PREHEATING FOR FURNACES

Field of application

This invention relates to a system for continuous charging and preheating of the charge for furnaces, in particular for electric furnaces used for the production of liquid steel with steel and cast iron scrap and pre- reduced ferrous material. The heat used for preheating the charge is recovered from the hot fumes aspirated by the furnace dedusting system and can, optionally, be partially provided by adapted burners installed on the system for continuous charging of the furnace.

The production of steel with electric furnaces constitutes a method of primary importance for the steel industry worldwide, because it makes it possible to recycle widely-available ferrous material and, compared with the process using a blast furnace, does not require large quantities of carbon coke, the production of which is still a cause of serious atmospheric pollution today.

Their intrinsic environmental qualities notwithstanding, electric furnaces are major consumers of energy, but in recent years advances have been made in containing consumption. The reduction of the consumption of energy, in addition to saving operating costs of electric furnaces, is now a universally felt requirement, because lower energy consumption reduces overheating of the Earth's atmosphere owing to the emission of greenhouse gases and, for this reason, the environmental authorization procedures in place in many countries now require the use of BATs (Best Available Techniques) in building new steel production plants.

The present invention eliminates the negative aspects of conventional systems for continuous charging and preheating of the charge for electric furnaces, powered both with alternating current and with direct current, and it also lends itself to use with other types of furnaces, such as for example submerged-arc, plasma or induction furnaces, both in the steel industry and in other sectors.

State of the art

In the current state of the art, one of the most widespread systems for continuous charging and preheating of the charge for electric furnaces is represented by the invention of John A. Vallomy, described by patent no. US 4,543,124, which has been developed over the last thirty years with the improvements described in patents nos. US 6,155,333, US 6,450,804 and US 7,767,136. The basis of the systems cited in the aforementioned patents is a conveyor of variable length which is connected to an opening provided on the side of the electric furnace. A first portion of the conveyor, which is open at the top, is used for the charging of the material from above, by way of a crane with a lifting electromagnet or other system, while the second part of the conveyor is enclosed in a tunnel for preheating, through which the hot fumes aspirated by the furnace dedusting system pass in the opposite direction. The position of the exhaust gas aspiration take-off divides the conveyor into its two main parts: the open part for the charging from above and the closed part for the transport of the exhaust gases aspirated from the furnace. In order to limit the intake of external air into the preheating portion, patent no. US 6,155,333 uses (Fig. 5) a combination of mechanical seals and a "dynamic seal", which is substantially another aspiration take-off located downstream of the conveyor charging region. The exhaust gases aspirated by this take-off, after treatment in a centrifugal dust separator, are discharged as "clean air". The operation of such sealing system has been found to be problematic and in the invention in US 7,767, 136 the "dynamic seal" has in fact been replaced by a series of mechanical seals, which are used in groups comprising up to five elements, the maintenance of which is burdensome.

It is evident that the infiltration of cold external air into the system results in a lower efficiency of recovery of heat from the exhaust gases generated by the furnace and in an increase in the gaseous volume processed by the dedusting system serving the furnace, with consequent increases in cost, maintenance and energy consumption of the system for aspirating and filtering the exhaust gases. Furthermore, the cooling caused by external air makes it difficult to reach the temperatures necessary in order to efficiently control carbon monoxide, volatile organic compounds and the dioxins which are formed inside the furnace and in the tunnel for preheating the material.

Patent no. US 6,155,333 claims the use of a conveyor constituted by a long vibrating channel for the continuous transport of the material into the furnace, a solution that poses the problem of sealing against infiltrations of external air along the two sides of the conveyor. Such problem has been addressed with longitudinal water seals, which are described in patent no. US 6,450,804 (Pos. 100 in Figs. 8, 9 and 10). Water seals are complex, because the water level must be continually restored owing to evaporation, and their maintenance is difficult.

In addition to the problem of the lateral seals, the vibrating channel conveyor is problematic owing to the fatigue breakage of the suspension elements and the generation of vibrations and noise, without taking account the extensive structural dimensioning of the entire system and of its foundations.

During some steps of the process, for example when the slag is discharged from the furnace, the furnace has to be able to rotate by a certain angle about itself, and thus the known systems mentioned above comprise a system of disengagement of the conveyor of material from the furnace, which is achieved in various ways. Such system is also used in order to facilitate maintenance. For example, in patent no. US 6,450,804 the entire system for charging and preheating is placed on trolleys with wheels, which move it back by running on rails, while in patents nos. US 6,155,333 and US 7,767,136 the inclination of the furnace is made possible by the retraction of only the end part of the preheating conveyor, which is moved back by means of a trolley that runs on rails. It is evident that all these solutions for disconnecting the system from the furnace, in addition to being expensive, pose problems of reliability owing to their complexity.

The most serious problem of the inventions mentioned above is constituted by their low heat exchange efficiency, which derives from the fact that the hot fumes extracted from the furnace pass over the bed of material transported by the vibrating channel and do not pass through it. The reduced surface of the material that comes into contact with the hot fumes and the minimal contact time do not make it possible to obtain a high recovery of energy from the hot fumes aspirated from the furnace. This consideration, which is a direct consequence of the heat exchange mechanism, is evident in the device designated with Pos. 36 of Fig. 7 of patent no. US 6,155,333, which consists of a deflector designed to direct the hot fumes toward the inner mass of the material. Assuming that this device could be used in practice, it is evident that the hot fumes can pass through only a small part of the material that is present over the entire length of the vibrating preheating channel. Furthermore, transport with the vibrating channel does not allow a high thickness of the layer of material, with the result that the contact time with the hot fumes is very limited, because the advancement speed of the material along the channel must be sustained in order to ensure the high volume flow required to ensure the productive capacity of the furnace. Attempts have been made to overcome this drawback by increasing the length of the preheating portion of the conveyor, with consequent increase of the space occupation of the system. The problem is particularly felt in revamping steelworks, when the space available is often insufficient to accommodate an excessively long system.

The recent publication "The Evolution of the Consteel EAF" by C. Giavani et al. (AIM Workshop, Milan, 29-30 March 2012), which describes the improvements made to the invention of John Vallomy, does not offer a solution to any of the above-mentioned drawbacks. This publication, in explaining the research carried out on the penetration of the flame into the material that is present on the vibrating channel, indirectly confirms the necessity of using high-penetration auxiliary burners in order to also heat the material that is not exposed to the flow of hot fumes aspirated from the furnace.

The present patent makes it possible to overcome all the disadvantages and drawbacks listed above, by providing a new system which is characterized by high efficiency of recovery of the heat of exhaust gases, an excellent seal against infiltrations of external air and the egress of pollutant exhaust gases, better control of atmospheric emissions of carbon monoxide and toxic organic compounds, limited space occupation, absence of vibrations, simplicity of construction, reliability, and low cost of construction and low management costs.

Object of the invention

The objectives of the invention are achieved according to the characteristics of claim 1 and/or of any other claim contained in this patent text.

The main aim of the present invention is to provide a system for continuous charging of the electric furnace with preheating of the charged material, which makes it possible to recover, with a high level of efficiency, the heat of the exhaust gases extracted by the dedusting system serving the furnace, while at the same time increasing the production capacity of the furnace.

The system utilizes a moving floor conveyor, which is constituted by longitudinal elements (slats) 3 with programmed cyclic movement, and is divided into two portions of variable length. The first portion of the conveyor 4, which is provided with lateral containment walls 9, is open at the top, where there is a hopper for facilitating the charging of the material from above by way of bridge cranes, or cranes with articulated arms, provided with a lifting electromagnet 10 or grabber. In the second portion of said conveyor 5, which is completely enclosed by two side walls and by a covering, the heat exchange occurs between the exhaust gases extracted from the furnace and the material that advances in the opposite direction. The end of the conveyor which is opposite to the end for charging from above interfaces with an opening provided in the side of the furnace shell 1 , and partially also in the roof thereof which can be opened. The slats of the moving floor transporter exit slightly from the preheating tunnel and discharge the material directly into the liquid bath contained in the furnace.

In the portion that comes into contact with the hot fumes aspirated from the furnace, the slats of the moving floor, the side walls and the covering of the conveyor all have a heat-resistant construction; for example they are water- or air-cooled, or they are made of steel that is resistant to high temperatures. In particular, the covering of the conveyor, which does not come into contact with the material and thus is not subject to wear, can conveniently be made of refractory material so as to limit the losses of heat through it.

The operation of the moving floor conveyor is known and takes place in four steps. The slats of the floor are moved longitudinally by electric, pneumatic or hydraulic actuators which control three, four or more groups of slats which are mutually integral. If there are three groups, then slat no. 1, slat no. 4, slat no. 7 and so on are integral. If there are four groups, then slat no. 1, slat no. 5, slat no. 9 and so on are integral. Alternatively, an actuator can be provided for each slat, without affecting the synchronized movement of a whole group of slats which are practically integral as indicated above. Moving one third, or one fourth, of the surface of the floor does not longitudinally move the material deposited on it, which remains still owing to the friction with the remaining two thirds, or three quarters, of the slats. The groups of slats are moved in sequence in the same direction by approximately 30-50 cm, according to the times determined by the control system of the floor. During the steps of moving the individual groups of slats, part of the material falls into the furnace through the openings that are created under the material when the slats move in the direction opposite to the one for loading the furnace. When the slats are all aligned again, they are moved simultaneously toward the furnace and a new advancement cycle, conducted in four or five steps, can begin.

The slats of the moving floor are supported by a set of transversal supporting stands 14, longitudinally spaced, on which the slats slide in contact with lubricated surfaces or surfaces with a low coefficient of friction. The slats are arranged in mutual contact, so as to provide a horizontal surface that is hermetically sealed against gases. Downstream of the region for charging material from above 4, positioned between two successive supports below the moving floor, there are sealed chambers which are provided with a heat-resistant construction (walls cooled by water or by air, or walls made of materials resistant to high temperatures). The lower chambers are placed in mutual communication through openings 15, so as to form a single plenum space for intake, which is connected by way of one or more pipes to the dedusting system of the furnace. In this way the chambers under the moving floor are kept at a negative pressure, so as to make the hot fumes aspirated from the furnace flow therein, which, after having passed through the material resting on the moving floor, pass through two longitudinal slots 8 which are arranged at the sides of said moving floor. The pressure difference, caused by the passage of the exhaust gases through the two slots communicating between the conveyor and the underlying plenum space, makes it possible to uniformly distribute the flow of exhaust gases along the entire length of the preheating portion of the moving floor.

The two slats that are found on the outer sides of the moving floor can be formed with two protrusions (Fig. 3), one of which slides inside a groove provided in the transversal support of the slats, so as to prevent their lateral movement. The upper protrusion, on the other hand, serves to prevent the entry of material into the underlying chamber.

As illustrated in Fig. 10, the longitudinal slots are absent in the portion for charging the conveyor from above 4, in which the end slats essentially slide in contact with the walls of the loading hopper 9.

One of the advantages of the system described above is the fact that the fumes pass through the entire thickness of the material and thus the surface of material affected by the heat exchange is much larger than that of the vibrating channel conveyor utilized in conventional systems. The heat exchange efficiency is thus higher, both owing to the fact that the hot fumes come into contact with a much larger surface of material, and because the time available for the heat exchange between the exhaust gases and the material is longer, thanks to the lower average speed of advancement of the material, which can be loaded onto the moving floor conveyor in a layer of greater thickness than is permitted by vibrating channel transporters.

The lower chambers 7 are provided with sealed access doors so as to allow their regular cleaning.

The lower chambers can be used as post-combustion chambers for the abatement of the pollutant compounds that are drawn from the furnace and caused by the heating of the material. In this case, there can be a controlled inflow of a certain amount of combustion air into the chambers. Otherwise the post-combustion can occur in an adapted chamber located downstream of the preheating system.

The problem of the precarious lateral seals, typical of the system with vibrating channel conveyor, is solved by the floor with moving slats, which does not require any special longitudinal seal. If the system is utilized only as a system for continuous charging of the furnace, without the specific goal of preheating the material, then the lower chambers and the associated longitudinal connection slots are absent and the longitudinal seal is ensured by two simple strip seals which are arranged between the side walls of the conveyor and the two outer slats of the floor. When the conveyor needs to be disengaged from the furnace so as to allow its inclination, as required by production requirements, all the slats of the floor are retracted by a certain distance (Figures 6 and 7). This occurs by the provision of three different stroke limit points for the slats: the first is the point furthest forward toward the furnace, the second corresponds to the operating position of the moving floor and is placed at approximately 30-50 cm from the preceding point (the slats move alternately between these two points), while the third stroke limit point, which is the furthest, is utilized only when the conveyor needs to be uncoupled from the furnace. In this last case, the heating tunnel also needs to be decoupled from the furnace and this occurs by way of a movable sleeve (Figures 6 and 7), or by the lifting and simultaneous retraction of the end part of the tunnel (Figures 4 and 5). The decoupling of the tunnel from the furnace occurs by way of electrical or hydraulic actuators, which are different from the ones that actuate the moving floor. For the retraction of the moving floor, hydraulic or electrical actuators can be provided with three stroke limit positions, two operating and one for decoupling from the furnace, or independent actuators can be used which operate in series with respect to the actuators used for the advancement of the material.

A seal system is provided for preventing the infiltration of external air at the opening for the entry of the material into the heating tunnel. The seal is provided by a group of laterally adjacent elements 19, individually counter-weighted, which can rotate upward by a certain angle when they are pushed by the material during the advancement step of the programmed transport cycle (Figures 8 and 9). The seal system described reduces to the minimum the free area between the material and the upper part of the heating tunnel and is capable of also compensating for an uneven distribution of the material in the transverse direction of the conveyor (Fig. 10). For an even more effective seal against the infiltration of external air, the system described above can be provided with air curtains 22 integrated in the moving devices that ensure the seal. In this manner, the operating distance of the air curtains is reduced, with respect to a system with a fixed air curtain, and thus the air flow necessary in order to achieve a good seal is appreciably lower, with consequent lower energy consumption of the fan serving the air curtain system.

The mobile air curtains, one for each seal element, are supplied by a conventional fan blower, whereas the distribution of the air through the various elements is achieved by coupling the hollow rotation axle 23 thereof with a duct that is pressurized by way of a fan. The seal between the hollow axle and the fixed duct is achieved with an axial seal system. Along the hollow rotation axle there is a series of openings, one for each seal element, which connect the inside of the axle with the various seal elements. In this manner, the air curtains are active in all the positions of the operating angular sector, without the necessity of resorting to flexible pipes in order to supply the air under pressure.

Brief description of the drawings

These and other characteristics of the present invention will become more apparent from the following description of a preferred, but not limiting, embodiment, and from the seven tables of accompanying drawings, wherein:

- Figure 1 is a plan view of the electric furnace with the system for continuous charging and preheating of the charge;

- Figure 2 is a plan view of the operation of the moving floor with slats moving alternately;

- Figure 3 is a cross-sectional view of the moving floor conveyor at a line crossing a lower chamber and the aspiration take-off of the exhaust gases;

- Figure 4 is a longitudinal sectional view of the electric furnace with the system for continuous charging and preheating of the charge;

- Figure 5 shows the same sectional view as Figure 4 with the system uncoupled from the electric furnace following the lifting of the end element of the preheating chamber;

- Figure 6 is a partial longitudinal sectional view of the system for decoupling the preheating tunnel from the furnace by way of a movable sleeve;

- Figure 7 is a partial longitudinal sectional view of the system for decoupling the preheating tunnel from the furnace by withdrawing the end section of the tunnel;

- Figure 8 is a longitudinal sectional view of an enlargement of the seal system placed at the entry to the preheating tunnel (seal system raised);

- Figure 9 is a longitudinal sectional view of an enlargement of the seal system placed at the entry to the preheating tunnel (seal system in the operating position);

- Figure 10 is a transverse sectional view of the seal device placed at the entry to the preheating tunnel.

As can be seen from the accompanying figures, this is a new system for continuous charging and preheating of the material for an electric furnace 1, which is characterized substantially in that it is formed by a moving floor conveyor 2, with slats 3 moving alternately. The conveyor is divided into two parts, an open part for charging the material from above 4 and a closed part for preheating (tunnel) 5. The two parts of the conveyor are separated by a system 6 for sealing against the infiltrations of external air and by a series of lower chambers (plenum space) 7 which are connected to the preheating zone of the material through two lateral longitudinal slots 8. The plenum intake spaces are connected to the system for aspirating and purifying the exhaust gases of the electric furnace, which is not shown in the drawings.

The system utilizes a moving floor conveyor 2, which is constituted by longitudinal elements (slats) with programmed alternating movement 3, and is divided into two portions of variable length. The first portion of the conveyor, which is provided with lateral containment walls, is open at the top, where there is a hopper 9 for facilitating the charging of the material from above by way of a bridge crane, or a crane with articulated arms, provided with a lifting electromagnet 10 or grabber. In the second portion of said conveyor, which is completely enclosed by two side walls and by a covering, the heat exchange occurs between the exhaust gases extracted from the furnace and the material that proceeds in the opposite direction 11. The end of the conveyor which is opposite to the end for charging from above interfaces with an opening 12 provided in the side of the furnace shell, and partially also in the roof thereof which can be opened. The slats 3 of the moving floor transporter exit slightly from the preheating tunnel and discharge the material directly into the furnace.

In the portion that comes into contact with the hot fumes aspirated from the furnace, the slats of the moving floor, the side walls and the covering of the conveyor all have a heat-resistant construction; for example they are water- or air-cooled, or they are made of a steel that is resistant to high temperatures. In particular, the covering 13 of the conveyor, which does not come into contact with the material and thus is not subject to wear, can conveniently be made with refractory material so as to limit the losses of heat through it.

The operation of the moving floor conveyor is known and takes place in four steps, as shown in Figure 2. The slats of the floor are moved longitudinally by electric, pneumatic or hydraulic actuators which control three, four or more groups of slats which are mutually integral. If there are three groups, then slat no. 1, slat no. 4, slat no. 7 and so on are integral; this solution is shown in Figure 2. If there are four groups, then slat no. 1, slat no. 5, slat no. 9 and so on are integral. Alternatively, an actuator can be provided for each slat, without affecting the synchronized movement of a whole group of slats which are practically integral as indicated above. Moving one third, or one fourth, of the surface of the floor does not advance the material deposited on it, which remains still owing to the friction with the remaining two thirds, or three quarters, of the slats. The various groups of slats are moved in sequence in the same direction by approximately 30-50 cm, depending on the times determined by the automatic actuation system of the floor. During the steps of moving the individual groups of slats (steps 1 , 2 and 3 in Fig. 2), the material that is on the end part of the floor falls, by gravity, into the furnace owing to the free spaces that are formed under the material when the slats move in the direction opposite to the direction of loading into the furnace. When the slats are all aligned again (position D), they are moved simultaneously toward the furnace (step 4: from position D to position A) and a new advancement cycle, conducted in four or five steps depending on the number of groups of mutually integral slats, can begin. The moving floor described above is already widely utilized for moving goods transported by semi-trailer trucks (both for loading and for unloading); the high reliability of the system has resulted in its widely distributed use also for the transport of materials that are particularly difficult to move with other types of transporters, such as for example waste and scrap metal. The movement volume capacity, in addition to depending on the dimensions of the system, depends also on the thickness of the layer of material and on the average speed of advancement, which is determined by the stroke of travel of the slats of the floor and by the times of the cycle for actuating them.

As illustrated in Figure 4, the slats 3 of the moving floor are supported by a set of transversal supporting stands 14, longitudinally spaced, on which the slats slide in contact with lubricated surfaces or surfaces with a low coefficient of friction. The slats are arranged in mutual contact, so as to provide a horizontal surface that is substantially hermetically sealed against gases. When the material loaded in the furnace does not entail the risk of blockage, as in the case of loading with pellets of pre-reduced material, the slats can be mutually spaced in order to allow the passage of the exhaust gases coming from the furnace through the longitudinal slots which are created between the slats. Below the moving floor, positioned between two successive supports, there are one or more sealed chambers 7, which are provided with a heat-resistant construction (walls cooled by water or by air, or walls made of materials resistant to high temperatures). The lower chambers are placed in mutual communication through openings 15, so as to form a single plenum space for intake, which is connected by way of one or more intake pipes 16 to ducting that takes the aspirated fumes to the dedusting system of the furnace. In this way the chambers under the moving floor are kept at a negative pressure, so as to make the hot fumes aspirated from the furnace flow therein, which, after having passed through the material resting on the moving floor, pass through two longitudinal slots 8 which are arranged at the sides of said moving floor. The decrease in pressure, owing to the passage of the exhaust gases through the two slots communicating between the conveyor and the underlying plenum space, makes it possible to uniformly distribute the flow of exhaust gases over the entire length of the preheating portion of the moving floor.

One of the advantages of the system described above is the fact that the fumes pass through the entire thickness of the material and thus the surface of material affected by the heat exchange is much larger than that of the vibrating channel conveyor utilized in conventional systems. The heat exchange efficiency is thus higher, both owing to the fact that the hot fumes come into contact with a much larger surface of material, and because the time available for the heat exchange between the exhaust gases and the material is longer, due to the lower average speed of advancement of the material, which can be loaded onto the moving floor conveyor in a layer of greater thickness than is permitted by vibrating channel transporters.

The lower chambers can be used as post-combustion chambers for the abatement of the pollutant compounds that are drawn from the furnace and produced by the heating of the material. In this case, there can be a controlled inflow of a certain amount of combustion air into the chambers. Otherwise the post-combustion can occur in an adapted chamber located downstream of the preheating system.

The problem of the lateral seals, typical of the system with a vibrating channel conveyor, is solved by the floor with moving slats, which does not necessitate any special longitudinal seal. If the system is utilized only as a system for continuous charging of the furnace, without the specific goal of preheating the material, then the lower chambers and the associated longitudinal connection slots are absent and the longitudinal seal is ensured by two simple contact seals which are arranged between the side walls of the conveyor and the two outer slats of the floor.

When the moving floor conveyor needs to be disengaged from the furnace to allow its inclination, all the slats of the floor are retracted by a certain distance. This occurs by the provision of three different stroke limit points for the slats: the first is the point furthest forward toward the furnace, the second corresponds to the operating position of the moving floor and is placed at approximately 30-50 cm from the preceding point (under normal conditions of charging the furnace the slats move alternately between these two points), whereas the third stroke limit point, the furthest, is utilized only when the conveyor needs to be uncoupled from the furnace. In this last case, the side walls and the cover of the preheating tunnel also need to be decoupled from the furnace and this occurs by way of a telescopic sleeve 17 (Figures 6 and 7), or by the lifting and simultaneous retraction of the end part 18 of the tunnel (Figures 4 and 5).

The decoupling of the end element of the preheating tunnel from the furnace occurs by way of electrical or hydraulic actuators, which are different from the ones that actuate the moving floor. For the retraction of the moving floor, hydraulic, pneumatic or electrical actuators can be provided with three stroke limit positions, two operating and one for decoupling from the furnace, or independent actuators can be used which operate in series with respect to the actuators used for the advancement of the material.

As a result of the operation of the fans of the system for purifying the exhaust gases produced by the furnace, the preheating tunnel is under slight negative pressure with respect to the atmospheric pressure and thus a seal system 6 is provided in order to prevent the infiltration of external air at the opening for the entry of the material into the preheating tunnel. The seal system is illustrated in detail in Figures 8, 9 and 10.

The seal is provided by a group of laterally adjacent elements, individually counter-weighted, which can rotate upward by a certain angle when they are pushed by the material during the advancement step of the programmed transport cycle (step 4 of Fig. 2). This seal system reduces to the minimum the surface of the free cross section between the material and the upper part of the heating tunnel and, as shown in Figure 10, is capable of also compensating for an uneven distribution of the material in the transverse direction of the conveyor. Thanks to the moving floor, the resistance that the seal elements offer to the material that advances inside the tunnel is negligible and thus cannot obstruct the advancement of the material, as occurs with a vibration conveyor.

The counterweight 20 is connected to each oscillating seal element by way of a steel cable and a system of pulleys 21. Instead of the gravity counterweights, it is also possible to utilize electronic systems for compensating the weight, which cancel out the weight of the seal elements 19 by exerting a force on the suspension cable which is slightly smaller than the one deriving from the weight of the element.

For an even more effective seal against the infiltration of external air at the point of entry of the material into the preheating tunnel, the system described above can be provided with air curtains 22 integrated in the moving seal devices 19. With the solution proposed by the present invention, the operating distance of the air curtains is reduced, with respect to a system with a fixed air curtain, and thus the air flow necessary in order to achieve a good seal is appreciably lower, with consequent lower energy consumption of the fan serving the air curtain system.

The mobile air curtains, one for each seal element, are supplied by a conventional fan, whereas the distribution of the air through the various seal elements 19 is achieved by coupling the hollow rotation axle 23 thereof with a pressurized duct connected to the fan. Along the hollow rotation axle there is a series of openings, one for each seal element, which connect the inside of the axle with the various seal elements. In this manner, the air curtains are active in all the positions of the operating angular sector.

As an alternative to the air distribution system described above, it is possible to use flexible pipes that connect the various (mobile) seal elements to a (fixed) duct that is connected to the delivery port of the fan that generates the air flow required for the operation of the curtain.

The seal system described above effectively reduces infiltrations of external air into the system and thus makes it possible to reduce the flow of exhaust gases treated by the exhaust gas purification system, thus limiting the consumption of the exhaust gas aspiration fans.

The passage of the exhaust gases through the material involves a certain loss of load for the circuit of exhaust gases, which must be taken into account in the calculation of the pressure of the fans of the exhaust gas purification system. If the nature of the material with which the furnace is charged should bring about a loss of pressure which is higher than desired, it is possible to transfer part of the exhaust gases aspirated from the furnace directly to the exhaust gas purification system by way of a by-pass duct 24, the opening of which can be automatically adjusted by operating on the gate 25 arranged on the by-pass duct. The opening of the by-pass gate can be automatically adjusted as a function of the value set for the partial vacuum inside the furnace. In this manner the egress of exhaust gases from the furnace is prevented when the material is not sufficiently permeable to the exhaust gases. The by-pass system described above can also be utilized in order to prevent the localized melting of the material in the preheating tunnel owing to an excessive heating temperature. When the by-pass gate is completely open, the flow of exhaust gases that pass through the material is practically nil and from this point of view the system behaves like conventional systems.

It is also possible to have another gate at the portion of ducting 26, upstream of the point of convergence between the intake ducting from the plenum space and the by-pass ducting (Fig. 3). This second gate makes it possible to exclude the entry of exhaust gases into the plenum space 7 through the longitudinal slots.

The invention, naturally, is not limited to the embodiment described above, based on which there can be other forms and other embodiments, and the execution details may in any case vary without for this reason departing from the essence of the invention as stated and as claimed hereinbelow.

In particular, the system according to the invention can also be utilized almost without modifications or with modifications that are evident to the person skilled in the art in sectors other than steel working; in all cases in which hot fumes are aspirated directly from the melting furnace and material is loaded to be preheated with the heat of the aspirated exhaust gases (such as for example the production of alloys of aluminum, magnesium, melting down scrap glass etc.).

In a particularly advantageous manner, the slats are cooled by way of forced circulation of a cooling fluid (water, air or other fluid) which circulates in an internal cavity that is present in each slat. Thus, flexible conduits connected to each of the slats are present, which allow the circulation of the cooling fluid in the interior of the moving slats. In this manner the invention envisages a floor of moving slats with forced cooling thereof and with the transition of gases downward from above.