Performance of a solar-heated,forced-air,fully automated lumber dryer

A 3.5 m³, external collector type kiln has been designed and constructed to dry green lumber to M.C. low enough for use in furniture. The kiln uses a hot air transfer-circulation system and its ratio of collector area to lumber capacity is 2.9 m²/m³. The kiln is equipped with a fully automated supervisor system. The energy efficiency and performance of the solar kiln have been evaluated through a series of operational tests. The solar collectors operate at 71 to 75% efficiency, while the drying chamber at an efficiency of 46%. The kiln has been found able to use 33% of the available solar energy for lumber drying, Control system has been proved able to run the process to the drying gradient required by any given species of wood. The performance of the dryer, evaluated by direct comparison with air drying, is satisfactory in respect of the final moisture content and drying time. A final M.C. of 10% was attainable at all times of the year.     

Drying rate and air circulation in a fully automated solar kiln

A fully automated solar kiln, operating on logical venting principles and controlled according to a drying schedule, was used to dry wood. The drying rates in the solar kiln and an air dryer were compared in the moisture content ranges above and below fibre saturation point (fsp). The effect ofair circulation on drying rates in the solar kiln was also investigated both above and below fsp.     

The influence of material properties on the amount of twist of spruce wood during kiln drying

The development of twist in Norway spruce boards (Picea abies Karst.) during normal temperature kiln drying was researched. Tangentially oriented boards were sawn from diametrical radial planks, from which the basic wood properties, i.e. linear shrinkage and grain angle, were determined. The unrestrained boards were kiln dried using a selected drying schedule. During the controlled drying process the moisture content (MC) and twist of the boards were measured. Twist was generally induced by the shrinkage of wood below the fibre saturation point, and increased in proportional to the decrease in MC. The earlier appearance of twist, with regard to MC and drying time, was confirmed in the case of boards sawn from the central part of logs. The final twist amounted to between 10 and 20°/m in the case of boards close to the pith, and decreased to less than 4°/m in the case of boards sawn at a radius of 70 mm from the centre of the logs. Ring curvature had the highest impact on the final twist of the dried boards, followed by grain angle and tangential shrinkage.     

Temperature control in a solar kiln

Realizing the effect of venting on temperatures in a solar kiln, average kiln temperatures were measured in a solar kiln with automatic vent control. These temperatures were found to be less than 2°C above ambient with the wood above fibre saturation point and increased to about 10°C when the wood reached 12% moisture content. The effect of these relative small differences between kiln and ambient temperatures on the drying rate were investigated as well as methods to increase the average temperature in a solar kiln.     

Economics of drying timber in a greenhouse type solar kiln

The economics of drying timber was assessed for a green-house type solar kiln along with the conventional drying techniques. Both the operating and capital costs of the solar kiln were much lower than those of the steam kiln. Compared to air drying the solar drying was cheaper and cost less than one-half of the steam kilning. The pay back period was about one year for the solar kiln against 31–45 years for the steam kiln. Solar drying was thus found economically attractive and viable technology in Bangladesh.     

Computerised control system for an experimental solar lumber dryer

The microcomputer based control system employed in an experimental highly efficient solar kiln and the advantages of using this approach in solar wood drying are discussed. The control system consisting mainly of a personal computer and a data acquisition unit, performs a number of measuring and control functions. The monitored drying variables are wood M.C., collector's temperature, kiln's temperature, and R.H. The M.C. is monitored by recording the weight of the 3.2 m³ timber stack, resting on a weighbridge. The direct and continuous monitoring of timber moisture and equilibrium moisture employed by the control system, makes it possible to run the solar drying process to a target value formed from these two measured drying variables (drying gradient). The developed control system is compared with a semiautomatic system controlling the same solar klin.     

Energy efficiency and drying rates during vacuum microwave drying of wood

The application of vacuum-microwave drying can dramatically decrease drying times in comparison to conventional kiln drying. Instead of thermal energy for the conventional drying, electric energy is used for heating the material in microwave drying. To investigate the energy efficiency specimens of beech, maple and spruce wood were dried in a semi-industrial and a laboratory vacuum-microwave kiln. Both allowed measurement of the energy consumption during the process. In addition, a continuous monitoring of the weight loss was possible in the laboratory kiln. In the first experiments energy input was controlled manually. To avoid material degradation the process was controlled by measurement of wood surface temperature in further experiments. Energy efficiencies of up to 80% were reached depending on the moisture content of the samples and wood species. Drying rates depended additionally on mass specific energy input. For beech drying rates between 1.0 and 4.5%/min were reached. The period of fairly constant drying rate extended to values below the fibre saturation point. The wood was in most cases free of checks and always free of colour changes because of the short process duration and the absence of oxygen.     

Experimental pressure steam dryer for lumber

This article describes the design and construction of an experimental kiln suitable for operation at pressure steam drying conditions of temperatures up to 130°C, pressures up to 300 kPa and air velocities up to 10 m·s^?1. The kiln could also be utilized for low, medium and high temperature convective wood drying at atmospheric pressure. It is, therefore, extremely suitable for the determination of the basic drying behaviour of wood under various combinations of temperature/pressure/air velocity, using full-sized sample boards of 2.1 m length.     

Prevention of brown discoloration in European oakwood occurring during kiln drying by a vacuum process: Colorimetric comparative study with a traditional process

The efficiency of two kiln drying processes on brown discoloration developments ofQuercus robus (L) andQuercus petraea (Lieb) was studied. A traditional atmospheric pressure system, was compared with other using vacuum and super heated steam. 103 green boards from three different trees were used. After each cycle (6), we analysed and compared the color of the boards using a spectrocolorimeter fitted with the CIELAB analysis. With the traditional system (in moist condition) we obtained 100% brown discoloration, in contrast to vacuum and super heating which did not develop brown discoloration. The brown discoloration is characterised by a decrease in lightness (-6 to-8 units) and color angle (-1 to-2 units) compared to non discolored wood. The discoloration is probably independent of wood origin and can be prevented by quick kiln drying without oxygen.     

Comparison of drying behaviour, quality and yield of copra processed in either a solar hybrid dryer on in an improved copra kiln

Drying copra in a solar hybrid dryer reduces the moisture content from around 50% to 7% after 71 h of continuous drying. The copra was graded as 73% white copra, 21% Milling Ordinary Grade II (M.O.GII) and the remaining 6% M.O.GIII (dusty copra). Thermal efficiency was about 10%. In the Coconut Research Institute copra kiln, the moisture content of copra was reduced from around 52% to 8% in 62 h of intermittent dying. The copra was graded as about 82% M.O.GI and the remaining 18% M.O.GIII (burnt copra). Thermal efficiency was about 15.5%. High quality white copra could be processed in solar hybrid drying. However, no white copra could be processed in kiln drying.     

Design and testing of a solar kiln simulator

To overcome the need for repeatable weather conditions when investigating solar wood drying, a solar simulation system was designed consisting of a climatic chamber for producing ambient wet and dry bulb conditions, a solar energy simulator to simulate the solar energy falling on the collector, and a small solar kiln to fit inside the climatic chamber. A fully automated control system, as used in an experimental solar kiln, controlled the simulation system. The design parameters, construction, and control logic of the system are discussed in Part 1 while the testing and performance of the system will be discussed in Part 2.     

Relationships between drying defect parameters and some growth characteristics in kiln-dried South African grown Eucalyptus grandis poles

Eucalyptus grandis wood is susceptible to serious drying defects and all possible measures before and during kiln drying of poles need to be taken to minimize the defects. In order to identify factors that may affect drying quality of poles, the objective of this study was to assess relationships between surface check total length and depth, MC gradient, honeycomb percentage of pole cross section area, extent of collapse, sapwood depth, heartwood percentage of pole cross-sectional area, green MC and pole diameter. The defects were measured on 3 loads of 10?kiln-dried E. grandis poles, using a different drying schedule for each load. After measuring surface check length using a measuring tape and check depth using feeler gauges and a ruler, poles were cross cut at the theoretical ground line (TGL) 1.5?m from the butt end, to measure the MC gradient between the shell and core of poles. Digital image analysis of cross sections of discs cut at TGL was used to measure sapwood depth, and the areas of honeycomb, heartwood and disc cross section. Collapse was assessed using qualitative methods. Results showed that surface check total length was positively correlated with sapwood depth, and the average depth of surface checks was greater than the average sapwood depth. MC gradient between the shell and core was positively correlated with green MC while the transformed honeycomb percentage and collapse class were both positively correlated with heartwood percentage. It was concluded that pole drying defects are related to sapwood depth, green MC and heartwood percentage, and if pre-sorting of poles by these characteristics is done and appropriate kiln schedules are used, drying defects can be minimised.     

Regelungstechnik für die Schnittholztrocknung heute

After explaining the requirements which must be fulfilled by modern microprocessor- or computer-operated control units for kiln dryers, four control methods are compared. Concerning these methods, the initial moisture content of the wood as well as the constantly changing moisture content play very different roles depending on whether it is above, below or within the so-called range of fibre saturation of the wood. The most highly developed method No. IV includes the distribution of moisture content over the cross section of the board as a comparative figure in the control data processed by the computer, with the aim of optimizing the drying time and guaranteeing a high drying quality.     

A mathematical model of the Torry fish drying kiln

Summary. Proposed developments of the Torry fish drying kiln have led to a need for a closer understanding of commercial fish drying processes. In order to investigate the effects on fish drying of air velocity, air temperature and humidity, and fish packing density, a mathematical model of the kiln has been proposed. This model establishes partial differential equations for fish weight, air temperature and humidity. the model is applicable to the drying of other materials. A solution is obtained numerically by standard finite difference techniques.     

A study of some factors promoting kiln brown stain formation in radiata pine

Kiln drying of radiata pine (Pinus radiata D. Don) sapwood induces a brown discolouration, commonly called kiln brown stain. Kiln brown stain generally occurs 0.5–1.5 mm under the surface of the dry sawn lumber. However, subsequent planing of the wood can expose the stain causing substantial revenue loss in the New Zealand highvalue export markets. In the present study, the effects of kiln schedule temperature, wood moisture content and kiln relative humidity on kiln brown stain formation were investigated. The results showed that kiln brown stain was greatly reduced by using kiln schedules of less than 50°C as opposed to 90°C and was also more pronounced in the kilning at 30% kiln relative humidity, than at 80% humidity. Furthermore, the occurrence of kiln brown stain tended to intensify with decreasing wood moisture content, irrespective of kiln temperature and kiln humidity used. The practical implication of this work and the mechanisms of kiln brown stain formation are discussed.     

Development of schedule to steaming prior to drying and its effects on <Emphasis Type="Italic">Eucalyptus grandis</Emphasis> × <Emphasis Type="Italic">E. urophylla</Emphasis> wood

To investigate the influence of moisture content (MC) and steaming temperature on defects developing during steaming prior to kiln drying, mass loss, heat transfer, microstructures, extractives, and chemical transformation of Eucalyptus grandis?×?E. urophylla specimens (with moisture content of 120, 70, 60, 50, 40, 30, 20%) were observed in this study. Specimens of each experimental moisture content were steamed at 80, 100, and 120?°C for 4 h after pre-heating at atmospheric pressure, respectively. Results revealed that it was most beneficial to conduct steaming at 100?°C when the moisture content was approximately 50% after air drying when stagewise heating-up and continuous steaming schedule were adopted. Under this condition, there was a notable decrease in defects including edge bends, surface splits, and inner splits. Steaming was also a drying period characterized by various extents of mass loss. Changes in microstructures and extractive contents proved the increasing permeability of steamed wood. Deacetylation and crosslinking reactions happened in the hemicellulose of the sample materials, which contributed to the loss of hemicellulose after treatment.     

A comparison between cold water and steam for SPF lumber conditioning

Conditioning is an effective method to release drying stresses and improve final moisture content (MC) distribution for spruce-pine-subalpine fir (SPF) lumber during drying. There are currently two methods used in the industry to add moisture to lumber during conditioning, namely low-pressure steam spray and cold water spray. Sixteen kiln drying runs were carried out in a laboratory kiln. Two types of conditioning treatment were investigated: cold water spray and steam spray. Temperature profiles, final MC distribution and drying stresses obtained for lumber conditioned by the two methods were evaluated and then compared. Results indicated that there were no significant differences on drying temperature profiles and lumber quality between the two humidification systems. Therefore, both humidification systems can be used for controlling humidity during kiln drying and for conditioning treatments.

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Vergleich zwischen Kaltwasser- und Dampf-Konditionierung von SPF-Holz

Zusammenfassung Konditionieren stellt eine effektive Methode dar, um Trocknungsspannungen abzubauen, und um die endgültige Verteilung der Holzfeuchte in Fichten- Kiefern- Tannen-Holz während der Trocknung zu verbessern. Gegenwärtig werden in der holzverarbeitenden Industrie zum Konditionieren zwei Verfahren zum Befeuchten des Holzes verwendet: Sprühen mit Dampf bei niedrigem Druck, oder Sprühen mit Kaltwasser. Sechzehn Trocknungsläufe in einem Labortrockner wurden durchgeführt. Zwei Konditionierungsverfahren wurden untersucht: Kaltwasser- und Dampfsprühverfahren. Temperaturverläufe, Endfeuchteverteilung und Trocknungsspannungen bei Schnittholz das nach den genannten zwei Methoden konditioniert wurde, wurden untersucht und miteinander verglichen. Es zeigten sich keine signifikanten Unterschiede bezüglich der Auswirkung der Temperaturverteilung und der Schnittholzqualität zwischen beiden Verfahren. Deshalb lassen sich beide Verfahren zur Regelung der Feuchte während der Labortrocknung und zum Konditionieren anwenden.

     

Vergleichende Untersuchungen über die Freilufttrocknung und Solartrocknung von Schnittholz unter mitteleuropäischen Wetterverhältnissen

For systematic investigations on the effectiveness of lumber solar drying under Central European weather conditions in direct comparison with air drying, the tests carried out last year were continued for another year, partly under modified adjustable drying conditions. The three comparative tests carried out between mid March and the end of October 1979 in general confirmed the results obtained in the previous year. Faster solar drying compared to air drying was noticeable only after lowering the average wood moisture content below 30%; from 30% to 20% solar drying on the average occurred more than twice as fast. While lumber with air drying could only be dried to 14.9%, 15.0% and 23.0%, with solar drying 6.8%, 8.6% and 12.9% could be achieved. Only in the period from approximately mid December till mid February the effectiveness of solar drying proved to be unsatisfactory. Based on the findings of the tests and on foreign experiences, the constructive and operational demands regarding a solar dryer are discussed and the possibilities for a suitable utilisation of solar energy for lumber drying in the highly industrialized Central European countries are estimated.     

Design and testing of a solar kiln simulator

The solar simulation system as previously described (Steinmann 1990) was used to simulate an actual drying run in a solar kiln. Adjustments necessary to compensate for difference in scale are described. The simulated weather conditions were in close agreement with the actual weather. The conditions in the solar kiln simulator and the solar kiln were so similar that the drying rates were virtually identical. Repeatability of runs in the simulator was good and it was concluded that valid results on solar drying could be produced in the solar simulation system.     

Der Einfluß des Trocknungsgrades der Holzspäne auf die Festigkeit zementgebundener Spanplatten

Cement bonded particle boards were made of undried, air dried and kiln dried particles of spruce and poplar wood and the bending strengths of the boards were measured after 3, 10 and 28 days. Air dried particles of both wood species gave boards with better strength properties than undried particles. Kiln drying of the particles was found to have a substantially improving effect on the bending strength of cement bonded particle boards made of poplar particles whereas the same procedure did not produce any effect with spruce particles.