US2408434A - Method and apparatus for drying materials uniformly throughout their mass - Google Patents

Description

MOISTURE COI ITEN 1' ,v J. w. MANN ETAL 'm METHOD AND APPARATUS FOR DRYING MATERIALS UNIFORMLY THROUGHOUT THEIR MASS Filed March 2, 1942 4 Sheets-Sheet 1 0 I JEPTH 1N INCHES Ir 1 MOISTURE CONTINT Q "a 3 x \Q 4 I up A n; :1 I Of/5x E I u ,4 1oz Q b 57 k T1. 9 5. 3 L a 0 INVENTORS .DEPTHININCIYES 5 3 5 F. Rxssib:

9 u LIUS A BYMI 6 MW ATTORNEYS Oct. 1, 1946. ETAL METHOD AND APPARATUS FOR DRYING MATERIALS UNI FORMLY THROUGHOUT THEIR MASS Filed llarch 2. 1942 4 Sheets-Sheet 2 VIIIIIIIIIIIIIIIIIIIIIIII I I'll!IIIIIIIIII'IIIII'III I a g u R s ,0 It I J I o l k N N v 1 e 2*- i Q Q 03s GEORG E F". RUSSELL. BY Jupnu's W. .MANN

MM M+Km ATTORNEYS Oct. 1, 1946. J. w. MANN ETAL 3 t 9 O h Y .w. L 6 M 6 W h F S m 4 L A I R mm? mm. GR NI IE2 uu RTh D C a n W" Fun d S B w n T P P A m D 0 H m lllllllllllllllllllllllllllllllllllllllllllllllllll r4 0 2 DEPTH IN INCHES 0 DEPTH IN INCHES INVENTORS GEORGE F. RUSSELL. BY JULIUS W. MANN 4ww1,fl mi-M ATTORNEYS I DEPTH IN INCHES Oct. 1, 1946- J. w. MANN ETAL 4 1L 6 9 h v. m L e M 8 m h F s m 4 S L A I ms T52 mum GR m z M h D c R w H d S E U01 Tani. H A P P A m D O H T E M iiiiiiiiiiiiiiiiiiiiiii i lllllllllllllllllllllllllllllllll \I Grease: F. RusssLL JQILIUS W MANN Km A'I'IORNEYS Patented Oct. 1, 1946 METHOD AND APPARATUS FOR DRYING MATERIALS UNIFORMLY THROUGHOUT THEIR MASS Julius W. Mann and George F. Russell. Tacoma, Wash.

Application March 2, 1942, Serial No. 432,936

Claims. (01. 34-1) The present invention relates to improvements in a method and apparatus for drying materials uniformly throughout their mass, and it consists of the steps hereinafter set forth and of the combinations, constructions and arrangements hereinafter described and claimed.

Under the present method of drying wood, heated kilns are used for drying the wood externally. The heat in the kilns is maintained at a relatively low temperature around 160 F. and not higher, because if the temperature is made too high in relation to the inside, surface checking of the wood will result. If lumber would shrink evenly throughout the thickness and in all directions, there would be no drying stresses and no tendency to check. Surface checking is caused by too great a moisture gradient between the surface and the interior of the wood. Where the temperature is maintained at around 160 F. it usually takes several days to dry thick wood to the point desired because little heat can be rapidly conducted to the inside of the wood. Surface checking is likely to result where low humidity and high temperature are combined for external drying. It is necessary to maintain a high external humidity and low heat to prevent shrink-' age of the outersurface without corresponding inside shrinkage.

Unless the vapor pressure of moisture in the surrounding air is less than that of the moisture in the wood, surface evaporation, or drying, cannot take place. Lumber drys more rapidly in hot windy weather because heat and air circulation as well as low humidity of the surrounding atmosphere are factors which are combined to create that differential in vapor pressure which is necessary to cause drying. Drying by applying external heat as in present dry kiln operation sets up a moisture gradient as soon as evaporation from the surface of the wood begins and moisture starts to move'out from the center of the wood. The moisture will tend to distribute itself uniformly through the wood by flowing from spots of high moisture content to those of low. The steeper the moisture gradient, 1. e., the greater the difference in'moisture content between the outer an dinner layers of wood, the more rapid the rate of drying. But the steeper the moisture gradient, the greater the danger of the wood checking. In drying by the present dry kiln methods, shrinkage differentials can not be eliminated, but the drying conditions can be; adjusted by the application of predetermined temperature and humidity conditions so as to lessen the stresses of shrinkage and minimize ents. A greatdeal of time is required for drying where the conditions are controlled to prevent checking.

The principal object of our invention is to dry wood rapidly to a given equilibrium moisture content without either surface or heart checking or cracking. To accomplish this we provide a method for simultaneously heating the inside and outside of wood or other material for raising the temperature uniformly throughout the mass, and

surrounding the heated mass with atmospheric conditions so controlled as to temperature and humidity as to create a moisture gradient and a vapor pressure differential between the entire mass of the uniformly heated wood and the surrounding atmospheric conditions so that the moisture will leave the wood due to the difference in vapor pressure in the wood and external of the wood. The vapor will leave the'wood without shrinkage differences and drying stresses in the wood. This method eliminates both surface and heart checking. If heat is applied to the interior as well as the exterior of the wood simultaneously with controlled humidity conditions, the temperature can be rapidly raised above the boiling point of water without causing the wood to check. There are two type sof water in green wood; hydroscopic water which is held in the cellulose fibre walls, and free water which is held in the fibre cavities or cells. In drying wood it is necessary to evaporate both the hydroscopically bound moisture as well as the free moisture. Internal heating by penetration of radio frequency waves will cause instantaneous increase in internal vapor pressure. The thus increased internal vapor pressure in the wood starts an immediate movement of moisture from areas of high vapor pressure within the wood toward areas of low relative vapor pressure surrounding the wood. By controlling the surrounding temperature and vapor pressure, a desired time period of drying can be obtained suitable to the material being dried. Our method utilizes varying degrees of external heat and low relative humidity with internal heat created by radio frequency wave penetration in such proportions as to maintain a relatively low, if any, moisture gradient between inside and outside fibres of the lumber or wood during drying. Because there is no fear of surface or heart checking in wood dried by our instantaneous method, the speed of drying is greatly increased over currently used dry kiln methods.

We are aware of methods for heating the interior of wood, but where this is done without the raising of the air temperature surrounding the wood, the center of the wood is dried more rapidly than the surface with the result that heart checking will occur. So far as we know we are the first to combine the interior heating of wood simultaneously with the exterior heating of wood so that the heat will be applied uniformly and instantaneously throughout the whole mass. Uniform drying of the wood results and this will obviate surface or heart checking.

'We provide novel means for controlling the humidity and the heat so that the temperature throughout the mass of wood will be raised uniformly and rapidly to dry the wood quickly without checking. The method adapts itself to the continuous drying of wood although it is possible to dry wood or other material in batches. The novelty of our methd is to combine external with internal heat at one and the same time in order to prevent losses in internal heating through surface radiation, while at the same time controlling the relative humidity of the surrounding heated atmosphere so as to drawofi' moisture in the wood as rapidly as possible without checking or cracking.

Other objects and advantages will appear in the following specification, and the novel features of the device will be particularly pointed out in the appended claims.

Our invention is illustrated in the accompanyin drawings forming a part of this application, in which:

Figure 1 is a diagram illustrating the moisture gradient in drying wood when external heat is used for drying;

Figure 2 is a cross-section through a block of wood that has been externally dried in the manner indicated in Figure 1, the section illustrating surface checks that have been made by too rapid a drying of the surface;

Figure 3 is a diagram illustrating the moisture gradient in drying wood when internal heat is used for drying;

Figure 4 is a section through a piece of wood that has been internally dried in the manner indicated in Figure 3, and illustrates the heart checking that results in too rapid a drying of the interior;

Figure 5 is a diagram illustrating the moisture gradient in drying wood when simultaneous external and intemal heating of the wood is accomplished;

Figure 6 illustrates a piece of wood that has been dried exteriorly and interiorly at the same time in the manner indicated by the diagram shown in Figure 5;

' Figure 7 is a longitudinal section through an apparatus illustrating one means for continuously drying wood;

Figure 8 is a transverse section taken along the line 8-8 of Figure '7;

Figure 9 is a diagram illustrating moisture gradients in the-wood which result from multiple stage drying of wood where the external heat is applied in each stage prior to the internal heat being applied;

Figure 10 is a diagram illustrating moisture gradients in the wood as the result of multiple stage drying of wood where the internal heat is applied to the wood in each stage prior to the external heat;

the drier and the electrodes for the internal heating of the wood are arranged at intervals along the compartment;

Figure 12 is a transverse section taken along the line l2-l2 of Figure 11, showing the electrodes;

Figure 13 is a section taken along the line l3-l3 of Figure 11, showing the rollers;

Figure 14 is a diagram showing moisture gradients in the wood as the result of the single compartment drier where the external heat is ap plied to the wood before the internal heat;

Figure 15 is a diagram showing moisture gradients in the wood as the result of the single compartment drier where the internal heat is applied to the wood before the external heat;

Figure 16 is a section through another modifled form of the invention illustratin batch drying when using our method of simultaneously applying external and internal heat; and

Figure 17 is a schematic showing of still an other modified form of the invention illustrating a continuous drying method for veneer.

While we have shown only the preferred forms of our invention, it should be understood that various changes or modifications which come within the scope of the appended claims may be made without departing from the spirit of the invention.

In carrying out our invention, we will first describe the external drying of lumber, then the internal drying and finally our method of the simultaneous external and internal drying. We will then describe four different means for carrying out our method, the first being multiple compartment drying; the second, single compartment drying; the third, batch drying; and the fourth, continuous drying of veneer. The multiple and single compartment drying, and the veneer drying are continuous processes, while the batch drying is not.

External Zum-ber drying (dry kiln method) In Figure 1, we illustrate a diagram with moisture gradient curves showing five different stages when drying wood in an ordinary dry kiln. The base line of the diagram designates the distance from the center to the surface of the wood and has been divided off into inches. The surface of the wood is indicated by the two zeros placed at the ends of the base line, while the center of the wood is indicated by the number 2. Halfway between the center and the ends we have placed the number I. These numbers refer to inches in depth from the surface to the center of the wood and would vary for wood of different thicknesses. The two outer parallel vertical lines are entitled Moisture content and are divided into percentages starting at zero and increasing to 35%. The vertical center dotted line refers to the center of the wood.

The wood, when it is Douglas fir, enters the dry kiln with a moisture content of about 36%, and this is indicated by the line A--A in Figure 1. The line indicates that the wood is green, that the moisture content is in equilibrium throughout, and that the amount of moisture content i 36%. After the wood has remained in the dry kiln for a certain length of time with the surrounding air at F., th surface layer of the wood begins to dry. This is indicated by the moisture gradient line B-B in the same figure. It will be noted that the outer surface of the wood has a moisture content slightly less than 20% while the moisture content of the wood cause of this steep moisture gradient and the checkin is shown in Figure 2 where the wood ID has surface checks II therein. It will be seen that the center of the wood is still green with a 36% moisture content while the surface is dry.

A later stage of drying is indicated by the line 0-0 in Figure 1. Here the surface of the wood has a moisture content of about 18% while the center of the wood still has a'moisture content of 36%. Line DD illustrates still a later stage of drying where the moisture content of the wood surface is about 12% and the center of the wood is over The line EE in Figure 1 indicates the final stage of drying where there is equilibrium of the moisture content throughout the body of the wood. Ther is no moisture gradient between the outside and inside wood fibres at this stage. However damage has been done duringthe stage BB and the stage C--C of drying with the result that the wood I0 has surface checking ll thereon. Figure 1 is a chart made by the West Coast Lumbermens Association and illustrates a prime reason for the surfac checking of wood. Figure 2 illustrates the physical result of surface checking.

Internal Lumber drying (radio frequency method) In Figure 3, we illustrate a chart showing the moisture content in drying wood when heating the interior of the wood only. The lower horizontal or base line in Figure 3 has zero markings at its ends for designating the surface of the wood with intermediate markings l and 2 representing distances from the surface to the center of the wood. Vertical side lines represent percentages of moisture content in the wood and these read from zero up to 35%. It is understood that the atmospheric air surrounding the wood is at a temperature of 70 F. and the relative humidity is 60%. Line A'--A indicates the condition of the wood when it is green with the moisture content of the wood at 36% and at equilibrium throughout. The line B'B' designates the gradient between the surface moisture content of the wood and the inside moisture content thereof. It will be noted that the line B'B' is not as steep as the line B--B in Figure 1 where the ordinary dry kiln method is used. However the heart checking of the wood is at a maximum at the line B'B' because the outer surface has a moisture content of approximately 35% while the heart has a moisture content of about Figure 3 illustrates stages of internal heating and the relative extent to which the moisture gradients at various stages may rise or fall under internal heating created by the penetration of radio frequency waves. Line B'B' illustrates the moisture gradient as the result of internal heat alone applied to the wood which, for example, raises the center of the wood to a temperature of 100 F., while the external surrounding atmosphere remains at 70 F. and 60% relative humidity. The dotted line above B'B' in Figure 3 illustrates the moisture gradient in the same piece of wood where the temperature applied at the center might be 90 F. and the surrounding atmospheric conditions remain at the same 70 F. and 60% relative humidity.

the drying process.

The dotted line below B'B' illustrates the moisture gradient in the same piece of wood where an internal temperature of F., for example, is applied to the center and the outside surrounding atmosphere remains at the same 70 F. and 60% relative humidity. 'I'hese differentials in moisture gradients illustrate the extent to which control of the gradient may be obtained. The dotted lines above and below C'-C' and DD' illustrate the same differentials in gradient, but at later stages of the drying.

In Figure 4, weshow the block of wood l2 dried by too rapid internal heating. The heart checks l3 are caused by the drying of the interior of the wood more rapidly than the drying of the exterior surface of the wood. The surface of the wood remains green and has a high moisture content during the initial stages of internal drying. The wood heart in drying shrinks and pulls away from the green surface wood. This causes heart checking as shown.

Simultaneous internal and external drying (perfect drying method) that lines A"-A" through to E"E" inclusive are straight and extend parallel with the base line. This will show that the moisture content throughout the mass of wood is in equilibrium at all times during the drying of the wood from a green moisture content of 36% down to a dry moisture content of 12%. This diagram illustrates the ideal in lumber drying; a condition during which there is no moisture gradient existing between exterior and interior fibres during The perfect drying of the wood is indicated by the piece of wood 14 in Figure 6. There are no surface or heart checks in this piece because there is a zero moisture gradient throughout the entire mass of wood during the entire drying operation. The uniform and simultaneous inside and outside drying of the wood obviates any chance for checking to take place in any part of the wood. It may be impossible to approach perfect equilibrium of all lines from A"-A" through E"-E" inclusive during the process of combining internal and external heat simultaneously, but the approach is so close that for all practical intents and purposes there is equilibrium.

Multiple compartment drying (continuous drying process) In order to carry out this method of the uniform construction with all of the others excepting the compartment K, and therefore a description of the compartment F, for example, will also suffice for the compartments G to J inclusive.

The compartment F has an entrance opening [6 for receiving wood, indicated generally at H. The compartment has rollers 18 therein for supporting the wood. These rollers may be power driven if desired for continuously moving the wood through the compartment. A conveyer l9 delivers the wood to the compartment. Near the exit end of the compartment we provide electrodes, indicated generally at 20. These electrodes preferably contact with the wood and are connected to a radio frequency generator; the electrodes should be adjustable so that they either may make direct contact with the wood between them or have an air gap between themselves and the wood; or may be in direct contact with cauls above and below the wood being dried. Any type of radio frequency generator may be used that is adapted to create heat within the interior of wood or other material. We contemplate using the radio circuit shown in our copending application, Serial No. 406,530, filed August 20, 1941, for generating heat within the wood.

In Figure 8 we show a closed hot air circuit for the compartment F. An outlet pipe 2| leads from the top of the compartment and carries the air ladened with moisture from the wood to a controlled moisture condenser, indicated generally at 22. This condenser may be of any well known type and is indicated only diagrammatically in the drawings. The condenser removes water vapor from the air, and the condensation drops upon a baffle 23 and is carried away from the closed air circuit. The dry air is conveyed to a cyclone air blower 24 where it is forced into a return pipe 25 that leads back to the base of the compartment F. The air passes through heater coils 26 that may be either electric or steam, and thence through additional heater coils 21 if need be before it is again forced into th compartment F. The dry hot air enters the bottom of the compartment and flows upwardly to surround the wood I! and remove water vapor therefrom.

The lumber while passing over the rollers l8 moves between a pair of output electrodes 20 connected to a radio frequency generator. Only a coil of the radio circuit is indicated at 28 in Figure 8, the balance of the circuit being shown in our copending application above referred to. The radio frequency waves passing through the wood heat it internally. The surface of the wood is heated at the same time by the flow of hot air thereby. The interior and exterior portions of the wood are simultaneously heated and the temperature of the wood is raised to the desired extent. The humidity of the air as well as its temperature is controlled for removing the desired moisture content from the wood without creating a suflicient moisture content gradient between the surface and the heart which will cause the wood to check at any place.

We will now explain the actual drying of a piece of lumber as it passes through the device, using purely illustrative temperatures and humidities which may vary widely. The temperature of the wood before it enters the first compartment F is about 70 F. and the atmosphere around the wood is about 60% relative humidity. When drying a piece of Douglas fir, for example, the moisture content of the green wood just before it enters the first drying compartment will be 36% throughout the mass of wood. The temperature in the first compartment F may 'be maintained at 130 F. with a relative humidity of 50%. The moisture content of the wood surface prior to the wood reaching the radio circuit output electrodes 20 will be reduced to 34%, and the moisture gradient line is shown in line L-L in the chart shown in Figure 9. This gradient line is not sufficient to cause surface checking of the lumber.

When now the wood passes between the electrodes 20, the inside temperature of the wood will be raised to plus F. The moisture content of the wood surface will be about 30% with the moisture content of the heard of the lumber about 28%. At a distance of about three-quarters of an inch in from the surface, the moisture content will be 32%. This condition is shown by the line M-M in Figure 9. Here again the moisture content gradient is not sufficient to cause either surface checking or heart checking of the lumber.

The wood now passes into compartment G where the air temperature is raised to F. and the relative humidity is 40%. The moisture content of the wood at its surface is reduced to 26% in compartment G prior to the wood reaching the electrodes 20' which are mounted in this compartment. The moisture content of the wood heart is 28%, as shown by the line NN, while three-quarters of an inch in from the surface the moisture content is 32%. It will be noted that the line N-N partly merges with the line M--M. When the wood reaches the electrodes 20', the internal temperature is raised to 175 F. plus. The moisture content of the wood surface is now reduced to 22%, the heart to 21%, and threequarters of an inch in from the wood surface, the moisture content is 23%. This moisture gradient condition is graphically illustrated by the line 0-0 of Figure 9. This line will tend to illustrate a wave in the moisture gradient which differs from the equilibrium lines described and explained in Figure 5.

The wood next enters compartment H, and it will be noted that wehave shown a break in the drawings of housing 15 between the compartments G and H. This is for the purpose of providing as many compartments as desired for raising the surface and the internal temperatures of the wood to the desired extent and through as many stages as required. In compartment H, the temperature is 220 F. plus and the relative humidity is 20% or less. The moisture content of the wood prior to its reaching the electrodes 20" is 18% for the wood surface, 21% for the heart, and 24% for three-quarters of an inch in from the surface. Reference is made to line PP in the graph shown in Figure 9. The line PP merges with the line O-O for a portion of its length. When the wood reaches the electrodes 20, the internal temperature is raised to 220 F. plus which will cause the moisture content to be as follows: surface 12%, heart 8%, threequarters of an inch in from the surface 16%. It will be seen that the temperature of the air in compartment H and the internal temperature of the wood while it is in this compartment is above the boiling point of water. and internal temperature will cause the water content in the wood to turn instantly to steam and to escape from the wood pores and fibres. The compartments J and K are for the purpose of reducing the temperature of the air around the wood and the temperature within the wood to a point where the wood can again be moved out into the atmospheric air without too sudden it drop in temperature being encountered.

The compartment J may have a temperature of 220 F. or less with a relative humidity of 12% or whatever relative humidity is desired at which the wood is to remain. The electrodes II" This high external in this compartmentmay maintain an internal temperature of 220 or less. The moisture content in the wood will equalize itself throughout at 12% while the wood passes through the remaining compartments. It will require time to acclimate the wood to atmospheric temperaing of the temperature both inside and outside of the wood is there a moisture gradient high enough to cause either .a heart or a surface checking. The wood will therefore be vrapidly and continuously dried in practically a uniform manner throughout. The'temperatures and relative humidities will be adjusted for the particular kind of material being dried. The figures given are merely by way of setting forth an example of drying lumber uniformly throughout and in a continuous manner. I

We have described the moisture gradient for the wood after it passes through the various compartments where the electrodes 20, 20 and 20" are placed at the rear. of the compartments. It is possible to place the electrodes at the en- .ture and therefore a number of compartments J trance to each compartment rather than at the rear. The diagram illustrated in Figure 10 shows the moisture gradients of the wood when the electrodes are moved to the entrance ends of the compartment. The line LL in Figure 10 shows the initial internal heating of the wood by a radio circuit and illustrates the reducing of the moisture content of the wood heart to about 32 2% while the surface of the wood still has a moisture content of 36%. This is due to the fact that the heated air which surrounds the wood immediately upon the entrance of the wood into the heated compartment, will require an appreciable length of time to dry the wood surface. The gradient line M'--M' shows the moisture content throughout the wood at the time the wood leaves the compartment F and enters the compartment G. It will be noted that the surface and the heart of the wood have been reduced to 32 and therefore a portion of the moisture gradient line MM will correspend with the portion of the moisture gradient line L-L'.

In like manner the line N'-N' indicates the moisture gradient of the wood just after it enters the compartment Gand comes into contact with the electrodesat the entrance to the compartment. The electrodes will dry the interior of the wood to. slightly less than When the wood leaves the compartment G, the surface of the wood will have av 26% moisture content and the heart .will still be at-29%, as indicated by the line O-O'. From compartment G to compartment H the electrodes placed at the entrance to compartment H will raise the center of the wood in temperature so that the moisture content will be 21% while-the surface moisture content will be almost 25%, as shown by the line O"-,-'-O". When the wood leaves the compartment H, the moisture content of the wood surface would be at about 20% as indicated by the line P -P. The line Q'Q' shows the wood entering the compartment Jwhere themoisture content of the Single compartment internal and external drying (continuous drying process) In Figures 11, 12 and 13'we show a wood drying apparatus where the housing 29 has one long compartment extending from end to end thereof instead of the housing being composed of a plurality of compartments. The housing has rollers 30 therein for carrying pieces of lumber 3| therethrough. Figure 13 shows the rollers 30 as being driven by worms 32 and worm gears 33. The wood enters through the entrance opening 34 and moves along the rollers to the exit opening 35. At intervals along the housing 29 we provide output electrodes 39 which are coupled to a radio circuit by a coil 31, see Figure 12. The radio circuit used is preferably the one shown in our copending application above mentioned.

We provide means for causing heated air of the desired humidity to flow from the exit end of the housing to the entrance end. A cyclone air blower 38 is used for this purpose and moves the air through a passage 39 in the direction of the arrow shown in the passage. Heater coils 40, either electric or steam, are placed in the passage 39 for raising the temperature of the air to the desired degree. The warm dry air is directed into the housing 29 and flows in a direction counter to that taken by the wood 3| as it is moved through the housing. The air will pick up moisture from the wood and therefore as the air reaches the entrance end 34- of the housing, it will carry an appreciable amount of water vapor and it will be cooled to a certain extent. The air will then pass into a return passage 4| which has a moisture condenser indicated diagrammatically at 42. The condenser will remove water vapor from the air and this water will drain off at the water outlet opening 43. The air then returns to the blower 38 to complete its cycle.

The temperature of the air entering the housing 29 is substantially the same as that for the compartment H in Figure 7 of the drawings; This means that the temperature will be about 220 F. with a relative humidity of 12% or below. The air when it reaches the entrance end of the casing 29 will have a temperature and humidity approximately the same as that for compartment F in Figure 7, i. e., a temperature of 130 F. and

a relative humidity of 50%. The electrodes 36 will step up the internal temperature of the wood from an atmospheric temperature of 70 to over 220 F. This can be done in stages, the first set of rollers raising the internal temperature to the second set of rollers raising the internal temperature to and the third set of rollers raising the internal temperature to 220 F.

If necessary, booster blowers 44 may be disposed along the housing. 29 at desired intervals. These blowers will draw in the air at their entrance ends 45 and expel it at their exhaust ends 46. The blowers may have humidifiers and heaters 46a therein for raising the air to the desired temperature and relative humidity.

The chart shown in Figure 14 illustrates the moisture content of the wood as it passes through the housing 29. It will be noted that the wood comes into contact with the heated air in the compartment 29 before it reaches the first set of electrodes 36. In Figure 14, the moisture gradient lines T-T to Z-Z inclusive indicate the moisture gradients in the wood from the time the green wood enters the compartment 29 until it leaves the same compartment as dry wood. A shifting of the electrodes so that the first set of electrodes will be at the entrance 34 will cause the moisture gradients to take the form shown at T'--T' to ZZ' inclusive in Figure 15. Since the base lines of the two charts in Figures 14 and '15 and the vertical lines are the same as illustrated in Figures 9 and 10, a further description Batch drying of wood internally (non-continuous drying process) It is possible to use our method in the batch drying of wood and this is illustrated in Figure 16. The device shown in this figure has a compartment 41 with a door 48 normally closing it. Wood 49 is placed on a plate electrode 50 and this is covered by another plate electrode The wood is then moved into the compartment and rests on standoff insulators 52. The plates 50 and 5| are connected to the radio circuit as already mentioned, the coil 53 alone being shown. An air passage 54 has its two ends communicating with the compartment. A blower 55 is mounted in the passage and directs air over a heating coil 58 so that heated air at the desired humidity will enterthe compartment 41 through the opening 51. After the heated air has passed over the wood and removed water vapor therefrom, it returns to the passage 54 through the entrance opening 58. Here the air is caused to flow over a condensing coil 59 where the water vapor will be removed from the air and will escape through the outlet 60. The air will be sucked into the fan 55 and thus complete its cycle.

In the batch drying process, the wood remains in the compartment during the entire drying period. It is possible to increase the temperature of the air from time to time so that the exterior surface of the wood will be gradually raised in temperature. This rise in temperature is so controlled that the wood will not receive surface checks. In like manner the radio circuit can gradually raise the heat in the interior of the wood 49 to correspond with the heated air surrounding the wood. In this way the entire operation of simultaneously heating the surface and interior of the wood is carried out in the same manner as that shown in Figures 7 and 11, except that the wood remains stationary throughout its entire drying period. The compartment 41 can be gradually cooled after the wood has been dried to the desired extent, whereupon the wood is removed from the compartment through the door 48.

Continuous internal and external drying of veneer In Figure 17, we show diagrammatically the continuous drying of a number of pieces of veneer. The pieces of veneer 6! are placed in contact with each other and are then continuously moved between two electrodes 62. The electrodes preferably contact with the pile of veneer and are in electrical connection with a radio circuit in the same manner as that designated for the electrodes 20, 36, etc. The radio waves pass through the stack of veneer and raise the internal temperature therein. Before any checking of the veneer can take place because of the raising of the internal temperature, it is passed into a heated compartment 63. Here the proper heat and relative humidity is maintained to dry the surface of the veneer. It will be noted that the pieces of veneer have been spaced from each other as they pass into the compartment 63 in order that all sides of the outer surfaces of the veneer be subjected to the hot air. We show spacing rollers 64 for separating the pieces of veneer.

The compartment 63 can be equipped in the same manner as the compartment H with an air duct whereby heated air at the proper temperature and relative humidity will be delivered to the compartment. It is also possible to provide a compartment similar to the one shown at 29 where the air is heated and maintained at the proper relative humidity by the means disclosed. The veneer will be internally and externally dried in a uniform manner by the device illustrated. The veneer drier electrodes 62 can be placed after the external heating rather than in front as shown. Also the separation of the plys can be either automatic as shown by the rollers 64 or the plys may be manually separated.

We claim:

1. The herein described continuous process of drying material which comprises continuously moving the materialthrough a zone containing radio waves that will pass through the material for raising the temperature within the material, controlling the temperature and relative humidity of the air surrounding the material as it passes through the zone for raising the surface temperature of the material in uniformity with the rise in temperature within the material to obviate the forming of moisture gradient in the material which will result in its cracking, the relative humidity of the air being lowered as the temperature of the material is raised for causin the vapor pressure within the material to exceed the vapor pressure in the air, whereby there is a rapid flow of water vapor from the material with a uniform dehydrating of the material resulting.

2. The herein described continuous process for drying wood which comprises continuously moving the wood through a plurality of zones, heating the air in successive zones at progressively increasing air temperatures and maintaining in successive zones progressively decreasing relative humidities, passing radio waves through each zone for penetrating the wood and raising the internal temperature of the wood while in the zone to substantially the same temperature as the air in the zone, whereby the uniform raising of the temperature of'the entire mass of wood as it moves from. zone to zone due to the radio waves and heated air and the decreasing of the relative humidity in the air will create a water vapor differential in the wood and air and will cause the vapor" to rapidly and uniformly leave the wood without checking the heart or surface of the wood.

3. A continuous wood dryer comprising a plurality of compartments, means for conveying wood through the compartments, means for circulating air through each compartment, means for controlling the temperature and relative humidity of the air in each compartment so that the temperature of the surface of the wood will be progressively raised as the wood passes from compartment to compartment while the relative humidity is progressively lowered, and means in each compartment for causing radio waves to p ss through the wood for creating a temperature in the wood substantially equal, to that of the air in the compartment, whereby the wood 14 nals and through the wood for raising the internal temperature in the wood, and means for progressively increasing the temperature of the air in the compartmentin step with the increase of the temperature in the Wood, and for decreasin the relative humidity in the air for causing a uniform and rapid drying of the wood without checking the heart or surface.

is uniformly dried throughout without heart or surface checking.

4. A continuous wood dryer comprising a plurality of compartments, means for conveying wood through the compartments, means forcirculating air through each compartment, means for controlling the temperature and relative humidity of'the air in each compartment so that the temperature of the surface of the wood will be progressively raisedas the wood passes from compartment to compartment while the relative humidity i progressively lowered, and means in each compartment for causing radio waves to pass through the wood for creating a temperature in the wood substantially equal to that of the air in the compartment, whereby the wood is uniformly dried throughout without heart or surface checking, the means for controlling the temperature and relative humidity of the air including condensers for removing excess water va por from the air, and heaters for maintaining the circulating air at the desired temperature.

5.'A continuous wood dryer comprising a compartment through which the wood is continuously moved and having an entrance at one end for the wood and an exit at the other end, means for moving wood through the compartment, means for heating and circulating dry hot air around all of the surfaces of the wood, the air entering the compartment adjacent to the exit end thereof and leaving the compartment at the wood entrance end, the air receiving moisture from the wood and being cooled by the wood as the air moves therealong, and means placed at intervals along the compartment for passing radio waves through the wood for raising the internal temperature of the wood substantially in step with the raising of the surface temperature, whereby the wood will be uniformly dried as it passes through the compartment without heart or surface checks developing, and air boosters arranged at spaced intervals along the compartment for aiding in causing the air to flow along the compartment, said air boosters including means for conditioning the air passing therethrough so that the air issuing therefrom will be maintained at the desired temperature and relative humidity.

6. A batch wood dryer comprising a compartment for housing a quantity of wood, terminals placed on opposite sides of the wood, means for causing radio waves to pass between the termi- 7. The herein described process of drying wood which comprises subjecting the wood to the penetration of radio frequency waves for raising the internal temperature of the wood to the desired degree for driving a portion of the moisture content in the wood to the surface of the wood for evaporation without creatin too great e, moisture gradient between the center and surface of the wood to cause checking, and in enveloping all surfaces of the wood with heated air maintained at a temperature substantially the same as the inner temperature of the wood and at a humidity which will create a vapor pressure differential between the moisture in the wood and on the surface of the wood for removing the moisture driven from the inside of the Wood.

8. The hereindescribed continuous process of dryin material which comprises moving the material through a zone containing radio waves that pass through the material and raise the temperature within the material; controlling the temperature and relative humidity of the air surrounding the material as it passes through the zone to raise the surface temperature of the material in uniformity with the rise in temperature within the material to obviate the forming of a moisture gradient in the material which will result in its cracking; lowering the humidity of the air as the temperature of the materiali raised to cause the vapor pressure within the material to exceed the vapor pressure of the surrounding air; whereby there is a rapid flow of water vapor from the material with a dehydrating of the material resulting, and in gradually reducing the temperature of the air surrounding the material after the material has been dried to the desired extent, to bring the temperature of the material back toward the temperature of the atmos-- phere.

9. A continuous wood dryer comprising a plurality of compartments, means for conveying wood through the compartments, means for circulating air through each compartment, means for controlling the temperature and relative humidity of the air in each compartment so that the temperature of the surface of the wood will be progressively raised as the Wood passes from compartment to compartment while the relative humidity is progressively lowered, and means in certain of the said compartments for applying a high frequency field of force to the wood for creating a temperature in the wood substantially approaching that of the air in the respective heating compartments.

10. A method of drying material which consists in adjusting the internal temperature of material to predetermined points by subjecting the material to a high frequency field of force and maintaining the external temperature of said -material at approximately the same predetermined points by circulating heated air about said material.

JULIUS W. MANN. GEORGE F. RUSSELL.

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