Design and Construction of Solar Kiln for Lumber Drying
Department of Chemical Engineering, Federal University of Technology Owerri
This research aims to design and fabricate a solar kiln dryer that uses a solar panel to trap the sun’s rich energy and also the design of an inverter that converts the direct current of the sun to an alternative current that will charge the battery and power heater element for drying. The principal mode of heat transfer is convection. The temperature of the incoming stream of air is raised by the heating element, a circular motion fan drives the heated air uniformly through the heating chamber, in this way heat is convectively transferred from the hot air to the cold air.
Keywords: Solar kiln, solar panel, inverter
Wood is a hygroscopic material which gains moisture content as a result of changes in humidity. Hygroscopicity is one of the most distinctive properties of wood. Any kind of wood product absorbs and desorbs moisture from the surrounding air until it reaches equilibrium moisture content (EMC), a balance point between the wood’s moisture content and that of the surrounding environment [1-5].
Fresh cut lumber contains a great deal of water. If the water is not removed, the lumber can’t be used to produce a high quality finished product. Properly dried lumber sells for a higher price and is much easier to work with than lumber that have not been dried. When lumber is well dried it machines better, glues better, and finishes better. Drying also improves the strength of the lumber, kills infestations, hardens pitch, preserves colour, reduces weight and controls shrinkage. Lumber that is not dried under controlled conditions is prone to warping, staining, and other degradations that diminishes its selling price and workability. Drying wood is simple, the objective is to reduce the moisture content of wood to the relative content of the climate where the wood will reside, while trying to keep the lumber from distorting, checking, honeycombing, . However, unlike many wet materials that must be dried, wood must be dried at specified rates to avoid degradation (value loss). If degradation were no concern, lumber could be dried in minutes. The dimensions of a wood specimen do not vary with MC [6-10]. We can achieve a moisture content of 10-15 percent by air-drying alone, although this will vary according to one’s local climate. The goal for commercially dried lumber is 6-8 percent MC.
The water or moisture content (MC) of wood is expressed in percent, as the weight of water present in the wood divided by the weight of dry wood substance. As an example, a green (freshly cut) wood may have a MC as low as 30% to as high as 250%. Sapwood usually has a higher MC than heartwood. Average green wood MCs may vary considerably from one tree to another, among boards cut from the same tree, and with the time of year the tree is cut. Most of this water must be removed in order to obtain satisfactory performance from wood that is to be processed into consumer and other types of useful products. As the MC decreases, wood shrinks; conversely, as the MC increases, wood swells or grows larger. Loss of water results in changes in many of the properties of wood, such as strength and both thermal and electrical conductivity. The basic cause of drying degrade is wood shrinkage, often 5% or more. Shrinkage parallel to the annual growth rings (tangential shrinkage) is twice as much as shrinkage perpendicular to or across the annual rings (radial shrinkage). Changes in MC result in strain and strain-induced stresses, the magnitudes of which are sufficiently large to produce configurationally strain construction of solar kiln for lumber drying is investigated.
Factors considered in the design are the dimensions for the drying chamber, optimum angular position for the solar panel for the efficient absorption of solar energy and the best position for the fan for uniform hot air circulation. The drying cabinet is rectangular in shape with dimensions of 120×50 cm. It is made up of plain carbon steel in a double walled pattern and lagged with an insulator known as polyurethane foam. Inside the drying cabinet are drying chamber, where sample specimens are kept for drying to take place, the base of the drying chamber is perforated so that convective heat transfer can take place. Also inside the drying cabinet is a rotating fan that distributes hot air uniformly so as to effect efficient drying. On top of the drying cabinet is a solar panel hanged on a Gurdon pin that helps in absorbing the solar energy. An inverter converts the direct current to an alternating current that charges the battery for powering heating element, the inverter is made up of a plain carbon steel. The heating element provides the heating medium by which the temperature of the incoming cold air is raised to a sufficient degree for drying to take place, there is an electric control box, a heater on/off switch all of which regulates and control the dryer, heater and fan respectively.
Table 1 material selection for components of solar kiln dryer