Investigation of the Responses of Acacia nilotica (sunt) Wood to Air Drying and Solar Drying

Abstract

For many end uses, wood must be formally dried under control to perform satisfactorily. If formal drying is not carried out before the final product is made serious defects will occur in service, often with disastrous result. Wood drying, however, is the most energy intensive of all wood manufacturing processes. Water is removed from wood by supplying a large amount of thermal energy. The different drying methods differ from one another by the source and the methods of supplying the required thermal energy. Solar energy applications began to look more attractive in the past decades. Air (natural) drying is simple and cheap, but it takes a long time and dries timber to equilibrium moisture content only. Kiln drying, on the other hand, is faster and dries wood to any moisture content but is very expensive to install and operate and requires skilled operators. Solar drying is expected to be in the middle and have most of the advantages of the two traditional methods mentioned above. The aim of this investigation is, therefore, to search for relatively simple, cost effective and energy-efficient solar dryers which will speed up air drying with minimum wood degradation. To achieve this goal an air dryer and two greenhouses type solar dryers were constructed. The designs of the two solar dryers were different with regards to the locations and orientation of the collectors, the fans and vents. The three different dryers were constructed at Suki sawmill, Sennar State. They were of wooden frame construction. All walls and roofs of the two solar dryers were made of one layer of transparent plastic sheet. The collectors in the solar dryers consisted of black-painted corrugated zinc absorber plates. The air dryer was a shed with corrugated zinc roof to shade the wood stack but with no side walls in order to facilitate the flow of natural air into the stack. The first solar dryer denoted (S H) had a total area of 4 by 4 meters. The collector was placed above the timber stack and below the roof. The area of the collector was approximately 12 square meters. The second solar dryer had a low flat collector placed on the dryers floor on the northern side of the timber stack and was denoted (SL). The area of the collector was only 12 square meters. The total area of this dryer was 4 by 6 square meters, running north/south. The research program consisted of two trials (Charges), one in summer and the other in winter. All stacks in all dryers and seasons had east/west orientation. In both charges the stacks in the three dryers consisted of 80 ( 2" x 4" x 10' ) sunt (Acacia nilotica) boards each. They were stacked in 10 rows with 8 boards in each row. The stickers between the rows were 1.5 inch thick. Three sample boards were selected in each stack for periodic weighing an m.c determination. The initial weight and initial m.c. of each board were recorded and the expected dry weight calculated. Each board was taken out of the stack every three days, weighed and returned to the stack. The dry-bulb and wet-bulb temperatures were obtained and the relative humidity worked out. The moisture content was calculated from weights obtained. In the summer charge solar dryer with high collector (SH) had an average initial m.c. (39.1%) which was approximately equal to that of the air dryer (39%), but the average final m.c. in SH (9.5%) was significantly lower than that of air drying (12.3 %) This was due to the significantly higher temperature in SH and may also be due to the air flow caused by the fans and the location of the vents. However, solar dryer (SL) started with a lower initial m.c. (35.4%), and reached a lower final m.c. (11.2%) than air drying. According to the environmental conditions in the two solar dryers, SH, with lower average temperature and higher average initial m.c. should have had a higher average final m.c. than SL, but the former ended up with a lower average final m.c. than the latter. This may mean that the circulation of the heated air in SL was not directed properly, which in turn may indicate that the orientation and location of the fans and vents should be adjusted in the coming trials. In the winter charge the progress of drying of samples with initial m.c higher than fiber saturation point in each dryer was followed. The samples comprised the following: Sample B in air drying showed the slowest rate of drying and ended up to 16.2% final m.c after 30 days. Sample A in SH reached a final m.c. of 8.5%, while sample A of SL had 12.3% final m.c. By looking at the samples with the lowest initial m.c in each dryer (sample C in all three dryers) we got a rough estimate of the equilibrium m.c. (EMC) under all three conditions. The EMC of Suki area (from sample C in air drying) was about 8.9%. In solar dryer (SH) EMC was 3.7%, whereas in solar dryer SL it was 6.3%. These results also indicate that the average final m.c. was lowest in case of SH (5.6%), followed by SL (10.4%), and highest in case of air drying (12.6%) this means that the EMC for Suki area in winter is about 8.9% and that the two solar dryers can dry timber to m.c. lower than the equilibrium m.c. of the area.