Blast Furnace Cooling System
Blast Furnace Cooling System
Fritz W Lurman, a well known blast furnace man of the time opined in 1892 that ‘irrespective of the use of so called refractory materials, the best means of maintaining the walls of the blast furnace is with cooling water’. Coolers with water circulating in them are installed between the shell of the blast furnace and the refractory lining in the upper part of the furnace to protect these components from heat radiation. In addition to having its own coolers, the part of the shell adjacent to the hearth and the bottom of the furnace is also cooled in some furnaces on the outside by water sprays.
Function of blast furnace cooling system is to cool the furnace shell and prevent from the overheating and subsequent burn through. Cooling system removes the excess heat generated in the blast furnace which is otherwise loaded on the shell. Cooling system thus prevent the increase of the shell and lining temperature. Various methods exist for cooling of the shell for the blast furnace.
In earlier times, cooling boxes of different size, number and design were used for transferring heat of the furnace to a cooling medium in conjunction with external cooling (spray cooling, double shell). Blast furnaces with cast iron cooling staves are operating since mid 1900s. Cast iron stave cooling was originally a Soviet discovery from where it travelled initially to India and Japan. By 1970s, cast iron cooling staves have attained world wide acceptance. Since the introduction of these cast iron stave coolers, the development work of blast furnace cooling got accelerated and today a wide variety of coolers are available for the internal cooling of the furnace shell to suit extreme condition of stress in a modern large high performance blast furnace. Some of the coolers used for cooling blast furnace shell are given below.
Cast iron staves coolers
For the cast iron staves coolers, initially alloyed perlitic lamellar grey iron was used but now a days it has been replaced by ductile iron or SG iron, since it is less subject to cracking at temperatures higher than 760 deg C. Recently cast steel staves in place of cast iron staves have also been successfully tested. The cooling effect of the cast iron staves is determined by the size and the shape of the cooling water tubes inside the stave cooler. The typical dimensions of cast iron staves consist of 1.8 m to 2.4 m of length, 0.8 m to 1.1 m of width and 0.25 m to 0.6 m of thickness.
Since acquiring of cast iron stave technology from former Soviet Union in 1969, Japanese have made various improvements for enhancing the durability of the staves. These include narrowing of the pipe spacing and the installation of corner cooling pipes and rear serpentine pipes. The fourth generation staves are characterized by the fact that they have two cooling planes, four vertical tubes in the hot side plane and one serpentine tube on the cold side plane. The staves are equipped with cooled noses and/or bracket for the support of refractory materials. Further the corners of the staves are intensively cooled. The refractory materials are cast into special support holes in the staves. The improvements carried out from first generation to fourth generation of cast iron stave coolers are shown in Fig. 1.
Fig 1 Development of cast iron staves in Japan
Different types of cast iron cooling staves are shown in Fig 2. Type C coolers are much thinner and are designed to save space inside the furnace in order to enhance the working volume of the furnace.
Fig 2 Different types of cast iron staves
The advantage of stave cooling over flat plate cooling is the blanket cooling effect of the staves as compared to point to point cooling effect of flat plate cooler. This ensures a more uniform cooling performance. Cast iron staves in the blast furnace bosh and stack area have an average life expectancy of around 8 to 10 years. Cast iron staves typically fail due to loss of cast iron material and exposure of internal pipe coil. Cast iron cracks in service due to high heat loads it is exposed to. Further cast iron has a low thermal conductivity (about 45 W/m.K). Random gaps between the cooling tubes and the cast iron reduce the amount of heat that is removed. Also the difference between the coefficients of thermal expansion of the materials of the tube and the cooler proper can cause the iron to separate from the tube and destroy the cooler. The use of cast-iron coolers in which the tubes are closer to the cast iron complicates the design of the shell while not necessarily increasing the life of the cooling system.
Copper flat plate coolers
Copper flat plate coolers have been used nearly in all the European blast furnaces. These coolers are either welded or cast in electrolytic copper. The usual plate sizes of copper flat plate coolers consist of 0. 5 m to 1.0 m of length, 0.4 m to 0.8 m of 3width and a height of approximately 0.75 m. The vertical spacing of the coolers is 0.3 m to 0.6 m. In the zones with high heat loads, especially in the bosh and lower stack areas, the spacing is often reduced to 0.25 m. Copper flat coolers have a greater uniformity of material properties over the complete cooling element. In those regions of the blast furnace which are subject to mechanical damage, the front side of the cooling elements is usually reinforced with special materials. These coolers are mostly welded to the blast furnace shell to ensure gas tight sealing. The copper flat plate coolers have normally multiple channels with one or two independent chambers. One of the designs of capper flat plate cooler has six pass with single chamber. These coolers are designed to maintain high water velocities throughout the cooler, thus have an even and high heat transfer coefficient. Typical copper flat plate coolers are shown in Fig. 3.
Fig. 3 Typical copper flat plate coolers
The failure of copper flat coolers is attributed to four failure mechanisms. They are (i) deflection, (ii) pipe weld failures, (iii) plug weld failures, and (iv) face abrasion. A comparison of copper plate cooler with cast iron stave cooler is given in Tab 1.
Tab 1 Comparison between cooling systems
Copper plate cooler
Cast iron stave cooler
|Specific surface area of cooling element per Square meter shell||Sq m/Sq m||1-2||2.5||0.8-1||1-2|
|Specific cooling water flow per Sq m shell||Cum/hr||5-10||3-5|
|Typical cooling water velocity||m/sec||0.5-1||2-2.5||1-1.2||2.5-3|
These are also known as copper jackets. Cigar coolers are used in the open areas between the plate coolers when more intensive cooling is required or there is insufficient existing spacing of the flat plate coolers. These are used sometimes for improvements to the existing cooling system during a campaign. Cigar cooler is normally machined from a solid copper bar to form a cylindrical core and a single channel is added by drilling and plugging. Cigar coolers are normally inserted on the centerlines between adjacent flat plate coolers on a horizontal and vertical plane. For the purpose of installation of a cigar cooler usually a cylindrical hole is drilled through the furnace shell and existing refractory lining with a core drill. A typical cigar cooler and its positioning between the plate coolers are shown in Fig. 4.
Fig 4 Typical cigar cooler (Left) and its positioning (right)
The cigar cooler use increases the cooling system area and increases the resistance of the refractory lining to chenmical and mechanical attack mechanism. However since the use of cigar coolers results in increase of appertures in the furnace, it is necessary the strength of the shell is checked before its use.
Copper staves use became prevalent in the mid 1990s but the majority of the installations are in or after 2000. The development of cooper staves was carried out both in Japan and Germany for use in the region of bosh, belly and lower stack to cope with high heat loads and large fluctuations of temperatures. While Japan has gone for cast copper staves, German copper staves are rolled copper plates having close outer tolerarnces and with drilling done for cooling passages. Drilled and plugged copper staves are typically designed for four water pipes in a stright line at the top and four water pipes in a stright line at the bottom. Materials for internal pipe coils include monel, copper or steel. Unlike cast iron staves, copper staves are intended to be bonded to the cooling pipe. A cast copper stave is shown in Fig.5.
Fig 5 Cast copper stave