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INDUCTION MELTING FURNACES
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Furnace Selection
FURNACE SELECTION PROCESS  
Step 1  
The furnace capacity selection is the most common question bothering for the buyer/prospects. Sometimes it creates confusion, because different manufacturers suggest different combinations. How should one select the furnace (Kw/KG/Hz) which fitted to one's requirement technically and commercially as well? So we suggest every one to follow the furnace selection process step by step. 

• Which metal or alloy do one's want to melt and at what pouring temperature? • Melt rates or melting speed is different for different metal & alloys at particular applied kw, this is due to difference in specific heat of different metal. • At the same time due to different latent heat of each metal, superheating of each molten metal will be different, so different melt rate at particular applied kW. 

For Example  
For Different Metal At particular kW Furnace Compare Brass & Steel Melting For Steel Melting 1000 Kgs / Hour @1600ºC Where as, Brass Melting will be, 1900 Kg / Hr @11000ºC 

Summary  
So At same kW, different metals, different Melt rate For super Heating case: At particular kW for steel melting: At 15000ºC the melt rate will be 1000 Kgs / Hour 

Summary  
For Same metal, At particular applied kW, Different Pouring Temperature, Different melt rate. Once you configure your requirement i.e. melt rate @ temperature degree centigrade ºC according to step one, Now switch to step 2. 

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Step 2  
You must specify the liquid metal requirement in tons or in Kgs, per month with working hours in a day.  
( A ) For Steel Ingot  
Suppose you required 1200 Tons / Month of M.S. Steel Ingot @ 1600ºC what the Furnace size should be? • 1st consider working days  25 days • 2nd 1200 Tons 1 25 days gives 48 Tons of liquid metal Day out of 24 Hrs working. • 3rd 48 Tons 1 24 Hrs i.e melt rate will be (48/24 = 2) 2 Tons per hour • 4th Now consider the unproductive time of cycle, i.e. time for deslagging, charging, sintering, lining etc. so the furnace utilization will be 80 % thus • 2 tons 1 hr divided by 0.8. is 2.0 ÷ 0.8 = 2.5 Tons / Hour should be the required furnace melt rate at 1600ºC . • 5th Now refer the furnace specification table. You will find the nearest melt rate can be achieved by 1500 kW 1 3 Tons Furnace i.e. 2.6 Tons 1 Hour Melt rate. • We have considered here 1500kw means 1500 KVA – Exclusive power at the furnace Input. So the Answer is By 1500 KW 13 Ton furnace you can produce 1200 Tons of M.S. Steel Ingots @ 16000C per month. 

B) For Castings (Step 2)  
Suppose you want to make 300 Tons / Month of finished steel casting, with an average yield of 65% and utilizing of furnace is about to 75%. Then what should be the furnace capacity? Pls suggest. • So now 300 Tons ÷ yield (0.65) • Gives total monthly molten metal requirement for casting is 465 Tons / month. • Now 465 Tons divided by 25 working days gives 18.6 Tons of molten metal per day of 24 hours working. • 18.6 Tons again divided by 24 hours gives required melt rate of molten metal is 775 kgs / Hour. Now considering the utilization of the furnace is 75 %, is due to sintering, lining deslagging, recharging, composition setting etc. • Thus actual molten metal requirement will be 775 ÷ 0.75 i.e. 1033 Kgs / Hour. • Next step now you refer the table & from the table the nearest suitable furnace, which can give 1033 Kgs/ Hour melt rate is FER 1. • So you should select 600kW & 1 Ton Induction Furnace, which will give you the suitable melt rate. • Another main factor which one foundry men has to be decided is, what should be the batch size? • The batch size will be depending upon the molten metal weight you required. 

For Example  
If you required 600 Kgs, 750 Kgs, 1000 Kgs, metal at a time then you can select 600 kw solid state M.F. Power source with 1000 Kgs crucible or otherwise, if your molten metal requirement is around 500 Kgs or less, then you can select 300 kw x 2 Nos solid state M.F. Power Source with 500 Kgs crucible. Which will cost you more, compare to buy 600 kw one unit.  
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Step  3  
Now what should be the right frequency? Normally frequency selection is not available, and is not an important constraint too. There are many combinations of Kw and Kgs available at different frequencies. Basically frequency is one parameter which affects stirring. In a coreless Induction furnace stirring is produced by magnetic forces acting on a molten metal because of interaction between the coil current and the current flowing in the molten metal bath. The force is the strongest at middle part of the coil so metal is forced at the central side of the bath from where it is resolved to upward and downward. Metal moves up because of upward resolved force. The upward movement of the metal in the center creates meniscus a unique characteristic know as a stirring effect is measured by h/D ratio (h by D ratio). Stirring effect is depending upon the power frequency applied, the induction coil & molten bath as well as density and viscosity & molten metal. The three major variable which Effect stirring are 1) Power 2) Frequency 3) Furnace Size Stirring can be change with charging anyone of the major variable. 

WHAT IS EFFECT OF STIRRING  
INADEQUATE STIRRING CAUSE  
1. Insufficient Mixing of Alloys 

EXCESSIVE STIRRING CAUSES  
1. Frequent lining erosion 

• Once you decide the kW/Kgof the furnace & according to your mixing or alloying requirement, metal properties, operating frequency can be decided.  
Role of Frequency in Melting  
High power density melting allows better utilization of the equipment, minimizing the amount of time needed to perform a melt. This also improves the efficiency since the energy loss due to heat conduction and radiation is also minimized because the molten metal is not kept in the furnace for a long time. This method of quick high power density melting and complete emptying of a furnace became known as "batch" melting. The older technique, called "heel" melting, involved large furnaces which were only partially emptied and then topped by a load of solid metal charge. The batch melting method requires larger power supplies operation on higher frequencies. High power density i.e. produced melting power kw/kg or per ton of charge, gives better utilization of the furnace, and so takes less time to melt the metal in particular batch size. This too improves the overall effective of the melting operation as energy loss due to conduction of heat through lining and radiation loss is reduced, reason is less holding time of molten metal in the furnace. The lining life of the furnace is also gone up and so over all efficiently of the melting operation is gone up so the melting cost per kg totality has to be reduced. This method of quick high power density melting or hatch melting requires larger power supplies (kw/kg) on high frequency. Because produced melting power or power density is a function of the product of sensitivity of the metal and the operational of frequency metal. Pm = Const × resistivity of metal × Frequency Plays significant role for effective melting at a particular power on a particular batch size. 

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HOW TO CALCULATE THE FURNACE.EFFICIENCY  


THEORETICAL HEAT REQD FOR MELTING:  






WHERE W = WEIGHT OF THE METAL TO MELT HERE, H T = H1 + H2 AND • ACTUAL CONSUMPTION OF THE MELTING CAN BE MEASURED FROM THE INPUT BUSBAR OF THE FURNACE PANEL IN KWH. (H A IN KWH) WHICH IS HIGHER THAN H.T. • SO DIFFERENCE BETWEEN HA AND HT (HA  HT) IS LOSS DUE TO CONDUCTION RADIATION AND OTHER LOSSES, • AND SO FURNACE EFFICIENCY IS RATIO OF THEORETICAL HEAT REQD FOR MELTING IN KWH TO ACTUAL CONSUMED HEAT IN KWH. i.e., 



55% TO 65% (MAX) OVERALL ENERGY EFFICIENT FURNACES ARE AVAILABLE. FOR M.S. SCRAP SPECIFIC HEAT = 0.682 KJ / KG DEG CENTI. SPECIFIC HEAT REQD TO MELT 1000KGS OF M.S SCRAP WILL BE, @ 1650ºC = 100 × 0.682 × (1650  30) LATENT HEAT REQD, HEAT FOR SLAGE TOTAL KWH THEORETICALLY REQD FOR MELTING IS 395 KWH. APPROX 400 KWH REQD TO MELT M.S OF 1000KGS TO 1650DEG CENTIGRADE. 

DISTRIBUTION OF LOSSES IN INDUCTION FURNACE:  
LOSSES IN INDUCTION FURNACE  
The theoretical energy require to melt one Ton of steel is 385 TO 400 KWH / Ton. However in actual practice, the specific energy consumption is remarkably higher to 550 KWH / ton to 950 KWH / ton. 

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THE POWER LOSS IN INDUCTION FURNACE SYSTEM 



Above figures can vary according to  
1. Manufacturer of furnace. 

FACTORS EFFECTING THE FURNACE EFFICIENCY.  
In foundry the tool or unit of efficiency measurement is kwh/ton. 1. Wrong selection of the furnace & operational practice: i. Small power supplied unit & big crucible. 

Results: a / slower melting For up to 1000 kw 1.5 times the kw applied 

2. Furnace down time: (1) molt sintering and pateching. 3. Break down time : due to poor maintenance the total production stops sometimes. Higher breakdown results in increasing the' cost of production 4. Low supplied power : some time supplied voltage is low, so furnace draw less power. Causes slow melting & inefficient operation resulting; increase cost of production 5. Wrong lining : some time lining material selection is wrong with respect to metal. Wrong lining selection increases breakdown, Resulting in inefficient operation. Besides above all points some more factors which can also effect the energy consumption per MT are: A ) poor coordination between melting staff & contractor 

Total absolute energy required to turn 1 Ton of different. Solid metals to melt @ different molten temperature. 



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