Drying 4/4 Red Oak by Solar heat

Transcription

1 FOREST PRODUCTS LABORATORY (Madison 5, Wis.) FOREST SERVICE, U. S. DEPARTMENT OF AGRICULTURE Approved Technical Article VOL. XII, NO. 3 FOREST PRODUCTS JOURNAL MARCH 1962 Fig. 1.-Experimental solar dryer with double glazed plastic Fig lnterior of experimental solar predryer showing walls and roof. location of test load. 1 Drying 4/4 Red Oak by Solar heat EDWARD C. PECK Forest Products Laboratory, 2 Forest Service U. S. Department of Agriculture OLAR HEAT IS USED TO A LIMITED Sextent in both air drying and kiln drying. The roadways and ground of an air-drying yard are heated by the sun as are the lumber piles themselves. Although the heat that is absorbed by these objects is transferred to the air in the yard, much of it is lost by radiation to the sky. The roofs and walls of a kiln also are heated by the sun. Much of this heat, too, is lost by radiation to the air and sky. The effect of solar heat on kiln drying, therefore, is not important. To utilize solar heat in the drying of lumber, it is necessary to trap the heat that is absorbed. One way of doing this is to place a transparent sheet or membrane between the sun and the object that is heated. The transparent sheet offers practically no resistance to the passage of sunlight, but does resist the passage of the heat waves that are radiated from the object that has absorbed the heat. This principle is incorporated into a drying structure, called a predryer, that has been designed by staff mem 1 Presented at the 1961 Annual Meeting of the Forest Products Research Society, Louisville, Ky., Session 10 (Drying B), June 21, FOREST PRODUCTS JOURNAL bers of the U.S. Forest Products Laboratory, Madison, Wis. A series of experiments was carried on using such a predryer with 4/4 red oak flooring stock at Sauk City, Wis., in Another series of such experiments was carried on with an improved version of this predryer on the Laboratory grounds at Madison, during In this report, the results of the latter experiments (charges 1, 2, and 3) are compared with the results of investigations on air drying that the Laboratory conducted at a small mill 25 miles from Madison during 1956 to Objectives The objectives of the experiments were to: 1) determine the time required to dry green 4/4 red oak flooring stock to a moisture content of 20 percent in a solar-heated predryer and to compare the time with that required for air drying, 2) determine the cost of drying in the predryer and compare it with that for air drying, 3) test a new method to control the operation of the fan in the predryer, 4) try out a new method of operating the dampers in the vents of the predryer, and 2Maintained at Madison, Wis., in cooperation with the University of Wisconsin 5) dry the lumber to a final moisture content of 10 percent and compare the time and cost with.those for kiln drying. Design of Predryer Figure 1 shows the predryer, which is 7 feet 6 inches wide by 12 feet 8 inches long by 8 feet high. The framing of the four walls consists of 2- by 4-inch studs, and the framing of the roof consists of 2- by 6-inch rafters. Sheets of corrugated aluminum, painted dull black on both surfaces, are nailed to the inner edges of the studs and rafters. To the outer edges of the studs and rafters are fastened sheets of transparent plastic, except on the end wall facing north, which is sheathed with plywood and includes a door into the predryer (see Figure 2). A second film of transparent plastic is mounted on frames to provide a space of several inches between the two plastic films on the roof, the enteringair side (east wall) and the leavingair side (west wall). Also, on these two sides and the roof, rectangular openings are cut in the aluminum and the metal is bent to form air deflectors. This causes some of the circulated air to pass over the outer as well as the 103

2 inner face of the heat-absorbing metal. Inside, a 24-inch fan, powered by a 5 /8-horsepower motor, is mounted in the central baffle that extends the entire length of the predryer. Operation of Fan: A light-sensitive cell is placed in the electrical circuit to the fan motor. Consequently, the fan operates only during the daylight hours. With this system of control, the number of hours per day that the fan operates is not a constant but varies with the season and the type of weather. The control element has been adjusted so that the fan will run for a certain number of hours even on the darkest days. The fan always runs in the same direction, so that the direction of circulation through the lumber pile is not reversed. Operation of Vents: The dampers of the four vents, which are 8 inches in diameter, are operated by electric motors, The current is turned on or off by a microswitch that is connected with a wood element. Because of the widely fluctuating temperatures inside the predryer, it is impossible to adjust the vent damper control with a high degree of accuracy. Temperature Recorder: A threeelement recorder was used to record the wet- and dry-bulb temperatures. One dry-bulb element was located in the entering-air duct inside of the predryer, and the other one was located outside the predryer to record the outdoor temperatures. The wet-bulb element was located close to the dry-bulb element inside of the predryer. Temperatures were also measured by means of thermocouples and an indicating potentiometer. Three of the thermocouples were placed close to the three elements of the recorder. An additional thermocouple was placed in the leaving-air plenum chamber and another just above the black metal element of the east side of the roof. The temperature readings made with the potentiometer were primarily for the purpose of checking the recorder. Experimental Procedure are outdoor temperatures. Figures 3 and 4 show zones of average daily so- The lumber used in the experiments lar heat in terms of British thermal was green 4/4 red oak flooring stock units per square foot for the United averaging about No. 2 Common in States during January and July. Madi grade, 8 1 /2 feet long, and of random son. Wis., receives little solar heat widths. during January but a moderate There were three charges, each conamount during July. sisting of about 425 board feet, in Figure 5 shows the amount of solar stickercd piles 40 inches wide and 17 radiation from May, 1960, through courses, or about 3 feet, high, which February in comparison io the average were fitted into the opening of the monthly amounts of solar radiation 3 central baffle. The stickers were /4 for the years 1953, 1956, and If inch thick and 1 1 /2 inches wide, placed it is assumed that the monthly aver- in three tiers. The lumber of charge I ages for these 3 years represent the was dried to a moisture content of 10 normal monthly amounts of solar ra percent so comparisons could be made diation for the Madison, Wis., area, with kiln drying as well as air drying. then the solar radiation during the pe- The lumber of charges 2 and 3 was riod of the solar-drying experiments dried only to a moisture content of 20 was somewhat above normal except for percent. An attempt was made to dry June and September. the lumber of charge 2 to a moisture The daily temperature and relative content of 10 percent, but the minimum that was attained was 10.5 humidity were derived by taking a percent. point midway between the maximum The drying rate of the lumber was and the minimum for each day. From determined by weighing specially pre- May through December, 1960, the weather was generally colder and wet- pared pile samples at certain intervals. ter than normal. In 1961, January was The current moisture content of the colder than normal, but February was lumber, as represented by the ten 3 1 /2 considerably warmer than normal, and foot samples, was determined by means of the current weights and the both were drier than normal. calculated ovendry weights. At the end Such data as the amount of precipi of the drying process, a moisture con- tation and wind movement had little tent section was cut from the middle effect on the drying of the lumber of each pile sample. The calculated within the predryer. Precipitation has ovendry weight of the sample was ad- an indirect effect because there is little justed by means of the weight of the solar radiation while it is raining. sample at the final condition of dry- The differences between the predryer ness and the moisture content as deter- and the outdoor temperatures (Figures mined by the oven method in the test 6, 7, and 8) were established by the section. amount of solar heat that was absorbed Surface checking that developed in and transferred to the circulating air the samples during drying was measwithin the predryer. The amount of ured in terms of total inches of length heat that was absorbed during any on both faces, End splits, when devel- particular day depended on the amount oped, were counted along with the of solar radiation, which depends on surface checks. the intensity of the radiation and the number of hours of sunshine. The Data temperature within the predryer de- Climatological: The most impor- pended not only on the amount of tant climatological data are solar ra- solar heat that was absorbed but on diation values and the next important the amount that was expended in heat- 104 Fig. 3. -lsopleths of average solar heal for January, British thermal units per square foot. Fig. 4. -lsopleths of overage solar heat for July, British thermal units per square foot. MARCH, 1962

3 ing the lumber, dissipated by evaporating and moving moisture from the lumber and dissipated by ventilation. Drying: Drying curves for the three charges (Figures 9, 10, and 11) show the moisture content of the five pile samples on the entering-air side and the five pile samples on the leaving-air side plotted against drying time in days. Air-drying curves were calculated on the basis of data obtained during the 1956 to 1958 investigations. Figures 9 and 10 also show calculated kiln-drying curves, which are plotted on a 20-day period, from green to 10 percent moisture content. Charge 1 Started May 19, 1960: Table Charge 1 reached a moisture content of 20 percent in 33 days, on June 20. The corresponding calculated airdrying period is 65 days. During the 33 days, the outdoor temperature averaged 2.2 F. below normal, the outdoor humidity averaged about 75 per-. cent as against a normal of 67 percent, and there was a trifle less solar radiation than normal, The temperature within the predryer was always higher than that outdoors (Table 1), and the relative humidity within the predryer was generally lower than that outdoors (Figure 6). The relative humidity within the predryer exhibited a downward trend as the drying progressed, which was to be expected. During the early stages, a great deal of moisture that was evaporated from the green lumber was retained within the predryer because the mechanism was adjusted so that the dampers of vents were closed during the daytime and open during the night. During the later stages, less moisture was being evaporated from the lumber and the vent dampers were open most of the time, The relative humidity also was affected by the temperature within the predryer and the difference between this temperature and the outdoor temperature. For example, on July 8, the mean temperature within the predryer was 103 F., the difference between it and the outdoor temperature was 20 F., and the relative humidity was 26 percent. In contrast, on July 10, the temperature within the predryer was 76 F., the difference between it and the outdoor temperature was 9 F., and the relative humidity within the predryer was 50 percent. It is possible that the predryer has been penalized to some extent in the comparison between the predryer s 33 day drying period and the air-drying 1. -TEMPERATURES WITHIN THE SOLAR-HEATED PREDRYER AND OUTDOORS AND SOLAR RADIATION DURING THE DRYING OF 4/4 RED OAK FOREST PRODUCTS JOURNAL Fig. 5. -Average solar radiation in Langleys for 1953, 1956, and 1957 and solar radiation for 1960 and 1961 during months of drying experiments. Langley is expressed os energy per square centimeter of horizontal surface per day (1 British thermal unit per square foot = 0.27 calorie per square centimeter). period of 65 days because the airdrying period was based on nearly normal weather conditions, while the weather was generally cool and damp during the drying of Charge No. 1. Charge 2 Started August 18, 1960: The 4/4 red oak lumber of charge 2 reached a moisture content of 20 percent on September 9, or after 23 days of drying. The corresponding airdrying period was 86 days. The outdoor temperature during the drying of charge 2 averaged 5.8 F. above normal, while the average relative humidity was 80 percent compared with the normal of 72 percent. During the 23-day period for charge 2, the precipitation averaged just a trifle above normal, but solar radiation was 5.6 percent greater than normal. Table 1 gives the differences in temperature from August 18 to 31, 1960 and from September 1 to 9, 1960, and the weighted average of these figures for the 23-day drying period is 16.6 F. The corresponding figure for the 33-day drying period of charge I is 14.3 F. The average drybulb temperature inside the predryer during the drying of charge 2 was 91.9 F., which compares with 80.6 F. during the 33-day drying period of charge 1. Figure 7 shows the temperatures and relative humidities within the predryer compared with the outdoor temperatures and relative humidities of charge 2 as obtained from Weather Bureau data. The relative hu 105

4 Fig. 6. -Comparison of temperalures and relative humidities outdoors and in the predryer during drying of charge 1. Fig. 7. -Comparison of temperatures and relative humidities outdoors and within the predryer during drying of charge 2. midity was at all times considerably below that of outdoors, except during the first day. The average relative humidity within the predryer during the drying of charge 2 was 63.6 percent, while the average relative humidity for 29 days of the 33-day drying period of charge 1 was 57.9 percent. The shorter drying period of charge 2, compared with charge 1 can be accounted for only by the difference in the temperature, which amounted to 11.3 F. The average initial moisture content of the 10 samples of charge 1 was 76.7 percent and for charge 2 was 72.3 percent. This lower initial moisture content in the lumber of charge 2 could be responsible for some shortening of the drying period. A theoretical calculation indicates that if the initial moisture content in the lumber of charge 2 were equal to that in the lumber of charge 1, the moisture content in the lumber of charge 2 would be 20.6 percent moisture content after 23 days of drying, instead of 20 percent. It would, therefore. require one more day of drying to reach a moisture content of 20 percent, thereby in 106 creasing the drying period for charge 2 from 23 to 24 days. After making allowance for the difference in initial moisture content, the lumber of charge 2 still dried much more rapidly than charge 1. Charge 3, Started on November 17, 1960: In charge 3 a moisture content of 20 percent was reached on March 1, 1961, or after 105 days. Lumber piled in the yard during November 1956 reached a moisture content of 20 percent in 173 days. The average outdoor temperature during the 105-day drying period was 1.1 F. above normal. The relative humidity of the outdoor air averaged 65 percent for the 105 days, compared to a normal 73 percent. The total amount of solar radiation during the drying period was appreciably above normal. The average daily solar radiation, based on the averages of the years 1953, 1956, and 1957, is Langleys, while during the drying of charge 3, the average daily solar radiation was Langleys. The weather during the drying period was warm, dry, and with a greater than normal amount of solar radiation. All of these would tend to shorten the drying period from that which would prevail under normal weather conditions. Figure 8 shows the temperature and relative humidity of the outdoor air obtained from Weather Bureau data and the temperature and relative humidity inside of the predryer for charge 3. During the first 4 days of drying, a high relative humidity was maintained inside of the predryer by keeping the dampers of the vents closed. After about the eighth day, the relative humidities within the predryer were generally lower than those outdoors. The temperatures within the predryer were always above those outdoors. Table 1 gives a value of 12.2 F. as the average difference between the temperature within the predryer and that outdoors. During the last 10 days, there were 4 days on which the amount of solar radiation was greater than that on any preceding day during the 105-day period. Cost One of the principal objects in these solar drying experiments was to find a more economical way than air drying of reducing the moisture content of the green oak to 20 percent. Table 2 gives a comparison of estimated costs between air drying and drying in the solar-heated predryer. The method of calculating the drying costs is explained in the footnotes of the table. It is not claimed that these costs are authentic, but they do provide a,basis for comparison. Figures that are truly representative of drying costs are not available, The loss in value caused by drying defects should be charged against the drying process, and this should work in favor of the lumber dried in the predryer. Since the lumber in the predryer is protected from the weather. there would be a resemblance to lumber dried in an open shed. Lumber piled in an open shed for air drying usually develops less drying defects and consequently tends to maintain its Table 2. -COMPARISON OF COSTS PER M BOARD FEET OF DRYING 4/4 RED OAK BETWEEN AIR DRYING AND DRYING IN THE SOLAR-HEATED PREDRYER MARCH, 1962

5 Fig. 8. -Comparison of temperatures and relative humidities outdoors Fig. 9. -Comparison of drying curves for kiln and air-dried lumber and in the predryer during drying of charge 3. with those for lumber in charge 1 dried in a solar-heated predryer. grade and value better than lumber air If 10 cents per 1,000 board feet per lumber dried in the predryer is generday is charged against air drying, then ally superior to that dried in the yard. dried in a yard. The experimental lumber was not graded, so the only index of drying defects and possible predryer for charge 1 was not short to a moisture content of 10 percent the drying period of 33 days in the The lumber of charge 1 was dried loss in value is the amount of surface enough to result in a financial saving considerably cheaper in the solarchecking. (Table 2). If 15 cents per 1,000 heated predryer than it could have The amount of surface checking for board feet per day is charged against been in a dry kiln; 27 cents 58 = red oak dried in the yard and in the air drying, then the predryer process $15.66, compared to $ = predryer is shown in the following was cheaper than air drying. $30. The 27 cents was the average cost tabulation. The starting date for each Table 2 also gives the costs of dry- per 1,000 board feet per day of preing charge 2 in the predryer compared drying, while the $1.50 represents an charge is noted, as well as the total inches of length on both faces of an to the costs of air drying lumber piled estimated cost per thousand board feet 8 1 /2 foot board. The average amount during August. Since lumber piled for per day for kiln drying. of surface checking for the lumber air drying during August does not Summary of Results dried in the yard is 63 inches, while reach a moisture content of 20 per- The drying of three charges of 4/4 the average for the lumber dried in cent before the advent of the less red oak Common lumber by solar heat the predryer is 74 inches. The lumber favorable drying weather of the fall demonstrated that the drying time to that was dried in the predryer starting months, the drying period is pro- a moisture content of 20 percent can in June 1958, however, was dried with longed. The relative freedom of sur- be reduced to about one-half that reface the vents wide open. If this one experiment checks in the lumber of charge quired to air dry the same lumber in is eliminated, then the average 2 indicates that the saving in drying a yard. It seems probable that this amount of surface checking in the cost would probably be greater than drying time ratio can be maintained lumber dried in the predryer is less that indicated in Table 2. on an average throughout all of the than that for lumber dried in the yard; Table 2, also, gives the costs of dry- months of the year. namely, 56 inches compared to 63 ing the lumber of charge 3 in the pre- The cost of the drying process using inches. dryer and corresponding costs for air solar heat is approximately equal to drying. Since the drying period in the the cost of air drying. The protection predryer was about 61 percent of that from the weather afforded by the in the yard, there was no opportunity solar-heated predryer prevents weather to show a savings on the cost of dry- staining and tends to restrict the deing by using the predryer if a basis of velopment of drying defects. 10 cents per 1,000 board feet per day The results of these experiments, for air drying is used. If 15 cents per conducted at a geographical location 1,000 board feet per day is charged where only a moderate amount of sohowever, the total lar energy is received, indicate that against air drying, cost of drying in the predryer is ap- there are places where drying lumber proximately equal to the cost of air by solar heat should be economically drying. Under this condition, drying feasible in competition with air drying. by solar heat might be more economi- The experiments also pointed out cally feasible since the quality of the the need for improvements in the design of the structure of the solarheated predryer and in operating procedures. Fig. 10 -Comportson of drying curves for kiln and air dried lumber with those for lumber in charge 2 dried in a solar heated Fig 11. -Comparison of drying curve for air-dried lumber with predryer those for lumber of charge 3 dried in a solar heated predryer FOREST PRODUCTS JOURNAL 107