US3097933A

US3097933A - Papermaking drying machine

Info

Publication number: US3097933A
Application number: US747039A
Authority: US
Inventors: Harry B Kellogg; William A Dickens
Original assignee: Kimberly Clark Corp
Current assignee: Kimberly Clark Corp
Priority date: 1958-07-07
Filing date: 1958-07-07
Publication date: 1963-07-16
Anticipated expiration: 1980-07-16

Description

y 1963 H. a. KELLOGG ETAL 3,097,933

PAPERMAKING DRYING MACHINE Filed July 7, 1958 4 Sheets-Sheet 1 AX AX Direction of Dner heat flux surfing wvcouonv 4 Sheets-Sheet 3 H. B. KELLOGG ETAL PAPERMAKING DRYING MACHINE July 16, 1963 Filed July 7, 195B July 16, 1963 Filed July 7, 1958 H. B. KELLOGG ETAL PAPERMAKING DRYING MACHINE 4 Sheets-Sheet 4 an .L

3,097,933 Patented July 16, 1963 3,097,933 PAPERMAKLNG DRYING MACHINE Harry B. Kellogg, Appleton, and William A. Dickens,

Neenah, Wis., assignors to Kimberly-Clark Corporation, Neenah, Win, a corporation of Delaware Filed July 7, 1958, Ser. No. 747,039 9 Claims. (Cl. 34-41) This invention relates to a process for the manufacture of paper or similar sheet material involving the use of a conventional Yankee drying drum or cylinder, commonly referred to as a Yankee drier or cylinder. The invention is particularly useful in operations incorporating the most modern type of Yankee drier system, involving unusually high rates of heat transfer through the drier shell.

In the conventional process for the drying of paper on a Yankee cylinder, the speed of production by the basic paper web forming (wet end) machine under certain conditions is reduced substantially below capacity due to limitations on drying capacity of the Yankee drier. The limitations on drying capacity of the drier are ordinarily not due to lack of potential of the drier to transmit sufiicient heat to the wet paper sheet to perform a complete drying operation at the desired higher speeds. Rather, the limitations are due to the fact that at the high speeds of operation, with the drier operating at the substantially higher internal steam and surface temperature required to provide the heat necessary to dry the sheet, water vapor is formed between the drier surface and the paper sheet at such a rate that it cannot escape through the pores in the sheet, resulting both in an undesirable blistered appearance being given to the sheet and a separation of the sheet from the drier surface with resultant reduction in drying eifect. The surface temperature must then be lowered, by lowering internal drum steam pressure to reduce the degree of steam formation and resultant blistering of the sheet, and the speed of the machine as a whole must then be reduced to provide the time interval necessary for drying at the lower drier surface temperature.

The loss of production, or inability to achieve increased rates of production, which thus results, is even more serious in view of the recent development of driers manufactured out of materials which provide substantially increased thermal conductivity and in view of modern hightemperature heating media, through the use of which great increases in paper machine speed could be achieved if it were possible to apply to the paper sheet available heat from the drier without adverse effect on the sheet or consequential eifects which prevent realization of the heat transfer capacity of the drier. This invention relates to apparatus whereby such increased drier potential may be utilized without attendant disadvanges or countereifect, and to a method whereby increased drying speed may be obtained.

Details of the above and other objects and advantages of the invention will be readily seen from the following descriptions in connection with the attached drawings, in which:

FIGURE 1 schematically shows a Yankee drier installation incorporating the present invention,

FIGURE 2 is an enlarged illustration of the division of a Yankee drier shell into increments, in explanation of certain calculations set forth below,

FIGURE 3 shows the surface temperature of a high heat transfer Yankee drier, based on calculations, during a complete revolution in a conventional installation, and in an installation incorporating the present invention, and

FIGURES 4 and 5 show relative temperatures of particular portions of a high heat transfer drier at selected intervals in the drying cycle and drier revolution, based on calculations.

Referring to FXGURE 1, it is seen that in the conventional apparatus the sheet 20 supported by a felt 21 passes over a tail roll 22 and is then pressed into contact with the Yankee drier 23 by a pressure roll 24, which may be of the suction type. Sheet 20 then adheres to the exterior surface of drier 23 while felt 21 is led back over a series of supporting rolls to the forming end of the paper machine. After being carried around through the major part of a complete revolution of drier drum 23, the dried sheet 20 may be removed from the surface of drier 23 in any suitable conventional manner, which in the case of the manufacture of creped paper would involve used of a creping doctor blade. The sheet is then Wound up on rolls (not shown) for further handling. In some operations the sheet is not fully dried on the first drier, but is carried over or around supplementary drying means after initial partial drying by the first Yankee cylinder. These supplementary means may comprise an additional Yankee cylinder, or multiple can driers, or other conventional means. In certain other operations, necessary drying following removal of the sheet from the first Yankee drum may be provided as a result of or following subsequent processing steps.

The Yankee drier drum 23 is heated by means of internally-introduced steam, hot oil, or equivalent means to an exterior surface temperature of about 160 to 200 degrees Fahrenheit, or even higher. The sheet 20 at the time of application to the exterior surface of drier 23 commonly comprises about 30 to 40 percent solids in the form of wood pulp or other fibrous or particulate material and 70 to 60 percent water, the composite sheet be ing usually at a temperature of about 60 to degrees Fahrenheit.

The speed of travel of the paper web is limited not only by the theoretical capacity of the paper web forming and drying apparatus as a mechanism, and by the theoretical ability of the drier to transmit heat to the sheet in quantities sufiicient to evaporate sufficient water therefrom to bring its moisture content to the desired level (as determined by total evaporation load, drier size, material of construction, shell thickness and maximum internal steam pressure), but also by a number of other factors having a definite effect upon the practical ability of the drier to dry the sheet. Among these factors, the exact effect of some of which cannot be exactly predicted in advance, but which in greater or lesser degree determine whether or not the sheet will blister and/or separate from the drier, are basis weight of the paper sheet, drier surface temperature, pressure with which the sheet is applied to drier 23 by roll 24, type of wood pulp employed, sheet moisture at time of application to drier, whether or not roll 24 is a suction roll, and amount of vacuum applied to roll 24 if it is a suction roll. The blistering problem ordinarily is more likely to be present when making sheets of the heavier basis weights.

The modern high heat transfer drier has a drying shell made of a material with a very high capacity for heat transmission, as compared to the conventional Yankee drier made, for example, of cast iron. One of the particular metals now available for manufacture of a high heat transfer drier is an aluminum bronze, having a thermal conductivity of about 40 B.t.u./hr.-ft. F./ft. and a specific heat of about 0.10 B.t.u./ih.- F., as compared to corresponding factors of about 17 and 0.13 for the cast iron commonly used in driers.

In order to determine the method and means for ob taining substantial utilization of the heat transfer (ipacity of the newer high heat transfer drier under desirable operatiug conditions it is necessary to ascertain and understand the various heat qualities of a Yankee drier during a drying cycle.

The surface temperature profile of a Yankee drier, as the function of rotational position of the drier, may be calculated by numerical integration of the basic differential equation describing heat flow through the drier. The formulation and solution of such an equation, for steadystate, one-directional conduction of heat through an isotropic solid is usually a simple matter, the assumption of a steady state implying static conditions at any point as a function of time, so that no accumulation or depletion of heat energy results in any given volume as a function of time. The equation is formulated from an elementary heat balance on the volume contained by two planes perpendicular to the direction of the heat transmission:

Input-output:accumulation for steady-state transfer Putting this heat balance into differential form and considering the volume between two planes separated by a differential distance, dx:

dT Input-kA( (l) (17 d dT Output-kA( (k/1- dx 2 The basic differential equation becomes:

d T aq 0 3) where k:thermal conductivity A=area for heat flow, perpendicular to the direction of heat flux T =temperature x=distance parallel to the direction of heat flow This basic differential equation is, however, not applicable to the drying of a sheet on a Yankee drier, since the heat flux from the drier varies with time, being high when the sheet is wet, falling as the sheet dries, and being low when the sheet is dry and also when there is no sheet on the drier. Thus, the heat transfer from a Yankee drier is unsteady, and for unsteady-state heat transfer the accumulation is not zero, but:

Accumulation: 1Cp(dr) (4) where =solid density Cpzheat capacity 6=time The basic differential equation for unsteady state transfer becomes:

Input-output=accumulation0 As shown by the above Equation 6. the temperature of the solid (the drier shell) is a function both of time and of distance parallel to heat flow. The dependence on time indicates that the temperature of a particular point on the surface or of a particular interior point on the drier shell will change with time, i.e., will change with rotation of the drier. In the above it has been assumed that heat flows only perpendicularly to the drier surface,

and that there is no change in area for heat transfer between the inner and outer surfaces of the shell of drier 23; obviously no appreciable error arises from these assumptions.

While the partial differential Equation 6 may be solved analytically for simple boundary conditions, and for more complex boundary conditions such as encountered here, a numerical solution of a finite difference approximation to the equation is simpler and is satisfactory for present purposes, that is, for ascertaining the temperature of the Yankee drier shell at various points on the surface and within the shell as a function of time.

The boundary condition of the heat flux from the drier 23 to the sheet 20 is a function of time for any point on the drier surface. Using a Taylor series, an neglecting the higher order terms of the Taylor series expansion, the partial differential Equation 6 is reduced to the finite difference approximation:

where The individual terms in Equation 7 will be understood from FIGURE 2, which indicates that the solid through which heat is flowing (the Yankee drier shell) is divided by a series of equidistant planes perpendicular to the direction of heat flux, the planes being Ax apart, and being designated at plane 0, 1, 2, m-2, m -l, m, m+l, etc. Similarly, the time variable is divided into finite increments, 0, l, 2, N-2, Nl, N-l-l. etc., each of these increments being no in length. Thus, Equation 7 indicates that the temperature at any given plane is approximated by a simple linear function of three temperatures at the previous time interval, these temperatures being those at and on either side of the plane of interest. The smaller the time and distance increments, the greater the accuracy of the finite difference approximation.

Use of Equation 7 is restricted only in the requirement that the modulus M have a value of at least 2.

The method of computation of temperatures at different times and for different positions becomes apparent by a consideration of Equation 7. Once the temperatures have been specified for all positions at the initial time 9:0, the temperatures at time 0+A6, i.e., N: 1, are completely specified by Equation 7 except at the boundaries of the drier shell. Here, however, the temperatures can be specified by reasonable boundary conditions. At the inside of the drier shell, the temperature will be essentially constant, maintained at that temperature, for example, by condensing steam. At the outer surface, the temperature is obtained from a knowledge of the heat flux necessary to cause experimentally observed drying rates. By writing a heat balance on the outer segment of the drier of differential thickness, there is obtained the relation:

l and a where F :the heat flux necessary to cause drying rates and =the temperature gradient at the surface Converting Equation 9 to a finite difference approximation:

AT F=LA 10) Therefore, with a knowledge of experimentally observed heat fluxes and the variations in these heat fluxes around the drier, it is possible to specify the surface temperature of the drier at any time increment by a knowledge of the temperature at plane 1 at the same time. Thus, by repeated use of Equations 7 and 11, it is possible to follow the changes in temperature at different positions in the drier shell and at the surface as a function of time. These calculations are admirably suited to solution by a digital computer.

A modulus value of two was chosen for the work because of the simplicity of the calculations that result from such a choice. Once the modulus has been chosen, the form of the modulus defining equation p 'P( V has allows only one of the two independent variables, x and 6, to be specified.

For a suitable aluminum bronze metal:

If A0=% sec., for M=2, Ax=0.0176 in. These values and a drier shell thickness of 1.76 in. were the basis for digital computer calculations.

It is known that the conventional drying of fibrous sheets on hot surfaces occurs first at a constant rate of drying and thereafter at a falling or decreasing rate. For a sheet applied to an 8 foot aluminum bronze drier at a sheet consistency of 37.5 percent, that is 1.667 pounds of water per pound of bone dry (B.D.) fiber, the critical moisture content at which the constant drying rate changes to a falling rate is estimated to be 0.41 lb. water per lb. B.D. fiber with the sheet removed from the drier at about 0.05 lb. per lb. B.D. fiber. (Dreshfield, A.C. Jr., Chemical engineering Progress, p. 53, No. 4:174, 1957.) The equilibrium moisture content would be about 0.01 lb. water per pound B.D. fiber. The falling rate of drying is approximately proportional to the difference between the actual moisture content and the equilibrium moisture content. (Walker, W.E., Lewis, W.K., McAdams, W. H., and Gilliland, E. R., Principles of Chemical Engineering, 3rd ed., pp. 644-645, McGraw-Hill Book Company.)

Based on the above, the relationship between the time for falling rate drying, 0,, and the time for constant rate drying, 0 becomes:

Therefore, of the period the sheet spends on the drier, 57 percent is constant rate drying and 43 percent is falling rate drying.

For the previously mentioned 8 foot drier at 1500 feet per minute sheet travel (i.e., 1500 f.p.m. circumferential speed of drier), internal drier steam pressure of 125 p.s,i.g., and drier shell thickness of 1.875", all of which are reasonably assumed figures, the time for one revolution, 9 becomes:

0 1500 =l.00 sec.

Assuming 80 percent of the drier surface is covered by the sheet being dried,

It was determined that a reasonable rate of production for the previously mentioned 8 foot drier was 0.09 lb. B.D. fiber per second per foot of drier width. Therefore,

6 during the period of constant rate drying the rate of water evaporation is:

0.09 lb. B.D. fiber 1 sen-ft. Width 11 ft.

lb. water 1257 lb. B.D. fiber x 3000 gi -s am lb. water/hn-ft.

Therefore, the heat flux is approximately 37,000 B.t.u./ hr.-ft.

For the period of falling drying rate, the average rate of water evaporation is:

lb. water 0.09 lb. B.D. fiber 1 lb. B.D. fiber sea-ft. width 9 ft.

3600 see. hr.

X =13.0 lb. water/hr.ft. width As previously determined, the heat flux from the surface, s, is,

During the period of constant drying rate, this surface temperature gradient is,

10A During the period of falling drying rate, the average surface temperature gradient is,

13,000 dz: kA During the unwrap period, the surface temperature gradient is,

From the above, solution of Equation 7 may easily be elfected by digital computer techniques to provide data set forth graphically in FIGURES 4 and 5. It should be noted that in these calcuations, it was assumed the wet paper sheet was applied to the drier at the drier surface temperature, which of course gives a result somewhat different from that which would obtain under typical operating conditions, but a result which nonetheless presents a valid picture of drier surface temperature variations for the present purpose. Referring to FIGURE 4, the abscissa represents the one second of time involved in one complete revolution of the drier, the axis position being the precise time of application of the sheet to the drier surface. It is observed that the drier exterior surface at the point in question undergoes approximately an 8 F. loss in temperature during the period of constant rate of drying of the sheet, which of course is regained during the period of falling rate of drying of the sheet and the period between removal of the sheet from the drier and reapplication of the sheet to the same point on the drier surface. As is noted, there is a corresponding, but sharply diminished change in temperature of the drier shell at a point only .07 inch in from the exterior surface, with the expected time occurrence lag clearly in evidence. As indicated thereon, the temperatures in FIGURES 3, 4 and 5 are all given above 7 an arbitrary datum, since for present purposes it is temperature differentials and changes, rather than absolute temperatures, which are under study. Although not critical to the study, the datum was assumed to be at about 180 F.

FIGURE illustrates the calculated variations in temperature of particular points spaced from the drier exterior surface at different times during a complete revolution of the drier, again above an arbitrary datum. The considerable variations in temperature at the various points immediately adjacent the drier surface during the revolution of the drier are clearly shown.

In FIGURE 3, the heavy broken line A represents the results of calculations corresponding to those previously discussed, with the exception that it was more realistically assumed that the paper sheet at the time of application to the drier surface would be at a temperature of 90 F., a condition commonly occurring in actual operation. From this it will be observed that immediately upon application of the relatively cold sheet to the drier surface, the drier surface suffers a loss of about 16 F., and then promptly recovers about 8 F. of temperature, at which time the temperature remains approximately constant during the time of constant rate of drying of the sheet. Thereafter, the temperature of the drier surface rises to the starting temperature which existed at the time of application of the sheet. It should be noted that in this instance a temperature variation at the surface occurs in the amount of 16, while under the different conditions of FIGURE 5 the surface temperature varied by only some 8 F. Again in FIGURE 3, the left end of the graph represents the precise time when the sheet is applied to the drier.

It is apparent from a consideration of both FIGURES 3 and 4 that at the time of sheet application the exterior drier surface is at the highest temperature existing during the drier cycle. If to this hot surface, a sheet of sufliciently heavy basis weight and/ or sufliciently high moisture content is applied, enough water vapor will be generated by contact with the hot surface to blister and/or separate the sheet from the drier. The basis weight and moisture content at which this blistering occurs is a function of the drier surface temperature at the point of sheet application. The higher the surface temperature, the lower the basis weights and moisture contents at which this blistering and/or separation will occur. Utilization of a nonporous pulp will also favor this unwanted blistering phenomenon.

This invention provides apparatus whereby the exterior surface temperature of the drier 23 (FIGURE 1) is reduced immediately prior to application of the wet sheet to a temperature sufficiently low to prevent blistering and/or separating of the sheet from the drier surface through the formation of steam between the exterior drier surface and the sheet at a rate greater than can be transmitted through the sheet.

Still referring to FIGURE 1, a preferred form of this invention for achieving the above purpose involves the inclusion of a cooling medium to be applied to the drier exterior surface immediately prior to the time when the sheet 20 is applied to that surface. In the particular embodiment of FIGURE 1, this means involves a cooling spray 30 of water sprayed onto the drier surface through a nozzle or series of nozzles 31 fed from a suitable source such as pipe 32. Under the conditions and with the drier previously discussed, excellent results have been achieved through the application of water at a temperature of 60 F. in the amount of one gallon per minute per foot of drier width. It is of course understood that only a portion of the heat absorption capacity of this volume of water is in fact completely utilized in cooling the drier surface, largely due to splashing and other deficiencies in water application. A doctor blade 33 may with advantage be used to remove excess Water from the surface of drier 23, so that an additional amount of water to be evaporated is not presented to the sheet at the point of its application to the drier, and to prevent splashing of the spray water against the sheet.

Combining the concept of drier surface temperature reduction into the method of calculating drier surface temperature previously set forth, and assuming the sheet to be at F. at the time it is applied to the drier and that steam pressure applied to the drier is increased sufficiently to overcome the reduction in drying capacity produced by introducing the coolant, there are obtained the results illustrated in the solid line B in FIGURE 3. The substantial temperature drop at the right end of the graph results from application of the coolant to the drier surface. As shown, the sheet is applied to the drier before the drier surface has completely recovered to the temperature existing just prior to introduction of the coolant. The contrast of the broken and solid lines A and B in FIGURE 3 graphically illustrates the unusual degree of initial reduction in and control of drier surface temperature through the use of this invention. It will be noted that in this particular comparison the end condition temperature, that is the temperature of the drier surface just at the time of appiication of the sheet, is approximately 7 below the corresponding temperature when the invention is not employed. However, during the remainder of the drying cycle the corresponding temperatures differ only slightly, in the order of some 2, and average about the same. Thus, there is no substantial change in drying effect, in spite of the very material reduction in temperature of the drier surface at the time of application of sheet 20 thereto. It will be appreciated that this apparent phenomenon results primarily from the high heat capacity and high thermal conductivity of the drier shell combined with the fact that only the surface of the drier shell is cooled by the cooling medium (see also FIGURES 4 and 5).

The light broken line C in FIGURE 3 represents the calculated results of introducing the cooling medium as before described and simultaneously increasing to a considerable extent (as compared to the steam pressure for line A) the steam pressure applied to heat the drier, thus to realize the substantial advantages of the invention. It will be observed that while the temperature of the drier surface at the point of sheet application has through application of the coolant been maintained at the level of the conventional installation (line A), the temperature during the main part of the drying cycle is increased by some 5 to 8 F., with corresponding substantial increase in drying capacity.

The light interrupted line D in FIGURE 3 illustrates the conditions resulting from introduction of the cooling medium as before described, but with no change in the steam pressure from that assumed in connection with graph line A. Again the cooling medium causes the anticipated great reduction in drier surface temperature only partially recovered as of the time the sheet is applied. The reduction in drier temperature throughout the main part of the drying cycle, due to the added load imposed by the coolant, and the corresponding loss in drying capacity, are less than initially result first application of the cooling medium.

In the calculations used in arriving at FIGURE 3 it was reasonably assumed that the heat flux from the drier surface immediately after sheet application was proportional to the difference between drier surface temperature and sheet temperature and that the sheet assumed a maximum temperature of F. during the constant rate period of drying. These assumptions allowed the calculation of the form of the surface temperature curve immediately after the point of sheet application. To determine the effect of water sprayed on the drier surface immediately before the point of sheet application, it was assumed that water was sprayed onto the drier at 60 F. in an amount of one gallon per minute per foot of drier width, the spray being applied in the area between 16 and 12 inches ahead of the point of sheet application, with an estimated resultant heat flux from the drier surface of 160,000 b.t.u./ft. This assumption led to a value of dT l-s52 F./1n.

Over the remaining one foot to the point of sheet application the value of was considered equal to 3S2Xr 9 where T is in F., 2: is in inches and B is in seconds. This assumption is most reasonable in terms of knowledge of heat transfer rates which prevail under similar circumstances, as indicated by McAdams, W. H., Heat Transmission," 3rd ed., McGraw-Hill Book Co., New York, NY. (1954). It is obvious that positioning of the sprays over an area of the drier surface different than that indicated by the previously stated assumptions of heat flux would lead to a different surface temperature profile of the drier immediately previous to the point of sheet application and that for the sprays to be most effective they should be located as close to the point of sheet application as possible. Under usual operating conditions, the cooling medium preferably should be applied not more than about three feet ahead of the point of sheet application, so that the cooling effect will not be dissipated prematurely; however, this distance will be determined by drum speed and other operation conditions actually presented. Under usual conditions, in order to obtain maximum effectiveness, the cooling medium is not removed from the drier surface until just ahead of the point of sheet application.

In an actual operation pursuant to the foregoing, a 14 /2 pound sheet (per ream of 2880 ft. before creping) was made on the paper machine, the sheet being dried on the aforementioned aluminum bronze Yankee drier of 8-foot diameter, 30-inch width, and shell thickness of 1.875 inches. A production speed of 1390 f.p.m. was maintained with steam provided to the drier at 110 p.s.i.g., and with a cooling medium being applied to the drier surface. In this instance the cooling medium was as in FIGURE 1, comprising a water spray of one gallon per minute per foot of width of the drier, the water being at about 50 F. The spray was applied across the drier in the area between about 16 and 12 inches ahead of the line of sheet application.

When the cooling sprays were turned off the sheet started to blister even though the steam pressure was lowered to 90 p.s.i.g. Although the blistering condition improved as equilibrium was reached under the new conditions, an unsatisfactorily damp and sloppy sheet was produced, due to incomplete drying. It would appear necessary to reduce sheet production to about 1200 to 1250 f.p.m. in order to regain a satisfactory drying condition. Upon restoring steam pressure to 110 p.s.i.g. the blistering condition returned, with adverse effect both as to appearance and incomplete drying; however, when the cooling sprays were again turned on completely satisfactory drying of the sheet resulted. It is thus seen that without application of the cooling medium the steam pressure and resultant drying ability must be reduced, with the requisite that machine speed must then be reduced if satisfactory drying is to be obtained. Conversely, through employment of this invention a greatly increased drying speed may be obtained.

The effectiveness of the spray continued in evidence in tests involving maintenance of production through an increase in sheet basis weight manufactured and attendant reduction in machine speed. For example, under the conditions described immediately above, a paper sheet of 15.5 pound basis Weight was produced at 1290 feet per minute, or a sheet of 17.2 pounds basis weight at 1130 feet per minute (with somewhat increased spray volume to effectively prevent blistering of the sheet from the drier surface), or a sheet of 24.4 pound basis weight at a speed of 920 feet per minute. In these operations the furnish to the paper machine was wood pulp comprising 65 percent kraft pulp and 35 percent sulfite pulp.

The blistering and reduced efiiciency problems described above are encountered to an increased degree when a high wet strength paper is produced, involving the use of wet strength resins in the pulp furnish. This apparently re sults from a restriction and closing of the pores in the sheet by the resin, thus reducing the moisture vapor transmissibility of the sheet upon being heated by the drier. When the pressure roll 24 (FIGURE 1) is a suction roll, it has been found that the most serious blistering occurs in the areas where the suction is applied by the roll, that is, in the areas where mechanical pressure is not applied by the suction roll; the adverse effect may be greatly reduced and under proper conditions eliminated through use of the present invention. While the limitations on drying speed due to blistering become more obvious and pronounced when considering utilization of the modern high heat transfer driers, the problem also exists and this invention is of great utility in connection with drying by conventional driers made of cast iron or other materials.

While in the preferred embodiment the cooling means comprises a liquid spray, preferably of water, applied to the drying medium at the point ahead of the area of initial drying contact where under the conditions extant the desired advantage will be attained, it will be clear that other means of cooling only the surface of the drying medium in accordance with the spirit of this invention may be utilized. For example, the means might comprise a brush continually supplied with a cooling liquid, or a belt or roller briefly in contact with the drying surface and also so supplied with a coolant, or other equivalent means. The primary requirement is that the cooling medium be applied so as to produce a cooling of only the surface of the drier, with no substantial reduction in the drying capacity, and that the cooling medium be applied in such proximity to the point of application of the material to be dried that the cooling effect will not have been dissipated at the time drying commences. Application of the coolant must be so controlled that no material amount thereof is carried by the drying means onto the material being dried, which would result in an increase in the drying load and a substantial dissipation of the advantages desired. While the invention is described in connection with the drying of paper sheet material made from wood pulp, or vegetable fibers such as flax, tobacco, abaca, synthetic fibers, mineral fibers, textiles, non-woven fabrics, it is obvious that it may equally well be utilized in connection with the drying of sheets made from other known materials, such as vegetable fibers, tobacco, and the like, and in the drying sheets of most weights through the light to heavy range.

Having now described the method and a preferred embodiment of apparatus for the practice of this invention, it is to be understood that limitations on the scope thereof are intended only as specifically set forth in the appended claims.

What is claimed is:

l. The method of increasing the drying capacity of a sheet drying apparatus, the apparatus comprising a heated rotatable drier drum and means for continuously applyin g a wet sheet to the surface thereof and for removing the dried sheet therefrom, comprising increasing the heat applied to the drum to raise its surface temperature and thereby increase its sheet-drying capacity, and simultaneously reducing the thus-increased temperature of the surface only of said drum primarily just ahead of and at the point of application of said sheet, said method of reduction comprising applying a cooling medium to the surface of the drum in close proximity to and rotationally ahead of the point of sheet application, said cooling medium being applied in amount and at the temperature to reduce the temperature of the drier surface at the point of sheet application to obviate formation of steam between the sheet and drier surface in amount sufiicient to substantially interfere with drying contact therebctween.

2. The method according to claim 1, further including the step of removing substantially all moisture from the drier surface just prior to the point of sheet application.

3. In a drying apparatus, the combination of a drum, means for rotating said drum, means for heating said drum, means adapted to apply a continuous sheet of material into contact with the exterior surface of said drum for drying the sheet as the drum is rotated, a source of liquid of a type adapted to evaporate on said heated drum, a sprayer connected to said source of liquid and directed to impinge the liquid against the surface of said drier drum immediately adjacent and rotationally ahead of said sheet as it is being applied on to the drum for cooling the drum by evaporation of the liquid thereby reducing the temperature of the surface of the drum immediately ahead of said sheet for inhibiting blistering of the sheet on the drum surface, and a doctor blade disposed between said sprayer and the point of application of said sheet on said drum for removing excess liquid from the surface of the drum and for shielding the sheet of material from splashing of the liquid.

4. In a drying apparatus, the combination of a drier drum, means for rotating said drum, means for heating said drum, means adapted to app-1y a web of paper into contact with the exterior surface of the drum as the drum is rotated for drying the web, a source of water, a sprayer connected to said source and directed to spray the water on the surface of the drum immediately adjacent and rotationally ahead of the paper web as the web is being applied on to the drum so that the water evaporates prior to application of the paper to the drum and cools the surface of the drum to inhibit blistering of the paper web on the surface of the drum, and a doctor blade positioned between said sprayer and the point of application of the web on said drum and coacting with the drum for removing excess liquid from the drum surface and shielding the web from splashing of the water.

5. The method of continuously drying a web of paper comprising continuously revolving a cylindrical drum. and applying the web of paper to the drum as the drum is rotated, heating the drum, applying a spray of water to the exterior surface of the drum in close proximity to and rotatably ahead of the point of application of the paper web to the drum, and removing excess water from the surface of the drum subsequent to its application to the drum and prior to the line of application of the paper web to the drum so that the Water evaporates from the drum prior to the application of the paper web thereto and cools the surface of the drum so as to inhibit blistering of the paper web at its line of application to the drum.

6. In a drying apparatus, the combination of a rotatable drier drum, means for heating said drum, means adapted to apply a continuous sheet of material into contact with the exterior surface of said drum, a source of a fluid cooling medium, means connected to said source of cooling medium and applying the medium on the sur face of said drum immediately adjacent and rotationally ahead of said sheet as the sheet is being applied onto the drum for cooling the surface of the drum immediately ahead of said sheet, and means for shielding the sheet of material from said cooling medium so that substantially none of the cooling medium is applied onto the sheet material as it passes onto said drum.

7. The method of drying a continuous length of sheet material comprising, applying the sheet material into contact with the exterior surface of a heated continuously revolving cylindrical drum, applying a cooling medium to the surface of the drum in close proximity to and rotatably ahead of the line of sheet application, said cooling medium being applied in amount and at a temperature to reduce the temperature of the drum surface at the line of sheet application to obviate formation of steam between the sheet and drum surface in amount sufiicient to substantially interfere with drying contact therebetween, and shielding the cooling medium from said line of sheet application so that substantially none of the cooling medium is applied onto said sheet.

8. The method of drying a continuous length of sheet material comprising, applying the sheet material into contact with the exterior surface of a heated continuously revolving cylindrical drum by running the sheet material over a pressure roll in contact with the cylindrical drum and through the nip of the pressure roll and drum, and applying evaporable cooling medium to the exterior surface of the drum in close proximity to and rotatably ahead of and spaced from the line of application of the sheet material to the drum in such amount and manner that substantially none of the cooling medium reaches the line of application of the sheet material to the drum.

9. The method of drying a continuous length of sheet material comprising, applying the sheet material into contact with the exterior surface of a heated continuously revolving cylindrical drum by carrying the sheet material on a belt passing through a nip of a pressure roll with the drum so that the sheet material is applied to the drum at the nip of the pressure roll and drum, and applying an evaporable cooling liquid to the exterior surface of the drum in close proximity to and rotatably ahead of and spaced from said nip in such manner and amount that substantially none of the evaporable liquid moves into said nip and onto said sheet material.

References Cited in the file of this patent UNITED STATES PATENTS 1,283,888 Pope Nov. 5, 1918 1,601,387 Cram Sept. 28, 1926 2,081,945 Massey et a1. June 1, 1937 2,157,388 MacArthur May 9, 1939 2,678,890 Leighton May 18, 1954 2,810,966 Bicknell Oct. 29, 1957 2,826,827 Metz Mar. 18, 1958 2,950,989 Freeman Aug. 30, 19 0

Claims

1. THE METHOD OF INCREASING THE DRYING CAPACITY OF A SHEET DRYING APPARATUS, THE APPARATUS COMPRISING A HEATED ROTATABLE DRIER DRUM AND MEANS FOR CONTINUOUSLY APPLYING A WET SHEET TO THE SURFACE THEREOF AND FOR REMOVING THE DRIED SHEET THEREFROM, COMPRISING INCREASING THE HEAT APPLIED TO THE DRUM TO RAISE ITS SURFACE TEMPERATURE AND THEREBY INCREASE ITS SHEET-DRYING CAPACITY, AND SIMULTANEOUSLY REDUCING THE THUS-INCREASED TEMPERATURE OF THE SURFACE ONLY OF SAID DRUM PRIMARILY JUST AHEAD OF AND AT THE POINT OF APPLICATION OF SAID SHEET, SAID METHOD OF REDUCTION COMPRISING APPLYING A COOLING MEDIUM TO THE SURFACE OF THE DRUM IS CLOSE PROXIMITY TO AND ROTATIONALLY AHEAD OF THE POINT OF SHEET APPLICATION, SAID COOLING MEDIUM BEING APPLIED IN AMOUNT AND AT THE TEMPERATURE TO REDUCE THE TEMPERATURE OF THE DRIER SURFACE AT THE POINT OF SHEET APPLICATION TO OBVIATE FORMATION OF STEAM BETWEEN THE SHEET AND DRIER SURFACE IN AMOUNT SUFFICIENT TO SUBSTANTIALLY INTERFERE WITH DRYING CONTACT THEREBETWEEN.