Patent Publication Number: US-2019186076-A1

Title: Method For Coating A Drying Cylinder

Description:
The invention is based on a method for coating a drying cylinder of a drying apparatus, which can be part of a machine for producing and/or finishing a fibrous web such as a paper, board or tissue web. 
     Generic drying cylinders are equipped with coated surfaces. They can be assigned a heating device, by means of which the drying cylinder can be heated, and are primarily used for smoothing and/or drying the fibrous web previously produced in the machine. Depending on the application, these drying cylinders are firstly subjected to considerable corrosion because of the surroundings to which these are exposed. They come into contact directly or indirectly with the fibrous web and the additives contained therein, such as chemicals, are exposed to high ambient temperatures and must have an overall high abrasion resistance, since, indirectly or directly, the fibrous web or clothing (e.g. transport belts) carrying such a fibrous web runs on their outer circumference. 
     Recently, novel methods for coating roll cores of steel, cast steel or cast iron, e.g., cast iron with lamellar graphite (GGL), have become widespread, producing a metallic, ceramic or cermet sprayed layer on the roll core by means of methods such as HVOF or flame spraying. This process provides that, to produce the corresponding layer, a material is partly or completely melted as a spray additive, for example as a powder, wire or in another suitable form by means of the input of thermal energy and is accelerated kinetically onto the roll core to be coated. The impinging material cools down, solidifies and forms a mechanical and form-fitting connection with the roll core. The properties and possibilities of the spraying process are substantially predefined by the ratio of kinetic to thermal energy. The factor common to all the methods is that the underlying substrate, that is to say, for example, the roll core of the drying cylinder, is not melted. 
     The known methods have the disadvantage that fuels in gaseous form, such as oxygen and hydrogen, are needed to melt and accelerate the spraying additive. Said fuels must be kept ready in appropriate tanks. In dealing with these substances, there is often an acute risk of explosion. The spray burners used are additionally very loud in operation, that is to say during the coating. In addition, the reaction gases arising as a result of combustion must be laboriously extracted. All in all, high safety regulations with regard to occupational protection and occupational safety of such coating units are needed for the operating personnel, in order to avoid accidents. In addition, comparatively high kinetic pressures are needed in order to accelerate the molten spray additive to a relatively high discharge rate of the spray additive onto the substrate to be coated. This means that it is necessary for more spray additive per unit time to be delivered. A high supply rate of spray additive therefore in principle necessitates more spray additive. In particular, here extraction that is dimensioned highly in terms of output of particles of spray additive that are not melted or bounce off the substrate must be carried out. The increased discharge rate likewise in turn entails an increased energy consumption. In addition, the thermal spraying does not represent an economical or ecological method because of the poor application efficiency of the spray additive with regard to the layer thickness that can be achieved per coating pass (μm per stroke) and the percentage of the spray additive that remains on the roll. 
     Furthermore, firstly the conventional thermally sprayed coatings are merely bonded mechanically to the coated substrate, such as the drying cylinder. Therefore, the coatings obtained in this way are not particularly corrosion-resistant, as a result of “only” such bonding. This is particularly critical when chemicals are used, as explained at the beginning. In addition, although the previously used materials of the coatings achieve an enormous abrasion resistance, they are less ductile and comparatively brittle. The coatings can therefore be detached from the substrate. Secondly, until now older drying cylinders reaching the limits of their service life had to be taken out of service. The reason for this is that the existing, worn coating must be removed before re-coating. To this end, the drying cylinder is generally ground down as far as the roll core to which the coating is applied. As a result, the wall of such drying cylinders becomes thinner and thinner from coating to coating. This is particularly problematical for drying cylinders that are or can be heated with oil or steam. This is because as a rule the same safety regulations apply thereto as to pressure containers. If the wall thickness falls below a minimum value, the result is often that such drying cylinders have to be taken out of service. With the previous thermally sprayed coatings, because of the comparatively higher brittleness, said coatings could not prevent the drying cylinder being taken out of service. 
     The present invention relates to such generic subjects. 
     Accordingly, it is an object of the invention to specify an alternative method for coating a drying cylinder, which in particular, avoids the disadvantages of conventional, thermally sprayed coatings. 
     The object is achieved by a method for coating a drying cylinder having the features of claim  1 . 
     According to the present invention, the coating is carried out within the drying apparatus itself. This means that the drying cylinder is (re-)coated in the drying apparatus within the limit of the drying apparatus itself (e.g. within its housing), that is to say without having to take the same out of the drying apparatus. Here, the coating of the drying cylinder is preferably carried out in a non-operating state of the drying apparatus. This can be achieved, for example, in that the coating unit needed for this purpose is likewise arranged at least temporarily within the limit of the drying apparatus or can be brought into such a position. Expressed in another way, a system comprising a drying apparatus and the coating unit then exists, at least temporarily. 
     If, according to the present invention, mention is made of the fact that the melting at least of the applied spray additive is carried out by means of a beam, it is then generally meant that the object—in the sense of laser spraying—is firstly able to melt only the spray additive or, secondly, that the radiation source is configured in such a way that the beam—in the sense of laser cladding—in addition to the spray additive is also simultaneously able to melt the substrate to be coated, that is to say, for example, the circumferential surface of the drying cylinder to be coated or the roll body. The radiation source can be appropriately adjustable for this purpose, for example with regard to its output (e.g. energy per unit length), beam geometry, or focusing of the beam. Expressed in another way, in the case of laser cladding under the following preconditions, one could also say: if the substrate to be coated, e.g. the roll body (or the circumferential surface of the drying cylinder to be coated) and the coating to be applied thereto are at least partly made of metal, then these can form an alloy as a result of the laser cladding. This is because the energy or output of the radiation source can be dimensioned such that the temperature of the melt made of both molten metals lies above the highest occurring temperature of the liquid line in a state diagram of an alloy to be formed from these two metals. 
     A radiation source is configured for laser cladding if it is capable of partly melting the circumferential surface, e.g. the roll core, the adhesive or functional layer arranged thereon during the production of the coating according to the invention. 
     Application direction means the spatial direction of the beam, which extends away from the radiation source to the circumferential surface to be coated. It can be described by three-dimensional vectors in a Cartesian coordinate system, for example. If, after leaving the radiation source, the beam is deflected in the direction of the circumferential surface to be coated, then the term application direction means the path actually traced (beam path) of the beam. 
     The drying apparatus according to the present invention dries and/or smoothes a fibrous web in intended operation (operating state). It can be part of a machine mentioned at the beginning for producing and/or finishing a fibrous web such as a paper, board or tissue web. For this purpose, it has at least one drying cylinder. Preferably, a plurality of drying cylinders is provided. These are arranged in the running direction of the fibrous web to be dried and/or to be smoothed and running through the same, parallel to and at a distance from one another with regard to its longitudinal axes. In the last-named case, the fibrous web, in each case viewed in the running direction, wraps around each individual drying cylinder alternately in a corresponding wrap area. 
     In intended operation, the at least one drying cylinder is therefore at least partly wrapped around by the fibrous web. In the sense of the present invention, a drying cylinder means a heatable or heated drying cylinder, such as a Yankee cylinder. However, the term drying cylinder can also include a calender roll. A factory-fresh drying cylinder produced for the first time generally has a roll core and a coating applied directly thereto. The coating wears over time, so that re-coating becomes necessary. If therefore, according to the invention, mention is made of drying cylinders, a drying cylinder having a roll core and a (new or existing, e.g. partly worn) coating is always meant thereby. Such drying cylinders for treating a fibrous web measure several meters both in length and also in diameter. 
     In a non-operating state of the drying apparatus, that is to say when the drying apparatus is taken out of operation, for example with the aim of maintenance, drying and/or smoothing of the fibrous web is not possible. The fibrous web is no longer led around the drying cylinder and no longer wraps around the same. The coating of the drying cylinder according to the invention is carried out when the drying apparatus is in the non-operating state. In this state, the coating unit according to the invention has its intended state (operating state of the coating unit). On the other hand, when the drying apparatus is in its operating state, the coating unit is in turn in a non-operating state. Coating unit and drying apparatus are thus each operated oppositely to each other, viewed in their intended operation. 
     In the wrap area, during the operating state of the drying apparatus, the fibrous web is in contact with the drying cylinder directly or indirectly (e.g. via a clothing). The fibrous web runs in the same direction of rotation of the drying cylinder driven in rotation about its longitudinal axis, together with the latter. In the drying apparatus according to the invention, the wrap area is that section of the circular path of the drying cylinder—respectively starting from a stationary observer looking at the axis of rotation of the drying cylinder in side view—around which the fibrous web wraps (at least partly). The wrap area can be delimited by two removal points, at which the fibrous web is firstly passed on to the drying cylinder and secondly taken off the latter in intended operation of the drying cylinder. The at least one removal point can be formed by a press nip, which a press roll and the drying cylinder form with each other. One of the two removal points can also be formed by a crêping doctor, which is pressed onto the circumferential surface of the drying cylinder—if the drying cylinder is formed as a Yankee cylinder—in order to remove the fibrous web from the drying cylinder (to crêpe the same). 
     An area of the drying cylinder located outside the wrap area of the fibrous web will also be called a coating application area here below. In the coating application area, the fibrous web is not in contact with the drying cylinder. As seen in a plan view of the longitudinal axis of the drying cylinder, it is, for example, that remaining part of the circular arc of the drying cylinder which preferably results after subtraction of the part circular arc that is bounded by the wrap area from the full circle of the drying cylinder, in each case viewed in the aforementioned side view. For instance, the coating application area can be that area which corresponds to the circular arc over which the fibrous web sweeps, as seen in the direction of rotation of the drying cylinder. In the aforementioned plan view, this is generally the circular arc which, in the direction of rotation of the drying cylinder, is covered between the two aforementioned removal points (press nip and crêping doctor). The fibrous web is guided through the drying apparatus over its length. It is supported on the drying cylinder over its entire width in the wrap area. Starting from this point, according to the aforementioned plan view of a stationary observer of the longitudinal axis of the drying cylinder the wrap area—viewed in the circumferential direction—therefore corresponds to part of the full circumferential surface of the drying cylinder. On the other hand, the coating application area corresponds to the difference of the part of the circumferential surface spanned by the wrap area from the full circumferential surface of the drying cylinder. The coating application area is also that physical area at which the beam is directed in the intended operation of the coating unit and the coating according to the invention is applied to the drying cylinder with said coating unit. As a result of this arrangement of the coating unit within the drying apparatus, the advantage results that the coating unit can remain within the drying apparatus even in its non-operating state (therefore in the operating state of the drying apparatus), without disrupting the treatment of the fibrous web. In this way, corresponding conversion work, which would lead to a stoppage of the drying apparatus, is dispensed with, which means that the operating costs of the drying apparatus are reduced. On the other hand, the coating unit can be used again immediately for subsequent re-coating. 
     In the sense of the invention, a fibrous web is to be understood to mean a laid or tangled web of fibers, such as cellulose fibers, plastic fibers, glass fibers, carbon fibers, additional materials, additives or the like. Thus, the fibrous web can be formed, for example, as a paper, board or tissue web. It can substantially comprise cellulose fibers, it being possible for small quantities of other fibers or else additional substances and additives to be present. This is left to those skilled in the art, depending on the application. 
     The coating according to the invention is preferably carried out permanently. A permanent coating in the sense of the invention is a coating carried out in the solid state (solidified) and preferably in one piece with the drying cylinder in the operating state of the drying apparatus. Thus, in the operating state of the drying apparatus, temporary coatings in liquid or pasty form, not bonded in one piece with the drying cylinder (permanently), such as paints, varnishes or lacquers which adhere to the drying cylinder (temporarily), for example as a result of chemical bonding, do not fall under the term permanent. Instead, according to the invention, as a result of the laser cladding, an integral connection (alloy) results between the drying cylinder or the roll core of the latter and the coating. Permanent also means that the coating remains firmly connected to the drying cylinder or the roll core of the latter over its service life in the operating state of the drying apparatus, and that over a multiplicity of revolutions of the drying cylinder. Permanent therefore means that, viewed over the service life, the coating does not have to be re-applied after each full revolution of the drying cylinder. The coating can be chosen in such a way that it has such a relatively high abrasion resistance with respect to objects coming into frictional connection therewith, such as the fibrous web, a clothing carrying the fibrous web or a cleaning or crêping doctor resting on the circumferential surface of the drying cylinder. The abrasion resistance can be chosen to be so high that, in the intended operating state of the drying apparatus, a service life of several hundred hours is achieved by the coating of the drying cylinder. And this without the coating having to be renewed. 
     The coating according to the invention is carried out within the drying apparatus, therefore in situ. The latter can mean that the coating of the drying cylinder is preferably carried out within the geometric limit of the drying apparatus, inside the housing wall of the drying apparatus. A coating unit according to the invention is, for example, accommodated appropriately within the housing wall of the drying apparatus. This has the advantage that the drying cylinder does not have to be removed from the drying apparatus for its re-coating. Instead, it remains in its place, which it assumes in any case in the operating state of the drying apparatus. 
     Material in the sense of the present invention is in principle understood to mean the starting material from which the coating, for example, the adhesive or functional coating, is to be produced. Spray additive means in this case that starting material which is applied to the substrate to be coated (here the roll core or the circumferential surface of the drying cylinder) for the purpose of producing the corresponding layer. 
     Main application direction of the spray additive means that direction which corresponds to the (statistically most frequently occurring) main movement component of the spray additive supplied to the circumferential surface of the drying cylinder. In the case of powders, it is the prevailing direction of the major part of the particles of the spray additive, in spray burners that are usual for spraying technology for laser or electron beam coating according to the invention, that have correspondingly known physical distributions of the particles applied thereby. The main movement component of a particle of spray additive thus accelerated is, in a Cartesian coordinate system, that of the three physical movement components of the particle which has the greatest magnitude. 
     The term “substantially counter to gravity” is intended to mean that the application direction of the beam and/or the main application direction of the spray additive is/are chosen such that it/they is/are counter to the direction of gravity, including with regard to only one movement component. The latter means that the smallest angle, e.g. which is enclosed by the application direction of the beam and/or main application direction and the direction of gravity is between 0° and ≤90°, preferably between 0° and 45°. Expressed in another way, this means that two straight lines each extending in, for example, the main application direction and direction of gravity enclose between themselves a minimum angle which lies between 0° and less than 90° or 0° and 45°, viewed with respectively opposite directions of main application direction and direction of gravity. The same is correspondingly true of the application direction of the beam. In relation to the coating unit, the latter can be arranged or set up relative to the drying apparatus in such a way that, in its installed position envisaged for the operation of the coating unit, said apparatus accelerates the spray additive in the main application direction onto the aforementioned coating area. Expressed differently, the term according to the invention means that the beam (in its extension in the application direction) and/or the spray additive always strike/s the circumferential surface of the drying cylinder below the axis of rotation of the latter. 
     The term functional layer in the sense of the present invention means a layer which comes into contact with the fibrous web directly or indirectly, for example via transport belts located between fibrous web and functional layer, such as clothings or filters running around the same. The functional layer therefore has properties which in particular avoid corrosion and abrasion to the greatest extent. 
     In the sense of the present invention, the adhesive layer is understood to be a layer which is used for adhesion promotion between the roll core, on the one hand, and a functional layer. 
     Therefore, both adhesive layer and functional layer are part of the coating applied to the drying cylinder, preferably permanent in the solid state, which is applied gradually to the roll core during the production. Functional and/or adhesive layer can in this case be built up from a plurality of individual layers. 
     In principle, in the sense of the invention, substrate—depending on the stage of the coating produced—is preferably understood to be the radially outermost material to be coated directly, such as the roll core, the adhesive layer, the functional layer or, where it makes sense, a combination thereof. 
     The term alloying region is understood to mean a metallic bond between at least two at least partly metallic materials, obtained as a result of the melting (with subsequent cooling) thereof, forming a crystalline lattice. For example, the applied coating can form a metallic compound with the roll core of the drying cylinder. Furthermore, the materials of the adhesive layer and of the roll core can be chosen such that they form such an alloying region if, during the production of the coating, they form with each other as a result of melting by the supply of energy, following solidification. In principle, the materials of the adhesive layer and of the functional layer can be chosen such that these do not form any such alloying region with each other. 
     Radiation source means a source which emits coherent light or particles, such as electrons, which can be focused to form a bundle of rays. Radiation sources in the sense of the invention are therefore lasers and electron radiation sources. Lasers of different types, such as CO 2  lasers, HDPL (high-power diode lasers) or DDL (direct diode lasers) or combinations thereof are possible. 
     According to the present invention, the term laser cladding or a method equal to the same is understood to be a coating method by means of which it is possible to begin to melt the surface to be coated—that is to say the substrate—itself (at least close to the surface). Such a high input of heat is not possible in the known thermal spraying method itself, such as for example flame-spraying, high-speed flame spraying, arc spraying or plasma spraying, which leads to incipient melting or melting of the substrate. Incipient melting or melting during laser cladding can be carried out, for example, by supplying thermal energy to the substrate to be coated and can preferably be implemented by radiation such as laser radiation. Thus, in principle, during laser cladding the material of the adhesive or functional layer (spray additive) is introduced into the beam path of the laser, is melted and applied to the substrate. At the same time, the laser beam melts the surface of the substrate at least partly with respect to the radial thickness. The advantage of this method, if it is used primarily for heated or heatable drying cylinders, is that the service life of the drying cylinder can be prolonged. This is because, by means of the laser cladding, given an appropriate choice of the spray additive—e.g. the same material as that of the roll core (or a material compatible therewith)—the wall thickness of an actually worn-out drying cylinder can be built up again. If, for example, the material of the roll core of the drying cylinder is at least partly made of metal, for example steel or cast iron, then the spray additive can be chosen appropriately and also at least partly be such a metal. The spray additive and the substrate, here the roll core of the drying cylinder, are melted together, mixed with each other and, following their cooling, form a type of welded bead (application layer) with each other. Thus, the wall thickness of an old, used drying cylinder can be increased again, so that the latter once more meets the safety requirements on pressure containers. After that, this can once more be coated in accordance with the invention. Although the term laser cladding implies the use of a laser as radiation source, the use of a radiation source emitting an electron beam is also conceivable. 
     A further advantage of the coating of a drying cylinder in accordance with the method according to the invention is that it is now possible to dispense with a highly explosive fuel in gaseous form, such as oxygen or hydrogen. This is because both the melting of the substrate and of the spray additive is now carried out alternatively by means of laser radiation. As a result, first of all the loudness of the coating method can be reduced considerably as opposed to the previously known thermal spraying method with burners. On the other hand, no more relatively high requirements on the extraction of the reaction gases and on the remnants of the spray additive are necessary, since it is possible to work with comparatively low application rates, so that it is possible to deliver less spray additive per unit time. Thus, less spray additive which is not melted gets into the drying apparatus, so that the effort for cleaning after such a coating can also be reduced considerably. 
     A gradual production of the coating is carried out if the entire circumferential surface of the drying cylinder is coated little by little, for example in a continuous spiral line. This is the case in particular when the surface over which the input of heat is carried out is considerably smaller than the entire surface of the drying cylinder that is to be coated. This is in principle the case in the generic drying cylinders when, for example, they are used in a paper-making machine. This is because, here, the applied spray additive volume flow, viewed in the width direction, is lower than the overall width of the drying cylinder. 
     To improve the adhesion of the functional layer to the roll core of the drying cylinder, according to one embodiment the functional layer can be applied to an adhesive layer lying underneath in the radial direction. The adhesive layer can then form with the roll core an alloying region with the roll core, given appropriate material pairing of roll core and adhesive layer. On the other hand, during the subsequent production of the functional layer, the adhesive layer lying directly underneath, as viewed in the radial direction, can be at least partly melted, so that functional layer and adhesive layer in turn form an alloying region with each other in the region of their transition. 
     The functional layer and/or the adhesive layer can be built up from a plurality of individual layers. The spray additive for producing the individual functional layers and/or adhesive layers can be chosen amongst each other in such a way that the layers differ. As a result, coatings tailored specially to the desired application can be achieved. 
     The spray additive for producing the functional layer can be at least partly a metal or an industrial ceramic, such as oxide ceramic. The oxide ceramic contains metal oxides which are chosen from chromium oxide (Cr 2 O 3 ), titanium oxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO 2 ), silicon oxide (SiO 2 ) yttrium oxide (Y 2 O 3 ) or mixtures thereof. As a result, a particularly abrasion-resistant coating can be specified. 
     The spray additive for producing the adhesive layer can be at least partly a metal and preferably a nickel-aluminum alloy, preferably with a mixture ratio of 95% nickel and 5% aluminum, in each case based on the weight of the mixture. This material and mixture ratio permits a particularly ductile-reacting coating, which is important for the aforementioned intended application of the invention. 
     The coating unit can be assigned its own drive, in order to drive the drying cylinder to be coated in rotation in its installed position in the drying apparatus, at least during the coating and preferably also during the preferably material-removing surface machining. This has the advantage that coating can be carried out autonomously and without the existing drive of the drying cylinder. This can be important when the coating of the drying cylinder is to be carried out with simultaneous maintenance of the drive of the drying apparatus. This has the advantage that the drying cylinder does not have to be removed. 
     The coating unit can be assigned an apparatus for the surface machining of the circumferential surface of the drying cylinder, preferably for removing an existing coating of the drying cylinder, or for machining the coating produced in accordance with the invention. The advantage thereof is that the aforementioned machining of the coating can also easily be carried out in situ in the drying apparatus, without the drying cylinder having to be removed from the drying apparatus for this purpose. 
     Furthermore, the invention relates to a coating unit and a system comprising coating unit and drying apparatus and a method for converting or producing a drying apparatus in accordance with the independent claims. The conversion is carried out in the non-operating state of the drying apparatus. As a result of the conversion, the system according to the invention comprising coating unit and drying apparatus is produced. If the conversion is made reversible by removing the coating unit from the drying apparatus, then the system comprising coating unit and drying apparatus is dissolved again. The coating unit can therefore remain within the drying apparatus only temporarily and preferably for the duration of the coating or surface machining. This can be carried out particularly simply and with little effort if the coating unit is designed such that it can be connected detachably to the drying apparatus. For this purpose, according to one embodiment, the coating unit can comprise means in order to be connectable detachably to the drying apparatus to be coated. These means can be configured in such a way that the coating unit is supported or fixed in direct contact with the drying apparatus, for example on a component of the drying apparatus, or is supported indirectly on the drying apparatus, for example via the floor on which the drying apparatus stands. These means can be, for example, cross-members, on which the coating unit is arranged such that it can be displaced along the longitudinal axis of the drying cylinder to be coated. 
     Finally, the present invention relates to a machine for producing and/or finishing a fibrous web such as a paper, board or tissue web, comprising a coating unit and a system comprising coating unit and drying apparatus. 
     Further advantageous embodiments of the invention can be gathered from the sub-claims. 
    
    
     
       The invention will be described in more detail below with reference to the drawings, without restricting generality. In the drawings: 
         FIGS. 1 a  and 1 b    show a highly schematic illustration in side view of two drying apparatuses suitable for the invention, the drying cylinders of which can be coated; 
         FIGS. 2 a  and 2 b    show a development of the embodiment of the subject of  FIG. 1   a;    
         FIG. 3  shows a highly schematic plan view of a system comprising drying apparatus and coating unit; 
         FIG. 4  shows an embodiment in a highly schematic sectioned view through a coating according to the invention. 
     
    
    
       FIGS. 1 a  and 1 b    show two embodiments of the invention in a schematic side view of an operating state of the drying apparatus. In both figures, a part of a drying apparatus of a machine for producing and/or finishing a fibrous web F, such as a paper, board or tissue web, is respectively illustrated. In both cases, the fibrous web F is formed in the machine, dewatered and then transferred to the drying apparatus for the drying and/or smoothing thereof. 
     In  FIG. 1 a   , the drying cylinder  1  is embodied as a Yankee cylinder. The fibrous web F formed in the machine is transferred here to the drying apparatus at a first removal point by a transport belt by means of a press roll  10 . The first removal point is formed here by a press nip, which the press roll  10  and the drying cylinder  1  form with each other. The drying cylinder  1  is of heated design. Also assigned thereto is a dryer hood  11 , in order to additionally dry the fibrous web F. After the fibrous web F has left the dryer hood  11 , it is removed from the drying cylinder  1  at a second removal point. This is carried out in this case by a crêping doctor  12 , which is pressed against the circumferential surface of the drying cylinder. As seen in the running direction of the fibrous web F through the drying apparatus (and in the direction of rotation of the drying cylinder  1 ), the crêping doctor  12  is arranged after the press roll  10 . In the further sequence, the crêped fibrous web F is wound up. In the direction of view in  FIG. 1 a   , the fibrous web is thus always in direct contact with the rotating drying cylinder and its circumferential surface only within the wrap area illustrated dashed. Expressed in another way, the wrap area is thus bounded in the direction of rotation of the drying cylinder by the first removal point and the second removal point. 
     According to  FIG. 1 b   , the fibrous web F is led over a plurality of drying cylinders  1  arranged one after another in the running direction of the fibrous web F. Said drying cylinders are arranged parallel to and at a distance from one another with regard to their longitudinal axes. In the present case, the drying cylinders directly adjacent to one another are spaced apart from one another horizontally and also vertically. As a rule, the fibrous web F does not come directly into contact with the drying cylinders  1  but is carried by a clothing (not illustrated). In this case, the clothing rests directly on the respective drying cylinder  1 . Here, the fibrous web F also wraps partly around the respective drying cylinder  1  in a respective wrap area (illustrated dashed). 
     The invention will now be explained in more detail by using the embodiment of  FIG. 1 a   . In principle, the explanations made also apply in a corresponding way to the embodiment of  FIG. 1 b   . To this end, the operating state is illustrated in  FIG. 2 a    and the non-operating state of the drying apparatus in  FIG. 2 b   . In the last-named case, the fibrous web F does not run through the drying cylinder  1 , as is shown in  FIG. 2 a   . Instead, only the path traced by the fibrous web F in the operating state of the drying apparatus is indicated dashed. 
     In order to coat the drying cylinder  1 , without removing the latter from the drying apparatus, it is coated in the drying apparatus itself. To this end, the drying apparatus is set into the non-operating state according to  FIG. 2 b    and possibly converted appropriately for the first time for the coating. For the conversion, the drying apparatus is provided with a coating unit  13 . The latter is arranged in an installed position in the drying apparatus, which is located outside the wrap area of the fibrous web F that is present in the operating state of the drying apparatus—thus close to the coating application area—here, therefore, underneath the drying cylinder  1 , between the two first removal positions, namely the crêping doctor  12  and the press roll  10 . If the coating unit  13  is installed at the aforementioned point in the drying apparatus, then the system according to the invention comprising the two is achieved. The coating unit  13  can thus be installed retrospectively in existing drying apparatuses. It can remain there permanently or else only temporarily for the duration of the coating or a surface treatment of the drying cylinder  1 , carried out previously or following thereafter. In the last-named case, it is then advantageous if the coating unit  13  can be connected detachably to the drying apparatus via means. These means include, for example, the cross-member  14  on which the coating unit  13  is supported. The cross-member  14  can be supported indirectly on the drying apparatus, for example via the floor on which the latter stands. However, the cross-member  14  could also be connected firmly, that is non-detachably, to the floor or the drying apparatus, therefore being part of the drying apparatus. 
     In order at the same time to remove a possibly already existing coating from the drying cylinder  1  within the drying apparatus or to re-machine the finished coating once more, the coating unit  13  can also be assigned a device for surface treatment  6 . This is likewise provided in the area of the aforementioned installation position of the coating unit  13 . The former can be embodied as a grinding machine and/or for blasting, such as shot-blasting of the drying cylinder  1 . In addition, the device  6  can be detachably supported on the drying apparatus via appropriate means, such as the cross-member  14  or via a dedicated cross-member (not shown). 
     Because the coating unit  13  is installed in the drying apparatus at the aforementioned point, neither the dryer hood  11  nor the crêping doctor  12  nor the press roll  10  has to be removed for the coating. In other words, the space which is normally free and which results outside the wrap area is provided for the installation of the coating unit  13 . The latter can therefore also remain at the point in operation of the drying apparatus, since it does not influence treatment of the fibrous web F by the drying apparatus. 
     As soon as the coating or retrospective surface treatment of the drying cylinder  1  has been completed, the system comprising the drying apparatus and coating unit  13  can be dissolved again. To this end, the coating unit  13  is removed from the drying apparatus again, so that the arrangement shown in  FIG. 2 a    results again. 
     In  FIG. 3 , the coating unit  13  is shown in detail in a highly schematic plan view. The longitudinal axis of the drying cylinder  1  extends in the drawing plane here. 
     The drying cylinder  1  rotating about its longitudinal axis is driven suitably. The coating unit  13  comprises an application device  7  supported on the cross-member  14 , displaceable to and fro relative to the drying cylinder  1 , parallel to the longitudinal axis thereof (see the double arrow). Said application device comprises a material supply  8  that can be optionally connected and disconnected for supplying a spray additive (indicated dotted), a beam  9  which is emitted by a radiation source, not illustrated, into which the spray additive is put, and also a protective gas supply, not shown, for supplying protective gas to the drying cylinder  1 . The application device  7  can be embodied as a spray burner. The spray additive in the present case is present in the form of powder. By means of the coating unit  13 , the entire surface of the drying cylinder  1  can be coated gradually, for example in a continuous spiral line. However, it is also possible to apply the coating in another way, for example in radial rings or axial stripes. Of course, it would be conceivable that multiple such application devices  7  could be arranged at a distance from one another along the cross-member  14  and the longitudinal axis of the drying cylinder  1 . In this way, the coating process could be carried out considerably more quickly. 
     The application of protective gas is indicated in the present case by the cone of the beam  9 . The protective gas can serve to entrain and/or accelerate the material such as spray additive, which is introduced into the beam path of the radiation source for the melting. The spray additive accelerated by means of protective gas and incipiently melted or melted is thrown onto the drying cylinder  1  to be coated, here for example the naked (that is to say freed of an existing coating) roll core  2  machined by means of the device  6  (see  FIG. 4 ). If the surface of the roll core  2  is also melted, then the melted spray additive also gets into the melt of the roll core  2 . The application device  7  can be designed in such a way that protective gas, beam  9  and spray additive emerge together, for example concentrically, from one and the same spray burner. In that case, the beam can be coupled into the spray burner, so that at least the longitudinal axis of the beam and the main application direction of the spray additive coincide. 
     According to the illustration in  FIG. 2 b   , the spray additive is thrown out of the application device  7  in the main application direction. Both the main application direction of the spray additive and the application direction of the beam  9  extend counter to the direction of gravity in the view illustrated, the latter extending along a vertical through the longitudinal axis of the drying cylinder  1  here. Expressed in another way, the spray additive is applied to the drying cylinder  1  from below—underneath the longitudinal axis/axis of rotation of the drying cylinder  1 , counter to gravity. It would also be conceivable for both beam  9  and spray additive to arrive from different directions, so that the application direction of the beam  9  is at an angle to the main application direction of the spray additive. Thus, for example, the spray additive can be brought tangentially up to the circumferential surface of the drying cylinder  1 , while the beam  9  still extends counter to gravity and therefore at an angle to the main application direction of the spray additive. 
     In  FIG. 4 , one embodiment of the present invention is illustrated in a highly schematic sectional view in a partial cross section at right angles to the longitudinal axis through the drying cylinder  1  from  FIG. 1 a    or  1   b . To simplify the illustration, the roll curvature has been disregarded. It should likewise be noted that the thickness, seen in the radial direction of the individual layers of the roll  1 , is not illustrated to scale. The intention is merely to symbolize the sequence of the layers. The respectively illustrated drying cylinder  1  usually has a radially inner roll core  2 , which can at least partly be produced from a metal such as steel. 
     In the present case, the coating  5  according to the invention is applied directly in the radial direction to the roll core  2 , to its circumference. It comprises a functional layer  4 . This can be applied directly to the roll core  2  or, as illustrated dashed, arranged on at least one adhesive layer  3 . The adhesive layer  3  is used for adhesion promotion between the roll core  2  and the layer following next toward the outside in the radial direction, here the functional layer  4 . The adhesive layer  3  is advantageously chosen when the coating  5  is intended to have improved adhesion between functional layer  4  and roll core  2 . Although this is not illustrated, on the one hand the roll core  2  and the adhesive layer  3  and/or the adhesive layer  3  and the functional layer  4  could together form an alloying region. 
     Seen in the radial direction of the drying cylinder  1 , the functional layer  4  is applied to the at least one adhesive layer  3 . The entire coating  5 , in each case produced in accordance with the embodiment illustrated, can be produced by means of the coating unit  6 . 
     LIST OF DESIGNATIONS 
     
         
         
           
               1  Drying cylinder 
               2  Roll core 
               3  Adhesive layer 
               4  Functional layer 
               5  Coating 
               6  Device for surface treatment 
               7  Application device 
               8  Material supply 
               9  Beam 
               10  Press roll 
               11  Dryer hood 
               12  Crêping doctor 
               13  Coating unit 
               14  Cross-member 
             F Fibrous web