US 3071869 A
Description (OCR text may contain errors)
Jan. 8, 1963 LATIMER ETAL 3,071,869
WEB DRYING APPARATUS 5 Sheets-Sheet 1 Filed Oct. 16, 1958 INVENTORS KENNETH B. LATIMER a MORRISON N. STILES Y J l 3w 1M their ATTORNEYS Jan. 8, 1963 K. B. LATIMER ETAL 3,071,869
WEB DRYING APPARATUS File d Oct. 16, 1958 3 Sheets-Sheet 2 INVENTORY. KENNETH B. LATIMER 81 MORRISON N. STILES BY W Jan. 8, 1963 K. B. LATIMER ETAL 3,071,869
WEB DRYING APPARATUS 3 SheetsSheet 3 Filed Oct. 16, 1958 LATI MER 8x STI LES I Fw,M ,Dpvl-4 INVENTORS. KENNETH B. MORRISON N lk LT Q their ATTORNEYS United States Patent Office 3,071,869 Patented Jan. 8, 1963 3,071,869 WEB DRYING APPARATUS Kenneth B. Latimer and Morrison N. Stiles, Westport, Conn., assignors to Time, Incorporated, New York, N.Y., a corporation of New York Filed Oct. 16, 1958, Ser. No. 767,587 18 Claims. (Cl. 34-114) This invention relates to methods and apparatus for drying rapidly moving webs.
This is a continuation-in-part of our copending application Serial No. 491,059, filed February 28, 1955 for Web Drying Methods and Apparatus.
Difficulties have been encountered in the printing, paper, motion pictures and textile fields in connection with the rapid drying of a continuously moving web, particularly when such web travels at speeds in excess of 1000 feet per minute. At lower speeds, the use of heat drying has satisfied many drying requirements but as the speed of the web increases, the application of heat thereto must be proportionately increased to afford effective drying. However, such a heat increase often results in a scorched web, or alternatively, if this is to be avoided, a far longer run of the web for drying purposes than is desirable or, in many instances, possible. In addition, the necessary increase in heat may be so costly as tooifset any advantages gained from the higher web speeds.
Considering, for example, the problems encountered in modern high speed printing methods, eflicient inking of the rapidly moving web may now be performed up to the limiting speeds of the web material, about 2000 feet per minute. In addition, cutting and folding stations accommodate the rapidly moving web at its other end. Accordingly, such efiicient operations on the moving web require greatly accelerated printing ink drying between the inking and the cutting and folding stations.
The factors involved in preventing satisfactory drying of the rapidly moving inked web are, among others, the nature of the solvents employed in the normally used heat-set inks and the phenomena of the boundary layer of air which follows the web. The solvents generally employed in the heat-set inks are kerosene fractions which, accordingly, are relatively high boiling point liquids with low vapor pressures. Therefore, once the solvents are raised out of the ink film on the web, they tend readily and rapidly to condense if not immediately removed therefrom.
Concerning the layer of air, termed the boundary layer, clinging closely to the moving web, such boundary layer exhibits an increasing tenacity against efforts to remove it with increasing speed of the web. This boundary layer, if undisturbed, soon becomes saturated with solvent greatly inhibiting the rate of evaporation from the ink film below it.' It will be readily understood that the solution to the drying problem must be concerned with the removal of this solvent laden boundary layer of air.
In discussing removal of the boundary layer of air, it will be understood that such language, while commonly employed in this art, may be misleading. There is always a layer of air at the-web surface, the so-called removed boundary layer being a saturated layer of air that is immediately replaced by unsaturated air to aid solvent evaporation.
Attempts have been made to remove the boundary layer from the web by a jet of air directed against the direction of web travel. However, this method has not proven satisfactory since the jet of air, directed against the web, entrains cool air and such cooling effect greatly hinders solvent evaporation. Further, it is almost impossible to get a kniflng action with such an air jet because the air issuing from an orifice tends to bell out and rapidly lose its velocity. Also, directed as it must be against the web direction, the jet tends to form eddies adjacent to the web having an unpredictable behavior.
In view of the above, it will be evident that to provide for the satisfactory drying of a rapidly moving printed web, a significant fraction of the solvent must initially be raised out of the ink film by heat. Subsequently, the solvent laden boundary layer of air clinging to the web must be removed by some means to permit enough solvent to evaporate from the ink film to leave it substantially dry. In addition, some type of system must be provided to exhaust the removed boundary layer of air containing the raised solvent, to preclude condensation of the vapor on the web.
Accordingly, it is an object of the present invention to provide for the effective drying of a moving element carried by a cylindrical surface.
It is another object of the invention to provide for the removal of the solvent laden boundary layer of air clinging to a rapidly moving web.
It is a further object of the invention effectively to dry a rapidly moving inked and heated web by removing the solvent laden boundary layer therefrom.
It is yet another object of the invention to provide a drying station on a rapidly moving web for removing the solvent laden boundary layer of air therefrom.
It is yet a further object of the invention to provide a plurality of drying stations having the above characteristics along a rapidly moving web.
These and further objects of the present invention are accomplished by providing a blade adjacent to and extending longitudinally along a roll carrying a heated rapidly moving web. Convergent and divergent sections at the inlet and outlet sides of the web are formed by the blade. A hood, directly communicating with the convergent section, encloses the web at the initial web drying station.
Means including an exhaust system produces a dilferential pressure across the blade, between the convergent and divergent sections, to induce air flow between the blade and the roll, in the direction of web movement. The passage configuration thus formed between the blade and web is such as to cause a rapid acceleration of the air flow'in the convergent section reaching a maximum velocity at a neckdown point that is sufficient to insure turbulent flow. As the roll curves sharply away from the main path of air flow, complete separation of the clinging solvent-laden turbulent boundary layer of air from the web is induced. Such separated air is removed immediately and permanently from the region of separation by the main flow of high velocity air which carries such solvent-laden air into the exhaust system. A counter flow of air along the web is provided to assist in flowing the solvent-laden air into the exhaust.
- A further feature of the invention resides in the provision of vacuum rolls, under the blades if desired, to prevent web flutter due to the turbulence and lift produced by the high velocity air flow by controlling web tautness throughout the drying process.
These and further objects and advantages of the present invention will be more readily understood when the following description is read in connection with the accompanying drawings in which:
FIGURE 1 is a side elevation, partially in sect-ion, of apparatus for drying a rapidly moving web constructed in accordance with the present invention;
FIGURE 2 is a view in perspective, partially broken away, of a drying unit employed in the web drier illustrated in FIGURE 1;
FIGURE 3 is a fragmentary side elevation of the apparatus of FIGURE 1 illustrating in some detail the mounting of a blade utilized in such apparatus;
FIGURE 4 is a schematic diagram useful in explaining the principles of operation of the present invention;
FIGURE 5 is a longitudinal section of a vacuum roll that may be used in the drier of FIGURE 1;
FIGURE 6 is a transverse section of the vacuum roll illustrated in FIGURE 5 taken along the view line 66 looking in the direction of the arrows; and
FIGURE 7 is an enlarged sectional view of a slot in the vacuum roll of FIGURE 5 taken along the view line 6-6 looking in the direction of the arrows.
Although the present invention will be described in connection with inked paper webs, it will be understood that the drying principles set forth may be equally well applied to other rapidly moving webs or other elements supported by rolls such, for example, as paper, motion picture film, printing plates, textiles and coated material of any nature.
Referring to an illustrative embodiment of the invention with particular reference to FIGURE 1, a rapidly moving web 10, newly inked at a printing station (not shown), travels in the direction indicated by the arrows over a vacuum roll 9 and rolls 11, 12, 13 and 14 to direction shifting rolls 15, 16 and 17, and then over rolls 18 and 19. The rolls 9 and 11 to 19, inclusive, are supported by any conventional structure (not shown) on the side frames 20 of the web drier. In addition, suitable means (not shown) may be provided for rotating the rolls '9 and 11 to 19 to drive the web 10, in accordance with conventional practices in this art.
A plurality of gas burners 21a, 21b and 210 supported in any desired manner (not shown), are positioned along and extend later-ally across the web 10. Pursuant to established practices, the burners 21a to 210, inclusive, are controlled in accordance with the speed of the moving web 10 to heat it to a predetermined temperature and to preclude scorching thereof.
A hood 21d, supported by angle bars 22, is disposed around the moving Web 10 and the gas burners 21a to 210, inclusive, in order to prevent cool air currents from interfering with a planned rise in temperature on the web 10. The hood 21a is constructed from top, bottom and side pieces 23 to 25, respectively, formed from heat insulating material. The top 23 and sides 25 may be interiorly lined with a layer of aluminum foil 26, although this has not proven necessary in many instances.
Upon reaching the end of the hood 21d, the rapidly moving web 10 encounters the roll 15 which carries it through a change in direction, shown here as a 90 degree turn.
A pair of brackets 27 are supported from the frames 20, as best shown in FIGURES 1 and 3, by bolts 27a in slots 27b. This permits longitudinal adjustment of the bracket 27 along the frames 20. Obliquely positioned supporting arms 28 provided with transversely extending slots 29 carrying bolts 39 are welded to the brackets 27. Threaded openings 31 receiving set screws 32 lead to the extremities of the slots 29. It will be evident that adjustment of the bolts in the slots 29 may be achieved by suitable rotation of the set screws 32.
A blade 33, obliquely suspended immediately above the roll 15, includes a lateraly extending supporting plate 34 having its ends formed with a pair of busses 35 joined by reinforcing bars 36. Threaded lateral openings 37 through the busses 35 receive the bolts 30, which also pass through spacers 38 between the arms 28 and the blade 33.
Fastened to the supporting plate 34 by a plurality of bolts 39 is a rectangularly shaped hollow chamber 40 formed with a flange 41 on the lower edge thereof, openings 42 and 43 being provided at both ends of the chamber 40. A medium such as air may be circulated through the openings 42 and 43 to maintain the temperature of the lower surface of the blade 33 at a level precluding condensation thereon of solvent vapors from the web.
Each end of the blade 33 mounts a side shield 44 arcuately shaped to fit the roll 15 closely, as best shown in FIGURE 3, and fastened by suitable means such as brazing or welding to clamping brackets 44a. Each of the clamps 44a extends laterally across the bottom of the blade 33 and is secured at one end to the supporting plate 34 by a foot 45 carrying set screws 46, and at its other end to the flange 41 by a foot 47 carrying set screws 48. In addition, an L-shaped bar 49, fastened by bolts or rivets 50 to the flange 41, provides for a continuation of the lower face of the blade 33 in the direction of web travel.
A plane surface adjacent to the roll 15 is provided by the blade 33. It will be understood that the term blade as used herein refers to any structure forming an equivalent surface.
Referring to FIGURES 1 and 2, an angle bar 51 fastened to the frames 20 by appropriate structure (not shown) supports a rectangularly shaped exhaust chamber 52 joined thereto by means of hinges 53. It will be apparent that this construction permits the exhaust chamber 52 to swing back about the angle bar 51. A pair of angle bars 54 and 55 on the frames 20 are joined by bolts 55a. A further angle bar 56, aflixed to the upper edge of the exhaust chamber 52, is clamped against the bar 54 by bolts 57 on the bar 55.
The exhaust chamber 52 communicates with ducts (not shown) leading to suitable exhaust fans or pumps. This arrangement permits the pressure in the chamber 52 to be suitably decreased to perform functions explained in detail below.
To form an exhaust mouth 58 in the exhaust chamber 52, adjacent rectangular portions 58a and 58b are cut from the top and one side thereof adjacent to the roll 15, such portions extending from one end of the chamber 52 to a position adjacent to one end of the roll 15. A lower lip 59 for the exhaust mouth 58, angled toward the roll 15, is secured by bolts 60 to coincide laterally with the cut away portion 53b. To complete the exhaust mouth 53, the L-shaped piece 49 extends into the cut away portion 58a, as shown in FIGURES 1 and 2, and a sealer strip 61, adjustably mounted by bolts 62 in slots 63 on top of the exhaust chamber 52, may be positioned to abut the plate 49.
The roll 15, the blade 33, the exhaust chamber 52 and associated mechanism form what will be termed a drying station for the web 10. It will be understood that any number of stations necessary to dry the rapidly moving web 10 satisfactorily may be employed along its path. In FIGURE 1, a web drying system utilizes three drying stations along the rapidly moving web 10, the first station being described above, the second station being associated with the roll 16 and the third station with the roll 17. The web 10, after leaving the first drying station, encounters the rolls 16 and 17 which carry it through further changes in direction and complete the drying process.
The roll 16 may comprise a conventional water cooled roll in order to initiate cooling of the residual ink film to room temperature, if the ink film is based on thermoplastic resins or other materials requiring such cooling. The roll 17 may comprise a vacuum roll and cooperate with the vacuum roll 9 to insure adequate tensioning of the web 10 under the blades 33 and 33' to prevent web flutter at the drying stations.
Other suitable tensioning devices may be substituted for the vacuum roll 9 such as a reel or other device exerting drag on the web. If desired, a suitable take-up reel or other device exerting a constant tension on the web 10 downstream of the drying stations may be used to replace the vacuum roll 17. Since the drying stations associated with the rolls 16 and 17 are similar in many respects to that at the roll 15, similar elements will be designated by primed reference numerals.
In each of the second and third drying stations, brackets 27, secured to the frames 20 by bolts 27a, are fastened to a blade 33', which may be identical to the blade 33 or may lack the chamber 40' as shown. Supporting arms 28 suitably position a plate 34' adjacent to the roll by means of bolts 30. Side shields 44, functioning for the same purpose as the shields 44 but slightly different in configuration due to the position of the blade 33 with respect to the roll 16 or 17 are secured by means of a clamp 44a to the plate 34. In addition, an L- shaped plate 49 effectively extends the face of the plate 34 into an exhaust mouth 58. The exhaust aperture 58 is further defined by a lower lip 59' and leads into an exhaust chamber 52 supported by angle bars 64 and 65 which are joined by angles 66 and 67, respectively, to lower frame members 20b, joined by blocks 20a to the frame 20. An adjustably mounted sealing strip 61' abuts the extension plate 49". It will be apparent that the elements in the second and third drying stations corresponding to those found in the first station differ therefrom because of mechanical design considerations.
In examining a typical operation of the present invention, it will be helpful to state the nature of the problems involved in drying heat set inks printed on rapidly moving webs. At the outset, it should be understood that it is a simple matter to provide sufficient heat to raise the solvent from the'ink film into the air immediately adjacent to the web. However, normal air will not hold the desired quantities of these relatively high boilingpoint 'called boundary layer of solvent laden air from a rapidly moving web. This boundary layer clings with great tenacity to the moving web and its saturated condition greatly inhibits the rate of evaporation of solvent from the ink film beneath it. In summary, the solvent saturated boundary layer of air must be removed effectively from the rapidly moving web to permit further drying of the ink and to prevent condensation of the solvent further along the line of web travel.
Examining the diagrammatic drawing of FIGURE 4, the roll 15 is illustrated together with the blade 33 and the exhaust mouth 58. Before the rapidly moving web engages the roll 15, it has been heated to vaporize the solvent found in the ink film. The. gas mixture pressure in the burners 21a, 21b and 210 will, of course, be increased as the web speed is raised in order to heat the web 10 sufficiently to perform this function. As mentioned above, the solvent vapor will saturate the boundary layer'of air clinging to the moving web 10.
Due primarily to the vacuum found in the exhaust chamber 52, a high velocity stream of air, identified by the arrows 68, will fiow into a converging section A formed by the blade 33 and a portion of the web 10 supported by the roll 15. The provision of a constricted portion or throat B, formed by the necking down of the blade 33 to the roll 15, will greatly accelerate the air stream 68. The maximumair velocity will be reached in the neckdown B and its magnitude may be controlled by adjustment of the vacuum in the exhaust chamber 52 to insure that Reynolds number for the flow at point B exceeds a critical value established by well known fluid dynamic principles. In one structure embodying the principles of the invention, the critical Reynolds number has beendetermined to be 5000, and the drier has been operated at 6000 as a safety factor. The boundary layer will accordingly change from laminar to turbulent flow about at point B and thus facilitate its subsequent separation from the web surface when subjected to a sufiiciently large pressure drop perpendicular to the web surface.
Turbulence is a function of the quantity known as Reynolds number which is expressed as where V=av. stream velocity in ft./ sec.
L=representative linear dimension in feet (L is height of neckdown) v=kinematic viscosity (a temperature and pressure function) It is apparent from the above formula that turbulence is a function of dimension, air speed, temperature and pressure (which together define the viscosity of the air.) Appropriate steps are taken to control the elements that contribute to the critical Reynolds number to provide turbulence which results in mixing and an attack on the stagnant boundary layer.
, Normally, fio-w above the rapidly moving web 10 is laminar. flow toward an exhaust system is opposed by the strong directional vector of the moving web. Laminar flow opposes mixingthe solvent tends to cling to the sheet, to stagnate in the boundary layer, and by its concentration there, preclude further evaporation from the hot ink film below it. Previous driers have attempted to overcome these problems by pouring in additional energy in the form of heat. But heat is dangerous for webs, expensive, and does little to offset the tendency of laminar fiow to re-establish a saturated solvent-bearing layer on the web. Turbulence provides a powerful mixing action so that solvent is distributed uniformly above the web 10. Such turbulence exists downstream from the neckdown line B to the lip 59 of the exhaust.
From the neckdown line B, the accelerated air stream passes into a divergent section or region C formed by the blade 33 and a portion of the web 10 supported by the roll 15. In section C, the rapidly moving turbulent air flow 68 tends to follow a linear path along the blade 33 which produces the desired pressure gradient or lifting action which, with the breaking away of the web 10 from this path .as it follows the roll 15, induces separation of the solvent laden turbulent boundary layer of air at the surface of the Web 10'. As shown by the small arrows 69 indicating separation in the divergent section C, the solvent laden air in the turbulent area is liftedv and enters the main stream of air 68, which carries it into the exhaust mouth 53. In other words, separation results in a direct and violent attack on the boundary layer tending to peel it off and move it into the main line of air flow into the exhaust chamber. In connection with boundary layer separation, the kinetic energy of the main air stream is reduced as it flows into the divergent section C since it encounters a rising pressure gradient. In addition, the kinetic energy of the boundary layer of air at the web surface, which has been at a much lower level than that of the main air stream due to frictional losses, drops to zero. Great shearing stresses are thus set up that induce separation of the solvent laden boundary layer from the web 10 and it is swept up into the main air stream and into the chamber 52. The magnitude of these stresses is increased by virtue of the turbulent flow set up at the neckdown B and continuing into the divergent section C. In addition, the turbulent flow insures transverse mixing and a thinner boundary layer of air.
Boundary layer separation is initiated on the web surface in a region at which a tangent to the roll 15 makes an angle with the blade 33 exceeding about 3 /2 degree or, in other words, when the air stream sufficiently exceeds an angle of about 3 /2 degrees to a tangent to the roll 15 parallel to the blade 33. Since this angle is determined by the diameter of the roll 15, initiation of separation in the region C is a function of the roll diameter. Further along the roll 15 separation of the turbulent boundary Any attempt to diffuse solvent through this 7 layer increases until maximum separation is reached at a point at which a tangent to the roll makes an angle of' about to degrees with the blade 33. Preferably the lip 59 extends to about such point of maximum separation to remove all of the separated air immediately from the web 10 and direct it into the exhaust chamber 52. The foregoing theoretical explanation should not be considered to limit the present invention to the exact. angles set forth since the region at which separation is. induced is not critical.
It is preferable to position the blade 33 so that the neckdown B occurs midway of the contact line along the web direction on the roll 15. This is true because theneckdown B is the point of maximum air velocity and therefore, it is also the point where the lift exerted upon. the web 10 will be greatest. Since the web 10 must hug; the roll 15 if the drying system is to operate efficiently, by providing the neckdown B substantially midway along the contact line, the lifting vector is opposed by the maxi-- mum vector of force on the web 10 acting towards the roll 15.
While a degree wrap of the web on the rolls 15, 16' and 17 has been shown in FIGURE 1, a greater or lesser amount of wrap may be used to dry the web effectively without web flutter. In other words, if web speed and air velocity are increased, a greater wrap of the web 10 on the rolls would be preferred to oppose the higher lifting force at the neckdown B.
The lifting force at the neckdown B is also combatecl by the use of the supporting roll 15 since atmospheric pressure is not free to act on the underside of the web 10 at this critical point. In addition, the lifting force on the web 10 is also counteracted by suitably tensioning the web 10 by the use of vacuum rolls 9 and 17, or some equivalent devices, as pointed out heretofore. Such a taut web resists the lefting forces encountered under the divergent blades 33 and 33'.
If a vacuum roll is used under the blades 33 or 33, the reduced pressure created under the web 16 by the vacuum roll 17, for example, furnishes a pressure differential across the web that urges it against the roll in opposition to the lifting force of the turbulent boundary layer. Such arrangement also assists in preventing web flutter.
One type of vacuum roll assembly that may comprise the roll 9, or that may be used at the drying stations, is disclosed in US. application Serial No. 635,005, filed January 18, 1957, for Apparatus for Tension Control. Referring to FIGURES 5 and 6, a cylindrical shell 161 is secured to end rings or plates 162 and 162' by threaded bolts. At one end of the shell 161 a hollow journal 171 is connected to a bearing (not illustrated) and through a center pipe to a suitable source of vacuum. The center pipe 165 extends axially into a vacuum chamber enclosure indicated generally at 190 which includes a vacuum chamber 175 enclosed within the cylindrical shell 161. The end of the vacuum chamber 175 remote from the pipe 165 is closed by a plug 176 and supported in a bearing 177 mounted in the rotating end plate 162. A bearing 180 suitably mounted in the rotating end plate 162' surrounds the pipe 165.
The vacuum chamber enclosure consists of a pair of side walls 191 and two end walls 192. Longitudinal channels 193 are formed by upstanding flanges 194 and auxiliary plates 195 and 195 bolted to the enclosure as shown in FIGURE 6. The upstanding flanges 194 are formed integral with the chamber housing 190. Transverse channels 196 at the outer extremities of the walls 192 are formed by upstanding plates 197 and 197' (FIG- URE 5) that are bolted to the vacuum chamber housing 190, and by the upstanding flanges 198 integral with the vacuum chamber housing 190.
Longitudinal sealing members 199 are mounted in the channels 193 and biased outwardly against and in frictional engagement with the inside surface of the cylindrical shell 161 by resilient tubes 200 mounted in the bottom of the respective channels. The biasing tubes preferably are formed of an age-resistant elastomeric material such as silicone rubber and, if necessary, may be inflated to produce the desired pressure for sealing the members 199 against the cylindrical shell 161. However, selection of tubing of the proper size and thickness makes it unnecessary to inflate those elements.
Lateral or transverse sealing members 201 are mounted in the end wall channels 196 and urged upwardly against the inside surface of the cylindrical shells by helical springs 202.
The longitudinal sealing members 199 and the transverse sealing members 201 are maintained in relatively air-tight contact with each other by adjustable spring screw units 204, two being located at each end of the chamber 175. The spring screw units 204, which include a threaded bolt 295 formed with a flanged head 206, pass through a washer 207, the plate 197 and the sealing element 201 without engaging those elements. Between the washer 207 and the head 206 is positioned a helical spring 208 acting on the washer 207 and the head 206 tends to pull the longitudinal sealing members 199 in an axial direction toward the lateral sealing members 201.
Shown at the opposite end of the vacuum chamber in FIGURE 5 is a bolt 210 threaded in the end plate 197 and provided with a lock nut 211. The bolt 210 is formed with a hollow cylindrical center using a helical spring 212 that presses against the lateral sealing member 291 thereby tending to maintain it in air-tight engagement with the flange 198. Several such bolts are provided at both ends of the vacuum chamber.
An upper vacuum chamber section 175" communicates with the central portion of the vacuum chamber 175 through apertures 213 best shown in FIGURE 6. A negative pressure may be maintained in the chambers 175 and 175 through the axial center pipe 165 and such negative pressure will be transmitted to slots 222 in the shell 161 to hold the web on the roll firmly against the outer surface of the shell 161.
The grooves 222 in the shell 161 are formed with an arcuate shape shown in FIGURES 5 and 7. Such grooves can be milled in the walls of the cylinder 161 by means of an 0.028" thick milling cutter resulting in a groove width of 0.028 to 0.013". A groove substantially wider permits the vacuum to mark the paper by pulling it inwardly. For processing heavier webs wider grooves may be used and, conversely, for lighter webs, narrower grooves may be used. In FIGURE 7 it may be observed that a sharp corner at the edge of the groove on the inside of the shell 161 is rounded off or chamfered to prevent damage to the sealing members 199. The edge of the groove on the outside edge of the periphery remains sharp as machined to assist in maintaining a firm hold on the web. The grooves in adjacent axial rolls are overlapped to obtain continuous coverage of the area of contact between the paper and the cylindrical surface.
To reduce noise originating from the successive sudden exposure of the slots 222 to atmospheric pressure, a structure designated generally at 225 (FIGURE 6) is provided. The longitudinal plate 195 extends upwardly almost to the inner surface of the cylindrical shell 161 and at that point is provided with a skirt 226 gradually tapering away from the inner cylindrical surface, moving in the direction of rotation of the shell. A wedge-like space i thereby formed between the skirt 226 and the shell 161 with the smallest clearance near the edge of the vacuum chamber. The structure described provides for gradual refilling of the evacuated slots, rather than a sudden refilling, with a consequent sudden reduction or even complete elimination of noise.
To maintain the tension of the web 10 to prevent Web flutter under the blades 33 and 33', the vacuum roll may be controlled in two ways: (1) the vacuum may be maintained at a uniform level and the surface speed of the rolls 9 and 17 increased or decreased as desired by means of conventional variable speed devices driving the vacuum rolls or (2) the surface speed of the vacuum rolls may be constant and the vacuum varied. The first alternative (the control of the speed of the vacuum roll with constant vacuum) is preferred. In the second alternative (control of the vacuum) changing the vacuum varies the surface friction between the rolls and the web 10, thereby permitting control of web tension.
With the above principles in mind, the illustrative embodiment of the invention shown in FIGURE 1 will be readily understood. The rapidly moving web 10 passes through the hood 21d Where it is heated by the gas burners 21a, 21b and 210. It should be noted that the hood 21d, being positioned adjacent to the blade 33 and side shields 44, actually constitutes part of a single enclosure extending along the web into the exhaust chamber 52. Thus, heated air provided by the burners 21a, 21b and 21c, increases the temperature of the air passing between the blade 33 and the web 10. The heated air from the hood 21d and the heat radiated from the web 10 tend to heat the blade 33 which, together with the chamber 40, prevents it from condensing solvent vapors.
As the web 10 travels over the roll 15, the high velocity stream of air changes the solvent laden laminar boundary layer to a turbulent boundary layer at region B. Further along the web 10, separation of the solvent laden turbulent boundary layer occurs, as explained above, in the divergent section C created by the blade 33 and a portion of the web 10 supported by the roll 15. This results in the removal of the turbulent boundary layer from the web 10, and it is immediately carried into the exhaust chamber 52 along with the stream of air as indicated by an arrow 70 in broken line in FIGURE 1. Additional solvent will now evaporate from the ink film since the restraining solvent laden boundary layer has been removed.
The positioning of the lower lip 59 on the mouth 58 assist in directing the solvent l-aden air into the chamber 52, and a suitable exhaust system creating a negative pressure or vacuum in the chamber 52 also removes the solvent carrying air therefrom.
It should be understood that the precise clearance between the blade 33 and the roll is not critical. However, it is preferably kept in excess of one-quarter inch to avoid trouble with pasters, air bubbles and Wrinkles. In addition, with a very high capacity system, a smaller neckdown may choke the flow of high velocity air. In one embodiment of the invention, it was found that the neckdown dimensions could be varied from between three sixteenths to three-quarters inch without seriously imr pairing its operation. An exemplary clearance i about three eights to one-half inch.
In connection with the lower lip 59, it is placed as close to the web 10 as possible, for example one-quarter inch, to permit the reduced pressure in the exhaust chamber 52 to act with maximum efiiciency. Such reduced pressure also induces a reverse or counter air flow into the divergent section C between the lip 59 and the web 10 (FIGURE 4) which aid in promoting and maintaining boundary layer separation.
In a typical drying station, the aperture of the exhaust mouth 58 may measure about two inches across with a sixteen inch diameter roll 15, these dimensions providing a sufficieut area for separation of the boundary layer of air from the rapidly moving web 10 before the pickup of such air.
In applying the invention to the printing art where, for example, heat-set inks with solvents consisting of kerosene fractions with relatively high boiling points and low vapor pressure are employed and the ink film is based on thermoplastic resins, it has been found that one drying station is adequate to dry the rapidly moving web 10 at speeds of, about 1000 feet per minute Beyond that speed, it
beyond that source.
has been found that the solvent continues to rise from the web 10 beyond the drying station. Accordingly, at higher speeds second and third stations such as those shown in association with the rolls 16 and 17 may be provided. However, it is generally not necessary to heat the rapidly moving web between drying stations because as the speed of the web 10 increases, the cooling effect of the rapidly moving air in each drying unit is less pronounced and the heat level persists for a longer distance However, if the speed is greatly raised or for other reasons, it will be understood that further heating means may be employed within the scope of the present invention.
Examining the operation of the drying station employed in connection with the roll 16, the rapidly moving web 10 passes under the blade 33'. Due to the exhaust chamber 52', a high velocity air stream passes between the Web 10 and the blade 33 creating turbulence and separation of the solvent laden boundary layer as discussed in connection with the first drying station. The separated saturated air immediately passes into the chamber 52 as shown by an arrow 70' in broken line, and is subsequently exhausted therefrom. The rapidly moving web 10 is then advanced to the drying station at roll 17 for further drying and from there may be carried to a conventional cutting and folding station by the rolls 18 and 19.
Rather than providing a roll at each drying station, two
or more drying stations may be positioned adjacent to a larger roll around which the web 10 is wrapped to a greater extent than on the rolls 15, 16 and 17.
While the inventive methods and apparatus have been described as particularly advantageous in connection with high speed webs, such methods and apparatus are also useful in drying of rapidly moving webs traveling at somewhat slower speeds such as in gravure or aniline (flexigraphic printing) printing presses. In such instances,
direct heating of the web may be eliminated in favor of heat requirements previously rendering high speed drying impractical and uneconomical. Of course, the abovedescribed embodiments of the invention are illustrative only and modifications thereof will occur to those skilled in the art. Therefore, the invention is not to be limited to the specific apparatus disclosed herein but is to be defined by the appended claims.
1. A drying unit for a rapidly moving web comprising a roll adapted to support and change the direction of travel of the web, a blade extending longitudinally along at least a portion of the roll and positioned adjacent to a supported portion of the web to form with the web convergent and divergent sections joined by a throat, a pair of side shields mounted on the blade and'conforming to the configuration of the roll for partially enclosing the convergent and divergent sections and the throat, means for creating a pressure differential across the blade to generate a high velocity flow of air through the throat in the direction of web travel, the air flow inducing turbulence and shearing and lifting forces at an area adjacent to the web in the divergent section, and means for immediately exhausting the air flow from the divergent section.
2. Apparatus as defined in claim 1, in which means are provided to induce a counter flow of air along the web into the divergent section.
3. Apparatus as defined in claim 1, wherein the pressure differential creating means comprises the exhaust means.
14. Apparatus as defined in claim 1, wherein the roll comprises a vacuum roll to prevent web flutter under the blade.
5. Apparatus for drying a rapidly moving web comprising a roll adapted to support and change the direction of travel of the web, a blade extending longitudinally along at least a portion of the roll and positioned adjacent to a supported portion of the web to form with the web convergent and divergent sections joined by a throat, a pair of side shields conforming to the configuration of the roll for partially enclosing the convergent and divergent sections, means for at least partially enclosing a portion of the web leading to the roll, said enclosure means communicating with the convergent section defined by the blade, the roll, and the shields, means in said enclosure means for heating the rapidly moving web, means for creating a pressure differential across the blade to generate a high velocity flow of air through the throat in the direction of web travel, the air flow inducing turbulence and shearing and lifting forces at an area adjacent to the web in the divergent section, and means for immediately exhausting the air ilow from the divergent section.
6. Apparatus as defined in claim 5, in which means are provided to induce a counter fiow of air along the web into the divergent section.
7. Apparatus as defined in claim 5, wherein the pressure differential creating means comprises the exhaust means.
8. Apparatus as defined in claim 5, in which means are provided for cooling the blade.
9. A system for drying a rapidly moving web comprising a plurality of drying stations along the web, each of the drying stations including; a roll adapted to support and change the direction of travel of the web, a blade extending longitudinally along at least a portion of the roll and positioned adjacent to a supported portion of the web to form with the web convergent and divergent sections joined by a throat, a pair of side shields mounted on the blade and conforming to the configuration of the roll for partially enclosing the convergent and divergent sections, means for creating a pressure differential across the blade to generate a high velocity flow of air through the throat in the direction of web travel, the air flow inducing turbulence and shearing and lifting forces at an area adjacent to the web in the divergent section, and means for immediately exhausting the air stream from the divergent section; means for at least partially enclosing a portion of the web leading to the initial station, said enclosure means communicating with the convergent section defined by the blade, the roll, and the shields found in the initial station, and means in said enclosure means for heating the rapidly moving web.
10. A drying unit for a rapidly moving web comprising a roll adapted to support and change the direction of travel of the web, a blade extending longitudinally along at least a portion of the roll and positioned adjacent to a roll supported portion of the web to form with the web convergent and divergent sections joined by a throat, said throat being between the blade and the roll supported portion of the web, means to create a sufiicient pressure differential across the throat formed by the blade to flow air through the throat in the direction of Web travel tangentially to the web to provide a turbulent boundary layer on the web flowing in the direction of web travel and to separate the turbulent boundary layer from the web at a region in the divergent section, and means adjacent to and downstream from the roll and blade to exhaust immediately the separated turbulent boundary layer from the region of separation between the blade and the web.
11. Apparatus as defined in claim 10, wherein the exhaust means comprises a chamber having a mouth communicating with the divergent section, a lip forming one wall of the mouth extending into close proximity with the web, and means to couple the chamber to a vacuum source to create said pressure differential and exhaust the separated turbulent boundary layer.
12. Apparatus as defined in claim 11, wherein the lip is positioned to create a counter flow of air along the web into the divergent section between the lip and the web.
13. Apparatus for drying a rapidly moving web com- .prising a roll adapted to support and change the direction of travel of the web, a blade extending longitudinally along at least a portion of the roll and positioned adjacent to a roll supported portion of the web to form with the web convergent and divergent sections joined by a throat, said throat being between the blade and the roll supported portion of the web, means to heat the Web prior to its passage over the roll, means to create a sufficient pressure differential across the throat formed by the blade to flow air through the throat in the direction of web travel tangentially to the Web to provide a turbulent boundary layer on the web flowing in the direction of web travel and to separate the turbulent boundary layer from the web at a region in the divergent section, and means adjacent to and downstream from the roll and the blade to exhaust immediately the separated turbulent boundary layer from the region of separation in the divergent section.
14. Apparatus as defined in claim 13, in which means are provided for varying the blade temperature.
15. Apparatus as defined in claim 13, in which is provided means to engage the web upstream and downstream of the roll to maintain the Web in tension and hold it against the roll in opposition to the lifting force of the turbulent boundary layer.
16. Apparatus to dry a wet surface comprising a roll adapted to support the wet surface concentric with at least a portion of the roll and to move the Wet surface in a desired direction, a blade extending longitudinally along the roll to form with the wet surface convergent and divergent sections joined by a throat, said throat being between the blade and the wet surface, means to create a sufiicient pressure differential across the throat formed by the blade to flow air through the throat in the direction of travel of the wet surface and tangentially to the wet surface to provide a turbulent boundary layer of air on the wet surface flowing in the direction of movement of the surface and to separate the tubulent boundary layer from the surface at a region in the divergent section, and means communicating with and downstream from the divergent section to exhaust immediately the separated turbulent boundary layer from the region of separation in the divergent section.
17. Apparatus as defined in claim 16, in which means are provided to heat the air prior to its fiow into the throat between the blade and the roll.
18. Apparatus as defined in claim 10, in which is provided means to engage the web upstream and downstream of the roll to maintain the web in tension and hold it against the roll in opposition to the lifting force of the turbulent boundary layer.
References Cited in the file of this patent UNITED STATES PATENTS 175,644 Becker Apr. 4, 1876 1,576,906 Greve Mar. 16, 1926 1,656,853 Bean Jan. 17, 1928 1,724,645 De Long Aug. 13, 1929 2,082,411 Merrill June 1, 1937 2,091,805 Chuse Aug. 31, 1937 2,186,032 Mann Jan. 9, 1940 2,209,760 Berry July 30, 1940 2,268,987 Hess et a1. Jan. 6, 1942 2,574,844 Roden Nov. 13, 1951 2,659,162 Katz Nov. 17, 1953 2,677,197 Lindgren May 4, 1954 2,775,046 Kabelitz Dec. 25, 1956 2,783,546 Armstrong Mar. 5, 1957 FOREIGN PATENTS 235,952 Germany June 6, 1911