US1593598A - Method of drying moisture-containing materials - Google Patents

Description

July 27 1926.

K. REDMAN METHOD OF DRYING MOISTURE CONTAINING MATERIALS 1925 2 Sheets-Sheet 1 Filed Feb. 24

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WMMEZKQ/M July 27 1926.

K. REDMAN METHODOF DRYING MOISTURE CONTAINING MATERIALS 1923 2 Sheets-Sheet 2 Filed Feb. 24

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UNITED STATES 1,593,598 PATENT OFFICE.

-KENNETH BEDMAN, OF WEST ROXBUBY, KASSACHUSETTS, ASSIGNOR, BY MESNE AS- SIGNMENTS, TO B. F. STURTEVANT COMPANY, 015 BOSTON, MASSACHUSETTS, A COR- POBATION OF MASSACHUSETTS.

METHOD OF DRYING MOISTURE-CONTAINING MATERIALS.

Application filed February 24, 1923. Serial No. 621,059.

The present invention relates to a method of drying moisture-containing materials and is more particularly concerned with the kiln drying of lumber. n

It has been appreciated by those skilled inthe art for a considerable length of time that the drying of moisture-containing materials should be initiated in a relatively humid atmosphere in order that the drying action might be caused to take place gradually, thereby obviating the danger of crack ing or warpin of the drying material by drying the sur ace too rapidly. To this end. the older methods of drying lumber consist in merely'subjectin the lumber to' heated air or other gas at Tig h relative humidity, this relative humidity being gradually decreased and the temperature being increased as the drying action progresses. This method, however, is rather slow, and according ly possesses the disadvantage that long continued heating periods are required, thereby utilizing excessive amounts of fuel to say nothing of the charges involved in the long use of the. kiln for one drying operation.

The principal object of the present invention is to provide a method of drying Wood or other moisture-containing materials whereby the total drying operation may be much more rapidly performed with an increased qualit of product.

Another ob ect of the-invention is to provide a method of drying moisture-containing material by which the entire drying operation may be more economically effected than in revious existing methods.

With t ese and other objects in view, the present invention consists of the method or steps hereinafter described and more particularly defined in the claims.

I have discovered that the drying of lumher or similar material may be greatly improved by alternately heating and cooling the lumber and providing after certain or erlods an extended rest period during which the lumber is not subjected to currents of either hot or cold air. Accordingly, the principal feature, of the present invention is the provision of the following cycle of operations in the drying of lumber: First, subjecting the lumber to a current of heated air or other gas of high humidity; second, passin a current of relatively cool air over the umber; and third,

allowing the lumber to remain in a still atmosphere for a considerable length of time. The cycle of operations may then be repeated, it being preferable, however, to lower the relative humidity and increase the dry bulb temperature in the first step of the cycle.

It is believed that the advantages accruing to this method depend largely upon differences in vapor tension which'may exist between moisture in the heated material and that of the surrounding air or gas and also to certain hygroscopic efl'ects existing within the materia For example, the initial step of assing heated air of high relative humi ity over the lumber thoroughly heats the lumber to a fairly high temperature and allows only partial drying of the wood, at the same time preventing excessive evaporation of the moisture from the-surface. The vapor tension ofthe moisture at the surface or shell of the wood is thus made relatively high. The second step which consists in passing relatively cool air over the lumber allows evaporation of the moisture from the surface of the lumber into the current of air, due to the lower vapor tension of the moisture in the air. The draft of air may then be shut OE and the lumber subjected to a rest period for a considerable length of time durin which a transfusion of the moisture ta es place from the central portion of the material to the outer surface, due to the fact that the central portion of the wood still retains its higher moisture content which tends to equalize with theisurface of the lumber which has previousl had its moisture content materially reduce during the preceding step. More moisture may then be removed from the wood by repeating the above cycle of. operations as many times as may be desired. It has been found that this method of inserting a rest period into the cycle of operations not only1 reduces the operating time required for t e kiln drying but produces an improved quality of product with greatly less liability of checking or otherwise injurin the lumber due to improper kiln operatlon. The method also ossesses the advantage of re uiring materially less heat and power for e entire o erationi ,h b

Be ore app ying any 0 .t e a ove steps, it is highly desirable, although not necessary, that the material to be dried be heated throu bout to a fairly hi h temperature but w1t-hout an substantial rying. To this end, another fzature of the invention provides for subjecting the material to a pre-.

I at the start of the drying operation.

Certain other features of the invention consist of certain novel features of construction and modes of operation hereinafter described and claimed, the advantages of which will be apparent to those skilled in the art from the 'fo'llowlng description.

In the accompanying drawings Figure 1 is a plan view in section of a fan type or forced draft kiln in which the im roved' method of the present invention may I ried out; Fig. 2 is a section on line 2-2 of Fig. 3 showin endwise piling of the lumber in the kiln; of the kiln shown in Fig. 1; and Fi 4 is a sectional elevation of a common orm of pipe coil kiln showing crosswise piling of the lumber. I y

The present invention resides in the cycle of operations and combinations of steps'in the method of dryin and the particular type of apparatus use may be of the usual formowith certain slight modifications which will hereinafter appear. The illustrated apparatus for carrying out the improvedmethod comprises both forced draft kilns and pi e coil kilns with either of which the metho of the present invention may be empltliyed.

eferring to Figs 1, 2 and 3, the forced draft kiln of the usual form consists of side walls 6, end walls 8, roof 10, floor 12 and suitable doors 14. The lumber is referably piled on trucks 16 and 18, the di erent layers of lumber being separated in the usual manner by stickers 20. In order that the air or gas may penetrate to all parts of the wood, the pile 1s preferably constructed with a central ta ring flue 22, as indicated in Fig. 2. e trucks 16 and 18 are run into the kiln over rails 24. The circulation of air or other gas in" the kiln is effected by a fan or blower 26 havin an inlet connected with heating coils 28 an ahumidity s rayer 30, and an outlet connected with supp y assages 32 and 34. The assages 32 an 34 communicate with supp y ducts 36 and 38,-

respectively, which are tapering in form and are located beneath the trucks. The ducts are rovided with louvers 40 and 42 of the usu form consistin of a series of u standing metallic vanes. t will be seen t at the carig. 3 is a sectional elevation through the assages 32 and 34 upwardly through the ues 22 by the louvers. A deflector 44 may be inserted at the joint between the passages 32 and 34 to control the downwardly through openings 46 in the floor 12 into return ducts 48 which are connected at openings 49-, and thence into an intake duct 50 leading directly to the inlet of the heater 28. In order to allow circulation of fresh air through the kiln, ventilators 52 and 54 are provided, the former leading directly to the inlet of the heater, as indicated Hi Fi ."1, and the latter 'being meraly connecte with an opening in the roof 10. The ventilator 52 is provided with a slide 56 and the ventilator 54 is provided with a damper 58 which may be actuated in any desired manner. Upon opening the slide 56 and damper 58 the circulation of fresh air from the atmosphere may be caused to take place within the kiln.

The humidity sprayer is fed with steam through'a steam p1pe 60 and a suitable valve valve 64 to perforated steam pi es 66 and 68 resting upon the floor 12 for t e purpose of injectlng live steam into the kiln for the preliminary step of thoroughly heating the lumber. The complete series of operations for d 1n lumber in the above desired type of kil n wi now be enumerated in detail. The prel mmary step consists of the admission of hve'steam to thoroughtly heat the lumber 62. The steam p1pe 60 also leads through a through its mass. The average time for this operation is from two to four hours for one lnch lumber and will, of course vary with the thickness of the lumber and its previous degree of moisture. The first step in the cycle of operations consists in the admission of humid heated air into the kiln. Tothis end,'the fan 26 is started and the valve 62 is o ened to permit steam to be injected into t e current of air through the humidity sprayer 30, the valve 64 being closed. In this step, it is desirable to approximate the humidity and temperature conditions recommended by the Forest Products Laboratory at Madison, Wisconsimof the U. S. Governmcnt Department of Agriculture. The rec- 'ommendations cover a considerable range of moisture content, thickness and kinds of lumber, and the humidity and temperature conditions may be readily adjusted to conform therewith. During this period it is air passing through the fan is directed desirable to re-circulate most of the air and remainder of the working da The second step of the eye e is comparatively short and consists in the rapid dryin of water vapor from the surface or shell the wood. To this end the humidit sprayer and heater coils are turned off an slide 56 and damper 58 are open wide to allow the circulation of fresh cool air through the kiln, continuing the operation of the fan 26 all the while. This step conveniently lasts from one to two hours. During this step the vapor tension of the moisture in the heated wood is much greater than that of the surrounding cooler air or gas. Since all pressures tend to equalize, it is believed that the vapor in the heated wood tends to create an equilibrium with the vapor tension in the cooler air. In so doin the surface moisture of the wood is rapidly given up, thereby cooling the wood until an equilibrium is reached, whereupon all moisture transference to the atmosphere substantially ceases.

The third and final step in the cycle of operations consists of a rest period during which time the blower 26 is shut off. Communication with the atmosphere is maintained by keeping the ventilators open but no forced circulation of air is necessary.

During this period, it is believed that due to the hygroscopic character of the wood, the moisture contained in the central portions tends to establish an equilibrium with the surface or shell which has just been dried durin descri d. That is to say, the surface or shell absorbs moisture from the center so that at the conclusionof the rest period, the surface again contains moisture which may be readily evaporated by repetition of the preceding cycle. This rest period preferab y continues over a number of hours and accordingly may last all night. It will be seen that the use of heat and power during the night is thus obviated and furthermore,

no attention is required.

Upon arrival of the workman in the mornin the kiln is once against started preferab y omitting, however, the preliminary steaming operation. The heating step wit humid air is carried throu h this time, however, with a reduced relative humidity and an increased dry bulb temperature. This is followed, as before with the drying by means of cool air delivered by the fan, and this in turn by the rest eriod. The cycle of these three steps may e repeated as man times as desired, the temperature prefera ly bein increased, and the humidity decreased with each recurrence of the first step.

It is believed that the foregoing remarks the second step of the cycle above correctly explain the theoretical operationof the improved method. Re ardless of the correctness of this -theory, known that this method produces a higher quality of product than previously existing methods and may be much more economically carried out.

A slight modification of the above rocedure may be carried out with thin lum er or veneer. In this case the passage of the moisture from the internal to the external layers does not require considerable lengths of time so that in the first cycle of operations the extended rest period may be considerably reduced or, in fact, omitted altogether. During the second cycle, however, the rest period should be used for maximum econom and rapidity of drying.

he features of the present invention are not alone applicable to the forced draft type of kiln but may be embodied in a pipe coil kiln of the usual form. In this ty e of owever, it is kiln, the heating is done by convectiona currentsof air which are heated over a series of pipes containing steam or other heating fluid. Referring particularly to Fi 4, the pipe kiln is constructed with side we s 6/and roof 10, as before. The trucks 16 are preferably run in over three rails 24 if the illustrated form of crosswisepiling is employed. As before, the successivelayers of lumber are separated by stickers 20. Fresh air is admitted to the kiln throu h air ducts 70 and is allowed to pass over t e rows of heated pipes 72. The air is raised to the ropcr umidity by steam from the per orated eration is eii'ected by closing the air ducts,

7 0 and the dampers 74 and turning on the steam in pipes 66 and 68 for a period of two to four hours. The first heating step in the cycle is then carried out as in the case of the forced draft kiln. The pipes 72 are heated by the passage of steam therethrough and the ducts 70 and dampers 74 are opened to allow the circulation of heated air upwardl through the lumber pile, the proper humid ity being maintains by regulation of the steam in pipes 66 and 68. As in the previous case, this operation is carried out for the remainder of the working day. The temperature and humidity maybe substantially the same as in the other case but it is desirable to have the temperature slightly higher and the humidity somewhat lower to compensate for the slower circulation. The second step in the cycle consists, as before, in passing the cool dry air over the lumber. This is caused to take place b natural instead of forced circulation. T e previously heated condition of the lumber and kiln building will tend to permit circulation of the air therethrough. The steam in pipes 66 and 68 is turned off during this step. As in the previous case, the third and final step consists in a rest period during which time a transfusion of the moisture occurs from the central portions of the wood to the surface. In the case of a pipe coil kiln, this rest period obtains as soon as the vapor tension of the moisture in the wood substantially equalizes with the vapor tension of thesurrounding air or gas, that is, when the temperatures of the lumber and gas have become practically equalized. During this rest period there is substantially no circulation of air through the kiln. It is to be pointed out that as in the case of the forced draft kiln, the several steps are separate and distinct and although the line of demarcation between the second and third steps may not be as pronounced in the pipe coil kiln as in the forced draft kiln, there is nevertheless no difierencein the principle ofoperation between the two types of kilns as in both types the second step consists in the evaporation of moisture from the surface of the wood and the third step consists in the transfusion of the moisture from the interior to the shell of the wood. The only difference is that in the case of the pipe coil kiln, the change from the second to the third step isgradual and takes place naturally due to the equalization of the temperatures rather than through any actual o eration of the apparatus as in the case o the forced draft kiln. After the completion of the rest period, the cycle of operations is carried on as before, preferably omitting the preliminarv steaming operation.

Although the resent invention has been s ecifically described in connection with the rying of lumber, it is understood that the invention should not be so limited, but

should extend to the dryin of other mate rials. Theinvention shoul also not be considered limited to the specific steps described herein except as limited in scope by the appended claims.

The invention 'having been described, what is claimed is a y .1. The method of drying moisture-containing materials which consists in sub'ecting the material to a. current of heate air or gas at high relative humidity, then to a' current of materially cooler as, allowing the material to-rest fora perio sufiicient to permit diffusion of the moisture from the central portion to the surface of the mateperiods to rial, and thereafter subjecting the material to a current of heated air.

2. The method of drying materials which in vapor tension between the moisture at the surface of the material and the moisture in the air; third, allowing the material to rest without further drying action for a period sufficient to ermit transfusion of the moisture outwardly from the central portion of the material; and fourth, passing a current of air over the material to absorb moisture from the surface thereof.

3. The method of drying materials which consists in repeatedly and alternately passing heated air of high relative humldity and materially cooler air over the material and allowing the material to rest without further drying after certain of the cooling ermit transfusion of moisture outwardly 50m the central portions of the material.

4. A method of drying materialswhich consists in thoroughly heating the material throughout its mass without drying and thereafter subjecting it to the following repeated cycle of operations; first, subjecting the material to a circulation of humid heated air, then passing a current'of relatively dry air at a lower temperature over the material to permit differences in vapor tension to cause evaporation of moisture from the surface of the lumber, and thereafter allowing the lumber to rest without further drying action for an extended period to permit I transfusion of moisture outwardly from the central portions of the-material.

5. The method of kiln drying lumber which consists in thoroughly heating the lumber throughout its mass by contact with steam, then repeatedly passing alternate currents of heated humid air and relatively dry air at a' lower temperature over the lumber, and thereafter allowing the lumber to rest without d ing action at the conclusion of certain o the cooling periods for extended periods to permit transfusion of moisture from the central portion toward the surface of the lumber.

6. The method of drying moisture-eontaining materials which consists in subjecting the material to a current of heated air or gas at high relative humidity, then to a current of materially cooler gas until the er gas to permit the diffusion of moisture from the central portion to the surface of memes the material, and then again subjecting the material to a current of heated air or gas and thereafter to a current of cooler gas.

7 The method of drying materials which consists in heating the material and drying, then cooling the same by subjecting it to a current of cool air, then permitting the cooled material to stand without drying, and thereafter again heating and drying the ma- 10 terial.

8. The method of drying moisture-containing materials which consists in suhjecting the material to a current of heated air or gas at high relative humidity, then to a current of materially cooler gas, thereafter allowing the material to remain in the cooler gas and then again subjecting the material to a current of heated air or gas but of higher temperature and lower humidity.

than the former current of heated air.

KENNETH REDMAN.

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