Abstract:
In order to produce a method of providing a workpiece with a coating comprising the following process steps: —coating the workpiece: and —drying the workpiece by means of a drying device which is of increased capacity and is also particularly suitable for very long workpieces, there is proposed a method wherein the workpiece is moved relative to the drying device after the process of coating the workpiece has begun and before the process of drying the workpiece has terminated.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation application of PCT/EP2010/056869 filed on May 19, 2010, the entire specification of which is incorporated herein by reference. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    The present invention relates to a method for providing a workpiece with a coating comprising the following processing steps:
       coating the workpiece; and   drying the workpiece by means of a drying device.       
 
       BACKGROUND 
       [0005]    It is known for a workpiece that is to be provided with a coating to be introduced into a processing booth and for the workpiece to be arranged statically in the processing booth whilst being coated manually or by means of automatic coating devices. Subsequently, the processing booth is heated in order to dry the coated workpiece. After the workpiece has been dried and the processing booth subsequently cooled, the workpiece is removed from the processing booth. During the entire phase of operation, which may also include a pre-treatment phase and an evaporation phase, the workpiece does not move relative to the processing booth which is also serving, inter alia, as a drying device. 
         [0006]    In this known method, the overall processing time for a workpiece is made up of the combined times needed for the coating of the workpiece, the heating of the processing booth, the drying of the workpiece and the cooling of the processing booth, this thus limiting the capacity of the coating plant and the coating process being carried out therein. 
       SUMMARY OF THE INVENTION 
       [0007]    The object of the present invention is to produce a method for providing a workpiece with a coating of the type mentioned hereinabove which is of increased capacity and is also particularly suitable for very long workpieces. 
         [0008]    In accordance with the invention, this object is achieved in the case of a method incorporating the features according to the preamble of claim  1  in that the workpiece is moved relative to the drying device after the process of coating the workpiece has begun and before the process of drying the workpiece has terminated. 
         [0009]    The present invention is thus based on the concept of not keeping the workpiece static relative to the drying device during the entire phase of operation comprising the processes of coating and drying the workpiece but rather of moving it relative to the drying device at least in the period between the beginning of the process of applying the coating to the workpiece and the conclusion of the process of drying the workpiece. 
         [0010]    It is thereby possible to significantly reduce the total amount of time needed for the treatment of a workpiece. 
         [0011]    In connection therewith, the relative movement between the workpiece and the drying device can be produced by a movement of the workpiece whilst the drying device is stationary, by a movement of the drying device whilst the workpiece is stationary or else by means of a simultaneous movement of the workpiece and the drying device. 
         [0012]    In a special embodiment of the method in accordance with the invention, the workpiece is moved both during the coating process and during the drying process. 
         [0013]    In connection therewith, provision may be made, in particular, for the workpiece to be advanced in a continual process, either intermittently or preferably continuously, through a coating zone and a drying zone located behind the coating zone in the direction of advancement of the workpiece. It is thereby possible to carry out the processes of coating and drying the workpiece simultaneously at different parts of the workpiece whereby a very considerable reduction in the total processing time that is necessary for each workpiece is obtained. 
         [0014]    Moreover, the continual process enables both the coating zone and the drying zone to be made significantly shorter than the workpiece that is to be coated. 
         [0015]    If, apart from the process of coating and the process of drying, yet further processing steps are effected on the workpiece such as a pre-treatment process and/or an evaporation process for example, then these additional processes can likewise be carried out in separate zones, thus for example in a pre-treatment zone and in an evaporation zone having a length which can be shorter than the length of the workpiece that is to be coated. 
         [0016]    A significant saving in energy is obtained due to the smaller booth size of the individual zones and the consequential reduction in the volume of air flowing through these zones. 
         [0017]    Furthermore, it is no longer necessary to heat the entire processing booth for the purposes of drying the workpiece and then having to cool it down again after the workpiece has been dried, this likewise contributing to a significant saving in energy. 
         [0018]    Furthermore, the coating plant used for carrying out the method does not have to be dimensioned in dependence upon the size of the workpiece, but can be designed in dependence on the desired throughput. 
         [0019]    In a further embodiment of the method, provision is made for the workpiece to be moved, preferably continuously, after the process of coating the workpiece is concluded and whilst the workpiece is being dried. 
         [0020]    In this case in particular, provision may be made for the process of drying the workpiece to take place in the continual operational mode, whilst the process of coating the workpiece is effected whilst the workpiece is stationary. 
         [0021]    In this case, further processing steps, such as a pre-treatment process and an evaporation process in particular, can also be carried out whilst the workpiece is stationary. 
         [0022]    In a further embodiment of the method, provision is made for the drying device to be moved, preferably continuously, whilst the workpiece is being dried. 
         [0023]    In this case, the coating process can be carried out with a stationary workpiece. 
         [0024]    In this case too, further processing steps such as a pre-treatment process and an evaporation process for example, can also be carried out whilst the workpiece is stationary. 
         [0025]    The workpiece can also remain stationary during the drying process. Since, in this case, the workpiece is not being advanced in the continual operational mode whilst it is being processed in the processing booth, a conveyor device of simpler construction can be used for advancing the workpiece since lesser demands are then imposed on the quiet running of the workpiece conveyance process. 
         [0026]    In a further embodiment of the method, provision may be made for the workpiece to be moved relative to the drying device after the process of coating the workpiece is concluded but before the process of drying the workpiece begins. 
         [0027]    In this case, both the process of coating the workpiece and that of drying the workpiece can be effected whilst the workpiece is stationary. It is sufficient for the workpiece to move from a coating zone into a drying zone between the coating process and the drying process. Since, in this case, no processing steps are being performed on a moving workpiece, the conveyor device for advancing the workpiece can be of simpler construction than it would be for continual operation due to the fact that lesser demands are being imposed on the smooth running of the workpiece moving process. 
         [0028]    In a preferred embodiment of the invention, the workpiece is advanced substantially continuously through a processing booth of the coating plant during the coating process and/or during the drying process. 
         [0029]    In this connection, the speed at which the workpiece is conveyed preferably amounts to between 0.2 m/min and 1 m/min. 
         [0030]    In particular, the workpiece can be moved relative to the drying device by means of a track-guided workpiece carriage. 
         [0031]    The conveyor device in the coating plant used for carrying out the method in accordance with the invention preferably comprises a longitudinal conveyor track along which the workpiece carriage is movable in a longitudinal direction of advancement, and a transverse conveyor track along which the workpiece carriage is movable in a transverse direction of advancement running transversely relative to the longitudinal direction of advancement. 
         [0032]    In order to enable the workpiece carriage to move outside of a processing booth of the coating plant and back from the exit of the processing booth to the entrance of the processing booth, the conveyor device preferably comprises a return conveyor track over which the workpiece carriage is moveable in the reverse direction from an end of the longitudinal conveyor track to the beginning of the longitudinal conveyor track. 
         [0033]    Preferably the return conveyor track runs outside the processing booth of the coating plant. 
         [0034]    In particular, the return conveyor track can run substantially parallel to the longitudinal conveyor track. 
         [0035]    In a preferred embodiment of the method in accordance with the invention, provision is made for the workpiece to be moved by means of a self-propelled workpiece carriage. 
         [0036]    In order to supply a drive device of the self-propelled workpiece carriage with the requisite electrical energy, provision may be made for the conveyor device of the coating plant to be provided with a device for transmitting energy to the workpiece carriage in non-contact-making manner. 
         [0037]    As an alternative or in addition to such a process of transmitting energy in non-contact-making manner, provision may be made for the workpiece carriage to have a storage means for electrical energy, in particular, an accumulator. 
         [0038]    In order to enable the workpiece carriage to move in both a longitudinal direction of advancement and in a transverse direction of advancement running transversely relative thereto, it is expedient for the workpiece carriage to be moved by means of a first set of running wheels in a first direction and by means of a second set of running wheels in a second direction which runs transversely relative to the first direction. 
         [0039]    In particular, the workpiece carriage may have longitudinally running wheels for the longitudinal transportation process in a longitudinal direction of advancement and transversely running wheels for a transverse transportation process in a transverse direction of advancement running transversely relative to the longitudinal direction of advancement. 
         [0040]    Preferably, the longitudinally running wheels and/or the transversely running wheels are arranged on the workpiece carriage in height adjustable manner so that the workpiece carriage can be switched over from the longitudinal transportation mode to the transverse transportation mode or from the transverse transportation mode to the longitudinal transportation mode by lowering or raising these running wheels. 
         [0041]    In order to achieve the effect that the workpiece carriage runs as smoothly as possible, provision may be made for the workpiece carriage to be guided on at least one rail which has a curved, preferably convex, bearing surface. 
         [0042]    In particular, such a rail can be in the form of a round rail. 
         [0043]    In this case, the workpiece carriage preferably has at least one running wheel which has a curved, preferably concave, running surface along the periphery thereof that is complementary to the curved bearing surface of the rail. 
         [0044]    In order to prevent the rail on which the workpiece carriage is being guided from becoming contaminated, the rail in the coating zone of the coating plant is preferably separated by a shielding element, for example a casing, from an application area of the coating zone within which the coating material is applied to the workpiece. 
         [0045]    In a preferred embodiment of the invention, provision is made for the workpiece to be pre-treated before the coating process in order to activate the workpiece surface that is to be coated. 
         [0046]    In connection therewith, provision may be made for the workpiece to be pre-treated prior to the coating process by means of a vacuum suction-jet device which is preferably fixed on a robot-guided or automatic carriage unit. 
         [0047]    In such a vacuum suction-jet device, an abrasive medium is blasted onto the surface of the workpiece and immediately sucked off again under a hood which is held on the workpiece by suction so that no dust is produced outside the hood. 
         [0048]    As an alternative or in addition thereto, provision may be made for the workpiece to be pre-treated prior to the coating process by means of a robot-guided brushing system incorporating a suction device. 
         [0049]    The process of coating the workpiece can be effected, in principle, using any form of coating material. 
         [0050]    Preferably, a lacquer and in particular a solvent-free lacquer such as a water lacquer for example is used for the coating process. 
         [0051]    The process of coating the workpiece is preferably effected in a coating zone in which excess coating material is picked up by an air stream, whereby the superfluous coating material is separated out of the air stream by means of a separation device. 
         [0052]    This separation device is preferably in the form of a dry separation device. 
         [0053]    In particular, such a dry separation device can comprise filter elements that are coverable with a pre-coat material. 
         [0054]    On the pre-coat layer consisting of such a pre-coat material, stone dust for example, sticky particles in the coating material can settle. 
         [0055]    As an alternative or in addition thereto, the dry separation device may comprise labyrinth filters for the separation of the coating material. 
         [0056]    The process of drying the workpiece can be effected by supplying warm air to the workpiece for example. 
         [0057]    The drying device by means of which the workpiece is dried can be in the form of a convection dryer for example. 
         [0058]    As an alternative or in addition thereto, provision may be made for the coating produced on the workpiece to be dried and/or hardened at least partially by means of an irradiation unit. 
         [0059]    In connection therewith, the irradiation unit can emit infrared radiation and/or UV radiation (in the case of a coating that is curable by UV irradiation) for example. 
         [0060]    Furthermore, a cooling device for cooling the at least one irradiation device can be provided. 
         [0061]    The at least one irradiation unit can be fixed in place in a drying zone. 
         [0062]    As an alternative thereto, provision may be made for the at least one irradiation unit to be moveable, preferably in the longitudinal direction of a processing booth of the coating plant, so that a larger area of the surface of the workpiece can be swept over by this irradiation unit without the workpiece itself having to be moved relative to the irradiation unit. 
         [0063]    Particularly in the case where the cross sectional geometry of the workpiece varies in the longitudinal direction thereof, it is of advantage for the irradiation unit to comprise at least one irradiation device which is movable relative to the coated surface of the workpiece so that the distance thereof from the coated surface of the workpiece is adjustable in a variable manner. In this way, the position of the irradiation device can be adapted to the varying cross sectional geometry of the workpiece thereby achieving a uniform intensity of irradiation on all the surfaces of the workpiece. 
         [0064]    The air being supplied to the processing booth of the coating plant is preferably fed through an air re-circulating system thereby achieving a considerable saving of energy because it is not then necessary to constantly warm up fresh air to the temperature needed in the processing booth. 
         [0065]    Furthermore, it is expedient for the coating plant to comprise a plurality of air re-circulating systems so that the atmosphere in the booth can be conditioned in differing manners in the different air re-circulating systems in dependence upon the requirements. 
         [0066]    In particular, it is expedient for there to be a first air re-circulating system which feeds a supply of air to a pre-treatment zone, and a second air re-circulating system which feeds a supply of air to a coating zone so that the air supply for the pre-treatment zone on the one hand and that for the coating zone on the other are conditionable in different manners. 
         [0067]    Furthermore, it is expedient for the processing booth to comprise a plurality of mutually separated zones to each of which the air is supplied via its own air supply line because the quantity of air being supplied in this way to a particular zone can then be precisely adapted to its particular requirements. A further saving of energy is obtained by such a sectionalised air supply system. 
         [0068]    The process of drying the workpiece is preferably effected in a drying zone having a longitudinal extent in the direction in which the workpiece is being moved relative to the drying device which is smaller than the longitudinal extent of the workpiece in this direction. 
         [0069]    In like manner, it is expedient for the process of coating a workpiece to be effected in a coating zone having a longitudinal extent in the direction in which the workpiece is being moved relative to the drying device which is smaller than the longitudinal extent of the workpiece in this direction. 
         [0070]    Preferably, all the processing steps that are to be carried out on the workpiece in the coating plant are effected fully automatically when the plant is operating normally. 
         [0071]    However, provision can be made for the processing booth of the coating plant to comprise at least one reserve zone or back-up zone in which it is feasible for manual treatment of the workpiece to be effected. 
         [0072]    In this way, it is then possible for the workpiece to be properly coated even if an automatically operated processing zone preceding it in the direction of advancement of the workpiece fails or if a defective product is produced so that a manual touching-up process is necessary. 
         [0073]    In particular, provision may be made for such a reserve zone or back-up zone to be arranged between a pre-treatment zone and a coating zone of the coating plant. 
         [0074]    As an alternative or in addition thereto, provision may also be made for such a reserve zone or back-up zone to be arranged between a coating zone and a drying zone of the coating plant. 
         [0075]    The coating process in accordance with the invention is suitable in particular for the coating of very long workpieces and especially elongated workpieces wherein the longitudinal extent thereof is significantly greater than the maximum extent thereof in a transverse direction running perpendicularly with respect to the longitudinal direction of the workpiece. 
         [0076]    Preferably, the longitudinal extent of the workpiece is at least five times greater than the maximum transverse extent of the workpiece. 
         [0077]    The workpiece that is to be coated is preferably an individual part of a given longitudinal extent, i.e. not a tape-like material of indefinite length. 
         [0078]    Furthermore, the present invention relates to a coating plant for providing a workpiece with a coating wherein said plant comprises at least one coating unit by means of which the workpiece is providable with a coating, and at least one drying device by means of which the coating on the workpiece is dryable. 
         [0079]    The further object of the present invention is to provide such a coating plant which is of increased capacity and is particularly suitable for the coating of very long workpieces. 
         [0080]    In accordance with the invention, this object is achieved in the case of a coating plant incorporating the features of the preamble of claim  16  in that the coating plant comprises at least one moving device by means of which a relative movement between the workpiece and the drying device is producible after the process of coating the workpiece has begun and before the process of drying the workpiece is terminated. 
         [0081]    Such a coating plant is particularly suitable for carrying out the method in accordance with the invention. 
         [0082]    The moving device can be constructed in such a way as to cause the workpiece to move and/or the drying device to move. 
         [0083]    In particular, the moving device can comprise a workpiece carriage and/or a movable irradiation unit for drying the workpiece. 
         [0084]    Further features and advantages of the invention form the subject matter of the following description and the graphical illustration of exemplary embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0085]      FIG. 1  shows a schematic plan view of a coating plant incorporating a processing booth extending in a longitudinal direction and a conveyor device which comprises a longitudinal conveyor track running through the processing booth, a return conveyor track running outside of the processing booth and parallel to the longitudinal conveyor track and also transverse conveyor tracks which connect the return conveyor track and the longitudinal conveyor track to one another; 
           [0086]      FIG. 2  a schematic sketch of the processing booth in the coating plant depicted in  FIG. 1  which comprises a pre-treatment zone, a first back-up zone, an air-lock zone, a coating zone, a further back-up zone, an evaporation zone and a drying zone which succeed one another in the longitudinal direction of the processing booth; 
           [0087]      FIG. 3  an air supply diagram which represents the supply of air to and the exhaust of the air from the processing booth depicted in  FIG. 2 ; 
           [0088]      FIG. 4  a further schematic sketch of the processing booth incorporating a workpiece carriage which carries a very long workpiece (in the form of a rotor blade for a wind turbine for example) and advances it through the processing booth in a longitudinal direction of advancement, while parts of the workpiece succeeding each other in the longitudinal direction of the workpiece are being treated at the same time in different zones of the processing booth; 
           [0089]      FIG. 5  a schematic side view of the workpiece carriage together with the workpiece that is being held thereon; 
           [0090]      FIG. 6  a partially schematic vertical cross section through the workpiece carriage and a longitudinal conveyor rail upon which a longitudinal conveyor running wheel of the workpiece carriage is rolling; 
           [0091]      FIG. 7  a schematic view of the workpiece carriage, of the workpiece being held thereon and of irradiation devices that are adjustable with respect to the workpiece, along a line of sight in the longitudinal direction of advancement through the coating plant; 
           [0092]      FIG. 8  a schematic section through a vacuum suction-jet device which is used for the pre-treatment of the workpiece in the pre-treatment zone; 
           [0093]      FIG. 9  a schematic sketch of a processing booth in a second embodiment of a coating plant wherein the pre-treatment and the coating of the workpiece and also the evaporation process are carried out in a common work zone whilst the workpiece is stationary whereafter the workpiece is advanced through a drying zone; 
           [0094]      FIG. 10  a schematic sketch of the processing booth in a third embodiment of a coating plant wherein the pre-treatment and the coating of the workpiece and also the evaporation process are carried out in a common work booth whilst the workpiece is stationary whereafter a drying device is moved along the workpiece whilst the workpiece remains stationary; and 
           [0095]      FIG. 11  a schematic sketch of the processing booth in a fourth embodiment of a coating plant wherein the pre-treatment and the coating of the workpiece and also the evaporation process are carried out in a common work zone whilst the workpiece is stationary whereafter the workpiece is advanced to a drying zone and dried in the drying zone whilst in a stationary state. 
       
    
    
       [0096]    Similar or functionally equivalent elements are designated by the same reference symbols in all of the Figures. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0097]    A coating plant which bears the general reference  100  and is illustrated in  FIGS. 1 to 8  comprises a processing booth  102  and a conveyor device  104  with the aid of which a workpiece  106  that is to be coated (see  FIGS. 4 and 5 ) is conveyable through the processing booth  102  in a longitudinal direction of advancement  108  (see  FIG. 1 ). 
         [0098]    In accordance with the invention, the workpieces that are to be coated are often one-piece units that are intended for use as rotor blades in wind-powered power stations, ships hulls, automobile bodies or aircraft wings or tail units. 
         [0099]    A coating plant in accordance with the invention displays special advantages in the case of substrates having a longitudinal extent of 10 m or more, as will be described in more detail hereinafter. 
         [0100]    The conveyor device  104  comprises a longitudinal conveyor track  110  which runs in the longitudinal direction of advancement  108  and has two longitudinal conveyor rails  112  that extend in parallel with the longitudinal direction of advancement  108  and are mutually spaced in a direction perpendicular to the longitudinal direction of advancement  108 . 
         [0101]    Furthermore, in order to enable the workpiece  106  to return after a first coating process in the processing booth  102  from the exit of the processing booth  102  to the entrance of the processing booth  102  for a second coating process, the conveyor device  104  comprises a return conveyor track  114  which runs in a reverse direction of advancement  116  that is parallel to the longitudinal direction of advancement  108  but is opposed thereto. 
         [0102]    The return conveyor track  114  comprises two return conveyor rails  118  which run in parallel with the reverse direction of advancement  116  and are mutually spaced in a direction perpendicular to the reverse direction of advancement  116 . 
         [0103]    In order to enable the workpiece  106  to be conveyed from the end of the longitudinal conveyor track  110  to the beginning of the return conveyor track  114 , the end of the longitudinal conveyor track  110  is connected to the beginning of the return conveyor track  114  by a first transverse conveyor track  120  which runs in a first transverse direction of advancement  122  that is oriented perpendicularly with respect to the longitudinal direction of advancement  108  and perpendicularly with respect to the reverse direction of advancement  116 . 
         [0104]    The first transverse conveyor track  120  comprises two transverse conveyor rails  124  which run in parallel with the first transverse direction of advancement  122  and are mutually spaced in a direction perpendicular to the first transverse direction of advancement  122  and which form a first rail crossing  126  with the longitudinal conveyor rails  112  of the longitudinal conveyor track  110  and a second rail crossing  128  with the return conveyor rails  118  of the return conveyor track  114 . 
         [0105]    Furthermore, in order to enable the workpiece  106  to be conveyed back from the end of the return conveyor track  114  to the beginning of the longitudinal conveyor track  110 , the conveyor device  104  comprises a second transverse conveyor track  130  which runs in a second transverse direction of advancement  132  that is oriented perpendicularly with respect to the reverse direction of advancement  116  and perpendicularly with respect to the longitudinal direction of advancement  108  and which connects the end of the return conveyor track  114  to the beginning of the longitudinal conveyor track  110 . 
         [0106]    The second transverse conveyor track  130  comprises two transverse conveyor rails  134 , which run in parallel with the second transverse direction of advancement  132  and form a third rail crossing  136  with the return conveyor rails  118  of the return conveyor track  114  and a fourth rail crossing  138  with the longitudinal conveyor rails  112  of the longitudinal conveyor track  110 . 
         [0107]    The processing booth  102  of the coating plant  100  is illustrated in greater detail in  FIGS. 2 and 3 . 
         [0108]    The processing booth  102  comprises a fully automatic pre-treatment zone  140 , a first reserve zone or back-up zone  142  for carrying out a manual pre-treatment, an air-lock zone  144 , a fully automatic coating zone  146 , a second reserve zone or back-up zone  148  for carrying out a manual coating process, an evaporation zone  150  and a drying zone  152 . 
         [0109]    The aforesaid zones follow one another in the said sequence in a longitudinal direction  153  of the processing booth  102  which corresponds to the longitudinal direction of advancement  108 . 
         [0110]    The pre-treatment zone  140  is in the form of a closed booth which is composed of self-supporting framework elements consisting of sheet steel sections having glass infill panels and sheet metal infill panels integrated into the elements of the framework. 
         [0111]    The side walls  154  of the pre-treatment zone  140  that are formed from these framework elements comprise, in the sequence from top to bottom, an upper sheet metal panel, an upper glass panel for the arrangement of lights (having a height of approximately 1 m for example), a central sheet metal panel, a lower glass panel (having a height of approximately 1.6 m for example) and a lower sheet metal panel. 
         [0112]    The glass infill panels of the side walls  154  preferably consist of single pane safety glass. 
         [0113]    The glass infill panels and the sheet metal infill panels are screwed or clamped into seals in such a way that cleaning media used for cleansing the booth cannot penetrate outwardly therethrough and such that the infill panels are prevented from falling out in the event of fire. 
         [0114]    The lights envisaged for the upper glass panels of the side walls  154  comprise asymmetrical reflectors and sheet metal housings which are pressed onto the glass panels externally. 
         [0115]    The lighting housings can be provided with adjustable fixing devices and clamping devices, with safety chains or a suspension device as well as a seal against the glass sheet. 
         [0116]    The lighting for the booth is controllable locally by means of a lighting cabinet. 
         [0117]    A drive rail  156  extending in parallel with the longitudinal direction of advancement  108  for an automatic pre-treatment unit  158  that is movable in parallel with the longitudinal direction of advancement  108  is arranged on the lower sheet metal panel. 
         [0118]    The automatic pre-treatment unit  158  may, in particular, be in the form of a pre-treatment robot. 
         [0119]    The side walls  154  are thus readied for the integration of the pre-treatment units  158 . 
         [0120]    Doors are provided in the side walls  154  so that maintenance or cleaning personnel can enter the interior of the booth. 
         [0121]    The doors may be in the form of steel doors incorporating viewing windows for example. 
         [0122]    For the purposes of operating the doors, push plates are preferably attached thereto on the inside of the booth and handles are provided on the exterior thereof. 
         [0123]    The doors preferably open outwardly. 
         [0124]    At the beginning and at the end of the booth, there is a respective end wall  160  of panel-like construction which is then ready for the attachment of further parts of the plant and in particular, further booths. 
         [0125]    The end wall  160  may be in the form of a zinc coated metal sheet for example. 
         [0126]    The end wall  160  is preferably smooth and is constructed as a double-walled structure and is thus ready for concealed cable runs. 
         [0127]    In order to enable the workpiece  106  to be advanced from one booth to the next-following booth, the end wall  160  is provided with a passage opening which preferably has a rigid outline. 
         [0128]    When the workpiece  106  is passing therethrough, the distance from the outline to the surface of the workpiece preferably amounts to at least 0.5 m. 
         [0129]    In order to detect misalignment of the workpiece  106  when passing through the passage opening and enable the coating plant  100  to be switched off in such a case, there are provided in the passage opening, hanging screens which are deflected from their rest position if an improperly oriented workpiece  106  comes into contact with them. The deflection of such a hanging screen is registered by a proximity switch associated with the hanging screen whereupon it sends a signal to the control centre of the coating plant  100  which leads to an emergency shutdown of the coating plant  100 . 
         [0130]    The proximity switches of the hanging screens are preferably integrated into the respective end wall  160 . 
         [0131]    The bottom of the booth is in the form of self-supporting structural steelwork incorporating removable gratings. 
         [0132]    A drip tray made of high-grade steel, preferably non-sloping, is installed under the gratings. 
         [0133]    The entrance doors to the booth are at the level of the gratings or are accessible from the level of the grating via platforms, preferably smooth sheet metal platforms, by means of a staircase. 
         [0134]    The work space of the booth is closed in the upward direction by a ceiling filter  162  (see  FIG. 3 ). 
         [0135]    The ceiling filter can be formed from galvanized and/or lacquered sheet metal elements having an integrated lacquered wire grating. 
         [0136]    The framework of the ceiling filter  162  is preferably configured as a walkway. 
         [0137]    The amount by which the filter of the ceiling filter  162  is contaminated is monitorable by a differential pressure manometer. 
         [0138]    The differential pressure manometer preferably has a display which is readable locally in the booth. 
         [0139]    The differential pressure across the ceiling filter  162  is determined in a zone of the booth representative of the filter contamination. 
         [0140]    A filter plenum  164  is located above the ceiling filter  162 , whereby the filter plenums of the successive booths in the longitudinal direction of advancement  108  are likewise separated from each other by partition walls  166  so as to allow air to be supplied separately to the zones of the processing booth  102  in sectionalised manner in accord with the work zone. 
         [0141]    The filter plenum  164  is provided with a lighting system which is capable of being switched on preferably from a central position of the filter plenum  164  by means of a switch incorporating a pilot lamp. 
         [0142]    The filter plenum  164  is accessible through an air-tight maintenance door by maintenance or cleaning personnel. 
         [0143]    The booth of the pre-treatment zone  140  is ventilated by means of an air re-circulating system; the ventilation of the booth will be described in detail hereinafter with reference to  FIG. 3 . 
         [0144]    For the purposes of pre-treating the workpiece  106 , each of the movable automatic pre-treatment units  158  is provided with a vacuum suction-jet device  168  that is illustrated schematically in  FIG. 8 . 
         [0145]    The vacuum suction-jet device  168  comprises a jet hood  170  which is open towards the workpiece  106  and is positionable on the surface  172  of the workpiece  106  in substantially air-tight manner by means of a seal which is arranged on the rim of the jet hood  170  facing the workpiece  106 . 
         [0146]    A blasting lance  176 , by means of which an abrasive medium  178  is appliable to the surface  172  of the workpiece  106  bounding the interior space  174 , opens up into the interior space  174  of the jet hood  170 . 
         [0147]    The surface  172  that is to be coated is activated by the impact of the abrasive medium  178  on the surface  172  of the workpiece  106 . 
         [0148]    The abrasive medium is sucked out of the interior space  174  of the jet hood  170  through an exhaust pipe  180 . 
         [0149]    The exhaust pipe  180  is attached to a source of negative pressure so that a negative pressure is produced in the interior space  174  of the jet hood  170  whereby the jet hood  170  is pressed against the surface  172  of the workpiece  106 . 
         [0150]    In order to cover the entire surface  172  of the workpiece  106  that is to be coated, the jet hoods  170  are moved over the entire surface  172  of the workpiece  106  that is to be coated by means of the pre-treatment units  158  in the form of movable robots that are respectively associated therewith. 
         [0151]    Due to the fact that the abrasive medium  178  and the respective part of the surface  172  of the workpiece  106  to which it is applied are separated from the environment by the jet hood  170 , there is no accumulation of dust in the pre-treatment zone  140  during the process of activation by means of the vacuum suction-jet device  168 . 
         [0152]    As an alternative to a vacuum suction-jet device  168 , a robot-guided brushing system incorporating a suction device could also be employed for the purposes of activating the surface  172  of the workpiece  106  that is to be coated. 
         [0153]    The first back-up zone  142  following the pre-treatment zone  140  in the longitudinal direction of advancement  108  is in the form of a closed booth in like manner to the pre-treatment zone  140 , and the construction thereof corresponds to that of the booth of the pre-treatment zone  140  so that to this extent, reference should be made to the preceding description thereof. 
         [0154]    The first back-up zone  142  is not however provided with automatic pre-treatment units  158 . Rathermore, the first back-up zone  142  serves for permitting the pre-treatment of the workpiece  106  to be carried out manually by workers  181  (see  FIG. 4 ) using suitable pre-treatment equipment in the event of breakdown or if the result of the work effected by the automatic pre-treatment units  158  in the pre-treatment zone  140  is unsatisfactory. 
         [0155]    The air-lock zone  144  follows the first back-up zone  142  in the longitudinal direction of advancement  108 , a vertically acting air curtain being producible in the air-lock zone  144  in order to separate the atmosphere in the pre-treatment zone  140  and the first back-up zone  142  from that in the coating zone  146  which follows onto the air-lock zone  144  and thereby prevent impurities from the pre-treatment zone  140  or the first back-up zone  142  from reaching the coating zone  146  or else coating material from the coating zone  146  reaching the pre-treatment zone  140  or the first back-up zone  142 . 
         [0156]    In like manner to the pre-treatment zone  140 , the coating zone  146  following the air-lock zone  144  in the longitudinal direction of advancement  108  is in the form of a closed booth, the structure thereof upwardly of the level of the grating being identical to the structure of the booth of the pre-treatment zone  140  so that to this extent, reference should be made to the preceding description thereof. 
         [0157]    However, instead of the pre-treatment units  158 , coating units  182  are used in the coating zone  146 , said coating units being movable in parallel with the longitudinal direction of advancement  108  on the drive rails  156  which are integrated into the side walls  154  of the booth. 
         [0158]    The coating units  182  can be in the form of coating robots, in particular in the form of 7-axes robots for example. 
         [0159]    The coating units  182  are provided with suitable application devices for the application of coating material to the surface  172  of the workpiece  106 . 
         [0160]    In particular, a lacquer, preferably a solvent-free lacquer and especially a water lacquer can be used as the coating material. 
         [0161]    Optionally, in addition to the coating units  182  which are located on the two long sides of the coating zone  146 , further coating units can be arranged at the ends of the zone over the inlet to the coating zone  146  and/or over the outlet from the coating zone  146 . 
         [0162]    Due to the mobility thereof, the coating units  182  enable the coating material, and in particular the lacquer, to be applied continuously to the workpiece  106 . 
         [0163]    A separating device  184  (see  FIG. 3 ) is provided under the coating zone  146  for separating out excess coating material from an air stream flowing downwardly through the coating zone  146 . 
         [0164]    The separating device  184  is preferably in the form of a dry separation device and comprises filter elements that are coated with pre-coat material, upon the pre-coat layer of which settle sticky particles from the coating material. 
         [0165]    Stone dust for example can be used as a pre-coat material. 
         [0166]    A supply of the pre-coat material is kept in a (for example funnel-shaped) storage vessel located below the filter elements and this is whirled up by means of air jets at intervals in order to coat the filter elements with fresh pre-coat material. 
         [0167]    If the pre-coat layer consisting of the pre-coat material on a filter element is saturated with coating material by more than a given amount, this pre-coat layer which is saturated with the coating material is loosened from the filter element by a burst of compressed air from the clean gas side of the filter element whereupon the mixture consisting of the pre-coat material and the coating material falls into the storage vessel and is then sucked out from there. 
         [0168]    Such a dry separation device is known from DE 10 2007 040 901 A1 for example, to which reference should be made in regard to the construction and functioning of such a dry separation device and which is hereby incorporated in this application in these respects. 
         [0169]    Such a dry separation device, which is arranged below the grating level of the coating zone  146 , may comprise, in particular, a casing for the filter elements on both sides of the vertical longitudinal centre plane of the booth, which is aligned in parallel with the longitudinal direction of advancement  108 , as well as filter modules which contain the filter elements, an accessible gangway arranged between the filter housings, a supply unit for the supply of fresh pre-coat material to the storage vessels located below the filter elements and an extraction unit for the removal of the mixture consisting of pre-coat material and coating material from the storage vessels. 
         [0170]    As an alternative or in addition to the dry separation device incorporating pre-coated filter elements described above, there could also be used a dry separation device which comprises cardboard labyrinth filters upon which the coating material from the air stream that is loaded with coating material is deposited. 
         [0171]    Moreover, instead of a dry separation device, it is also possible to use a wet washing out device as the separating device  184 . 
         [0172]    In like manner to the coating zone  146 , the second back-up zone  148  following the coating zone  146  in the longitudinal direction of advancement  108  is in the form of a closed booth, the structure thereof upwardly of the grating level being identical to the structure of the booth of the coating zone  146  so that to this extent, reference should be made to the preceding description thereof. 
         [0173]    The second back-up zone  148  does not however comprise automatic coating units  182 . Rathermore, the second back-up zone  148  serves for permitting the coating of the workpiece  106  to be carried out manually by workers  181  (see  FIG. 4 ) using suitable coating equipment in the event of breakdown or if the result of the work effected by the coating units  182  in the automatic coating zone  146  is unsatisfactory. 
         [0174]    Below the second back-up zone  148 , there is provided a separating device  186  for separating out excess coating material from a stream of air flowing downwardly through the second back-up zone  148  from above in like manner to the coating zone  146 . 
         [0175]    Since, however, only a brief emergency coating action is intended to be carried out in the second back-up zone  148 , the performance of this separating device  186  can be designed for a lesser degree of separation. 
         [0176]    Consequently, it will in general be sufficient for the separating device  186  to be in the form of a dry separation device using a cardboard labyrinth system. 
         [0177]    Such a cardboard labyrinth system for example comprises extraction ducts which are arranged below the level of the grating and are equipped with vertically arranged, hinged supporting frames for coating-media separators. The separation of the coating material is effected by glass fibre fleece filters and downstream cardboard labyrinth filters. 
         [0178]    The evaporation zone  150  following the second back-up zone  148  in the longitudinal direction of advancement  108  is in the form of a closed booth in like manner to the previously described zones and preferably comprises a housing of galvanized steel sheet which is provided with an interior lighting system and a closely fitting door. 
         [0179]    An air supply duct incorporating filter frames that are replaceable from below is provided in the ceiling region of the evaporation zone  150 . 
         [0180]    The process of exhausting the air that has been supplied the evaporation zone  150  is effected in the floor region of the evaporation zone  150 . 
         [0181]    The drying zone  152  following the evaporation zone  150  in the longitudinal direction of advancement  108  is in the form of a closed booth in like manner to the previously described zones and it is preferably installed in the manner of a self-supporting structure consisting of prefabricated housing segments. 
         [0182]    The entire internal contour of the drying zone  152  is preferably implemented in a cleaning-friendly and maintenance-friendly manner. 
         [0183]    In order to prevent fluff from settling in the drying zone  152  during a cleaning operation, all the edges of the sheets and the welding seams in the working area of the drying zone  152  are preferably deburred and the dryer tunnel is preferably implemented as a smooth structure. 
         [0184]    In the drying zone  152 , irradiation units  188 , of which one is illustrated in detail in  FIG. 7 , are arranged on both sides of the conveyance path of the workpiece  106 . 
         [0185]    The irradiation unit  188  comprises a pedestal or a stand  190  upon which a plurality of, for example three, irradiation devices  192  are held. 
         [0186]    Radiation is appliable to the coated surface  172  of the workpiece  106  by means of the irradiation devices  192  so as to dry and/or harden the coating. 
         [0187]    This radiation may for example be infrared radiation and/or UV radiation (in the event of the coating being one that is curable by a UV irradiation process). 
         [0188]    The drying zone  152  together with the irradiation units  188  contained therein thus forms a drying device  189  relative to which the workpiece  106  is moved, whilst the workpiece  106  is being coated and whilst the workpiece  106  is being dried. 
         [0189]    The irradiation devices  192  are adjustable, preferably independently, in regard to the vertical position thereof and in regard to the position thereof in the transverse direction  194  of the drying zone  152  (preferably by motorised, hydraulic or pneumatic means) in order to automatically set a desired spacing between the respective irradiation device  192  and the part of the surface  172  of the workpiece  106  that is currently being irradiated. 
         [0190]    Since the cross section of the workpiece  106  varies in the longitudinal direction  196  thereof, the irradiation devices  192  are thus moved towards the workpiece  106  or away from the workpiece  106  whilst the workpiece  106  is passing by. 
         [0191]    The effect is thereby achieved that the average amount of radiation falling on the coated surface  172  of the workpiece  106  remains substantially constant despite the varying cross section of the workpiece  106  since the position of the irradiation devices  192  is automatically adapted to the geometry of the workpiece  106 . 
         [0192]    Furthermore, it is possible to switch-out or switch-in individual irradiation devices  192  and/or to alter the power output of the irradiation devices  192  in order to produce the desired quantity of radiation falling on the workpiece surface. 
         [0193]    In order to produce a uniform surface temperature over the surface of the workpiece  106 , the temperature of the workpiece surface is detected and the radiant power emitted by each of the irradiation devices  192  is regulated, as is also the distance of the irradiation devices  192  from the surface  172  of the workpiece  106 , in dependence upon the result of the temperature detection process. 
         [0194]    Detection of the temperature can be effected in particular by means of a pyrometer. Such a pyrometer detects the radiant heat emitted by an object in non-contact-making manner and evaluates it in order to determine the temperature of the object, whereby use is made of the fact that the intensity of the radiant heat emitted by an object depends on its temperature. 
         [0195]    The irradiation devices  192  are coolable by means of cooling air which is blown over the surfaces of the irradiation devices  192  that are to be cooled by means of flexible hoses. 
         [0196]    Following this description of the individual zones of the processing booth  102 , the system for supplying air through the processing booth  102  will now be described with reference to the air supply diagram depicted in  FIG. 3 : 
         [0197]    The coating plant  100  comprises a first air re-circulating unit  198  which supplies the pre-processing zone  140 , the first back-up zone  142  and the air-lock zone  144  with air via the air supply lines  200 , and a second air re-circulating unit  202  which supplies the coating zone  146 , the second back-up zone  148  and the evaporation zone  150  with air via the air supply lines  204 . 
         [0198]    The drying zone  152  receives its air supply indirectly from the evaporation zone  150 ; exhaust air from the drying zone  152  is exhausted from the drying zone  152  through an exhaust air duct  206  and fed off into the surroundings. 
         [0199]    The booth air that is supplied to the pre-treatment zone  140  and the first back-up zone  142  via the respective filter plenum  164  and the respective ceiling filter  162  is sucked out of the booths through excess-air openings that are arranged in the floor region of the respective side walls  154  for example, and is then fed back into the first air re-circulating unit  198  by way of exhaust air ducts  208  which flow into a main exhaust air duct  210 . 
         [0200]    The excess-air openings through which air is sucked out of the booths can be equipped with hinged supporting frames. 
         [0201]    Fibre filters consisting of glass fibre fleece for example, can be clamped into these supporting frames. 
         [0202]    A fan  212  which sucks in the exhaust air from the pre-processing zone  140  and the first back-up zone  142  and supplies it to the first air re-circulating unit  198  is arranged in the main exhaust air duct  210 , whereby the air re-circulating system is closed by the first air re-circulating unit  198  and the pre-treatment zone  140  or the first back-up zone  142 . 
         [0203]    In the first air re-circulating unit  198 , the circulating air is modified to meet the desired air conditions by conditioning devices (a heating device, a cooling device, a humidifying device and/or a dehumidifying device for example) and supplemented if necessary with fresh air which is supplied to the first air re-circulating unit  198  by way of a fresh air supply line  214 . 
         [0204]    The energy necessary for the conditioning of the booth atmosphere (for the heating, cooling, humidifying and/or dehumidifying processes) is kept to a minimum due to the air circulation system. 
         [0205]    The ventilation of the coating zone  146 , the second back-up zone  148  and the evaporation zone  150  is likewise effected by means of the air circulation system. 
         [0206]    In connection therewith, the exhaust air from the coating zone  146  that has been released from excess coating material in the separating device  184  is sucked through an exhaust air duct  216  into a main exhaust air duct  220  by means of the fan  218  and supplied from there to the second air re-circulating unit  202 . 
         [0207]    The exhaust air from the second back-up zone  148  which has possibly been released from excess coating material in the separating device  186  and the exhaust air from the evaporation zone  150  which has been sucked from the floor region of the evaporation zone  150  are sucked through an exhaust air duct  222  by means of the fan  224  into the main exhaust air duct  220  and likewise supplied from there to the second air re-circulating unit  202 . 
         [0208]    The air circulating system is thus closed by the second air re-circulating unit  202  and the coating zone  146 , the second back-up zone  148  or the evaporation zone  150 . 
         [0209]    In the second air re-circulating unit  202  too, the circulating air is modified to meet the desired air conditions by conditioning devices (a heating device, a cooling device, a humidifying device and/or a dehumidifying device for example) and supplemented if necessary with fresh air which is supplied to the second air re-circulating unit  202  by way of a fresh air supply line  226 . 
         [0210]    Due to the air circulation system for the coating zone  146 , the second back-up zone  148  and the evaporation zone  150 , the energy necessary for the air conditioning process (in particular, for the heating, cooling, humidifying and/or dehumidifying processes) is further reduced. 
         [0211]    By allocating the zones of the processing booth  102  to two different air re-circulating units  198  and  202 , the atmosphere in the booths can be conditioned in different ways as required, and in particular, it can be conditioned in a different manner for the pre-treatment zone  140  on the one hand and for the coating zone  146  on the other. 
         [0212]    Due to the fact that the air supply for each booth is supplied by way of its own air supply line  200  and  204 , the quantity of air supplied to each booth can be precisely matched to the respective needs thereof. As a result of this sectionalised air supply system (selective ventilation of the individual zones of the processing booth  102 ), a further saving of energy is obtained. 
         [0213]    The air-lock zone  144  represents a connecting link between the two air re-circulating systems of the coating plant  100  since, although it does receive its air supply from the first air re-circulating unit  198 , the exhaust air from this air-lock zone  144 , which may be loaded with excess coating material from the coating zone  146 , cannot be fed back to the first air re-circulating unit  198 , but rather, it is supplied to the second air re-circulating unit  202  by way of the separating device  184 . Nevertheless, the air supply to the air-lock zone  144  is always supplied thereto via an air circulation system, this thereby entailing a saving of energy. 
         [0214]    Surplus air from the air circulation system of the first air re-circulating unit  198  is delivered to the environment through an outgoing air line  228  which is attached to the air supply lines  200  of the first air circulation system. 
         [0215]    Surplus air from the air circulation system of the second air re-circulating unit  202  is delivered to the environment through an outgoing air line  230  which is attached to the air supply lines  204  of the second air circulation system. 
         [0216]    The transportation of the workpiece  106  that is to be coated through the processing booth  102  is effected by means of a workpiece carriage  232  which is illustrated in  FIGS. 4 to 7 . 
         [0217]    The workpiece carriage  232  comprises a substantially rectangular framework  233  consisting of two box shaped side members  234  which are connected to one another by transverse beams  236  running perpendicularly to the longitudinal direction  238  of the workpiece carriage  232 . 
         [0218]    The space remaining between the transverse beams  236  following each other in the longitudinal direction  238  is sufficient for allowing the air flow for the booth to pass through the workpiece carriage  232  in the vertical direction. 
         [0219]    At the front ends thereof, the two side members  134  together carry a workpiece holder  240 . 
         [0220]    The workpiece  106  that is to be coated, a rotor blade for a wind turbine for example, is fixed at one end to the workpiece holder  240 , by a bolting or clamping arrangement for example. 
         [0221]    Hereby, the workpiece  106  is arranged on the workpiece carriage  232  in such a way that the longitudinal direction  196  of the workpiece  106  substantially corresponds to the longitudinal direction  238  of the workpiece carriage  232 . 
         [0222]    As can be seen from  FIGS. 4 and 5 , the longitudinal extent L of the workpiece  106  in the common longitudinal direction is significantly greater than the longitudinal extent I of the workpiece carriage  232 . 
         [0223]    The workpiece  106  thus projects for a considerable length beyond the rear end of the workpiece carriage  232 . 
         [0224]    In particular, provision may be made for the protrusion of the workpiece  106  beyond the workpiece carriage  232  to amount to at least half, and preferably, to at least two thirds of the longitudinal extent L of the workpiece  106  in the longitudinal direction  196  thereof. 
         [0225]    For the purposes of forward movement in the longitudinal direction of advancement  108  and as can best be seen from  FIG. 5 , each side member  234  of the workpiece carriage  232  has two longitudinal running wheels  242  which are rotatable about axes running in parallel with the transverse direction  244  of the workpiece carriage  232 . 
         [0226]    Since the workpiece carriage  232  must run very smoothly in order to ensure that an even coating is obtained, the longitudinal conveyor rails  112 , upon which the longitudinal running wheels  242  are rolling, are in the form of round rails having a convexly curved bearing surface  246  (see  FIG. 6 ). 
         [0227]    In preferred manner, the longitudinal running wheels  242  are made from a hardened steel, an aluminium alloy or from a thermosetting synthetic material and the peripheries of their concavely curved running surfaces  248  are complementary to the convexly curved bearing surfaces  246  of the longitudinal conveyor rails  112 . 
         [0228]    Due to the smooth surfaces of the longitudinal conveyor rails  112  and the longitudinal running wheels  242  that are ground in complementary manner thereto, very smooth running of the workpiece carriage  232  in the longitudinal direction of advancement  108  is achieved. 
         [0229]    In alternative exemplary embodiments, the longitudinal running wheels are equipped with solid rubber tyres or with gas-filled tyres. 
         [0230]    The diameter of the longitudinal conveyor rails  112  can amount to approximately 60 mm for example and the diameters of the longitudinal running wheels  242  to approximately 250 mm. 
         [0231]    The return conveyor rails  118  of the return conveyor track  114  are constructed in like manner to the longitudinal conveyor rails  112  of the longitudinal conveyor track  110 . 
         [0232]    For the transverse transportation of the workpiece carriage  232  from the longitudinal conveyor track  110  to the return conveyor track  114  or from the return conveyor track  114  to the longitudinal conveyor track  110 , a respective transverse running wheel  250  is arranged on each end face of each side member  234  of the workpiece carriage  232  (see  FIG. 5 ). 
         [0233]    The transverse running wheels  250  are held on the framework of the workpiece carriage  232  in height adjustable manner and the height thereof relative to the framework  233  and thus relative to the longitudinal running wheels  242  can be adjusted (by means of a motor or hydraulically for example). In all other respects, the details of the transverse running wheels can be similar (or identical) to that of the longitudinal running wheels. 
         [0234]    During the movement of the workpiece carriage  232  in the longitudinal direction of advancement  108  or in the reverse direction of advancement  116 , the transverse running wheels  250  are located in their upper position in which the transverse running wheels  250  are lifted off the underlying ground so that the workpiece carriage  232  is supported on the longitudinal conveyor rails  112  or on the return conveyor rails  118  by means of the longitudinal running wheels  242 . 
         [0235]    When the workpiece carriage  232  reaches the vicinity of the first rail crossing  126  or the third rail crossing  136 , the transverse running wheels  250  on the end faces are lowered until the workpiece carriage  232  is supported on the transverse conveyor rails  124  or  134  by means of the transverse running wheels  250  and the longitudinal running wheels  242  are raised up from the longitudinal conveyor rails  112  or the return conveyor rails  118 . In this position of the transverse running wheels  250 , the workpiece carriage  232  is free for the transverse transportation process in the first transverse direction of advancement  122  or in the second transverse direction of advancement  132 . 
         [0236]    In principle, the transverse conveyor rails  124  and  134  could also be provided with a convexly curved bearing surface in like manner to the longitudinal conveyor rails  112  and the transverse running wheels  250  could be provided with a concavely curved running surface. 
         [0237]    However, the demands made on a smooth running process for the transverse transportation of the workpiece  106  outside the processing booth  102  are less stringent so that the transverse conveyor rails  124  and  132  can be in the form of flat rails having a substantially flat bearing surface and the transverse running wheels  250  can be provided with cylindrical running surfaces. 
         [0238]    The use of transverse running wheels  250  having cylindrical running surfaces has the advantage that they can be narrower than running wheels having concavely curved running surfaces and therefore only need a smaller amount of space in order to achieve the necessary wheel load. 
         [0239]    The diameter of the transverse running wheels  250  can amount to approximately 400 mm for example. 
         [0240]    The workpiece carriage  232  is preferably self-propelled. 
         [0241]    Preferably, at least two longitudinal running wheels  242  and at least two transverse running wheels  250  per carriage are motor driven. It is particularly preferred hereby that all of the longitudinal running wheels  242  and all of the transverse running wheels  250  be driven. 
         [0242]    The drive devices needed for this purpose are preferably accommodated in the interior space of the box shaped side members  234  of the workpiece carriage  232  so that the free cross section of the workpiece carriage  232  available for the through-flow of air through the booth in the vertical direction will remain as large as possible. 
         [0243]    The transmission of the drive movement from the drive device to the respectively driven running wheel can be effected by means of a drive belt  252  (see  FIG. 6 ), and in particular, a toothed belt for example. 
         [0244]    The drive devices for the running wheels preferably comprise at least one electric motor. Optionally however, each driven wheel may have an individually associated gearless electric motor. 
         [0245]    The electrical energy needed for the supply of the electric motor is preferably transmitted in non-contact-making manner by electrical induction from a conductor system  254  laid between the conveyor rails of a conveyor track to a pickup coil  256  on the workpiece carriages  232  (see  FIG. 6 ). 
         [0246]    In order for this inductive process of transmitting energy to be possible, the environment of the pickup coil  256  and the conductor system  254  must be iron-free. 
         [0247]    This necessarily iron-free space is achieved in all the zones of the processing booth  102  by raising the level of the grating by at least approximately 20 cm for example, above the floor level in the vicinity of the conductor system  254 . The pickup coil  256  is led below the level of the grating. 
         [0248]    The pickup coil  256  is held on the framework  233  of the workpiece carriage  232  in height adjustable manner (by means of a motor or hydraulically for example) so that the pickup coil  256  can be lowered relative to the framework  233  during the transition from the longitudinal transportation mode to the transverse transportation mode in order to maintain the same distance from the conductor system  254  as that during the longitudinal transportation process. 
         [0249]    The conductors  258  of the conductor system  254  are fed through plastic tubes within the processing booth  102  and can be supported on cross beams of the processing booth  102  by means of plastic blocks for example. 
         [0250]    When using a non-contact-making system for the transmission of energy to the workpiece carriage  232 , it is preferred that use be made of a solvent-free coating material, in particular a solvent-free lacquer. 
         [0251]    A high frequency panel having a frequency of 25 kHz for example is preferably used for the non-contact-making, and in particular inductive, transmission of energy. 
         [0252]    Instead of utilising a non-contact-making system for the transmission of energy to the workpiece carriages  232 , provision could also be made for the workpiece carriage  232  to be provided with a storage means for electrical energy, in particular with an accumulator. The drive elements required for this variant of a drive means are available in an explosion-proof form. 
         [0253]    In the particular case where a plurality of workpiece carriages  232  are being used in the coating plant  200 , at least three of them for example, then there is adequate time available for charging the accumulator during the return of a workpiece carriage  232  along the return track  114 . 
         [0254]    In order to prevent the longitudinal conveyor rails in the processing booth  102  from becoming contaminated, the longitudinal conveyor rails  112  and the side members  234  of the workpiece carriage  232  equipped with the longitudinal running wheels  242  are separated at least in the coating zone  146  and the second back-up zone  148  by a casing  260 , which can be in the form of a sheet metal cladding for example, from the application area in which the coating material is applied to the workpiece  106  so that the longitudinal conveyor rails  112  and the longitudinal running wheels  242  are protected from contamination by the coating material overspray and also from contact with the exhaust air stream from the booth which is loaded with excess coating material. As can be seen from  FIG. 6 , the casing  260  only has a narrow passage gap  262  through which the cross beams  236  of the workpiece carriage  232  extend from the application area outside the casing  260  into the protected interior of the casing  260 . Thereby, only a small gap remains between the workpiece carriage  232  and the edges of the passage gap  262  so that substantially no coating material can get through this narrow gap into the interior of the casing  260 . 
         [0255]    The longitudinal conveyor rails  112 , the longitudinal running wheels  242  and the side members  234  of the workpiece carriage  232  are accommodated in their entirety in the protective casing  260 . 
         [0256]    The previously described coating plant  100  is suitable, in particular, for the coating of very long workpieces, and especially elongated workpieces having a longitudinal extent L which is significantly greater than the maximum extent B in a transverse direction running perpendicular to the longitudinal direction  196  (see  FIG. 5 ). 
         [0257]    Preferably, the longitudinal extent L of the workpiece  106  is at least five times greater than the maximum transverse extent B. 
         [0258]    In particular, the coating plant  100  is suitable for the coating of workpieces having a length of at least 10 m, preferably of at least 30 m, of approximately 50 m for example. 
         [0259]    A suitable workpiece  106  may be in the form of the rotor blade of a wind turbine for example. 
         [0260]    A method for providing a workpiece with a coating is carried out using the previously described coating plant  100  as follows: 
         [0261]    The workpiece  106  that is to be coated is fixed to the workpiece holder  240  of the workpiece carriage  232  in such a manner that the longitudinal direction  196  of the workpiece  106 , i.e. the direction of the largest longitudinal extent of the workpiece  106 , substantially corresponds to the longitudinal direction  238  of the workpiece carriage  232  and the longitudinal direction of advancement  108  in the coating plant  100 . 
         [0262]    The workpiece carriage  232  together with the workpiece  106  held thereon, which is supported by the longitudinal running wheels  242  on the longitudinal conveyor rails  112  of the longitudinal conveyor track  110 , is then set in motion from the region of the first rail crossing  126  so that it enters the pre-treatment zone  140  of the processing booth  102  and successively passes through all the other zones of the processing booth  102 . 
         [0263]    The conveying speed of the workpiece carriage  232  preferably amounts to 1 m/min or less, as long as the workpiece  106  is being treated within the processing booth  102 . 
         [0264]    During the treatment of the workpiece  106 , the conveying speed preferably amounts to approximately 0.8 m/min. 
         [0265]    As soon as the workpiece  106  has completely left the processing booth  102 , the conveying speed, in particular along the return conveyor track  114 , can be increased to a conveying speed of more than 1 m/min, for example, to approximately 12 m/min. 
         [0266]    The workpiece carriage  232  can move through the processing booth  102  at variable speeds, and in particular can be temporarily stopped and started again. Preferably however, the workpiece carriage  232  moves through the processing booth  102  in a continuous manner, preferably at a substantially constant speed. 
         [0267]    Whilst the workpiece  106  is passing through the successive zones of the processing booth  102  in this way, each section of the surface  172  of the workpiece  106  is treated successively in these zones in the respectively scheduled manner. 
         [0268]    The part of the surface  172  of the workpiece  106  currently in the pre-treatment zone  140  is activated by means of the movable pre-treatment units  158  whilst this part of the workpiece  106  is moving through the pre-treatment zone  140 . 
         [0269]    That part of the surface  172  of the workpiece  106  that is currently located in the coating zone  146  is provided with the coating material by means of the movable coating units  182  whilst the relevant part of the workpiece  106  is moving through the coating zone  146 . 
         [0270]    Volatile components of the coating material evaporate off that part of the surface  172  of the workpiece  106  that is currently located in the evaporation zone  150 , whilst the relevant part of the workpiece  106  is moving through the evaporation zone  150 . 
         [0271]    The part of the surface  172  of the workpiece  106  that is in each case located in the drying zone  152  is dried and/or hardened by means of the irradiation units  188 , whilst the relevant part of the workpiece  106  is moving through the drying zone  152 . 
         [0272]    As can best be seen from  FIG. 4 , the longitudinal extent L of the workpiece  106  is significantly greater than the longitudinal extent of the pre-treatment zone  140 , the coating zone  146 , the evaporation zone  150  and the drying zone  152 . 
         [0273]    In particular, the longitudinal extent of the workpiece  106  is so large that different parts of the surface  172  of the workpiece  106  are being treated in different manners, at least occasionally, at the same time in different zones of the processing booth  102 . 
         [0274]    Thus, the front end of the workpiece  106  (for example the rotor blade root of the rotor blade for a wind turbine) is already in the evaporation zone  150  whilst, at the same time, a middle section of the workpiece  106  is being coated in the coating zone  146  and the rear end of the workpiece  106  (for example the tip of the rotor blade) is being simultaneously activated in the pre-treatment zone  140 . 
         [0275]    As a result of this simultaneity of the processes being performed on the workpiece  106 , the entire processing time for the workpiece  106  in the processing booth  102 , i.e. the time required for the workpiece  106  to pass through the processing booth  102 , is significantly reduced. 
         [0276]    Due to the continuous running principle being used, the individual processing zones of the processing booth  102  can be significantly shorter than the workpiece  106  that is to be processed therein. 
         [0277]    Thus, in the case of an exemplary longitudinal extent L of the workpiece  106  of approximately 50 m, the pre-treatment zone  140  has a longitudinal extent in the longitudinal direction of advancement  108  of approximately 6 m, the first back-up zone  142  a longitudinal extent of approximately 3 m, the air-lock zone  144  a longitudinal extent of approximately 3 m, the coating zone  146  a longitudinal extent of approximately 6 m, the second back-up zone  148  a longitudinal extent of approximately 3 m, the evaporation zone  150  a longitudinal extent of approximately 24 m and the drying zone  152  a longitudinal extent of approximately 10 m. 
         [0278]    As a result of the smaller booth size of the individual zones and the sectionalised supply of air as well as the air circulating system that is used in most zones of the processing booth  102 , a significant saving of energy is obtained. 
         [0279]    The coating plant  100  does not have to be dimensioned in dependence upon the size of the workpiece, but can be designed in dependence upon the desired throughput. 
         [0280]    When the workpiece  106  has passed through the processing booth  102  in its entirety, the rear end of the workpiece  106  has left the drying zone  152  and the workpiece carriage  232  has reached the first rail crossing  126 , the workpiece carriage  232  is changed from the longitudinal transportation mode to the transverse transportation mode by lowering the transverse running wheels  250 . 
         [0281]    Subsequently, the transverse running wheels  250  of the workpiece carriage  232  are driven in order to move the workpiece carriage  232  in the first transverse direction of advancement  122  from the longitudinal conveyor track  110  to the return conveyor track  114 . 
         [0282]    After reaching the second rail crossing  128 , the workpiece carriage  232  is changed over from the transverse transportation mode to the longitudinal transportation mode by raising the transverse running wheels  250  and the workpiece carriage  232  is moved back along the return conveyor track  114  up to the third rail crossing  136  by driving the longitudinal running wheels  242  in the reverse direction of advancement  116 . 
         [0283]    After reaching the third rail crossing  136 , the workpiece carriage  232  is changed over from the longitudinal transportation mode into the transverse transportation mode by lowering the transverse running wheels  250  and the workpiece carriage  232  is moved along the second transverse conveyor track  130  in the second transverse direction of advancement  132  from the return conveyor track  114  to the longitudinal conveyor track  110  by driving the transverse running wheels  250 . 
         [0284]    After reaching the fourth rail crossing  138 , the workpiece carriage  232  is changed over again from the transverse transportation mode into the longitudinal transportation mode by raising the transverse running wheels  250 , and the workpiece carriage  232  together with the workpiece  106  held thereon is moved once more through the processing booth  102  by driving the longitudinal running wheels  242  in order to carry out a second coating process on the workpiece  106 . 
         [0285]    After its second run through the processing booth  102 , the workpiece  106  can be removed from the workpiece carriage  232 , for example, when the workpiece carriage  232  has reached the first rail crossing  126 . 
         [0286]    However, it is also possible for the workpiece carriage  232  together with the workpiece  106  held thereon to be moved back in the previously described manner to the starting point at the fourth rail crossing  138  and for the workpiece  106  to be removed from the workpiece carriage  232  only at this point. 
         [0287]    Furthermore, it is also possible for the workpiece  106  to be removed from the workpiece carriage  232  immediately after its first passage through the processing booth  102  if it is only intended that a single coating layer should be applied to the workpiece  106  without an intermediate drying process. 
         [0288]    A second embodiment of a coating plant  100  which is illustrated in  FIG. 9  differs from the previously described first embodiment which is illustrated in  FIGS. 1 to 8  in that the processing booth  102  in this second embodiment has only one work zone  264  before the drying zone  152 , wherein the longitudinal extent of said work zone in the longitudinal direction of advancement  108  corresponds at least to the longitudinal extent L of the workpiece  106  so that the workpiece  106  can be driven into the work zone  264  in its entirety. 
         [0289]    The work zone  264  serves for pre-treating and for coating the workpiece  106  as well as for carrying out the evaporation process after the coating process. 
         [0290]    To this end, the workpiece  106  disposed on the workpiece carriage  232  is driven into the work zone  264  in the longitudinal direction of advancement  108  until the workpiece  106  is completely accommodated in the work zone  264 , and then it is brought to a stop. 
         [0291]    The basic construction of the work zone  264  corresponds to the construction of the coating zone  146  of the processing booth  102  in the first embodiment of the coating plant  100 . 
         [0292]    However, in addition to the coating units  182  which are movable on the drive rails  156  running in parallel with the longitudinal direction of advancement  108 , the work zone  264  also comprises pre-treatment units  158  which are movable on the same drive rails  156  or on other drive rails that likewise run in parallel with the longitudinal direction of advancement  108 . 
         [0293]    After the arrival of the workpiece  106  in the work zone  264 , the pre-treatment units  158  are driven over the entire length of the workpiece  106  in order to carry out an activation pre-treatment of the entire surface  172  of the workpiece  106 . The coating units  182  follow closely upon the pre-treatment units  158  and so conduct in succession a process of completely coating the areas of the surface  172  of the workpiece  106  that were activated just before. 
         [0294]    When both the pre-treatment units  158  and the coating units  182  have moved over the entire length of the workpiece  106  (from right to left in  FIG. 9  for example), the pre-treatment and the coating of the workpiece  106  are concluded and the workpiece carriage  232  is set in motion in the longitudinal direction of advancement  108  in order to move the completely coated workpiece  106 , preferably continuously, through the drying zone  152  which is significantly shorter than the longitudinal extent L of the workpiece  106 . 
         [0295]    The drying zone  152  thus forms a drying device  189 , relative to which the workpiece  106  is moved after the process of coating the workpiece  106  has concluded and whilst the workpiece  106  is being dried. 
         [0296]    Within the work zone  264 , volatile components of the coating material evaporate off that part of the workpiece  106  which has not yet reached the drying zone  152 . 
         [0297]    Once the entire workpiece  106  has passed through the drying zone  152  and the entire coating of the workpiece  106  has been dried and/or hardened in the drying zone  152  by irradiation by means of the irradiation units  188 , the first coating process of the workpiece  106  is concluded, and the workpiece can be moved back to the entrance of the processing booth  102  in order to carry out a second coating process in the manner described hereinabove in connection with the first embodiment. 
         [0298]    This second embodiment of a coating plant  100  has the advantage that the workpiece  106  is not moved during the coating process, this thereby enabling the coating to be applied in a particularly uniform manner. 
         [0299]    In contrast to the first embodiment, the significant aspect of this embodiment is that use is made of a long work zone  264  which can accommodate the entire workpiece  106 . 
         [0300]    Furthermore, very long drive rails  156  for the movable pre-treatment units  158  and for the movable coating units  182  are used so that these units can sweep over the entire surface  172  of the workpiece  106 . 
         [0301]    In the event that there are no back-up zones, manual emergency action is only possible to a limited extent in the case of this embodiment. 
         [0302]    The pre-treatment and the coating processes preferably take place at the same time in the same booth, whereby the air supply for the pre-treatment process and for the coating process is conditioned in an identical manner. 
         [0303]    The use of a dry separation device incorporating pre-coated filter elements is envisaged when the length of the work zone  264  that is used for the coating process is less than 10 m. 
         [0304]    The conveying speed at which the workpiece  106  is advanced through the drying zone  152  is preferably at least 1 m/min, and is approximately 1.6 m/min for example. 
         [0305]    In all other respects, the second embodiment of a coating plant  100  which is illustrated in  FIG. 9  corresponds in regard to the construction and functioning thereof to the first embodiment illustrated in  FIGS. 1 to 8 , and so to this extent, reference should be made to the preceding description thereof. 
         [0306]    A third embodiment of a coating plant  100  which is illustrated in  FIG. 10  differs from the second embodiment illustrated in  FIG. 9  in that a dryer portal  266  which is movable in parallel with the longitudinal direction of advancement  112  is arranged in the work zone  264  in the processing booth  102  in this third embodiment, the irradiation devices  192  for the irradiation of the coated surface  172  of the workpiece  106  being held on said dryer portal. In preferred manner, one can therefore dispense with a drying zone  152  adjoined to the work zone  264 . 
         [0307]    The dryer portal  266  may be movable on the same drive rails  156  as the pre-treatment units  158  and the coating units  182 , or separate drive rails for the dryer portal  266  which extend in parallel with the longitudinal direction of advancement  108  can be provided. 
         [0308]    In this embodiment, the work zone  264  is significantly longer than the workpiece  106  as there has to be adequate space available for parking the dryer portal  266  between the front end of the workpiece  106  and the rear end wall of the work zone  264 . 
         [0309]    In this embodiment of a coating plant  100 , the workpiece  106  is driven completely into the work zone  264  on the workpiece carriage  232  in the longitudinal direction of advancement  108 , and is brought to a stop therein. 
         [0310]    Subsequently, the workpiece  106  is pre-treated in its entirety by the pre-treatment units  158  that are being driven in parallel with the longitudinal direction of advancement  108  and is coated by the coating units  182  that are being driven in parallel with the longitudinal direction of advancement  108 . 
         [0311]    After the coating process comes to an end, the dryer portal  266  is driven in parallel with the longitudinal direction of advancement  108  over the entire length of the workpiece  106 , whereby the irradiation devices  192  arranged on the dryer portal  266  irradiate the coated workpiece  106  and thereby dry and/or harden the coating. 
         [0312]    The dryer portal  266  thus forms a drying device  189  which is moved relative to the workpiece  106  whilst the workpiece  106  is being dried. 
         [0313]    The workpiece  106  can be held immovably in the work zone  264  during the entire process, from the pre-treatment up to the conclusion of the drying process, or it can be driven at a particularly low speed of less than 0.1 m/min. 
         [0314]    After the conclusion of the drying process, a second coating process can be carried out in the work zone  264  whilst the workpiece  106  continues to remain motionless, this being followed by a drying process which is effected by moving the dryer portal  266 . 
         [0315]    After the conclusion of the second coating process including the second drying process, the workpiece  106  is moved out of the work zone  264  on the workpiece carriage  232  in the reverse direction of advancement  116  and is thus moved out of the processing booth  102 , and it can then be removed from the workpiece carriage  232 . 
         [0316]    Since, in this embodiment, a second coating of the workpiece  106  can be effected without the workpiece  106  having to be moved back to the entrance of the processing booth  102  outside the processing booth  102 , one can dispense with the return conveyor track  114  and the transverse conveyor tracks  120  and  130  in the case of this embodiment of a coating plant  100 . 
         [0317]    Furthermore, the workpiece carriage  232  in this embodiment need only be suitable for the longitudinal transportation process, so that one can also dispense with the height adjustable transverse running wheels  250 . 
         [0318]    In the case of this embodiment, the workpiece  106  can remain stationary during both coating processes and it does not have to be moved between the two applications of a coating. 
         [0319]    As was the case for the second embodiment, the workpiece  106  in the third embodiment of a coating plant  100  is not moved during the process of applying the coating material, this thereby enabling the coating material to be applied in a particularly uniform manner. 
         [0320]    The significant aspect of this embodiment is that use is made of a particularly long work zone  264  in which the dryer portal  266  can be parked. 
         [0321]    Moreover, it is also significant that the dryer portal  266  is moved through an area of the work zone  264  in which an application of a coating has taken place. 
         [0322]    For the purpose of moving the pre-treatment units  158 , the coating units  182  and the dryer portal  266 , very long drive rails  156  are used so that all these devices are movable over the entire length of the workpiece  106 . 
         [0323]    In the event that the pre-treatment and the coating of the workpiece  106  take place in the same zone of the processing booth  102 , a common, identical and/or unitary conditioning of the air supply is provided for the pre-treatment process and for the coating process. 
         [0324]    In all other respects, the third embodiment of a coating plant  100  that is illustrated in  FIG. 10  corresponds in regard to the construction and functioning thereof with the second embodiment that is illustrated in  FIG. 9  so that to this extent, reference should be made to the preceding description thereof. 
         [0325]    A fourth embodiment of a coating plant  100  which is illustrated in  FIG. 11  differs from the second embodiment which is illustrated in  FIG. 9  in that there is provided a long convection drying zone  268  which can accommodate the workpiece  106  in its entirety instead of a short drying zone  152  through which the workpiece  106  is continuously moved. 
         [0326]    In this convection drying zone  268 , the workpiece  106  is not dried by irradiation by means of irradiation devices  192 , but rather, by means of warm air which is passed through the convection drying zone  268  by the air circulation system. 
         [0327]    In this embodiment, the entirety of the workpiece  106  is first driven into the work zone  264  on the workpiece carriage  232  in the longitudinal direction of advancement  108  and is then brought to a stop therein. 
         [0328]    Subsequently, a pre-treatment of the entire workpiece  106  is carried out by means of the movable pre-treatment units  158  and coating of the entire workpiece  106  is carried out by means of the movable coating units  182 . 
         [0329]    After the process of coating the workpiece  106  has been completely concluded, the workpiece  106  is moved on the workpiece carriage  232  from the work zone  264  into the convection drying zone  268  and is again brought to a stop therein. 
         [0330]    The convection drying zone  268  thus forms a drying device  189 , relative to which the workpiece  106  is moved after the process of coating the workpiece  106  has concluded and before the process of drying the workpiece  106  begins. 
         [0331]    After the drying time necessary for the process of drying by means of warm air, the workpiece  106  is moved out of the convection drying zone  268  on the workpiece carriage  232  in the longitudinal direction of advancement  108  and is thus moved in its entirety out of the processing booth  102  and thereafter, it is returned to the entrance of the processing booth  102  by means of the first transverse conveyor track  120 , the return conveyor track  114  and the second transverse conveyor track  130  in the manner described hereinabove in connection with the first embodiment of a coating plant  100 , whereupon the workpiece  106  can be driven into the work zone  264  again for a further coating process. 
         [0332]    After the second coating process including a second drying process whilst the workpiece  106  remains stationary in the convection drying zone  268 , the workpiece  106  is again driven completely out of the processing booth  102  on the workpiece carriage  232  and can then be removed from the workpiece carriage  232 . 
         [0333]    This embodiment has the advantage that the workpiece  106  does not have to be moved during the process of applying the coating material, whereby a particularly uniform application of the coating material is attainable. 
         [0334]    Warm circulating air and/or an IR emitter (infrared emitter) are provided for the process of drying the workpiece  106  in the convection drying zone  268 . 
         [0335]    In the event that the coating plant  100  in accordance with this embodiment has sufficient space available, two zones arranged one behind the other, namely the work zone  264  and the convection drying zone  268 , can each exhibit at least the full length of the workpiece  106 . 
         [0336]    Drive rails  156  are provided over the entire length of the plant for the purposes of moving the pre-treatment units  158  and the coating units  182  over the entire length of the workpiece  106 . 
         [0337]    In the event that there are no back-up zones, manual emergency action is only possible to a limited extent in the case of this embodiment. 
         [0338]    In the event that pre-treatment and coating of the workpiece  106  take place at the same time in the same zone of the processing booth  102 , provision is made for the air supply for the pre-treatment and the air supply for the coating to be conditioned in unitary manner. 
         [0339]    Furthermore, use of a dry separation device incorporating pre-coated filter elements is envisaged for a length of the zone used for the application of the coating of less than 10 m. 
         [0340]    In all other respects the fourth embodiment of a coating plant  100  that is illustrated in  FIG. 11  corresponds in regard to the construction and functioning thereof to the second embodiment that is illustrated in  FIG. 9  and so to this extent, reference should be made to the preceding description thereof.