Abstract:
A method and apparatus for controlling local flow along a slot jet with applications to airknives is disclosed. The airknife provides improved options for on-line control over coating thickness and edge build-up prevention. The gap of the slot nozzle of the airknife can remain constant while the outflow pattern is controlled by a series of preset throttle valves in combination with a single moving component, the multi-port aero-valve. This valve can be actuated locally or remotely, in rotation or translation, thereby changing gradually from a conventional uniform outflow to one which increases in velocity and mass flow rate along the airknife span to produce a bow effect. The valve can also change gradually from a conventional uniform outflow to one which increases in velocity, mass flow rate and outflow angle along the span to simulate bow effect in combination with fan-like outflow.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The embodiments of this invention generally relate to online flow control along the span of a high aspect ratio slot jet with applications to airknives as used in industry to apply coatings, dry coatings or to control the thickness of coatings. Most problematic is controlling the coating thickness distribution in the hot-dip galvanizing industry, where excess zinc coating of sheet steel is an expensive waste of material. With frequent changes in sheet thickness-width-and-speed, together with changes in furnace temperature, zinc pot temperature-and-chemical composition, coating thickness control is an ongoing problem for the operator. Currently the operator&#39;s options are limited to changing sheet speed, airknife supply pressure, distance between slot jet and sheet, and the blowing angle onto the sheet. To prevent coating edge buildup with associated coiling problems of the finished sheet goods and to improve coating uniformity, the operator has the option to change offline, the “bow” setting in the slot jet nozzle lips. To change the bow setting offline requires taking the slot jet to a machine shop for nozzle lip gap adjustment. A “bow” setting in the slot jet nozzle lip gap, is used to increase the jet mass flow rate or momentum, and thereby the wiping action, towards the edges of the sheet where the lack of flow blockage deflects the flow outward, thereby locally reducing the stagnation pressure on the sheet and thus wiping action. 
         [0002]    Airknife technologies from the 1990&#39;s incorporated on-line controllable internal swiveling elbows to produce a fan shaped outflow angle of the airknife slot jet. This method proved to be effective in reducing edge build-up. However this mechanism was complex with numerous moving parts and often unreliable. After fixing the position of the outflow generating elbows, such airknives remained in service over the past two decades. Other operators resort to: (1) fences placed near the edges of the sheet to minimize edge build-up and coiling problems or (2) a bow-like setting in the airknife lips, to increase the mass flow rate and thus wiping action near the sheet edges. Figures from U.S. Pat. No. 5,683,514 are shown in  FIG. 5  to illustrate the obtainable fan-like outflow pattern by adding swiveling elbows inside the airknife inlet plenum. The ever increasing cost of coating materials increases the demand for new technologies with online control over coating thickness distribution. This is likely to be in the form of online control over the distribution of any or all local mass flow rate or velocity or outflow angle along the length (span) of the slot jet of an airknife. 
       THEORY OF OPERATION 
       [0003]    Herein is disclosed a “Method and Apparatus for Online Flow Control Over the Span of a High Aspect Ratio Slot Jet”. This technique involves placing two sets of throttle valves in series, all along the span of the slot jet in at least one embodiment. The upstream set of valves is installed within the airknife body, where it provides control over the spanwise distribution of supply air, thereby replacing the need for conventional baffles. This upstream set of valves is adjusted to supply a smoothly increasing air-pressure, with distance from the center of the airknife towards each slot jet end. If this pressure distribution remains preserved downstream, via a series of individual nozzles, which discharge just upstream of the slot jet with uniform gap setting, then it produces a slot jet with maximum bow like velocity or mass flow distribution. To reduce this bow effect gradually down to zero through online adjustment, a single cylindrical shaft in each airknife is either manually or remotely actuated. This shaft is machined in the form of a multi-port aero-valve in one embodiment. It is either rotated or axially translated within a stationary sleeve which serves as a housing, and has its multi-ports machined to perform either of two functions: 1) gradually throttle-off the excess pressure produced by the upstream set of valves, thereby reducing the simulated bow effect to zero, resulting in uniform velocity and mass flow rate; 2) gradually direct the flow to discharge through a series of fixed flow direction elements such as flow elbows which deflect the flow away from the middle of the span of the airknife, resulting in a combined fan like outflow angle with bow effect. This multi-port aero-valve shaft can easily be actuated remotely from a control room in one embodiment or manually in another embodiment. The optimum amount of bow effect and outflow angle required to improve coating uniformity for any particular line varies with sheet width and thickness, coating thickness, line speed, coating material and chemistry. Currently employed airknives can only alter the amount of bow effect by adjusting the lips in a machine shop, after the airknife has cooled. The herein disclosed valve arrangement, machined within a single shaft, can be adjusted online. One simple technique for remote control is a spring loaded pneumatic actuator, supplied with shop air at the desired pressure using a pressure-regulator. The valve arrangement can also be locally manually controlled in another embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  shows an embodiment of the apparatus configured as an airknife assembly. 
           [0005]      FIG. 2  shows the cross-section of an embodiment of the apparatus with the herein claimed elements which are a series of throttle valves inside the airknife inlet plenum and a multi-port aero-valve, within a stationary housing which discharges either 1) through a series of straight nozzles exiting upstream but in close proximity to the gap in the slot jet or 2) through a series of elbow type nozzles with gradually increasing flow turning angle with distance from the middle of the slot jet. 
           [0006]      FIGS. 3A ,  3 B and  3 C each show a different position of the multi-port aero-valve.  FIG. 3A  shows the aero-valve in a position required to produce a uniform outflow velocity. This is achieved by throttling off all bow-like excess pressure provided by fixed throttle valves inside the airknife inlet plenum. 
           [0007]      FIG. 3B  shows the aero-valve in the position required to provide the maximum bow-like pressure distribution as is provided by the fixed throttle valves inside the airknife inlet plenum. 
           [0008]      FIG. 3C  shows the aero-valve in the position required to provide a combination of maximum bow-like pressure distribution with fan-like outflow angle, via a series of elbows, to minimize edge build-up. 
           [0009]      FIG. 4  shows a schematic of pneumatic air-lines used to remotely control the multi-port aero valve within each airknife and also control the blower air supply pressure. 
           [0010]      FIG. 5  shows a prior art (U.S. Pat. No. 5,683,514) fan-like outflow pattern obtainable by adding swiveling elbows inside the airknife inlet plenum. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0011]      FIG. 1  shows an assembly view of the herein disclosed high aspect ratio, slot jet configured as an airknife. A high aspect ratio, slot jet typically comprises a substantially rectangular cross-section jet with its length or span being at least five times its width. The air is supplied to airknife inlet plenum  10  via the airknife blower inlet  12 . Inside the airknife inlet plenum  10  is a set of manually pre-set throttle valves  22  to provide a velocity distribution along the span of the airknife nozzle exit slot, thereby simulating the performance of a bow-like exit slot gap setting. The multi-port aero-valve  20  is located downstream of the set of throttle valves  22 . This multi-port valve  20  is designed to modulate the pressure distribution provided by throttle valves  22  and acts a proportioning valve in at least one embodiment. If this pressure distribution is left unchanged, then the airknife jet velocity or mass flow rate and momentum exiting the uniform gap exit slot gradually increases with distance from the mid-span of the airknife nozzle exit thereby simulating a bow effect. By translating or rotating the multi-port aero-valve  20 , the airknife produced pressure profile, and thus bow effect, can be gradually eliminated resulting in a uniform exit velocity. Another option with the multi-port aero-valve  20  is to direct the flow through a series of at least partially spanwise facing flow direction elements such as elbows to produce a fan-like airknife outflow pattern, having spanwise velocity components directed away from the mid-span. This has also proven to be effective in preventing edge build-up on a coated sheet. Edge flanges  19  are used to seal off the ends of the exit slot forming lips and to support the airknife. 
         [0012]      FIG. 2  shows a typical cross-section of the airknife embodiment of the invention. The bottom lip  18  is attached to the airknife inlet plenum  10  via a bottom saddle  16 . The side lip  39  is attached to plenum  10  via a side lip saddle  37 . Sheet metal panels  34  are used to minimize air leakage from the slot nozzle cavity  31  in one embodiment. Throttle valves  22  inside airknife plenum  10  are used to pre-set the desired spanwise supply pressure distribution. Plenum screen  36  prevents flow blocking particles from entering the narrow passages of the airknife. Downstream of throttle valves  22  is located the multi-port aero-valve  20 , which discharges into the slot nozzle cavity  31  through individual nozzles (nozzles  24  and  32  as shown in  FIG. 2 , for example). To simulate a bow effect, with a constant gap slot jet, the pressure available for fluid acceleration must increase with spanwise distance from the mid-span of the slot jet. Because the slot nozzle cavity  31  cannot support a spanwise pressure gradient, the pressure profile supplied by the multi-port aero-valve must first be transformed into a spanwise velocity profile using nozzles  24  and  32 . These nozzles discharge their velocity profile into the slot nozzle cavity  31  and in close proximity to the uniform gap exit slot  14 , so that the spanwise, velocity distribution of the slot jet reflects the upstream individual nozzle velocities. The multi-port aero-valve stationary housing  30  has a plurality of two different outlet nozzle types or flow direction elements. The outlet nozzle  24  provides straight outflow with either a uniform velocity or with a bow simulating velocity profile, and the outlet nozzle  32  type comprises outward pointing flow direction elements (elbows in one embodiment) to produce spanwise flow components directed away from the mid-span and the flowfield may include some bow effect. 
         [0013]      FIG. 3A  shows the multi-port aero-valve in the position required to produce a uniform airknife discharge velocity, thus without bow effect. The multi-port aero-valve  20  can be mechanically actuated, by either a small amount of rotation or translation, or a combination thereof.  FIG. 3A  shows the multi-port aero-valve  20  actuated in translation. The airknife inlet plenum  10  is shown with the multi-port aero-valve  20  below it and downstream of it. A plurality of throttle valves  22  is required to simulate the desired bow effect. For clarity, only three valves are shown. Valve  21 , located adjacent to the centerline  38  of the airknife, (and any other such valves so located) is adjusted to reduce supply pressure the most. Valve  22 , (farther from the centerline  38  and any other such valves so located) is adjusted to reduce supply pressure to a lesser degree. Valve  23  (farther still from the centerline  38  and any other such valves so located) reduces the supply pressure the least, and is only required to assure symmetry in outflow from the airknife. Downstream of each of these throttle valves is attached a stationary housing  30  containing a multi-port aero-valve. For straight flow, it discharges through outlet nozzles ( 24 ,  26 , and  28 ) also known herein as a type of flow direction element. In this position of the multi-port aero-valve, passage  25  entirely uncovers the inlet to outlet nozzle  24 , positioned near the airknife centerline  38 . Passage  27  partially blocks the inlet to outlet nozzle  26 . Passage  29  partially blocks and to a greater extend the inlet to outlet nozzle  28 . The amount of flow blockage into these nozzles ( 24 ,  26 , and  28 ) is designed to cancel any supply pressure in excess of that supplied by throttle valve  21  adjacent to the centerline  38 . The result is a uniform mass flow rate and velocity discharged by each outlet nozzle ( 24 ,  26  and  28 ) and likewise uniform velocity out of uniform gap exit slot  14 . This position of the multi-port aero-valve simulates an airknife without bow setting. It is understood that many more than three throttle valves, straight and curved outflow nozzles may be employed in various embodiments. The single-headed arrows shown in  FIGS. 3A ,  3 B, and  3 C illustrate the fluid velocity vectors produced by the apparatus as a function of varying position of the multi-port aero-valve system (a proportioning valve system). 
         [0014]      FIG. 3B  shows the multi-port aero-valve in the position required to simulate the maximum bow effect. The airknife inlet plenum  10  is shown here with the multi-port aero-valve  20  below it and downstream of it. A plurality of throttle valves  22  is required to simulate the desired bow effect. But for clarity, only three valves are shown here. Valve  21 , located adjacent to the centerline  38  of the airknife, (and any other such valves so located) is adjusted to reduce flow rate the most. Valve  22 , (farther from the centerline  38  and any other such valves so located) is adjusted to reduce the flow rate to a lesser degree. Valve  23  (farther still from the centerline  38  and any other such valves so located) reduces the local flow rate the least, and is only required to assure symmetry in outflow from the airknife. Downstream of each of these throttle valves is attached a stationary housing  30  containing a multi-port aero-valve. For straight flow, it discharges through outlet nozzles ( 24 ,  26 , and  28 ). In this position of the multi-port aero-valve  20 , all inlet ports to the outlet nozzles are totally uncovered by passages ( 25 ,  27  and  29 ). The amount of flow passing through outlet nozzles  24 ,  26 , and  28  is proportional to the pressure supplied by the upstream throttle valves ( 21 ,  22  and  23 ). The result is maximum airknife bow effect as indicated by arrows leaving outlet nozzles ( 24 , 26  and  28 ) and also leaving uniform gap exit slot  14 . Due to the movement of the multi-port aero-valve (a proportioning valve) from position shown in  FIG. 3A  to that in  FIG. 3B , the airknife outflow velocity transitions smoothly from uniform flow to maximum bow effect flow. 
         [0015]      FIG. 3C  shows the multi-port aero-valve in the position required to simulate a combination of maximum bow effect with fan-like outflow. The airknife inlet plenum  10  is shown here with the multi-port aero-valve  20  below and downstream. A plurality of throttle valves  22  is required to simulate an accurate bow effect. But for clarity, only three valves are shown here. Valve  21 , located adjacent to the centerline  38  of the airknife, (and any other such valves so located) is adjusted to reduce flow rate the most. Valve  22 , (farther from the centerline  38  and any other such valves so located) is adjusted to reduce the flow rate to a lesser degree. Valve  23  (farther still from the centerline  38  and any other such valves so located) reduces the local flow rate the least, and is only required to assure symmetry in outflow from the airknife. Downstream of each of these three throttle valves, is attached a stationary housing  30  containing a multi-port aero-valve. The flow is directed through elbow shaped nozzles ( 32 ,  33  and  34 ) also known herein as a type of flow direction element. The inlets to these elbows are totally uncovered by the passages ( 25 ,  27  and  29 ) of the multi-port aero-valve  20 , and in at least one embodiment, each elbow may have a different outlet flow angle. The amount of flow leaving these elbows is proportional to the pressure supplied by the throttle valves. The result is the maximum airknife bow effect in combination with a fan-like outflow angle, as indicated by the arrows. By moving the multi-port aero-valve from position shown in  FIG. 3B  to that in  FIG. 3C , the airknife outflow velocity with bow effect transitions smoothly from straight outflow to fan-like, spanwise outflow by engaging selected flow direction elements. Such a slot jet profile has proven to be beneficial to minimize edge coating build-up on sheet goods. 
         [0016]      FIG. 4  shows a schematic for remote control over the multi-port aero-valve and the blower supply pressure. For application for a hot dip galvanizing line, two airknives (right air knife  40  and left airknife  42 ) are used. The airknives  40  and  42  are supplied with air from blower  43 , via a pressure controlling damper  44 . The blower supply pressure can be monitored inside the control room by pressure gauge  46 . Air is supplied to airknife  40  via pipe  48  and to airknife  42  via pipe  50 . For remote control of the multi-port aero-valves and the blower supply pressure, compressed air is used as supplied to the inlet of valve  52  the outlet pressure of which is shown on pressure gauge  54 . A spring loaded, high temperature, piston-type, pneumatic actuator  66  is used to control the multi-port aero-valve on airknife  40  in one embodiment. Compressed air is supplied to the pneumatic actuator  66  via air-line  64 . The piston position within the actuator depends on the pressure supplied by a regulator and shown on pressure gauge  56 . A shaft connects the piston within the actuator to the multi-port aero-valve  20  such that the piston position controls the position of the multi-port aero-valve and thus the flow pattern exiting the airknife  40 . A spring loaded, high temperature, piston-type, pneumatic actuator  60  is used to control the multi-port aero-valve on airknife  42  in one embodiment. Compressed air is supplied to the pneumatic actuator  60  via air-line  58 . The piston position within the actuator depends on the pressure supplied by a regulator and shown on pressure gauge  62 . A shaft connects the piston within the actuator to the multi-port aero-valve  20  such that the piston position controls the position of the multi-port aero-valve and thus the flow pattern exiting the airknife  42 . 
         [0017]    A spring loaded high temperature pneumatic actuator  70  is used to control the blower damper setting  44 . This pneumatic actuator piston position depends on the pressure supplied by a regulator as shown by pressure gage  68 . 
         [0018]    The various embodiments described within are merely descriptions and are in no way intended to limit the scope of the invention. Modifications of the present invention will become obvious to one skilled in the art in light of the above descriptions and such modifications are intended to fall within the scope of the appended claims. It is understood that no limitation with respect to the specific apparatus and methods illustrated herein is intended or should be inferred.