Blast Furnace Cooling System
Blast Furnace Cooling System
In consideration of the huge capital investment needed for blast furnace (BF) relining, enormous efforts have been made in the past to extend the campaign life of the BFs. Development in the BF ironmaking process and advances in material sciences have improved the productivity, fuel consumption, product quality, and the campaign life of the BF. The duration of a BF campaign until 1990s was influenced mainly by the lifetime of the lower shaft, i.e. the area of the highest thermal load. If heat transfer, thermal stress, and furnace campaign life all such parameters are not analyzed then it can lead to the failure of the BF.
Efficient cooling is needed to balance the thermal load and associated wear of the inner lining of the furnace, and to protect the shell and their cooling elements. The cooling systems of a BF play a key role in the campaign life of a BF and hence the operating costs. Cooling elements with water circulating in them are installed between the shell of the BF and the refractory lining in the upper part of the furnace to protect these components from heat radiation.
Fritz W Lurman, a well known BF 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 BF is with cooling water’. Function of BF 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 BF which is otherwise loaded on the shell. Cooling system hence prevent the increase of the shell and lining temperature. There are several methods which exist for cooling of the shell of the BF.
BF cooling systems are being developed since 1884. Earlier (till 1920s) the cooling was applied only to hearth and bosh areas. By 1930s and 1940s, cooling was also applied to the shaft. Simultaneously, external cooling methods like shower and jacket cooling of the furnace shell were tried. This method relied on extracting the heat through the furnace shell to the cooling medium, generating high thermal stresses during the heat transfer and hence jeopardizing the integrity of the shell.
In the past, in addition to having its own coolers, the part of the shell adjacent to the hearth and the bottom of the furnace was also cooled in some furnaces on the outside by water sprays. Also, 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).
BFs with cast iron cooling staves are operating since mid 1900s. A stave is a cooling device having one or more internal water channel, and is installed in numbers on the inner surface of a BF to protect its steel shell from the high-temperature gas and molten burden material in the furnace, and maintaining the profile inside the furnace. The three properties which are mainly needed from a stave are (i) long life and reliability, (ii) appropriate cooling capability, and (iii) thermal insulation ability.
The first is the long life and reliability. Since it is impossible to repair a stave from outside the BF due to its structure, extensive replacement work is r needed when the staves are damaged. Damaged staves have serious adverse effects on the BF operations, causing a long blowing-stoppage and the temperature drop inside the furnace due to water leakage, or changing the profile which can result in operational failure. For this reason, stable long life is needed for the staves.
The second is the appropriate cooling capability. To protect the shell from high-temperature gas around 1,200 deg C and liquid material, appropriate cooling capability is needed of the staves. Since high cooling capability is needed for cooling the furnace part between the bosh and the lower part of the shaft which is exposed to high temperature, a copper stave is used for this part in several cases.
The third is the thermal insulation ability. A BF in which the iron oxide is reduced and melted at high temperature is desired to have a heat insulation structure so as not to waste thermal energy. On the other hand, as described above, staves are cooled to maintain the profile inside the BF and to protect the shell. Hence, it removes thermal energy from the high-temperature gas and material. Removing heat by a stave involves the equivalent fuel (coke) consumption, directly causing an increase in the reducing agent rate (RAR).An increase in the RAR can in turn lead to an increase in the carbon di-oxide emissions and raise the unit price of liquid iron. For this reason, staves are needed to have appropriate heat insulation (heat removal restriction) i.e. ability to minimize heat energy taken from inside the furnace, as well as appropriate cooling ability. Conventionally, bricks with low thermal conductivity are embedded at the front of a cast iron stave to get both cooling ability and heat insulation ability. Meanwhile, a copper stave is used to form a heat insulation layer from semi-liquid material located in front of the staves by cooling such material with the high cooling ability to make it adhere to the inner surface of the staves.
Stave coolers first began to be developed around mid 1900s. Cast iron stave cooling was originally a discovery of former Soviet Union from where it travelled initially to India and Japan. By 1970s, cast iron cooling staves have attained world wide acceptance. Since the introduction of the cast iron stave coolers, the development work of BF cooling got accelerated and today a wide variety of coolers such as plate cooler, cigar cooler etc. are available for the internal cooling of the furnace shell to suit extreme condition of stress in a modern large high performance BF. 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 and cigar coolers. This ensures a more uniform cooling performance.
For the majority of large capacity BFs operating today, design engineers have decided in favour of using staves for the cooling system since they enable intensive and, above all, uniform cooling of the furnace. Classically, staves are made from nodular cast iron, which is cast around the cooling water pipes. They are installed over the entire furnace shell, from the bottom plate upto the throat. Frequently, however, the staves are subjected to heavy stresses caused by high heat loads, particularly in the bosh and belly areas, which can limit the length of the furnace campaign. This can lead to loss of the entire stave body, with only the water conducting pipes remaining. It is believed that both the thermal conductivity of the cast iron material and the heat transfer between the piping and the cast body can be the problem. Even with the best known cooling systems, the lower shell area remained the weak point of the BF.
The staves were made conventionally of cast iron, but the viability of manufacturing them of copper, which has excellent heat conductivity, was confirmed in Germany around mid-1990s, and hence copper staves have come to be used for several BFs ever since. In 1993, copper staves were introduced for the first time in Germany at the BF number 2 of Thyssen Krupp at Schwelgern and the BF ‘B’ at Salzgitter. Producing staves from copper, using either drilled water passages in place of pipes, or providing suitable channels when casting the copper slabs, has proven to be a significant step in the design of the modern BFs.
The theory that the high thermal conductivity of copper leads to an increased dissipation of process heat has been disproved. The high conductivity of copper results in low surface temperatures and a rapid formation of a thin layer which reduces the heat removal from the furnace. The cooling effect is so intense that a protective layer forms within a few minutes, even in front of an unprotected stave. Recent results have shown that the level of heat removed from the furnace is even lower than experienced with cast iron staves.
The insulating effect of such layers maintains the heat losses at a minimum. For this reason, when the BF number 2 Stahlwerke Bremen was relined, no permanent refractory material was installed in front of the staves above the tuyeres. Rather, only, a thin, blown-in protection layer was gunned on. These staves proved to be so successful that today the use of copper staves in the area of high thermal load is the state-of-the-art blast furnace technology. The lower stack is no longer considered to be a limiting factor to the campaign life of the BF. Instead, the status of the hearth dictates the BF campaign life. Present day copper stave coolers insulate the outer shell from the process heat generated in highly stressed furnaces. Yet, depending on where they are positioned within the BF, they are subjected to differing levels of the thermal load.
The key to successful operation of a cooling system in a high heat load BFs is the formation of a skull on the hot face of the cooling element. This skull is composed of condensed vapours, solidified slag and metal that attaches the cooling element surface by splashing, dripping and freezing onto it. The thickness can vary by upto 20 mm. The stability of the skull mainly depends on the cooling capacity and mechanical adherence ability of the cooling element to which it adheres. The severity of gas streaming at the wall also affects the skull retention. The skull is a natural insulator if metallic content is low. During periods of extreme heat load (i.e. high temperature gas jets or process upsets) the skull can spall off, then build up again afterwards. The falling and building up of skulls causes significant fluctuations in heat loads on the furnace wall with peak loads in the range of 300,000 watts per square meter (W/sq m) to 500,000 W/sq m.
Hence, the lining and cooling design normally is to be capable to handle such heat loads to avoid premature failures. Detailed investigations have revealed that the higher the cooling efficiency of the cooling element, the more stable the skull and the longer it adheres to the cooling element and retain its insulating and protective properties. The outcome is that highly efficient cooling systems normally result in lower overall heat losses from the furnace.
While majority of the BFs are free standing type with no supports on the furnace shell, some furnace designs incorporate a mantle where the furnace is supported from ground level either by the steel or the concrete columns. The use of a mantle normally eliminates the belly area. The mantle is an integral part of the furnace design of these furnaces and is to be protected against overheating under the same conditions as experienced in the belly area of the free standing BFs. Cooling of the furnace lining in the mantle area using copper plates is difficult because of the limited access for changing and the length of plate coolers which is required to be used. The use of staves in the area has the advantage of giving a complete cooling coverage of the mantle. Fig 1 shows the stave cooling arrangement at the mantle as well as the different types of cooling arrangements in the BFs.
Fig 1 Different types of cooling arrangements in blast furnace
Modern BFs can have production rates higher than 3 tons of hot metal per cubic metre of working volume per day. This level of productivity in the BF is achieved by using improved burden materials, burden distribution techniques, process control, high hot blast temperatures, oxygen enrichment and auxiliary fuel injection. However, these high productivity practices result in high heat loads and heat load fluctuations to act on the walls of the BF. Although the specific zone and the magnitude of peak heat loads can vary considerably between furnaces, the belly and lower stack region normally encounter the highest heat fluctuations as shown in Fig 2.
Fig 2 Heat load fluctuations in various regions of blast furnace
The area in which the highest heat load is experienced is closely related to the position and shape of the cohesive zone and specific charging pattern of the furnace. The major cause of high heat fluctuations is irregular high velocity gas jets which are venting toward the furnace walls through coke slits in the burden. High and fluctuating temperatures are the main loads which the cooling elements in the BFs have to cope with. Burden composition and quality have been observed to affect considerably the heat loads and the heat fluctuations. BF burden with more than 70 % sinter can result in peak loads of 100,000 W/sq m to 200,000 W/sq m, while burden with a high percentage of pellets or lump can generate peak heat loads of over 400,000 W/sq m. Depending on the expected heat load, different cooling elements have to be applied. Tab 1 gives overview of different cooling and refractory designs with their maximum peak heat load capabilities.
|Tab 1 Overview of different cooling and refractory designs with their maximum peak heat load capabilities|
|Design||Peak heat load in W/sq m|
|Plate coolers with alumina refractories||50,000|
|Cast iron staves, first generation||100,000|
|Thicker cast iron staves with multiple cooling water circuit||200,000|
|Plate cooler, closer spacing (300 mm) with special refractories||400,000|
|High performance copper staves||500,000|
It is required to emphasize here that these peak thermal load is the primary determining factor for the long-term survival of the furnace lining or cooling elements.
The various cooling elements developed for the BF cooling system are (i) cast iron staves of different generaton , (ii) densely spaced copper plate cooler, (iii) cigar cooler, and (iv) copper cooling staves.These cooling elements are described below.
Cast iron stave coolers
For the cast iron stave coolers, initially alloyed perlitic lamellar grey iron was used but these days it has been replaced by ductile iron or nodular 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. Different types of cast iron cooling staves are shown in Fig 3. Type C coolers are much thinner and are designed to save space inside the furnace in order to enhance its working volume.
Fig 3 Different types of cast iron staves
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 4.
Fig 4 Improvements in cast iron stave coolers
Cast iron staves in the bosh and stack areas of the BF have an average life expectancy of around 8 years to 10 years. Cast iron staves typically fail due to the 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. Random gaps between the cooling tubes and the cast iron reduce the amount of heat which 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.
Failure mechanism of cast iron staves under high fluctuating heat loads is because of the low thermal conductivity (about 45 W/m.K) of cast iron. Cast iron staves have a lower cooling efficiency compared to copper staves due to the relatively low conductivity of cast iron and the presence of an insulating layer. This layer results in a thermal barrier between the water-cooled tube and cast iron stave body reducing the heat transfer. Inefficient heat transfer results in a considerably higher hot face temperature of the cast iron stave (over 700 deg C) and subsequent thermal deformation of the cast iron stave. The cast iron body also experiences phase volume transformation at elevated temperatures, resulting in fatigue cracking, loss of stave body material, and exposure of steel cooling pipes directly to the furnace heat.
Copper flat plate coolers
Copper flat plate coolers, as the name describes, are flat plates which are arranged horizontally into the furnace shell. These flat plate coolers have been used nearly in all the European BFs. These coolers are either welded or cast in electrolytic copper. With the latter, there are then no problems at the weld seams and there is a greater uniformity of the material properties over the complete cooling element. This type of cooler is normally used in the area from the bosh upto and including the lower stack. It is normally designed to maintain high water velocities throughout the cooler, thus giving both an even and high heat transfer coefficient. Typical copper flat plate coolers are shown in Fig 5.
Fig. 5 Copper flat plate cooler
The normal 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 width and a height of around 75 mm. 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 frequently 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 BF which are subject to mechanical damage, the front side of the cooling elements is normally reinforced with special materials. These coolers are mostly welded to the BF shell to ensure gas tight sealing. The copper flat plate coolers have normally multiple channels with one or two independent chambers. Minimum losses of water pressure are ensured in both the piping and the element itself.
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.
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 2.
|Tab 2 Comparison between cooling systems|
|Description||Unit||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 square meter (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|
Cigar coolers are used for more intensive cooling or with insufficient existing spacing of the flat plate cooler. These are also known as copper jackets. Cigar coolers are used in the open areas between the plate coolers when more intensive cooling is needed or there is insufficient existing spacing of the flat plate coolers. These are also 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 normally a cylindrical hole is drilled through the furnace shell and existing refractory lining with a core drill. The use of cigar coolers in the bosh, belly, and lower stack areas increases the cooling system area. Since this area of the BF has the highest temparature and temperature fluctuations, the use of cigar cooler can increase the refractory lining resistance to chemical and mechanical attack mechanisms. However since the use of cigar coolers results in increase of appertures in the furnace, it is necessary that the strength of the shell is checked before its use. A typical cigar cooler and its positioning between the plates coolers as seen from the outside of the BF shell are shown in Fig 4.
Fig 6 Cigar cooler and its positioning between plate coolers
In consideration of the vast capital investment needed for the BF relining, great efforts have been made to extend the campaign life of the BFs. The copper stave technology is one of the products of such efforts.
The cast iron stave, due to its material characteristics, causes material deterioration at the lower part of the blast furnace, which is exposed to high heat load. This has made it difficult to achieve stable furnace life of 20 years using the cast iron stave. As an alternative cooling means to the cast iron stave considering the high heat load, the rolled copper stave was developed. This type of stave is made from a rolled copper plate on which holes are drilled and water supply and drain pipes are welded to form water channels. The use of rolled copper staves was started in Germany in the mid-1990s, and has been spread to the BFs in several countries.
Other copper staves developed to date include cast copper staves with water channels formed using a core in the casting process, and cast-in monel pipe copper staves with water channels formed using monel pipes bent into a channel shape and cast.
Copper staves were first utilized in BFs in the late 1970s as the best high heat-load wall cooling element. As the BF productivity intensified and desired campaign life goals increased to 20 years, it became apparent that copper staves had the best potential to meet or exceed these demands. Though the copper staves use became prevalent in the mid 1990s, the majority of the installations are in or after the year 2000. The development of copper 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 straight 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.
The water channel of a copper stave was formed mostly by drilling a hole into a rolled copper plate and welding water pipes at the ends of the hole, and it was formed sometimes by casting using a disposable sand core, however, copper welding work was indispensable in either of the cases. While conventional copper staves proved excellent in the cooling capacity, their manufacture needed several work steps, and hence, they were expensive.
Presently, the most popular type of copper stave is the rolled copper stave, the manufacturing process of which involves drilling holes on a copper plate. The water channel ends of this stave are plug-welded. The cast-in steel pipe copper stave, which has been developed, is made by casting bent steel pipes into the copper, a completely different manufacturing process from that of the conventional rolled copper stave. This unique manufacturing method has enabled achieving high energy efficiency and long life of BFs, which cannot be achieved using the rolled copper stave.
Rolled copper staves sometimes have the three problems namely (i) deformation, (ii) weld cracking due to thermal fatigue, and (iii) wear. The first problem is deformation. Rolled copper staves are warped due to the difference in thermal expansion between the stave inner surface, which is exposed to high temperature gas, and the stave outer surface which is cooled. They are seriously deformed when a stave is too long or when the positions of fixing bolts are not appropriate. Such deformation can cause a wear of a protruding portion and breakage of a weld due to high temperature gas flowing to stave joints and back surfaces. The second problem is weld cracking. Due to thermal fatigue, this takes place since the welds of rolled copper staves are subjected to repeated thermal stresses. Due to the temperature fluctuation, rolled copper staves result in cracking and breakage. The third problem is wear. Iron ore, sinter, and coke have higher hardness than that of copper. These materials abrade copper staves when they contact the stave surface and descend. In general, the wear rate of a copper stave depends on the contact force and descending speed of the material in contact with the stave surface, hardness of the copper and the material, and the shape of the material.
The development of the cast-in copper stave has considered the following aspects. As per the first aspect, for the prevention of deformation, appropriate design of the stave length and bolt constrained points is important. The first aspect is that the use of the cast-in steel pipe copper stave with its own design is beneficial for effectively reducing the risk of deformation. Fig 7 shows the constrained points of a rolled copper stave and the cast-in steel pipe copper stave. A rolled copper stave is constrained to the shell by mounting bolts and pins. To prevent the weld at the base of a rising pipe from being damaged by stresses, rising piping is connected to the shell by an expansion joint. Due to this structure, the upper and lower ends of the stave are freely displaced, causing the stave to be easily deformed. The large thermal load which is repeatedly applied to the copper stave in the course of the fluctuation in the BF operations etc., causes plastic strain to be gradually accumulated, and results in large deformation. There are cases in which the deformation at the upper end has reached 50 mm or more and a weld has been broken, under the condition of an overly long stave, an in-appropriate bolt position, or high heat load exceeding the design condition.
Fig 7 Constrained points of rolled and the cast-in pipe copper stave
High reliability and low manufacturing costs are realized in case of the cast-in steel pipe copper stave by a casting technology to embed a steel pipe in a copper casting applied to the production of the new-type copper stave. This casting technology has been developed based on the manufacturing of cast-iron staves. The new-type copper stave having the embedded steel pipe has the advantages of (i) high cooling capacity since the casting of high-purity copper ensures a cooling capacity as high as that of a conventional copper stave, (ii) high reliability since embedding a steel pipe in a copper casting eliminates welding of copper in the formation of a water channel and the possibility of water leakage, (iii) wide flexibility in design since the manufacture by casting and embedding a steel pipe allows a far greater flexibility in the stave design than that of conventional copper staves in terms of the stave shape and the arrangement of the water channel, and (iv) low costs.
The cast-in steel pipe copper stave has the following deformation-resistant features. As shown in Fig 7, for the cast-in steel pipe copper stave has, gas seal boxes in addition to bolts which are used to fix the protective pipes at the ends of the stave. This applies the displacement constraint to the upper and lower ends of the stave. Furthermore, since the protective pipe is casted in the body of the copper stave, no welds with a breakage risk are used.
Further, the cast-in steel pipe copper stave uses steel pipes, which are stiffer than copper and serve as the frame work. The use of steel pipes provides a structure which is more deformation-resistant than conventional copper staves. The inner surface of the cast-in steel pipe copper stave is made bumpy. Since rolled copper staves have a rectangular cross-sectional shape, the temperature increases at locations on the stave inner surface far from water channels. In contrast, the cast-in steel pipe copper stave uses a bumpy surface to render the distance between the stave inner surface and each water channel virtually constant around the water channel. This allows the stave inner surface to be uniformly cooled. Such uniform cooling in turn reduces the temperature difference between the stave inner and outer surfaces, and suppresses thermal stresses and deformation.
In addition, under large thermal load, compression plastic strain is caused on the stave inner surface of a rolled copper stave by the temperature difference between the stave inner and outer surfaces, which can lead to stave deformation. In contrast, the inner surface of the cast-in steel pipe copper stave is isolated at each bump, thereby making compression stresses less likely to act on the stave and suppressing plastic strain. Hence, the bumpy surface of the cast-in steel pipe copper stave reduces stresses and strains which act on the stave, and suppresses deformation. Also, since the cast-in steel pipe copper stave forms water channels using steel pipes, plug welding or pipe connection welding, which are indispensable for making rolled copper staves, are not necessary. By avoiding the use of welding, which is structurally weak parts, the risk for breakage of welds can be eliminated.
Copper staves prevent wear by the scabs formed by using its high cooling ability, thereby avoiding direct contact with the descending material in front of the stave. However, such accretion frequently falls off due to the fluctuations during furnace operations. Without accretion, the wear of a copper stave depends on the contact force and descending speed of the material in front of the stave, hardness of copper and the material, and shape of the material. Given this, counter-measures against wear feasible for a stave include reducing the contact force and descending speed of the material when there is no accretion, in addition to stably retaining accretion, as well as not allowing the hardness of copper to be reduced. While grooves for rolled copper staves is formed by machining, the cast-in steel pipe copper stave forms ribs by integrally casting, allowing for forming as-desired rib shapes. Based on this feature, cast-in copper stave normally has an upward rib structure with wear resistance. The hardness of copper depends on the cooling ability.
If there is no accretion inside-furnace surface of a rolled copper stave, material once entered between ribs hardly moves because the ribs are small. Hence, material in front of the stave descends without being influenced by the ribs. In contrast, since the ribs of the cast-in steel pipe copper stave face upward and are large, material which has entered between the ribs is discharged back into the furnace, creating a flow (load transfer). At this time, the material is discharged upward. This upward flow pushes the material in front of the stave back to the furnace, causing the contact force and descending speed of the material to be reduced at the rib tips.
In addition to the technical advantages in extending life and saving energy, the cast-in steel pipe copper stave has also the characteristic of the high design flexibility. When a cast iron stave or cooling plate which a BF uses is broken and something needs to be done in order to extend the life, the replacement with a copper stave using the existing shell opening may be needed. In the case of rolled copper staves, since water channels are formed by drilling, the water channel layout is restricted, making it difficult to freely form water channels in a manner tailored to the existing shell opening. In contrast, water channels of the cast-in steel pipe copper stave, which are formed using bent steel pipes, allow for flexible layout adopting steel pipes for the existing opening of the shell. Fig 8 shows high flexibility of cooling channel layout in cast-in steel pipe copper stave.
Fig 8 High flexibility of cooling channel layout
Layouts of cooling systems
Water cooling systems are normally designed to operate in a closed loop rather than the conventional open systems. This allows the pipe work to be chemically cleaned, and by controlling water chemistry throughout the campaign, this clean surface can be maintained, thus ensuring maximum heat transfer. The development of the sealing of the cooling tubes to the shell is towards the use of ever, thinner, ’softer’ metallic expansion joints. For both systems, i.e. flat plate coolers or staves, there is an increasing use of rubber bend and hoses.
Independent series are typical for water distribution in flat coolers. With stave coolers, it is normal to provide a number of independent flow and return headers in accordance with the number of tubes. This ensures that in the case of failure one feed system, the remaining stave tubes receive sufficient cooling. Nose and corner tubes are connected to the additional water circuits.
The rise in the BF productivity has been accompanied by a greater demand for efficient cooling systems. High-performance cooling systems are needed to ensure that the operational process runs smoothly even when under such stress. Reliable and effective cooling system solution is needed in the high loaded zones of the BF. There are three types of cooling circuits which are normally used for the BF cooling systems. These are (i) closed loop cooling circuits, (ii) combined closed loop cooling circuits, and (ii) semi-closed or open loop cooling circuits or evaporation cooling circuit
Closed loop cooling circuits – These cooling system circuits are normally designed with two or three nitrogen pressurized closed loop water circuits with forced recirculation. The advantages of such a system, as compared to an open loop cooling circuit are (i) circuit can be operated with treated, softened water, (ii) no corrosion, fouling, and clogging of pipes and cooling elements, (iii) low cost for chemical additives, (iv) low water consumption, (v) no contact with oxygen of ambient air, (vi) very sensitive leakage detection is possible, (vii) increase of the evaporation temperature of the cooling water due to operation under adjustable nitrogen pressure (e.g. pressure – 8 bar, evaporation temperature – 170 deg C), (ix) low electrical power consumption, since only the pressure drop is to be covered by the pumps and the differential height is not to be considered, (x) easy flow adjustment and control for the different cooling elements, (xi) low maintenance costs, and (xii) longer life time of circuit equipment and cooling elements resulting in high availability of and hence high productivity.
Combined closed loop cooling circuits – These circuits constitute an economic, but still efficient, reliable, and safe solution. The cooling water is used to cool serially different blocks of cooling elements, still respecting the operation requirements of the individual cooling elements. The total flow rate is considerably reduced while temperature difference of the cooling water is still in an acceptable range and the cooling tower operates efficiently.
Semi-closed or open loop cooling circuits or evaporation cooling circuit – These can also be used as an option in accordance with site conditions. This solution is mainly good for minimum cost upgrades at the existing BFs.