Following the introduction of new developments in the last few years [1][2][3][4][5][6][7] it has become possible to improve the thermal operation of tunnel kilns. However, the required uniformity of firing the goods is not always provided. This article assesses certain design and regime factors that influence this parameter. The influence of the shape of the setting and the properties of the goods being fired is not considered.Let us examine the irregularity of the firing, which is connected with the cooling of the products of combustion during movement along the kiln channel in the section between the two adjacent side burners. We shall use the following scheme for the heat work in the i-th section.At the exit of the port, combustion products are formed from the fuel whose temperature t com equals i the maximum permitted in the firing conditions for the stated type of article. The combustion products are exhausted into the burner chamber (gap in the setting opposite the burner), where they are mixed with the departing flow. As a result of the blending process, products are formed with an average temperature [cc 1 ' filling the entire volume of the combustion chamber. We consider that in the combustion chamber there is thermal equilibrium, i.e., [cc is constant for the entire volume. As the combustion products move they 1 give up heat to the material, and compensate for heat losses. As a result of this, the temperature of the combustion products is reduced.Let us call the section between the columns of the setting opposite the burners the combustion section. The section in which the combustion products are cooled shall be called the setting section. We note that the boundaries of the combustion section necessarily coincide with the boundaries of the combustion chamber. The minimum of the mean temperature of the combustion products'~. TM does not coincide with the boundaries 1 of the section of the setting, but is displaced by a distance Al because of the axial radiation of the neighboring combustion chamber.The value of Al depends on many factors and is selected on the basis of experimental data. In firing Dinas the value of •l can be taken as 0.2-0.3 of the length of the column.The heat-work scheme is illustrated in Fig. 1 in which the longitudinal section through the tunnel kiln is shown schematically. Fuel is fed into the combustion chamber with a consumption B i and primary (cold} air ' The secondary, hot air with a consumption V'.' with a consumption V i.1 moves from the cooling zone. At each section the fuel combustion uses up a certain amount of hot air V". The consumption of combustion products formed as a result of the burning of the fuel in each section we denote by V cp. cThe total consumption of combustion products passing from the section (the removal current) we indicate by VZi. The combustion products move toward the material. The length of the combustion section is l c, and of the setting section l s.We consider all the temperatures as average and constant in the transverse section of the kiln...

The distribution of fuel and air consumption at various positions in a tunnel kiln and the proper arrangement of the combustion process affect the quality of firing and the consumption of fuel for the kiln as a whole. A method was developed for calculating the rational distribution of gas and air consumption at various positions in a tunnel kiln [1]. In [1] equations are given for computing the heat load and volume of flue gases at different kiln positions for a stated gas temperature at that position. In this case it is assumed that the coefficient of consumption of primary air is known.The proposed method is suitable for computing the fuel consumption and also the primary and secondary (channel) air at each position of the firing zone, starting from the conditions for obtaining the minimum consumption of fuel at the position Bmi n (task 1) and a stated change in the temperature of the products of combustion over the length of the position Ats (task 2). It is assumed that the firing temperature of the material is given, the temperature of the combustion products in the soaking zone tcp is 10-20~ higher than the firing temperature, and the consumption of hot air moving along the channel from the cooling zone is stated.The formulas were obtained by using the following simplified scheme for the heat work at the position. Tentatively we can divide the position lengthwise into two sections: the combustion section and the section for which there is a fall in the temperature of the combustion products. We shall consider that in the combustion section the total heat absorption is compensated by the chemical heat separation, i.e., the temperature of the combustion products is constant. In the temperature-fall section the process of combustion is complete, and the temperature of the combustion products falls smoothly over the path of movement as a result of heat absorption for the material and the heat loss.The maximum change in the temperature of the combustion products in this section is indicated through At. The fall in the combustion product temperature causes a corresponding fall in the temperature of the setting over the gas path, as a result of which there forms an irregularity of firing of the goods in an a~ial direction. Since the firing irregularity is proportional to the magnitude At we shall tentatively consider At as the parameter characterizing the uniformity of firing over the length of the setting. We note that the firing uniformity over the setting length depends on the dimensions and thermophysical properties of the goods, the conditions of heat exchange, and the duration of heating. However, the scheme used permits, as a first approximation, an assessment of the firing uniformity without resorting to cumbersome calculations.The consumption of fuel at the n-th position in an oxidizing or neutral gaseous medium (~ >-1) may be determined from the heat balance for the combustion section from the equation ( 1) where B n is the consumption of fuel, m3/h; QZ = QK + QM + Qa + QO kJ/h (QK heat lost through the s...

Following experimental work with the firing of large dinas articles [1][2][3], the Pervouralsk dinas factory has constructed and put to use a new tunnel kiln for these products. The project was designed by the All-Union and East institutes.The technical specifications are: length 243 m; number of cars in the channel (positions) 81; car's plan size 3.0 • 3.2 m; height from floor of car to Mln roof 1435 mm; firing temperature --up to 1450~ fuel --natural gas with a combustion heat of 34.4-35.7 mJ/m3; kiln output up to 40,000 tons a year, depending an the type of product.The kiln roof in the preheat and cooling zones is made of fiat-thrust chamotte and dinas goods. In the firing zone the roof is of the suspension type, made of high-alumina shapes. The arrangement of the zones in the tunnel kiln is shown in Fig. 1. Figure 2 shows the cross sections of the preheat and firing zones.Drying and Preheating in the Kiln. Drying and warming up of the kiln with a hydraulic valve in winter time are affected with heat generators designed by the East Institute, heated with natural gas. The generator was connected to the suction pipe of the fan designed for recireulation in the cooling zone. The gas--air mixture (125~ is impelled by fan into the kiln channel at the end of the firing zone. The front gaps of the kiln channel and the lower apertures in the walls were closed, and the connecting pipes for taking off the flue gases, and only the flue tract choke for the natural draft were left open.The temperature in the kiln channel was raised to 100~ on account of the heat from the thermal generator. This prevented freezing of the water in the hydraulic valve and the worldng of the kiln in winter time. The kiln channel was filled with cars and a setting of nonstandardized products. On the extreme car we established a temporary burner (Fig. 3). The rising temperature was recorded with Chromel--Alumei thermoelectric thermometers.The temperature was raised in line with the graph shown in Fig. 4. Up to 1200~ the kiln operated on natural draft. The draft at the floor level was 3-4 Pa. When 750~ was reached the first pair of main burners was lit. Further lighting of burners occurred at 700-800~After lighting the intermediate burners at the end of the firing zone the gas to the temporary burner was switched off, and the temporary gas duct was demolished. At the same time, the cars began to be pushed every 8 h. When the end of the firing zone reached 1250~ car-pushing started to be done every 4 h on this working schedule until the kiln was at the working temperature. Above 1000~ the temperature was measured with an optical pyrometer sighted on a brick placed in the middle column of the setting at a height of 600-700 mm from the car floor.When the burners were switched on a drop in pressure was established on the air consumption diaphragms of 0.7 kPa (about 83 m3/h). To ensure the same rise in temperature on both sides of the kiln, the gas and air consumptions an opposing burners were maintained at the same levels.When the suspension was weak...

No abstract

No abstract