EFFECT OF ENVIRONMENTAL CONDITIONS OF TWO NATURAL WOOD SEASONING MEDIA
CHAPTER ONE
INTRODUCTION
1.1 Background
Wood is a natural versatile and variable material. There is an urgent need to recognize the virtues and develop means of protecting and rationalizing the use of such a valuable resource. Winger,(1977) stated that When used for construction, furniture millwork and other similar uses wood will perform better if it is dried prior to use to the equilibrium m.c.
As wood comes from the tree it contains appreciable amount of water frequently as much as 100 – 200 per cent in terms of its ovendry weight. Wood begins to lose moisture as soon as it is exposed to atmospheric conditions. As wood dries below the fiber saturation point it shrinks. The shrinkage is not equal in all directions. This inequality in shrinkage in three direction at right angles to one another sets up strains which are unavoidable and which may cause fracture in the wood tissues (Panshin and de Zeeuw, 1980). 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 under controlled drying will occur in service, often with disastrous results. Wood
is dried in a variety of shapes and sizes. The appropriate drying technique depends on size and species. Because there are thousands of tree species or generic groups in the world with some commercial importance , we are faced with formidable range of possible drying techniques.
Nasroun,(1979) reported that Observations made at different timber depots in Sudan as well as sawmills, showed that splitting and distortion of timber, resulting from uncontrolled drying, are among the major causes of timber losses and failures if not the major ones. This is due to two main reasons: lack of proper drying and the hot dry conditions prevailing in many parts of Sudan. Proper wood drying is, therefore, an essential process in nearly all wood utilization practices. This means that timber must be dried to the proper moisture content for end- use requirements. This is essential because the inherent value of timber and its potential commercial uses are generally affected by the nature and magnitude of the defects that occur during drying. This is why timber markets are demanding greater emphasis on improved quality of timber drying and in-service moisture content.
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 method of supplying this energy. Air (or natural) drying is simple and not costly, but it takes a long time and dries timber to the equilibrium moisture content only. Kiln drying, on the other hand, is faster and dries wood to any moisture content desired, but it 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. So solar energy applications began to look more attractive in the past decades (Simpson and Techernitz, 1977).
Solar dryers should improve drying conditions because the temperature available for drying is raised above ambient temperature, air circulation is sustained over the drying period and some control is exercised over the relative humidity (Troxell, 1977) .If solar drying reduces drying time over that of air drying and if drying defects are within acceptable limits, then this will be a great achievement, since only the sun will provide all the energy required. When compared with conventional steam or direct-fired drying kiln solar drying shows significant savings in capital and operating costs, but longer drying time is required. Fortunately experience and certain generalities make the choice easier ( Simpson, 1983 ). 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 ( Simpson and Tschernite,1977). It is somewhere in the middle between traditional air drying and Kiln drying with regards to rate of drying and costs. The two conventional methods- air drying and Kiln drying are the most widely used throughout the world. Air drying is the most economical practice in hot dry conditions like Sudan. This method uses the heat carried by the natural air current for drying the timber stack. It is therefore cheaper, but takes a long time. Drying Kilns are chambers of special type with adequate source of thermal energy and sophisticated control devices for controlling temperature, relative humidity and air circulation at every location inside the Kiln. All this indicates that drying Kilns are expensive to buy, install and operate. ( Nasroun and Shommo 1983) .
1.2 Research Problem:-
Because of the great values in wood, there is urgent need to recognize these virtues and develop means of protecting and rationalizing the use of this valuable resourse. Splitting and distortion of wood due to lack of proper drying are the major causes of timber losses and failure. Many locallygrown hardwoodsare particularly susceptible to these defects. Proper wood drying is therefore an essential processes to nearly all wood utilization practices. Wood drying, however, is the most energy intensive of all wood manufacturing processes. 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 it 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 a relatively simple, cost effective and energy- efficient solar dryers which will speed up air drying with minimum wood degradation.
EFFECT OF ENVIRONMENTAL CONDITIONS OF TWO NATURAL WOOD SEASONING MEDIA
