Industrial electric heating furnaces – Resistance furnace device – Lining
Patent
1997-01-30
1998-11-10
Hoang, Tu B.
Industrial electric heating furnaces
Resistance furnace device
Lining
373109, 373119, 373122, 392435, 392437, F27D 100
Patent
active
058355251
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to furnace linings and furnaces using them.
BACKGROUND OF THE INVENTION
In gas-powered metal melting furnaces the non-luminous products of combustion heat the lining of the furnace. This lining, or radiant surface, then radiates heat to the crucible. Some heating of the crucible occurs by convection but this on a minor scale unless the gas has a high velocity and this does not tend to occur in this type of furnace.
Conventional linings are made of brick, castable refractory, or ceramic fiber. The lining is provided as a continuous structure and if a refractory failure occurs at any point in the lining the operator of the furnace is invariably obliged to replace the entire lining. This involves shutting down the furnace during the relining process, which takes at least some days, and this incurs costs in addition to the cost of the new lining and the cost of having it fitted. After a new lining has been fitted the furnace must be heated up again which incurs a further use of energy.
The materials used to line conventional gas powered furnaces have specific features as follows:
Brick linings (and the insulation behind the linings) take a considerable amount of time to heat up to the desired temperature. Consequently, they use considerable amounts of energy. Another disadvantage is that the bricks are cemented together into a wall and if even a small area cracks or otherwise fails to provide a uniform heating surface the entire wall must be broken and rebuilt. Thirdly, building brick linings requires specialist brick-laying skills as the bricks must be carefully laid in a short tower-like structure of small diameter (from 0.75 m).
Castable refractory linings are easy to make. However, it is difficult to prepare a reliable castable refractory lining as the refractory material tends to crack and then break up as temperature fluctuations occur at the radiant surface. Castable refractory and dense brick linings have approximately similar heat capacities.
Ceramic fiber linings have advantage in that their heat transfer and thermal insulation properties give thermal efficiency 20% higher than brick or castable linings. However their being fiber requires quite onerous safety precautions be observed during handling. Also, linings comprising ceramic fiber are easily contaminated, for example by metal splashing out of a crucible or glaze from the crucible itself. The fiber can react to form a glass which severely reduces the ability of the lining to radiate heat, and a contaminated lining must therefore be replaced.
Electrically heated furnaces, though the energy is costly, are used as alternatives to gas-powered furnaces. Electric resistance bale-out furnaces for example find application in aluminium and zinc alloy pressure and gravity foundries. As in gas-powered furnaces, the principal mode of heat transfer is radiation. The heater panels and element wires rise in temperature to around 1150.degree. C. when protective controls arrest further increase, and heat is transferred according to the temperature difference and the surface areas of transmitter and receiver.
One type of electrical furnace is type HE Electric Resistance Bale Out Furnace manufactured by the present applicant. The radiant surface of the lining is heated by means of electrical heating elements which are part embedded in a castable refractory. The lining is split into panels, each of which is separately removable so that if a fault develops in one panel, that panel can be replaced without shutting down the furnace.
In both kinds of furnace thermal strain and shock to the crucibles need to be avoided as far as possible and an atmosphere conducive to good melting maintained.
Gas-powered furnaces are generally more economical than electrical furnaces for heating the contents of the furnace to the desired temperature from room temperature as this usually requires a great deal of energy. However, electrical furnaces are advantageous for holding the furnace at the desired temperature. The temperature of th
REFERENCES:
patent: 2525882 (1950-10-01), Ferguson
patent: 4435819 (1984-03-01), Plume
patent: 5218615 (1993-06-01), Wieland et al.
Brochure entitled "Morgan Electric Resistance Bale Out Furnace Type HE/BA" (four pages; undated).
Brochure entitled "Morgan High Efficiency Bale Out Furnance" (four pages; undated).
WPI Accession No. 90-355220/48 (English-language abstract of DD 280313).
Hoang Tu B.
Morganite Thermal Ceramics Limited
Russell Dean W.
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