US3199213A

US3199213A - Method of changing the moisture content of wood

Info

Publication number: US3199213A
Application number: US123535A
Authority: US
Inventors: Frederick H Milligan; Charles W Hoyt
Original assignee: Crown Zellerbach Canada Ltd
Current assignee: Crown Zellerbach Canada Ltd
Priority date: 1961-07-12
Filing date: 1961-07-12
Publication date: 1965-08-10
Anticipated expiration: 1982-08-10

Description

Aug. 1965 F. H. MILLIGAN ETAL 3,199,213

METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Filed July 12, 1961 4 Sheets-Sheet l MIVENTOES FREDERXCK H. MILLIGAN h lvm b CHARLES W. HOYT.

F. H. MILLIGAN ETAL 3,199,213

METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Aug. 10 1965 Filed July 12, 3.961 4Sheets-Sheet2 DRYING TIME FOR SECTIONS OF WOOD /3 INCH THICK ALSO SHOWN ARE WOOD SURFACE TEMPERATURES) MOISTURE CONTENT VS.

N M.OOOOOOOOOOOOOOO 5 OOOOOOOOOOOOOOOO MPO OOOOO O O OOO O 0 0 0 7 9 m5w 3SJ QW w E D U N O W WS5W5W5WW555555 P 3333335366666666 M E W ABCDEFGH JKLMNOP l HGFE DRYING TIME MINUTES PONML K J WOOD SURFACE TEMPERATURES, DRYER OPERATING AT 650% -----'--WOOD SURFACE TEMPERATURES, DRYER OPERATING AT 350F mil/EH?! CHARLES W. HOYT 1965 F. H. MILLIGAN ETAL 3,199,213

METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Filed July 12, 1961 4 Sheets-Sheet 5 Ioo I MOISTURE CONTENT vs.

so IB4F DRYING TIME FOR SECTIONS O OF wooo V INcI-I THICK so 205 F DRYER TEMPERATURE-450F VELOCITY zooorpm.

0 30 ZIQ F 2 8 o m 20 245 I: U1 0.

*- 2 LL! Z 8 o m 10 268 F D: J 5 s 6 z 0 as? F o I 2 s 4 DRYING TIME- MINUTES lNl/EATOQS FREDERICK H. MILLIGAN CHARLES w. HOYT rray/5V; v

4 Sheets-Sheet 4 F. H. MILLIGAN ETAL 6 FREDERICK H. MILLIGAN CHARLES W. HQYT METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD Aug. 10, 1965 Filed July 12, 1961 TIME HOURS B B DF w W L m nU M O m N m B W o B om V B FL 0 O B Y M U 4 D BFE L o 6 IR E U B M D RT OR E D AA E NM 0 BY W I H w H OP mm M Y Y NM? flw D R "4. @E MO D m WmM T 5 m mmA cL E 8 ml. A V T P N o m M w E IG R W R N A .L L 0 i N C A W E T A OW B N L B l O T @E 0E 0mm V N T TR R M A Y q A H i T HE P H 1 m m w m wm 11- E 0/ v H D M W M O NO aw A B M H H h p 0 M L S L L O a a a w mw m fl R R R T #0 W T B 0 4 Tom 01.. 67 m a m. 9 1 3 8 O A 3 a. A A 3 M 4 Q L Q Y A u 7 3w E W -l'lu 6 5 4 3 2 O O 0 w w 3 m 9 P2528 E3202 9 IiTO United States Patent 3,199,213 METHOD (9F CHANGING THE MOHSTURE CGNTENT 0F WQ'OD Frederick H. Miiligan, Port (Ioquitlam, British Columbia, Canada, and Charles W. Hoyt, Cambridge, Mass, assignors, by direct and mesne assignments, to Qrown Zellerhach Canada Limited, Vancouver, idritish Columhis, Canada, a corporation of the Province of British Columbia, Canada Filed July 12, 1961, Ser. No. 123,535 8 Claims. ('Cl. 34-133) This invention relates to a method of changing the moisture content of wood, that is, to reduce or to increase the moisture content to a desired point.

The term wood as used herein is meant to include wood in the form of strips, sheets, veneers, boards, shingles and box material whether produced from logs, cants, blocks, or other pieces of wood by sawing, slicing, peeling or splitting, and it also includes products commonly known as fiakeboard, particle board, hardboard, plywood and composition boards, the materials of which are primarily pieces or particles of wood or other lignocellulosic fibre bonded together naturally or by means of resins or glues. All of the foregoing materials are, of course, generally planar in configuration having two major planar surfaces and lesser edge surfaces.

Standard methods of changing the moisture content of wood are generally referred to as air drying, kiln drying, oven drying, seasoning, curing, conditioning or humiditying. In most cases, the moisture content of the wood is reduced; although in the manufacture of such products as flakeboard, particle board, fiber board and some composite boards, it is often necessary to increase the moisture content as one of the final steps in manufacture.

Seasoning, curing, conditioning and drying are all terms which refer to the establishment of desired mechanical, physical and chemical properties in wood and wood products. Conditions of time, temperature, circulation rate and humidity when tempreatures are below 212 F. govern the extent of the moisture change and also the way the change is brought about, which in turn affects the properties of the wood or wood products.

Air drying is accomplished simply by stacking the material in a yard or under a shed for a period of time long enough to permit evaporation of moisture to take place.

Kiln drying, or oven drying, as presently practiced, is essentially a process of heating the wood or wood product in a closed cabinet or chamber to accelerate the removal of internal moisture by a circulating atmosphere. Regulation of temperature and humidity is used to control the degree and rate of moisture removal.

Conventional methods and equipment such as kilns, ovens and humidifying chambers presently used for changing the moisture content of wood and wood products are faster and subject to better control than is air drying. Drying lumber in a kiln, for example, is a matter of days instead of weeks or months. Drying thick softwood veneer takes to minutes in the roller type oven dryer widely used in the plywood industry in contrast to the several days that would be required for air or loft drying. Conditioning or increasing the moisture content of flakeboard, particle board and fibre boards of all densities is accomplished in a few hours in a humidifying chamber as compared to days or weeks in a loft or warehouse where conditions of temperature and humidity approximate that of the prevailing weather.

Conventional wood drying equipment operates at temperatures ranging from 100 F. to 425 F. For example, lumber and shingle dry kilns commonly operate at temperatures below the boiling point of water, while most plywood veneer dryers operate at a range of 300 to ice 425 F. These dryers also circulate the atmosphere at low velocities of the order of 600 f.p.m. in lumber and shingle kilns and 600 to 3000 f.p.n1. in veneer kilns with the flow of air parallel to the surface of the drying wood.

Drying wood and wood products at temperatures in excess of those mentioned above has been attempted in the past. However, it has been asserted that use of high temperatures of the drying atmosphere as a technique for reduction of drying time also results in reduction of strength, softening, chemical changes, and drying stress defects such as case hardening, checks, splits, warping and honeycomoing of the wood.

In order to avoid harming or discoloring the wood, it is advisable not to allow the surface temperature to rise above 350 F., and for some species it is safer to keep this temperature below 300 F. Both time and temperature are involved in the harming and discoloring of wood. Wood may be subjected without ill effects to much higher temperatures for a short period of time than for substantially lower temperatures and extended periods of time. However, there are some occasions on which a slight discoloration would not be objectionable. This would be the case, for example, when the veneers are used with a glue other than the phenolic resin used for producing exterior grade plywood.

The main object of the present invention is the provision of methods for drying, humidifying or equalizing the moisture content of wood which are faster, more economical and provide closer control of the conditions affecting the chemical, physical and mechanical properties of the wood being processed than is possible with existing methods.

Another object is the provision of a method of changmg the moisture content of wood by the utilization of high temperatures without harming or discoloring the wood.

The method according to the present invention, contrary to general opinion, utilizes high temperatures to dry wood without any harmful effects. It has been found that it is not the high temperature of the dryer atmosphere per se that directly causes the wood defects referred to above, but it is the high temperatures induced in the wood by exposure to the high temperature atmosphere or contact with parts of the dryer that are at high tcmperatureQ In the present method, the moisture in the wood is evaporated exceedingly rapidly, producing an evaporative cooling effect at the wood surface. This enables the wood to be dried to a low moisture content by a high temperature atmosphere without exceeding safe wood temperatures. In order to create high rates of evaporation, it is necessary to circulate the drying atmosphere over the surface of the wood at high velocities. This is preferably accomplished by directing hot gas against the surface of the wood at substantially right angles thereto. The hot drying medium is directed through one or more nozzles against the surface of the wood, but it is preferable to direct it against the wood by a plurality of nozzles throughout the entire surface of said wood. The previous research in high temperature drying has been done on simulated conventional equipment which circulates the drying atmosphere parallel to the wood surface at low velocities. Low velocity, parallel-flow does not evaporate the moisture rapidly enough to result in a significant lowering of the wood temperature. When the high temperature atmosphere is directed through a plurality of nozzles, the moisture-laden air or gas can get away quickly from the surface. In other words, the gas wipes the moisture off the surface without unduly raising the surface temperature. As the moisture is wiped olf the surface, it is replaced by other moisture from the interior of the wood. What is actually happening is each impinging stream of hot gas against the surface of the wood substantially norp be used to obtain satisfactory results.

7 3 mal thereto strips oif or scrubs the moisture-laden boundary layer of the wood to cause rapid evaporation of moisture from said surface and simultaneously cool the surface. Thus, the hot gas stream is removing the moisture and at the same time setting up a cooling action that prevents the wood from being harmed in any way. One of the advantages resulting from the use of the present method is the evenness of the treatment over the entire surface of the wood. In the preferred form of the process, jets of hot gas are impinged against both the major surfaces of the wood throughout the entire areas of said surfaces. This results in the wood being dried evenly throughout both of the major surfaces thereof. Use'of the prior parallel-flow types of dryer results in uneven treatment of the wood. For example, if the dryer is of'the cross-circulation type, the inlet side of the dryer will be at a higher temperature than the outside side thereof. This arrangement can result in large variations in the moisture content of the dried product. Furthermore, with the prior dryer, it is difficult to control the velocity of the drying atmosphere over the wood surface. Irregularities in the flow passages around the wood deflect the gas stream with the result that the wood receives an uneven treatment. With the present method, the drying atmosphere'is directed in a plurality of jets against the entire wood surfaces substantially at right angles thereto so that every part thereof receives exactly the same treatment.

' The basic method of changing the moisture content of wood in accordance with this invention comprises impinging atleast one stream of a moisture conditioning gaseous medium against the surface of the wood substantially normal thereto at a velocity of at least 1000 f.p.rn.

harming or discoloring the wood. For example, for dry- 4 ing purposes, temperatures of from 250 to 750 F. may It is believed that higher velocities and temperatures may be used, but they are at present not considered economic since special materials are required in the equipment for strength at high I With uniform moisture distribution, the average residual moisture content can be set only slightly below the maximum allowable level. On the other hand, with a wide variation in residual moisture content, the average must be well below the maximum limit, and the net effect is that more moisture must be removed from wood dried in a conventional dryer than from wood dried by the present process; Furthermore, the wide distribution results in overdrying with resulting degrade to a substantial part of the total quantity dried. I

It has also been found that the moisture content wood may be'raised very quickly and accurately by impinging one or more jets, preferably a plurality of jets, of a gaseous medium against the wood surface substantially normal thereto at a velocity of at least 1000 f-.p.m., said gaseous medium having a dry bulb temperature less than V chambers so thatboth surfaces of each piece of wood are 212 F. and a wet bulb temperature adjusted to establish the desired equilibrium moisture content. this method may be used to balance the moisture content of wood which initially is much wetter in some areas than in others.

In the accompanying drawings:

FIGURE 1 is a side elevation of one form of apparatus for carrying out this method, most of the near wall of the apparatus being broken away,

FIGURE 2 is an enlarged fragmentary section taken on the line 2-2 of FIGURE 1,

FIGURE 3 is an enlarged fragmentary section taken on the line 33 of FIGURE 1,

FIGURE 4 is an enlarged vertical section taken on the line 4-4 of FIGURE 1,

FIGURE 5 is a graph showing the moisture content reduction of wood sections /s" thick at specified drying medium temperatures-and velocities, indicating the time required and the surface temperature of the wood,

FIGURE 6 is a graph showing the time required for reducing the moisture content of a wood section A" levels for a range of wet and-dry bulb temperatures, and

FIGURE 8 is a graph showing the time to humidify A" hardboard, comparing drying by the present method with drying by standard procedures.

FIGURES 1 to 4 illustrate apparatus for. carrying out this method. A dryer 10 has one or more drying sections, the illustrated dryer'including drying sections or

zones

12, 13 and 14. The apparatus includes an upper casing 13 spaced above a lower casing 19 to form a thin passage 21 therebetween. 7 Upper and lower conveyors 23 and 24 have adjacent

horizontal runs

26 and 27 travelling in the direction of arrow 28 inFIGURE 1 into, through and out of passage 21. Conveyors 23 and 24 are designed to permit as much gas or ,air'as possible to pass therethrough, and are preferably formedof wire mesh having relatively large openings therein, as shown. These conveyors are moved by a suitable source of power, not shown, so that they move pieces of wood 30 between the runs .26 and 27 thereof through the dryer 10 from its entrance end 32 to the outlet end 33.

As drying

sections

12, 13 and 14 are exactly the same, only one, namely section 12, will now be described in detail. Upper casing 18v at drying section 12 forms an upper plenum chamber 36, the bottom of which is defined by a ,wall 37 extending from an end 38 of the casing to a separator 39 forming the inner end of said section. A plurality of jet nozzles 42 project downwardly from wall 37 and terminate just above the horizontal run 26 of conveyor 23. Each of these nozzles opens into chamber 36;. Similarly, lower casing vl forms a lower plenum chamber 45 between an end wall 46 and a separator 47. A wall 49 extends across the plenum chamber, and a plurality of jet nozzles 51 extend from said wall and terminate near the run 27 of lower conveyor 24.

Hot gas, such as air, is directed at high velocities to upper and

lower'plenum chambers

36 and 45 in any suitable manner, such as by pipes Stand 55. -The hot gas is directed in a plurality of jets through

nozzles

42 and 51 to the upper and lower surfaces of wood pieces 30 being moved through passage 21 by conveyors 23. and 2 It is preferable that

nozzles

42 and 51 direct the gas against fully covered by said jets.

. The drying medium or gas escapes laterally from passage 21. Although the gas may be allowed to escape to the surrounding atmosphere, it is preferable to provide d-

ucts

57 and 58 along opposite sides of the apparatus and Furthermore,

and 19 to close the sides of passage 21. The gas is taken away from ducts 5'7 and 53 through suitable pipes, and is either discharged to atmosphere or completely or partially recirculated in the system.

The operation of dryer is obvious from the abov description. The wood pieces 30 are moved by the conveyors through passage 2. In drying section 12, the hot drying medium or gas is directed in a plurality of jets at high velocity to opposite faces of the wood pieces. Section 12 only may be used, but under some circumstances, it is desirable to subiect the wood to two or more diiferent sets of drying conditions and, therefore, drying section 13 and/ or 14 may also be used with section 12. The operation is exactly the same in each drying section, and it is only the drying conditions of the method that are changed. For example, the temperature and/or velocity of the drying medium in each section may be different from that of each other section. The temperature and velocity of the drying medium in the dryer are selected in accordance with the condition and type of wood being dried.

TABLE 1 Time (minutes) to dry Douglas Fir heartwood Ai-inclz veneer from 35% to 5% moisture content at various atmosphere temperatures and impingement velocities Velocity, f.p.m. Temperature, F.

4. 38 3. 33 Y 2. 1. 70 1. 37 1.14 0. 99 0. 86 3. 97 2. 87 1. 85 1. 36 1. 08 0. S9 0. 76 0. 67 3. 64 2. 53 1. 57 1. 1d 0. 89 0. 75 0. 63 0. 54 3. 35 2. 24 1. 36 0. 98 0. 76 0. 62 0. 53 0. 46 3. 11 2. 04 1. 21 0. 85 0. 66 0. 54 0. 46 0. 39 2. 88 1.85 l. 08 0. 76 0.59 0. 48 0. 0. 35 2. 73 1. 70 0. 98 0. 68 0. 53 0. 43 0. 36 0.31 2. 54 1. 57 i 0. 89 O. 62 0. 48 0. 38 0.33 0. 28

Table 2 shows some drying times for /8" Douglas Fir veneer in accordance with this process and with standard practice. From this table it is apparent that this method can dry thin wood sections in as little as the time required by conventional parallel circulation methods.

TABLE 2 Comparison of drying times for conventional and high velocity dryers Conventional Roll-Type 15- High Velocity Dryer Section Veneer Dryer, Material Front operating at 320 F. Back operating at 450 F., 550 F., 650 F., 750 F.,

340 F. 9,0001'.p.rn. 9,000 f.p.m. 0,000 f.p.m. 9,000 f.p.m.

%; Inch Douglas Fir Heart- 13. min 1.36 min 0.98 min 0.76 min 0.62 min.

wood, Average M. C. of 35%.

450 F., 7,000 f.p.m.

% Inch Douglas Fir Sap- 20 min 3.3 min.

wood, Average M. C. of 100%.

If different drying times are required in drying

zones

12, 13 and 14, each zone may be provided with its own conveyors which are independent of the other conveyors so that the time the wood is in each zone may be varied as desired. Another way is to move the wood in batches, in which case the conveyor speed would be adjusted f each drying zone. However, it is preferred to make the apparatus so that the temperature and velocity of the gaseous medium may be controlled in each zone independently of the others, and to have the conveyors operate at a constant speed. With this arrangement, the timethe wood is subjected to a given set of conditions is determined by the number of zones in which said conditions exist. For example, one zone may have one set of conditions, and the next two Zones another set of conditions so that the wood sections would be subjected to the second set of conditions longer than to the first set.

5 is a graph showing a group of representative drying curves determined by drying thin wood sections in accordance with this moisture conditioning method. For example, curve A shows that wood with an initial moisture content of 35% can be dried to 5% moisture content in 3.3 minutes in a dryer atmosphere of 350 F. impingin g against the wood at 3000 r".-p.m. Curve K shows that wood can be dried from 35% to 5% moisture content by the utilizing of a drying medium at 650 F. and 7000 fpm. in 0.89 minute without raising the wood temperature above 305 F. The graph indicates wood surface temperatures with the dryer operating at 650 F. and with it operating at 350 F.

The following Table i has been prepared from accumulated data and shows the time required to dry thin Wood sections from 35% to 5 moisture content at various dryer atmosphere temperature and velocities.

The curves shown in FIGURE 5 provide the basis for drying schedules applicable in a commercial dryer. example, a single pass, ft. long dryer 10 (with one drying section only) operating at 650 F. and 7000 f.p.m. (curve K) would dry thin wood sections from 35% to 5% MC. in 0.89 minute at a conveyor speed of 84 f.p.m. A conventional roller-type, 5-line, 75 ft. long counter-current circulation dryer operating in the range of 320 F. to 340 F. would require 13.0 minutes (see Table 2) to dry the wood and the rate of output would be 5.8 f.p.m. per line or 29 f.p.m. for the entire dryer.

The curves presented in FIGURE 5 also provide the necessary information for establishment of zone conditions to secure minimum drying times when a specific limit is placed on allowable wood temperature and final moisture content. For example, it may be desired to dry thin sections of heartwood from 35% to 5% MC. in min mum time without exceeding a wood temperature of 305 F. Using the data in FIGURE 5, a dryer with two independent drying sections may be used as follows:

Zone 1650 F. at 1l,000 f.p.m. for 0.50 minute (curve M) Zone 2650 (curve K) This two-section schedule enables the wood to be dried in 0.71 minute as compared with 0.89 minute for a single zone dryer operating at 650 F. and 7,000 f.p.m. Wood temperatures would not exceed 290 F.

The drying processes widely employed in the plywood industry use higher atmosphere temperatures at the dry end than at the green or wet end. Consequently, the driest wood is acted upon by the hottest gases. In the present process, the practice is reversed. That is, the high temperature atmosphere is applied to the green or wet wood until the wood tempenature approaches the maximum allowable limit; then the wood enters a lower temperature F. at 7,000 f.p.m. for 0.21 minute For atmosphere. In prior dryers, evaporative cooling does not take place appreciably; however, in this process, the effectiveness and controlof the drying operation depends upon this phenomenon.

This process is also eifective in drying thin Wood sections containing a high percentage of moisture. FIGURE 7 TABLE 3 5%. Other combinations of dry and wet bulb tempera- 'tures to establish a range of desired equilibrum moisture contents are shown in FIGURE 7.'

Equilibrium moisture content can be reached by exposing the wood to air where there is natural circulation or by exposing to air with forced circulation at low velocities as in conventional kilns or conditioning chambers. However, reaching equilibrium moisture content in a low circulation rate atmosphere requires considerable tirne. In contrast, direct impingement of an atmosphere moving at high velocity and at proper conditions of temperature and relative humidity permits establishment of equilibrium conditions in the wood in much shorter times than is possible by other methods now in use. For example, assume the piece to be dried con- Boil/shcar stress, percent of wood failure of plywood (3-ply) samples constructed with Douglas Fir veneers dried in a high velocity dryer Percent Moisture Dryer Tests Content Veneer Drying Thickness, Time,

Inches Minutes Temp, Velocity, Boil IShear, Wood Initial Final Zone F. t.p.m. lbs/sq. in. Failurc, Percent Adhesive used in test panels was American Marietta AM5581 with Furafil 1005 as compounded and used for regular plywood production. Glue spread was 54 lb./M sq. ft. double glue line.

All test panels of 3-ply construction pressed at 200 p.s.i., 290F.; 3% min. for i o veneers; 4 min. for veneers.

Sometimes it is necessary to increase the moisture content of wood, and particularly with flakebo'ard, particle board and fibre boards of all densities. This procms may be utilized to increase the moisture content of the wood. Increasing moisture content is known as conditioning or humidification and can be accomplished by this process in much less time and more uniformly than in conventional equipment. By maintaining the gaseous medium at temperatures of 200 F. dry bulb and 170 F. wet bulb (relative humidity about 50%), the moisture content of oven dry wood can be raised to 5% moisture content in much less time than is requiredwith conventional humidification equipment as shown in FIGURE 8.

This process is valuable in equalizing the moisture content of wood. The outer layer or sapwood of the tree contains a much larger percentage of moisture than does the inner or heartwood portion. When a wood product is manufactured, a single piece often consists of mixed heartwood and sapwood. This situation creates a drying problem for if conditions of temperature and velocity are established to quickly dry the sapwood to the desired mois-' ture content, the heartwood will often be over-dried. Conversely, if the heartwood is quickly dried to the desired moisture content, the sapwood will often remain wet.

The moisture content of Wood exposed to air ordinarily is dependent upon the temperature and'relative humidity of the air. This moisture content balance of wood with the surrounding atmospheric conditions is known as its equilibrium moisture content. 'By selecting a specific temperature and relative humidity, it is possible to bring all portions of a wood product ultimately to the desired equilibrium moisture content regardless of the ini-tial moisture content of the wood. For example, if an atmosphere of 186 F. dry bulb and 156 F. wet bulb is maintained, wood will reach an equilibrium moisture content around minutes.

sists of a mixture of sap and heartwood with moistures ranging from 106% to 35%; the maximum allowable temperature for the wood during the process is 350 F. and the desired final moisture content is 5%.

In dryer l0, drying section 12 would be at 650 F. at a'velocity of 9,000 f.p.m., the exposure time 1.0 minute. Upon leaving this section, the heartwood would be about 2% M.C. with a surface temperature of 350 F., and the sapwood would be at 35% M.C. with a temperature of 210 F. The section 13 conditions would be 325 F. at a velocity of 9,000 f.p.m. and an exposure time of 1.5 Upon leaving section 13, the heartwood would be at 0.75% M.C. with a temperature of 325 F. and the sapwood would be at 10% M.C. with a temperature of 220 F. Section 14 conditions would be at 210 F, 50% relative humidity and a velocity of 7,000 ipm. Exposure time would be 1.5 minutes. In this third zone the heartwood would increase its moisture content to approach 5% while the sapwood would continue to decrease its moisture content to approach the desired'level of 5%. The total elapsed time would be 4.0 minutes.

This is longer than the optimum time for drying sapwood alone. However, the method permits drying of wood containing areas of both high and low moistures in the same piece in as little as one-fifth the time required by other methods and without damage to the low moismoisture present to be higher than this in some cases, but whatever the moisture level, it will rarely have a spread in values greater than this example.

Schedule I Dryer schedule for %-inch Douglas Fir heartwood veneer, moisture content: max, 43%; avg. 35%; mm. 27%

Zone Percent Mois- Time in ture Content Zone Zone (ruins) Temperature Velocity, In Out f.p.m.

8 5. 2 350 F 9, 000 0. 4D 3 0. 80 1 0 25 q o 5. .5 3 7, 000 0. 20 0.80 4. 0 160 wet bulb- 0. 25 3.6 J 4. 5 4. 7 4 80 F. (cooling 5, 000 0.20 4. 0 4. 3 l 3. o a. 9

Total time in dryer, 1.58 mins. Speed of dryer proportional to dryer length.

Schedule ll Dryer schedule for K-inch Douglas Fir intermixed sap and heartwood veneer moisture content: max-150%, sapwood mode-115%; heartwood ave.35%; him-27% Total time in dryer, 5.38 mins.

Speed of dryer proportional to dryer length.

From the above it will be seen that this relatively simple method or process may be used quickly and uniformly to reduce the moisture content, raise the moisture content or balance the moisture content of wood, simply by varying the temperature, velocity, and/or the relative humidity of the drying medium, and controlling the time of exposure of the wood to said medium.

What we claim as our invention is:

1. The method of changing the moisture content of wood of generally planar configuration which comprises drying the wood by impinging a gas at a gas temperature ranging from about 350 F. to about 950 F. against a major planar surface of the wood to be dried substantially normal thereto at a velocity ranging from about 3000 f.p.m. to about 11000 f.p.m. to strip 005 the moistureladen boundary layer of air at the wood surface to cause rapid evaporation of moisture from the wood surface and simultaneously cool said surface to protect it from harm or discoloration and for a time sufiicient to reduce the moisture content of the Wood to a desired level without raising the board temperature above 350 F., said stripped-off moisture being replaced by moisture from the interior of the wood thereby rapidly adjusting the moisture throughout the wood without harmful effects.

2. The method of changing the moisture content of wood of generally planar configuration which comprises drying the wood by directing a plurality of individual small jets spaced from each other in all directions of a gas at a temperature ranging from about 350 F. to about 950 F. against a major planar surface of the wood to be dried substantially normal thereto at a velocity ranging from about 3000 f.p.rn. to about 11000 f.p.-m. to strip off the moisture-laden boundary layer of air at the wood surface to cause rapid evaporation of moisture from the wood surface and simultaneously cool said surface to protect it from harm or discoloration and for a time suflicient to reduce the moisture content of the wood to a desired level without raising the board temperature above 350 F., said stripped-off moisture being replaced by moisture from the interior of the Wood thereby rapidly adjusting the moisture throughout the wood Without harmful effects.

3. The method of changing the moisture content of wood of generally planar configuration which comprises drying the wood by directing a plurality of individual jets of a gas at a temperature ranging from about 350 F. to about 950 F. against a major planar surface of the wood to be dried substantially normal thereto at a velocity ranging from about 3000 f.p.m. to about 11000 fprn. to strip off the moisture-laden boundary layer of air at the wood surface to cause rapid evaporation of moisture from the wood surface and simultaneously cool said surface to protect it from harm or discoloration and for a time sufiicient to reduce the moisture content of the wood to a desired level without raising the board temperature above 350 F. and quickly removing said gas from the wood, said stripped-off moisture being replaced by moisture from the interior of the wood thereby rapidly adjusting the moisture throughout the wood without harmful eflects.

4. The method of changing the moisture content of wood of generally planar configuration which comprises raising the moisture content of the wood by impinging a gas at a dry bulb temperature of from to 212 F. against a major planar surface of the wood to be treated substantially normal thereto at a velocity ranging from about 1000 f.p.m. to at least 11000 f.p.m. for a time sufiicient and with the relative, humidity of the gas adjusted to establish a desired equilibrium moisture content.

5. The method of changing the moisture content of wood of generally planar configuration which comprises raising the moisture content of the wood by directing a plurality of spaced individual small jets of a gas at a dry bulb temperature of from 140 to 212 F. against a major planar surface of the wood to be treated substantially normal thereto at a velocity ranging from about 1000 f.p.m. to at least 11000 f.p.m. for a time sufficient and with the relative humidity of the gas adjusted to establish a desired equilibrium moisture content.

6. The method of changing the moisture content of wood of generally planar configuration which comprises raising the moisture content of the wood by directing a plurality of spaced individual small jets of a gas at a dry bulb temperature of from 140 to 212 F. against a major planar surface of the Wood to be treated substantially normal thereto at a velocity ranging from about 1000 f.p.m. to at least -1 1000 f.p.m. and with the relative humidity of the gas at least 35% and for a time sufficient to adjust the moisture content of the wood to a desired level.

7. The method of changing the moisture content of wood of generally planar configuration which comprises a first stage of drying the wood by impinging a gas at a gas temperature ranging from about 350 F. to about 950 F. against a major planar surface of the wood to be dried substantially normal thereto at a velocity ranging from about 3000 f.p.m. to about 11000 f.p.m. to strip 05 the moisture-laden boundary layer of air at the wood surface to cause rapid evaporation of moisture from the wood surface and simultaneously cool said surface to protect it from harm or discoloration and for a time sufficient to reduce the moisture content of the wood to a desired level without raising the board temperature above 350 F., said stripped-off moisture being replaced by moisture from the interior of the wood thereby rapidly adjusting the it! moisture throughout the Wood without harmful effects; and a second stage of raising the moisture content of said wood by impinging a gas at a dry bulb temperature of from 140 to 212 F. against the planar surface of the wood to be treated substantially normal thereto at a velocity ranging from about 1000 f.p.m. to at least 11000 f.p.rn. for a time sufficient and with the relative humidity of the gas adjusted to establish a desired equilibrium 7 moisture content.

8. The method of changing the moisture content of wood of generally planar configuration which comprises a first stage of drying the wood by directing a plurality of individual small jets spaced from each other in all directions of a gas at a temperature ranging from about 350 F. to about 950 F. against a major planar surface of the wood to be dried substantially normal thereto at a velocity ranging from about 3000 f.p.m. to about 11000 f.p.m. to strip off the moisture-laden boundary layer of air at the wood surface to cause rapid evaporation of moisture from the wood surface and simultaneously cool said surface to protect it from harm or discoloration and for a time sufficient to reduce the moisture content of the Wood to a desired level without raising the board temperature above 350 F., said stripped-off moisture being replaced by moisture from the interior of the wood thereby rapidly adjusting the moisture throughout the wood without harmful effects, and a second stage of raising the moisture content of said wood by impinging a gas at a dry bulb temperature of from 140 to 212 F. against the planar surface of the Wood to be treated substantially normal thereto at a velocity ranging from about 1000 f.p.m. to at least 11000 f.p.m. for a time sufficient and with the relative humidity of the gas adjusted to establish a desired equilibrium moisture content.

References Cited by the Examiner UNITED STATES PATENTS 835,843 1l/O6 Baetz 34-l3.8 2,590,850 4/52 Dungler 34l60 2,758,386 8/56 Cobb 343l 2,791,039 5 /57 Rosen'baum 34l60 3,099,541 7/63 Hildebrand 34l8 WILLIAM F. ODEA, Acting Primary Examiner.

NORMAN YUDKOFF, Examiner.

Claims

4. THE METHOD OF CHANGING THE MOISTURE CONTENT OF WOOD OF GENERALLY PLANAR CONFIGURATION WHICH COMPRISES RAISING THE MOISTURE CONTENT OF THE WOOD BY IMPINGING A GAS AT A DRY BULB TEMPERATURE OF FROM 140 TO 212*F. AGAINST A MAJOR PLANAR SURFACE OF THE WOOD TO BE TREATED SUBSTANTIALLY NORMAL THERETO AT A VELOCITY RANGING FROM ABOUT 1000 F.P.M. TO AT LEAST 11000 F.P.M. FOR A TIME SUFFICIENT AND WITH THE RELATIVE, HUMIDITY OF THE GAS ADJUSTED TO ESTABLISH A DESIRED EQUILIBRIUM MOISTURE CONTENT.