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Lining is the term for the fireproof cladding of furnaces and ladles.

Furnace covers serve essentially to avoid heat losses, and as safety devices for the operating personnel, to screen them against the heat radiation and prevent the danger of falling. Furnace covers are made of steel plate with a ceramic cladding, which is as a rule cast. An imaginary furnace cover is set in place under processor operation of a crucible furnace at the end of the melting process, and all stored values of the melt are processed, and displayed on the screen or printed out for the information of the operator (ready for further measures).

Larger furnaces with cover diameters of over approx. 1,200 mm have an additional small cover that can be swung open by hand. Through this opening, additives can be added to the melt or the temperature measured.

Furnace control is the operation of a furnace, particularly a melting furnace, in order to fulfil certain metallurgical conditions. This is also referred to as process control.

Furnace bottom construction varies in design according to the furnace size and the application in question. Although the heat losses can be reduced by the installation of heat insulation blocks, this also reduces the strength and rigidity. Cast concrete elements have greater strengths and somewhat higher heat conduction properties. This trend has lead to longer working life, thanks to the use of high-quality concretes. In most cases, the masonry underpinning of induction coils has been replaced by concreting for cost reasons. With furnace sizes of up to approx. 5 t, the crucible removal device can be installed in a central position. Above 5 t capacity, the installation is displaced excentrically in the direction of the casting spout.

Furan resin binders are cold- or warm-hardening mould material binders, which are usually used for the production of the core.

Crucibles up to approx. 1.5-t capacity are made of graphite clay or silicon carbide, and in the case of metal crucibles, of cast iron, cast steel, steel plate and plated steel plate. These crucibles are used in metal foundries for non-ferrous metals. Acidic crucibles with capacities of up to 13 t are made from SiO2 material. The crucibles are rapped in in special moulds with the addition of binding agents, and after being allowed to cool in air are dried in drying furnaces, or even pre-sintered so that they can be transported by road or rail. These crucibles are “fixed” with back-fill compound after being installed in the furnace, patched at the upper edge, and then sintered in the same way as a normally lined crucible. The working life of such crucibles is comparable to that of conventionally lined crucibles.

Flow speed in lines carrying water, as in cables, pipes and hoses, should not exceed 2.5 m/sec. In the crucible induction furnace coils the flow speed should be approx. 2 m/sec. In connection areas and necessary restrictions in pipes caused by the design, flow speeds of up to 4 m/sec. can sometimes occur. The lengths of line where this occurs should be kept within a maximum range of 100 mm. In extraction systems, speeds of approx. 40 m/sec. are achieved in the pipelines. In a hood extraction system of a 13-t furnace, the flow speed at the extraction slot in the crucible cover should be approx. 6 m/sec., in order to create an adequate under-pressure.

Fireproof describes the property of a ceramic material to withstand high temperatures without softening. Under German standard DIN 51060, the term is defined via the Seger cone fall point: the softening point of a standardised test piece (Seger cone) under defined heating conditions.

Classification under DIN 51060

DIN 51060 classifies refractory ceramic raw materials and products into three levels by their measured Seger cone fall point:

Level	Seger cone fall point
Heat-resistant	up to 1,500 °C[2]
Fireproof	1,500 °C to 1,800 °C[2][3]
Highly fireproof	above 1,800 °C[2][3]

A material is therefore considered fireproof once its Seger cone fall point reaches at least 1,500 °C.^[3]

Distinction from lining

Fireproof is a material property, not a component name. Lining is the physical refractory brickwork of a melting or holding furnace, while furnace lining is the general term for a furnace’s refractory lining as a whole. A material can be fireproof without being installed as a lining at all, for example as a brick or raw mix in storage.

References

1. DIN 51060:2000-06, Feuerfeste keramische Rohstoffe und feuerfeste Erzeugnisse – Definition der Begriffe feuerfest, hochfeuerfest (refractory ceramic raw materials and refractories – definitions of the terms refractory, high refractory). Language-independent standard, cited by number.
2. H+M Refractories GmbH: Feuerfeste Werkstoffe nach DIN 51060 (refractory materials under DIN 51060, German only, no English edition available). Product page, accessed 27 August 2026.
3. Gießerei-Lexikon: Feuerfeste Erzeugnisse (refractory products, German only, no English edition available). Accessed 27 August 2026.

Energy / Energy consumption is the designation for the product of performance (output) x time, and is specified in kWh. In melting, the concept of “energy consumption” per tonne of melted material is usually of interest. Here however, a distinction must be made between theory and practice. The theoretical requirement is around 390 kWh per tonne of iron. The grid energy demand of a well-designed high-output casting furnace is 510 to 540 kWh/tonne at the tapping temperatures usual in foundries (approx. 1,450 to 1,500 °C), while typical plants in practical melting operation require 585 to 640 kWh/tonne.^[1] If a crucible furnace is operated as a “melting machine” with optimum charging, extraction and process control, energy consumptions of 560 kWh/tonne at the higher tapping temperature of 1,550 °C can be achieved. The surcharge compared to the high-output range is due to temperature: approx. 15 kWh/t per 50 K of additional temperature, on the grid side (calculation check: 0.83 kJ/(kg·K) x 50 K = 11.5 kWh/t of useful energy, roughly 15.3 kWh/t of grid energy at an efficiency of 0.75; cf. /melting-time/), so that 560 kWh/t at 1,550 °C is consistent with the high-output range at 1,450 to 1,500 °C.

References

1. Dr. Erwin Dötsch (ABP Induction Systems GmbH): Induktionsofentechnologie und Energieeffizienz (induction furnace technology and energy efficiency). GIESSEREI 06/2011, published on guss.de (Bundesverband der Deutschen Gießerei-Industrie e. V.): specific enthalpy 390 kWh/t for melting and superheating cast iron to 1,500 °C, grid energy demand 520 kWh/t at 1,500 °C, practical melting operation 560 to 650 kWh/t. Accessed 27 August 2026.

Earth shorting insulation test is restricted as a rule to the area of the furnace and there mainly to the immediate area of the furnace coil. The first thing to do is check the function of the earth shorting indicator with a 500 Ohm resistance, which is installed in the measurement line. In order to establish whether the earth short lies between the melt and the coil, a significantly higher resistance value must be displayed after disconnecting the bottom electrodes. If the earth short is still present with the same value, the short must lie outside the furnace coil. In a dark furnace area, the short may even be visible to the naked eye. If the short lies in the area of the supply lines or the power supply, this might also be visible in dark working areas. In mains frequency systems, earth shorts often occur in the area of the starter resistors. One “rough method” is a check using a bridged insulation detector, the earthed neutral point of the transformer using a 25 or 35 A fuse. This method can even be used over a longer period to arrive at an audible diagnosis, e.g. in the balancing choke or between an iron pile and the coil.