WO2002034430A1

WO2002034430A1 - Device for heating casting moulds

Info

Publication number: WO2002034430A1
Application number: PCT/IB2001/001884
Authority: WO
Inventors: Erich Rauch
Original assignee: Ing. Rauch Fertigungstechnik M.B.H.
Priority date: 2000-10-25
Filing date: 2001-10-10
Publication date: 2002-05-02
Also published as: DE10052760A1; AU2001292179A1

Abstract

The invention relates to a device (1) for heating casting moulds (2), comprising a heating station (5). The inventive device is associated with a heating device which is used to heat at least one heating chamber, and a conveyor device (11) which supplies heating moulds (2) to the heating station (5) and removes heated casting moulds (2). The heating station (5) is surrounded by first walls (4, 4a) and is provided with a blower device (23) for hot gas and at least one heating chamber which can be sealed in a substantially tight manner by a sealing device (17) and comprises a receiving device for charging at least one mould (2). It is also thermally insulated by the wall of a supply station (7) which is at least partially embodied as an insulating wall, said supply station being at least partially surrounded by two walls (6). A manually actuated device (11) is provided as a conveyor device. Said device enables moulds (2) to be removed from the supply station (7) via openings in both stations (5,7) and to be supplied to the heating station (5) or removed therefrom. An associated heat station (5) can comprise a blower device (23) having a first and second hot gas line associated therewith. The hot gas is selectively guided through a valve via the first or second hot gas line.

Description

DEVICE FOR WARMING CASTING MOLDS

Field of the Invention

The invention relates to a device for heating casting molds according to the preamble of claim 1 and to a heating station with the preamble features of claim 6.

“Casting molds” are understood here to mean a mold, such as blocks, ingots or the like, cast metal or hollow molds intended for casting, which must be preheated for certain purposes. For example, ingots are heated before they are used are melted in a melting furnace in order not to cause an excessive temperature drop in the latter or to prevent undesirable reactions.

Background of the Invention

Devices of this type known on the market are built in the manner of ceramic tunnel ovens, in which a belt conveyor conveys the molds to be heated into a tunnel, where the heating takes place, and then conveys them out again in a heated state to a delivery station. This has several disadvantages. On the one hand, such arrangements are relatively space-consuming, especially since the tunnel must be long enough to achieve the desired heating in the throughput time. It also does not make sense to operate the system only for a short time, i.e. it will have to be provided in a foundry in a central location from where the heated pigs or other molds are to be removed. This means relatively heavy traffic with industrial vehicles, such as forklifts, which is not conducive to safety, especially in a foundry. Numerous special safety measures, such as protective grilles, light curtains, etc., must be provided for the furnace itself, which must be adapted on site and cannot be planned and manufactured from the outset.

Then such an arrangement is also energy-intensive, because the continuous supply of the, generally electrically heated, tunnel oven or the removal makes it necessary that the two end faces of the tunnel oven are at least temporarily open and so thermal energy escapes. A further heat loss arises from the fact that the respective transport device, for example with a chain, drivers etc., is warm and has to go through cold zones and a lot of thermal energy is lost in the process. This transport device and its bearings can often give rise to malfunctions and operational downtimes, precisely because of the frequent temperature turmoil, which of course places an additional burden on the energy balance and reduces the availability of the systems.

In addition, today one tends rather to plant small, self-sufficient casting cells, in order to avoid long transport routes and to rationalize the operation. In such a small space, a tunnel oven is practically impossible to accommodate. On the other hand, one has to recognize that a central furnace is much more likely to meet the needs of casting cells of different sizes, i.e. much more adaptable to different requirements, such as differently designed and large molds.

Summary of the invention

The invention is therefore based on the object to design a device of the type mentioned so that it

• is designed to save space

• and yet easily adaptable to different requirements,

• can be operated with less energy or less energy losses.

According to the invention this object is achieved by the characterizing features of claim 1.

Characterized in that the heating station has a blower device for hot gas, the heat can be distributed more evenly, even if the molds do not - as in the tunnel furnace - move relative to it. Of course, this does not mean that the heating station only has to work with hot gas, rather additional heaters can be provided which are operated electrically or with gas, as can also be the case for the heating of the hot gas. If "hot gas" is used here, this will generally be air, but for certain applications it may also be expedient to use other gases, such as CO ₂ , or an inert gas, such as nitrogen. In any case According to the invention, the heating takes place essentially convectively, the temperature of the “transport medium”, ie the hot gas, and of the parts that come into contact with the ingots or casting molds never being close to the Melting point can come, which is of course a particular advantage of the training according to the invention. Because even in the event of business interruptions, the molds or ingots cannot overheat. However, this also provides increased safety - regardless of the throughput and the shape of the parts to be heated, because existing casting lugs can be very thin and then easily ignite when heated by means of radiators according to the prior art.

From the above it is clear that thus the very space-consuming and failure-prone transport device can be omitted. The device according to the invention thus becomes significantly smaller. At the same time, however, it is also possible to provide the heating station, which is surrounded by the first walls, and thus closed, with at least one heating space which can be sealed essentially tightly by a sealing device and a receiving device for charging at least one casting mold, so that the heat losses are reduced to a minimum ,

Instead of the feed section of the transport device, a feed station is now provided, which is thermally insulated by a wall of the heating space formed as an insulating wall and is at least partially surrounded by second walls. These second walls can now be designed in a slightly protective grid-like manner, these two units thus being constructed in the manner of individual modules and being connectable to one another, so that the device can also be easily adapted to different needs. Of course, it is advantageous if the heating station is also surrounded by a type of protective grille, because with such a structure, it is no longer necessary to adapt to local conditions.

As a transport arrangement, a handling device is provided according to the invention, with which molds can be removed from the supply station via an opening of the two stations and fed to or removed from the heating station, so that this device is not exposed to the entire heating and is therefore more reliable in the Operation is.

[0012] A heating station used according to the invention advantageously has the features of claim 6.

Brief description of the drawing

[0013] Further details of the invention can be found in the following descriptions. Description of a preferred embodiment shown schematically in the drawing. Show it:

Fig. 1 is a perspective view of a modular sequence of a feed station, a heating station and a delivery station, which from one into the

Module arrangement integrated protective grille are surrounded, which is why

Fig.la illustrates an enlarged detail from the circle A of Fig. 1;

FIG. 2 shows a side view of the arrangement according to FIG. 1 after the protective grille has been removed and in a slightly modified form;

Figure 3 shows the heating station in detail and on a larger scale.

4 shows an alternative embodiment of the heating station with two pairs of heating spaces; and

5 shows an arrangement composed of more than the three modules shown in FIG. 1.

Detailed description of the drawing

In Fig. 1, a device 1 for heating cast into blocks and brought into a customary shape pigs 2 can be seen, the core of which on the one hand insulating walls 4a (Fig. 3) and on the other hand, preferably as a protective grid 4, first formed Has heating station 5 having walls. To the left of the heating station 5 (with reference to FIG. 1) there is a feed station 7 which is at least partially surrounded by a protective grid 6 as a second wall and in which the still cold pigs 2 are preferably mounted on a frame 8. The protective grille 4 or 6, as can be seen, is preferably square in plan view, in order to enable easy attachment of a further module, as required. Feet 9 are expediently attached to the respective lattice frame.

[0015] In order to be able to replenish the stock of pigs 2 in the feed space again and again in good time, it is advantageous to provide the second walls 6 as a grid to enable a view and at the same time for protection. On one of the four sides it is advisable to have a door, such as a door (which can be moved to the left in FIG. 1) Sliding door 10 to be provided to allow refilling of ingots 2 into the frame 8. However, the door 10 can be designed on each side that appears to be expedient and, if appropriate, also as a door that can be pushed up or as a pivoting door. It should be mentioned that, although it is preferred if the walls 4 and 6, for example in the manner of FIG. 1A described later, are rigidly connected to one another, it would be possible to make the feed station 7 mobile so that it is possible is to fill a feed station 7 with pigs 2, while another feed station in the manner shown in FIG. 1 is currently in spatial communication with the heating station and is in operation.

During this operation, individual ingots 2 are removed from the frame 8 by a handling device 11, which is advantageously located above the two stations, and brought into at least one heating chamber enclosed by the insulating walls 3. For this purpose, the handling device 11 has an approximately horizontal travel rail 12 and a vertical rail 13, on which it can run up in a manner not shown, known per se by means of a schematically indicated travel drive unit 14, while the travel unit 14 also on the rail 12 engages in such a way that it can be moved like a cross slide along both guide devices 12 and 13.

The handling device 11 has, moreover, expediently two pliers-like gripping pliers 15 which reach under the respective ingot 2 like a shovel and are thus capable of being lifted. The distance between the two gripping tongs 15 accordingly corresponds approximately to the dimension of the ingots 2. The gripping tongs 15 can be actuated in a manner known per se for handling devices in such a way that they can reach under the respective ingot with their lower, shovel-like ends and then lift them when starting up , It can be seen that the walls 4 and 6 are open at the top for unimpeded access of the pliers 15. This is particularly favorable to save space, although it would be conceivable to provide lateral openings in the grid walls 4, 6 and to set up a handling device arranged outside the same.

The above description makes it clear that the handling device 11 when gripping (and then also when parking) the pigs 2 is to be positioned fairly precisely. This can be done in various ways. The exact position of the tongs 15 above the ingots 2 in the feed station 7 can be read, for example, by mechanically, optically, magnetically or inductively or capacitively provided on the rail 12 incremental or binary markings are determined. However, a stepping motor can also be provided for moving the handling device 11, the number of steps of which is counted and which stops at a certain number of steps (which corresponds to the position above the frame 8). But since the presence of a ingot 2 in the frame 8 can also be determined, it is preferred if the handling device 11 is provided with at least one position sensor, for example at 16, which at least for determining the position of the tongs 15 and / or the ingot to be gripped 2 is formed. This sensor 16 can be an optical sensor, for example.

With the help of the handling device 11, the pigs 2 are brought from the feed station 7 to the heating station 5. This heating station 5 will be described in detail later with reference to FIG. 3. However, as already shown in FIG. 1, the heating station has at least one heating space, but here two, which can be sealed at the top by a cover 17 as a closing device.

The slightly modified variant of the feed device according to Fig. 2 shows a feed station 7 with two juxtaposed frames 8. This makes it possible, for example, for an operator to fill up a frame 8 with fresh ingots while the handling device 11 ingots 2 from the takes another frame. The changeover of the removal by the handling device can either be controlled by hand, or takes place automatically when the sensor 16 detects that no frame is contained in a frame 8. In this case, it is advantageous if the sensor 16 triggers a corresponding alarm of an alarm device 16 '(optically and / or acoustically) in order to draw the operator's attention to the need for refilling. Another variant could provide that the frame 8 is designed to be mobile so that it is filled outside the grid walls 6 and then inserted into the feed station 7 instead of an empty frame. This design is particularly useful when a feed station contains two such racks 8, one of which is "being processed" by the handling device 11, ie pigs are fed from this rack to the heating station while the other is being filled. The adaptation of the handling device 11, Once removed from one frame and then from the other, either by manually switching the operator, by providing a turntable (which changes the frames), or by appropriate software in connection with the sensor 16, as mentioned above. In this embodiment variant, each frame 8 has laterally protruding storage surfaces 18 which, although they can be folded upwards in accordance with arrow a, but can hold a pig safely in the position protruding from the respective stand, as shown. It should be mentioned that such storage areas are not absolutely necessary if the casting molds to be transported by means of the handling device 11, such as the ingots 2, are shaped in such a way that the handling device 11 can grip them easily and safely. This is particularly the case with pigs with sloping faces, with which they are often formed, so that even more pigs of this type can then be placed standing one on top of the other in the feed station.

2 also shows the outer structure of the heating station 5 more clearly. In the embodiment shown, it has two separate heating spaces 19.1, 19.2, each of which is closed at the top by means of the cover 17. Each lid 17 has, appropriately graded, upstanding struts 17 ', on which the other lid, as shown in dash-dotted lines, can be placed when opening the associated heating space 19.1 or 19.2, so that no separate parking space has to be provided. In addition, it is advantageous if the heating station 5 is also provided with a clipboard 20 for heated pigs 2 removed from the respective heating space 19.1 or 19.2, so that the handling device 11, after removal of a heated pig 2, place it on the tray and then can still close or put on the respective cover 17. Only then is the respective ingot 2 removed from the clipboard 20 and advantageously fed to a delivery station 21 which, for example, via motors provided in the interior of transport rollers 22 and driving it (not visible), the respective ingot 2 to a processing station, that is to say, for example to one Die-casting cell assigned to the melting furnace. A common blower 23 is expediently assigned to the heating spaces 19.1, 19.2, the operation of which is described in connection with the heating spaces using FIG. 3. It should be mentioned that the arrangement could alternatively do without a delivery station 21 if the handling device 11 supplies the respective heated ingot directly to the consumer in a production cell, that is to say to the melting furnace located there.

The two modules or cells 5, 7 can be joined together so that there is a substantially continuous surface from their two walls 4, 6, for example, by telescopically inserting the frame of the grid into one another are. For example, the feed station 7 and the heating station 5 can be designed as detachable cells. 1A, however, illustrates an advantageous possibility of detachable assembly on the basis of section A of FIG. 1.

The grille frames of the protective grille are provided with a T-slot 24 profiles 25, e.g. extruded profiles. This corresponds approximately to the design of curtain rails into which a clamping piece 26 can be pivoted in the longitudinal direction of the groove 24 and then 90 ° in the groove 24, whereupon it can be clamped by means of a screw 28 inserted on a second clamping piece. Of course, any other releasable fastening - such as fastening screws that can be screwed into the screw holes of one frame on the other frame, dowels or other form-fitting connecting devices - is also possible. In any case, such a releasable fastening enables greater stability on the one hand and on the other hand the greatest possible flexibility in the assembly of the individual modules 5, 7 and 21.

The operation of the heating station 5 will now be explained with reference to FIG. 3. As can be seen, a fan rotor 23.1 of the fan 23 is mounted in a shaft 29 below the two heating spaces 19.1, 19.2 and can be driven by means of a drive part 23.2. On the suction side opposite the drive part 23.2 there are provided hot gas lines 30, 31 which are each connected to slot-shaped suction openings 32, 33 in a manner not shown. This ensures an energy-saving recirculation mode.

In the way of the slots 32, 33, heating devices, e.g. at 105, (gas and / or electric heating) to compensate for heat losses. As a result of any leakage losses and the possibly necessary delivery of gas that is too moist (normally air, but possibly also inert gas, such as nitrogen), it will be necessary to add a certain amount of gas. This can be done through a respective intake duct 34 with an adjustable supply valve (flap 35) or in another corresponding manner.

At the top of the shaft 29 branches to form supply lines 36, 37 for the individual heating rooms 19.1, 19.2. A flap valve 38 (or another valve) is provided in each supply line 36 or 37, through which the respective heating space 19.1 or 19.2 can be cut off from the hot gas supply via the shaft 29. Of course, it would also be possible to open the respective heating space simply to turn off the heating device 105 or the blower 23. However, this would involve temperature fluctuations that should be avoided. ^ Therefore, the line 37 is assigned to the heating space 19.1 as a shunt line, via which the hot gas flow can be continued even when the valve 38 assigned to the space 19.1 is closed. Analogously, the line 36 forms a shunt line to the heating space 19.2, via which the hot gas flow can be conducted when the supply to the heating space is closed by its valve 38.

In the embodiment shown in Fig. 3, the respective shunt line 36 or 37 is now connected to a further heating space, so that optimal use of the thermal energy and heating up a second ingot 2 is possible if from the other heating space Completely heated ingot 2 is removed after opening the associated lid 17 by means of the handling device 11 (cf. FIGS. 1, 2). This means that it is preferred if at least one pair of heating spaces 19.1, 19.2 is connected to a common hot gas supply (blower 23), because the hot gas is not uselessly conducted through the shunt line and, moreover, the time of removal of the already heated ingot for heating the ingot in the other heating room can be used.

If we are talking about "the mess", it is because each of the heating spaces 19.1, 19.2 according to FIG. 3 has only one receiving device for charging a casting mold or ingot 2 in the form of a parting stand 39. In theory, it would be it is possible to provide a frame for holding a number of ingots 2 within each of the heating rooms 19.1, 19.2, for example a frame (8) similar to that provided in the feed station 7. Should there be more than 1 frame or more than one trestle 39 in a heating room 19.1 or 19.2 are provided, a cover 17 could be provided above the respective frame or trestle, so that, if necessary, more than one such closure member 17 is also assigned to a heating space.

As can be seen from the above, the respective flap valve 38 is to be closed to avoid heat losses when the associated cover 17 is opened. As a result, the handling device 11 is also exposed to a lower thermal load. As such, it would be conceivable for the respective flap 38 to be switched from its open state to the approximately horizontal closed state by the control of the handling device 11, which "knows" when the handling device - according to its program or on the basis of a control commands - the cover 17 picks up. However, it is simpler to provide a sensor for opening the cover 17, in particular a mechanical sensor. Thus, in the arrangement shown, a rod 40 is articulated to a handlebar 41, which in turn engages a crank 42 for rotating the associated flap 38. This arrangement is only shown on the left flap, but is provided in both flaps 38. When the cover 17 provided with insulations 17b is placed on it, the rod 40 is pressed against a spring 43 or a weight by a cover plate 17a projecting like a flange over the circumference of the cover, and the flap 38 is thus pivoted into the open position.

A control circuit 44 can be provided for regulating a drive motor 23.3 for the blower 23, in which case this blower has at least two different speeds. The design is advantageously made such that at least one temperature sensor 45, e.g. is provided in a hot gas duct 31, which only switches the blower 23 to a higher speed when a certain minimum temperature has been reached, which is determined via a threshold switch 46. In this way, unnecessary loads or overloads of the drive motor for the air circulation can also be avoided. Of course, a stepless regulation could also be accomplished in order to avoid that too cold gas or too cold air is unnecessarily circulated and for a short time also comes into contact with the heating device provided in lines 30 and 31. Furthermore, such a temperature sensor could influence the heater 105, but in the opposite sense, by switching to maximum thermal energy output when the temperature determined by the sensor 45 is below a setpoint level (possibly from several such setpoint levels) and is switched to minimum heat output as soon as the desired temperature has been reached.

Fig. 4 shows an embodiment of a heating station, which is advantageous for larger amounts of metal to be heated. Three heating stations 5a, 5b of the type shown in FIG. 3 are assembled such that the blower shaft located in bearings 23.2 can be extended to the right beyond motor 23.3 by one blower rotor 23.1 (see FIG. 3). in each of the two units 5a, 5b, each having a pair of heating spaces. If desired, the two hot gas circuits of the units 5a, 5b could also be combined into a single circuit. In the case of separate hot gas circuits, a temperature sensor 45 (FIG. 3) may be assigned to one of the two units 5a, 5b, whereby the output signals of both sensors are fed to a mixer for joint fan control. The mixing stage may be a simple addition stage, with the threshold switch 46 (FIG. 3) then being connected downstream of this mixing stage.

5 is to be shown how the modular construction according to the invention can be used to adapt to different size requirements. It should be assumed that the preheating system shown for loading two, e.g. neighboring consumers, such as two die casting machines, should be used. A single feed station 7 may suffice for this, which of course must then be replenished more frequently by an operator. Of course, it is also possible to provide several feed stations 7.

[0034] This feed station 7 is followed by two heating stations 5.1, 5.2, from which the handling device 11 alternately removes the heated pigs via rails 12.1, 12.2 which are inserted into one another. This is advantageously done in a regular sequence. These heating stations 5.1, 5.2 can then be followed by two delivery stations 21.1, 21.2, of which station 21.1 has, for example, the roller conveyor 22.1 which is shown in FIGS. 1 and 2 and runs approximately perpendicular to the plane of the drawing, whereas the delivery station 21.2 has a (or may have in a different direction) roller table 22.2.

Claims

1. Device (1) for heating molds (2) with a heating station

(5), which is associated with a heating device (105) for heating at least one heating space (19.1 or 19.2), and a transport arrangement (11) for feeding casting molds (2) to the heating station (5) and for removing heated molds (2) characterized in that the heating station (5) surrounded by first walls (4, 4a) has a fan direction (23) for hot gas and at least one heating space (19.1 or 19.2) which can be essentially sealed by a sealing device (17) is provided with a receiving device (39) for charging at least one casting mold (2) and is thermally separated by a wall at least partially formed as an insulating wall (4a) from a feed station (7) at least partially surrounded by second walls (6), and that a handling device (11) is provided as the transport arrangement, with which molds (2) can be removed from the feed station (7) via openings at the two stations (5, 7) mbar and can be supplied to or removed from the heating station (5).

2. Device according to claim 1, characterized in that at least one of the following features is provided: a) at least one group ^'of the first and second walls (4 and 6) are, viewed in cross section, arranged to form a square space; b) the feed station (7) and the heating station (5) are each designed as cells which can be detached from one another by a detachable connecting device (24-28); c) a rail frame (12, 13) is arranged above the feed station (7) and heating station (5), on which the handling device (11) can be moved; d) the handling device (11) has two gripping tongs (15) arranged on it at a distance approximately corresponding to a dimension of the casting molds (2); e) the handling device (11) with at least one position sensor (16) for the

Provide location of the device (11) and / or a mold (2); f) at least one of the openings is provided on the top of the relevant station (5, 7, 21).

3. Device according to one of the preceding claims, characterized in that the feed station (7) with at least one frame which is stationary during operation (8) is provided for the storage of at least one casting mold (2), and that the at least one frame (8) can preferably be moved from a rest position into an operating position, for example is arranged on rollers and / or a turntable.

4. Device according to one of the preceding claims, characterized in that the blower device (23) is assigned a first hot gas line (36) and a second hot gas line (37), and via a valve (38) optionally hot gas via the first or the second hot gas line (36 or 37) is feasible, and that preferably a) at least one second heating space (19.2), which is essentially the same as the first (19.1) and is connected to the second hot gas line (37), is provided, whereas the first heating space (19.1) is located on the first hot gas line (36) and / or b) the closing device (17) is connected to the valve (38) in the sense of an essentially simultaneous actuation, so that hot gas is conducted via the second hot gas line (37), when the closing device (17) of the first heating space (19.1) is open and vice versa, with closed heating spaces (19.1, 19.2) both of which can be supplied with hot air, wobe i Preferably, the respective heating chamber (19.1 or 19.2) is assigned a sensor (40) for opening the closing device (17), via which the valve (38) can be switched so that when the closing device (17) is closed, the heating chamber (19.1 or 19.2) can be supplied through the open valve (38) through hot gas, otherwise the valve (38) is closed.

5. Device according to one of the preceding claims, characterized in that in addition to the supply station (7) and heating station (5) at least one delivery station (21), in particular on the opposite side of the heating station (5) of the heating station (5), is provided and via an opening can be loaded from the handling device (11) and that an intermediate storage (20) for heated molds (2) is preferably provided between the heating station (5) and the delivery station (21).

6. Heating station (5) for a device according to one of the preceding claims, with a heating device (105), which is associated with a blower device (23) for hot gas for supply in at least one heating space (19.1 or 19.2), characterized in that the Provide heating station (5) surrounded by first walls (4, 4a) with at least one heating chamber (19.1 or 19.2), which can be closed essentially tightly by a closing device (17), with a receiving device (39) for charging at least one casting mold (2) and has a wall formed as an insulating wall (4a), and that the blower device (23) has a the first hot gas line (36) and a second hot gas line (37) are assigned, and hot gas can be routed via a valve (38) either via the first or the second hot gas line (3Q or 37).

7. Heating station according to claim 6, characterized in that at least a second, the first (19.1) substantially the same heating space (19.2) is provided, which is connected to one of the hot gas lines (37), whereas the first heating space (19.1) on the another hot gas line (36), and that preferably the spill device (17) is connected to the valve (38) in the sense of an essentially simultaneous actuation, so that hot gas is conducted via the second hot gas line (37) when the closing device ( 38) of the first heating space (19.1) is open and vice-versa, both of which can be supplied with hot air when the heating spaces (19.1, 19.2) are closed.

8. Heating station according to claim 6 or 7, characterized in that the respective heating space (19.1 or 19.2) is assigned a sensor (40) for opening the closing device (17), via which the valve (38) can be switched such that When the closing device (17) is closed, hot gas can be supplied to the heating chamber (19.1 or 19.2) through the open valve (38), otherwise the valve (38) is closed.

9. Heating station according to one of claims 6 to 8, characterized in that at least one temperature sensor (45) is provided, that the blower device (23) and / or the heating device (105) can be switched in at least two states of different power, and that the blower device (23) can only be switched to the state of higher power by the temperature sensor (45) when a predetermined minimum temperature has been reached, whereas the switching characteristic is reversed in the case of control of the heating device.

10. Heating station according to one of claims 6 to 9, characterized in that it is at least partially surrounded by an attached protective grid as a wall (4) and / or in that it comprises at least a pair of heating stations (5a) each surrounded by the first walls (4a) , 5b), and that this pair is assigned a common blower (23) for hot gas (Fig. 4).