US3434222A

US3434222A - Method and an apparatus for the drying of wood in a channel

Info

Publication number: US3434222A
Application number: US658213A
Authority: US
Inventors: Lars Malmquist
Original assignee: Svenska Flaktfabriken AB
Current assignee: Svenska Flaktfabriken AB
Priority date: 1966-08-22
Filing date: 1967-08-03
Publication date: 1969-03-25
Anticipated expiration: 1986-03-25
Also published as: AT283216B; NL158609B; BE702920A; CH470642A; SE368740B; FI52629C; DE1729259B2; FI52629B; DE1729259A1; ES344108A1; DE1729259C3; GB1149985A; NL6711495A; NO124560B

Description

March 25, 1969 l.. MALMQUIST 3,434,222

METHOD AND AN APPARATUS FOR THE DRYING OF WOOD IN A CHANNEL Filed Aug. .'3, 1967 Sheet of 4 Sheet of 4 March 25, 1969 l.. MALMQUIST METHOD AND AN APPARATUS FOR THE DRYING OF WOOD IN A CHANNEL Filed Aug. 5, 1967 o N GOOM *IO .LNLNOD H'LLSOW BVLNHBCI INVENTOR'. LARS MALMQUST ATTYS.

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United States Patent O M' 3,434,222 METHOD AND AN APPARATUS FOR THE DRYIN G F WOOD IN A CHANNEL Lars Malmquist, Huddinge, Sweden, assignor to Aktiebolaget Svenska Flaktfabriken, Stockholm, Sweden Filed Aug. 3, 1967, Ser. No. 658,213 Claims priority, application Sweden, Aug. 22, 1966, 11,280/ 66 Int. Cl. F26b 21/10, 21/04, 3/04 U.S. Cl. 34-16.5 4 Claims ABSTRACT 0F THE DISCLOSURE The present invention relates to a method and an apparatus for the drying of wood in a channel, through which the wood is fed by steps while being exposed to a gaseous drying medium being advanced in the longitudinal direction of the channel.

It is well known that the drying of sawed wood goods must be carried out very carefully for obtaining an yacceptable quality. The defects caused to the wood by careless drying are primarily cracks in the wood and twist or other deformations of the individual wood pieces. Both of said defects are caused `by the deformation, to which the wood is subjected when the water in the wood cells is being removed. In order to prevent the deformation of the cells from giving rise to stresses of such a magnitude that the wood cracks, the moisture ratio gradient within the wood must be sufficiently small.

Formerly, when articial drying was applied, the wood was dried mostly in so-called chamber dryers, i.e. dryers in which a wood batch lies still and the condition of the drying medium surrounding the wood can be varied optionally. In a dryer of this type there are no difficulties of ybeing able at any occasion to adjust the drying speed of the wood to its moisture ratio so that a high capacity can be obtained without subjecting the wood to drying damages.

In a modern industry with a high production of sawed goods, however, the chamber dryers are less suit-able, because of their high degree of discontinuity in operation. So there is a tendency of changing over to dryers, which operate more continuously. Among the latter certain advantages are offered by the channel dryers, in which the drying medium is circulated in the longitudinal direction and the wood stacked in so-called packages is fed through them on rail-bound cars or on roller beams. The channel dryers are much less expensive in relation to their capacity than the chamber dryers with respect both to the initial costs and the operation costs, and they allow for good drying continuity. It is, however, not possible to select as freely as in a chamber dryer the drying schedule of an individual wood package for its passage through a channel dryer with longitudinal circulation.

A channel dryer operates substantially in such a manner that a certain number (l0-20) of wood packages are simultaneously enclosed in a channel. In connection with removing the package, which had been the longest time in the dryer and is completely dried, at the discharged end, all of the remaining packages are advanced by one step 3,434,222 Patented Mar. 25, 1969 towards the discharge end and a new package is inserted at the receiving end. The condition of the air, to which a wood package in a channel dryer for longitudinal cir culation is subjected in an arbitrary position within the drying channel, cannot be controlled individually, but is determined by the position and the drying speed of the other packages in the channel. It is understood, thus, that it is very difficult to provide the control and ow arrangement for the drying medium in such a manner that an individual wood package is made the subject of a certain ideal drying schedule.

Due to the fact that the drying speed at a given condition of the drying air is greater at a high moisture ratio of the wood than at a low moisture ratio, a change of the ingoing wood moisture ratio will cause a change of the condition of the drying air through the dryer (the drying schedule). Such changes in the ingoing wood moisture ratio may arise, for example, when wood from different plant areas or with different cutting times is sawed, when lboth floated wood and truck-transported wood is sawed, or when boards of different dimensions are mixed in one and the same channel. It is very complicated and in practice scarcely applicable to measure the ingoing mean moisture ratio and, therefore, it is dicult to adjust the drying schedule in a. suitable way. By holding the drying climate constant in such a point in the dryer, where the drying medium already has passed through a number of packages and, thus, its condition has 'been changed due to a change in the ingoing moisture ratio, it is, however, possible to utilize said constant climate for an automatic adjusting of the drying schedule to the ch-anged ingoing moisture ratio.

In usual types of channel dryers with longitudinal circulation the drying medium ows through the drying channel in a counter-flow to the wood. The Condition of the drying medium is maintained constant either prior to its contact with the wood or subsequent to its passage through all of the wood or the major part thereof. Each of the two control methods involves apparent disadvantages. Irrespective of the control method, both systems show the disadvantage that the drying speed of the wood becomes too high in the section, where the moisture ratio has become so low as to be close to the bre saturation moisture ratio, i.e. that ratio in the wood where the free moisture has been eliminated. It is here, namely, that the wood commences to shrink heavily in its surface, and in order to prevent crack formation, the wood must shrink as homogenously as possible. In other words, the moisture ratio gradient in the surface layer of the wood must be not too great. Consequently, the wood must dry very slowly within this section.

The solution of the problem according to the invention is characterized in that in a drying channel being divided into two sections and wherein the drying medium is divided into two partial flows, one of which is caused to flow through the rst section of the channel in a direction counter-current to the direction of motion of the wood, and the second partial flow is caused to ow through the second section of the channel in the direction concurrent to the direction of motion of the wood, there is imparted to the partial flows of the drying medium prior to their contact with the wood such -a condition in common that the one partial flow subsequent to its passage through the second section of the drying channel has achieved the desired condition.

By such a method a better adjustment of the drying operation of the individual wood packages to the ideal drying schedule is obtained than by conventional methods, and at the same time a simple and accurate control of the moisture ratio of the outgoing wood is obtained. The invention is particularly adapte-d for the drying of so-called export wood, which prior to its shipment is dried down to a moisture ratio of about 20%, i.e. not substantially below the fibre saturation moisture ratio, which is about 30%, but it is also adapted for use -as a pre-dryer in a plant comprising both a preand an after-dryer, wherein the wood in the pre-dryer is dried down to immediately `below the fibre saturation moisture ratio.

An apparatus for carrying out the method according to the invention comprises a drying channel adapted for stepwise feeding therethrough of wood, which on conveyors is stacked in packages separated from each other, said packages being passed through by a gaseous drying medium being advanced in the longitudinal direction of the channel, which channel is divided into two sections, between which there is provided a longitudinal intermediate space, into which a gaseous drying medium can be introduced transversely and be caused to flow longitudinally through the two sections in two partial flows. The apparatus is characterized in that means for measuring thetemperature and the humidity are mounted in the flow path of the drying medium subsequent to its passage through the second section.

The properties of the drying medium will be dealt with in connection with the description of the figures, whereof FIG. 1 shows a longitudinal section through a drying channel with the invention applied therein, and

FIGS. 2-7 show the drying operation under different conditions, both according to the invention and in a conventional drying arrangement.

In FIG. 1 a number of wood packages 1 wait in front of the inlet door 2 of a drying channel for wood. The packages 1 are supported on cars 3 adapted to be moved on a conveyor 4 running through the entire channel. To the inside of the inlet door 2 three wood packages 5 are located in a first section of the channel, and four packages 6 in a second section of the channel. An intermediate space 7 between said sections is provided, in the roof of which a number of axial fans 8 is disposed. An outlet door 18 is provided after the packages 6 in the second section. Between the inlet door 2 and the packages 5 in the first section a free space 19 is provided. Between the outlet door 18 and the packages 6 in the second section a free space 20 is located. To the outside of the outlet door 18 some completely dried wood packages 21 are located.

Above the roof 9 of the drying channel two air passages 10 and 11 are arranged, in which steam-heated elements 12 and 13 are mounted. The steam flow to the elements 12 and 13 can be controlled by valves 14 and 15. The air -passage 11, which is located nearest to the outlet door 18 comprises a ventilation air outlet 22 provided with a damper 17 and an exhaust fan 23. In the air passage 11 below the outlet 22 are mounted in the ow path of the drying medium two temperature sensing members 24 and 25, one 24 for sensing the dry temperature and the other 25 for sensing the wet bulb temperature of the outgoing ventilation air. Both members 24 and 25 are connected to a governor 26, to which also the steam valves 14 and 15 and the damper 17 are connected.

In continuous operation the function of the drying apparatus is as follows:

The axial fans 8 effect the circulation of the drying medium through the wood packages 5 and 6. The drying medium is moist air. From the fans 8 the air is pressed down into the intermediate space 7, where it is divided into two flows, the left-hand flow flowing to the left in a counter-flow through the wood packages 5 in the first section of the channel, where it takes up humidity from the wood and continues therefrom into the space 19 and upwards into the passage 10. Here the air, which now is very moist, is mixed with relatively dry outside air flowing in through the inlet 16. The mixture continues to the right through a heating element 12 to the fans 8, in which it is mixed with air arriving in a fiow to the left through the passage 11,

The right-hand flow of the two air flows in the intermediate space 7 is pressed to the right through the wood packages 6 in a flow concurrent with them and thereby takes up humidity. In the space 20 after the packages 6 the air is deflected upwardly by the discharge door 18 and is there divided into two partial flows, whereof one iiows to the left through the heating element 13 in the passage 11 and back to the circulation fans 8. Thereby this partial ow flows past the temperature sensing members 24 and 25, which measure the dry and the wet bulb temperature of said partial flow and forward the measuring values to the governor 26, which by actuating the valves 14 and 15 in the steam lines to the heating elements 12 and 13 can maintain constant the dry temperature at the point 24, and by actuating the damper 17 in the ventilation air outlet 22 can maintain constant the wet bulb temperature at the point 25. The second partial flow is sucked out 'by the exhaust fan 23 through the ventilation air outlet 22 into the atmosphere.

The

doors

2 and 18 are opened at equal time intervals, and the wood package located closest to the discharge door 18 is pushed out to the completely dried packages 21, whereafter all of the wood packages 5 and 6 are advanced by one step, and a new package 1 is inserted among the packages 5. Thereby, the package, which was located farthest to the right in the first section has been moved past the intermediate space 7 to the position farthest to the left in the second section. As, thus, the wood is moved in this way intermittently in a definite direction, the definitions concurrent and countercurrent between the drying medium and the wood can be used.

In the drying of wood the drying speed must be low in certain phases of the drying for obtaining a high quality. One of these phases takes place immediately subsequent to the insertion of the wood into the dryer, while it is being warmed up, when in the wood the development of a moisture ratio gradient required for the drying is being started. If the drying-out occurs too rapidly in this phase it results in so-called surface dryness, which causes complications in the latter phases of the drying. The second phase takes place, as mentioned, when the moisture ratio in the wood surface has decreased to the fibre saturation moisture ratio. It is then, namely, that the hygroscopically bound water in the cell walls of the wood commences to dry out, where-by said cell walls will shrink. If the shrinking develops in an entirely homogenous manner, no cracks or warps will be caused. However, as heat is being supplied and water vapour is removed only at the surface of the wood in convective heat supply, a homogenous drying is not possible. A certain moisture ratio gradient in the wood must be accepted, but it should 'be kept low.

The drying speed, or the time derivatives du/ dt of the moisture ratio u, according to empirical support, depends in a pure convection dryer a.o. the socalled psychrometer difference of the drying medium, i.e. the difference (t- V) between the dry temperature t and the wet bulb temperature 6v of the drying medium. This relation is utilized to the extent possible for controlling the drying process in a drying channel.

By evaluation of a number of measuring results from a wood dryer the inventor has drawn up an analytic relation between drying speed, moisture ratio and psyehrometer difference. The relation has been utilized with the help of the water balance for drying medium and wood to determine a drying procedure which better adapts t0 what can be regarded desirable with respect to wood quality and drying speed. FIGURES 2-7 show in a graphic manner certain results of the study.

The FIGS. 2 and 3 are directly comparable to each other. In FIG. 2 the psych-rometer difference of the drying medium and the moisture ratio of the wood are shown for an arrangement according to the invention as a function of a longitudinal coordinate in the dryer, which was made equal to zero in the recetiving end and equal to 18 meters in the discharge-end, In FIG, 3 corresponding curves yare shown for a conventional counter-flow dryer, in which the condition of the drying medium is maintained constant in the discharge end of the dryer. I-t applies to both dryers that they have the same length, the same passing time for the wood, the same drying capacity (i.e. the

' same Wood ow), the same fan effect, the same drying medium speed, the same drying medium ow past the wood, the same moisture ratio of the ingoing wood and the same moisture ratio of the outgoing wood. The letters F.S. indicate the points in the channels, at which the moisture ratio has decreased to 30%, i.e. approximately to fibre saturation. It is evident from the figures that both dryers have a low psychrometer difference in the receiving end, i.e. low drying speed, which is needed for preventing damages of the wood. About the libre saturation point, however, where also a low drying speed is desirable, the counter-tlow dryer has a psychrometer difference of more than 13 degrees, while the dryer according to the invention only has 6.5 degrees and, thus, a substantially lower drying speed.

When studying a surface layer of the wood a decrease occurring in the psychrometer difference will result in a momentary decrease of the evaporation from the surface. The surface layer, on the other hand, is supplied by the existing moisture ratio gradient at the beginning with the same water flow from the inner parts of the wood, i.e. the moisture ratio of the surface layer will tend to increase. Besides, the drying of the surface layer towards the equilibrium moisture ratio corresponding to the psychrometer difference will be retarded, because this equilibrium moisture ratio increases at decreased psychrometer diffrence. Owing to the concurrent flow part provided according to the invention the psychrometer difference decreases successively, and therefore said difference will effectively contribute to a decrease in the drying-out of the surface layer of the wood so that the concurrent flow part will be characterized by a drying with small varying surface moisture ratio and decreasing moisture ratio gradient in the wood. This is of essential importance in that phase of the drying, when the wood surface proceeds towards fibre saturation and a thin surface layer commences to shrink. This shrinking is obstructed -by wood parts located farther inwardly, which have not yet commenced to shrink, and as long as the shrinking surface layer is thin, it has a low strength against tensile stresses produced and develops indications of fractures, which during continued drying-out -result in crack formation. When drying with counter-How in this moisture ratio range the situation is reverse in that an accelerated surface drying takes place, which is unfavourable to crack formation.

The concurrent part arranged according to the invention further involves the advantage that a relatively high psychrometer difference (drying speed) can be applied in the intermediate phase of the drying, where the moisture ratio in the wood surface still is above the bre saturation moisture ratio and no shrinking yet has commenced. A great lbut in this drying phase not dangerous moisture ratio gradient is then developing and results in a quick drying in this phase, which compensates for the lower drying speed in the later phase of the drying.

The concurrent part, furthermore, offers the advantage that the psychrometer difference at the end of the drying (at the discharge end) is only 5 C., compared with 14.6 C. in the counter-fiow dryer. At a wet bulb temperature of, for example, 3035 C., this corresponds to an equilibrium moisture ratio of the wood of `about 12% and about 6% respectively, which moisture ratios the wood surface endeavors to assume. In view of what is said above on the effect of concurrent and counter-current respectively on the surface moisture ratio of the wood, this ratio after completed drying can be expected to lie for the dryer according to the invention not insignificantly above 12%, while for the counter-How dryer it can be expected to lie close to 6%. It is understood that the remaining moisture ratio difference between the interior of the Wood and its surface (at 20% mean moisture ratio) is substantially more favourable in the dryer according to the invention than in the conventional dryer, which is of great importance for the existence of remaining stresses in the wood after the drying, which stresses give rise to complications in the use of the wood.

It further was investigated for both arrangements, how much the mean moisture ratio of the wood supplied to the dryers can be increased without causing the moisture ratio of the outgoing wood to increase more than by 1%- unit from 20% (full lines) to 21% (dashed lines). As appears from the tigures, in the counter-flow dryer the ingoing moisture ratio can be increased by only 1.2%- units, while in the dryer according to the invention the increase can amount to 15.9%-units. As the mean moisture ratio of the ingoing wood at times can show considerable variations, the reported capacity of balancing such variations is of particular value.

FIGS. 4 and 5 are also directly comparable with each other. FIG. 4 refers to the same dryer as in FIG. 2, .and FIG. 5 refers to the same dryer as in FIG. 3. In the FIGS. 4 and 5 an investigation has been made on how an increase of the feed speed by 10 percent (for example in order to temporarily increase the dryer capacity at the expense of a deviation from the desired final moisture ratio) affects the moisture -ratio of the wood delivered from the dryer. FIG. 4 shows that in the dryer according to the invention a decrease by 10 percent of the drying time will result in an increase of the moisture ratio in the outgoing wood by 1.2%-units from 20% to 21.2%, while FIG. 5 shows that the corresponding values for the counter-flow dryer are 8%-units from 20% to 28%. Also in this respect the dryer according to the invention is substantially more favourable than the conventional arrangement.

It may now be alleged that the comparison had turned out to be unfavourable for the conventional arrangement, 'because the drying medium flow was chosen so small that the drying medium was close to saturation prior to its leaving the channel of the counter-flow dryer. rIhis allegation can be opposed by the argument that the chosen relation between the drying medium flow and the wood flow is representative for counter-dow dryers, as they exist today, and furtheras will be shown rbelow--that the comparison is to the advantage of the invention at much greater (unrealistically great) air flows.

FIG. 6 shows, therefore, the procedure in a conventional counter-ow dryer with the drying medium condition being maintained constant in the discharge end of the dryer, i.e. the same arrangement as referred to in FIG. 3. The circulating air quantity, however, in FIG. 6 has been made twice as great as in the preceding figures, but the pressure drop was maintained. This can be achieved by giving the dryer a greater height and at the same time stacking the wood thinner. The initial costs of the dryer are, of course, higher and lalso the fan effect, which is twice as high as before. In comparison with FIG. 2, therefore, FIG. 6 refers to a dryer having the same length, the same passing time for the wood, the same drying capacity (ie. the same wood flow), twice as great a fan effect, the same drying medium speed, twice as great a drying medium flow past the wood, the same moisture ratio of the ingoing wood and the same moisture ratio of the outgoing wood.

A comparison between FIG. 2 and FllG. 6 shows that FIG 6. operates with a somewhat higher, i.e. more unfavourable psychrometer difference in the receiving end (1.8 degrees) than FIG. 2 (1.2 degrees). The situation is the same at the bre saturation point (8 degrees and 6.5 degrees respectively). FIG. 6 also shows that for an increase of the final moisture ratio by 1%-unit from 20% to 21% at unchanged feed speed an increase in the mean moisture ratio of the ingoing wood by only 3.9%-units is required, while the corresponding value in FIG. 2 -was 15.9%-units, i.e. the capacity of correcting variations in the mean moisture ratio of the ingoing wood is in this case four times greater for the invention than for the conventional arrangement.

FIG. 7 iinally shows how much the moisture ratio of the outgoing wood increases, if the feed speed is increased by 10%. The value is 4%-units (from 20% to 24%) in comparison to 1.2%-units (from 20% to 21.2% for the invention (FIG. 4). The invention, thus, still is superior in this respect. Moreover, taking into consideration that the dryer according to FIG. `6 requires a greater building vol-ume for the same production and twice as much fan energy per ton of wood, it is easy to understand that the invention constitutes a step forward in the art.

I claim:

1. In a method for drying wood to a certain moisture ratio in a drying channel having gaseous drying medium recirculated longitudinally therethrough, wherein the channel is divided into two sections, the wood is fed by steps longitudinally in sequence through said sections while being exposed to the drying medi-um, and the drying medium is introduced between said sections and is divided into two partial flows, the first of which is caused to ow through the first section of the dryer countercurrent to the direction of motion of the wood and the second of which is caused to ow through the second section of the dryer in the direction of motion of the wood, said partial flows being merged for re-introduction into said channel between said sections upon completion of their ow through said sections; the improvement including the steps of sensing the temperature and humidity conditions of only the second partial ow after its passage through the second section and prior to its merger with the first partial flow, and, upon occurrence of deviations from a predetermined value of the condition of said second partial liow, regulating the temperature and humidity condition of the merged partial flows prior to introduction into said channel so that said predetermined value is restored to said second ow, said predetermined value being chosen so that regardless of changes in the average moisture ratio of the wood entering the first section and/ or regardless of changes in the feeding velocity of the wood, the moisture ratio of the wood leaving the second section is substantially at said certain ratio.

2. A method according to claim 1 wherein the temperature of the merged partial Hows is regulated by adjusting the temperatures of both partial ows prior to the merger, the temperature of the second partial flow being adjusted after its conditions are sensed.

3. A method according to claim 1 wherein the humidity of the merged partial ows is regulated by exhausting the portion of one of said partial flows prior to said merger and introducing a like portion of fresh drying medium without directly affecting the condition of the second partial flow prior to said sensing.

4. Apparatus for drying Wood to a certain moisture ratio comprising a drying channel divided into two sections, means to feed the wood stepwise longitudinally in sequence through said sections, fan means to introduce drying medium into said channel intermediate said sections to eect two partial flows, the first of which is caused to flow through the rst section of the dryer countercurrent to the direction of motion of the wood and the second of which is caused to flow through the second section of the dryer in the direction of motion of the wood, conduit means extending from each end of the dryer to said fan means to effect recirculation of said medium and merger of said partial flows for re-introduction into said channel, temperature and humidity sensing means mounted in the flow path of the second partial flow after passage through said second section and prior to its merger with the first partial How, and temperature and humidity regulators in said conduit means responsive to said sensing means to regulate the temperature and humidity of the merged partial flows so to maintain the temperature and humidity of the second partial tiow after its passage through the second section substantially constant.

References Cited UNITED STATES PATENTS 1,598,980 9/1926 Munroe 34-31 X 2,202,143 5/1940 Cobb 34--29 JOHN J. CAMBY, Primary Examiner.

U.S. Cl. X.R.