WO2020239390A1

WO2020239390A1 - Method and device for heating a pressed material mat

Info

Publication number: WO2020239390A1
Application number: PCT/EP2020/062844
Authority: WO
Inventors: Andreas Krott; Benedikt Reehuis; Klaus-Peter Schletz; Dipl.-Holzwirt Günter STAUB
Original assignee: Siempelkamp Maschinen- Und Anlagenbau Gmbh
Priority date: 2019-05-24
Filing date: 2020-05-08
Publication date: 2020-12-03
Also published as: DE102019114016B4; CN113840702A; WO2020239390A8; CN113840702B; DE102019114016A1

Abstract

The invention relates to a method for pre-heating a pressed material mat (2) before feeding same into a press (4), in particular in the course of producing wood material boards. The pressed material mat (2) runs through a double-belt pre-heating device (3) with an upper continuously circulating gas-permeable transport belt (5a) and a lower continuously circulating gas-permeable transport belt (5b), wherein the pressed material mat (2) is supplied with steam or a steam-air mixture in the double-belt pre-heating device (3) by means of at least one steam supply device (7), and the pressed material mat is thereby heated. The method is characterized in that the transport belts (5a, 5b) in the double-belt pre-heating device (3) are supplied with hot air by means of a drying device (9) arranged downstream of the steam supplying device (7) in the transport direction (T) such that the moisture deposited on the transport belt (5a, 5b) by means of the steam pre-heating process is picked up by the hot air (L) flowing along the transport belts (5a, 5b) and is discharged together with the hot air.

Description

Method and device for heating a pressed material mat

Description:

The invention relates to a method for preheating a pressed material mat before it enters a press in the course of the production of wood-based panels, the pressed material mat running through a double-belt preheating device with an upper, continuously circulating, gas-permeable conveyor belt and a lower, continuously circulating, gas-permeable conveyor belt, wherein the pressed material mat in the double-belt preheating device is acted upon by steam or a steam-air mixture by means of at least one steam supply device and is thereby heated.

The invention also relates to a double-belt preheating device for heating a pressed material mat and a system for producing wood-based panels.

The pressed material mat to be heated (which is also simply referred to as “mat”) is preferably a pressed material mat for the production of wood-based panels. In principle, however, the heating of other pressed material mats or material webs is also recorded. A pressed material mat for the production of wood-based panels usually consists of chips or fibers, in particular wood chips or wood fibers, preferably glued wood chips or wood fibers which, for. B. be sprinkled on a scatter belt conveyor or the like to a pressed material mat. As a result, a continuous pressed material mat and consequently a pressed material mat strand is generated, which passes through a double belt preheater for heating and then in a press, for. B. in a continuously operating double belt press, using pressure and heat to form a plate or a strip of panels is pressed. To optimize the pressing process, the material to be pressed is preheated in the preheating device.

The mat preheating in the course of the production of wood-based panels has long been implemented in a wide variety of embodiments in practice. So you know z. B. double belt preheaters, in which the press mat by means of a steam supply device with steam, z. B. is heated with saturated steam at temperatures between 100 ° C and 130 ° C, which is injected into the mat surface directly before the press inlet, so that the top layers are heated immediately. The steam can be injected into the mat above and below. The circulating conveyor belts, which are designed as sieve belts, fix the mat surfaces above and below during the steam injection.

Alternatively, the preheating is implemented in a double-belt preheating device by alternately applying a steam-air mixture to the mat surfaces, the penetration of the mat by the mixture being supported by a suction device or negative pressure on the opposite side.

In practice, it is also possible to vent the mat after it has been preheated and before it enters the continuously operating press, either using a separate prepress or a compactor integrated into the preheating device. The compactor displaces air from the mat through several rollers so that the infeed of the continuous press is approached with a higher density and the risk of blowouts is reduced. This enables operation at a higher feed rate.

A method and a system for preheating of pressed material mats from glued pressed material is z. B. from DE 197 01 596 C2. The mat of pressed material is subjected to a flow treatment in a preheating system in which it is flowed through by an air-conditioned fluid consisting of air and water vapor at a temperature of below 100 ° C with a set dew point and a set dew point difference.

The treatment of a litter mat with a steam-air mixture is also known in connection with the production of biodegradable insulation boards (cf. DE 196 35 410 A1). The mat or fleece is first compressed to the desired plate thickness and then a steam-air mixture is introduced into the compressed fleece in a heating zone over a period of 10 to 20 seconds, avoiding pre-hardening of the binding agent. In a further treatment step, a stream of hot air is passed through the compacted fleece for curing and drying. The system known in this respect is therefore not used to preheat a mat for subsequent pressing in a separate press, but in the double-belt press both preheating takes place via a steam-air mixture and curing and drying of the mat using hot air.

Methods and devices for preheating pressed material mats during the production of wood-based panels are also known from EP 2 213 432 A1 and EP 2 588 286 B1.

Preheating the mat in front of a continuously operating wood-based panel press has proven itself in practice. The endlessly circulating conveyor belts, which are preferred, are of particular importance are designed as sieve belts and through which the steam or the steam-air mixture is injected into the mat. Since the sieve belt experiences a certain cooling in the return and is heated up again to the process temperature and consequently to the dew point in the steam injection area, steam condenses on the sieve belts with each circulation. The moisture in the sieve belt increases until the moisture withdrawn from the belt with each revolution is as great as the moisture supplied to the belt. As a result, so much condensate can collect in the belt under unfavorable conditions (e.g. weather-related or seasonal) that condensate droplets form that are so large that they get onto the product surface (i.e. onto the pressed material mat) and cause stains there on the surfaces of the final product (ie the wood composite panel). There is consequently a need to avoid the stains caused by condensation on the product surface.

Since these problems are directly related to the principle of preheating with steam or a steam-air mixture, alternative methods for preheating pressed material mats have also been proposed, e.g. B. preheating by means of microwaves (cf. e.g. DE 10 2016 110 808 A1). Regardless of this, however, there is still considerable interest in the fundamentally advantageous and economical mat preheating by applying steam or steam-air mixtures.

Proceeding from this, the invention is based on the technical problem of creating a method and a device for preheating a pressed material mat which, on the one hand, uses the advantages of steam preheating or preheating by means of a steam-air mixture and, on the other hand, uses those observed in practice Avoids or at least significantly reduces problems with condensation stains on the product surface.

To solve this problem, the invention teaches in a generic method for preheating a pressed material mat before entering a press, in particular in the course of the production of wood-based panels, that the conveyor belts (or at least one of the conveyor belts) in the double-belt preheating device after steam preheating by means of a the (respective) steam supply device downstream of the drying device in the transport direction are acted upon with hot air, preferably in such a way that the moisture precipitated or precipitated by the steam preheating on the conveyor belts is absorbed by the hot air flowing along the conveyor belts and removed with the hot air. The conveyor belts are preferably designed as sieve belts.

According to the invention, the mat is preheated in a basically known manner by applying steam or alternatively also with a steam-air mixture in the double-belt preheating device. Subsequently, however, the belts are immediately dried with the aid of hot air, which absorbs the moisture from the sieve belt, so that problems with condensation spots on the product surface can be reliably avoided. The invention is based on the knowledge that with the aid of hot air immediately after the application of steam and consequently also in advance of the conveyor belts (in particular sieve belts) in which the mat is held between the belts, simple and effective drying of the conveyor belts can take place. For this purpose, hot air is used, which, due to its nature, is able to absorb moisture from the sieve belt, in that hot air with a high temperature and low relative humidity is preferably used. Hot air with a temperature of more than 110 ° C, preferably more than 120 ° C, z. B. more than 130 ° C used. This not only has a high absorption capacity for the moisture of the sieve belt, but also prevents the sieve belts from cooling down to temperatures below 100 ° C during drying, which would counteract the principle of preheating the mat.

For proper drying of the sieve belts with the help of hot air, the use of hot air with low relative humidity is necessary. In this respect, the invention proposes that the hot air preferably has a relative humidity of at most 10%, preferably at most 5%, particularly preferably at most 2%. These values refer to the hot air before contact with the conveyor belt. B. be measured in the area of the air inlet, z. B. just before or just behind its mouth.

In practice, the suitable hot air for drying the conveyor belts can be made available very easily and economically by adding ambient air, e.g. B. from the production hall, which is usually at a temperature of about 10 ° C to about 50 ° C and (under atmospheric pressure) at these temperatures may have a relatively high relative humidity. By heating this air (e.g. with the help of a steam-air heat exchanger) to the desired temperatures of more than 110 ° C, e.g. B. more than 120 ° C, particularly preferably more than 130 ° C, the relative humidity of the drying air drops. In order to achieve the desired, very low values of relative humidity, additional drying of the air can be provided. The air has a very high drying potential due to the heating (and optionally due to drying).

Overall, the drying of the sieve belts with (dry) hot air in the area of the advance of the conveyor belts of the continuously operating double belt preheating device is of particular importance. The hot air is used exclusively to dry the sieve belts and not to treat the product and consequently not to treat the press mat, because the press mat is preheated exclusively via the steam or a steam-air mixture previously introduced into the press mat. The hot air is consequently not blown into the product or sucked through the product, but rather the conveyor belts are exposed to the hot air in such a way that the hot air only flows along the conveyor belts and due to the high absorption capacity with their own low relative humidity absorbs moisture adhering to the conveyor belts without penetrating the press mat and without influencing the mat temperature by preheating. For this purpose, the drying device z. B. have an upper hot air tunnel for heating the upper conveyor belt and a lower hot air tunnel for heating the lower conveyor belt, each hot air tunnel having an air inlet at one end of the tunnel (or in its vicinity) and an air outlet at the opposite end of the tunnel (or in its vicinity ), so that each hot air tunnel forms a hot air section for applying hot air to the respective conveyor belt. There is consequently not the blowing in of hot air from one side of the mat and the suction of the hot air from the other side of the mat, but the hot air is supplied and discharged on the same side of the mat, so that it is only along the conveyor belt within the hot air tunnel flows without penetrating the product. For this purpose it is useful if the hot air is passed through the respective hot air tunnel with a low pressure or overpressure, which is less than 10 millibars, preferably less than 5 millibars. It is preferable to work with very low flow speeds of the hot air, the flow speed referring to a direction of propagation of the hot air oriented parallel (or anti-parallel) to the transport direction, i.e. the hot air flows essentially parallel (or anti-parallel) to the transport belt along the transport belt, and preferably with a flow velocity of less than 5 m / s., particularly preferably less than 3 m / s. Overall, a geometry of the heat tunnel and an operating mode are selected that result in a relatively low flow speed and thus a relatively long interaction time of the hot air with the conveyor belt. The duration of interaction or contact time between hot air and conveyor belt is at least 2 seconds, preferably at least 3 seconds, e.g. B. 3 to 5 seconds. The hot air can flow along the conveyor belts via a hot air section with a length of at least 1 m, preferably at least 2 m. For this purpose, the above-mentioned upper hot air tunnel and / or the lower hot air tunnel can have a length along the transport direction of at least 1 m, preferably at least 2 m, particularly preferably at least 3 m.

In addition, it has been found to be advantageous to guide the hot air along the conveyor belts in a flow (main) direction opposite to the transport direction. This means that the air inlet is arranged in the area of the rear end of the tunnel and the air outlet in the area of the front end of the tunnel, specifically in relation to the transport direction of the pressed material mat. Conveyor belts and hot air are therefore preferably brought into contact with one another in countercurrent.

The method described can preferably be carried out with a double-belt preheating device, which has an upper, continuously revolving, gas-permeable conveyor belt and a lower, endlessly circulating, gas-permeable conveyor belt, with a treatment gap being formed between the upper conveyor belt and the lower conveyor belt through which the mat can be passed along the transport direction (by means of the driven conveyor belts). Furthermore, the double-belt preheating device has a steam supply device (arranged between the inlet and outlet), with which the mat within the treatment gap between the inlet and the outlet can be acted upon with steam or with a steam-air mixture and thereby heated. According to the invention, this double-belt preheating device has a drying device which is arranged downstream of the steam supply device in the transport direction, but which is also arranged in front of the conveyor belts and consequently between the inlet and outlet, i.e. the drying device is arranged between the steam supply device and the outlet of the double-belt preheating device . With this drying device, hot air can preferably be applied to the conveyor belts in such a way that the moisture precipitated by the steam preheating on the conveyor belts is absorbed by the hot air flowing along the conveyor belts and removed with the hot air. The conveyor belts are as gas-permeable and preferably as air-permeable and water-vapor-permeable conveyor belts, e.g. B. designed as sieve belts.

Particularly preferably, a steam supply device is provided for the mat, in which steam is simultaneously pressed into the mat from above and below, so that at least one steam supply is provided both above the upper conveyor belt and below the lower conveyor belt. The respective drying devices for the conveyor belts are preferably arranged downstream of these steam supplies. ok

In principle, however, it is possible that a steam supply is assigned to only one conveyor belt and that a drying device is then assigned to only this conveyor belt. Finally, there is basically the possibility that a steam supply is initially arranged along the transport direction on one of the conveyor belts with a subsequent drying device and that in the transport direction a further steam supply is only connected to the other conveyor belt, which in turn is followed by a drying device for this conveyor belt.

The drying device can - as already mentioned in connection with the method according to the invention - have an upper hot air tunnel for heating the upper conveyor belt and a lower hot air tunnel for heating the lower conveyor belt, each hot air tunnel having an air inlet (at one end of the tunnel) and an air outlet ( at the opposite end of the tunnel). In terms of construction, the upper hot air tunnel and the lower hot air tunnel are preferably designed in such a way that the hot air flows through the tunnel at a relatively low flow rate and consequently a long interaction time with the sieve belt, and at the same time perfect moisture absorption and moisture mixing of the air within the heat tunnel is achieved.

The hot air tunnel preferably has a width which extends (essentially) over the entire width of the conveyor belts or also beyond. The length of the hot air tunnel along the transport direction is selected in the manner already described so that a relatively long interaction time between the hot air and the transport belt is ensured. Furthermore, it is optionally expedient to also select the height of the hot air tunnel to be relatively large in order to deal with large amounts of air and a high one To be able to work through mixing of the air so that the hot air absorbing the moisture does not remain in the immediate vicinity of the conveyor belt over the entire transport path, but instead is mixed with the air above or below to improve the moisture absorption.

Based on these considerations, it is optionally proposed that each hot air tunnel each have a plurality of blocking elements reducing the flow cross-section, e.g. B. has partitions which form flow openings in the area of the conveyor belts. The hot air consequently flows through the hot air tunnel from the entrance to the exit via corresponding barriers or bottlenecks, which on the one hand result in the hot air reaching the bottlenecks and the throughflow openings formed thereby in the area of the conveyor belts and on the other hand at the bottlenecks for an increase in the Ensure speed directly on the conveyor belt. This leads to a subsequent turbulence at the barriers in the respective adjoining tunnel zones and thus to an intensive mixing of the hot air, so that the moisture extracted from the conveyor belt is well distributed over the entire amount of air inside the tunnel or within the respective tunnel zone. The drying potential of the air flowing past the sieve belt thus remains high over the entire interaction path, so that a high degree of drying is guaranteed.

According to a further proposal of the invention, the mat of material to be pressed is not only preheated in the double-belt preheating device, but also compressed, with the aim of venting the mat before the subsequent pressing process. The use of such a compacting device within a double-belt preheating device is known in principle from the prior art, but according to the invention integrated into the claimed double belt preheating device, namely in the transport direction behind the drying device. Subsequent to the steam preheating and the subsequent drying of the sieve belts, there is consequently a subsequent compression to vent the press material mat. In terms of construction, this is realized by a compacting device that z. B. designed as a roller device and an upper grain pacting roller and a lower grain pacting roller can have. The upper grain pacting roller and / or the lower grain pacting roller can with at least one suitable power means, for. B. at least one press cylinder (preferably hydraulic cylinder) or with several power means / press cylinders are applied. The compacting rollers are integrated into the feed of the conveyor belts, ie they act on the conveyor belts from both sides. The material to be pressed is consequently subjected to pressure by the compacting rollers with the interposition of the conveyor belts.

Optionally, the conveyor belts can be preheated with a belt heating device upstream of the steam supply device before the mat is applied with steam. These

Belt heating device is e.g. B. arranged in the return of the conveyor belt, preferably immediately before the transition from the return to the flow and consequently immediately before the inlet of the double-belt preheating device. The band heating can e.g. B. be done by applying hot air.

In the context of the invention, the area of the conveyor belt, which extends from the inlet to the outlet and in which the pressed material mat is consequently guided, is referred to as the forerun and the area of the conveyor belt connected to it, starting at the outlet up to Infeed forms the return of the conveyor belt, which in turn merges into the forerun at the infeed.

Furthermore, in a preferred development, cleaning of the conveyor belts with the aid of a belt cleaning device is provided, with a separate belt cleaning device preferably being assigned to each conveyor belt. This belt cleaning device is preferably in the return of the respective conveyor belt, for. B. arranged immediately behind the outlet of the double belt preheater. After the press mat has run out, the conveyor belt is cleaned by removing adhering press material particles, e.g. B. wood fibers or wood chips are removed. Such a cleaning device can have at least one cleaning brush. Optionally or in addition, the cleaning device can work with the aid of (warm) cleaning air, i. H. be pressurized with air for the purpose of cleaning. For this purpose, an additional suction device is usually provided for the cleaning air, so that the particles removed from the conveyor belt with the aid of the cleaning air and / or the cleaning brush are suctioned off via the suction device. Of particular importance is the fact that the cleaning is optimized by the (previous) drying of the screen belts according to the invention. On the one hand, fiber adhesions are avoided from the outset due to the lower screen belt moisture. On the other hand, due to the lower moisture content, the adhering particles can be removed more easily during cleaning. Overall, this contributes to the trouble-free operation of the preheating system.

According to the invention, the double-belt preheating device described and its operation for preheating the pressed material mat are in the foreground. However, the invention also relates to a system for the production of wood-based panels, with at least one scattering device for producing a Press material mat and with a double belt preheating device of the type described and with a press for pressing the preheated material mat. This press, which is preferably designed as a continuously operating double belt press, is consequently preferably arranged behind the double belt preheating device according to the invention. The double-belt preheater consequently does not serve to produce a finished product, but only to preheat a press mat within the manufacturing process, ie the preheated mat is then pressed into the finished product in a separate press using pressure and heat.

The press downstream of the preheating device, which is preferably designed as a double belt press, has, for. B. an upper heating plate and a lower heating plate and in the upper part of the press and in the lower part of the press endlessly rotating press belts, for. B. steel press belts, these press belts z. B. with the interposition of rolling element assemblies (z. B. roller bars) are supported on the heating plates or press plates. One of the heating plates or both heating plates are acted upon by press cylinders, which are supported on the press frame (e.g. on the press frame). The preheating device according to the invention can consequently be integrated into a conventional manufacturing process and combined with known double belt presses.

In the following, the invention is explained in more detail with reference to drawings, which only represent an exemplary embodiment. Show it

Fig. 1 shows a section of a plant for the production of

Wood-based panels in a simplified representation, 2 shows a double belt preheating device according to the invention from FIG

Plant according to Fig. 1 in a broken perspective view and

3 shows a schematically simplified side view of a double-belt

Preheating device according to Fig. 1 (detail).

In Fig. 1 is a simplified system for the setting of Flolzwerkstoffplatten shown in a continuous flow. First of all, with the aid of a scattering device 1, which is only indicated, the grit to be compressed (e.g. wood fibers or wood chips) is sprinkled onto a belt conveyor to form a grit mat or pressed material mat 2. The grit mat produced in this way is pretreated in a double belt preheating device 3 and then in a continuously operating press 4 using pressure and heat to form a plate or a plate-shaped strand, e.g. B. pressed a chipboard or fiberboard. The press 4 is preferably designed as a double belt press, which has an upper heating plate and a lower heating plate and endlessly circulating press belts (e.g. steel press belts) in the press upper part and in the press lower part, these press belts with the interposition of roller assemblies (e.g. roller bars) the press plates (Fleizplatten) are supported. One of the sausage plates or both sausage plates are acted upon by press cylinders which are supported on the press frame (e.g. on the press frame).

In order to optimize the pressing process within the press 4, according to the invention, the pressed material mat 1 is preheated with the aid of the preheating device 3, which is only indicated in FIG. 1. In the preheating device 3, the pressed material mat 2 is preheated with the aid of steam or with the aid a steam-air mixture which is injected into the pressed material mat 2 and / or sucked through the pressed material mat 2.

The double belt preheating device 3 is shown in FIGS. It has an upper, endlessly circulating, gas-permeable conveyor belt 5a and a lower, endlessly circulating, gas-permeable conveyor belt 5b. The conveyor belts 5a, 5b are designed as sieve belts 5a, 5b in the exemplary embodiment. The conveyor belts or sieve belts 5a, 5b are each guided around a plurality of rollers or rollers 6, of which at least one roller is designed as a drive roller for the upper part and the lower part. A treatment gap is formed between the conveyor belts 5a, 5b, through which the mat 2 is guided along the conveying direction T from an inlet E to an outlet A. The area of the conveyor belt 5a, 5b, which extends from the inlet E to the outlet A and in which the pressed material mat 2 is consequently guided, is referred to as the forerun V and the subsequent area starting at the outlet A to the inlet E forms the return R, which in turn merges into flow V at inlet E.

The double-belt preheating device 3 has a steam supply device 7 immediately behind the inlet E, which has an upper steam supply box 8 and a lower steam supply box 8, with which the mat 2 within the treatment gap is acted upon with steam or alternatively with a steam-air mixture is heated thereby. It is possible to provide a steam injection box 8 both on the upper side of the belt and on the lower side of the belt, so that steam is pressed into the mat from both sides. Alternatively, however, the mat can be acted upon from one side and the steam drawn off from the other side. Details are not shown. According to the invention, the preheating device 3 has an additional drying device 9, which is also integrated into the double-belt preheating device 3 and which is arranged downstream of the steam supply device 7 in the transport direction T. This drying device 9 is not used to treat the mat 2 to be pressed, but exclusively to dry the conveyor belts and consequently the sieve belts 5a, 5b. Because with the drying device 9, hot air can be applied to the sieve belts 5a, 5b in such a way that the moisture precipitated by the steam preheating on the conveyor belts 5a, 5b is absorbed by the hot air L flowing along the conveyor belts and removed with the hot air L. The drying device 9 is consequently also provided in the lead of the conveyor belts 5a, 5b and consequently between the inlet and outlet in the area of the press mat. The drying device 9 has an upper hot air tunnel 12a and a lower hot air tunnel 12b, each hot air tunnel having an air inlet 13a, 13b at one end of the tunnel and an air outlet 14a, 14b at the opposite end of the tunnel. Through the hot air tunnel 12a, 12b hot air L with z. B. a temperature of 130 ° C and a relative humidity of a maximum of 5%, preferably a maximum of 2% on the sieve belts 5a, 5b, in such a way that this hot air flows along the sieve belts 5a, 5b for the purpose of drying without to penetrate into the pressed material mat. For this purpose, the construction and mode of operation of the drying device are designed so that the hot air L moves along the conveyor belts 5a, 5b at a relatively low flow speed, the flow speed in a main flow direction which is oriented approximately parallel to the transport direction T, preferably less than 3 m / sec. amounts. This ensures that there is a relatively long contact time and, consequently, interaction time between hot air L and conveyor belt 5a, 5b to absorb the moisture, this contact time e.g. B. can be 3 seconds to 5 seconds. The length X of the hot air section, which is defined by the length X of the upper hot air tunnel 12a and the lower hot air tunnel 12b, is relatively large, so that a correspondingly long interaction time is achieved taking into account the flow velocity mentioned.

Furthermore, it can be seen in the figures that each hot air tunnel 12a and 12b each have a plurality of blocking elements, e.g. B. partitions 15 which reduce the flow cross-section and are consequently aligned transversely to the flow direction. In the area of the conveyor belts 5a, 5b, these blocking elements or partition walls 15 are interrupted, so that throughflow openings 16 are formed in the immediate vicinity of the conveyor belts 5a, 5b. On the one hand, this ensures that the hot air L is reliably guided directly along the conveyor belts 5a, 5b. In addition, there is an increased speed in the area of the constrictions or through-flow openings 16 and thus turbulence, which leads to good mixing of the air over the height of the tunnel. Although it is primarily a matter of direct hot air application to the conveyor belts, the tunnels 12a, 12b have a relatively large height H, so that sufficient air is available for thorough mixing and thus for proper removal of the moisture from the conveyor belts. The construction with the blocking elements 15 and the throughflow openings 16 results in an improved mixing and thus the drying effect. The tunnels 12a, 12b are moreover constructed in such a way that the hot air can of course come into contact with the conveyor belts to a large extent. The housing-like resp. box-like tunnels 12a, 12b formed with openings on their sides facing the conveyor belts and consequently the mat or formed entirely open. The tunnel wall directly adjoining the respective screen belt 5a, 5b is consequently preferably provided with a large number of openings. You can z. B. be designed as a grid plate or be equipped with grid plates. Details are not shown in the figures.

The hot air L is preferably supplied as ambient air or removed from the ambient air, e.g. B. from the production hall in which the press system is located. Such ambient air typically has a temperature of 10 ° C to 50 ° C at atmospheric pressure. The ambient air is heated to the desired temperature of e.g. B. 130 ° C heated, z. B. with a steam-air heat exchanger, not shown, and this results in a very significant reduction in the relative humidity to the specified values, so that the hot air has an excellent moisture absorption capacity. The hot air is guided by suitable means and fed into the respective tunnel 12a, 12b via the air inlet 13a or 13b at a relatively low pressure of less than 5 millibars and discharged at the opposite end of the tunnel via the outlet 14a, 14b. In the exemplary embodiment shown, the hot air flows along the conveyor belts in a main flow direction which is opposite to the transport direction T of the conveyor belts, ie the air is applied in countercurrent. It is always of particular importance that the hot air is not pressed into the mat, but that both the supply of the hot air and the corresponding discharge of the hot air are arranged on one and the same side of the mat 2, so that the hot air flows along the conveyor belt . The figures also show that the double-belt preheating device 3 in a preferred embodiment is additionally equipped with a compacting device 10, which is arranged downstream of the drying device 9, but which is also integrated into the double-belt preheating device 3. With this compacting device 10, the mat is compressed (in advance) in order to displace air from the mat, so that the subsequent pressing process in the press 4 is optimized and the risk of blowouts is reduced. In this way, an increased feed rate of the system and thus a higher profitability is achieved. The compacting device 10 has an upper compacting roller 11 and a lower grain compacting roller 11, the compacting rollers 11 acting on the pressed material mat 2 with the interposition of the respective screen belt 5a, 5b. One of the compacting rollers 11, e.g. B. the upper compacting roller 11 can with suitable power means, for. B. be equipped with one or more press cylinders 19.

The preheating device 3 is additionally equipped with belt cleaning devices 18 with which the conveyor belts 5a, 5b behind the outlet A are cleaned. Such belt cleaning devices 18 can, for. B. be equipped with an air supply and / or with a brush and / or suction. Details are not shown. In the case of belt cleaning with air admission, this air admission in return R must be distinguished from the hot air admission in the course of drying and consequently in curve V.

Furthermore, an additional belt heating 17 of the conveyor belts 5a, 5b is provided in the double belt preheating device 3, namely in the return R of the conveyor belts before the inlet E of the device, ie the conveyor belts 5a, 5b are before contact with the pressed material mat 2 and consequently also preheated before the application of steam, for example by the application of hot air to keep the amount of steam condensing on the conveyor belt low from the outset.

In addition, suitable devices for recording, monitoring, controlling and / or regulating the essential parameters, e.g. B. the temperature, pressure or humidity. Some sensors are indicated by way of example in FIG. 3. The temperature of the respective conveyor belt 5a or 5b can be measured with the temperature sensor 19, preferably in the return R. Furthermore, the humidity of the conveyor belt 5a or 5b can be measured with a sensor 20, e.g. B. in the return R. At least one temperature sensor and / or one pressure sensor can be integrated into the steam supply device 7, which are only indicated with the reference number 22 in FIG. Furthermore, one or more temperature sensors 21 can be provided for monitoring the temperature of the hot air, with which the temperature of the hot air can be measured in particular in the area of the air inlet 13a or 13b. The sensors in the upper part of the double-belt preheating device 3 are shown in FIG. 3 merely by way of example. Of course, corresponding sensors can be provided in the lower part in the same way.

Claims

Patent claims:

1. A method for preheating a pressed material mat (2) before it enters a press (4), in particular during the production of wood-based panels, the pressed material mat (2) having a double-belt preheating device (3) with an upper, endless, gas-permeable conveyor belt (5a) and a lower, endlessly circulating, gas-permeable conveyor belt (5b) runs through, wherein the pressed material mat (2) in the double-belt preheating device (3) is subjected to steam or a steam-air mixture by means of at least one steam supply device (7) and is thereby heated, characterized in that at least one of the conveyor belts (5a, 5b) or both conveyor belts (5a, 5b) in the double-belt preheating device (3) after

Steam preheating by means of at least one drying device (9) downstream of the steam supply device (7) in the transport direction (T) is exposed to hot air (L) in such a way that the moisture precipitated by the steam preheating on the conveyor belt or belts (5a, 5b) is removed from the is received on the conveyor belt or belts (5a, 5b) flowing along hot air (L) and discharged with the hot air.

2. The method according to claim 1, characterized in that the hot air (L) has a temperature of more than 110 ° C, preferably more than 120 ° C, particularly preferably more than 130 ° C.

3. The method according to claim 1 or 2, characterized in that the hot air has a relative humidity of a maximum of 10%, preferably a maximum of 5%, particularly preferably a maximum of 2%.

4. The method according to any one of claims 1 to 3, characterized in that the conveyor belts (5a, 5b) are acted upon by the hot air (L) in such a way that the hot air flows along the conveyor belts without penetrating the mat.

5. The method according to any one of claims 1 to 4, characterized in that the flow speed of the hot air (L) parallel to the transport direction (T) is less than 5 m / s, preferably less than 3 m / s. amounts.

6. The method according to any one of claims 1 to 5, characterized in that the contact time between hot air (L) and conveyor belt is at least 2 seconds, preferably at least 3 seconds, e.g. B. 3 to 10 seconds, particularly preferably 3 to 5 seconds.

7. The method according to any one of claims 1 to 6, characterized in that the hot air (L) flows along a hot air section with a length (X) of at least 1 m, preferably of at least 2 m along the conveyor belts (5a, 5b).

8. The method according to any one of claims 1 to 7, characterized in that the hot air (L) flows along the conveyor belts (5a, 5b) in a flow direction opposite to the transport direction (T).

9. The method according to any one of claims 1 to 8, characterized in that the pressed material mat (2) is compacted with a compacting device (10) downstream of the drying device (9).

10. The method according to any one of claims 1 to 9, characterized in that the conveyor belts (5a, 5b) are preheated before the steam is applied to the mat with a belt heating device (18) upstream of the steam supply device (7).

11. The method according to any one of claims 1 to 10, characterized in that ambient air is used for the hot air (L), which preferably has a temperature of 10 ° C to 50 ° C, which is heated to the hot air temperature while reducing the relative humidity .

12. Double-belt preheating device (3) for heating a pressed material mat (2), in particular with a method according to one of claims 1 to 11, with an upper, continuously circulating, gas-permeable conveyor belt (5a) and a lower, continuously circulating, gas-permeable conveyor belt ( 5b), between which a treatment gap is formed, through which the pressed material mat (2) can be passed along the transport direction (T), with at least one steam supply device (7), with which the pressed material mat in the treatment gap with steam or a steam-air mixture can be acted upon and thereby heated, characterized by at least one drying device (9) downstream of the steam supply device (7) in the transport direction (T), with which one of the conveyor belts or both conveyor belts (5a, 5b) is supplied with hot air can be acted upon so that the moisture precipitated or precipitated by the steam preheating on the conveyor belts (5a, 5b) is absorbed by the hot air (L) flowing along the conveyor belt or conveyor belts (5a, 5b) and removed with the hot air.

13. The device according to claim 12, characterized in that the conveyor belts (5a, 5b) are designed as sieve belts.

14. The apparatus of claim 12 or 13, characterized in that the drying device (9) has a compacting device (10) for the

Compression of the press mat (2) is arranged downstream, which preferably has an upper and a lower grain pacting roller (11), wherein the upper and / or the lower grain pacting roller with at least one power means, for. B. a press cylinder, can be acted upon or can be acted upon.

15. Device according to one of claims 12 to 14, characterized in that the steam supply device (7) for preheating the conveyor belts (5a, 5b) each one

Belt heating device (17) is arranged upstream, with which the conveyor belts (5a, 5b) can be preheated before steam is applied to the pressed material mat.

16. Device according to one of claims 1 to 15, characterized in that the conveyor belts (5a, 5b) are each assigned a belt cleaning device (18), which are preferably each equipped with an air supply and / or a brush and / or a suction device.

17. Device according to one of claims 1 to 16, characterized in that the drying device (9) has an upper hot air tunnel (12a) for heating the upper conveyor belt (5a) and a lower hot air tunnel (12b) for heating the lower conveyor belt (5b ), each hot air tunnel (12a, 12b) having an air inlet (13a, 13b) at one end of the tunnel and an air outlet (14a, 14b) at the opposite end of the tunnel.

18. The device according to claim 17, characterized in that each hot air tunnel (12a, 12b) each have a plurality of the flow cross-section reducing blocking elements, for. B. partitions (15) which form or have flow-through openings (16) in the region of the conveyor belts.

19. Plant for the production of wood-based panels, with at least one scattering device (1) for producing a pressed material mat (2), a double belt preheating device (3) according to one of claims 12 to 18 for preheating the pressed material mat (2) and a press (4) for pressing the preheated material mat (2) using pressure and heat.