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INDUCTION MELTING FURNACES

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Furnace Selection

Magnalenz Induction Furnace

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
At 16000ºC the melt rate will be 925 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.
 
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.
 
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
2. Temperature difference in molten metal bath
3. difficulty to melt light weight scrape

  EXCESSIVE STIRRING CAUSES

1. Frequent lining erosion
2. Metal oxidized and splashing of metal
3. Lining or slag inclusions in casting found

  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.

 
HOW TO CALCULATE THE FURNACE.EFFICIENCY
 
FURNACE EFFICIENCY = THEORETICAL TOTAL HEAT REQD FOR MELTING (H T) IN KWH. FOR THE MELTING OF THE METAL IN KWH
--------------------------------------------------------------------------------------------
ACTUAL CONSUMED ENERGY BY THE FURNACE FOR MELTING OF THE METAL IN KWH (H A)
 
THEORETICAL HEAT REQD FOR MELTING:
 
H.1 IN KWH = W X SPECIFIC HEAT OF THE METAL X ( T2- T1) + LATENT HEAT OF
THE METAL XW
--------------------------------------------------------------------------------------------
3600 ( 3600 KJ = 1KWH)
 
H2 IN KWH = HEAT REQD FOR SLAG TO.HEAT AND MELT WILL BE 1.65X WSLG X3.6 ( WHERE 3.6 MJ = 1 KWH)

 

WHERE

W = WEIGHT OF THE METAL TO MELT
T2- = MELTING FINAL TEMP OFTHE METAL.
T1 = INITIAL OR CHARGING TEMP OFTHE METAL.
WSLG = WEIGHT OF SLAG GENERATED IN OPERATION.

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.,

FURNACE EFFICIENCY = HT
--------------------------------------------------------------------------------------------
HA
 

55% TO 65% (MAX) OVERALL ENERGY EFFICIENT FURNACES ARE AVAILABLE.

FOR M.S. SCRAP

SPECIFIC HEAT = 0.682 KJ / KG DEG CENTI.
LATENT HEAT = 272.0 KJ / KG

SPECIFIC HEAT REQD TO MELT 1000KGS OF M.S SCRAP WILL BE, @ 1650ºC

=   100 × 0.682 × (1650 - 30)
   -------------------------------------- KWH
                      3600
= 307 KWH

LATENT HEAT REQD,
272 × 1000 / 3600
76 KWH.

HEAT FOR SLAGE
1.6 × 25 / 3.6
12 KWH.

TOTAL KWH THEORETICALLY REQD FOR MELTING IS 395 KWH.

APPROX 400 KWH REQD TO MELT M.S OF 1000KGS TO 1650DEG CENTIGRADE.
IF THE FURNACE CONSUMES 625 KWHITON THAN THE FURNACE IS OVERALL ENERGY EFFIECIENT 65%.

[ i.e. 400 ÷ 625 = 64 % ]
 
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.

 

THE POWER LOSS IN INDUCTION FURNACE SYSTEM

 

1.

Power loss in generator / panel

2% to 4%

2.

Power loss in capacitor Bank

1.0% to 3%

3.

Power loss in Crucible
(Water cooled cables, Bus bar, change over switches)

18% to 25%

4.

Radiation loss

7% to 9%

 
Above figures can vary according to

1. Manufacturer of furnace.
2. Size of the furnace.
3. Scrap quality.
4. Plant operational specific issue.

 
FACTORS EFFECTING THE FURNACE EFFICIENCY.
 

In foundry the tool or unit of efficiency measurement is kwh/ton.
In foundry most of the power is consumed by the induction furnace is around 65% of the power.
The factors, which effects the furnace efficiency are as under;

1. Wrong selection of the furnace & operational practice:

i.   Small power supplied unit & big crucible.
ii.  furnace top extension and relative slow charging.
iii.  wrong lining thickness.

Results:

a / slower melting
b / reduced lining life
c / poor utilization of power due to increase in furnace losses.
d / poor power transfer/coupling increase melting time.
Optimum furnace size, approximately to be;

For up to 1000 kw -1.5 times the kw applied
Above 1000 kw furnace - 2.0 times the kw applied.

2. Furnace down time:

(1) molt sintering and pateching.

      Higher sintering & patch in g increases the cost of production and reduces the efficiency.

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.

Basic lining is better conductor of heat compared to acidic lining

Wrong lining selection increases breakdown,
Furnace down time and furnace losses,

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
B ) absence of material handling equipment
C ) poor molding efficiency so furnace on hold
D ) poor quality scrap, reducing lining life, takes more time to melt.
E ) absence of thermal insulation between lining & coil.

 
Total absolute energy required to turn 1 Ton of different. Solid metals to
melt @ different molten temperature.
 
Type of metal Specific heat require Latent heat require Total require kwh/Ton

Mild steel @ 1650ºC Melting temp.

1000 x 0.682 x 16200C ÷ 3600
∆T=1650-30
Speci heat = 0.9 kj/kg0C
= 307 kwh

272 x 1000 ÷ 3600 kwh
Latent Heat = 272 kj/kg
= 76 kwh

307 + 76 = 383 kwh

Aluminum @ 710ºC Melting temp.

1000 x 0.9 x 6800C ÷ 3600 kwh
∆T=7100C-300C
Speci heat = 0.9 kj/kg0C
= 170 kwh

396.9 x 1000 ÷ 3600 kwh
Latent Heat = 396.9 kj/kg
= 110 kwh

170 kwh + 110 kwh = 280 kwh

Copper @ 1130ºC Melting temp.

1000 x 0.386 x 11000C ÷ 3600 kwh
∆T=11300C-300C
Speci heat = 0.386 kj/kg0C
= 118 kwh

212 x 1000 ÷ 3600 kwh
Latent Heat = 212 kj/kg
= 59 kwh

118 kwh + 59 kwh = 177 kwh

Gold @ 1130ºC Melting temp.

1000 x 0.131 x 11300C ÷ 3600 kwh
∆T=11300C-300C
Speci heat = 0.131 kj/kg0C
= 36.38 kwh

67.62 x 1000 ÷ 3600 kwh
Latent Heat = 67.62 kj/kg
= 18.78 kwh

36.38 kwh + 18.78 kwh = 55.16 kwh

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