US 3176412 A
Description (OCR text may contain errors)
April 6, 1965 'r. A. GARDNER MULTIPLE NOZZLE AIR BLAST WEB DRYING 4 Sheets-Sheet 1 Filed Jan. 4. 1961 MENTOR. 20m" 4. Gaza/are W! m M April 1965 T. A. GARDNER A 3,176,412
MULTIPLE NOZZLE AIR BLAST WEB DRYING Filed Jan. 4, 1961 4 Sheets-Sheet 2 n 5 INVENTOR.
M1, MA Q ATTORNEY! April 1965 T. A. GARDNER MULTIPLE NOZZLE AIR BLAST WEB DRYING 4 Sheets-Sheet 3 Filed Jan. 4, 1961 IN V EN TOR. Tim/w?! '9. Gaza/vex April 6, 1965 T. A. GARDNER 3,176,412
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o .5 L0 1.5 2.0 2.5 3.0 IN VEN TOR. DI5TANcl EETWEEN NozzLES- mcnes TQ ,q GHQDNEE A 7' 7' GENE Y5 United States Patent 3,176,412 MULTIPLE NOZZLE Am BLAST WEB DRYlNG Thomas A. Gardner, 513 Clark St, Neenah, Wis. Filed Jan. 4, 1961, Ser. No. 80,610 8 Claims. (Cl. 34-422) This invention relates to multiple nozzle air blast web drying and is a continuation in part of application Serial No. 662,891, filed May 31, 1957, now abandoned, and application Serial No. 743,437, filed June 20, 1958, now abandoned. The invention seeks to provide increased drying capacity by operating at low Reynolds number values upon the laminar boundary layer of air and vapor in immediate proximity to the Web to be dried. (Any drying gas may be used, the term air being employed generically since air is the gas most commonly used for drying.) The invention has been tested successfully in the drying of webs of newly-formed paper, the drying of paper coatings, and the drying of inks, dyes and the like on the surface.
While it is convenient to refer to the nozzles which create the jets, it should be noted that it is the jets of dehydrating gas that do the drying and are required to have the critical dimensions and spacing. Thus the figures given above may be stated in terms of the jets themselves. The jets should be spaced from each other not less than three-quarters of an inch nor more than two inches. They should originate at a distance of no more than three-quarters of an inch and preferably one-eighth to three-eighths of an inch from the surface of the work to be dried. The jets should have a total cross section at the said distance of origination which amounts to one and one-half percent to five percent of the area acted upon, the preferred percentage area being two to four percent.
The invention provides increased drying capacity by operating on the boundary layer film in immediate proximity to the web being dried in such a manner that its resistance to the flow of heat and mass is reduced.
The invention is based on critical relationships of nozzles including specifically the ratio of total orifices area to total work area; nozzles spacing from the work to be dried; and the spacing of the nozzles from each other. The distance between nozzles is particularly critical. It should be not less than of an inch nor more than 2 inches, with peak performance at a space slightly in excess of one inch. The percentage of total area of nozzle orifice to the total heat transfer area of the surface acted upon should be not less than 1 /2 nor more than 5 /2%, with a preferred range from 2 to 4%. The distance of the nozzle from the surface of the work is less critical but should be between /s" and /s" for best results. The dryer shows no improvement over previously known dryers if this spacing is increased beyond of an inch.
The invention further seeks to provide equal drying across the entire width of continuous webs used in industry. Only if drying is uniform is it possible to operate at high temperature. The large flow area in the preferred plenum compared to the flow area of the orifices ensures that the pressure drop in the orifices will exceed by many times the pressure variations in the plenum. Thus the flow through all orifices is extremely uniform. Uniform drying across wide machines is dilficult to maintain. Performance of the machine herein disclosed is very superior.
The invention also provides more economical drying by the conservation of heat and power ordinarily used in drying.
I A major complaint made with respect to previously known dryers is the massive loss of air into the room in which the dryer operates. On virtually all dryers, 50%
more or less, of the discharge from the first nozzle and the last blows out into the atmosphere after impinging on the surface, because of its high energy. Because of the multiplicity of small capacity nozzles used in the present dryer, the percentage of air lost to the room becomes very small. In the present dryer the air loss with nozzles 1 inch apart is only one third as great as the loss from a dryer of equal size and air capacity with nozzles 3 inches apart.
The matter of air loss is important. Such air carries away both heat and vapor. With small air loss the dryer may be operated at high temperatures and humidities. High humidity operation requires less air to be heated to secure the same amount of drying. Thus the thermal efiiciency is high. High humidity also provides high concentration of latent heat, which is easier to reclaim if desired. In the present dryer heat reclamation is economically feasible; formerly this has not been true. In the drying of costly solvents, high solvent humidity permits easier reclamation of the solvent.
There is a further advantage of the present dryer in that it accomplishes a given drying job in very much less space than is required by any previously known machine of comparable capacity. This not only reduces the demands on floor space in the building in which the dryer is used, but it reduces heat losses attributable to radiation and convection.
Another advantage of using very small opacity nozzles with high jet energy is to provide the beneficial effect of high velocity flow without high kinetic energy such as might disrupt fragile webs or unstable surface coatings.
In practice, the web may be supported either by a relatively flat plate or by a drum or floated between opposed sets of drying nozzles. The speed of the web is broadly immaterial since the flow of drying air is preferably at a rate so high as to make conventional rates of web movement immaterial.
It is conventional to use heat in the drum or plate over which the web is trained for drying purposes. In the use of the present invention, evaporation of moistures or other evaporized liquids will occur so rapidly from the surface of the web as to make it possible to increase the amount of heat used in the drum, at the same time eliminating by evaporative cooling the difliculties experienced in some dryers when the heat is excessive for the particular materials involved.
Laterally elongated non-circular nozzles preferably extending the full width of the web are so organized that the jets of drying gases issuing from the nozzles are directed against the web substantially at right angles to its surface or at an angle which, allowing for web movement, results in a pattern of flow and pressure gradient which is substantially symmetrical from the stagnation point opposite each nozzle. Each nozzle may provide a substantially continuous opening which is as long as the width of the web, broken only by spacers which maintain its side margins at uniform distances.
A feature of the present invention is the provision of means for the escape from the vicinity of the drying surface of air which has been spent in impinging on the surfaces, the escape being accomplished without impairing the uniform drying effect or in any manner impairing the dried web or its coating, and without limiting the length, width, or number of nozzles. The flow of impinged drying air along the surface acts as a curtain which isolates the surface from the spent air. The arrangement To provide a multiplicity of nozzles with their respective pressure gradients extending across the entire width of a web trained over a' heated dmm results in holding the web more tightly to the drum, air and vapor being purged from the space between the web and the drum,
thereby increasing'heat fiow from the drum to the web. Also the mass transfer rate of vapor. away from the web is increased and the vapor pressure and the related saturation temperatureon the surface of the web are reduced, thereby increasing heat flow from thedrum to the web and making it possible to use higher drum temperatures without blowing off the web. The vapor mass transfer coefficient is efficient.
There are essentially three primary features of the apparatus. They are:
proportional to the heat transfer e 1 In theidr awings':
by these opposing side walls and the interior spaces'of the respective channels open at' their ends to permit the escape of the vapor bearing air. In a preferred organization the air is recirculated through'a filter and heater with controlled continuous withdrawal of some of the saturated air and its'replacement with unsaturated air. There is nothing fundamentally new about recirculation but, as will be shown hereafter, the drying results produced by the instant invention arevery. greatly superior to the drying achieved with any' other known device for the purpose.
.FIG. lis a view taken in section of FIG. 2 through a typical installation. Y v 1 FIG. 2 is 'a'view'inrear elevation of the device shown in FIG- 1, portions of the housing being broken away.
FIG.'3 is a greatly enlarged detail view of some of the nozzle structure'shown in FIG. 1.
FIG. 4 is a view showing the drum in side elevation and the nozzle structure in section on line 4-4 of FIG. 1, the'section also being indicated at 4+4 in FIG. 3 to show that it passes th'roughone of thenozzles.
FIG. 5 is a fragmentary detail view taken in section on the line 55 of FIG. 3.
proaching closer than one-eighth of an inch, even if it .i
were practical to do so.
The nozzles cannot be brought so close to thework that contact might occur through ir= regularities in mechanical tolerances. Neither can the nozzles be withdrawn from the work substantially beyond one-half inch without largely destroying the advantage which this apparatus has overconventional dryers. Any
7 FIG. 6 is a fragmentary view in-perspective of the apparatus as it appears from the inner ends of the nozzles, portions of a roll and the web to be dried being fragmentarily illustrated. a v V FIG. 7 is'a view in perspective fragmentarily illustrating the ends of a bank ofnozzles and the adjacent end wall of the plenum chamber all of the air, collecting chamber walls' being broken away.
loss of velocity at'the point of impingementofthe drying gas on the surface of the work can be'compensated only i by increased power. I i
Whateverthe kinetic energy of the, air in any particular installation, thenoz'zle arrangement disclosed herein will produce at leastthirty'percent more heat transfer with e correspondingly increased mass vapor transfer than can be achieved in any drying organization which does not employ the arrangement'of the present invention.
Nozzle spacings up'to two inches are practical. How'- ever, there are marked advantages if a spacing closely approximating one inch is used with the preferred area ratio of nozzle orifice to total area. The relationship befer area 'o'f'thework upon which the jets from the nozzle is operating is veryimportant. As shown by the' charts included in the drawings, the percent of orifice area should be'approximately three percent of the heat tran sfer areaat all noz'zlespacings, the outside limits'b eing-a 7 minimum of;one ,anda half percent and a. maximum of five and a halfpercent with a preferred range of twofto four percent. I T The drying gas, herein called air for convenience, is sup- FIG. 8 is a view in perspective of a fragmentary portion of one of the individual troughs that make up the nozzle bankp g i FIG. 9 is a diagrammatic view on areduced scale showing the circulatory. system- I I FIG. 10 is a'view in cross section showing a modified application of the invention to a web passing over a fixed p ate.
, FIG. 11 isa diagrammatic view' in cross section through d a series of drying rolls to one of which a bank of drying nozzles embodying the present invention has been applied.
tween the'open area of the nozzle and the total heat trans:
FIG. 12 is a diagrammatic view showing the effect of the ets of drying gases produced accordingto the invent10n.. V
FIGS. 1 3.and 1 4 are pressure charts showing the corn parative effect" of locating the. nozzles at different distances fromfthe work.
plied to the nozzles at such pressure as to'producevery hlgh Velocltles, a e fimg 09 to'be dried 'isrepresented by'a web 20 passing about a drying roll 2-1 which may be assumed to be the steam-heated feet'per minute. I M Sincethe'volum'e delivered from the'very narrow 'nozf-.
zles is small, the power requirementis not excessive and? the kinetic energy. expended" on the surface of the web is not destructive; The small volume flow from an individual-nozzle has'little'energy in spiteof the high'velocity of the jet? The net result-of these-factors is to disrupt the laminar boundary region with high frequency to promote high heat a and mass transfer rates. A convenient mechanical V 7 nection of the bases. of generallychannel-shaped members :with slightly flared sides 'tothe otherwise'iopen side of a plenum chamber so that the bases of :the respective ,chane construction involves the coni 7 FIG. 15 isfa chart. showing theefiect of the. distance of the nozzle from the surface of the work. FIG. 16 is a chart showing .the effect oftotal orifice areain relation to the total heat transfer meant the work. FIG. 17 is a chart showing theeffect of distance between nozzles upon the heat transfer coefficient and upon the kinetic energy of air required to produce a given rate of heat transfer; a i
In theembodiment shown in FIGS. to 9 the work roll of a conventional dryer towhich the drying apparatus of the'present invention has'been added. As already noted, the inventionis not concerned either with the manels close the chamber except for nozzle openings between the opposing andoutwardly converging side "wallsof consecutive channels. Thusthe nozzles are constituted terial of the web or. theliquidto be'vaporized. The invention is applic'able to. primary drying which involves the removal from a freshly formed paper web of the liquid Qjjcornponent ofthe furnish. 'It is alsoapplicable to machine felts and to crepe. It has been tested in thedrying of the printing done bya four-color rotary printing press. It is "also. usable tolp'reset aqueous coatings and to re i'move thevolatile, constituentsof, coatings; inks, and dyes.
The foregoing particulars 'are given byway of example and not by way of limitation. 'Also, because air is the most readily available drying'medium, the invention will be described with. reference to air but with the, underpartitions 33, 34, 35 as shown in FIG. 3.
standing that other drying gases are equivalent so far as the present invention is concerned.
For convenience, the entire apparatus is mounted within a hood or shroud 25 bodily movable to and from the roll 21 which not only provides a platen for the support of the web but also desirably contributes heat to expedite the drying. One way of mounting the hood for advancing and retracting movement is to support it on the arms 26 from trunnions 27 about which it may be withdrawn pivotally from the drum by cables 28 to facilitate training the web about the drum, or to give access to the nozzles when cleaning is desired.
The rim 29 about the margin of the hood is in proximity to the periphery of the drying cylinder 2]; but is usually spaced about one-half inch from the end of the cylinder and about one-half inch radially inwardly from the surface of the cylinder. This rim reduces the area through which fresh air is drawn into the system thereby providing more effective seal. it also serves to deflect the stream of air flowing out of return spaces between the nozzles hereinafter described.
Within the hood 25 is an inner wall 3% which provides a plenum chamber 31 to which the drying air is admitted through pipe 32. The plenum chamber is shorter than the hood, its end walls 33, 34 being spaced from the ends of the hood. Support for the nozzles is provided by the walls 33, 34 and by an intermediate partition or partitions 35 provided with a suificient number of apertures 36 to permit ready flow of air throughout the plenum chamber from one end to the other. The plenum chamber is so sized that the cross sectional area of flow at any point is considerably greater than the total orifice area downstream of the point. This arrangement provides uniform distribution of air to all nozzle orifices if air is supplied under some pressure to the plenum. Since the orifices represent but a small open area the pressure plenum need not be excessively large.
The internozzle channels, hereafter described, lead the saturated air from the surface of the web to the ends of hood 25, externally of walls 33 and 34. From this external space the air is exhausted through a pipe 38. Some of it is discharged through the blower 39 and replaced by make-up air which leaks in to the extent that circulating air is discharged. The rest passes through a filter 4t and heater 4i and is recirculated by blower 42,
which delivers the air back into the pipe 32. Desirably, the return air temperature may be about 300 F. and its pressure sufficient to develop a flow through the nozzles at the rate of about 16,000 f.p.m. This head is about 16" water gauge presure at 70 F.
The two pipes may have beveled flanges at 43 which seat with complementary flanges of the circulatory pipes when the housing is lowered to the operative position of 7 FIG. 1. The seated connection is shown in FIG. 4 and the flanges separately in FIG. 2.
'The inner wall of the plenum chamber 31 is composed entirely of the eighty-four channels 45 which in practice have been used to form the respective nozzles 50 and the vent spaces 51 as best shown in FIGS. 3, 6
I and 8. The number of channels is, of course, optional,
depending on the size of the bank of nozzles. The bases of the respective channels are welded to the bulkheads or As already indicated, the otherwise generally parallel side walls 53, 54 of the respective channels 45 flare slightly adjacent their ends and the proximate side Walls of consecutive channels converge to form the nozzles 553. While not critical, the included angle between the converging side walls which form the nozzles 50 has been made about 20. It has been found that the change in angle between the generally parallel portions of the side Walls and the converging portions 53, 54' stiifens the nozzle and there is less friction loss in the nozzle than would be the case if the side walls converged uniformly from the bases to the ends.
These nozzles are spaced from each other three-fourths of an inch to two inches or desirably about one inch center to center. Importance of nozzle spacing is clearly shown in FIG. 17 which demonstrates that the spacing is critical, the permissible range being three-fourths of an inch to two inches and the greatly preferred spacing being very close to one inch.
FIG. 17 shows the effect of distance between nozzles when the nozzles are mounted one-fourth of an inch from the work surface, the orifice openings aggregating 2.8 .ercent of the exposed surface and the air temperature eing 400 F. The curve 77 shows the heat transfer coefficient at various nozzle spacings with air energy of 0.4 H.P./ft. The curve 78 shows kinetic energy of air requ'red to maintain a heat transfer coefficient of 65 Btu/hr. ft. F. The preferred range of operation is at nozzle spacings between the broken lines indicated by the arrow 79. The area between nozzles can be kept small so that the velocity of the exhaust air is quite high, this being permissiblebecause the path of flow of the discharge air and vapor is isolated from the drying surface by the dynamic air curtain along the surface produced by the high velocity nozzle discharge.
It has also been found practicable that each nozzle have an opening approximately .023 inch wide and ten feet four inches long interrupted by spacers hereinafter referred to. However, while the dimensions are critical within limits hereafter explained, the specific dimensions here given are merely illustrative.
The relationship between nozzle opening and nozzle spacing can be expressed in terms of a ratio of nozzle opening to heat transfer area. The total orifice area should be not less than one and a half percent or more than five and a half percent of the heat transfer area, the preferred range being two to four percent for reasons clearly shown in FIG. 16.
In FIG. 16 the curve 71 shows the heat transfer coefficient in relation to the percentage of orifice area to heat transfer area when the nozzles are spaced one inch apart, the air energy being constant at 0.4 H.P./Ft. The curve 72 is a similar curve showing the relationship when the nozzles are spaced one and a half inches apart. The curve 73 is based on a nozzle spacing of two inches and the curve 74 is based on a nozzle spacing of three inches from each other.
The preferred range of percentages lies between the broken vertical lines indicated by the arrow 75 while the broken lines designated by arrow 76 show the outside limits within which practicable operation according to the invention is achieved.
As best shown in FIGS. 2 and 7 the respective channels 45 extend beyond the respective plenum chamber end walls 34, 35. The spaces 55 leading to the nozzles 50 are closed off outwardly of the end wall flange 56 by plates 57, 58 welded between the outer margins and ends of side wall portions 53, 54 of successive channels. To facilitate the escape of saturated air from the vent spaces 51, the bases of the channels are cut away beyond the end wall flanges 56 as best shown in FIG. 7.
Each of the channels is provided at intervals along one of its flared side Walls with the spacers 59 which are shown, by way of example, applied to the side wall 54 of the channel 45 separately illustrated in FIG. 8. It is practicable to use spacers 3 in width and 2 /2" center to center. When the several channels 45 are assembled in a bank, as shown in FIGS. 1, 3 and 6, their opposing side walls 53, 54 converge to form a narrow nozzle slot 50, the width of which is determined by the spacers 59. The spacers should be present in the minimum number required to support the margins of side walls at the desired spacing. The spacers facilitate assembly. They may comprise weld metal and later are securely fused to the side walls at the tips of the nozzles by a spot welding operation, without, however, appreciably changing the desired orifice opening. The length ,known. 7 V V I The'removal of the nozzle beyond three-fourths 'of .7 r of the spacers is so slightthat they do not materially affect operation. Thus the effective length of each nozzle is desirably at least equal tothe width of the web'to be dried.
The desirable range of nozzle opening widths in relation to work area has been found to be .015 to .055 In practice, these tend to lie within the range of .020 to .030. The opposed side walls 53 and 54 of consecutive channels constitute the nozzle and then converge with an angle up to 20 between them. Actually an angle of 3". or 4 is satisfactory. This angle is not critical but is largely a matter of design, the 'jet expansion anglev being independent of nozzle design. r V
The nozzles 50 are located in very close proximity to the web. The preferred range of spacings is from oneeighth of an inch to three-eighths of an inch, although spacing of one-eighth of an inch to one-half of an inch have been used successfully. FIG. 15 shows that beyond three-fourths of an inch there is no advantage over conventional dryers.
FIG. 13 is a chart showing actual measurementf surface pressures exerted by a blast'of .airunder 12.2 water pressure issuing from a nozzle .015" in width spaced one-eighth of an inchfrom a surface 60. The
line 60 not. only represents the surface but represents the zero. pressure. b
As shown in FIG. 13, .the area-61 in which the" work surface 60 is subject to near-maximum pressure is only slightly in excess of the nozzle width and the pressure over this area is quite high, being nearly equal to the stagnation point pressure at the apex of the curve. From this maximumthe pressure falls off sharply to subatmospheric pressure in the areas 62.at each side of the pres sure area. itresults in a very. steep pressure gradient along the The sharp reduction is important because one-eighth of an inch to three-eighths of an inch as shown by the vertical broken lines identified by arrows 83 and 84 in FIG. 15. Arrow'85'designates a vertical broken line representing a distance of three-quarters of an inch of travel of the jets from the'nozzles to the work, this being the top of what is regarded as a practicable range of jet travel. FIG. 15 shows the effect 'of varying the distance between the nozzles and the surface of the work with the air temperature and energy constant. In making these curves, the air was held to a temperature of 400 F. and the air energy was held at 0.4 H.P./ft. The curve 80 a was taken with the nozzles of 0.028 inch orifice one inch apart. The curve 81 was made with the nozzles one and a half inches apart and having openings 0.042 inch.
In the device of FIG..11 I have shown a whole series.
. In the construction of FIG. 10 the rolls 213, 214 are mere idlers and the web 20 to be dried is fed over a supporting plate 70 with which the hood 251 is associated. Although the work-supporting platen constitutes the stationary plate 70 instead of a roll such as those indicated ,at 21 and 210, the arrangement of the nozzles 50 in a surface of the work in the plane of the laminar boundary layer. This tends to subject the molecules constituting the laminar boundary layer to greater fluid shear, thereby increasing the rate of molecular flow in a direction such as to makethe boundary layer thinner'and increase the rate of heat exchange and of drying. 7
With the nozzles spaced in'accordance with preferred practice, the resulting fiowis as shown in FIG. 12, the maximum continuum along the surface between consecutive nozzles being no greater than half thedistance between nozzles symmetrically in bothdirections from thepoint 66 of impact to the intermediate areas of escape as indicated by the arrows 67. This is important in view of the analogy hereinafter made to the Pohlhausen theory of heat transmission through the boundary layer and'because it means that every partof the work surface is constantly beingexposed to fresh" and unsaturated drying air whereas previous dryers have developed a pattern of flow in which the continuum is with the entire surface.
ence in pressure curve resulting from spacing the nozzle one-half inch from the work-surface instead .of onepractically coextensive r V i e W V 55 Comparison of FIG. 13 with FIG. 14 shows'th'e diiferbank within hood 251 is essentially the same operation is similar to that above described. 7
The principles underlying the invention are as follows: The problem is to remove a, vaporizable liquid from the surface of the work which, in this instance, is represented by a moving web. The vaporizable'liquid may be and the "present inIthe'web as a result of manufacture thereof or it may be present on the surface of the web as, for example, the result of a coating or a printing operation. Regardless .of. the depth to .which the liquid penetrates the web, the' vapor mustalways be removed from the surface. f
. Wherever there is relative motion between a surface and a surrounding gas there exists the phenomenon known as a boundary layer resulting frommolecular adhesion of molecules of gas to each other and to the surface. In the case of'most dryers using air, the boundary layer is turbulent at its surface, and isv generally considered to consist of three" distinct layers or strata, the innermost, close to. the surface, of-the drum, being laminar. 1 Outside of the laminar stratum there is a transition or buffer straturn' in which the molecules are less closely bound and outside of this there is a turbulent outer'stratum wherein the molecules are bound to the drum but capable of turbulent movement respecting each other. To remove the moisture from the web on the surface ofthe drum, heat is caused to. pass inwardly through the boundary layer and the vapor must move eighth inch, everything elsebeing the same. It will be noted from FIG. 14 that there is no longer any area of sub-atmosphericpressure-such as th'at'shown at 62 in FIG. 13. The area-610 which is subject to super atmospheric pressure is much broader vdue to the expansion of the jet from thenozzjle. (Theex pansion angle is about 6 /2.) For'the same reason the pressure is also muchless. However, evaporation is still several in the use oftechniques previously times as i great as aninch fromthe surface so far diffuses the jet'andreduces its pressure as to result in-a dryingloss so great in nozzle ata distance in excess of three-fourths. of an inch from tlie work See P161115 The "preferred rangeis outwardly through .that layer; Heat-and vapor transfer through the boundary 'layeris relatively slow due to-the insulating effect of the gas.' It' is desirable to reduce thethickness of the boundary layer and thereby its insulating effect.
In 'the-laminar inner'stratum' transfer takes place extremelyfislowly by moleculardiffusion. 'Thermal gradicats are very steep. In' the turbulent strata particles of fiuid do not move in orderly fashion but move in eddies.
This action causes rapid diffusion and therefore heat transfer rapidly occurs with relatively low gradients.
'The inner-laminar stratum presents the primary obstacle to the movement of both heat and vapor.
In the past, the effort has generally been'to supply substantial volumes of gas at 'relatively high velocity.
:However, in-such arrangements the velocity has to be limited because, at least in the, drying of paperor the like, there is a limit-to the amount of velocity and mass 59 which the paper can stand without displacement of fiber, and even disruption of the surface. By placing the nozzles extremely close to the surface of the work, in accordance with the arrangement previously described, I am able to use higher velocities and lower masses of air in a way to change the characteristics of the boundary layer to maintain a laminar layer which is unusually thin because of its short continuum, whereby to bring about tremendous increases in drying rate without endangering the surface of the work. However, if the nozzles are placed too close together or if the ratio of total nozzle area to treated work area is too large, the flow from adjacent jets mutually interferes so that a sudden loss in drying occurs.
In preferred practice the velocities at the nozzle orifice are in the range of 10,000 to 20,000 feet per minute. However, since the volume of air from each nozzle is small, the amount of power required is not excessive, and at a ratio of nozzle opening to surface area of two to four percent the kinetic energy expended on the surface is within safe limits.
It is also a feature of the present invention that the nozzles are quite close together about the surface of the drum. This is true even though relatively greater space is provided for the exhaust of air and vapor between nozzles than is possible in some of the prior art devices. The advantage of having the nozzles close together is that the more frequently and thoroughly the boundary layer can be disrupted and its saturated air withdrawn, the lower will be the Reynolds number and the greater will be the evaporation rate.
Primary purpose of the nozzle arrangement is to provide high heat and mass transfer coefficients in the film of air on the surface of the material being dried. Since mass transfer in an air film or the capability of the air film to allow evaporation is largely a function of the heat transfer coefficient, the latter will henceforth be used for simplification but with the understanding that it is essentially identified with the drying potential. Furthermore, since most air dryers are capable of being attached to an air circulation system which may provide air or other gas at various temperatures and humidities, the chief criterion of the efiiciency of an air dryer is the heat transfer coefiicient which depends to a large extent on the geometry of the given dryer.
As has previously been shown, the continuum of the boundary layer with respect to the surface of the web consists of a length or distance originating at a point on the surface directly under the nozzle which is called the stagnation point and continuing to a point on the surface midway between the nozzles. The boundary layer film developed under my nozzle arrangement is highly analogous to the proposition stated by Pohlhausen in his development of a formula applicable to heat transfer to or from a thin fiat plate immersed in a deep uniform fluid stream. (McAdams, Heat Transmission, 3rd edition, page 224.) The chief difference between the Pohlhausen proposition and the case of this dryer is that in the former a uniform flow exists at the origin of thecontinuu-m and no pressure gradient exists in the direction of flow, whereas in this dryer a pressure potential exists at the origin which is rapidly converted into high velocity flow with an accompanying steep pressure gradient in the direction of flow. Beyond the point where pressure potential has been converted into high velocity flow and for the greater part of the continuum the analogy is nearly exact.
. Little is known about the effect of a steep pressure gradient on the boundary layer.
If a pressure gradient is imposed on the boundary layer in the direction of flow, all particles in the film or boundary layer are subjected to the pressure gradient in addition to the usual viscous drag and pull of vertically" adjacent particles. Therefore, in the practice of the present invention, it is believed that there is a region of very high fluid shear close to the surface in the impingement area. This high shear is believed to be the cause of the very high heat transfer and mass transfer rates achieved by the present invention, as compared with those predicted by the Pohlhausen formula cited in McAdams supra.
I have rearranged the Pohlhausen formula and grouped all of the quantities related only to the internal state of the fluid in a single constant It. The formula thus modified is as follows:
As in the Pohlhausen formula L represents the length measured from the source of flow over which the heat transfer occurs. In my apparatus L necessarily is equal to one-half the distance between nozzles, since the flow under consideration is the fiow between the point of impingement against the work and the point at which the gases and vapor escape from the surface of the work (see FIG. 12).
In the Pohlhausen formula V is the stream velocity which is comparable to the impingement velocity in my apparatus. From this abridged formula it is apparent that a greater heat transfer coefficient, h will result with higher impingement velocity and/or shorter continuum length. In the actual measure of heat transfer, l have found that the length L has an even greater effect than indicated by the modified formulaprobably due t the high shear effect above referred to.
The impingement velocity is related to the velocity at the nozzle orifice, the size of the orifice, and the distance of the nozzle from the surface. The formula,
V D 1/2 V D relates these quantities according to my measurements. V/ V is the ratio of impingement velocity to nozzle velocity with a limiting value of 1.0; D is the width of the nozzle orifice; Y is the distance of the nozzle from the surface. And 6 is a constant with a value of about .13 related to the expansion angle of the jet. It is evident from the formula that increasing the distance of the nozzle from the surface will decrease the impingement velocity, and reducing the nozzle orifice will also decrease the impingement velocity though not to the same degree.
FIG. 15 shows the effect of varying the distance of the nozzle from the surface. As will be apparent, the heat transfer coefficient reaches a maximum at a point near one-eighth of an inch spacing of the nozzle from the surface. The chart shows that there is no material advantage in spacing the nozzle more than three-fourths of an inch from the work. The spacing should preferably be kept lower than three-eightl1s of an inch.
The effect of varying the orifice opening in percentage ratio to heat transfer area is very clearly shown in FIG. 16 for different nozzle spacings. At all spacings the heat transfer coefficient falls off very sharply below one and a half percent and is seriously impaired above five and a half percent, the preferred percentage range being between two and four percent.
It will be observed that in the above equation it 1s assumed that the quantity in parenthesis is less than unity. If it were above unity, the equation would cease to have significance from the standpoint of the present invention. Unity is virtually reached with a nozzle .032 in width and spaced from the Work by one-eighth of an inch. The formula is verified in FIG. 15. The invention contemplates as one of its requisites that a relatively high impingement velocity of the stream of drying gases against the work, together with the relatively close spacing between such jets of drying gases, be used to reduce the continuum and minimize the thickness of the laminar portion of the boundary layer which is proximate the work. If the spacingof the nozzles from the work exceeds a value such that the value of the above equation exceeds unity,.no further advantage is gained.
Thus there is little advantage in spacing closer than about one-eighth of an inch. Ideally the value ,of unity for this quantity would be desirable. a
Note further that it is desirable that the nozzle spacing from the Work be several times the orifice width-in order' that maximum gas velocity will be developed at the impingement point rather than between the nozzle edge a and the work. 7 I v As compared with competitive equipment my drying apparatus exhibits several advantages 1 not heretofore regarded as possible of achievement. In addition to;
figures assume that the nozzles'will be of the transversely elongate type herein shown. Y 7
My dryer has consistently effected removal-from 'a web surface of vaporizable liquid at the, rate of eight pounds'to twenty pounds per, hour per square footof surface (depending on the application) by theuse of air only,'without any other source of heat.v One unit has removed sixty'pounds of water per hour per square thejnozzles'is quite uniform over the entire drying area. The flow from the nozzles. impinges on the surface and is thusredirected to flow on the surface from the stagnation point opposite a nozzle to a point. midway'between nozzles where the air, leaves the surface. This flow of air at high energy on the surface insulates the surface from the effect of spent air leaving the area through the internozzle channels. The surface is therefore contacted only by the primary stream of air atiuniform-temperature and humidity from the nozzles. The critical test of the uniformity of drying for any air dryer is operation at high temperature since, non-uniformity of drying is accentuated by high temperature. In practice my apparatus has dried a web onehundredforty-eight inches wide with The character of the foot of surface in combination with a heated cylinder and there appears to be a" possibility .ofstill further improvement. a
an air temperature of approximately 680 Fahrenheit without measurable variation. in dryness across the width of the'web leaving the dryer. 7
' V y device provides high drying capacityand uniform drying, and is at the same time economical to operate. Although it is conventional touse relatively high pressure in the supply to :the drying unit, the flow through the nozzles is low; and at the specified ratios of nozzle area to work area, low fan'power is required (see FIG. 17).. In my apparatus the drying potential varies approximately as the .24 power of the fan horsepower in a given design 'of the device. Therefore excessive use of power produces little benefit; In practice my apparatus requires air driving, power of approximately one-half horsepower per square foot of drying surface with an air temperature ;of 200 F. i
My apparatus lends itself to the conservation of heat in the drying process} The first and last nozzles effectively seal the drying area from the surrounding atmosphere. In so doing half. of the flow from each of these nozzles escapes to the atmosphere, but in a drying apparatus such 'as mine with arelativ ely large number of nozzles, the
The previously demonstrated high drying capacity of my apparatus has been successfully proven. In one case the addition of two of my devices to a paper machine dryer section composed of twenty forty-eight inch cylinders increased the drying capacity of the machine inexcess of 30%. In another case a new design, of rotogravure press with greatly reduced length of draw between decks :was-made possible by the use of my drying device. In each of thesecases and in others the high drying rate in a short space has beena decisive advantage.
percentage of the leakage is .very low. The remaining edges of the drying area are sealed by maintaining close clearances between the exhaust curtain and the web support. Thus it'is' possible to recirculate a large percentage of the drying airwithout excessive loss. The air ex- 7 hausted fromsuch a system contain's'large amounts of vapor which is conducive to high thermal efficiency.
7 Exhaust humidities in excess of .6'pound of water vapor The greatheat and mass transfer coefficients of my drying apparatus and method have, several incidental advantages. When my dryer is'used in conjunction with theusual steam-filled cylinder of a conventional dryer, a large portion of the heat required for evaporation is supplied tothe web from the surface-of'the cylinder. In some instances in the prior art the cylin'derisurface is web and the cylinder sufficient to force the web away to' one pound of air'have been measured. Furthermore, the very compactness of my dryer, doing a given drying job in a very small area leads to high thermal efficiency since losses due to radiation and convection are less per pound of waterevaporated;
In practice my dryer can operate with as little as ten percent of makeup air. Typical steam consumption in the use of my invention is 1.25 lbs. per. pound of evaporation. My device requires little space, has low heat loss,
,so hot as to develop high vapor pressure between the Q from the cylinder surface, thus sharply reducing. heat. transfer. Theuse of my invention so reduces the'vapor:
' pressure on the outer surface of the web that the resulting gradient of vapor pressure through the web reduces the tendency forthe web to "blow off'the cylinder.' Moreover, the-increased rate of evaporation from the surface of the web sharply reduces'the temperature to which the web is subjected andrnakes vit-p'ossible even to increase the amount of heat-in theycylinder beyond the .point previously thought to be damaging to'the web.
web secured in my apparatus'is. an; important result of requires little maintenance, is easy'and safe to, operate,
hashigh production capacity,- and is inexpensive to pro- 1. In aewebdryeri of type in'whi'ch the work to be dried is exposed to jets of drying gas, the combination with means for advancing .upon a predetermined path the work to be dried of a plurality of nozzles extending continuously for substantially the fullwidth of the work transversely of such path and 'very narrow ingthe direc- The uniformity of drying across'the entire width of a my nozzle arrangement. The reason that other attempts I to obtain high drying capacities have failed to be prac-. tical is because they were unable to dry uniformly across the width of webs commonly used by industry. In my.
app'aratus-theturbulent mixing or: air under pressure in tion of movement of the work, said nozzles being" directed toward the surface of the work to be dried at a distance of not more, than three-fourths of an inch therefrom and spaced not less than three-fourths inch and not more than two inches from each, other, the ratioof total nozzle orifice area to total exposed work area being no less than one and'one-half pe'rcent and no morethan five and onehalf percent.
2. ,Thejdevice of claim 11 inwhichthe nozzles com- "prise elongated .o'pposing walls and means closing the nozzles at the, ends of said' walls,.lthe side margins of said walls. being angularly convergent to provide nozzle slots greatly elongated ascOmparedwith their width, the said 13 opposing walls of each nozzle being spaced from the opposing walls of adjacent nozzles to provide clearance for the Withdrawal of gases from the surface of the work against which jets of drying gas are directed through said nozzles.
3. In a web dryer of the type in which the work to be dried is exposed to jets of drying gas, means for jetting gas upon the work and including a plurality of generally parallel nozzles elongated transversely of the work to be dried and directed toward the surface of the work to be dried and having spaces between them for the escape of gas which has acted upon such work, said nozzles being spaced from the work at a distance of not more than three-fourths of an inch therefrom and spaced not less than three-fourths of an inch nor more than two inches from each other, said nozzles having an aggregate open orifice area totalling between l /2% and 5 /2% of total heat transfer area.
4. In a web dryer of the type in which the work to be dried is exposed to jets of drying gas, means for jetting gas upon the work and including a plurality of generally parallel nozzles elongated transversely of the work to be dried and directed toward the surface of the work to be dried and having spacings between them for the escape of gas which has acted upon such work, the ratio of the total orifice area to the total area of the work exposed to said jets being between two and four percent, the distance between said nozzles approximating one inch between centers and the distance from the nozzles to said surface being not more than three-eights of an inch.
5. In a dryer, the combination with a hood and means providing a plenum chamber within the hood, of a plurality of nozzles opening from the plenum chamber toward the surface of the work to be dried at a spacing not to exceed three-fourths of an inch from such work and mutually spaced from each other at a spacing not to exceed approximately two inches to provide inter-nozzle passages communicating with the hood externally of the plenum chamber, means for supplying a drying gas under pressure to said chamber for delivery at high velocity from said nozzles, the width of each such nozzle being less than the spacing between the nozzle and the work, whereby the velocity of gas impingement on the work exceeds the velocity of gas movement along the work, and a circulatory system of which said means constitutes a part, said system including a return conduit from said hood and a supply conduit to said plenum chamber, the plenum chamber having a wall opposite the work to be dried which comprises a series of channels having base portions connected to the plenum chamber, said channels having opposing flanges, means connecting the flange of one channel to the flange of the next at the ends of said flanges to constitute said nozzles between successive flanges, the space within the channels constituting said return passages accommodating flow of gases from the work to the hood, the side margins of the channel flanges 'being mutually divergent whereby the opposing walls constituting respective nozzles are convergent at the tips of said nozzles, the nozzle openings being greatly elongated as compared with their said width and being spaced from each other between three-fourths inch and two-inches,
center to center, the ratio of total nozzle opening to total l 4 work area being in the approximate range of 1.5 percent to 5.5 percent.
6. The device of claim 5 in which the ends of respective channels extend beyond the plenum chamber, the said means connecting the ends of the channel flanges being disposed externally of the plenum chamber and the base portions of respective channels being relieved externally of the plenum chamber, whereby the interior portions of respective channels are opened into the hood.
7. In a web dryer of the type in which the work to be dried is exposed to jets of drying gas, the combination with means for advancing along a predetermined path the work to be dried of a plurality of jets elongated transversely of said path and continuous for substantially the full width of the work, said jets being very narrow in the direction of the work and directed toward the surface of the work to be dried and originating at a distance of not more than three-fourths of an inch therefrom and at a spacing of not less than three-fourths inch and not more than two inches from each other, the ratio of the total cross sectional area of the jets at their origin to the total exposed work area being no less than one and one-half percent and no more than five and one-half percent.
-8. In a web dryer of the type in which the work to be dried is exposed to jets of drying gas, the combination with means for advancing the work to be dried along a predetermined path of means immediately adjacent said path for establishing a plurality of jets directed toward the surface of the work to be dried, said means comprising nozzle slots elongated transversely of the direction of advance of the work to be dried and continuous for substantially the width of such work and being at a distance of not more than three-fourths of an inch from the work surface and constituting means for spacing the jets not less than three-fourths of an inch and not more than two inches from each other and for giving them a total area at said means of not less than one and one-half percent or more than five and one-half percent of the total exposed work area.
References Cited by the Examiner UNITED STATES PATENTS Re. 24,144 4/56 Dungler 34-l60 1,724,645 8/29 De Long 34-122 X 1,933,960 11/33 Brabaek 34-122 2,306,019 12/42 Hanson 34-160 2,622,343 12/52 Metcalfe 34-122 2,627,667 2/53 Gillis 34-37 X 2,828,552 4/58 Brendel 34-l22 X 2,928,185 3/60 Drew 34-122 X 3,077,675 2/63 Dickens 34-122 FOREIGN PATENTS 309,650 9/55 Switzerland. 727,058 3/55 Great Britain. 773,908 5/57 Great Britain.
NORMAN YUDKOFF, Primary Examiner.
GEORGE D. MITCHELL, CHARLES OCONNELL,
BENJAMIN BENDETT, Examiners.
UNITED STATES PATENT OFFICE CERTIFICATE ()F ,;CORRECTION Patent No. 3,176,412
April 6, 1965 Thomas A. Gardner It is hereby ent requiring corr certified that error appears in the above numbered patcorrected below.
ection and that the said Letters Patent should read as Column 10, lines 11 to 13, shown below inst the formula should appear as ead of as in the patent:
1/2 r m (T) same column 10, lines 33 to 37 the formula sh shown below instead of as in t ould appear. as he patent.
- l/2 V D v MP same column 10, line 42, for "6" read oz Signed and sealed this 28th day of September 1965.
ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner :of Patents