13.. THE DRYING RATE OF SUGAR MAPLE AS AFFECTED BY RELATIVE HUMIDITY AND AIR VEECCITy Information Reviewed and Reaffirmed April 1951 No. R12C4 DbrGSi UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin THE DRYI1T3- RATS OF SUC-.-iR IIAPIS A S AFF5CTRD 5Y RELATIVE HUMIDITY Al T D AIR VELOCITY By 0. T .V. TOHGSSON, Engineer Introduction Lumber dry kilns now in use vary frora natural circulation kilns with hand control to the latest type of fan kilns with automatic control cf "both temperature and humidity. In between are remodeled kilns equipped with various types and sizes of fans, and various types of control. Some slow circulation kilns are used for drying green stock where rapid circulation would be best, and some rapid circulation kilns are used for drying air-dried stock where a much slower rate would be satisfactory. Circulation is needed to replace heat losses through vails and around doors and where these losses are large a brisk rate is helpful in maintaining uniform drying conditions. Another important consideration, however, is the supplying of heat for evaporation, and, therefore, more efficient designing and drying could be accomplished if more specific information was available concerning the air needs for various species and items. How long does it take to kiln dry sugar maple? This seems like a simple question, but it can be answered satisfactorily only after many factors are taken into consideration. These factors are: (l) original and final moisture content, (2) sapwood or heartwood, (3) temperature of kiln air, (4) size of stock, (5) relative humidity of kiln air, (6) velocity of kiln air, and (7) length of air travel. To correlate various combinations of these factors and drying time requires considerable knowledge of the effect of each. Recently the Forest Products Laboratory completed a series of kiln runs on the heartwood and sapwood of 1- oy 8-inch sugar maple primarily for the purpose of determining the effect of air velocity on drying time, runs were made in a wind tunnel type of drying unit within which the temperature, relative humidity, and air velocity were maintained constant during each of the various runs. The boards were piled on 1-inch stickers to a width of 4 feet in the direction of air travel. The original green moisture content averaged, approximately, 70 percent and the drying period Report ITo. R1264 was from this condition down to a moisture content of 20 to 30 percent. Only one temperature, 130° ?. , was used for all runs, "but the relative humidity, because of surface checking tendencies, ranged from 20 to 76 percent for the sapwood boards and from 76 to 91 percent for the heart- wood boards. The air velocity ranged from 155 to 1,640 feet per minute. The data obtained in these kiln runs arc- presented here in discussing the seven factors enumerated in the preceding paragraph. (1) Original and Final koisture Content The drying rate of a given piece of wood is proportional to the moisture gradient. 3y gradient is meant the rate of increase in moisture content from the surface fibers toward the center. This gradient is greatest at the beginning and, under any given relative humidity, is a maximum only when the air velocity is sufficiently high to bring the sur- face fibers down to a moisture content in equilibrium with tne surrounding air. As the interior dries, the gradient decreases causing corresponding changes in the drying rate. To illustrate the importance of moisture content, the average drying rate cf sugar maple under a temperature of 130° F. and a relative humidity of 76 percent is given in table 1 for 10 percent moisture con- tent changes. The highest air velocity run (1,640 feet per minute) is used to eliminate as much as possible the velocity effect. The drying rate at any specific moisture value is influenced by the original green moisture content. Tor that reason a slight error is intro- duced in comparing the drying rates of the sapwood which has an average original moisture content of 70 percent with that of the heartwood which had an average original moisture content of 65 percent. At 55 percent moisture content, this error amounts to approximately 15 percent, but at 20 percent moisture content the error is negligible. In other words, if the average original moisture content of the heartwood had been 70 instead of 65 percent, the average drying rate between 60 and 50 percent moisture content would have been approximately 15 percent less than that shown. jt these constant drying conditions, more time was needed to dry from 50 to 20 percent moisture content than from 70 to 30 percent. This illustrates the necessity of specifying quite definitely the moisture content limits in estimating drying time. Report ko. kl264 -2- Table 1. — Dr ying time of 4/4 sugar ma ple under a temperature of 130° I a relative h umidity of 76 percent, and an air velocity of 1,64Q feet per minute Sapwood : Heartv/ood Moisture content ui j xxig, •■■v^j.w.g,^ mvxovL^^ tJi. ^ -LUg, ?rom To time content loss per hour time content loss per hour Percent Percent Hours Percent ■ Hours Percent 70 60 3.2 3.1 60 50 4.4 2.3 50 40 6.1 1.6 40 30 10.9 9 * - 30 20 34.9 .3 60 20 56.3 33.3 6.3 1.6 10.0 1.0 20.0 .5 47.0 .2 (2) Sapwood Versus Heartv/ood In general, heartv/ood dries slower than sapwood, and in the case of the sugar maple the ratio of drying rates was approximately two-thirds. This is illustrated "by the data given in table 1. Under the conditions given, the drying time from 60 to 20 percent moisture content was 33.0 hours for heartv/ood and 56.0 for sapwood. (3) Temperature of Kiln Air Only one temperature (130° P. ) was used but some data on oak have indicated that between 130° and 160° P. drying rate increases approxi- mately 2 percent for each 1 degree increase in temperature. On this basis, the sugar maple sapv/ood at 160° F. would dry from 60 to 20 percent mois- 56 ^ ture content in — — or 35 hours. This computed drying time is only an 1.60 approximation and is given merely as an illustration that temperature must be considered in estimating drying time. (4) Size of Stock Although only one size was dried in these particular experiments, it might be well to explain how the data can be used to estimate the drying time of other sizes. Drying time is not directly proportional to thickness, Report No. Pi 264 but, for a:, infinite width, is more nearly proportional to the square of the thickness. Width is a factor also "because as the width decreases the edge drying becomes relatively more important as compared to the amount of drying from the faces. A mathematical method of computing this has been used at the Laboratory and has been found to check well with empirical methods. Some of these computed ratios are shown in table 2. Table 2. — Theoretical drying time of various si?es based on that of 1-inch stock of infinite width as unity Thick- ness Width Infinite Inch 1 2 3 4 0.50 Helative time : 0.80 C.9C 0.94 0.97 0.99 : 1 : 2.00 3.77 •7, Of| «... Hl\^ 3. SO 3.77 4.00 4.50 5.76 7.20 7.89 9.00 S.OO 11.08 12.80 ' 16.00 The values show the relative drying time as compared to that of 1-inch stock of infinite width. Estimates can be made for narrow widths only when the air circulates freely around all sides. When the stock is piled edge to edge in a wide solid layer, an infinite width may be assumed. (o) Relative Humidity of Kiln Air Previously, it was stated that the drying rate is proportional to the moisture gradient. One factor limiting the gradient is the equilibrium moisture content at the surface of the wood and this, in turn, is a function of the relative humidity of the air. The confusing thing, however, is that the relative humidities at the surfaces and at the leaving-air side are not the same as that of the conditioned air. as heat passes from the air stream to th r ood surface and is used for evaporation, a temperature drop occurs which, together with r . ad ition of the evaporated moisture, results in an increase in reir-tive humidity at the wood surface. The main air stream is thus affected and by the time it reaches the leaving air side, it has a lower temperature and higher relative humidity than when it entered the load. Th;- magnitude of the difference between the air stream and the surface is governed mainly by the air velocity while that between the entering and leaving-air sides of the load is governed by the volume of air supplied and the length of air travel. Both differences (one perpendicular and one parallel to the board surfaces) are affectp-d by the rate at which the moisture is given off. Report ITo. R1264 -4- This is illustrated graphically in figure 1. The drying data were collected from a series of heartwood runs where the relative humidity and air velocity of the individual runs were as follows: 76, 90, 36, and 91 percent at 235 feet per minute, and 76, 30, and 36 percent at 450 and 930 feet per minute. On the charts, the lines drawn through the data points v/ere extended to the zero drying rate line at 100 percent relative humidity. It night be well to mention here that the moisture content values identifying the curves in figures 1 to 4 are average values and that in each case a moisture gradient existed from the interior of the wood to the sur- face and from the entering to the leaving-air sides of the load. At the same average moisture content, differences in the slope of these gradients accounted for the differences in drying rate. A comparison of the three charts of figure 1 shews that at 60 percent moisture content the relative humidity effect on drying rate under an air velocity of 930 feet per minute was quite different from that at 235 feet per minute. , At a moisture content of 30 percent, however, the difference was very much less. At a moisture content of 60 percent, a constant drying rate of 0.43 percent moisture content loss per hour was obtained under each of the following drying conditions, 76 percent relative humidity and 235 feet per minute air velocity, 85 percent and 450 feet per minute, and 91 percent 8 980 feet per minute. Each of these three conditions, then, produced the same average effective equilibrium moisture content on the surface of the wood at that particular moisture content of stock. Figure 2 shows the drying data for sapwood boards when dried under relative humidities from 76 down to 20 percent. The air velocity was 1,540 feet per minute, which was sufficiently high to prevent any appreciable humidity rise next to the wood surface especially at the low moisture con- tent values. The curves, then, represent the relative humidity effect on drying rate and show how much more important it was at the higher moisture value s . The curves also show that the importance of changes in relative humidity became increasingly greater as the humidity increased above 60 or 70 percent. Below 60 percent, the humidity effect became relatively unimportant. (6) T elocity of Kiln Air No kiln-drying time records are complete without showing the air velocity and volume as well as the temperature and relative humidity. The artance of this is shown by the constant drying rate curves of figure 3. Each curve repr d< finite drying rate of a 4-foot pile of 1- by 8-inch sugar maple sapwood boards when at 60 percent moisture content and Report No. H1264 „5- when subjected to various combinations of air velocity and humidity. The chart shows that wb I ie wood was at this high moisture content, the sur- face was subjected to the relative humidity of the conditioned air only when the air velocity was very high. 3elow this maximum air velocity, a constant dryin rate was maintained only by lowering the relative humidity of the conditioned air. Tor instance, when the stock was at 60 percent moisture content an air velocity of 1,600 feet per minute and a relative humidity of 64 resulted in a moisture loss of 2 percent per hour, '.'.lien the air velocity was reduced to 400 feet per minute the relative humidity had to be reduced to 43 percent to maintain the same average drying rate. In both cases the effec- tive average humidity at the wood surface must have been approximately 34 percent, but in the case of the lower velocity the humidity varied from the 43 percent of the conditioned air to the average of 34 percent next to the wood surface. ?or this reason, the results in various types of kilns apparently using the same drying schedule may vary widely because of a dif: r nee in air velocity. The effect of air velocity on drying rate at definite moisture-content values is shown by the curves of figure 4. The obvious conclusion is that air velocity is most important at hi h moisture-content values, and becomes relatively unimportant below 20 percent moisture content. Tor thoroughly air-dried stock very little velocity is needed except to establish uniform drying conditions in all parts of the kiln. Another important conclusion can be made from the data as presented in figure 5. This graph shows the average drying time in hours of a 4-foot wide pile of 1- by 8-inch sugar maple sapwood boards when dried from 70 to 25 percent moisture content under a constant temperature of 130° F. and under the indicated relative humidities and air velocities. The data indicate that much higher air velocities are needed for high than for low relative humidity schedules. The reason for this is that, at high Ldities, changes in the equilibrium moisture content of v/ood become increasingly greater with unit increases in humidity, and as a result the drying rate is more affected by the changes in humidity brought about by an iition of moisture from the wood and the drop in temperature across the load. Defining optimum air velocity as being some velocity beyond which the effect on drying time becomes relatively unimportant , the optimum values for relativ . ...laities of 20, 50, and 76 percent might be selected from - ie curves of figure 5 as being 20 , 400, and 500 feet per minute, respectively. Of course, for slower-c species such as oak, th. values would be lower. Report I\ T o. HI 2 64 _6- (7) Length of Air Travel As conditioned air passes through a load and heat is used for evapo- ration, the progressive changes in temperature and humidity result in changes in drying rate, and, therefore, in a drying lag across the load in the direction of air direction. Under otherwise fixed conditions, the amount of this lag is governed "by the length of air travel, cut is not directly proportional. To illustrate; data are given in table 3 to she'.-; the tine to dry from 70 to 40 percent moisture content at even intervals across a 4-foot pile of 4/4 sugar maple sapv;ood. As the air velocity effect on the entering-air side is quite different from that on the leaving- air side, the data given are from three separate air- velocity runs. Table 3. — ' Drying time of 4/4 sugar maple sapwood from 70 to 40 percent moisture content at definite intervals across a 4-foot wide pile under an entering-air temperature of 130° relative humidity of 76 percent and a Air velocity Length of air travel Average feet 1 foot 2 feet - 3 feet 4 feet : Leaving air . ntering air Fee t T>er minute : hours Hours Hours hours Hours : Hours 256 545 352 15 12 10 40 24 15 54 23 19 bo 30 21 69 31 22 49 27 13 Ehe dryfng time lag in each run is considerably greater than that w Lch might be expected from the average temperature drop across the load. The air, as it enters the load, is uniform in temperature and humidity, and, consequently, the velocity effect on the drying rate of the entering- air edge is relatively small. The directional force of the air, however, prevents a uniform distribution of the evaporated moisture and heat loss, and, as a result, the air at the leaving-air edge varies greatly in .peraturo and humidity from the wood surface to the center of the air stream. The velocity of the air influences this nonuniforraity and conse- quently has its greatest effect on the leaving-air side. Report H1264 -7- The greatest difference in drying tine occurred within the first 2 feet of the 4-foot air travel and in each of the three rums the average drying tine of the fv.li load was represented by the drying time of the wood located approximately 1.6 feet (or 0.4 of the total width) in fro;:, the entering-air side. At the high velocity this decreasing effect of length of air travel on drying time was such as to indicate that the width of load could have "been considerably greater with only a small change in average drying time or in that of the leaving-air edge. The effect at the low velocity was relatively high and might be taken as suggesting the desir- ability of introducing several entering-air edges within the load by means of vertical flues. In other ^ords , it suggests that two 4- foot loads of sugar maple sapwood placed side by side with a space between might have a lessor average drying time than one 3- foot load. Aside from the velocity phase it might be well to point out here that the sapwood boards dried without checking even under a relative humidity of 20 percent and an air velocity of 1,600 feet per minute, v/hereas the heartwood boards checked some at a humidity as high as 80 percent. Tor this reason, if, in sawing and kiln drying 4/4 maple, the all- sapwood boards could be sorted from the heartwood boards and dried separately under a low relative humidity schedule then their drying time would be greatly reduced from that ordinarily allowed for log run maple. SuiTiCiary In general, air needs arc proportional to drying rates. For that reason, air needs are least for the lover moisture content stock and for the slower-drying species and items. An exception to this rule occurs when relative humidities are increased above, approximately 70 percent. Such a procedure reduces drying rate, but requires a higher air velocity to prevent excessive increases in the moisture content of the wood surface and, consequently, excessive loss of drying time. By inference, then, the most efficient kiln from a drying time standpoint would be one equipped to furnish a great deal of air at the beginning and then lesser amounts a s the moisture content of stock decreases and as reductions are made in relative humidity. Report HI J Digitized by the Internet Archive in 2013 http://archive.org/details/dryOOfore U -H 3 13 O (b cd -P 3 t4 C! O •H tO s- a) ft 4-> a) o CO o> T3 Q. as a 05 in to T3 CM o Cm O a> Q-, W I t 3 a> o .-h o (lN33#3d) dDOH d3d J\T07 IN 3 IN 00 3iM±?IOlH X '4 \ W 'ol ^ ^ <\] w ^ ^ v£ r\j Uq M- °0 (iNBOtiSd) dHOH V3d i\ft?7 1N31N0D 3df)±SI0lA) o u od ffj M Id ~ Hi '. T" Hh 'A o d >o U 60 nd H a c,-. Q) X O -P -p 3 !D £ cl Ih •H o P 6 £ ■■- •H O ft rJ ft •H 6 -p H ; a: a! «h o o > P ^ •H aJ CD ■P - od 'd r-\ . ' O a^ o Ch fn ft O Cm O cri w • H P (D O i— 1 O ID ft i-H i I GVJ ID U 3 h.l •H fe \ \ \ \ °\ o\ — i\ <\1 -P 1 cd o: ■ -i fl o i) ^~ S si 5 Q) ID X. xJ tn V o 2 o P ti 5 X o CXi tw od 1) >> w a, 4^ S • H 0) Tl r-H 4-' • H Pll £ crt crt 3 S x •d U a ii U od > bO • H 3 P 4-i w Cj 31 ID H t^ G CD O S- u 0) fn Pj T3 3 S O o X tc >» Sh H o L. -P o >H Pj • 01 a; a^ p* p f~ «M a 3 • o -P Vi c CO CD +3 •H Xl (D O '1) t* e O c^ 3 CO «i-| p 31 rH H i • w ID fn 3 ttO • H [H (±N3dX3d) AiwiiAinH BAiivny I \ \\l \\l °0 «"> .-) ft > CCS -H S -p a Sh ,-H CtS hO *-. 3 w si Cm t3 o a aJ -P • aJ En fn 6j0 bO m CD •H T3 bo T3 to i— i X Cm -P O £ CD o t* • 3 -P >>-P £ -P -P CO t- *-. ai •H Cm a5 -P O a! Cm >> O T3 -P o •H -P o "3 o 5 •H ft £ Cm aS p Cm W ,C 1 <# • H En (iNdDVBd) VnOH V3d ?$01 1N31N00 jxnisiow 90 80 10 60 -50 r^ .20 7. ~^2%_ — — ZOO 400 600 800 1000 1100 1400 AIR VELOCITY (fEET PER MINUTE) 1600 37625 F Figure 5. — Effect of air velocity on the drying time of sugar maple sapwood in drying from 70 to 25 percent moisture content at a temperature of 130 deg. F. and relative humidities of 20, 50, and 76 percent. UNIVERSITY OF FLORIDA 3 1262 08866 6242