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Construction and evaluation of a timber-drying solar kiln
Bioresource Technology 52 (1995) 283-285
ELSEVIER
0960-8524(95)00030-5
Elsevier Science Limited Printed in Great Britain 0960-8524/95/£9.50
Short Communication
about 185 W/m2/day of insolation (Simpson & Tschernitz, 1980). Solar-kiln drying gives a higher quality of seasoned timber than open-air seasoning (Johnson, 1961). The main types of solar kiln include: the greenhouse type (Plumptre, 1979), semi-green-house (Young, 1979) and solar kiln with external collector (Tschernitz & Simpson, 1979; Off, 1982). There are other modified designs of solar kiln: semi-greenhouse kiln with insulated sides (Lumley & Chong, 1979), aluminium frame demountable green-house (Plumptre, 1983), solar kiln fitted with dehumidifier (Rosen & Chen, 1980). Some of these solar kilns have well insulated structures with sophisticated control of air circulation and humidity. The objective of this work was to construct and test the performance of a simple solar kiln designed for small-scale sawmills in Nigeria and other tropical countries.
Construction and Evaluation of a Timber-drying Solar Kiln Abstract A solar timber kiln with the capacity to season 3"24 m 3 of lumber was designed, constructed and used for seasoning commercial sawnwood. The kiln was constructed of a timber frame covered with transparent polythene sheet on the top and black leatherette on the sides. Corrugated, galvanized-iron sheet painted black and tilted at 30 ° to the horizontal served as a heat collector. Two fans were fitted for air circulation. The kiln attained a maximum temperature of 24 ° above the ambient temperature. The average efficiency of the heat collector was 38"5%. The solar kiln reduced timber drying time by 33.3-57"1% when compared with an air-drying technique. Sawnwoods (25 mm by 300 mm by 360 ram) of Mansonia altissima and Terminalia superba were dried from 46.16 to 15.02% and from 52 to 15.4% moisture content respectively in 12 days.
METHODS Design and construction The greenhouse type of solar kiln was designed with the collector fused within the drying chamber. The kiln's frame was constructed with timber and covered a land area of 16.65 m 2. The kiln's floor was covered by a 5 mm-thick layer of dark-coloured gravel which ensured heat retention in the kiln. The roof truss and rafters used in constructing the kiln were assembled as shown in Figs 1 and 2. The structure was covered by black, corrugated, sheet on the timber-stack part of the top and with transparent polythene sheet in the solar collector section. The collector was tilted at an angle of 30° to the horizontal to allow for maximum hours (about 6/day) of sun-ray interception, according to the stipulation for a fiat-plate collector (Duffle & Beekman, 1974). The short waves of solar radiation that passed through the transparent polythene sheet were converted to long waves by the black-painted collector. The heated air was then transmitted through the timber stack by convection enhanced by air flow from the fans (2 x 185W motor). The fans were operated for 10 h, between 8 a.m. and 6 p.m. every day. Air intake into the kiln was provided by a vent at the lower end of the collector. The air inlet was opened for 10 h during the day. The vents at the
Key words: Solar kiln, solar collector, seasoning, sawmill, lumbers, Mansonia altissima, Terminalia superba. INTRODUCTION Adequately dried timbers are in short supply in most developing countries because the small-scale sawmills do not have the appropriate facilities for wood seasoning. In most Nigerian sawmills, wood is dried in the open air using the natural draught to effect drying. This method is very slow and frequently results in highly degraded timber. The conventional kilns (compartment and progressive kilns) are very expensive to operate and are used by very few largescale sawmills. Solar kilns, which are relatively cheap to construct, may alleviate the problem of timber drying in small-scale sawmills in tropical countries (Plumptre, 1983). The use of solar energy for drying wood in the tropical latitudes is feasible because at least 6 h of sunshine are received everyday at 500-800 W/m 2 of radiation per hour, resulting in *Author to whom correspondence should be addressed. 283
Short communication
284 BInck
Air outlet
/~ I
,
,
\~__
z
%_
SoW collector
..2 ~
Fan assembLy
Fig. I. Side view of timber solar kiln. rear (end) of the kiln provided an outlet for water vapour. The actual dimensions of the kiln were: Side Front Low side Ridge High side Door leaf
3725 4500 600 2000 1750 800
mm mm mm mm mm mm (wide) × 1400 mm (high)
Evaluation of the idin's performance The assessment of the kiln's performance was based on the efficiency of the solar collector and the capacity of the kiln to dry wood to a required moisture content at a short time interval when compared with the open-air drying method. The solar collector's efficiency was calculated by measuring the kiln's temperature, ambient temperature, surface area of the collector and radiation intensity, and applying the values obtained in the equation: E-
tK--tA - x 100 AcI
where E = collector efficiency (%); A c = surface area of collector (mZ); I = radiation intensity (W/ m2); tK = kiln temperature; ta = ambient temperature. A tube solarimeter (Pyranometer) was used to monitor solar insolation, while the readings from wet- and dry-bulb thermometers in the kiln and in the open air were used in calculating relative humidity. Drying of lumber or lumber drying-rate The solar kiln and the open-air shed were stacked with sawnwood of Terminalia superba and Mansonia altissima. The initial moisture contents of the wood species were determined by an oven-drying method. Six samples were collected from each plank at inter-
vals of 3 days for the determination of moisture content (MC). Statistical analysis of variance was carried out to test for significant differences and interactions between the rate of drying of species, drying method and drying time. RESULTS AND DISCUSSION Efficiency of collector The efficiency of the collector was about 38.5% but varied considerably with solar radiation as illustrated in Table 1. An earlier observation was that the energy received on the collector's surface was affected by its tilt and orientation to direct impact of sun rays (Off, 1982). The efficiency of the kiln was low in the early hours of the morning, but increased as the radiation intensity increased between 11 a.m. and 12 noon. The collector's efficiency was directly proportional to the radiation intensity, and the solar intensity was highly correlated with the temperature in the kiln. Drying factors A maximum temperature of 54°C was recorded in the kiln at 2 p.m., this was 24°C higher than the corresponding ambient temperature (Table 1) and better than the report that the Griffith kiln had 22°C above the ambient (Simpson et al., 1979). Statistical tests showed that the temperature in the kiln was significantly higher than that of the open shed, and the kiln's temperature was positively correlated (0.88) with the solar insolation. The kiln retained some heat, with the temperature ranging from 29 to 35°C, after active insolation ceased at 6 p.m. This was probably due to the gravel which stored heat and the black covering which conserved the heat in the kiln. The relative humidity in the kiln also varied with the temperature in the kiln but was regulated by forcing circulating air through the wood-stack with
Short communication
285
Table 1. Average temperature (TR), relative humidity (RH) and solar insolation (SI) in the solar kiln
Time
8 a.m. 10 a.m. 12 p.m. 2 p.m. 4 p.m. 6 p.m.
Solar kiln
Ambient
Temp. °C
RH%
SIK (W./i 2)
SIK (MV)
Temp. °C
RH%
SIA W/m2
SIA (MV)
27 38 43 54 41 35
88 71 63 62 71 86
2.31 3.33 7.81 9.22 5.25 2.69
0.18 0.26 0.61 0.72 0.41 0-21
26 29 30 30 27 26
88 76 67 69 77 86
1-73 1.88 2.35 3.59 0.99 0.62
0.14 0-15 0.19 0.29 0.08 0.05
Table 2. Comparative decrease in moisture content (%) of sawnwood in solar kiln and open shed
Period (days)
Average moisture content (%)
Mansonia altissima
1 3 6 9 12 15
Terminalia superba
Solar kiln
Drying shed
Solar kiln
Drying shed
46.16 38.06 18.63 16.31 15.02 14.56
47.00 46.00 29-00 23.00 20.10 20.00
52.23 44.16 25.85 19.11 15.79 15.14
52.23 45.03 25.73 24.75 18.89 18.59
the fans, whilst the air-outlet vents in the kiln were opened between 2 and 4 p.m. everyday. Drying rate of sawnwood
The sawnwood species inside the kiln dried faster than those in the open shed, as shown in Table 2. The solar kiln was found to be better than the open-shed drying method. The timber drying-rate was found to be statistically higher in the solar kiln than the open shed. The solar kiln reduced the drying time of Terrninalia superba and Mansonia altissima by a third when compared with the open-shed during the wet season (May-July). The average efficiency of the kiln was 38.5%; this is reasonably high when compared with earlier reports of dryers (Off, 1982; Duffle et al., 1974).
REFERENCES
Duffle, Y. A. & Beckman, W. A. (1974). Solar Energy Thermal Process. John Wiley & Sons, New York, pp. 1-386. Johnson, C. L. (1961). Wind-powered solar-heated lumber dryer. Southern Lumerman, 203 (25), 41-4. Lumley, T. C. & Chong, E. T. (1979). Technical and economic characteristics of two solar kiln designs. Forest Products J., 29 (7), 49-56.
Off, O. (1982). Construction and evaluation of a solar dryer. Nigerian J. Solar Energy, 2, 47-59. Plumptre, R. A. (1983). Some thoughts on design and control of solar timber kilns. Paper presented to Wood Drying Workshop of IUFRO Division V Conference, Madison, WI. Plumptre, R. A. (1979). Simple solar-heated timber dryers, design, performance and commercial viability. Commonwealth Forestry Rev., 58 (4), 243-51. Rosen, H. N. & Chen, P. Y. S. (1980). Drying lumber in a kiln with external solar collectors. Am. Inst. Chem. Engng Symp. Series, 200 (16), 82-9. Simpson, W. T. & Tehernitz, J. L. (1980). Design and performance of a solar lumber dryer for tropical latitudes. Proc. Wood Drying Party, IUFRO Division V Conf., Oxford, UK, pp. 59-70. Tschernitz, J. L. & Simpson, W. T. (1979). Solar-heated forced-air lumber dryer for tropical latitude. Solar Energy, 22, 563-6. Young, K. C. (1979). Solar kiln performance at high latitudes 48°N. Forest Products J., 30 (3), 37-41.
Ibiyinka A. Fuwape* & Joseph A. Fuwape Federal University of Technology P.M.B. 704, Akure, Nigeria (Received 20 September 1994; revised version received 22 February 1995; accepted 2 March 1995) *Author to whom correspondence should be addressed.
ELSEVIER
0960-8524(95)00030-5
Elsevier Science Limited Printed in Great Britain 0960-8524/95/£9.50
Short Communication
about 185 W/m2/day of insolation (Simpson & Tschernitz, 1980). Solar-kiln drying gives a higher quality of seasoned timber than open-air seasoning (Johnson, 1961). The main types of solar kiln include: the greenhouse type (Plumptre, 1979), semi-green-house (Young, 1979) and solar kiln with external collector (Tschernitz & Simpson, 1979; Off, 1982). There are other modified designs of solar kiln: semi-greenhouse kiln with insulated sides (Lumley & Chong, 1979), aluminium frame demountable green-house (Plumptre, 1983), solar kiln fitted with dehumidifier (Rosen & Chen, 1980). Some of these solar kilns have well insulated structures with sophisticated control of air circulation and humidity. The objective of this work was to construct and test the performance of a simple solar kiln designed for small-scale sawmills in Nigeria and other tropical countries.
Construction and Evaluation of a Timber-drying Solar Kiln Abstract A solar timber kiln with the capacity to season 3"24 m 3 of lumber was designed, constructed and used for seasoning commercial sawnwood. The kiln was constructed of a timber frame covered with transparent polythene sheet on the top and black leatherette on the sides. Corrugated, galvanized-iron sheet painted black and tilted at 30 ° to the horizontal served as a heat collector. Two fans were fitted for air circulation. The kiln attained a maximum temperature of 24 ° above the ambient temperature. The average efficiency of the heat collector was 38"5%. The solar kiln reduced timber drying time by 33.3-57"1% when compared with an air-drying technique. Sawnwoods (25 mm by 300 mm by 360 ram) of Mansonia altissima and Terminalia superba were dried from 46.16 to 15.02% and from 52 to 15.4% moisture content respectively in 12 days.
METHODS Design and construction The greenhouse type of solar kiln was designed with the collector fused within the drying chamber. The kiln's frame was constructed with timber and covered a land area of 16.65 m 2. The kiln's floor was covered by a 5 mm-thick layer of dark-coloured gravel which ensured heat retention in the kiln. The roof truss and rafters used in constructing the kiln were assembled as shown in Figs 1 and 2. The structure was covered by black, corrugated, sheet on the timber-stack part of the top and with transparent polythene sheet in the solar collector section. The collector was tilted at an angle of 30° to the horizontal to allow for maximum hours (about 6/day) of sun-ray interception, according to the stipulation for a fiat-plate collector (Duffle & Beekman, 1974). The short waves of solar radiation that passed through the transparent polythene sheet were converted to long waves by the black-painted collector. The heated air was then transmitted through the timber stack by convection enhanced by air flow from the fans (2 x 185W motor). The fans were operated for 10 h, between 8 a.m. and 6 p.m. every day. Air intake into the kiln was provided by a vent at the lower end of the collector. The air inlet was opened for 10 h during the day. The vents at the
Key words: Solar kiln, solar collector, seasoning, sawmill, lumbers, Mansonia altissima, Terminalia superba. INTRODUCTION Adequately dried timbers are in short supply in most developing countries because the small-scale sawmills do not have the appropriate facilities for wood seasoning. In most Nigerian sawmills, wood is dried in the open air using the natural draught to effect drying. This method is very slow and frequently results in highly degraded timber. The conventional kilns (compartment and progressive kilns) are very expensive to operate and are used by very few largescale sawmills. Solar kilns, which are relatively cheap to construct, may alleviate the problem of timber drying in small-scale sawmills in tropical countries (Plumptre, 1983). The use of solar energy for drying wood in the tropical latitudes is feasible because at least 6 h of sunshine are received everyday at 500-800 W/m 2 of radiation per hour, resulting in *Author to whom correspondence should be addressed. 283
Short communication
284 BInck
Air outlet
/~ I
,
,
\~__
z
%_
SoW collector
..2 ~
Fan assembLy
Fig. I. Side view of timber solar kiln. rear (end) of the kiln provided an outlet for water vapour. The actual dimensions of the kiln were: Side Front Low side Ridge High side Door leaf
3725 4500 600 2000 1750 800
mm mm mm mm mm mm (wide) × 1400 mm (high)
Evaluation of the idin's performance The assessment of the kiln's performance was based on the efficiency of the solar collector and the capacity of the kiln to dry wood to a required moisture content at a short time interval when compared with the open-air drying method. The solar collector's efficiency was calculated by measuring the kiln's temperature, ambient temperature, surface area of the collector and radiation intensity, and applying the values obtained in the equation: E-
tK--tA - x 100 AcI
where E = collector efficiency (%); A c = surface area of collector (mZ); I = radiation intensity (W/ m2); tK = kiln temperature; ta = ambient temperature. A tube solarimeter (Pyranometer) was used to monitor solar insolation, while the readings from wet- and dry-bulb thermometers in the kiln and in the open air were used in calculating relative humidity. Drying of lumber or lumber drying-rate The solar kiln and the open-air shed were stacked with sawnwood of Terminalia superba and Mansonia altissima. The initial moisture contents of the wood species were determined by an oven-drying method. Six samples were collected from each plank at inter-
vals of 3 days for the determination of moisture content (MC). Statistical analysis of variance was carried out to test for significant differences and interactions between the rate of drying of species, drying method and drying time. RESULTS AND DISCUSSION Efficiency of collector The efficiency of the collector was about 38.5% but varied considerably with solar radiation as illustrated in Table 1. An earlier observation was that the energy received on the collector's surface was affected by its tilt and orientation to direct impact of sun rays (Off, 1982). The efficiency of the kiln was low in the early hours of the morning, but increased as the radiation intensity increased between 11 a.m. and 12 noon. The collector's efficiency was directly proportional to the radiation intensity, and the solar intensity was highly correlated with the temperature in the kiln. Drying factors A maximum temperature of 54°C was recorded in the kiln at 2 p.m., this was 24°C higher than the corresponding ambient temperature (Table 1) and better than the report that the Griffith kiln had 22°C above the ambient (Simpson et al., 1979). Statistical tests showed that the temperature in the kiln was significantly higher than that of the open shed, and the kiln's temperature was positively correlated (0.88) with the solar insolation. The kiln retained some heat, with the temperature ranging from 29 to 35°C, after active insolation ceased at 6 p.m. This was probably due to the gravel which stored heat and the black covering which conserved the heat in the kiln. The relative humidity in the kiln also varied with the temperature in the kiln but was regulated by forcing circulating air through the wood-stack with
Short communication
285
Table 1. Average temperature (TR), relative humidity (RH) and solar insolation (SI) in the solar kiln
Time
8 a.m. 10 a.m. 12 p.m. 2 p.m. 4 p.m. 6 p.m.
Solar kiln
Ambient
Temp. °C
RH%
SIK (W./i 2)
SIK (MV)
Temp. °C
RH%
SIA W/m2
SIA (MV)
27 38 43 54 41 35
88 71 63 62 71 86
2.31 3.33 7.81 9.22 5.25 2.69
0.18 0.26 0.61 0.72 0.41 0-21
26 29 30 30 27 26
88 76 67 69 77 86
1-73 1.88 2.35 3.59 0.99 0.62
0.14 0-15 0.19 0.29 0.08 0.05
Table 2. Comparative decrease in moisture content (%) of sawnwood in solar kiln and open shed
Period (days)
Average moisture content (%)
Mansonia altissima
1 3 6 9 12 15
Terminalia superba
Solar kiln
Drying shed
Solar kiln
Drying shed
46.16 38.06 18.63 16.31 15.02 14.56
47.00 46.00 29-00 23.00 20.10 20.00
52.23 44.16 25.85 19.11 15.79 15.14
52.23 45.03 25.73 24.75 18.89 18.59
the fans, whilst the air-outlet vents in the kiln were opened between 2 and 4 p.m. everyday. Drying rate of sawnwood
The sawnwood species inside the kiln dried faster than those in the open shed, as shown in Table 2. The solar kiln was found to be better than the open-shed drying method. The timber drying-rate was found to be statistically higher in the solar kiln than the open shed. The solar kiln reduced the drying time of Terrninalia superba and Mansonia altissima by a third when compared with the open-shed during the wet season (May-July). The average efficiency of the kiln was 38.5%; this is reasonably high when compared with earlier reports of dryers (Off, 1982; Duffle et al., 1974).
REFERENCES
Duffle, Y. A. & Beckman, W. A. (1974). Solar Energy Thermal Process. John Wiley & Sons, New York, pp. 1-386. Johnson, C. L. (1961). Wind-powered solar-heated lumber dryer. Southern Lumerman, 203 (25), 41-4. Lumley, T. C. & Chong, E. T. (1979). Technical and economic characteristics of two solar kiln designs. Forest Products J., 29 (7), 49-56.
Off, O. (1982). Construction and evaluation of a solar dryer. Nigerian J. Solar Energy, 2, 47-59. Plumptre, R. A. (1983). Some thoughts on design and control of solar timber kilns. Paper presented to Wood Drying Workshop of IUFRO Division V Conference, Madison, WI. Plumptre, R. A. (1979). Simple solar-heated timber dryers, design, performance and commercial viability. Commonwealth Forestry Rev., 58 (4), 243-51. Rosen, H. N. & Chen, P. Y. S. (1980). Drying lumber in a kiln with external solar collectors. Am. Inst. Chem. Engng Symp. Series, 200 (16), 82-9. Simpson, W. T. & Tehernitz, J. L. (1980). Design and performance of a solar lumber dryer for tropical latitudes. Proc. Wood Drying Party, IUFRO Division V Conf., Oxford, UK, pp. 59-70. Tschernitz, J. L. & Simpson, W. T. (1979). Solar-heated forced-air lumber dryer for tropical latitude. Solar Energy, 22, 563-6. Young, K. C. (1979). Solar kiln performance at high latitudes 48°N. Forest Products J., 30 (3), 37-41.
Ibiyinka A. Fuwape* & Joseph A. Fuwape Federal University of Technology P.M.B. 704, Akure, Nigeria (Received 20 September 1994; revised version received 22 February 1995; accepted 2 March 1995) *Author to whom correspondence should be addressed.