Despite huge efforts in the development of new ceramic materials, this group of materials has yet to achieve a significant breakthrough, although numerous applications in the fields of mechanical engineering, electronics, electrical engineering and production technology constitute a huge market potential for ceramic materials. One of the main reasons for the reluctance to employ ceramic materials is their extreme brittleness. Ceramic materials usually have either covalent or ionic bonds, i.e. a sliding displacement which, in the case of metallic materials results in good plastic mouldability, is not given with ceramic materials. Therefore, it is not possible to reduce mechanical and thermal tensions by is not possible to reduce mechanical and thermal tensions by means of plastic deformation. The main goal in the development of ceramics must therefore be to improve the stability of the material by optimising the structural conditions in order to achieve an increase in the acceptance of ceramic materials as a construction material. Improved material properties can be demonstrated first and foremost on the material itself. Furnace manufacturers and material developers must develop and improve the constructional design and the application of ceramic components jointly. For each particular application, stability and durability as well as design requirements and profitability must be taken equally into consideration. More specifically, this can mean, for instance, the improvement of the manufacturing and processing technologies for series production. From the material developer to the user and production engineer down to the furnace and plant manufacturer, comprehensive thought and development is required. Nabertherm has taken on this challenge and offers tailor-made furnace solutions for the field of research and development and for the industrial series production of high-tech ceramics.

2. Problem and Solution

Due to their sensitivity, quartz and ceramic components to be sintered cannot be exposed to any significant vibration. Nabertherm Lift-Top furnace have proven exceptionally useful in this area of application. An improvement in this technology has been achieved at Nabertherm by using mobile tables which facilitate charging of the furnace from both sides (Fig. 1). As soon as the charge in the furnace has cooled down sufficiently to allow the hood to be fully raised, the tables can be changed over and a new firing process commenced without any further delay. 

Fig. 1  Lift-Top Electric High temperature furnaces with mobile bogie (Nabertherm HT400/17/LTS2)

Raising the furnace hood, which can be carried out gradually in stages, allows the components to cool down very quickly. Already at temperatures above 1000°C the furnace hood can be lifted without any hesitation and the tables moved, provided that the components have sufficient thermal shock resistance. When designing thermal technology plants, the legal requirements with regard to environmental protection must also be complied with. Nabertherm furnaces also offer solutions for the exhausts given off during the binder removal process. Exhaust gas cleaning systems are in a position to convert harmful substances into water vapour and carbon dioxide through an environmentally-friendly catalytic combustion process.

3. Profitability

The two tables of the Lift-Top Furnace can be loaded and moved under the furnace hood alternately (Fig. 2). The tables can be loaded from three sides, reducing charging times considerably. The controller recognises the next table to be fired and moves this automatically under the furnace hood. The furnace tables can be loaded during normal production times, with the actual thermal process carried out at night or even on weekends to take advantage of cheaper off-peak electricity rates. This results in the optimum utilization of production capacities and a significant reduction in operating costs. Furthermore, Lift-Top furnaces render better utilization of residual heat possible. Heat losses can be considerably reduced and thus the production process carried out in a more economical manner. The plant technology presented here can reduce processing times by up to 40% compared with conventional charge furnaces equipped with just one table. This plant technology is therefore predestined for integration in production processes.

Fig. 2  The Nabertherm Lift-Top Electric High temperature Furnace

4. Furnace Technology

4.1 Construction - Lift-Top furnaces are high temperature hood-type furnaces which can be equipped with two or more mobile tables (Fig. 3). The maximum charge weight can be adapted to meet the requirements of the particular application in question.

Fig. 3  Construction drawing (Nabertherm HT 400/17/LTS)

Fig. 4  Construction of the Nabertherm Lift-Top Furnace table

To guarantee a fast cooling down process, each table is equipped with a bottom slide and cooling ducts. The cooling ducts in the table, lined with ceramic plates of high-purity aluminium oxide, prevent dust particles penetrating the furnace chamber (Fig. 4). Four flue gas vent holes with automatic flap control are located in the furnace ceiling. Furthermore, an extraction hood is installed above the furnace to carry off the exhaust air.

4.2 Casing and Insulation - To increase corrosion protection, the sheet-metal of the casing is zinc galvanized. All frame parts are made of aluminium profiles. The insulation of the furnace consists of several layers of high quality asbestos-free fiber material. The multilayer insulation combined with fan cooling of the double wall casing keeps the outer wall temperature low. All insulation materials are distinguished by their excellent insulation properties, high resistance to temperature change and low shrinkage. The hoods of bigger high temperature furnaces often constitute a weak point, as these hoods usually have to carry not only the insulation but also a large number of heating elements. The furnace hood is weakened even more by the bore holes for the exhaust air flaps. The problem posed by the free-hanging ceiling has been overcome by a new construction developed at Nabertherm. This is a self-supporting composite suspended system with a thickness of 90 mm. To prevent the ceiling from slumping, is is held on a frame by means of clips. An additional blower integrated below the table is available as an option which, by means of an air-duct system, ensures the uniform and gentle cooling down of components without erosive damage. Furthermore, the blower guarantees that an adequate supply of air is led into the furnace chamber which may be necessary, for instance, for the chemical reactions occurring during the binder removal process and for carrying off the gases formed during this process more rapidly out of the furnace. In addition, during these processes, the supply of air can be adapted exactly to the temperature/time program. The exhaust air is lead off through automatically controlled exhaust air flaps and can be subsequently treated in a catalytic combustion unit.

電氣高溫窯爐之詳細目錄資料可參考 Nabertherm 網頁.

4.4 Hood and Bogie Drive - The hood of the furnace is moved electrically up and down in vertical direction by means of recirculating ball screws with lift and angular gears (Fig. 5). The two mobile furnace tables are also moved electrically. All movements are controlled by a Siemens programmable controller. To ensure a gentle start and stop (soft start) the motors are controlled by a frequency converter. The speed at the ends can be set between 20% and 100% of the motor speed. To accelerate the cooling process, the hood can be raised in 3 freely selectable stages (e.g. at 1000°C by 100 mm. at 800°C by 200 mm, and at 500°C full opened). Moving the bogie horizontally into the furnace is accomplished without vibration by using a slide-bearing-supported rail system in soft-drive mode. 

Fig. 5  Nabertherm Lift-Top Furnace screw drive

4.5 Switch and Controlgear - The switch and controlgear (Fig. 6) is marked by its simple, straightforward operation, in keeping with the Nabertherm philosophy practised for decades. A clear text display renders on-line instant recognition of momentary operating and fault indications, thus permitting fast intervention in the process at any time. The automatic pre-setting of three different process cycles means that the furnace can be used as a production plant by the operator at the press of a button. The hood and tables can be operated either manually or automatically as desired. The power and control technology is installed in a free-standing cabinet which is kept at a temperature of below 35°C by means of a switchgear cooler. This guarantees a long service life of the electronic components installed. With the installation of an inspection window over all controls and display elements, together with the switchgear cooler, compliance with protection standard IP54 is achieved.

Fig. 6  Switchgear of the Nabertherm Lift-Top Electric High-temperature Furnace

Temperature measurement is carried out by Nabertherm thermocouples type B, class 2 (in compliance with DIN IEC 584 part 2). The measuring range is 40 ~ 1,800°C. Control of the thermal proceses is executed by a Eurotherm program controller type 900 EPC. This program controller with PID control algorithm can process 20 programs with a maximum of 500 segments. A self-optimization facility means the control parameters can be adapted exactly to meet the requirements of the process. Control of the heating elements is carried out by power transformers and thyristor controllers in phase control mode. The thyristor controllers are equipped with two current limiters which trigger individually according to temperature. This permits an exact temperature profile which not only leads to a uniform heating up of the components but, most importantly, protects the heating elements from overload, thus prolonging their service life. Speed control of the hood and table drives is carried out by means of a frequency converter. The integrated Siemens' programmable controller takes over all control and monitoring functions. The digital interface RS485 allows connection to a PC and thus optimum recording of the process in accordance with ISO 9000.

4.6 Plant Safety - ISO 9000 and VDE - Temperature selection limiter, mains contactor and emergency-off wiring for the electric drives of hood and tables protect man and machine from electric and thermal overload and comply with all current safety regulations such as DIN, VDE, VBG, EN. Furthermore, each furnace is subject to final inspection and testing in accordance with VDE 0113. Power cable filters for the thyristors and the frequency converter prevent, on the one hand, the infiltration of high frequency interference via the mains cable into the protected unit and, on the other, interference escaping from the inside of the unit into the mains cable. Electromagnetic compatibility (EMC) has also been taken into consideration in the designing of this plant in compliance with the requirements for the CE symbol. Process and data recording in accordance with ISO 9000 can be achieved through connected master computer or a multi-channel recorder. Furthermore, via ISDN or modem, on-line monitoring from a decentralised control desk or Nabertherm service monitoring is possible. This means faults are quickly traced and production standstills minimized.

5. Environmental Protection

Not only the TA Luft (Technical Regulation, Air) reduces the emission of hydrocarbons considerably, but also the Bundesimmisionsschutzgesetz (BImSchG) (Federal Emission Protection Act) and the Gefahrstoffverordnungen (GefStoffV) (hazardous goods regulations) in the workplace regulate the handling of materials harmful to the environment. Nabertherm therefore offers cleaning systems which clean exhaust air in the appropriate manner. Nabertherm Lift-Top furnaces can be equipped with an extraction hood for sucking off the exhaust air. A fresh air flap in the exhaust air duct regulate the concentration of the exhaust air mixture. The exhaust air first passes through an adsorption stage which binds the inorganic substances. The organic substances are heated up to 400°C in a three-stage catalytic exhaust air cleaning unit, consisting of pre-cleaning, main cleaning and follow-up cleaning, and converted on the surface of the catalyst into carbon dioxide and water vapour. Heating up is necessary so that the incoming gas mixture reaches the temperature required for the catalytic decomposition process. The cleaned exhaust air is blown by a hot air fan through the extraction hood out into the open (Fig. 7). As a standard, the cleaning units are equipped with honeycomb catalysts which guarantee a low pressure loss and therefore ensure reliability in operation, proven a million times over in the car industry. Control of the exhaust air cleaning unit is carried out by measuring the exhaust air temperature. When the gross calorific value of the exhaust gas is sufficient the temperature of the catalyst is maintained without the need or additional energy. Through a heat exchanger integrated in the exhaust air flow a heat recovery of up to 20 kW can be achieved, corresponding to an energy recovery of up to 10% of the energy required for the whole sintering process. 

Fig. 7  Exhaust air cleaning system for Lift-Top Electric High-temperature Furnaces (Nabertherm catalyst KAT450)

6. Conclusion

This article presents a furnace technology developed by Nabertherm which, through the use of mobile furnace table in lift-top operation, offers a lot of advantages:

• Automatic table change. Utilization of residual heat.

• Loading from three sides. Temperature distribution of ± 3K

• Fast cooling down of components. Excellent plant safety.

• Environmentally-friendly cleaning of exhaust gas.