Electric Arc Furnace Cooling System
Electric Arc Furnace Cooling System
The history of electric arc furnace (EAF) steelmaking is not very old. It is only slightly more than a century when the first furnace was commissioned to melt steel by utilizing electric power. The initial development of the technology took place, since these furnaces made it possible to easily achieve highest temperatures and ensured the best conditions for the production of high quality special steel grades and alloys. Since that time, a large number of advancements have been made in the furnace equipment, EAF technology, melting practice, raw materials, and steel products.
Initially, EAF steelmaking was developed for producing special grades of steels using solid forms of feed materials such as scrap and ferro alloys. Solid material were firstly melted through direct arc melting, refined through the addition of the appropriate fluxes and tapped for further processing. EAFs were also used to prepare calcium carbide for use in carbide lumps. The technology of EAF steelmaking has now developed from a slow process to a rapid melting process which performs at a level that approaches the productivity level of a BOF (basic oxygen furnace).
There have been several advances in EAF technology which have allowed the EAF to compete successfully with the integrated production route. The electric arc furnace (EAF) has quickly become large scale industrial equipment to produce liquid steel.
Since the 1960s, the technology has undergone rapid development moving from a ‘boutique’ technology to the second largest steelmaking technology behind basic oxygen steelmaking technology. Since the late twentieth century, the production of steel by the EAF has grown considerably. There have been several reasons for this but primarily they all relate back to product cost and advances in technology.
In 1878 – 79, Carl Wilhelm Siemens took out patents for electric furnaces of the arc type. Though De Laval had patented an electric furnace for the melting and the refining iron in 1892 and Heroult had demonstrated electric arc melting of ferro-alloys between 1888 and 1892, the first industrial EAF steelmaking process only came into operation in 1900. Development was rapid and there was a ten-fold increase in production from 1910 to 1920, with over 500,000 tons being produced in 1920, though this still only represented a very small proportion of the global steel production at the time.
Emerging new technology was put into commercial use in the beginning of the twentieth century when wide ranging generation of relatively cheap electric energy started at that time. In 1906, the first electric-arc melting furnace in the United States was installed at the Halcomb plant. This ‘Old No. 1’ Heroult electric-arc furnace now stands as a designated ASM (American Society for Materials) historical monument at Station Square in Pittsburgh, Pennsylvania (Fig 1a).
A schematic cross section through a Heroult arc refining furnace is shown in Fig 1b. In this Fig, E is an electrode (only one shown), raised and lowered by the rack and pinion drive R and S. The interior is lined with refractory brick H, and K denotes the bottom lining. A door at A allows access to the interior. The furnace shell rests on rockers to allow it to be tilted for tapping.
Fig 1 Heroult electric-arc furnaces
The emerging new technology of EAF started in the beginning of the twentieth century when wide-ranging generation of relatively cheap electric energy started at that time. First-generation EAFs had a capacity in between 1 ton (t) and 15 t. The EAF had Bessemer / Thomas converters and Siemens-Martin furnaces as strong competitors, initially. But its niche was the production of special alloy steels needing high temperature, ferroalloy melting, and long refining times. EAF based operations have gradually moved into production of those steels which were traditionally made through the integrated route. The first of these areas was long products (reinforcement bar, merchant bar, and wire rod). In the 1960s, with the advent of billet casting, the EAF occupied a new niche and became the melting unit of choice for the so called mini mills, feeding billet casting machines for the production of long products. This was followed by advances into heavy structural and plate products and by 1985, a new niche for electric steelmaking began to be taken to the flat products area with the advancement of thin slab casting and direct rolling process.
In the following two decades, to better support the short tap-to-tap time needed by billet casting machines, the EAF reinvented itself as a melting-only unit. Steel refining was left for the recently introduced secondary steelmaking. Large transformers were introduced and ultrahigh-power (UHP) furnaces were developed, which were made possible by adopting foaming slag practice. This way, tap-to-tap time became close to casting time at the continuous casting machines.
EAFs have greatly improved at a fast pace. Only 20 years to 30 years ago today’s EAF performance was impossible to imagine. Owing to the impressive number of innovations the tap-to-tap time has been shortened to 30 minutes (min) to 40 min for the best 100 t to 130 t furnaces operating with scrap. Accordingly, their hourly and annual productivity increased. Electrical energy consumption has reduced to around half, i.e. from 580-650 kilowatt hours per ton (kWh/t) to 320-350 kWh/t. Electrical energy share in overall energy consumption per heat has dropped to 50 %. Electrode consumption was reduced 4times to 5 times with this level of performance expected as normal for the majority of the steelmaking shops in the immediate future. Fig 2 shows evolution of EAF technology during the period 1965 to 2010.
Fig 2 Evolution of EAF technology during the period 1965 to 2010
Majority of the recent EAF technology has focussed on increased energy efficiency, furnace availability, and production. For example, foaming slag control systems provide increasing volume of slag layer, hence, shielding of the electric arc radiation, and efficient heat transfer from the arc to the melt. Bottom stirring systems improve thermal and chemical homogeneity of the liquid steel. Oxygen injectors and oxy-fuel burners which are installed at the EAF walls increase the use of chemical energy and decrease power-on time. However, with increasing energy intensity in the EAF, the thermal load of refractory lining is increasing, in particular at the slag zone area. Frequently, hot spots are identified by increased wear of refractory lining because of the electric arc radiation, gas burners, and oxygen injectors.
Today, cooling water system is a system which is integral to the EAF operation. Typically, there are several cooling systems. The main classes of the different developed design of cooling system are spray coolers, plate coolers, stave coolers, internal block, panels and external jackets.
Some operations need extremely clean, high quality cooling water. Transformer cooling, delta closure cooling, bus tube cooling and electrode holder cooling are all such applications. Typically, these systems consist of a closed loop circuit, which conducts water through these sensitive pieces of equipment. The water in the closed loop circuit passes through a heat exchanger to remove the heat. The circuit on the open loop side of the heat exchanger typically flows to a cooling tower for energy dissipation. Other water cooled elements such as furnace side panels, roof panels, exhaust gas system ducting, furnace cage etc. typically receive cooling water from a cooling tower.
The cooling circuit typically consists of supply pumps, return pumps, filters, a cooling tower cell or cells and flow monitoring instrumentation. Sensitive pieces of equipment normally have instrumentation installed to monitor the cooling water flow rate and temperature. For the majority of the water-cooled equipment, interruption of the flow or inadequate water quantities can lead to severe thermal over loading and in some cases catastrophic failure.
Present day EAFs are an advanced technology in the production of iron and steel, which began to be used in the late seventies and early eighties of the last century. EAFs are used to heat the materials installed in the furnace by an electric arc. The temperature of the EAF can be up to 1,800 deg C. The EAF is composed of three parts, the shell which includes walls and lower bowl, the main part, and the roof part which is well insulated or water cooled. In its original constructive conception, the EAF had refractory lining in the lower shell, the upper shell, and the roof, as shown in Fig 3.
Fig 3 Typical refractory lined electric arc furnace
In refining stage, the electric arc exposed to the part above the liquid bath, or called freeboard, can considerably heat up the wall of the furnace. In general, higher operating temperature means high power to melt and refine the scraps more quickly, so that the productivity is increased. On the other hand, the considerable electric energy can also damage the furnace wall because of the high temperature. In fact, one of the major problems in the EAF is the high temperature which results in thermal stresses and cracks within the material of the furnace surrounding walls. An effective cooling technique is needed to avoid such problems.
One of the most important innovations in EAF design was water cooling. Although this was used to a limited extent in older furnace designs for cooling of the roof ring and door jambs, modern EAFs are largely made up of water cooled panels (WCP) which are supported on a water-cooled cage. This allows for individual replacement of panels with a minimum of downtime. By water cooling the cage structure, it can be ensured that thermal expansion of the cage does not occur. Thus warping of the cage due to the thermal stresses is avoided as are the resulting large gaps between the panels. Water-cooled panels allow very large heat inputs to the furnace without damaging the furnace structure. In the older EAF designs, these high power input rates had resulted in increased refractory erosion rates and damage to the furnace shell.
Water cooling has been applied to the arc furnace for some years at isolated points such as the extraction elbow but, apart from a few smaller furnaces, widespread cooling of the sidewall and roof has only evolved over the last few decades. Japan started the trend with panels set behind the refractories at the hotspots, but by 1974 one system replaced 50 % of the sidewall, and today 75 % of the sidewall and 85 % of the roof can be water cooled. Different designs for sidewall cooling have been developed, and with few exceptions they can be divided into five types namely (i) box type, (ii) tubular type, (iii) sandwich system, (iv) water cooled block, and (v) copper panels .
The box type consists of a welded steel box with inlet and outlet, normally having internal baffles and external studs or slag-catchers on the hot face. Systems have emerged from different, the most successful being the Japanese DAIDO system and the German Korf / Fuchs type. Tubular type panels consist of steel tubes arranged in either horizontal or vertical rows and connected at the ends by U pieces, through which the water flows to present a cooling face to the furnace interior. Sandwich system uses smaller areas of water cooling integrated with high thermal conductivity refractory, but which has proved relatively unpopular. Water cooled blocks, which are normally cast iron with internal steel cooling tubes, and are more common in the USA than in Europe. Copper panels, have a higher thermal conductivity and hence suffer less thermal loading and conduct heat away more rapidly than steel panels. Majority of copper panels are cast, but fabricated panels have been used successfully.
To reduce the overheating, WCPs are required to be installed on the side wall and the roof to cool down the overheated wall by water. WCPs were introduced initially in specific locations of the sidewalls and upper shell of the EAF with the purpose of reducing the downtime of the EAF for refractory repairs. The good results achieved led to the expanded use of WCPs throughout the EAF sidewalls, upper shell, and roof. However, high temperature can still concentrate on the certain area on the EAF shell, called hot spot. The hot spot can overheat the WCP and make it perforated, which can lead to serious results like explosion. In recent years, sensors have been set up to monitor the condition of cooling water and alarms trigger if the excessive temperature appears, so that the EAF operator can take issues, like reducing arc power reasonably to control the WCP temperature.
The present day furnace design includes three water-cooled parts namely (i) roof, (ii) panels, and (iii) exhaust gas duct. Although some heat is lost due to the heat extraction by the cooling water, this design makes it possible less refractory consumption (because they replace refractory linings) and the use of high power. At the time the panels were first introduced, some fears had arisen on safety risks, but after realizing the cost advantage, almost all EAF adopted them. They can be made of steel or copper (much longer life) and with different designs (conventional, flip, and turn, etc.).
Today, WCPs used in the water cooling systems are the major component of electric arc furnaces and installed in the vessel and the roof of the furnaces. The panels transfer the heat surrounding the furnace by means of water flow through the panels. Since the WCPs of EAFs are exposed to considerable thermal, mechanical, and chemical stresses, they tend to crack and leak easily. A main concern of the designers and manufacturers of water cooling systems for the EAFs is to achieve the maximum efficiency for the panels. Increasing the useful life and efficiency of the cooling panels decreases the repairing time and the time needed for exchanging parts, and hence it increases the tonnage of productive fusion, saves time and expense and the cost of producing expensive new panels.
WCPs allow the furnace to withstand high temperatures without suffering any structural damage. In old design electric arc furnaces, such high temperatures can have caused a higher erosion rate of the refractory walls and damages to the furnace shell.
WCPs are typically installed around 350 mm above the liquid steel level, and their connection to the water cooling system is located at the rear of the panel, through hoses and ball valves. The initial water-cooled panels were made of special steel plate with an internal system of baffles to direct the flow of water, as shown in Fig 4. These WCPs were known as ‘box panels’. Presently, WCPs are made with steel or copper tubes. These panels are subjected to a pressurized water flow, which typically varies between 10 cum/h (cubic meters per hour) and 15 cum/h for each square metre of panel. The panels are designed so that, during the normal operation, the water in its entire path undergoes a temperature rise of between 10 deg C and 35 deg C, depending on their mounting position. The use of tubular panels allowed a reduction in the volume of circulating water, when compared to their predecessor, the box panel.
Fig 4 Schematics of the installation of the initial water cooled EAF panels
Since several years, WCPs for the side wall and furnace roofs are used at the majority of the EAFs as the appropriate technologies in the EAF steelmaking. WCPs are being used for the extreme thermal conditions in the upper part of the EAF which are due to the (i) radiation from the electric arc, (ii) use of oxy-fuel burners, (iii) post combustion of high CO exhaust gas, and (iv) oxidizing to reducing gas compositions during charging, melting and tapping. 10 % to 15 % of the electric arc power which is of the order of 30 MVA to 100 MVA is transferred to the EAF wall by radiation during the melting stage. However, few furnaces use water cooled furnace roof but entirely refractory lined furnace vessels in order to easily change complete EAF vessels during production and to avoid contamination of distinct sorts of special alloy steels during scrap melting in the EAF.
Parameters which have a strong influence on panel life include water quantity and quality, water flow rate and velocity, inlet water pressure and pressure drop across the panel, pipe / panel construction material, and pipe diameter.
Steel pipes with circular cross-sections are normally used in panels to cool the EAFs. These pipes are produced world-wide. The quality of the raw materials used to make the panels, their design, arrangement of the panels (water inlet and outlet, fittings and the height of the structures), thickness and the diameter of the pipes, water hardness and impurities present in the water, speed and the input rate of the water are factors which effect on the efficiency and lifetime of the panels.
For the majority of the boiler tube grade water-cooled panels, a pipe diameter of 70 mm to 90 mm is normally selected. For copper panels, the choice of tube diameter is normally is to be a cost based decision but frequently falls into the same range. In some high temperature zones (e.g. lower slag zone), a smaller diameter copper pipe is used with higher water velocity to prevent steam formation.
In majority of applications, boiler tube (grade ‘A’) steel is used for the WCPs. These steel grades are normally reasonably priced, are easy to work with, and provide a suitable thermal conductivity for heat transfer (around 50 W/ m.K) for a high thermal loads (up to seven million kJ/sqm.hr). However, this material is also susceptible to fatigue caused by thermal cycling. In areas which are exposed to extremely high heat loads, copper panels can be used. Copper has a thermal conductivity (383 W/m.K) which is around seven times that of boiler tube enabling it to handle very high thermal loads (up to 21 million kJ/sqm.hr).
Copper panels result in much higher heat transfer to the cooling water and hence to optimize energy consumption in the EAF, copper panels are only to be used in areas where excessive heats loads are encountered (e.g. close to the bath level, oxy-fuel burner ports etc.). This of course is dependent on the quantity of slag build-up over these panels. Normally, slag build-up is thicker on copper panels due to the greater heat transfer capacity. Alternatively, the distance between the start of water-cooled panels and the sill level can be increased if steel panels are to be used in the lower portion of the water-cooled shell. Fig 5a shows water cooled wall panels of the EAF.
Fig 5 Water cooled wall panel and energy balance of electric arc furnace
Water cooling systems at EAF roof and wall provide an economic solution to achieve acceptable lifetimes of the EAF wall under the extreme thermal loads but present also a considerable heat sink of the EAF (Fig 5b). There is evidence from complete energy balances of EAFs (Fig 5b), that the energy flow rate to the cooling system (water cooled EAF wall panels and furnace roof) and the share of energy loss due to cooling compared to total energy demand vary from furnace to furnace and range between 10 % and 20 % [for example (i) 100 t EAF – 83 kWh/t, (10.6 %), (ii) 145 t EAF – 143 kWh/t, (18.9 %), (iii) 125 t EAF – 134 kWh/t (17.3 %), and (iv) 150 t EAF – 135 kWh/t, (17.4 %)]. Hence, a considerable potential for energy saving is related to efficient control of heat transfer to EAF cooling systems. Economic calculations of EAF wall technology, e.g. high quality MgO-C refractory or water cooled steel panels, depend sensitively to energy losses and the (electric) energy price.
Heat flux densities to the EAF wall of the order of 35 kW/sqm (kilowatt per square metre) to 50 kW/sqm have been reported for furnaces with foaming slag technology in a study 10 years ago. Another study gives heat flux density values to the EAF wall panels between 200 kW/sqm and 600 kW/sqm for a modern UHP furnace. Recent measurements of water cooling systems resulted in values in the same order of magnitude (100 kW/sqm to 330 kW/sqm).
Besides the high thermal load to the EAF vessel lining and erosion from steel and slag movement due to the stirring and splashing by electric arc and injectors, respectively, slags with calcium silicate to fayalitic compositions provide chemical attack of the magnesia refractory lining at the slag zone area. In particular at hot spots, the lining at the slag zone area has to be regularly repaired with gunning mixes. Normally, the life time of the refractory lining at the slag zone restricts the life time of the entire EAF lining and determines shutdown of the furnace for relining.
Recently, additional water cooled blocks in the refractory lining at the slag line are suggested as latest EAF development with the aim to decrease refractory wear by generating a protective coating and solidifying infiltrating slag and melt. In the majority of the cases copper blocks are installed at the permanent refractory lining, preferably at hot spots below gas burners or supersonic oxygen injectors, but plant trials with advanced water cooled blocks in the wear lining at steel / slag level were also reported. Water cooled elements are not applied below slag / steel level due to safety restrictions. Fig 6 shows panels making the upper part of the shell.
Fig 6 Panels making the upper part of the shell
Some of the advantages achieved with the use of WCPs in the EAF include (i) increased productivity, (ii) elimination of the need to reduce power during refining, (iii) reduced downtime to repair refractory material, (iv) reduced time to erect a new furnace (refractory lining), and (v) reduced consumption of refractory, an important economic advantage. Despite the advantages achieved by the use of WCPs in the EAF, at least two factors caused concern among users. The first factor involves the increase of heat losses during the production of liquid steel because of the extraction of heat by water-cooled panels. However, subsequent studies have rejected this argument and proved that the increase in heat loss with the use of WCPs is compensated by the increase of availability of the furnace and the economy in the use of refractory material.
The second factor refers to the risk of water leaks from the panels into the EAF. The flow of water to the panels is continuous, but the heat input which focuses on the panels varies over the heat, making the panels suffer from thermo-mechanical fatigue of the tubes, resulting from the different expansions and contractions. The thermal fatigue of tubes gives rise to micro-cracks which propagate and cause minimum of the water leakage into the EAF. Over time, these micro-cracks expand, increasing the quantity of water leakage into the EAF. Other factors can cause water leakage into the EAF, such as holes in panels due to rebound flame, splash oxygen, electrical short circuit, and hose breakage.
Even small water leaks inside the EAF can cause accidents. These small leaks are typically very difficult to detect even by operational experts and by the detection system, and cause hydration of the refractory lining, bringing water to the refractory / shell interface, which can cause explosions and / or holes in furnace.
Large water leaks are more easily observed by the operators, but can quickly flood the EAF when occurring during a heat. Perhaps the only signal emitted by this type of leakage which can be visibly observed during the heat (also for dedusting exhaust) is the presence of a blue flame between the electrodes. Large water leaks in an EAF, in general, are responsible for explosions and projections of incandescent material out of the EAF. This occurs when the excess water present inside the EAF is covered with high-temperature liquid material (liquid steel and / or liquid slag), triggering the occurrence of two chemical phenomena namely (i) the increased speed of boiling of water to form steam, and (ii) the dissociation of hydrogen from water. These phenomena cause a volumetric expansion that it is quickly pressurized within this ‘blanket’ of steel and / or slag. The disruption of this layer of steel and / or slag means an explosion without notice, a function of the speed of the reactions.
Normally the detection of water leaks in an EAF is based on a visual inspection of the EAF, a practice totally dependent on the expertise of the EAF operators. There are also automated systems to detect water leaks from water-cooled panels, but typically these systems are based on measuring pressure and water flow, or on the analysis of exhaust gases from the EAF. These systems, in general, tend to detect only large water leaks, have a low response time and do not indicate the exact location of water leaks. The new leak detection technology developed by ‘Lumar Metals’, presents an innovative approach in the monitoring and detection of water leaks in water cooled panels. By analyzing and combining data collected in each individual water-cooled panel, new leak detection technology has proved capable of quickly detecting leaks of 3 litres per minute up to 10 seconds after their occurrence.
WCPs are required to withstand both high thermal loads as well as high mechanical loads. The highest mechanical load occurs during furnace charging. Scrap can strike the panels causing denting of pipes or even splitting and rupture. Hence, the pipe selection is to allow for a wall thickness which can withstand these forces. At the same time, a minimum wall thickness is needed in order to maximize heat transfer to the cooling water. This tradeoff is required to be evaluated in order to arrive at an optimum pipe thickness. Normally, the minimum pipe thickness used is 8 mm. Maximum pipe thickness depends on the thermal load to be removed and the quantity of thermal cycling. In practical application the wall thickness is normally in the range of 8 mm to 10 mm.
Cooling panel life is primarily dependent on the quantity of thermal cycling which the panel is exposed to. During flat bath conditions, the panels can be exposed to very high radiant heat fluxes. Following charging, the panels are in contact with cold scrap. Hence, the quantity of thermal cycling can be substantial. The optimum solution to this issue involves providing maximum cooling while minimizing the heat flux directly to the panel. In practice, it has been found that the best method for achieving these two goals is to promote the buildup of a slag coating on the panel surface exposed to the interior of the furnace. With a thermal conductivity of 0.12 W/m.K to 13 W/m.K, slag is an excellent insulator. Cups or bolts are welded to the surface of the panels in order to promote adhesion of the slag to the panels.
It is important that the temperature difference across the pipe wall in the panels does not get very high. If a large temperature differential occurs, mechanical stresses (both due to tension and compression) build up in the pipe wall. Most critically, it is important that the yield stress of the pipe material is not to be exceeded. If the yield stress is exceeded, the pipe deforms and does not return to its original shape when it cools. Repeated cycling of this type leads to transverse cracks on the pipe surface and failure of the panel.
Panel layout is related to several factors including thermal load, desired temperature rise for the cooling water, desired pressure drop across the panel, and desired pipe diameter. Normally, designers aim for a water velocity which results in turbulent flow within the pipe. Hence, the minimum needed water velocity varies depending on the inside diameter of the pipe which is used. A minimum flow velocity of around 1.2 m/s is quoted by several suppliers. The maximum velocity depends on the desired pressure drop but it is normally to be less than 3 m/s (metres per second). The range given by several suppliers is 1.2 m/s to 2.5 m/s. This helps to minimize the possibility of steam formation in the pipe.
Some systems are designed so that a water velocity of at least 2.5 m/s is provided to ensure that the steam bubbles are flushed from the panel. Failure to remove smaller steam bubbles can result in a larger bubble forming. Steam generation can lead to greatly reduced heat transfer (by a factor of ten) and as a result possible panel overheating and failure in the area where the steam is trapped. In extremely high temperature regions such as the lower slag line, water velocities in excess of 5 m/s are used to ensure that steam bubbles do not accumulate in the panel.
A vertical configuration is preferred in most modern designs since it minimizes scrap holdup and also can help to minimize pressure drop across the panel. Normally water cooling systems on EAFs are designed for an average water temperature rise (i.e. outlet to inlet) of 8 deg C to 17 deg C with a maximum temperature rise of 28 deg C. Majority of the furnace control systems have interlocks which interrupt power to the furnace if the cooling water temperature rise exceeds the maximum limit. In high powered furnaces, it is desired that a water flow rate of 0.15 cubic metres per minute per square metre (cum/min/sqm) is available for a side wall panel or a water flow rate of 0.17 cubic metres per minute per square metre is available for a roof panel. For DC (direct current) arc furnaces an additional 0.01 cubic metres per minute per square metre is to be made available.
Pressure drop across the panel is a parameter frequently ignored by most furnace operators. In order to get uniform flow to each panel, it is imperative that the pressure drop across panels be as even as possible. It is desired that the water pressure exiting the panels be at least 0.14 MPa. Frequently, panels of different design are mixed within the same supply system. This occurs since replacement panels are frequently sourced from local fabricators as opposed to the original furnace supplier. Slight changes in materials, dimensions and configuration can lead to big changes in the panel pressure drop. Mismatched panels within the same supply circuit lead to some panels receiving insufficient flow and ultimately panel failure.
Majority of the WCPs are designed to be airtight. However, replacement panels are sometimes made without complete welds between pipes. This can be a cost saving when considering the fabrication of the panel but leads to increased operating costs and lost production. The EAF is operated under negative pressure and as a result, air is pulled into the furnace through any openings. Thus openings are to be minimized, especially those in and around the WCPs. Fig 7 gives typical water cooled panel designs.
Fig 7 Typical water cooled panel designs
Water cooled roofs have followed on from water cooled sidewalls, and similar advantages, as detailed below, have been recognized. Additionally, the structure of the roof is stable, removing the danger of collapse present with refractory. The roofs are normally of the box or tube type, and they retain a refractory centre to prevent arcing between the electrodes and the steel panels.
One factor which has slowed the progress of all water cooling systems is safety as, traditionally, the combination of water and liquid steel has been regarded with apprehension. However, the safety precautions recommended by the manufacturers of WCPs (as given in the next paragraph) are readily implemented and there have, been no major problems recorded by users of water cooling systems.
Safety Requirements for the use of WCPs are (i) distance from bottom of panel to the melt surface is not to be too small ( 400 mm is a typical minimum), (ii) for tilting furnaces, this is to be greater in the area of the taphole and there has to be a safety hole above the taphole to indicate level of liquid steel in case it approach the panel, (iii) the panels are to be substantial enough to withstand small arc-backs without leaking, and in case a leak into the furnace takes place, safety measures as for any water leakage (e.g. electrode cooling) are required to be taken, (iv) the panels are to be positioned as far set back from the lower sidewall as possible so that any major leaks tend to run outside of the furnace hearth, (v) cooling water flow and temperature measurements are necessary, (vi) an emergency water supply is to be available in case of water pump failure, and (vii) safety pressure release valves are to be provided on each panel.
Refractory practice for the lower sidewall has changed since the introduction of WCPs, with high thermal conductivity bricks being used to allow conductive cooling of the sidewall down to the slag line (normally 500 mm below the panels). Magnesia-carbon bricks have been adopted in several furnaces with carbon levels from 5 % to 20 % and sometimes as high as 35 %. The optimum carbon level depends upon operating practice, particularly on the volume of oxygen blown.
The cost benefits of operating water cooled panels are consistently large enough to endorse their use on furnaces of all sizes and product types. The initial impetus for their use was the considerable savings possible on the refractories which were replaced, combined with reduced downtime from quicker and less frequent relines. Other benefits were recognized as more experience of WCP operation was gained.
The refractory savings are immediately apparent, and although they vary with furnace size and practice, for a medium or large UHP furnace, 60 % brick and 50 % gunning material savings are reported. A hard driven furnace can achieve a 80 % brick reduction, but conversely, a less intense gunning practice can show only 20 % materials savings. Increases in steel output of between 5 % and 19 % have been recorded, due primarily to the increase in the number of heats per campaign, from 100 / 200 to 250 / 500. Actual savings in downtime vary widely from 25 % to 75 %. Productivity is also increased by the ability, when water cooling is fitted, to use higher powered arcs during melting, hence reducing the overall tap-to-tap time. These shorter melting times and the use of longer arcs have resulted in reduced electrode consumption.
The effect of WCPs on the energy consumption of arc furnaces is not clear, as some users claim a saving while other users report increases to a level of 20 kWh/t maximum. It appears that, provided furnace practice is modified to fully utilize the WCPs, the energy consumption normally remains constant, although the type of system used can have a major effect. Heat losses to the water cooled lining account for 10 % to 20 % of the total input, and furnace practice can be adapted to try and reduce this, for example using foamy slags to minimize arc radiation to the wall. An alternative strategy is to recover the heat, either by providing hot water or more recently by hot cooling to produce wet steam. The latter needs some redesign of the water supply system and improved panel quality because of the higher temperatures and pressures involved.
The Korf / Fuchs type WCP were originally used with a sprayed on refractory coating which was intended to insulate the panel from electrical arcing and reduce heat losses, although it has been found in practice that the gunning material was soon replaced with splashed-on slag and metal. Majority of plants now fit the panels bare, relying on the slag build-up, but unlike the gunning material the slag layer has an unknown thermal conductivity and a variable thickness.
EAFs using part or complete charges of direct reduced iron have suffered more severely from sidewall refractory wear due to the longer periods without shielding from the arcs, and using WCPs has increased wall lives considerably. The water cooled panels themselves have a lifespan dependent on their design, material of construction, position in the furnace and the mode of failure. Majority of the WCPs fail by cracking, the suggested causes being hot face shrinkage and cyclic thermal shock, and hence copper panels have longer lives as their higher thermal conductivity reduces the thermal stresses.
WCPs can be scrapped when the slag catchers have eroded away, but several users make minor repairs to prolong panel life. Failure can also occur from arcing onto the panel, careless oxygen blowing, or partial immersion in liquid steel, but these failures can be reduced or prevented by careful scrap loading and oxygen practice, and the use of a safety hole set above the tapping spout. When failure does occur, the resultant water leak can damage refractories and so water supply to the failed panel is cut off and the panel is replaced at the end of the heat, or even after several heats.
Some manufacturers have guaranteed a minimum life of 1500 heats for their panels and for the Korf / Fuchs type panels this is normally exceeded by at least 1000 heats. Other box panels are not as durable, although this can be due to their position in the furnace. Cast cooling blocks have shown similar lives, but there is considerable range from 250 heats to 5000 heats. Copper panels have lasted over 10,000 heats, but this drops to 3300 heats for lower wall hot spot positions.
The use of water cooling for large areas of the furnace shell has resulted in considerable advances in the productivity, cost-effectiveness, and operating practices of EAFs.
Recently, more attention has been paid to safety with water cooling. Firstly, to detect, limit, and avoid the possibility of water leakage and secondly, to cut the need of repairing work in the hot furnace. Exhaust gas analysis, when hydrogen is included, is a useful tool to detect leakage. To limit leakage and maintenance work, solid cast or machined WCPs have been introduced. Split shell, with spray-cooled upper shell means less risk as non-pressurized water tends to penetrate less in case of leakage.
Other water cooled elements, such as furnace side panels, roof panels, off gas system ducting, furnace cage etc. typically receive cooling water from a cooling tower.
The water cooled panels typically used in the electric arc furnaces are made of mild steel. These mild steel-based WCPs normally have low life. Over a period of use, the effectiveness of heat transfer reduces considerably, resulting in higher refractory consumption.
The mild steel-based water cooled panels can be replaced with copper-based water cooled panels. The advantages of copper-based water cooled panels include (i) increase in life of panels (up to 6 times), (ii) decrease in number of panel failures, and (iii) reduction in refractory consumption.
WCP overheating issue
As a main energy source of EAF, electric arc takes up around 45 % to 65 % of the total energy. Arc power is delivered in 3 different forms namely (i) convection,(ii) radiation, and (iii) electron flow. Among these three ways, extremely high temperature of the arc makes the share of radiation can be up to around 34 % to 80 % according to different studies. Once the electric arc is exposed to the furnace walls, the huge power brought by electric arc in form of radiation can heat up these walls, leading to the excessive temperature on the water panels, and WCP overheating issue appears. This is also an important reason why the EAF operator can take reducing the electric arc power as an approach to reduce the overheating issue.
The slag foaming phenomenon is effective to protect the side wall from being heated up by the exposed electric arc. The covered electric arc will deliver most of the energy to slag layer and liquid steel rather than emitting heat to the furnace shell. To obtain a scheme to reduce the WCP overheating issue, it is applicable to find out a detailed relationship of slag foaming phenomenon with arc exposure and WCP overheating issue.
The free burning arc exposed to the freeboard can lead to a waste of energy and a risk of overheating issue on water-cooling panel.
Spray cooled equipment
Spray cooling has also shown itself to be a viable means for cooling EAF equipment. Spray-cooled components are typically used for roof and sidewall cooling. The main components of a spray cooled system are the inner shell, spray nozzles and water supply system. Water is sprayed against the hot surface (inner shell) of the cooling element. The outer shell acts purely for containment of the water and steam. The inner shell is fabricated from rolled plate. A uniform spray pattern is essential for proper heat removal from the hot face. Maintenance of a slag coating on the exposed side of the hot face is also integral to regulating heat removal and protection of the cooling panel. In general, at least half an inch of slag coating should be maintained.
The water supply system for spray-cooled panels is simpler than that used for conventional WCPs because the operation is carried out at atmospheric pressure. Strainers are provided to ensure that suspended solid material is not carried through the circuit where nozzle clogging could become a concern.