state dual-output power tomaximize a batch system's pouring rate
In early 1991, Inductotherm introduced an inductionpower supply engineered specifically to maximizethe efficiencies of butterfly batch melting. Named"Dual-Trak®," it was a unique single power unitwith two outputs.10
With its two outputs, a Dual-Trak unit is able tofeed continuous and completely controllable powerto two furnaces at the same time. (Fig. 21) Thisallows the furnace operator to melt in one furnaceand simultaneously apply the power needed tomaintain temperature in a pouring furnace. As aresult, it is no longer necessary to interrupt meltingin order to reheat the metal in the pouring furnace,a common but inefficient practice with single-outputpower supplies. Consequently, with dual-outputpower, the overall time needed to complete a meltcycle is reduced. This can increase metalproduction by up to 20 percent, depending on the
FIGURE 21.
15
нн ^ нщм Н
number of times a furnace operator had to reheatthe pouring furnace during a batch cycle with astandard power unit. (Fig. 22)
Also, because h o l d i n g power is appl iedcontinuously to the pouring furnace, precise metaltemperature is maintained, an important factor inmany applications.
A dual-output power unit gives the furnace operatorcomplete control of both furnaces. With separatecontrols, he can allocate power in any way betweenthem, up to the unit's total power rating, just byturning the power knobs. For example, one furnacecould be run at full power, with no power beingsent to the second furnace, or both furnaces couldbe run at holding power levels - or even superheatwhen required. The holding furnace power settingalways dominates so that the lesser power goingthere is exactly set by the operator and the remainingpower is then available to go to the melter. Unlikesingle-output power units, there is no need formechanical switching or a second power unit witha dual-output system.
Another significant advantage of this technology isits ability to sinter or cold-start two furnaces at thesame time or to sinter one furnace while melting inthe other. This reduces production downtime and in-creases system output. It also features the ability todirect full rated power to one furnace while fully iso-lating the other during maintenance or lining changes.
While a single dual-output power unit provides thebatch production capacity of two separate power sup-plies, it offers a number of advantages over two-unitsystems. First, there is just one set of power and wa-ter connections and the line KVA of a single unit, asignificant savings in installation and maintenancecosts. Second, it takes up less room in the melt shopthan two separate units. Third, it offers a level of equip-ment utilization approaching 100 percent because itis designed to use its full power capacity throughoutthe batch melting cycle. Finally, it offers a minimuminvestment per ton of metal poured.
Pour comparison dual output 1650 KW batch melter vs. 2211 kW 1st Iled in
i-ftmtf АНШ!, рп змв i| iiI Secsse т па ml ciwpl Ц minute,
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2500
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This schedule te- nittteinti It
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to 20 so АО so во mm7з00вт a 00am
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FIGURE 22. This chart shows how a 1650 kWDual-Trak unit with a pair of 3000 Kg furnaces maintains thesame pouring rate as a 2200 kW single output power unit with an 4000 Kg heel melting furnace.
16
Balancing variable metal demands
Before the advent of Dual-Irak power units, batch-melting foundries with highly variable metal de-mands during the operating day relied on separateholding furnaces to provide the metal reserve theyrequired. But with a dual-output system's ability toprovide steady and reliable holding power duringmelting operations, it is practical to configure a two-furnace batch system able to meet even the most vari-able demand for molten metal.
The key to this system is to size the dual-output powerunit to meet the overall daily demand for metal and tomatch it with furnaces sized to hold enough metal tomeet the day's greatest demand. Take, for example, afoundry requiring 480 metric tons of metal in a 16-hour period. That's an average demand of 30 metrictons of metal per hour. However, its actual hourly de-mand varies greatly, ranging from 15 metric tons inan hour to 45 metric tons in an hour. (Fig. 23) Thisfoundry would be able to operate efficiently with a
Dual-Trak unit able to melt 30 metric tons per hourusing a pair of 60 metric ton furnaces. No additionalholding furnaces would be required.
With this arrangement, metal is always ready in theamount needed and at the temperature required. Bal-ancing the varying power needs for melting and hold-ing are what Dual-Trak does best.
Dual-output and multiple-outputinduction melting
In the early part of the last decade, an induction powersupply engineered specifically to maximize the pro-duction output of batch melting was introduced. It wasa unique single power unit with dual outputs able toallocate continuous and completely controllable powerto two furnaces at the same time up to the unit's totalpower rating. Now multiple-output power units withthree or more outputs are in operation in foundrieswhere they are achieving the high levels of metal pro-duction previously the preserve of cupolas.
Pourtat©
30
fymetceA
itTONS
Furnace8
METRCTONS
16,Ш kW Dual-Tralc111 with two Ш IT holdersmelting iron
41 45 MT 45 MT
it т зо MT15 MT 1SMT 1EMT
I.
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HOURS
FIGURE 23. Variable metal demand
17
FIGURE 24. A vibratory conveyor mounted on the melt deck loads charge materials into an induction fur-nace.
Dual-output power units allowed foundries to simul-taneously melt in one furnace and hold with powerin a second pouring furnace. This could increasemetal production by up to 40 percent, compared to asingle furnace/single power unit system and by up to20 percent compared to a butterfly batch melting sys-tem with two furnaces and a single power supply.At Benton Foundry in Pennsylvania, a 7000 kW dual-
output power supply runs two 10 metric ton furnacesin a typical batch melting operation. (Fig. 24) Chargeis delivered, via bucket, from the scrap dryer to a vi-bratory conveyor that serves both furnaces. Whileholding power is directed to the furnace being tapped,melting power is applied to the furnace being charged.(Fig. 25)
FIGURE 25. Ten metric ton induction furnace pours into a ladle.
18
FIGURE 26. Oneof a pair of 1 met-ric ton furnacespowered by adual-output in-duction powersupply is tapped.
5.000KW ACI
.
5,OOOKWInverter
FIGURE 27. Schematic of a 5000 kW triple-output induction meltingsystem.
Small foundries also can take full advantage of dual-output systems. At a foundry in Washington, two 1metric ton furnaces operate with a 750 kW dual-
output induction power supply. (Fig. 26)
The first use of a triple-output induc-tion power system was in Ohio. Installedin 1997, this was a 5000 kW inductionpower unit able to direct full power toany of three furnaces or to allocate thatpower in any way among the furnaces.(Fig. 27)
At a more recent installation inGeorgia, three 12.5 metric tonfurnaces are powered by a single20000 kW triple-output powersupply able to direct up to 10000kW each to any two furnaces.(Fig. 28)
A company in Tennessee is us-ing a 9000 kW triple-output unitto power three 10 metric tonwide-bodied furnaces. This unitallows the foundry to melt in onefurnace and hold at the desired
19
1G.GQOKWInverter
CapacitorBank
•;" '..and••: :££i.;
Bank
Inverter
Capacitor
FIGURE 28. Schematic of a 20000 kWtriple-output induction melting system.
and maintenance costs.
• Less space required in the meltshop than separate units.
• Equipment utilization approach-ing 100 percent.
• Minimum investment per ton ofmetal poured.
Inductotherm's development of ca-pacitive isolation allows each furnaceto be protected by its own ground leakdetector system, a crucial safety re-quirement, and provides the ability tofully isolate a furnace during mainte-nance or lining changes. It also allows
temperature in one or both of theother furnaces, providing maxi-mum alloy flexibility to meetvarying requirements for gray andductile iron.
The production advantages oftriple-output systems are not dimin-ished when these systems are scaledup in size. For example, a 35500kW triple-output batch melting sys-tem with three 20 ton furnaces witha 36 minute melt and 18 minutepour will produce 65 metric tons ofmetal per hour. And the power uti-lization continues at 100 percent.(Fig. 29)
Other significant advantages ofmultiple-output induction powersupply technology include:
• The ability to sinter or cold-startmultiple furnaces at the sametime or to sinter one furnacewhile melting in others.
• The unit's single set of powerand water connections and theline kVA of a single unit, a sig-nificant savings in installation
FIGURE 29. Graph showing a 65T/hour production cycle for a 32000kW triple-output system.
35,500KW Triple-Output Induction Power SupplyIn Large Batch Melting Operation
17,75»
0 18 36 54 72 SO 108 126 144
0 18 36 94 72 SO 10* 126 144Jtam in minutes
OOKW
Power Utilization Curve (100% U.F.)
0 18 3« 54 72 90 108 126 144
Pelt time for 20 Ton batch »3S mine.Pouring time for 20 Ton batch =18mins.Tap«fo-Tap cycle time «54 mineTotal metal poured = 65T/Hr
20
Dual-output power supply "-4=with capacitively isolated furnaces
FIGURE 30. Capacitive furnace isolation supportsmultiple-output induction power systems.
multiple-output power systems to operate any num-ber of furnaces connected to a common DC bus. (Fig.30)
Fig. 31 shows a 100 ton per hour furnaces poweredby a 50 MW system able to direct up to 20 MW toany furnace. With this system, for example, two fur-naces could be melting at 20 MW each with theremaining power allocated to one or both of the otherfurnaces for melting, holding or sintering.
More than 500 Inductotherm dual and multiple-out-put induction power systems are operating todayworldwide.
Powerful induction systems spurcomputer controls
Industrial designers used to focus on theman-machine interface when it came to control tech-nology. Today the focus is on the man-computerinterface and the computers control the machines. Inairplanes this type of control system is called "fly bywire" and many advanced planes, such as the stealthfighter, could not stay in the air without it.
The world of the computer is a digital one and con-trol systems are becoming increasingly digital.Digital controls offer advantages which simply werenot easily obtained with their analog predecessors.These include:
• Direct connection to a computer.
• The ability to link multiple devices and have themcommunicate with each other locally or at anydistance.
• Maximum accuracy and repeatability of instruc-tions and information.
For the foundry worker, these properties of digitalcontrol systems translate into valuable operationalbenefits:
50Multiple-Output
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FIGURE 31. Schematic of 100 ton/hour induction melting system.
21
FIGURE 32.
Computer Control - - Because today's high powerdensity batch melting systems melt the charge so rap-idly, they have driven the development ofcomputerized melting operation control systems de-signed both to provide precise control of the meltingprocess for enhancedquality and to reduce therisk of accidental super-heating. Some of thesesystems operate on specialcomputers, some are builtinto the melting equip-ment itself and some arePC based, including thelatest system running un-der Windows NT®. ThisWindows-based systemoffers the full advantagesof the Windows operatingsystem for the customiz-ing of reports andinterfacing to other appli-cations. (Fig 32)
Batch melting is an idealprocess for computerizedcontrol. A typical controlsystem uses the weight ofthe furnace charge, either
from load cells or asentered by the opera-tor; the melt rate; andthe desired pour-ing temperature toautomatically calcu-late the kilowatthours needed tocomplete the melt. Itthen turns off thesystem or drops toholding power whenthe melt is complete.Thermocouple read-ings can betransmitted to thecomputer to furtherenhance accuracy.(Fig. 33) This pre-cise melting controloptimizes power us-age by minimizingtemperature over-shooting, saves time
by reducing frequent temperature checks and en-hances safety by reducing the chance of accidentalsuperheating of the bath, something which can hap-pen very quickly in a high power density system andwhich can cause lining failure.
Щ MeUminder 200
Furnace 1 Ш1 j Misceflaneom [F5] |
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0 25 50 75 1001 i 1 i I i 1 i JL
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Material Type Iron
Temperature Settings Hold Power Settings
Pour Temp [1480 Weight kWatts
Sample Temp J1450 J1000 f?40
Ной Power Offset flOO pOQO J144
Charge Warning Т emp High |1200 J4006 p48
Charge Warning Temp tow J1100 [5000 JTJS
Scrap Temp p JIOOOO p65
Inductance Energy Loss Test Model
MoS*"*0"8 OwseForra Archive Database
Save
FIGURE 33.
22
Meltininder 200
Furnace 1 {Ft) Miscehneous [F5]FIGURE 34.
Furnace DetaaiFSril Big Display JF71 T VIP(F8J T kWHour Mode F91 T ColdStgt(F101 | Sinter {Ft Ц T'jJ
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ReSneDate | March 2,199910:2E i § am Numbw Of Heals | о Weight PoutedManual ReiieStamp
The most advanced foundry melting automation sys-terns also provide fully programmable control of
•i Meltinindei 200
Furnace 1[F1J | Miscellaneous p5j ]
Furnace Oel [F6)f Bis Display PI T VIP|FS! T kWHowMode[F91 | Cold Start [F10} T Shtei(FH) ]
1 wit ^ Inductance History [Furnace 11 E3I
15_ % Inductance Historytnd« 125€tan 101)сe
Z 75
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'19 10/20 10«1 tO/23 1B/24 10/25 10/26 10/28 10/29 10/30
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FIGURE 35. Tracking inductance helps monitor lining life.
sintering and the ability to schedule and controlfurnace cold-start procedures. In addition to
operational control,computerized meltingsys tems can of ferreal-time informationabout system diagnos-tics and operation.(Fig. 34)
System Diagnostics -Diagnostic checks are animportant part of thisinformat ion. Sometechnologically ad-vanced systems are ableto identify specific prob-lems before any power isapplied, protectingequipment from damage.
Maintenance informa-tion can help the furnaceoperator keep track ofkey maintenance require-ments such as liningreplacement, an impor-
23
tant safety consideration. (Fig. 35)
Digital Control Networks — Because informationin a digital format can be easily communicated andshared by a variety of systems, digital control sys-tems in a foundry can link and control many piecesof equipment. Typically, this can be done with just asimple cable containing a pair of control wires pluspower and shielding. This is in contrast to complexcabling required for non-digital control systems. Linkyour induction power supply to several remote con-trol stations. Store important operational informationon your melt shop computer as well as on yourcompany's mainframe computer at corporate head-quarters. Coordinate charge makeup with meltingoperations and even with production much furtherdown the line. (Fig. 36)
OperationsManagement
Control JRoom /^
Harmonics
"Harmonics" is a general term often used to describemany of the effects a piece of electrical equipmentmay feed back into its power source. Now, with thefoundry industry operating solid state power convert-ers of up to 50000 kW, power utility companies havebegun to study what effect such high power may haveon utility lines. Power interface problems are oftendifficult to resolve and may include:
• Low Power Factor causing current to flow fromthe induction power supply back into the powerline due to reactive loading.
• High Frequency Current Harmonics generat-ing excessive heating and other adverse effectson power utility facilities.
• Line Voltage Notching caused bysemiconductor or contacts switching,producing severe voltage on the line.These spikes are short in duration, butmay carry significant energy. They alsogenerate radio frequency noise thatmay jam communication equipment inthe area and sometimes cause arcingand coronas, damaging line utilities.
ф о•Е?5*
IB >
.0 I
DigitalMelt Shop
Control andData Network
Induction:PowerSupply
RemoteStation
..• *RemoteStation
Furnaces
FIGURE 36 - A melt shop control network.
• Inter-Harmonic Distortions occurwhen static power converters injectcurrents into the power line at frequen-cies that are not a multiple of linefrequency but a multiple of the inverteroperating frequency. These harmonics,when superimposed on the AC line,may induce fluctuation in line voltage,causing flickering of lights in theneighborhood of the induction powersupply.''
Induction power converters may pro-duce some or all types of linedistortions.
Line distortions represent a series ofproblems that must be addressed bydesigners and users of induction melt-ing equipment. Failure to consider theeffect of static converters in early plan-ning may become very costly, if notimpossible, to correct later, when theequipment is commissioned. Thehigher the generated power, the moreacute is the problem.
24
Two types inverters are avail-able on the market: current-fedinverters and voltage-fed (Fig.37). Analysis of equipmentand techniques of static powerconversion shows that a volt-age-fed inverter with multiplefull-wave rectifiers is the besttechnology today for largemegawatt level induction melt-ing systems. Voltage-fedinverters provide:
• Minimal line voltagenotching. Notching is of-ten the source of powerspikes and radio fre-quency noise.
• High load matching whichminimizes high frequencyharmonics on power lines.
• High efficiency with apower factor of 95% orbetter.
• Minimal inter-harmonicdistortion due to the ca-pacitive energy storagedevices.
Therefore, full-wave, voltage-fed systems generate theminimum possible distortionback to the power lines thatfeed them. 12(Fig. 38)
APhassRectffi
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Current-fed power supply driving a parallel-resonant furnace circuit.
Voltage-fed power supply driving a series-resonant furnace circuit.
FIGURE 3 7. Current-fed and voltage-fed induction power supplies.
Power Quality Comparisons (Single-Rectifier-Bridge Configuration)
Characteristics
Line-Voltage Notching
Harmonic Generation
System Power Factor
Generates Inter-Harmonics
Current-Fed Inverter
Yes (Caused by Phase Control)
High
0.7-0.95 (Depends on Phase Control)
Yes (Depends on Furnace Frequency)
Voltage-Fed Inverter
No
Moderate
0.95
No
Source: Electric Power Research Institute (EPRI), Inc., Telecommentary (TC -114625) 1999
FIGURE 38. Current-fed and voltage-fed induction power supplies.
25
FIGURE 39. Voltage-fed 16500 kWbatch melter.
Powerful induction systems nowchallenge arc furnaces
With their continually increasing size, high powerdensity induction melting systems today present adirect challenge to the original electric batch melter,the arc furnace. Induction furnaces offer a numberof advantages over arc furnaces in most foundry op-erations. These include:
• Better control of bath metallurgy. Induction heatsthe metal from within so there is little or no metalloss. Electric arc furnaces heat by temperature dif-ferential, heating the surface with the arc andburning off appreciable amounts of metallic ele-ments, making it difficult to maintain precise metalspecifications.
• Better control of carbon. With induction furnacesthere are no carbon arcs and no carbon gets intothe metal as a result of the melting process.
• Better metal homogeneity. Inductive stirringmixes the elements in the metal bath.
• Cleaner melting. Induction furnaces melt cleanly,producing little or no smoke or fumes. Arc fur-
naces generate heavy metal fumes, carbon dustand other pollutants.
• Quiet operation. Compared to arc furnaces, in-duction furnaces are quiet in operation.
• Lower cost of refractory and other consumables.Induction furnaces use less refractory in their lin-ings and do not require the purchase of carbonelectrodes.
Illustrative of this challenge to the arc furnace bymodern high powered induction systems is the newinstallation at the Foundry Division of John Deere Wa-terloo Works, Waterloo, Iowa. John Deere, a leadingmanufacturer of farming machinery, has replaced itssix 16 ton arc furnaces with three 16500 kW solidstate, medium frequency induction units poweringthree 20 metric ton steel shell coreless furnaces. (Fig.39) Completed in the summer of 2000, it is the larg-est, solid state, medium frequency induction meltinginstallation in the world.
Overall melt shop control is provided by a dedicatedcomputer system in a central, elevated control roomwith line-of-sight monitoring of each furnace. Localoperation is via a full remote control station adjacent
26
FIGURE 40. Vibratory conveyor at the John Deere Waterloo Works adds charge to a 20 metric ton furnace.
to each furnace.
The new installation also includes integrated and au-tomated alloy makeup and charge deliveryequipment. (Fig. 40) The alloy makeup system op-erates from preset recipes to achieve the desiredconsistent bath chemistries and temperatures. Eachof the three furnaces is served by its own charge con-veyor. As each charge car is loaded, a weigh framebuilt into the melt deck indicates when a full 20 met-ric ton charge is on board. At that point, the car moveson rails to the furnace for unloading.
John Deere's new, high power density induction fur-naces include back tilting for easy slag removal,integral fume collection covers and lining removalmechanisms. These new furnaces generate minimaleffluences and produce substantially less noise thanthe six existing arc furnaces they replaced, provid-ing John Deere employees with a cleaner and quieterworking environment.
High power density furnace design forbatch melting
There is a wide range of engineering and manufac-turing challenges involved in building a high powerdensity induction batch melting system. It's not suf-ficient simply to build a higher output power supplyand attach it to a standard induction furnace. The en-tire system, from charging equipment to the designof the furnace, must be engineered as a high powerdensity system.
The coil
The first consideration in engineering a high powerdensity induction furnace for batch melting is the coildesign.
Calculating the amount of copper needed to carrythe current needed for a high power density induc-tion coil is only a small part of the coil designchallenge. The coil also requires more cooling ca-
27
,50 inch (12.7 mm)Copper Tubing
1250015000750010000
1125001500017500
•20000
Maximum Column StrengthProvides Zero Deformation
Under LoadFIGURE 41.
pacity due to the higher power levels,greater mechanical strength and a morecomplex configuration program to deter-mine the optimum coil turns anddimensions.
To meet the power transmission require-ments, water-cooling needs and structuralstrength necessary for a high power den-sity coil, the coil must have a high sectionmodulus.
This provides enough copper to carry thecurrent, a large open area within the tub-ing for water flow, and inherent structuralstability. A rectangular cross-section withrigid walls of uniform thickness providesthe maximum ability to withstand thestresses imposed by the thermal expan-sion of the refractory and the weight ofthe molten metal in the furnace as well asthe supporting forces from the furnacestructure. (Fig. 41)
In addition to heavy copper tubing, highpower density furnace coils require strong
and rigid structural support elements to limit the coilexpansion and flexing often responsible for shortrefractory life. These elements include nonmetalliccoil supports, which lock each coil turn solidly intoplace, and adjustable tie rods that control verticalmovement. The furnace's magnetic shunts also playa vital structural role by keeping the coil and fur-nace lining under compression without yielding.High power density furnaces require extensive shuntcoverage to redirect the strong inductive field intothe charge material and molten bath. By makingthese shunts an integral part of the overall structureof the furnace, the coil's rigidity is greatly enhanced.(Fig. 42)
Advanced coil insulation
Another important consideration in the design ofmodern furnaces for batch melting is the careful andeffective insulation of the coil. Proper coil insula-tion has always been crucial in vacuum furnaces,but many of today's high power density air meltingfurnaces also need an insulated coil to help preventarcing.
OutwardDeformationForces
ОInwardSupportForces
FIGURE 42. This drawing shows a high power density induc-tion furnace with 65 percent shunt coverage of the coil. Theseshunts are structurally tied to the heavy steel shell of the fur-nace, providing extremely rigid support of the coil. Shuntcoverage of 65 percent or greater, together with the steel shellof the coil, reduces EMF emissions below today's most rigor-ous international standards.
28
Lines of Xх
magnetic flux
Shunts oryokes
inductionmil
Furnacesteel shell"
fa} Flux patternat shunt
(b) Flyx patternbetween shunts
FIGURE 43. (a) Magnetic flux is fully captured in areascovered by shunts or yokes, (b) Flux which escapes betweenareas of shunt coverage is blocked by the furnace's steel shell,preventing EMF emissions.
The heavy steel shell furnace
In high power density batch melting furnacesbased on the advanced steel shell design, theinherent hoop strength of the heavy steel shellprovides an extremely strong and rigidall-around structure that minimizes distortionduring tilting and pouring operations. Thisshell forms a firm base into which the otherkey furnace structural elements are tied.
EMF emissions
While extensive shunt or yoke coverage pro-vides substantial structural strength to thefurnace, the most important job of the shuntsis to capture the magnetic field which is notgoing into the metal charge and redirect itback into the charge. But with anything lessthan 100 percent shunt coverage, some of themagnetic field is missed. However, in a steelshell furnace, the shell itself prevents emis-sions from escaping. (Fig. 43) That is whyheavy steel shell furnaces easily comply withtoday's most rigorous international EMF stan-dards.
Noise reduction
A variety of traditional coil insulation materials andtechniques continues to be available, but today's mostadvanced materials are monolithic coatings. The bestof these materials offer a dielectric or insulating valuesurpassing conventional coil insulation as well asstrong mechanical properties, such as resistance toheat and abrasion. They also remain highly flexibleto move with the coil as it expands and contracts.
Foundries can be noisy places, but noise can be con-trolled. This can be accomplished first by the carefulselection of inherently quiet equipment. Inductionfurnaces, as noted earlier, are much quieter than arcfurnaces.
Second, choosing equipment with built-in acousticsuppression will reduce unnecessary noise. (Fig. 44)
FIGURE 44. Theseconveyors are builtwith beds lined withsound absorbing ma-terials and withacoustic enclosuresthat move with them.
29
FIGURE 45. The furnace's fume reduction cover captures smoke, even while open during slagging.
Steel shell furnaces, by design, keep noise inside thefurnace shell. And they can be constructed withsupplementary sound insulation within the shell.Similarly, charge conveyors can be built with bedslined with sound absorbing materials.
Finally, acoustic enclosures can be incorporated inequipment and installation designs. Acoustic enclo-sures can either be in fixed locations to containindividual pieces of equipment or they can be attachedto a piece of equipment and move with it. Enclosuresalso can isolate complete areas of the foundry. Suchenclosures are extremely effective.
Noise reduction will increasingly become importantin equipment and foundry design.
Metal splash protection and fumereduction systems
Among other benefits of steel shell furnace construc-tion are the effective protection from metal splash itprovides the coil and its compatibility with fume cap-turing systems.
Fume collection — Unlike cupolas and arc furnaces,induction furnaces themselves produce no smoke orfumes. However, the charge materials being meltedcan generate undesirable emissions. These can be assimple as smoke or dust from oily or dirty scrap. Orthe emissions can be the inevitable by-products ofmelting certain metals. To capture smoke and otherfumes associated with melting, induction furnaces canbe equipped with highly effective fume collection cov-ers with integral furnace lids.
These covers can be designed to operate effectivelyboth when closed and when open for charging. (Fig.45) They also can be engineered to be compatible witha wide variety of automated charging systems.
Fast and complete slag removal
As induction furnaces grow larger and faster, effi-cient removal of slag becomes more difficult. To beeffective, the furnace crucible must allow enough re-verse tilt so that sufficient surface area of the bath isexposed and at the proper height in relation to therear slag spout.
30
FIGURE 46. Fur-nace equipped withfull back-slaggingtilt and slag re-moval spout.
Because of varying metallevels in any operation, aback-slagging furnace that isequipped with separate cyl-inders and an additionalfurnace frame allows asmuch as a 33 degree reversetilt for slagging. (Fig. 46)This allows the furnace op-erator, regardless of metalheight, to remove all of theslag from the furnace directlyinto the slag cart in a quick ._and efficient manner. The ad-ditional furnace framepermits a high furnace hearthto allow a slag cart to be po-sitioned under the rear slagspout. (Fig. 47)
This dedicated back-slaggingsystem is more effective thanfurnace back-tilting whichuses only the forward tiltingcylinders in a single furnaceframe and has a lower fur-nace hearth in relation to themelt deck. With such ar-rangements, the amount ofback-tilt will be restricted to10 or 12 degrees.
ГFigure 47. Back-slagging furnace makes the bath fully accessible for easierslagging.
FIGURE 48. Furnace back-tilting provides little slagging advantage.
31
This may mean that it is more dif-ficult to reach the metal bath orutilize a slag cart under the rearspout, resulting in dragging theslag on the melt deck before lift-ing into a slag hopper. (Fig. 48)
For most applications, furnaces of6 tons and larger should includeback-slagging.
Mechanized slag removal
Large furnaces that do not haverear slagging spouts should use amechanized slagging device tospeed up slag removal.
Push-out linings
Large coreless induction furnacesalso benefit from lining push-outsystems. These systems generallyconsist of a moveable block in thebottom of the furnace and a hydrau-lic cylinder. (Fig. 49) When a liningis ready to be replaced, the furnaceis tilted to a 90-degree angle andthe cylinder is attached to push theblock toward the front of the fur-nace, pushing the old lining aheadof it. (Fig. 50) When the old lining drops away into upright position for relining. The advantages offeredthe waste bin, the pusher block is returned to the bot- by these systems include the speed of lining removal,torn of the furnace, which is then restored to an fewer man-hours and reduced silica dust exposure.
FIGURE 50. Furnace lining being pushed out.
FIGURE 49.йшиншс! LinjRQ
Coll 1
taMM
Fumoc*
32
Melt DiameterWide Body vs. Standard Steel Shell
FIGURE 51.
"Wide-Body" s StandardSteel Shell Furnace Steel Shell Furnace
(53 1 /2" Diameter) (41" Diameter)Both have 10 Metric Jon Capacity
Wide-bodied furnaces
The typical coreless induction furnace is taller thanit is wide. This is an appropriate shape for most melt-ing applications and the vast majority of furnacesare built to this model. But for some applications,this is not the ideal shape. Rather, the ideal shapecalled for might be a wide-bodied furnace. (Fig. 51)A wide-bodied furnace is wider than it is tall andfeatures a significantly larger crucible opening thana standard furnace of comparable capacity. (Fig. 52)The advantages offered by this design relate to theapplications for which it was engineered.
Generally speaking, a wide-bodied furnace's largercrucible opening and proportionally shallower furnacedepth provide:
• The ability to load larger, bulkier (and conse-quently cheaper) scrap.
• Better access for removal of slag or dross buildupon furnace walls.
• Greater overhead clearance (or a reduced require-ment for overhead clearance)
• Minimal depth of pit.
FIGURE 52. A 10 metric tonwide-bodied furnace.
33
FIGURE 53. The wide furnace opening accommo-dates the counter-gravity casting device.
The first wide-bodied furnace was de-veloped in the late 1980s to support aninnovative counter-gravity casting pro-cess. (Fig. 53)
A company in Tennessee recently in-stalled three 10 metric ton wide-bodiedinduction furnaces to allow it to use longraisers as part of its normal furnacecharge. These internally generated rais-ers required a larger than normal furnaceopening to avoid tangling during charg-ing. The wide-bodied furnace offered anopening that was almost 20 percent largerthan a standard furnace of the same ca-pacity.
Automated charging systems
The growth of batch melting has spurredthe development of remotely controlled,mechanized charging. Batch meltingsystems typically use advanced induc-tion furnaces that provide high powerdensities and are able to run at full powerthroughout the charging process. Thesefurnaces require rapid charging to keep
pace with the melting power of the system. Manualcharging simply cannot support a large induction fur-nace able to melt a full charge in less than 30 minutes.Mechanized charging systems are engineered to de-liver charge materials to the furnace quickly andefficiently, allowing maximum utilization of the melt-ing system. They also permit manpower to be usedmore efficiently.
In batch melting, the vehicle emptying the chargeinto the furnace ideally should hold a full furnaceload. This allows additional charge materials to becontinually added as melting drops the level of coldcharge in the furnace. This takes maximum advan-tage of the higher efficiency of cold charge melting,prevents wasteful delays in charge delivery duringthe melting process and enhances safety by intro-ducing cold charge materials on top of solid materialalready in the furnace rather than directly into themolten bath.
The enhancement of worker safety is an importantreason for the growth in automated charging systemsassociated with induction furnaces. Many seriousfoundry accidents have occurred during manual fur-nace charging, when foundrymen were close to the
FIGURE 54.
34
molten bath. Splashes caused by dropping largepieces of scrap and by water/metal explosions causedby wet or damp scrap have proven to be anever-present danger. But these dangers can be re-duced through the use of charge drying andpreheating systems to reduce moisture and remotelycontrolled charging systems to keep the furnace op-erator away from the molten metal during hazardouscharging operations.13
In general, in-foundry charge transportation systemscan be divided into four categories: electromagnetcranes, belt conveyors , buckets and vibrating con-veyors (Fig. 54). These in turn are available in a widevariety of configurations and modes of motion. Forexample, vibrating conveyors, the most versatile andrugged of all furnace charging devices, may be infixed positions for holding, consolidating, weighingand transferring charge materials. They also may be
extremely mobile. They may move along tracks inany direction, may pivot and/or may index forwardand backward. In fact, vibrating conveyors have beenbuilt to traverse, pivot and index, all in the same unit(Fig. 55). This mobility enables a vibrating conveyorto be built to service a single furnace or any numberof furnaces. Largely unaffected by heat, vibratingconveyors are ideal for feeding charge materials di-rectly into the furnace. Vibrating conveyors also canbe fitted with integral fume collection covers overthe discharge chute to enhance fume removal duringcharging.
Ultimately, whether you use belts or buckets or vi-brators or cranes, the final configuration of anycharging system depends largely on the physical lay-out of the melting facility. Ceiling height willdetermine if your facility can handle buckets, floorspace and elevations will largely dictate the types of
FIGURE 55. This charge conveyor traverses from the charge makeup area to the melt deck and pivots to eachfurnace.
35
FIGURE 56, A reducing flame provides optimum temperature with minimal oxidation of charge material. Adryer/preheater raises the charge to the desired temperature to remove moisture from the scrap and willreduce furnace melt time and overall energy costs.
FIGURE 57.A scrap dryer inoperation.
36
FIGURE 58. Here, charge materials are weighed in the hopper that feeds the charge conveyor.
conveyors required for the job. Key considerationsremain safety and the ability to keep pace with themelting furnaces' need for charge materials. Thereis a charging system that's right for every foundry.14
Dry scrap is essential to safety
Wet or damp scrap exposes melt deck workers to thevery real and life-threatening danger of furnace erup-tion or explosion. The best way to ensure that there isno water or moisture on your scrap is to dry it in a gasor oil fired charge dryer or preheater. Charge preheat-ing may also significantly increase productivity inmelting operations.
Drying and preheating systems pass scrap through anoil or gas fueled flame runnel, heating the scrap andminimizing moisture that could cause a water/metalexplosion. (Fig. 56) These systems also burn off muchof the dirt, producing a cleaner charge and reducingthe energy required in the furnace to melt the scrap.These systems, however, cannot remove trapped liq-uid, such as oil in cans and cylinders. Such materialsmust be shredded before they are used.
The use of drying and preheating systems and re-motely operated charging systems can significantlyreduce accidents related to furnace charging opera-tions. (Fig. 57)
Scrap weight
Accurately measuring the weight of the furnacecharge has always been important to avoid acciden-tal overheating and to determine the proper amountof additives needed to maintain the desired metallur-gical properties. The advent of high power densitybatch melting and advanced computer control sys-tems, however, has made accurate charge weight ab-solutely crucial. Computers can only function as de-signed when provided accurate data.
Accurate charge weight can be obtained through fur-nace load cells and/or via weighing hoppers locatedon the charge conveyor or at the charge makeup loca-tion. (Fig. 58)
37
Batch melting continues to evolve into the 21st centuryMuch has been written in recent years about the sig-nificant and compelling advantages of induction batchmelting. These include reduced furnace capacity re-quirements (compared to comparable heel melters),easier alloy changes, safer charging and energy-relatedefficiencies in units able to deliver full power into thecharge from the start ofthe melt.
charge is largely molten lengthen the duration of stage2 and may create oxidation problems in the bath.15
Reducing oxidation also may play a role in increas-ing lining life, according to a phenomenon observedin the field. One report, for example, indicates thatmelting a layer of oily borings in the bottom of the
As is true of any viable op-erational process, batchmelting will continue tochange as new and more ad-vanced techniques and tech-nologies are developed andas those running the systemsgain knowledge andexperience.
The following looks at someof the more recent opera-tional advantages being dis-covered by batch meltingpractitioners.
Batch melting's opera-tional advantages
/. Reduced charge oxidationThe most significant advan-tage of batch melting is theability to maintain full powerthoughout the melt, espe-cially in the early stages, andits importance in the reduc-tion of oxidation in ferrousmetals.
Ferrous charge materialsoxidize rapidly once thecharge reaches Curie, thepoint at which a solid chargebecomes non-magnetic. Thisis stage 2 in the melting cycleillustrated in Fig. 59. Thepower supply must have suf-ficient flexibility to quicklytake the charge through thisstage. Power units unable toachieve full power until the
A CONSTANT OUTPUT POWER UNIT PROVtOfS FULLPOWER f HOW THE START OF THE MELT TO THE FINISH,
TIMEif TOWfR UNITS CAMNOT SUPPLY FULL POWER IN
THE EARLY STAGES OF THE MfcLTiNG CYCLl.
STAGE ONE - THE CHARGE is SOLID AND «AGNETIC.
STAGE TWO - THE CHARGEBECOMING NON-MAGNETIC,
STAGE THREE - THE CHARGE STARTS TO MELT,
STAGE FOUR - THE CHARGE ts .NOW FULLY MOLTEN,
FIGURE 59.
38
7000
6000
5000(Л
g 4000
Average Tons/Lining
2000
1000
0
ВоппдсаснМto charge mixAugust 1998
I Fumaoi f 1i Furnace #2I Fumae© #3
1995 1996 1997 1998 1999 2000
The above represent three ?mw medium frequency power supplies on three 9 metric ton furnces melting gray iron.Note the eonttmious lining life improvement over a sbc year period. Lining life continued to improve after adding15% oily borings to charge mix. System operates 24 hours a day, в days a week, all metal duplexed into channelholder via movable launder.
FIGURE 60.
furnace may play roles both in reducing free oxygenand increasing lining life by 30 percent to 50 per-cent, depending on the amount of borings used inthe initial charge. The hypothesis is that when the oilon the chips ignites at the beginning of the melt, oxy-gen in the furnace is depleted, creating a nonoxidizingenvironment, much as melting in nitrogen gas or otherreplacement atmosphere would. This phenomenonrequires further study.16
2. Greater lining life
Increased lining life is also a significant batch meltingadvantage.
With greater kilowatts being placed on a medium fre-quency furnace, it is important for foundries to maxi-mize lining life so that relines may be scheduled dur-ing a nonproduction period of time, such as a week-end. Batch melting operations, where a complete batchis melted and poured with little holding time, typi-cally allow much longer lining campaigns than com-parable heel melting operations. That's because batchmelting minimizes the amount of time the lining is incontact with molten metal. For example, where a com-
plete batch can be duplexed into a holder or ladle thesame size as the furnace, foundries can double thelining life of the equivalent medium frequency heelmelter.
The average lining life in a 10 metric ton furnace isbetween 300 and 400 batch heats or 3000 and 4000tons. The maximum lining life that has been reportedon a 10 ton furnace running gray iron is 8000 tons.
As with everything, there can be trade-offs depend-ing on application.
Lining life in dual-output or multiple-output systemswith more than one furnace will not realize the samelining life as a single batch melting furnace connectedto a single power supply. While these systems allowyou to achieve 100 percent utilization of the connectedpower, they are designed to operate with one or morefurnaces acting as a holder during part of the batchmelting cycle. For example, a Dual-Trak® dual-out-put system with two furnaces cannot have an effec-tive load factor of more than 50 percent. Therefore,lining life will be determined by how long a particularuser is holding iron and at what temperature.
39
Refractory Performance Curve700
600Number of Heats
Per Lining,Iron Melting 500
-Excellent furnace practices-Low temperature alloys-Less aggressive alloys-Around the clock operations-Limited cold starts-Limited refractory temp, cycling-Larger furnace capacities
MultipleFurnace
Capacities
400
300
200
100
Average
-Poor furnace practices•High temperature alloys-Highly aggressive alloys•Low daily operating hours-Many cold starts•Long holding times•Smaller furnace capacities
0%
FIGURE 61.
20% 40% 60% 80% 100%
Furnace Load FactorAverage lining life would be equivalent to a mediumfrequency heel melter and would be expected to rangebetween 250 and 350 heats for a 10 ton furnace. Allefforts should be made to maximize the furnace loadfactor to achieve maximum lining life. Operations thathave low furnace load factors will have poor lininglife.
Basic lining considerations
With high power density batch melting, it is particu-larly important to closely control the factors whichdetermine lining life. Special care must be taken dur-ing lining installation and sintering, during dailymelting operations and in the performance of liningmaintenance tasks. (Fig. 61)
There are some basic fundamentals in installing thelining:
• Lining forms must be held down mechanically byeither cross-bracing over the top of the furnace orsome welded tabs to keep the form from moving upwhen vibrating. It should be noted that the liningforms will move up very easily. In fact, if not helddown, simple forking of the material on the first layerwill cause the form to move up. The form movingup will cause an air gap between the melt-out formand refractories and it doesn't take very much airgap to make an excellent insulator. A 1/16" gapwill prevent the bottom refractory from properly be-ing sintered.
40
• Erosion is causedby two basic fac-tors, mechanicaland chemical. Fewlinings fail becauseof mechanicalabuse. When me-chanical damageoccurs, it tends to benoticed. Chemicaldamage, however, ismuch more subtle.
It is important to payclose attention tochemical erosion,
especially on theElephant's Foot Lining Erosion first heat. The
FIGURE 62. first heat is the
most importantbecause if we were to start a furnace up with lowsilica charge, steel and some high carbon pig, wewould have some initial erosion as the charge wouldattempt to take silicon from the refractory, as allelements will try to go to their neutral state. (Fig.62) One solution is to have at least a 2% or highersilicon formula at least halfway up the furnace.
FIGURE 63.
•4
Silicon will also help reduce the oxygen within themetal, reducing the chances of oxidation. Elementslike manganese reduce at low temperatures andcan be very detrimental to a new lining. The addi-tion of such elements should always be late in thecharge when the temperatures are elevated.
• Experience with channel furnaces has shown thatholding at low temperatures for long periods of timewill develop manganese oxide and cause excessiveerosion at low temperatures. The addition of carbonshould not be early in the first melt, as it will reducethe silicon oxide and cause early erosion and spalling.Carbon gas can penetrate a new lining and condensetoward the coil as carbon. This also can happen whentorching the furnace with a rich flame developing COgas, causing the carbon to deposit as far back as thesteel shell.
The better care we take of the initial charge, the betterthe overall lining will run. This type of problem canbe seen easier with ductile iron where there is a lowsilica content in the batch at an elevated temperature.
A simple detail of when to add carbon and silicon to anew batch can give good life or unsatisfactory life.17
(Fig. 63)
U.
JШ
1.0 2.0 3.0Catbon % o
4,0
Isotherms of Equilibrium between Silica (SiO2) Of the Lining and Carbon (C) of the Iron Charge.
41
Bottom refractory problems
It has been noted that some iron melting furnaceshave had premature bottom wear equal to half or lessof sidewall wear during a lining campaign. This hasbeen attributed to insufficient initial sinter of thebottom of the lining form. This can occur becauseof two reasons:
1. Bottom of form not in contact with bottomrefractory.
2. Bottom of form not attaining sufficient tem-perature to achieve hot face sinter.
Present sinter instructions do not allow for monitor-ing the temperature of the bottom of the steel melt-out form. If the initial sinter charge does not havesufficient density to allow a minimum of one hour at1900°F temperature, premature failure will occur. Toeliminate this problem, heavy charge pieces such as asolid sinter block should be set on the bottom of theform to attain a mass heating source. A thermocoupleshould be placed at the bottom of the form to verifytemperatures. The initial sinter cycle should be heldfor whatever time is necessary to achieve a 1900°Fbottom temperature to eliminate premature bottomfailures.
Top refractory problems
It is not uncommon to have top cap or top refractoryfinning problems until the necessary refractory instal-lation and maintenance procedures are established.Close capture furnace hoods that continuously pullcool air across the top hot refractory surfaces furthercomplicate this problem.
In establishing the proper written procedures for eachapplication, it is very important to know what you havedone and where you are going. Some important stepsin solving top finning problems include:
1. Shut off or turn down fume collection sys-tems during sintering procedures.
2. Ensure that sufficient temperature is attained(1600°F to 1900°F) and held for one hour atthe top of the lining form during sintering. Ifthe lining form has a tendency to drop in thefurnace before the top sinter is complete, tackweld tabs on the top to hold the form in place.
3. Use a gas torch as an induction assist to evenlyheat the top of the form.
4. Bring the metal level to two to three inches
below the spout when increasing to finalmaximum sintering temperature.
5. Make sure the bath is clean and slag off, ifnecessary, to ensure molten metal and not slagis at the preferred height.
6. If possible, eliminate or reduce the amount ofwet material used for the top cap, spout, etc.
7. Patch and washcoat the spout and top cap of-ten to eliminate metal penetration through anyrefractory cracks.
8. Reduce fume extraction flow when holdingor otherwise not actively melting.
Contact your refractory supplier representative for in-formation on all of the above to ensure you are work-ing with the correct materials.
3. Lower raw material costs
With diminishing ideal charge materials for heelmelting applications, the batch melter again excels inreducing purchased raw material cost by allowing thefoundryman to melt less than desired scrap. Thisincludes loose and bundled bushelings, shavings,borings or oily ferrous chips.
It is important that this material be automatically andcontinuously fed into the furnace from a chargesystem capable of feeding the entire furnace contentson demand. The most common charge systemutilized is a dedicated charge car for each furnace.One documented case was to add 7-1/2 to 15% oilyborings to a 10 ton charge.
The savings amounted to more than $80.00 percharge. In a 6000 ton per month operation, the savingswould be $48,000.00 a month. Of great importance tothe user was that no special equipment had to beadded to handle the chips and no adverse effects werecreated metallurgically. In fact, lining life actuallyincreased as reported above under reduced chargeoxidation.
4. Reduced electrical power costs
Power cost is a very important consideration not onlyfor new foundries, but also vintage induction meltshops utilizing older line frequency melt systems. Instudy after study, batch melting systems haveconsistently proven to have the lowest electrical costper ton produced compared to even mediumfrequency heel melting systems.
42
Because of the ever increasing electrical cost,Inductotherm has developed batch melting meltsystems that effectively run 100% of the productionperiod. As a result, total overall kilowatts are reduced,resulting in a much smaller induction system.
The early systems that provided this ability were theDual-Trak batch melters which have now evolvedinto the Tri-Trak® batch melter. While older linefrequency systems may be retrofitted with modernsolid state medium frequency power supplies, theoverall improvement may be limited because of theexisting charge scheme or ladle transfer size.
A detailed study of the operating impact of anyimprovement under consideration should be performed
and understood by everyone involved.
SUMMARY
In summary, where many melt systems may meetyour production requirements, batch melting will doso at the lowest cost. Operating cost becomes a fixedcost whether it's your monthly power bill, or the typescrap you must melt, or the cost of relines you haveper year. Batch melting systems have proven toprovide the foundryman with the lowest cost systemin all operating areas. Batch melting systems havealso proven to be more economical to purchase andleast costly to install.
References
1. D.P. Kanicki, M.J. Lessiter; "Consolidations Re-shaping Foundry Equipment Business," ModernCasting (M. 1998)
2. J.H. Mortimer; Research (1976)
3. J.H. Mortimer, Research (1975)
4. H.M. Rowan; "Relating Furnace Size to Frequency,A New Frequency Selection Chart for Coreless In-duction Furnaces," Foundry Management &Technology (Apr. 1987)
5. J.H. Mortimer, R.T. Ruble; Research (1980)
6. R.Q. Sharpless; Foundry Management & Technol-ogy (Feb. 1985)
7. J.H. Mortimer; "Batch Melting, Advancing Induc-tion Furnace Technology," Modern Casting (Sep.1987)
8. L.J. Kikkert, J.W. Schut, P.S. Norton; BCIRA In-ternational Conference Progress in Melting of CastIrons, Session 5, "Achieving Success with a 7 Tonne7 MW High-Powered Medium-Frequency Furnace,"University of Warwick, England (Mar. 1990)
9. Patent by J.M. Cartlidge
10. Patent by J.M. Cartlidge, O.S. Fishman, J.H.Mortimer, B. Potter, S. Rotman
11. O.S. Fishman; "AC Line Distortion for StaticPower Converters Used In Induction Melting," Induc-totherm (May 2000)
12. Electric Power Research Institute (EPRI); "PowerQuality for Induction Melting of Metals Production,"Telecommentary (TC-l 14625, 1999)
13. R.C. Turner, J.J. McKelvie; "Molten MetalSplash," Foundry Management & Technology (July1997)
14. D. Remalia; "Charging Systems for Today's Hun-gry Furnaces," Foundry Management & Technology(Nov. 1996)
15. K. Copi; "Reducing Oxidation in Medium Fre-quency Induction Melting Furnaces," AmericanFoundry Society, Induction Melting Holding and Pour-ing of Iron Conference (Jan. 1999)
16. T. Kretz, Melt Manager, Auburn Foundry, Plant2
17. P.B. Cervellero, Inductotherm (2000)
Acknowledgments
The author gratefully acknowledges the assistance of the following individuals in the preparation of this paper: (Listed alphabetically)
Joseph T. BelshPaul B. CervelleroMark. T. EckertOleg S. FishmanJohn J. McKelvie
John C. ThorpeFrancesco TirilloRobert C. TurnerVictoria Xiang
43
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