Analysis of Methods for Heating the Lining of High-Temperature Units

Refractories and Industrial Ceramics - An analysis of the methods of increasing the temperature of the lining during its drying and heating is given. Three options for increasing the temperature...     

State-of-the-art plants for drying and high-temperature heating of ladles

The Stal’proekt Institute has developed state-of-the-art plants for drying and high-temperature heating of steel-teeming ladles. Two new plants for drying tundish ladle lining have been designed for machine No. 6 for continuous billet casting at the Oskol’skii Electrometallurgical Works. Plants for drying and heating 30-ton teeming ladles to a temperature of 800°C and for high-temperature heating of ladle lining (1200°C) have been designed for a currently constructed metallurgical works. All these plants recover flue gas heat and use it for heating air supplied for combustion, thus decreasing the consumption of fuel. The process of drying and heating of lining is analyzed using a mathematical model developed at the institute. The plants are equipped with automatic control systems controlling the temperature schedule of drying and heating the lining. The specified plants ensure cost-effective operation in an automatic mode __________ Translated from Novye Ogneupory, No. 10, pp. 21–25, October, 2006.     

Damage of monolithic refractory linings in steel ladles during drying

Abstract

Safety linings of steel ladles are made of high content alumina castable. In situ observations confirm that during drying a network of macrocracks appears in the monolithic castable lining, owing to both dehydration of the cement paste and temperature. Crack formation kinetics have been characterised over the life of the lining. To simulate the behaviour of the lining, the thermomechanical behaviour of the castable was characterised in compression and three point bend tests, thermal dilatation tests and TGA analysis in the 20–600°C temperature range. The change in properties is shown to be due mainly to transformation of hydrates in the castable. Numerical simulations of the lining drying process were also performed using a finite element code to verify the causes of cracking. A parametric study was carried out to examine the possibility of reducing the stresses induced by thermal loading. Improved thermal insulation and better control of heating reduce the risk of failure in the lining.     

Drying and firstheat up of a kiln unit with cyclone heat exchangers with a lining of refractory concretes

Conclusions An accelerated drying and first heat up cycle was developed for a kiln unit for dry production of clinker with a capacity of 3000 tons/day with cyclone heat exchangers with a lining of refractory concretes of high-alumina cement with a chamotte aggregate. The drying of the lining and the heating of the unit were done in 4 days. The results of the work indicate the desirability of use of refractory concretes for lining the cyclone heat exchangers of kiln units for dry production of clinker.Translated from Ogneupory, No. 3, pp. 46–49, March, 1985.     

Service of fireclay blocks with clay — Phosphate bonds in blooming soaking pits

Conclusions A technology was developed for making chamotted concrete blocks with clay-phosphate bond. A cycle was selected for thermal processing.Blocks were tested in the walls of the working cells of soaking pits used for blooming. The resistance of the block lining is 50% higher than that of the brick lining.The blocks wear out mainly because of slag formation at the surface and the edges, and also because of mechanical failure. Scaling was noted during service of the blocks after natural drying.The phosphate bond during prolonged heating contributes to the decomposition of the mullite, and has a mineralizing effect on the polymorphic inversions taking place in the silica.Chamotte concretes with clay-phosphate bonds can be recommended for use in heat-treatment furnaces.Translated from Ogneupory, No. 2, pp.21–26, 1971.     

Drying behavior of dense refractory castables. Part 2 – Drying agents and design of heating schedules

Drying is the most critical process of the first heating cycle of monolithic dense refractories, as the reduced permeability of the resulting microstructure may lead to explosive spalling and mechanical damage associated with dewatering. The first part of this review series pointed out the various drying stages, the role of the binder components and the techniques that can be used to follow the water release in as-cast refractory materials, when they are exposed to heat. Although defining a suitable heating schedule is a great challenge, some tools can be applied to minimize the spalling risks associated with steam pressurization. In this context, this second review article points out (i) the main drying agents and how they affect the resulting castables’ microstructure (organic fibers, metallic powders, permeability enhancing active compounds, silica-based additives and chelating agents), and (ii) the effects related to the procedures commonly applied during the designing of heating routine (i.e., the role of the heating rate, ramp versus holding time), as well as the influence of the castable’s dimension on the overall drying behavior. Considering the recent advances regarding the design of refractory formulations and their processing, one may expect that incorporating suitable drying additives into the prepared composition should lead to a suitable and safer water release in such dense consolidated structures. Besides that, novel engineering opportunities, such as the use of in-situ based experimental techniques (i.e., neutron and X-ray tomography) to obtain more accurate data and the development of numerical models, might help in simulating and predicting the steam pressure developed in refractory systems during their first heating. Consequently, instead of designing conservative drying schedules based on empirical knowledge, the novel optimized heating procedures should be based on technical and scientific information.     

Mathematical models for the drying of rigid porous materials

This paper presents a resume of literature on theories and mathematical models for drying of rigid porous materials. Key work on drying by soil physicists has been neglected in the engineering drying literature. We have included these works here to bring this literature to the attention of engineers. A new and general model for moisture and energy transport in rigid porous media during drying is presented. It is demonstrated that under certain simplifying assumptions, the general model reduces to less general models which have previously been proposed. Experimental and simulation results are given for the drying of Valentine sand. Under the drying conditions studied, the drying rate during the falling rate period is controlled by the capillary flow of water to an evaporation zone in the porous media. The models simulated here are of varying complexity and rigor. The capabilities and limitations of these models are discussed.     

Thermal expansion of dinas — The basis of drying and heating the lining of coke batteries

Conclusions To determine the optimum condition for drying and heating up of coke batteries accurate data on the thermal expansion of dinas parts intended for lining the various zones of the coke ovens is necessary.A design of quartz dilatometer providing of dinas in the 20–750° range has been developed.Translated from Ogneupory, No. 5, pp. 42–46, May, 1989.     

EFFECTS OF HEATING METHODS ON VACUUM FREEZE DRYING

The heat and mass transfer during freeze drying of raw beef by different heating methods is studied theoretically and experimentally. The difference between radiant heating and microwave heating in freeze drying process has been analyzed. The results obtained are important for the selection of the heating methods in practical freeze dryers.     

Heating of a steel-pouring ladle bottom concrete lining under workshop conditions

A review is provided for processes that occur within refractory concrete during hydration and heating. Theoretical data are referred to practical cases connected with concrete explosive cracking. On the basis of physicochemical behavior of concrete with high-temperature heating and thermotechnical calculation it is possible to correct a heating curve in order to prevent concrete lining breakage during warm-up.     

A Study of the Power Density Distribution generated during the Combined Microwave and Convective Drying of Softwood

ABSTRACT

Investigations into new and innovative drying strategies can lead to the development of more efficient and effective drying processes. The commercialisation of these processes would prove invaluable to the drying industry as a whole and the associated technology would generate worldwide interest. Combined microwave and convective drying is one such process which offers great potential, with benefits that include : reduced drying times and increased drying rates; volumetric heating; higher fluxes of liquid to the drying surface; high temperature and internal pressure buildup within the material which enhances the overall moisture migration rate; and preferential heating of wetter areas. Numerical simulation can elucidate on the intricate details of the heat and mass transfer henomena that occur during the drying process, thus eliminating the need for performing numerous time consuming and expensive experiments. The simulations can predict the evolutionary behaviour of the moisture, temperature and pressure distributions, and can provide a detailed analysis of how microwaves interact with materials during drying and heating operations at a fundamental level. The research presented in this paper uses a comprehensive mathematical model to study the behaviour of the iniernal microwave power density distribution that is generated during the microwave enhanced convective drying of softwood. The configuration understudy concerns a plane wave microwave source irradiating the wood in the transverse direction.     

EFFECTS OF HEATING METHODS ON VACUUM FREEZE DRYING

ABSTRACT

The heat and mass transfer during freeze drying of raw beef by different heating methods is studied theoretically and experimentally. The difference between radiant heating and microwave heating in freeze drying process has been analyzed. The results obtained are important for the selection of the heating methods in practical freeze dryers.     

ANALYSIS AND MODELING OF BROWNING OF THE GRANNY SMITH APPLE DURING DRYING

After proposing simple empirical models for drying kinetics and for sorption isotherms, the authors show that browning during drying of Granny-Smith apple samples can be calculated from models developped in static conditions. The time at which the browning begins (t_B) is similar to the induction time deduced from the static conditions ; it corresponds to a very low sample water content (around 1% dry basis) and to a sample temperature equal to the heating temperature. The “thermic past” (drying method, drying rate, heating temperature before t_B) has no influence on the browning rate, only the heating temperature after t_B is important     

Determination of local moisture profiles during microwave and convective drying Bestimmung lokaler feuchtegehalte bei der mikrowellen- und konvektionstocknung

Several methods for the determination of local moisture profiles are presented and discussed. The γ-ray attenuation technique is suitable for the determination of local moisture concentrations in cylinders and spheres during drying. Measurements of moisture profiles were obtained during convective and microwave drying using samples of different material geometry and size. The results show that internal volumetric heating with microwaves greatly influences the moisture profile. During drying of aerated-concrete spheres, microwave heating shifted the location of minimum local concentration from the surface (found in convective drying) to the center.     

Study of the properties of ceramic surfacing material used for restoring coking chamber linings

Properties are given for original raw components and a production scheme is provided for preparing ceramic surfacing for repairing the dinas lining of coking furnaces. Data are presented for x-ray phase and petrographic analyses of ceramic surfacing. Adeformation - temperature diagram is provided. Recommendations are given for use of the proposed compositions for repairing heating units.     

Influence mechanism of radio frequency heating on moisture transfer and drying stress in larch boxed-heart square timber

Abstract

Radio frequency heating combined with convection (RF/C) drying of larch boxed-heart square timber and its influence on drying kinetics such as rate, moisture content distribution, and stresses was explored. Results revealed that RF heating increased the drying rate and in RF/C drying was twice as high as in conventional drying. Below fiber saturation point, RF heating reduced internal moisture gradients, especially around moisture content of 20%. The effect of RF heating on moisture transfer was strongly associated with moisture content. Specifically, above the fiber saturation point, RF heating played a minor role in moisture transfer however, it reached maximum around fiber saturation point and thereafter, it largely decreased with moisture content. RF heating relieved some drying stresses during RF/C drying and reduced residual stresses in the timber surface layers. Furthermore, it changed the original development pattern of drying stresses in conventional drying.     

US-Assisted Convective Drying of Biological Materials

The aim of this work is to promote ultrasound as an energy source that is suitable for the enhancement of drying processes, and in particular of biological materials. The study aims at a more profound recognition of the interaction between ultrasonic (US) waves and the tissues of fruits and vegetables, which may contribute to an intensification of moisture removal during their drying. Absorption of acoustic energy causes heating and structural changes of the drying material due to a series of rapid material compressions and decompressions. The research hypothesis is based on the expectation that the ultrasound waves may enhance moisture removal from the fruits and vegetables during drying due to a special “heating effect,” “vibration effect,” and “synergistic effect.” Convective ultrasonic-assisted drying tests were conducted experimentally on a new hybrid dryer with ultrasonic equipment, and the effects of ultrasonic enhancement by drying are presented based on a drying model and assessed numerically.     

Influence of heating mode on drying behavior of capillary porous media: Pore scale modeling

Invasion percolation (IP) rules under non-isothermal conditions are applied to model the pore-scale events occurring during drying of capillary porous media, namely displacement of immiscible phases and cluster formation. A saturated two-dimensional network with a bimodal pore size distribution is dried by applying two different heat transfer boundary conditions; one corresponds to convective drying and the other to less resistive contact drying. Simulated macroscopic drying behavior is presented in conjunction with freely evolved microscopic temperature fields and phase distributions for both heating modes. Convective heating exhibits similar invasion patterns as those in isothermal simulations; both are dominated by the spatial distribution of pore radii. However, in contact heating, temperature dependency of surface tension produces significantly different invasion patterns.     

Experimental proof of moisture clog through neutron tomography in a porous medium under truly one-directional drying

Structural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive spalling on partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in-operando drying behavior of a high-alumina refractory castable, proposing a novel experimental layout aimed at a truly one-dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.     

陶瓷纤维炉衬的研究

通过对全纤维棉式、隔板式、空气隔层式等三种形式的陶瓷纤维炉衬在加热过程中温度场的测量,并据此对上述三种形式炉衬的热损失计算表明:在纤维棉内层加一薄隔热板可使该处在加热过程中降温达150℃,热损失减少20％。若采用空气隔层式炉衬,炉衬厚度从全纤维棉炉衬的150mm厚降至110mm,且热损失减少30％。