Chen, Peter Y. S.

Helmer, Wayne A.

Rosen, Howard N.

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (31st : 1980)

Undetermined

ScholarsArchive@OSU

SOLAR-DEHUMIDIFICATION LUMBER DRYINGPeter Y. S. Chen andHoward N. RosenNorth Central Forest Exp. StationForest Service, USDACarbondale, IllinoisWayne A. HelmerDepartment of Thermal andEnvironmental EngineeringSouthern Illinois Univ.Carbondale, IllinoisBecause of the high cost and limited supply of oil andnatural gas, many industries are seeking ways to conserve energyand are also exploring alternative energy sources. This is par-ticularly true for those industries such as the lumber industrythat must dry their product before marketing. Lumber dryingconsumed 60 to 70 percent of the energy used in lumber processing,or approximately 7.5 x 10 13 Btu in 1972. This was about 0.1 per-cent of all the energy used in the United States in that year (7).Although solar lumber drying kilns have been designed andtested for more than 20 years, few commercial dryers are inoperation (4). Drying lumber with solar energy takes longer thanconventional kiln drying because of limited kiln temperatures(generally less than 120°F), lack of heat supply at night orduring cloudy weather, and unpredictable drying conditions due tochanges in the weather. Dehumidification-type wood dry kilns, inwhich moist air from the lumber is drawn over refrigerator coilsto condense out the water, have been in operation commercially inEurope for more than a decade but are just gaining acceptance inNorth America. Dehumidification kilns dry lumber slower (espe-cially below 20 percent moisture content) and at lowertemperatures but require less energy than conventional kilns (3).The combination of solar-dehumidification drying offersadvantages not found when each of these methods is applied sepa-rately: initial heat-up of the kiln and wet lumber charge isfaster and more economical, drying times are shorter, and dryingconditions are less dependent on the weather. Also, the effi-ciency of the solar collector is increased because of the loweredtemperature of air entering the collector. If a nonconventionalrefrigerant such as R-114 is used, the performance of the de-humidifier is improved by allowing higher operating temperatures.This paper describes the design of a solar-dehumidifier kiln,and presents the preliminary results on drying rate, power con-sumption, collector efficiency, and wood quality of 4/4yellow-poplar (Liriodendron tulipifera) lumber.Solar-Dehumidifier Kiln ConstructionDrying ChamberThe kiln has a wooden frame and inside dimensions of 8 feetby 8 feet by 6.5 feet high. The walls, ceiling, and floor areinsulated with 6 inches of Fiberglas ) wool and 2 inches of styrenefoam. The interior of the kiln is covered with an exteriorplywood that is coated with a sealer. Aluminum siding covers the51outside walls. The air inside the kiln is circulated by a 27-inchfan attached to a 1-hp motor (2).Solar CollectorsSurface area of the solar collectors is 122 square feet. Theabsorber plates are made from aluminum beer cans cut in half andpainted black. Double layer Fiberglas sheets form the coverplates. Blowers circulate the kiln air through the collectorswhen the collector outlet temperature exceeds the kiln tempera-ture (2).DehumidifierThe dehumidifier consists of a 1-1/2-hp open compressordriven by an 1,800 Rpm, 2-hp, 220-volt motor. The compressor andmotor are housed outside of the kiln so that they are not exposedto the high temperatures in the kiln (Figure 1). A check valve inthe discharge line prevents liquid from backing up into thecompressor during off cycle operation.The condenser has 2.5 square feet of face area and 193square feet of surface area and is mounted so that the kiln fanwill continually draw kiln air through it. The liquid refrigerantleaving the condenser goes through a thermo expansion valve thatproduces 10°F superheat at the compressor inlet. The evaporatoris four rows deep and has a surface area of 78 square feet and aface area of 1 square foot. Moist air from the kiln is drawnthrough the evaporator, which cools and removes the moisture fromit. Then the cool, dry air is divided into two flow streams--oneto each of the two solar collectors. The refrigerant leaving theevaporator returns to the compressors through 1/2-inch coppertubing. All refrigeration tubes are insulated to reduce heatlosses. A suction line accumulator is upstream of the compressorto reduce slugging during start-up.System ControlTwo control devices are used in the solar-dehumidifier kiln:(1) the solar collector blowers activate whenever the outlettemperature of the collector exceeds the kiln temperature and(2) the dehumidifier activates whenever the relative humidity (RH)in the kiln exceeds a preset value on the humidistat (e.g., 80percent). However, when there is no sunshine but the RH in thekiln exceeds a preset value, the blowers will be on but the airflowing through the evaporator will return to the kiln withoutgoing through the solar collectors (Figure 1). Thus, the controlof the solar collectors and the dehumidifier are independent ofeach other. A thermocouple on the compressor discharge lineshuts off the compressor motor when the refrigerant temperatureexceeds a preset value (about 225°F) to avoid damage to thecompressor discharge valve.52Materials and MethodsA 500-board-foot charge of green 4/4 yellow-poplar boardswere dried from 121 percent moisture content (MC) to 8 percent MCin a solar-dehumidifier kiln in December 1979. The boards were 8feet long, 4 or 6 inches wide, and 1 inch thick.Kiln air velocity was set at 250 feet per minute (fpm), andthe solar collector air velocity was set at 520 fpm. The humidi-stat was set from 25 to 80 percent RH depending on the MC of thewood (Table 1). An attempt was made to relieve drying stresses inthe wood by resetting the humidistat back to 80 percent RH for 14hours when the three wettest sample boards reached 15 percent MC.Boards were weighed before and after drying to calculate thetotal weight of water removed. Weights of six sample boards werechecked twice a day until the three wettest boards reached 8 per-cent MC. Twenty percent of the boards were measured for thicknessand width at three different points before and after drying tocalculate shrinkage due to drying. All boards were graded beforeand after drying to determine grade drop due to drying. Afterkiln drying, those 20 percent randomly selected boards weremeasured for crook, bow, and twist, and all boards were inspectedfor surface checking, end checking, and collapse. Two stress pinswere cut from each of the six moisture sample boards and from eachof six randomly selected boards to examine the degree of dryingstresses.The electric energy consumed by the compressor motor for thedehumidifier was recorded by a watt-hour meter. The power con-sumption rates of the collector blowers and the circulation fanwere measured by an ampere meter. The hours of operation of thefan and blowers were automatically recorded.The ambient temperatures (both dry and wet bulb), the kilntemperatures (both dry and wet bulb), and the inlet and the outlettemperatures of solar collectors were recorded at 10-minuteintervals. Also, solar radiation was recorded throughout the runby a pyranometer mounted on the solar collectors.Results and DiscussionThe time required to dry green 4/4 yellow-poplar lumber from121 percent MC to 8 percent MC in the solar-dehumidifier kiln was18 days during December 1979. This was approximately twice asfast as drying in a solar kiln during the winter months of theyear (6) but was only half as fast as drying in a conventionalsteam- or gas-heated kiln (5). Drying from green to 20 percentMC took 12 days, during which the drying rate was constant atabout 8 percent MC/day (Figure 2). Then the drying rate sloweddown considerably from 20 percent MC to 8 percent MC. Twoapparent causes contributed to the decrease in drying rate duringthis period. First, the humidistat was set back to 80 percent RHfor 14 hours between days 13 and 14 to equalize and therebyrelieve drying stresses in the wood (Table 1). Second, thedehumidifier was shut down due to a refrigerant leak for 21 hoursbetween days 15 and 16, which greatly reduced the drying rateduring that period. The average kiln temperature during the runwas only 94°F because two-thirds of the days were cloudy. This53low kiln temperature reduced the dehumidifier's capacity to removewater from the wood and, thus, slowed down the drying rate duringthe entire run. The dehumidifier only removed 4.0 lb of water/hrduring the run but it is capable of removing 20 lb/hr at 180°F.The efficiency of the solar collector during sunny days wasabout 43 percent, which was expected because the air velocitythrough the collectors was reduced to decrease power consumptionand to increase the outlet temperature.The quality of solar-dehumidifier dried wood was excellentand the light color of the lumber was aesthetically pleasing.No collapse, surface checking, or end checking was noted andvolumetric shrinkage was similar to solar kiln-dried but 20 per-cent less than conventional kiln-dried yellow-poplar (Table 2).The dry warpage was slightly less than solar and slightly morethan conventional kiln-dried yellow-poplar. Drying degradationwas less than solar drying. The 14-hour reversal of RH in thekiln to 80 percent did not remove the moderate drying stressesthat occurred in the solar-dehumidifier dried wood (Figure 3).Although the run consumed approximately three-fourths of thepower compared to a conventional kiln (1), it consumed the sameamount of power as a solar kiln (6). High air velocity across thecondenser and continuous running of the kiln circulation fanaccounted for about 40 percent of the total power consumption.Future PlansWe plan to test at least one run per season for 1 year toexamine the affect of season on the drying time and rate of 4/4yellow-poplar. We expect a better performance of the dehumidifierin spring through fall. Operating at a higher kiln temperaturewill increase the capacity of the dehumidifier to remove water aswell as increase the drying rate of the lumber.We also plan to try to reduce power consumption throughbetter control of blowers and the circulation fan.Footnote'Mention of trade names does not constitute endorsement ofthe products by the USDA Forest Service.Literature Cited1. Cech, M. Y., and Pfaff, F. 1978. Dehumidification dryingof spruce studs. Forest Prod. J. 28(3):22-26.2. Chen, P. Y. S., and Rosen, H. N. 1979. Drying yellow-poplarin a highly efficient solar kiln, In Proceedings AnnualWestern Dry Kiln Clubs Meeting, Coeur d'Alene, Idaho, May 10,1979. p. 23-32.3. Huber, H. A. 1977. Dehumidification drying of wood. Wood-working and Furniture Digest 79(5):51-52.544. McMillen, J. M., and Wengert, E. M. 1978. Drying easternhardwood lumber. Forest Products Laboratory, USDA ForestService, Madison, Wisconsin. Agric. Handb. No. 528, p. 70.5. Rosen, H. N. 1977. Impinging air jets and high temperaturesto dry lumber. In Proceedings of 5th Annual Hardwoods Sympo-sium of the Hardwood Research Council, Asheville, NorthCarolina. p. 18-27.6. Rosen, H. N., and Chen, P. Y. S. 1980. Drying lumber in akiln with external solar collectors. AIChE Symposium Series(In press).7. Skaar, C. 1977. Energy requirements for drying lumber. InPractical Application of Solar Energy to Wood Processing,Workshop Proceedings, Virginia Polytechnic Institute andState University, Blacksburg, Virginia. p. 29-32.55Table 1.--Drying conditions and moisture content (MC) of solar-dehumidifier dried 4/4 yellow-poplar lumber Elapsed	 :time	 :RH	 :setting	 :PercentDrying chamberMinimum	 :daily RH : InsolationPercent	 Btu/ft2-dayMC of	 •	 Maximum	 :lumber	 :	 daily temperature : Percent	 °FDays0 80 121 85 66 1,6811 80 113 103 56 1,0132 80 102 88 70 1003 80 96 71 72 4594 80 91 101 65 1,9325 80 79 95 60 1,8486 80 69 77 66 4917 80 63 92 58 1,4988 80 55 98 68 1,8849 75 47 108 65 1,79410 65 37 110 57 1,76311 35 27 89 52 9312 251/ 20 91 44 8013 80- 15 100 41 9514 25 13 98 41 772/15-- 25 11 86 38 8016— 25 10 105 46 1,22817 25 9 94 37 15218 25 8 95 37 486—/ Humidistat was set back to 80 percent RH for 14 hours.12/Table 2.--Comparison of properties of solar-dehumidifier,solar (6), and conventional kiln-dried (5) 4/4 yellow-poplarPropertySolar-	 :: dehumidifier :12180.080.110.19%	 9.63.0Solar	 :kiln	 :99140.170.130.199.2 1/5.0Conventionalkiln10280.090.050.0312.1Moisture content, SGreenDryDry warpage, inchesTwistBowCrookVolumetric shrinkage,Grade drop, 51/Corrected  to 8 percent moisture content.— Dehumidifier inoperational for 21 hours.56MOTORREFRIGERANTSUCTION LINEFigure 1.--Solar-dehumidification lumber drying kiln.1201008060402002	 4	 6	 8	 10	 12	 14	 16	 18DRYING TIME (Days)Figure 2.--Drying curve for solar-dehumidification drying of 4/4 yellow-poplar during December 1979 in southern Illinois.57Figure 3.--Demonstration of internal stresses remaining in solar-dehumidifier dried lumber.

Solar-dehumidification lumber drying

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Solar-dehumidification lumber drying

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