Heat Loss of Liquid Metal

7/30/2019 Heat Loss of Liquid Metal

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HEAT LOSS LADLESENERGY SAVINGS

Ir G.D HENDERIECKX GIETECH BV OKTOBER 2006 1

HEAT LOSS OF LIQUID METAL

1. INTRODUCTION2. CONDUCTION HEAT TRANSFER3. HEAT RADIATION4. CONCLUSION

1. INTRODUCTION

This paper will consider the amounts of energy that are lost from liquid iron duringtypical foundry operations, look at some preventive measures as well as the benefitsof more effective heat conservation in liquid iron.

During processing liquid iron in ladles and holders, there will be a continuousreduction of temperature due to heat losses from conduction and radiation.In order to keep a usable pouring temperature into the mould, these heat losses mustbe compensated for by excess tapping temperatures at the furnace.This in turn leads to increased cost of heating the iron, as well as higher alloyconsumption and refractory wear. By means of effective heat conservation, thelosses and the consequences can be minimised, and thereby reduce the overall costof produced iron.

The heat losses comprise conduction heat transfer through refractory linings andheat radiation from hot surfaces, as will be presented in more details in the followingwith the ladle design as shown in next figure applied for calculation purposes.


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2. CONDUCTION HEAT TRANSFERConduction heat transfer is governed by Fourier s law of conduction:

Q = - k * dT / Dx = -k * (T2 T1) / L = k * (T1 T2) / L

Q heat transfer per unit area, in W / mK thermal conductibility, in W / m * KT1 temperature of hot surface, in CT2 temperature of cold surface, in CL refractory thickness, in m

The equation is negative because heat transfer is contrary to the direction ofthe heat gradient.

Thermal conductivity varies between different refractory materials, and with

temperature, as indicated by Table 1 below. Data on specific materials is available inreference books or from the supplier of the refractory material in question.

Table 1: Some Typical Thermal Conduct ivity values.

Material T [C] k [W / m * K]

Al2O3-SiO2Refractories

Low Al2O3 600 800 0,80 1,00

High Al2O3 700 1000 1,20 1,25

Silic oncarbide (90 % SiC) 1000< 1,30 1,40

Insulating brick 200 700 0,30 1,40

Ceramic fibre board 100 500 0,30 0,80

Mineral wool blanket 0,04

Vermiculite 0,05-0,06

Steel 50 250 0,04 1,06

Single component wall

An example of heat transfer through a single component lining:

T1 =1480 C =1753 K,T2 =45 C = 318 K;k = 1 W / m * K for high alumina lining;L =51 mm = 0,051 m

Q = 1 * (1753 - 318) / 0,051 = 28,1 kW / m

For a single component alumina lining, the heat loss is 28 kW per squaremeter.


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Figure: Heat conduction througha single component wall a mult iple component wall

Doubling the refractory thickness will cut the heat loss in half, but is normally not auseful solution for foundries that are trying to cut the weight of refractory to aminimum. A better alternative is to combine different materials in a multiplecomponent lining. In this case, the heat transfer can be stated as follows:

Q = (T1 T2) / { (L1 / k1) + (L2 / k2) + }

Indices indicating for material 1, 2 and so on

An example of heat transfer through a multiple component lining, composing a highalumina inner lining, an insulating brick layer and a outer ceramic paper layer:

High alumina: L1 = 25 mm, k1 = 1 W / m * KInsulating brick: L2 = 25 mm, k2 = 0,5 W / m * KCeramic paper: L3 = 6 mm, k3 = 0,05 W / m * K

Q = (1753 - 318) / (0,025 + 0,050 + 0,12) = 7,4 kW / m

For a multiple component lining, the heat loss is 7 kW per square meter.

Thus, by changing the refractory materials, the heat loss through ladle walls are

reduced by 75 % with only 5 mm increase in wall thickness.

A reduction of 28 C in tapping temperature whilst maintaining the same pouringtemperature has been reported by a foundry by a similar change of ladle linings.

It should be noted that, in this case, the ceramic paper is the controlling factor, havinga far lower thermal conductivity than the other two components. To take advantage ofceramic papers, the service temperature should not be exceeded and this can becontrolled by the application of the refractory used at the working face. These should


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not only protect the ceramic paper, but also have a low heat capacity to reduce thetime and energy required for preheating. Further, the working face refractory shouldbe porous to allow escape of moisture during curing.

Other considerations in the selection of a refractory system include:

1. Knowledge of chemical reaction with metal or slag2. Consideration of the refractory service temperature and the actual

temperature of operation in the foundry3. Cost of installation and maintenance equated to the lifespan of the

refractory.

It is an unfortunate fact that these factors often override the best refractory systemsin terms of thermal efficiency. On many occasions, the use of a highly conductiveworking face material must be used, in which case selection of the baking materialsbecomes more critical.

A telling example of this is ductile iron where, due to an interaction betweenmagnesium and silica based lining, an alumina working face refractory is preferred.Alumina refractory tends to be more conductive to heat and thus the selection ofother components in the system is more important.


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3. HEAT RADIATIONHeat radiation is the main cause of heat loss from a hot surface (metal or inner ladle)and is given by the following:

Q = * * (T14 -T24)

the emissivity of the radiating body the Stefan-Boltzmann constant (5,67 * 10-8 W / m* K4),T1 the temperature of the radiating bodyT2 the temperature of the receiving body

The emissivity for a black body is 1, and for grey bodies between 0 and 1.Some common values are given in Table 2 below.

Table 2: Some Common Emissivity values.

Surface T [C]

Sheet steel 25 50 0,81 0,83

Molten iron 1400 1600 0,25 0,40

Al 2O3-SiO2Refractory

Low Al2O31000 1500

0,65 0,80

0,45 0,60H i g h Al2O3

An example of heat radiation from an exposed metal surface:

T1 =1480 C =1753 KT2 =45 C = 318 K

= 0,33

Q = 0,33 * 5,67 *10-8 * (17534 3184) = 176,6 kW / m or 29,4 kW for 1/6 m

From the exposed metal surface the heat loss will be 29,4 kW.Covering the ladle by a refractory lid will greatly reduce the heat losses, since afterthe initial heating of the lid, the net losses are restricted to conducted heat, rangingfrom 4,7 kW using a single component lid, down to 1,2 kW using a multiple

component lid.

A foundry with a 2,5 m long launder spout from a cupola, gained 22 C by coveringthe previously open launder.

After emptying the ladle, the hot refractory lining will be the emitting body. Accordingto Figure 1, the surface is 1 m. The net emitting surface, however, is limited to theladle opening, which is approx. 1/6 m.Compared to a metal surface, a ladle lining will rapidly loose temperature.


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Examples of heat radiation from a ladle lining at 1480, 1000 and 500 C:

T1 = 1480 / 1000 / 500 C = 1753 / 1273 / 773 KT2 = 45 C = 318 K

= 0,45

Q(1480 C) = 0,45 * 5,67 *10-8 * (17534 3184) = 240,8 kW / m or 40,1 kW (1/6 m)Q(1000 C) = 0,45 * 5,67 * 10-8 * (12734 3184) = 66,8 kW / m or 11,1 kW (1/6 m)Q(500 C) = 0,45 * 5,67 * 10 -8 * (7734 3184) = 8,9 kW / m or 1,5 kW for 1/6 m

Thus, the heat radiation from an empty ladle will start at very high values (40 kW), butwill rapidly decrease and will at about 500 C be of the same magnitude as theconductive heat loss through a insulating cover.Hence, the cover must be put on the empty ladle rather quickly after the pouring inorder to give significant heat conservation.

Our experience has shown that by covering the ladle between fills with a goodinsulator, tapping temperature reductions up to 30 C can be achieved, most notablythis can be seen in such systems as fixed top covered tundish ladles.Most of the energy lost by radiation can be saved using a cover.


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4. CONCLUSIONThis paper has addressed the means of which heat and subsequently temperatureare lost, and it is apparent that the major advantages of reducing these are:

1. Reduce the energy costs associated with melting as lower furnacetemperatures are possible;

2. Increased productivity, due to reduced superheat requirements and therebyless time in the furnace.

The gains are however not restricted to these, and the following benefits should alsobe noted:

Increased refractory li fe:As stated above, the use of an insulating cover between fills will reduce thetemperature variations during the pouring cycle. This is advantageous toreduce thermal shocks imposed on the lining, as well as ease slag removalwhen it is more fluid and does not freeze to the walls. This increases refractorylife and reduces labour time / cost in repair.

Reduced alloy costs:Lower tapping temperature leads to increased magnesium yield duringtreatments, and allows for less treatment alloy usage. From this, it may bepredicted that a reduction of tapping temperature of 25 30 C can increasethe Mg-recovery by about 10 %.

Covered tundish ladle:

Use of a tundish cover increases the efficiency of MgFeSi treatmentsconsiderably. Typically, Mg recovery in an open ladle is 30 70 %, whereaswith the tundish 50 80 % could be expected.

Casting quality:Thermally ineffective ladles can result in variable pouring temperatures anddifficulties in controlling the temperature during the day. Such variations maylead to sand burn-on or porosity from hot metal, and cold misruns or slagdefects from cold metal.

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Heat Loss of Liquid Metal