FR2512189A1 - Preheating scrap charge using exhaust gas - from steel-making plant which is passed through preheating units in series - Google Patents

Abstract

Several preheating units are used for preheating a charge for steel making equipment with heat extracted from exhaust gas from the equipment. A supply duct connects an exhaust discharge duct of the equipment to a gas inlet portion of each unit. Connecting ducts each connect the gas outlet portion of each unit to the gas inlet portion of a downstream unit so that the exhaust gas passes through the units in series flow. Shutoff valves are provided in the supply duct, connecting ducts and discharge ducts to control the flow of gas through the units. Approx. 70% of the heat of the exhaust gas is utilised. Air pollution is reduced.

Description

 The present invention relates to an apparatus for preheating the charge to be introduced into a steel manufacturing unit.

 In the manufacture of steel, it is advisable to heat the charge of constituents, such as hedgerow, alloyed iron, directly reduced iron and lime, before introducing it, for example, in electric ovens.

 This preheating is intended to prevent, for safety purposes, accidents due to sudden vapor releases during the introduction of the charge of constituents into the furnace, as well as to improve the steelmaking yield by promoting the merger of the charge.

 Most often, the load is heated by auxiliary burners, but these do not promote energy savings. Consequently, recourse has been had to apparatuses capable of recovering the heat of the exhaust gases from electric ovens, interposed between the oven and a dust sensor forcing the gases out of the oven by force. However, these devices have the drawbacks listed below.

 Mainly, their heat recovery efficiency does not exceed 20 to 30%, and they fail to fully preheat the load, which prolongs the duration of the steel making operation. On the other hand, if an attempt is made to improve the recovery efficiency, the dimensions and the complexity of the preheating apparatus are increased and therefore the cost. As a result, the device does not provide the desired energy savings. In addition, if the preheating device is large, it is necessary to extend the duration of the cycle for bringing the load to the device and for extracting the load from the latter, which counteracts the tendency which has arisen in steel foundry to reduce the melting time, and is therefore troublesome.

 In addition, the current preheater, which must satisfy the imperative of reducing the duration of the cycle, is nevertheless difficult to control, while leaving much to be desired as regards operating safety.

 Another problem is the current preheater. Due to the low heat recovery efficiency, the gases released from the device into the atmosphere are at a relatively high temperature (for example 1500C) and therefore contain a large amount of oil mist etc. When it is put into the atmosphere with exhaust gases, this fog generates white fumes causing atmospheric pollution.

The subject of the present invention is
- to use efficiently (for example around 70%) the heat of the burnt gases leaving a steel manufacturing equipment to ensure full preheating of the charge introduced into the equipment in order to reduce the time necessary for the load melting, that is to say steel manufacturing;
- to propose a device for preheating the load which has a simple and compact structure and a moderate cost price;
- to allow to operate the introduction of the load in the preheating device and its extraction out of this device with ease and in a reach time while letting the device ensure the full preheating of the load to an extent compliance with the melting time;
- to propose a preheating device which makes it possible to rid the exhaust gases of polluting products and to break them down in order to limit pollution as much as possible.

We will now describe by way of examples certain embodiments of this invention with reference to the appended drawings in which
Figure 1 is an overall perspective view of an apparatus according to an embodiment of the invention;
Figure 2 shows from the front and partially in section the same device seen along line II-II of Figure 1;
Figure 3 is a plan view of the part of the apparatus shown in Figure 2;
Figure 4 is a side elevational view of the apparatus portion shown in Figure 2;
Figure 5 is an enlarged view, partially in torque, of a preheating unit;
Figure 6 is a front view of a modified transfer means ;;
FIG. 7 is a front view of this transfer means, of which it illustrates the operation, and
Figure 8 is an overall perspective view illustrating a variant of the invention.

 A first embodiment will be described with reference to FIGS. 1 to 5.

 FIG. 1 represents a pre-heating device 1 provided between a steel manufacturing equipment 2, such as an electric oven, and a dust-collecting chamber 3 intended to collect the dust present in the burnt gases leaving the equipment 2. The device 1 is connected to the unit 2 and to the dust-collecting chamber 3 by a pipe. The exhaust gases flow through this pipe under the effect of an exhaust fan 4 interposed between the device 1 and the sensor chamber =: dust 3.

 The charge to be introduced into the unit 2, formed for example of hedgerow, alloyed iron, directly reduced iron and lime, is preheated by the apparatus I with the heat of the burnt gases leaving the equipment 2. The apparatus comprises several units preheating 5, two in number in the example illustrated, intended to temporarily receive the load.

 A burnt gas discharge section 6 of the equipment 2 is connected to gas inlet sections 7 of the preheating units 5 by branched supply conduits 8.

In the vicinity of the equipment 2, the supply conduits 8 are connected to a vertical gas distribution tower 9. Between the exhaust section 6 and the intake sections 7 of the various preheating units 5, each supply duct supply 8 is provided with a first shut-off valve 10, of the butterfly type, controlled electrically or by a jack.

 Exhaust ducts 11, ensuring the evacuation of the burnt gases having passed through the preheating units 5, connect the gas outlet sections of the units 5 to an exhaust fan 4, with the interposition of a vertical confluence tower of gas 13. Each of the evacuation conduits It is provided with a third stop valve 14 of the same type as the first stop valve 10.

 In order for the burnt gases to pass successively, in series, the preheating units 5 by flowing from the supply duct 8 into the exhaust duct 11, a second gas outlet section 12a of each unit 5 is connected to the gas inlet section 7 of the other unit 5 by an individual connection duct 15. In the example illustrated, the connection duct 15 extends between the two preheating units 5 and is provided in intermediate seconds positions stop valves 16 of the same type as the first stop valve 10. The second outlet section 12a communicates with the outlet section 12 inside the preheating unit 5.

 We will now describe in more detail the connecting conduits 15. The second outlet section 12a of each preheating unit 5 is connected by a segment of connecting conduit 15a to the supply conduit 8 at an intermediate diaphragm point between the inlet gas 7 from the unit concerned and the first stop valve 10. Each duct segment I5a has first and second outlet valves 16a, 16b in series. The sections of the pipe segments 15a situated between the first and second outlet valves 16a, 16b respectively are connected to each other by a common pipe 15b.

The first outlet valve 16a is closer than the second of the unit 5. Of the two preheating units 5, the one located on the left in FIG. 1 is called A and the other, located on the right, is called B. L one of the conduits 15 connecting the units 5 comprises the segment of conduit 15a connected to the second outlet section 12a of the unit A comprising the first outlet valve 16a, the common conduit 15b, and the segment segment of conduit 15a-connected to the supply conduit 8 of unit B and comprising the second outlet valve; ib. The first and second outlet valves 16a, i6b mounted on this conduit 15 constitute the second stop valves 16 of this conduit. Similarly, the other connecting conduit 15 extends between the second ga-z outlet section I2a of unit B and the supply conduit 8 of unit A.

 The valves 10, 14 and 16 are open or closed so that the burnt gases pass successively, in series, the preheating units 5 in a selectively determined order, for example under the control of an electrical program. The mode of gas flow will be described with reference to FIG. 1. When the first stop valve 10 and the first outlet valve 16a of the unit A, as well as the second outlet valve 16b and the third stop valve 14 are all open, while the other valves are closed, the burnt gases arriving from the steel manufacturing equipment 2 pass through the supply conduit 8 in the unit A, pass through the load present in it, then enter the unit B through the connecting duct 15, pass through the interior of the unit B, take the exhaust duct 11, and are attracted by the fan 4 in the sensing chamber -dust 3 where they are freed of dust (see arrows in dashed lines). These gases are then put into the atmosphere. If the stop valves 10, 14 and 16 occupy open or reverse closure positions, the burnt gases pass through unit B in series, then unit A.

While the gases are flowing through the apparatus, fractions of the dust which they contain are collected by inertia in the distribution tower 9 and in the confluence tower 13, the deposited dust being extracted at will from the towers 9 and 13.

 The present embodiment also includes a conduit 18 ensuring a direct connection between the gas distribution tower 9 and the gas confluence tower 13 through a fourth shut-off valve 17, usually closed. When the preheating device 1 is not in service, the fourth stop valve 17 is opened and the first stop valves 10 are closed to evacuate the burnt gases through the fan 4 and the collecting chamber 3 .

 The conduits are advantageously provided with a lining: refractory and / or water cooling means.

Although the present embodiment comprises only two preheating units 5, these units can be three in number or more. Whatever the arrangement, the preheating units 5 are connected to each other in series by the supply conduits 8, the connection conduits 15 and the discharge conduits 11 and the shut-off valves 10 are provided. , 14 and 116 which are opened or closed as prescribed so that the burnt gases successively pass through, in series, the units 5 in a selectively determined order.

 Referring to Figures 2 to 5, it can be seen that each preheating unit 5 comprises a fixed container 20 formed by a vertical hollow cylinder open at the top and mounted in a fixed position on a floor 19; a movable container 24, intended to receive a load to be preheated, having a load inlet 21 at the top and an open bottom 23 provided with an opening bottom cover 22, capable of taking and leaving the position of vertical movement insertion into the fixed container 20; and an upper cover 25 movable horizontally to open or close the inlet 21 of the movable container 24.

 As best seen in Figure 5, the movable container 24 has a hollow cylindrical body 24a with a vertical axis, intended to contain the load. On the outer periphery of the body 24a projects an annular cover 24b intended to close the annular opening formed between the containers 20 and 24 when the movable container 24 is placed in the fixed container 20. The bottom cover 22 is in the form of a cup circular divided into two segments 22a each of which has, in plan view, a semicircular foundation. The load is reviewed in the body 24a and is retained there when the bottom cover 22 is in the closed position. When the bottom cover assumes the open position indicated in broken lines in FIG. 5, the load is poured. On the body 24a is articulated a handle 27 on which a crane 26 can engage to move the body 24a provided with the bottom cover 22. The top cover 25 is in the form of a hollow truncated cone and its large lower base is removably fixed to the body 24a with the interposition of a tight seal.

 The supply duct 8 comprises a fixed duct segment 8b, which starts from the equipment 2 and has an end 8a with a horizontal axis, and a movable duct segment 8c which can be coupled to the fixed segment 8b. The movable segment 8c has a downwardly directed end which is mounted in the small upper base of the top cover 25, constituting the gas inlet section 7. The top cover key 25 is connected with the possibility of vertical sliding to the segment mobile 8c, with interposition of a waterproof seal. A tubular connector 28 is placed around one of two extracts: end 8a of the fixed duct segment 8b or horizontal end of the movable segment 8c and slides axially on the interested end under the effect of jacks 29. moved to slide, the connector fits on the other of said ends to couple the two duct segments 8b and 8c to each other.

 At the lower part of the fixed container 20 are connected at points diametrically, opposite the connection and discharge pipes 15. As indicated by arrows in solid line in FIG. 2, the burnt gases flow from the supply conduit 8 into the casing 24a through the somnet cover 25, pass through the load (not shown in tee} contained in the body 24a , flow through the household gap between the body 24a and the bottom cover 22 and are evacuated by the connection conduit 12 or the evacuation conduit 11.

 A transfer means 30 comprises two horizontal rails 31 which run along opposite sides of the two neighboring preheating units 5, interposed between them and on which a carriage 32 of the self-propelling type is movable. The carriage 32 has wheels 33 rolling on the rails 31 and an electric motor 34 for driving the wheels 33.

 The movable duct segment 8c is fixed to the carriage 32 and therefore movable horizontally along the rails 31.

The top cover 25 can be moved vertically by a lifting unit 35 mounted on the carriage 32. Thus, the movable duct segment 8c and the top cover 25 can be brought by the transfer means 30 into a position where they allow the movable container 24 to move vertically to penetrate into the fixed container 20 and out. In addition, the carriages 32 individually assigned to the units 5 can alternately take, along the rails 31, an intermediate position between the units 5 (as indicated in dashed lines in Figures 2 and 43).

 In the example illustrated, the connector 28 is placed on the movable duct segment 8c and the jacks 29 ensuring its movement are mounted on a frame carried by the carriage 32.

The lifting unit 35 mainly comprises an electric motor 36 mounted on the carriage 32, a winch drum 37 which the motor 36 rotates via a chain, and chains or cables 38 wound on the drum 37 and connected to the top cover 25.

 The movable container 24, when it is removed from the fixed container 20, is uncoupled from the supply conduit 8, the connection conduit 15 and the evacuation conduit On the other hand, when it is placed in the fixed container 20 and that the top cover 25 is fixed on it, the mobile container 24 is placed in communication with the conduits 8, 11 and 15. The components of the two units 5 are identical in shape and in size. Thus, the containers 24 of the two units are interchangeable.

 The preheater having the structure described above operates in the following manner.

 A load, for example of bocage, is deposited by a tilting wagon or a magnetic crane in the mobile container 24, extracted by the crane 26 from the fixed container 20. The mobile container 24 is then placed in the fixed container 20. The carriage 32 , preceding horizontally offset from the unit 5, returns above the movable container 2e the top cover 25 descends, under the effect of the lifting unit .35, close the inlet 21, and the conduit supply 8 is connected to the container 24. Simultaneously, the connector 28 is to slide, then placed on the con duitfixe segment 8b to couple with the latter the movable duct segment 8c.

 In this way, the mobile containers 24, each of which contains a load, are placed in the corresponding fixed containers 20, and the conduits 8, 11 and 15 are put into communication with the mobile containers 24. Then the first, second and third shut-off valves 10, 14 and 16 are actuated so that the burnt gases leaving the equipment-2 pass in series through the units A then B and are evacuated from the dust-collecting chamber 3 as shown in the figure 1. In this case, unit A is preheated with hotter gases than those which heat unit B.

When the preheating of the load located in unit A is finished, the top cover 25 of unit A is moved away from it at the same time as the movable duct segment 8c by the transfer means 30, the container mobile 24 is brought above the equipment 2, and the bottom cover 22 is opened to pour the preheated load into the equipment 2. Meanwhile, another mobile container 24 containing a load is introduced into the container 20 of unit A in communication with conduits 8, 11 and 15.

The first, second and third stop valves 10, 14 and 16 are then operated so that the burnt gases pass in series first through unit B, then unit
A, the order thus being inverse to the previous one.

 In this case, the preheating, already started, of the load present in unit B can be completed in a short time.

In addition, the sensible heat of the burnt gases passing through unit B is fully transferred to unit A, which ensures efficient recovery of the heat from the burnt gases. Given that it is possible to operate simultaneously the introduction of the mobile container 24, filled with a new charge to be preheated, into the fixed container 20 and the discharge of the preheated charge, from the preceding mobile container 24, into steel manufacturing equipment 2, the process idle times are shortened so that the warm-up time is sufficient and the operation efficiency is high.

 While the burnt gases are flowing, their temperature decreases, for example up to around 1000C, while the load located in the downstream preheating unit 5 is at low temperature, so that at this level the temperature gases fall below the dew point, allowing oil mist and other aqueous components of the gases to condense on the load surface at low temperatures; therefore, impurities, such as oil mist, which can cause secondary pollution are extracted from the gases. During the preheating of the load in the next stage, the oil mist and other pollutants thus collected are decomposed by oxidation under the effect of the gases burned at a high temperature, for example around 7500C. This results in an overall reduction in the release of polluting bodies to the atmosphere.

 Figures 6 and 7 show a modified transfer means 30. A pillar 39 extending from the floor 19 is provided at the top with an articulated member 41 which can pivot around a vertical axis under the effect of a set of gears 40.

As in the previous embodiment, the articulated member 41 is integral with the movable duct segment 8c and comprises a lifting unit 35 intended to vertically move the crown cover 25. The movable duct segment 8c and the crown cover 25 can be bring, along a path in an arc, the fixed segment 8b and the movable container 24, be respectively coupled to the latter (Figure 6) and deviate from it along said path (Figure 7).

 FIG. 8 illustrates a variant comprising preheating units 5 each having a single gas outlet section 12, and connection conduits 15 each of which connects a discharge conduit 11, going from the gas outlet section 12 of each unit 5 to a third stop valve 14, with a supply pipe 8 which goes from the gas inlet section 7 of the other unit 5 to a first stop valve 10. The connection pipe 15 comprises a second stop valve 16. The discharge conduits 11 are directly connected to a lower part of the gas distribution tower 9 by means of a fourth stop valve 17, usually closed. By operating the stop valves 10, 14 and 16, the burnt gases are sent through the preheating units 5 in series, for example according to the arrows in dashed lines shown in FIG. 8. Subject to the arrangement which has just described, the variant is substantially identical to the previous embodiment by its structure and its operation.

 The variant according to FIG. 8 implies, during preheating, the following temperature and gas flow distributions, indicated by way of example. Unit A is used upstream of unit B.

Measuring location Temperature Gas flow
(C) (Nm / min)
Gas distribution tower 800 900
Supply duct 750 1000
Gas outlet section
from unit A 350
Gas inlet section
of unit B 300 1200
Gas outlet section
of unit B 100 1400
The flow gradually increases downstream due to the penetration of air into the pipeline from the outside.

 The present invention described above offers the advantages set out below compared to the prior art.

A comparison was made between a PA installation comprising a preheating unit according to the prior art and an IN installation comprising two preheating units connected in series according to the present invention
I Efficiency 1) Sensitive heat of the burnt gases sent to the PA preheating unit: qi = 1000 Nm = min x 750 C x 0.34 kcal / Nm3.0C x 50
860 x 100 "ton" of load
= 148 kWh / "ton # * of load
IN: qi = 1000 x 750 x 0.34 x 50
860 x 100
= 148 kWh / "ton * of load The word" ton "designates here, and in the rest of this description, British tons of 1016 kg
907 kg. J 2) Sensible heat of the burnt gases leaving the preheating unit
PA: q0 = 1200 x 350 x 0.34 x 50
9o = 860 x 100 = 83 kWh / "tonX of load
IN q0 = 1400 x 100 x 0.34 x 50
860 x 100 28 28 kWh / "load tonne
Difference [(1) - (2), i.e. heat absorbed per charge:
PA: qi - qO = 148 -83 = 65 kWh / "load tonne
IN :: qi - qO = 148 - 28 = 120 kWh / "ton" of load 3) Reduction of energy consumption (# E) (Thermal efficiency of the installation S / P #H = 80%
PA = # E1 = 65 x 0.8 = 52 kWh / "ton" of load = 58 kWh /
"ton" of ingots IN = # E2 = 12 x0.8 - 96 kWh / "ton" of charge -. 107 kWh /
"ton" of ingots # E2 - # E1 = 49 kWh / "ton of ingots (new reduction-
energy consumption).

4) Pressure loss (AP) in the unit or both preheating units
PA: tP k 2.7 kPa IN: # P2 = 5.4 kPa 5) Power of the supercharging fan motor provided to compensate for the pressure drop suffered in the preheating unit or units PA: LFl 1000 x 250 = 226 kW
6120 x 0.5
IN: 1600 x 1.73 x500 = 452 kW
LF2 = 6120 x 0.5 LF2 - LF1 6 226 kW 6) Increase in energy consumption due to the motor
PA: 2.1 kWh / "ton" of ingots
IN: 4.2 kWh / "ton" of ingots
II. Overall benefits 1) Reduced energy consumption
PA: 58 kWh / "ton" of ingots (for the implementation of good quality hedging. For usual hedging, the gas supply is limited to avoid secondary pollution. The reduction in consumption then obtained is around 40 KWhX "ton" - ingots.

V15 / kWh x 40 kWh = V600 / "ton" - of ingots
IN: approximately 100 kWh / "ton" of ingot (variable depending on the operating conditions of the furnace)
Y15 / kWh x 100 kWh = 11500 / "ton" of ingot 2) Reduction of electrode consumption
PA: about 0.4 kg / "ton" of ingots
600 / ka x 0.4 = X240 / "tonne'Lde ingot
IN: about 1 kg / "ton" of ingot
s600 / kg x 1 = 1600/11 ton "of ingot = = yenJ 3) Reduction of oxygen consumption
PA: 2 Nm / "ton" of ingots
Y45 / Rn3 x 2 = 190/11 ton "of ingots
IN: 4.5 Nm2 / "ton" of ingots
Y45 / Nm x 4.5 = Y202 / "ton" of ingots 4) Reduction of oil consumption
PA: 0.5 lR / "ton" of ingot
X65 / # x 0.5 = Y32 / 'ton "of ingots
IN: 1.25 l / "ton" of ingot
Y65 / ex 1.25 = Y81 / "ton" of ingot 5) Reduction of refractory consumption
PA: 0.6 kg / "ton" of ingots
1150 / kg x 0.6 = 190 / "ton" of ingots
IN: 1.6 kg / "ton" of ingots
X150 / kg x 1.6 = Y240 / "ton" of ingots
Total reduction in the cost of energy and supplies
PA: 12623 / "ton" of ingots
IN: Y2623 / "ton" of ingots 6) Reduction of steel manufacturing time
PA: about 7 minutes
IN: about 13 minutes 7) Improvement in productivity through preheating
(Without preheating: 17 flows / day, casting interval to casting 85 min)
PA: casting interval 78 min; 18.5 flows / day
Annual flow = 18.5 castings / day x 90 "tonnes" / cast
x 350 working days / year
= 583,000 "tonnes" / oven
IN: casting interval to casting 72 min, 20 flows / day
Annual flow = 20 x 90 x 350
= 630,000 "tonnes" / oven
Augmentaticn (IN - PA): 4,700 "tonnes" / year 8) Reduction in annual costs (energy and supplies expenses only)
PA: Y1052 / "ton" x 583,000 "tons" / year
About 600 million yen / year (The cost of the equipment is recovered in 6 months).

IN: 2633 / "ton" x 630,000 "tons" / year
About 1,640 million yen / year (cost reopened in 4 months).

9) Measures to prevent secondary pollution (smoke, white, odors) .-.

PA: (1) NAPEC (micro gas flow control
calculator)
(2) Combustion of incompletely burnt gases
IN: (1) Whereas two preheating units operate in series with alternating gas flow reversal, the gases are at a reduced temperature of approximately 1000C when they are finally put into the atmosphere, hence the absence of secondary pollution.

 (2) NAPEC if desired.

Claims (9)

 1- Apparatus for preheating a load of steel manufacturing equipment with the heat of the burnt gases emanating from the equipment, characterized in that it comprises several preheating units (5) intended to temporarily receive a load , at least one supply duct (8) connecting a gas discharge section (6) of the manufacturing equipment (2) to a gas inlet section (7) of each of the preheating units (5 ), at least one conduit (11) for the final discharge of the burnt gases having passed through the preheating units, and connection conduits (15) each connecting a gas outlet section (I2a) of one of the preheating at the gas inlet section (7) of another preheating unit so that the burnt gases successively pass, in series, the preheating units by flowing from the supply duct to the exhaust duct.  2- preheating device according to claim 1, characterized in that the gas discharge section (6) brdlés is connected to the gas intake sections (7) of the preheating units (5) by supply conduits (8) each of which comprises a first shut-off valve (10), the conduits of: house comprising: each a second shut-off valve (16) and connecting the preheating units to one another, exhaust ducts (11) , each comprising a third stop valve (14) being each connected to the gas outlet section of the corresponding preheating unit, the stop valves being able to open or close in order to send the burnt gases through successively, - in series the preheating units in a selectively determined order.  3- preheating device according to claim 1 or 2, characterized in that each of the preheating units (5) has a load receiving section removably coupled to the supply conduit (8), to the connecting conduits (15 ) and at. exhaust duct (11) and interchangeable with the corresponding section of another preheating unit.  4- preheating device according to claim 3, characterized in that each of the preheating units comprises a fixed container (20) which has an upper opening and is mounted in a fixed position, a mobile container (24), serving to receive the load , which has a load inlet at the top (7) and at the bottom an opening bottom cover (22) and which can assume and leave vertically the insertion position in the fixed container, and a top cover (25) which can be move horizontally to open or close the load inlet of the mobile container, the top cover being connected to the supply duct (8), the fixed container being connected to the connection duct (15) and to the evacuation duct ( 11) to receive the burnt gases which arrive via the supply duct and pass through the mobile container (24).  5- preheating apparatus according to claim 4, characterized in that the supply duct (8) comprises a fixed duct segment (8b) and a movable duct segment (8c) separate from the movable duct segment, but being able to be coupled to it, the movable duct segment being fixed to a transfer means (30) movable horizontally, the crown cover (25) being coupled to the movable duct segment with the possibility of vertical movement and being supported by the means of transfer with the possibility of vertical movement, the movable duct segment and the top cover being able to be brought by the transfer means into a position allowing the movable container (24) to enter and exit the fixed container (20). .  6- preheating device according to claim 5, characterized in that a tubular connector (28) is threaded with the possibility of axial sliding around one of the fixed (8b) and mobile (8c) duct segments, and can join sliding the other segment to couple the conduit segments to each other.  7- Preheating apparatus according to claim 5 or 6, characterized in that the transfer means (30) comprises a carriage (32) with wheels (33) movable on horizontal rails (31).  8- Preheating device according to claim 7, characterized between that the horizontal rails extend from one of the preheating units to another neighboring preheating unit, and in that wheeled carriages (32) assigned to these units can alternatively move on the rails between the units.  9- Preheating apparatus according to claim 5 or 6, characterized in that the transfer means (30) comprises an articulated member (41) pivoting about a vertical axis.