Solar kiln

Abstract

The importance of having timber of the correct moisture content for a specific job in a building may be summarised as follows: a. to reduce moisture content; b. to allow easier and more efficient application of finish - paint, polishes, etc.; c. to allow more effective treatment with a preservative. It is therefore highly essential that users of timber should have some knowledge of what happens to dry timber when received on site and afterwards in use. This introduction thus mainly stresses the advantages of kiln-dried timber, and the need for specifying the correct moisture content of timber for each application. (N.B. Moisture content = mass of water per unit mass of dry timber). Why Kiln-Dried Timber: three major factors influence the seasoning of timber, namely: a. heat; b. air circulation; c. humidity. Where timber is seasoned in the open air, these are supplied by the sun, normal movement of the air assisted by the method of stacking, and the amount of moisture present in the air. Because of seasonal variations, timber which is seasoned outdoors varies in its final moisture content from about 15% in summer to about 22% in winter, averaging about 18% overall. Also, some years ago, it was usual for timber merchants to stack timber which had been in their yards for sufficient time allow the moisture content to have become reduced to the seasonal average. Today, such stacking is less common, so much greater care is required in the purchase and selection of timber, whether in constructional work or in joinery. Moreover, with air-conditioning and central heating in schools, hotels, offices and homes, timber is subjected to far more stringent conditions than it meets in the open air. With higher temperatures and lower humidities, this timber continues to reduce moisture, and as loss of moisture is coupled with the loss in volume, shrinkage of timber takes place, thus tending to cause the opening of joints and splitting. The answer is kiln-drying. Moisture-content Equilibrium: Timer will always reach a point of stability with the atmosphere in which it is situated. For example, if the temperature of the air is maintained constant at, say 27°C, and the humidity is held first at 40°C, a piece of timber would attain a moisture content of about 9%. If the humidity is then altered to 70%, the moisture content would rise to 15%, i.e. a change of 6% for a perfectly normal fluctuation in humidity. Therefore to obtain maximum efficiency from the use of timber, it is important that it be dried to a specific moisture content for each application. [...] It is highly essential, therefore, that architects and builders specify these moisture contents when ordering timber. [...] Moisture content equilibrium varies considerably with its function in the building. Interior flooring and wood-block flooring are examples of uses where the correct moisture content is important if ugly open joints, splits or worse troubles are to be avoided. Not all species of timber reach the same equilibrium moisture content under given conditions. A few species have slightly lower moisture content for such purposes as timber joinery and hardwood flooring. [...] Woodwork for interior use should be seasoned to a moisture content of about 12%. If installed in a new building still far from dry, it will absorb moisture and swell. The condition may be easily so damp that is would be quite possible for some of the timber installed to obtain the moisture content approaching 20%. It may do one of the two things, or both. It may attempt to swell or bulge. If moisture is reabsorbed to something approaching 20%, the timber will eventually dry out at about 12%, and this will give rise to badly fitting joints, etc. In either case, it means permanent damage. Thus, the importance of kiln-dried timber can now be appreciated. Two types of kilns are most commonly in use today: a. the traditional high-temperature kiln; b. the low-temperature kiln. Both types of kilns have the disadvantages of high capital investment and high fuel consumption. Bearing this in mind, an attempt is to be made to design and construct a solar kiln, i.e. a kiln whose heating and drying effect is derived almost entirely from the energy of the sun. It is required that this kiln be comparable in terms of drying capacity and quality of dried product to the conventional kilns. At the end of this report is included a feasibility report to study the economic viability of such a project, namely, the solar kiln.