US2324519A - Drying of distended fibrous products - Google Patents

Description

lJuly 20, 19.43. o. J. KRUEGER ETAL 2,324,519

DRYING OF DISTENDED FIBROUS PRODUCTS 'Filed mig. s. 1940 Y 2 sheets-sheet 1 July- 20, 1943. o. .1. KRUEGER ETAL f 2,324,519

DRYING OF DISTENDED FIBROUS ERODUCTS Filed Aug. s, 1940 V 2 sheets-sheet g Y l. v t Patentedlllly 2,0'

I DRYTNG oF nIs'rENDED mnous raonnc'rs Otto J. Krueger. Western Springs, andjohn Careslo, Chicago, lll., 'asllgnors to Masonite (llorporation, a corporation of Delaware Abpucauon august s, maserati No. 351,030

4 Claims.

'I'he invention relates to the drying of distended fibrous products, and relates more particularly to the drying of such products made from fibrous materials suchas kraft pulp, having for example foaming agents such as saponin mixed therewith and the mixture agitated with provision for air supply, sewing to produce 'a condition wherein relatively stable bubbles are incorporated with the fibers of the pulp in such manner as to produce distention. and maintain the ilbres in distended relation, in which relation' they are xed by drying.

The principal object of the invention is the provision of an improvement in the drying of distended fibrous products lby means of' heated air and products of combustion whereby an improved product can be made, and increased speed of'production obtained. i

When relatively thick sheets, as for-example,

sheets one inch thick of distended fibrous material andfcontaining a considerable proportion of water are traveled along on a reticulated support, asa Fourdrinier screen,.with excess or free water leaving the sheet from the bottom, the content of water retained in the sheet is least at and near the top surface, and the water content increases downwardlyand is greatest at and near the bottom of the sheet.

which the lower' part of the sheetgets'from the carrying belt; blanketing of this upper partof vthe sheet by water vapors is less than for the Alower part of the sheet because there i-s less wav ter present to be vaporized; and heat conducted lfrom theupper surface of the sheet downwardly is decreased relative to the'corresponding eiect at and near the bottom of the sheet, because of v decreased heat conductivity due toreducedway ter content.

` Consequently in the htfairdrying. of light weight sheets of distended fibrous materials, it has been necessary to keep the temperaturesv no higher than can be withstood by -the upper surface of the sheet without -charring or burning, i and this in turn has made it necessary to keep production speed correspondingly low or to make use oi very long and highly-expensive ,drying 1 apparatus. These diiilculties are avoided by the prior to its introduction into the drier.'

present invention thrqugh the addition'of water to the top surface and upper -part of the sheet By the addition of such water the temperatures are no longer limited by the ready charring. of J the upper surface, as is the case without the add` 4ed water. A small proportion of ilre retarding material, such as boris acid for example, can ad- 7 vantageously be dissolved in and supplied to the No great difficulty is encountered in drying the lower part of such a sheet by passingit through a hot air drier and subjecting it therein to temperatures suiliciently high to secure rapid drying and good qualities in the sheet, without scorching or burning. The lower surface of the sheet is shielded to somev extent by the wire belt onwhich it is taken through the drier, and also by the roller supports for supporting the belt. It is also more or less protected by water vapor 'formed from the water in this part of the sheet. The' water in'this part of the sheet appears togive good heat conductivity. so that-.the lheat is effectively conducted within` the interior of this part of the sheet, and the drying extends'upward- Ily into the sheet from beneath.

If, however, thesheet is dried in a comparatively short drier, which can be operated at high temperature, the y'saine speed of travel which gives good results fat and near the bottom of the sheet because of considerations as above'described will cause the upper surface of the sheet tobe charred or even burned, and at and about themedian zone of the sheet the drying'will'not be complete,

This is apparently due lto the relatively .low water content in the upper part of the sheet. The upper part of the sheet lacks the shielding top of the sheet with such added water, but good results can be obtained with or` without such dissolved soluble material. It might be supposed that such addition of water or water containing boric acid orother fire retarding material would slow down the drying and increase the fuel cost of drying, but'the reverse of this is really the case. In the drying of the sheet, water vapor is developed rapidly at the top surface of the sheet to which added water has been supplied, and this vapor appears to have a protecting effect against ready chai-ring. The heat penetrates downwardly` into the interior of the sheet or board, as is evi- `denced by thorough drying throughout the sheet.

This effect appears to be produced by the greater heat'conductivity due to the presence of the water added to :the sheet from above;- The fuel cost can be reduced, notwithstanding the fact that more water' has to be dried out. and this again appears to lbe'due to the added water by which heat is conducted `i`nto the interior of the board from above.' such heat conduction effect and the drying 'of the inner part of the sheet being the more efllcient 'since eil'ective heat conduction to the interior of the sheettakes place not only from the bottom` ofthe sheetupwardly. but alsoA downs. wardly fromthe top of thesheet.

It is-'thus an object of the invention to secure uniform drying of the sheets of distended fibrous material through the indirect, but nevertheless effective route by adding to the quantity of water which has to be dried out ofr the sheet.

The accompanying drawings forming a part y of the specification show an illustrative embodiment of apparatus for carrying out the invention. In said drawings,

Fig. 1 is a schematic side view of forming apparatus and also showing a portion of the' drying apparatus;

Fig. 2 is a side view on an enlarged scale of spraying apparatus;

Fig. 3 is a cross-section on line 3-3 of Fig. 2;

Fig. 4 is a side view of a part of a drier; and

Fig. 5 is a cross-sectional view on line 5-5 of Fig. 4.

Referring first to Fig. 1, reference character I0 designates a flow box to which the mix, consisting preferably of fiber, foaming agent, sizing materials, water and air from which the sheet I is to be formed, is supplied through the pipe I2. This mix is kept in agitation by an agitator I4, located behind theintermediate dam I6 in the flow box, and after passing over the dam I6 the mix is further agitated by the roll agitator 28. The mix from the ow box is delivered ,through the passage I8 on to the Fourdrinier Wire 22, the depth of the layer so delivered being regulated by a slice plate 24, which can be adjusted for height and for thickness of sheet, as by the regulating screws 26. The upper run 21 of the Fourdrinier wire 22 is supported in the usual manner by means of the spaced rollers 28 shown only in part but provided throughout the length of the Fourdrinier wire belt, and the sheet formed from the mix is designated by reference character 30. At the respective ends of the forming apparatus the Fourdrinier wire 22 runs around a breast roll 32, and a couch roll 34. The return or under-run 38 of the Fourdrinier wire 22 runs over rollers 38 and beneath other rollers as 48, which may be made adjustable and used for tensioning the Fourdrinier wire as indicated by the arrows 48' placed near these rolls. Suction boxes 4I, 4| may be provided, if desired, for compacting the extreme lower portion of the sheet 38 somewhat in order to give increased strength to this part of the sheet. The roll 34 is preferably perforated, and air under light pressure supplied at the interior 'thereof to ensure that the sheet 38y is freed from the Fourdrinier wire 22.

Transfer means are provided for delivering the formed sheet from the Fourdrinier wire to the drier belt, aslit passes oi the Fourdrinier wire belt and over the couch roll 34, and may consist, in the simplest form, of a short transfer belt 42, running over the rollers 44, 44, 44 arranged in triangular relationV so as to'guide the belt 42 to perform the transfer function.

'I'he drying mechanism is' illustrated only in part. This drying mechanism, which is of the hot air. type and does the drying by circulating heated gases including products of combustion, is preferably composed of several sections, the number ofv sections and total length of the drier varying with the conditions for the particular installation, and in the showing herein made, the first and. second sections only are illustrated. Such drier comprises essentiallyaf long drying chamber or tunnel 50, and heating and air circulating chambers which are preferably arranged partially over and partially alongside the drying tunnel 58. In the form shown, the first drying section, indicated generally by the reference numeral 52, contains a blower fan 54, and an outlet Inlet stack 60 supplies air for combustion and delivery thereof to the gas burners G2. A number of such burners are arranged across the width of the drier. In the form shown five of such burners are provided, the same being regulated as to supply of .combustible mixture of gas and air by the thermostatically operated controls 10 and the valves 12. As indicated by the arrows on Fig. 4, the passage of circulating gases is in the first drier section 52 counter to the direction of travel of the sheet 30. The means provided for travelling sheet 38 through the tunnel drier preferably comprises the wire belt 14, which may advantageously be of the double spiral, or hinged spiral type well known for such use, the same being carried at the entrance end of the drier around the tail roll 18, and at the opposite ends of the drier carried by, a corresponding head roll, not-show n. The belt 14 for carrying the sheet through the drier is fairly. substantial and the spirals thereof made of fairly heavy wire, and such belt aifordsquite a considerable shielding to the under side of the sheet 38, as it passes through the drier,- this shielding effect being increased by the relatively closely spaced rollers 88 on which the upper run of the drier belt 14 is supported as it passes through the drier. The return run 82 of the drier belt 14 Ais supported by therollers 84, which however do not need to be so closely spaced. The succeeding sections of the tunnel hot air drier which is preferably of n the single pass variety, are preferably arranged to direct the circulating gases so that the same pass through the tunnel 50 in the same direction as the sheet 38. As already stated, but one such further section is shown, and same is illustrated and heated by the gas burners are supplied to av conduit 90 which has branches 92 and 94 to the opposite sides of the tunnel 58, and gases are directed into the tunnel 50 by means of nozzles 96 and 98 arranged to supply said gases above and below the sheet 38 respectively.

Except as they are combined with the provision made for spraying water, preferably water containing la percentage oi.' soluble re arresting material, upon the sheet before it is dried, the particular construction and arrangement of the Fourdrinier apparatus and the drying apparatus constitute no part of the presentinvention, and various other forms of apparatus suited for these purposes can be utilized.

'I'he water delivery means are located over the sheet 38 on the Fourdrinier wireI and are located at such a distance from the entrance to the drier as to give opportunity for the water to even out and penetrate into the upper part of the sheet 38 before it reaches the drier. The preferred relationship is substantially as shown in Fig. 1.

The provision which is made for delivering the water, preferably containing fire resistant materials in solution, on the sheet ,may be of various kinds, but the preferred form shown inthe drawing consists of a transverse pipe I 80, which is of relatively large diameter, so that its contents other than lcut oil, as for example 4cut off during the time can be held ,atK substantially uniform pressure throughout its length, and this pipe III is provided with a series of spaced'spray nozzles ill, Ill. Thesenozzles il! are arranged to secure a uniform delivery of the iluid'upon thev sheet when the pipe |09 is located relatively close to the sheet SII.` These spray nozzles can be directed laterally and downwardly as indicatedat I, Fig. 1. A convenient mode of securing uniformdistribui tion of fluid from each of the nozzles III, consists in the formation thereof as 'simple spray nozzles centrally bored, at i, and selection thereof to give substantially' identical nozzle openings vadapted for uniform fluid-delivery. A convenlent mode of testing lthe fluid delivery is by supplying fluid to the nozzles at uniform pressure,

which is substantially the .pressure used in oper-A l are relatively much more weak less effective in .such undried portions than in ation, and causing each nomic todischarge'into agraduate for a selected period of time, and selecting from the supply of nonies those Vwhich deliver substantially equal quantities of water insuch time.

While it is not essential in all cases to supply dissolved ilre resistant materalswitli the spray water, itis preferred to incorporate a smallpercentage of such materialwith the water' to be sprayed on the sheet. The preferred material y Whens the sheet is first formed on the Ii'ourdriniery for this purpose is boric acid which is not decom tion consist primarily in making it possible to secure desirably higherl speeds of travel of the sheet being formedl and dried, and the production of a good qualityacceptable,"ilnished sheet prod-uct at such increased production speeds.

The advantages secured will be clearly under- The portions ofthe material which have gone through the drier without being' and Vfragile than the remainder'of the sheet, and cause a considerable reduction in the strength and resistivityl of the whole sheet.` If, for example, such partly' undried sheets are stackedon one another until any considerable weight is imposed on the lower sheets in the stack, the-undried parts will colf lapse, which, of course, ruins the sheet for commercial purposes. The sizing material used, and preferably consisting of waxor resin sizes. are

the parts of the sheet which have been thoroughly driedinthe drier. I

' Without commitment to particular theory, it appears that the beneficial effects of the spraying of the water onLthe sheet prior to delivery thereof to the drier may be explained as follows:

wire, it consists of an evenly distributed mixture of water, fiber and fine air bubbles. `As the sheet proceeds along thev Fourdrinier wire a considerable proportion of the water drains from it, passing through the wire mesh to drain pans beneath. When no spray is used, upon entering the drier the amount of water remaining in the sheet is least at and near .the upper surface. This 'upper part of the sheet having low water content, has poor heat conduction properties.-

Y As a result of this vuneven distribution of water y in the sheet. the upper portion oi'` thesheet containing least water is very difficult to dry by reason of its low heat conductivity.-

When the sheet with the water so unevenly distributed, that is to say, with much less water stood from the following. comparison of operation with and without the water lspray respectively:

Using the water spray, the apparatus is oper-4 ated under speeda'nd temperature etc.'conditions adapted to produce a good, acceptable sheet which is substantiallly bone dry at the exit end of the drier. The sheets of distended fibrous material are very light, weighing only a few ounces per square foot. Dried with the use of A thewater spray, the sheet is light-colored and free from char.

lf, then, for testing theA effect of nthe water spray, the operation is continued. with no change that the water spray is temporarily that a section of sheet I0, perhaps 25 Vfeet in length, has passed through the drier, it will be found that when that part ofthe sheet to which added water was not supplied issues from the drier, instead of being light colored and free from charring on theupper surface as before, this part of the sheet will havea charred surface, and

`the interior of the sheet, instead of being subi stantially bone dry, will contain sufficient moisture so as to be perceptibly damp to the touch. Furthermore, the undried part of the sheet so produced, even if the sheet is subsequently furl ther dried, is defective and unacceptable for purat and near the top than at and near the lower surface, is exposed to the hot gases in the drier, the sheet will dry very rapidly on the extreme upper surface, which then becomes even less heatconductive. 'Ihis further retards the pas-4 sage of heat into the sheetV from above to evaporate the water within the sheet, and upon con- Y tinued exposure to the hot circulating gases of the drier, the topsurface willV scorch and .char before the water is removed from the center of the sheet throughapplication of heat from above.

While this is going on near the upper surface of the sheet, a substantially eillcient drying is being performed at andI near the lower surface of the sheet, which has relatively highheat conduction properties due to presence of water. with such emcient drying operation taking place at the lower part of sheet, any' tendency to scorch in this lower part is reduced by the sheet being protected by the spiral linkbelt ofthe drier and bythe relatively closely spaced rollers over which the belt is passed. Another factor which tends both to emicient drying and protection from charring at the bottom of the sheet 4appears to consist inthe further protection afforded; by thepresence of water vapor in subi stantial quantities as the'drying proceeds.

The conditions just described are present and the effects just described are obtained when the sheetis passed into the drier without auxiliary spray treatment of its upper surface. .Quite dif ferent conditions and results are obtainedv when the spray is used, andthe explanation thereof.

appears to be as follows: The spray` creates on the upper surface and lin the upper portion of the sheeta condition with respect to water content generally similar to that :lust described as existing normally at or naar the lower surface poses for which good uniform sheets are desired.

thoroughly dried of the sheet. or in other words,.a more even distribution of water from bottom to top of the sheet. 'I'he water in the upper portion of the sheet makes it more highly heat conductive. A better and more rapid water removal from the interior of the sheet takes place from above. Also, water vapor formed over the sheet during the drying operation appears to protect the upper surface against the scorching and charring that would take place without use of the spray.

The inclusion of various fire retardant chemicals of which boric acid is an example, serves to increase the protection aiorded against char. The quantity of water to be sprayed on lthe i sheet and the amount of boric acid or equivalent tire resisting material to be included will vary considerably under varying conditions and with different thicknesses of sheet. However, it may be stated, for. example, that when making a sheet eight feet wide and substantially one inch thick, and operating the apparatus at a speed of approximately eight feet per minute with a temperature of approximately '150 F., in the rst drying zone, and progressively lower tempera-y tures in subsequent zones, excellent effects can be obtained by delivery of water to the upper sur face oi' the sheet at the rate of approximately 1.2 gallons per minute, and a desirable propor tion of boric acid for use therewith is in the neighborhood of from .2% to .5% on the quantity of water sprayed, or an average of approximately .35%. It is a feature oi' great advantage that the use of the spray as has been described permits use of much higher drying temperatures than without the spray, and makes it possible to operate at increased speed, giving greatly increased production of good quality and commercially acceptable product with use of drying apparatus which would otherwise have to be operated at lower drying temperatures and much lower speed and output.

We claim:-

l. Process of treating relatively thick wet sheet material containing fibers held in distended relationship by relatively stable bubbles, which comprises delivering added water to the upper part of the sheet, and then drying the sheet by directing hot circulating gases toward the sheet surface to obtain uniformand increased speed in drying of the sheet whereby the vfibers are substantially fixed in their distended relationship and improved strength obtained with substantially no charring.

`2. The process of treating relatively thick wet sheet material containing iibers held in distended relationship by relatively stable bubbles, which comprises delivering added water to the upper part of the sheet substantially uniformly throughout its area, and then drying the sheet by directing hot circulating gases toward the upper and lower surfaces of the sheet to obtain uniform and increased speed in drying of the sheet whereby the ilbers are substantially fixed in their distended relationship with substantially no charring.

3. The process of treating relatively thick wet

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