Assembly for preventing the plugging of a coater nozzle

Abstract: The invention relates to an assembly for preventing plugging of a narrow, slot-orifice-type jet nozzle 6 used in a paper and board coater in which jet nozzle extends over the entire cross-machine width of the web being coated, said coater comprising an application chamber (11), a coating mix infeed channel (10) connected thereto and a slot orifice (6) as well as at least one infeed port for feeding the coating mix into said application chamber (11). According to the invention, an easily cleanable strainer element (12) is placed close to the applicator apparatus, said strainer serving to separate aggregates of the coating mix that otherwise could plug the slot orifice (6).

Description: The present invention relates to an assembly according to the preamble of claim 1 for preventing the plugging of the narrow, slot-orifice-type nozzle used for coating paper and board in which the nozzle extends over the entire cross-machine width of the web being coated. A coater type conventionally used in paper-making is an applicator apparatus called a jet applicator. This apparatus is a variant of nozzle applicators, wherein the coating mix is applied in a noncontacting manner to the web surface via a very narrow slot orifice. One of the advantages of jet-application techniques is the small pumping rate (3 - 4 liters coating mix per second and linear meter of slot orifice) and the small amount of excess coat return flow. The method is particularly suited for web speeds slower than 1000 m/min, because at high web speeds the air film travelling on the web surface begins to disturb the stability of the impinging jet. As the apparatus is rather sensitive to air entrained in the coating mix, an air separator is required in the coating mix circulation, because otherwise the air bubbles of the applied coating mix could cause uncoated spots. The narrow and low-impact discharge from the slot orifice of the jet applicator apparatus does not stress the web being coated and achieves some degree of coat penetration into the web being coated. However, jet applicators are hampered by being highly sensitive to plugging of the slot orifice, which can be traced to the narrow opening of the orifice. Hence, even very small impurities or hardened coating paste aggregates can get trapped in the slot orifice causing coat defects and requiring cleaning of the orifice. Obviously, a production shut-down is necessary for opening and carefully cleaning the nozzle chamber. To avoid coating mix aggregates from reaching the applicator apparatus and therefrom the web, the coating mix circulation is in most coater installations equipped with strainers designed to remove aggregates and lumps from the circulating coating mix. The strainers are placed between the coating mix tank and the coater unit in the coat circulation. While the screening capacity in the circulation and the separation efficiency of strainers conventionally used in the circulation are sufficient for a majority of coating methods, these screening techniques may pass coating mix aggregates which in jet applicator apparatuses can plug the slot orifice. A source of such aggregates is the coating mix infeed piping section between the strainer and the coater unit. Some amount of the circulating coat easily adheres to this part of the piping or hardens therein so as to become later dislodged as lumps or strips which travel in the coating mix circulation and plug the slot orifice of the jet applicator if allowed to reach that far. Such hardening of the coating mix is chiefly due to small inflow rate to the jet assembly, whereby also the volume change rate and flow velocity of the coat in the piping remain small. While other types of coaters not using the jet-application technique are relatively insensitive to small amounts of coating mix aggregates, the jet applicator has been found to require an almost zero content of aggregates in the coating mix because of the narrow nozzle slot. It is an object of the present invention to provide an assembly capable of preventing the access of aggregates into the slot orifice of a jet applicator apparatus. The goal of the invention is achieved by adapting an easily cleanable strainer into the coating mix application chamber of the applicator apparatus so that the strainer serves to screen away aggregates from the coating mix prior to their access into the slot orifice. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. The invention offers significant benefits. By virtue of the invention, it is possible to eliminate the plugging problem of the application nozzle even when the slot orifice opening is very narrow-gapped, whereby unnecessary shutdowns for opening the coater assembly are avoided. Thus, the uninterrupted run periods of the coater station can be extended, which contributes substantially to improved profitability of the coater. The embodiment according to the invention does not essentially alter the pressure head of the coating mix circulation nor require changes in the equipment construction. In the following the invention will be examined in greater detail by making reference to the appended drawing showing the cross section of an assembly according to the invention. Referring to the diagram, the main parts of a jet applicator apparatus are a main beam 2 with a wall 3 which is attached thereto so as to form an application chamber 11 in cooperation with the main beam. To the main beam 2 is attached the upper lip 7 of a slot orifice 6 so that the upper lip can be moved with the help of an actuator 4 in order to adjust the opening of the slot orifice 6. The lower lip 5 of the slot orifice 6 is attached to the wall 3, whereby its function is to border, besides the slot orifice, also a meandering coating mix infeed channel 10 passing from the application chamber 11 to the slot orifice 6. The coating mix is fed into the application chamber 11 via an inlet port 13. During coater operation, a required amount of coating mix is fed into the application chamber, wherefrom the coat passes via the coating mix infeed channel and the slot orifice 6 to the surface of the web supported by a backing roll 1. A portion of the coating mix jet adheres to the web surface forming a coat layer 8, while the excess coat forms a return flow 9 passing reverse to the web travel direction. According to the invention, into the application chamber is adapted a strainer 12 suitable for separating aggregates from the coating mix flow before they can plug the slot orifice 6. The strainer 12 can be implemented as a slotted, perforated-hole or mesh strainer, and when adapted into the application chamber 11, its width will be really substantial, whereby the active area of the strainer 12 passed by the coating mix flow becomes very large and the pressure loss over the strainer drops to an insignificant value. Thus, the separation efficiency of the strainer can be substantially high provided that the openings of the strainer are made smaller than the minimum opening of the slot orifice. Alternatively, the strainer can be adapted, e.g., to the coating mix infeed port 13, or immediately in front thereof, so that the coating mix can pass directly from the strainer into the infeed port. However, a strainer placed into the application chamber, as close to the slot orifice as possible, offers a higher performance in the separation of particulate matter, because in this location the strainer can also separate coating mix lumps possibly formed within the application chamber. The strainer should in any case be placed so close to the applicator apparatus that no section of the coating mix infeed piping remains between the strainer and the applicator apparatus, which means that the strainer must be located on the coating mix path in the section remaining between the infeed port, to which the coating mix circulation piping is terminated, and the slot orifice. According to this design rule, the coating mix infeed piping can be directly connected the strainer, wherefrom the coating mix can immediately reach the infeed port of applicator apparatus.

Claims: An assembly for preventing plugging of a narrow, slot-orifice-type jet nozzle (5, 7) used in a paper and board coater in which jet nozzle extends over the entire cross-machine width of the web being coated, said coater comprising an application chamber (11), a coating mix infeed channel (10) connected thereto and a slot orifice (6) as well as at least one infeed port for feeding the coating mix into said application chamber, characterized by a strainer element (12) placed in the space remaining between the terminal point of said coating mix infeed channel and said slot orifice (6) so that the strainer element (12) is mounted within the application chamber (11) in a flow path of the coating mix from the inlet port (13) to the slot orifice (6) such that the coating mix fed through the inlet port (13) flows first into a lower portion of the application chamber (11) then through the strainer element (12) and into an upper portion of the application chamber (11). An assembly according to the claim 1, characterized in that the strainer element (12) is a slot-orifice, perforated slot or mesh strainer. An assembly according to the claim 1, characterized in that the openings in the strainer element (12) are smaller than the minimum opening of the slot orifice (6).

Inventors: KOSKINEN JUKKA, NISSINEN VILHO, KOSKINEN, JUKKA, NISSINEN, VILHO
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1997
Main classification: D21H23
IPC codes: B05C5, D21H23