|Journal of the Network of African Countries on Local Building Materials and Technologies - Volume 2, Number 3 (HABITAT, 1993, 42 p.)|
* Submitted by Central Building Research Institute (CBRI). Roorkee, India.
Solar energy could easily be used in air-seasoning of timber. Much heat is however, lost by radiation from the surface that absorbs it. It could be utilized more effectively if the heat energy could be entrapped inside a chamber. In this way, continuously transmitted heat is available for the drying process. This can be achieved by preparing a chamber with transparent glass plates. Such a kiln has been designed in the Central Building Research Institute, Roorkee, India (see figure 1). It consists of three main parts, viz., (a) solar-energy collector, (b) seasoning chamber, and (c) chimney.
Black-painted galvanized-iron sheeting of 22 SWG is used as solar energy collector. With the help of a wooden frame, a transparent glass sheet is fitted around the collector leaving an air gap of 5 cm for the movement of fresh air. It is attached to the bottom of the south wall of the seasoning chamber at an angle of 30° with the horizon.
A double-wall chamber is constructed with transparent glass except for the north wall which is of brick masonry. Black-painted aluminium fins of 24 SWG are fitted in the east, west and south walls at an angle of 45° with the horizon. The roof is made of black-painted corrugated galvanized-iron sheeting and has a slope of 1 in 3.
To provide a stack effect inside the seasoning chamber, a chimney (30 x 30 x 80 cm) is fitted vertically over an opening provided in one of the corners of the roof of the seasoning chamber.
Performance of the solar kiln
Performance of the solar kiln was studied in India by seasoning various species of wood, e.g., mango (Mangifera indica), haldu (Adina cardifolia), deodar (Cedrus deodara), teak (Tactona grandis), jamun (Eugenis jambolana), shisham (Dalbergia sissoo) and sal (Shorea robusta).
The seasoning of mango, jamun and haldu timber was carried out in the months of August and September. The time taken in seasoning from green stage to 10 per cent moisture content for the above three of thickness 3.75 cm in the solar kiln was found to be 17, 27 and 18 days respectively while in the shed it was 35, 62 and 40 days. Teak, deodar and shisham timber was seasoned in the month of February and March and the thickness of the planks was 5.0 cm. The time taken in seasoning by the respective woods in the kiln was found to be 20, 14 and 28 days while in a shed they took 42, 25 and 60 days. Sal wood is considered highly refractory from a seasoning point of view and is generally used in the form of scantling. Therefore, the size taken for study was 7.5 x 15 cm and the study was carried out from November to January. It was found that in the kiln it took 67 days while in the shed the seasoning was completed in 4 months. On average, it can be concluded that the time taken for seasoning in the kiln was about half of that taken in air seasoning.
The high temperature inside the kiln helped in lowering its relative humidity. However, in the initial stages it was somewhat more than the prevailing atmospheric relative humidity due to a quick decrease of moisture content. During rainfall, the moisture content of the plank kept inside the shed increased due to high relative humidity of the atmosphere. However, in the kiln there was no appreciable change as the entry of fresh air was prevented. The maximum difference of temperature inside and outside the kiln was 15°C.
Normal practice in air-seasoning is to keep the timber without proper stacking on an uneven platform in the field or under a tree or shed. In these situations the material cannot be protected properly from sun rays and rain showers and neither can the direction of air flow be regulated. Thus no proper air-seasoning can be achieved in this manner, besides the increase in time of seasoning. The seasoning losses such as warping, cracking or shrinkage are also high in such a situation and sometimes the material is not of any use except as fire wood.
Capacity and cost of construction of a solar kiln
The kilns designed in India were of two different capacities, viz., 3 and 15 cubic metres of wood. The cost of construction of the smaller kiln is about US$ 1600 while that of the larger one is about US$4200 (1989 price quotations).