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The most important steps in the ceramic manufacturing process arguably lie in drying and sintering. Drying becomes especially critical if the amount of moisture is high or if the product is large or a complicated shape. Drying also depends on the method of fabrication. Dry pressing or cold isostatic pressing components can result in < 2% moisture, in contrast to slip-cast components that have more than 15% moisture.
All of the above problems can be overcome through the use of environmentally friendly microwave technology for ceramic processing. The heating rate can be substantially higher during microwave processing, which results in shorter cycle times and lower energy costs. The use of 2.45 GHz microwave energy has been used by many researchers worldwide, mainly for sintering various ceramic materials at high temperature. However, though microwave drying has been extensively studied by many worldwide materials researchers, few reports are available on the use of the technology for processing ceramic materials.
We initiated the microwave drying of high-voltage insulators at a small segment level1 and demonstrated the technology’s effectiveness in a larger scale at the 1 metric ton (MT) level. A 30 kW microwave drying system was developed for drying 1 MT of material up to a height of 2.5 m.
One question arises regarding whether the fast microwave drying process creates defects. This has been tested, and the results indicate that no deterioration takes place for the porcelain components that were dried using the fast microwave process.
Most of the available literature has discussed the processing of these materials in the laboratory scale; very little information is available for the large-scale processing of these components. This article describes our effort in drying porcelain insulators using microwave energy in an industrial scale. In addition, developments from a laboratory level to the establishment of a pilot-scale facility, as well as a demonstration of the technology in the large-scale processing of electro-porcelain insulators, are highlighted.
Insulator DetailsDrying is a critical processing step for ceramic components. The process assumes significance for large and complicated components with high moisture content. The porcelain insulators are alumina, silica and flux-bearing compounds that undergo complex reactions during sintering. BHEL manufactures insulators in the range of 11-800 kV. Available types include post and pin insulators; disc insulators, including HVDC disc insulators up to 530 kN rating; solid core insulators; and hollow insulators of different ratings. Insulator sizes vary, with diameters ranging from 100-450 mm, and complicated creepage to 2.3 m long for solid core station post insulators of core and shade structure.
The process for manufacturing these insulators enables the green components to contain 15-18% moisture after the fabrication stage. The components need to be dried to < 1% moisture prior to glazing and sintering.
In the conventional process, the drying of the insulators is carried out using a humidity-assisted dryer in order to avoid the formation of defects during drying. Due to the insulators’ complex shapes and sizes, the drying times varied from 50-60 hours for disc insulators, and 250-300 hours for solid core insulators. It was therefore essential to develop a technology for the fast drying of these components without introducing defects.
Overcoming ComplexityMoisture content during drying is critical, and care is needed to maintain appropriate heating cycles in the low temperature range, as well as the final drying stage that takes place in the 110-130°C temperature range. Microwave-assisted drying has been proven to be the best method for reducing the drying time for such components. It has been shown that the drying time can be substantially reduced for high-voltage porcelain segments (in the range of 15-20 Kg of green weight) by 40-50% using a 3 kW pulsed microwave furnace. The drying of all types of insulators have been tested and yielded similar results, indicating that the components’ complexity is not an issue in microwave drying.
The moisture determination was carried out in all experiments after drying at different locations of shade and core. Moisture uniformity ranging from 0.9-1.0% throughout the 0.5 m long and 145 m diameter solid core samples was noted. This indicated that the process is capable of uniform moisture removal in a complex configuration like that of solid core porcelain insulators.
The other major issue is the effect of fast microwave drying on the green properties of insulators. It has been shown elsewhere that no deteriorating effect on the microwave-dried components’ green properties (e.g., bend strength, modulus, phase and microstructure) is evident.1 This was further substantiated in another study by measuring the reliability of microwave-dried components. It was reported that the Weibull modulus of microwave-dried components is higher compared to that of conventionally dried components, confirming that microwave drying of porcelain material is a feasible process.2
The microwave-dried components were also conventionally fired and their microstructure compared with conventionally dried components. The components that were dried in the microwave were found to be better in terms of a defect-free structure.3
Based on observations following detailed lab-scale experimental studies, a 30 kW batch-type microwave drying system was designed and fabricated using the multiple magnetron concept and 3 kw microwave sources. The system consists of a PLC-controlled 30 kw microwave source, a steel chamber measuring 1.1 x 1.1 x 2.8 m, a multiple temperature measurement system, a chilling water circulation system, and a programmable temperature controller. A wooden trolley interfaced with a metallic base was designed with a proper grounding system in place for loading the green high-voltage insulators inside the chamber.
The system uses a patented sine wave reflector design for maintaining a uniform microwave field inside the chamber, which can hold six solid core insulators up to 2.5 m long with a green weight of 1000 Kg. Similarly, up to 800 Kg of high rating disc insulators can be dried in the chamber. Temperature measurement is carried out at multiple points on the actual and dummy products using both infrared pyrometers and sheathed k-type thermocouples interfaced with the programmable controller. A large fan at the top of the system is used to act both as a mode stirrer and to assist in the exhaust of water vapor during the drying process. The drying cycle is configured in such a way that no extra humidity is needed inside the chamber, as is required in the conventional drying process. Using this system, it has been demonstrated that the drying cycle time can be reduced in the range of 30-40% compared to that followed in the conventional process for actual high-voltage porcelain insulator components. The moisture content was noted to be in the 0.9-1.2% range throughout the length of the component, which is a unique achievement due to the volumetric nature of heating using microwave technology.
Summary and Future OutlookA systematic approach has been made to understand the microwave drying of high-voltage electro-porcelain materials. A 30 kW pure microwave-based drying system was developed, and detailed experiments were carried out to establish the 1000 Kg drying of porcelain insulators with cycle time reduction and energy efficiency. The drying time can be reduced in the range of 30-40%, resulting in overall energy efficiency in the range of 20-30%, compared to the conventional process. This system is successfully running in a facility that produces high-voltage insulator products.
For more information, contact the lead author at Bharat Heavy Electricals Limited, Ceramic Technological Institute, Malleswaram Complex, Bangalore-560012, India; (91) 80-22182403; fax (91) 80-23466714; email email@example.com; or visit www.bhelceramics.com.
AcknowledgementsThe authors thank BHEL management for permitting the publication of this manuscript. The authors are also thankful to Sri Sushil Chandra and Sri K. V. Ravishankar of the BHEL Insulator Production department for drying trials and regular use of the drying system for commercial production.
References1. Vinayashree, Prabhu, and Satapathy, L. N., “Effect of Microwave Drying on the Green and Fired Properties of High Voltage Electrical Porcelain Insulator,” Am. Cer. Soc. Bull., 87(6), (2008), 36-44.
2. Hemanthakumari, P. N., and Satapathy, L. N., “Reliability in Microwave Drying of Porcelain Insulator Components,” Intl. J. Appl. Cer. Tech., 5(1), (2008), 94-100.
3. Satapathy, L. N., “Microwave Sintering of High-Voltage Porcelain Material and its Characterization,” J. Cer. Proc. Res., 10(5), (2009), 637-642.