Microwave drying of pine and spruce

Drying rates and power densities are determined for pine-and sprucewood when dried from green to 8% moisture content by microwave power. The process is controlled by measurements of internal wood temperature, internal vapour pressure and rate of moisture evaporation. Microwave power densities ranged from 25 to 78 kW/m³, microwave energy consumption from 365 to 760 kWh/m³. Internal wood temperatures up to 140 °C were used. Internal vapour pressure in the wood could rise to about 20 kPa without checking. Maximal drying rates of 0.20 to 0.45% moisture content per minute are possible to obtain when drying above fiber saturation (fsp). Below fsp the feasible drying rates ranged from 0.10 to 0.20% moisture content per minute. Spruce dried approximately 1.6 times faster than pine. No conditioning of the wood was necessary since the wood was free of stresses. The wood was free of checks but colour changes occured in the interior of some specimens.     

Evolution of internal stress during drying, cooling and conditioning of rubberwood lumber

In an attempt to develop a real-time measurement technique for an internal stress perpendicular to grain in the radial direction of rubberwood lumber during drying and conditioning, a novel design of a force measuring device, consisting of a restrained half-sawn specimen and a restrained free shrinkage specimen, was constructed. The measured force was verified with a differential released strain obtained from the conventional McMillen slice technique. The device was then employed to investigate the internal stress evolution during drying and conditioning of rubberwood lumber. It was shown that the force measured using the device strongly correlates with the differential released strain. The extents of the internal stress characteristics such as the maximum tensile and compressive forces and the reciprocal of time at stress reversal obey the Arrhenius equation and the adapted equation of sorption for the temperature and humidity dependences, respectively. The results also indicate that cooling under relatively high humidity after drying improves the internal stress relaxation process during conditioning. Possible application of the technique in kiln drying of rubberwood has been demonstrated. The measured force could be used as controlling parameter for an adaptive wood drying control system taking into account real-time material response of wood during drying and conditioning.     

Solar drying of sawn lumber in Spain

A solar heated experimental drying kiln was installed in Madrid (INIA-Forest Industries Department) to study the efficiency of solar drying of sawn lumber in Spain, as compared with normal air drying. The kiln has been designed as a low cost semi-greenhouse type, but was equipped with a semi-automated control of the drying schedules. The performance of the solar kiln has been studied during ten test runs over a period of two years particularly with respect to drying rate, initial and final wood MC, reliability and efficiency. The results have proven that the solar kiln is capable of drying (economically) from an initial moisture content of 70 to 80%, to a final MC of 8 to 10% up to 3.5 times faster than air drying. With respect to checks, splits and internal stresses, the final wood quality is mostly better, not only compared to normal air drying but also to conventional kiln drying.     

Effects of high temperature drying in nitrogen atmosphere on mechanical and colour properties of Norway spruce

Heat treatment and drying processes of wood are always accompanied by material changes. These changes are partly caused by oxidative reactions. This study aims to compare high temperature drying in inert gas atmosphere with conventional kiln drying of spruce wood. The mechanical properties as well as colour changes were measured after drying. In general, higher compression strength was observed for samples dried at high temperatures in nitrogen atmosphere. However, tensile and bending strength proved no significant difference. Corresponding to the strength values increased stiffness values were also observed for the compression samples, whereas for tension and bending samples no significant effect was visible. Up to a temperature of approx. 125 °C discolouration is prevented by drying samples in nitrogen atmosphere.     

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.     

干燥和粉碎方法对杨梅渣中总酚和花色苷含量的影响

为了尽可能多地保存杨梅渣中的功能成分,研究了干燥和粉碎方法对其中总酚和花色苷含量的影响。杨梅渣经干燥后,总酚和花色苷分别损失了11.4%～20.3%和43.8%～48.3%,微波干燥或真空微波干燥比普通的热风干燥能提高其中总酚和花色苷的保存率,并能显著提高干燥速率。将干燥后的杨梅渣分别经过普通粉碎机粉碎和超微粉碎,经超微粉碎后的杨梅粉平均粒度达到5～20μm,而且测得的总酚和花色苷含量比普通粉碎样品中的含量更高。说明微波干燥再经过超微粉碎可以较好地保存杨梅渣中的总酚和花色苷成分。     

木材真空过热蒸汽干燥的应用前景

简述了过热蒸汽干燥的主要特征、优点与局限性.列举了木材真空过热蒸汽干燥松木和红栎木的干燥温度、干燥时间、能耗与成本等参数.真空过热蒸汽干燥比常规快3～7倍,且干燥质量好,能耗低.但设备投资比常规干燥高25%～30%.今后此项技术用于干燥厚的难干材有一定的应用前景.     

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.     

Twisting force during drying of wood

The object of this study is to investigate when wood begins to twist and the extent of the twisting force during drying. Japanese larch (Larix leptolepis Gord.) and hiba (Thujopsis dolabrata Sieb. et Zucc. var. hondae Makino) which tend to twist significantly when drying, were used in experiments to obtain multiple examples of grain angle (GA) variation in the R-direction. A measuring system to measure the twisting force during the drying of wood in real time was developed. It is thought that the time when twisting of timber by drying starts is subject to the variation in R-direction of GA; timber begins to twist when the portion with large GA begins to dry. There is concern that timber with large GA in the inner portion may become twisted during storage after kiln drying. The twisting stress measured in this study is effective for examining the load on the timber to control the twisting.     

马蹄豆锯材干燥工艺试验研究

以22 mm厚马蹄豆锯材为研究对象,采用百度试验法对马蹄豆锯材进行干燥特性研究,并制定初步干燥基准,采用低温除湿干燥窑对马蹄豆锯材进行干燥工艺优化试验.结果表明,在保证干燥质量前提下,22 mm厚马蹄豆锯材干燥前期适宜低温干燥,含水率下降速度控制在0.14％/h;干燥中期含水率下降速度控制在0.11％/h;干燥后期含水...     

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.     

Influence of low and moderate temperature kiln drying schedules on specific mechanical properties of Norway spruce wood

The knowledge of the influence of different temperature ranges on mechanical properties of kiln dried timber is very important in structural applications. The current paper deals with an investigation of the influence of low and moderate temperature drying schedules on the selected mechanical properties of Norway spruce wood (Picea abies?L.). The drying experiments were performed at four different temperatures (45, 55, 65, and 80°C) and clear wood specimens were prepared for the mechanical tests, three-point bending test, impact bending test, and tensile test perpendicular to the grain, respectively. The results of this systematic investigation were statistically analysed and as a conclusion, it can be stated that the mechanical properties investigated were not influenced by the drying temperatures applied.     

Effects of pre-steaming on the drying quality of Rubberwood

In this study, the kiln drying yield of 30 mm Rubberwood (Hevea brasiliensis) furniture stocks with pre-steaming treatment was evaluated. The results revealed that pre-steaming increased the permeability of the wood, hence increasing the drying rate while reducing drying defects. It was found that the pre-steaming treatment should be included in the drying of Rubberwood in order to reduce the overall drying costs as well as improving its quality.     

Effect of high-temperature drying on properties of Norway spruce and larch

In timber drying, mechanical properties may be changed due to treatment temperature and treatment duration. In general, when increasing the kiln temperature, drying time is decreased and some timber properties are negatively affected. In this study, the effect of different drying temperatures (80, 120 and 170 °C) on equilibrium moisture content and sorption rate, on bending strength and stiffness was investigated for Norway spruce and larch from four proveniences and the results were compared to those obtained for heat-treated wood (Thermowood ^®). The experiments confirm earlier research that both treatment temperature and treatment duration affect the properties. High-temperature drying or treatment can be optimized for several applications, yielding strong but not so durable timber or vice versa.     

Untersuchungen über den Energieverbrauch bei der technischen Schnittholztrocknung und Möglichkeiten zu dessen Verringerung

Part two of this study presents in short the results of investigations on energy consumption of a practical used lumber dry kiln. Recording and evaluating the drying process have been done by means of a speacial computer controlled data acquisition system. Simulating drying behavior and calculating energy consumption of this process on account of realistic data characterizing dry kiln, kiln load and drying schedule are combined in a program package. By this way, without extensive tests, investigations on measures of reducing energy consumption (improved isolation of dry kiln, preheating of air by heat exchange systems, variable airvelocity by controlled fanspeed regulation, changed data of drying schedule) are feasible only with very little effort in time and costs. The results can be used for evaluating the economy of appropriate investments.     

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.     

Effect of drying method on the surface wettability of wood strands

An experiment was conducted to investigate the effects of drying methods, namely rotary drum, lab oven, microwave and air-drying, on the surface wettability of wood strands by the Wilhelmy plate method. The measurements included the surface contact angle, free energy, and liquid sorption capacity. Thirty industrial strands including southern yellow pine (Pinus spp.) and poplar (Populus spp.) were randomly collected for each drying method. The results from this study showed that the surface contact angle of a strand depends on the liquid used as well as the drying method and wood species. Liquid sorption capacity of the pine strands increased with decreasing surface contact angle, but this trend was not observed with the poplar strands. The total surface energy was almost the same for strands within species, regardless of the drying method, but it was higher on poplar strands than on pines. The polar and disperse components of surface free energy varied with both drying method and wood species. The surface polar energies of oven-dried pine strands and rotating drum-dried poplar strands were higher than on strands dried by other methods. For lower temperature drying methods (air and microwave), the lowest polar component was observed on the radial face of the pine strands. For the higher-temperature drying methods (oven and rotary-drum), the lowest polar component was found to on the tangential face of the pine strands.     

Industrial paddy drying and energy saving options

Paddy drying is an energy-intensive process and influences rice quality. In this study, the energy consumption of paddy drying in a large-scale milling plant was investigated. Furthermore, some drying experiments were conducted in a laboratory. The aims were to gain practical information and to propose more economical drying options to the industry. The results indicated that the current drying in the plant consumed specific primary energy of between 3.874 and 4.421 MJ/kg of water evaporated. The experimental results showed that two-stage drying with tempering by using a fluidised bed dryer at 100–110 °C in the first stage and drying with ambient air using a solar dryer for the second stage provided rice quality that was comparable to that of the plant. Also, from the calculation, the energy cost of the plant could be reduced if an in-store dryer was used after the first-stage drying by LSU dryer.     

Drying rate control in the middle stage of microwave drying

Microwave drying of apples at constant temperatures follows typical drying curves. The middle stage of the drying process shows a faster drying rate and accelerated moisture evaporation. Meanwhile, more flavors are lost, surface color is degraded, and charring often occurs in this stage. To improve the drying effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically varying drying temperatures in the middle stages. The controlled drying process therefore led to an optimized temperature profile. To simplify the drying methods, apples were further dried with the obtained temperature profile, while drying curves were online monitored but not controlled. It was proved that slowing down the drying rate in the middle stage could improve the product quality in terms of color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable (within 200 min and 22 kJ/g, respectively).     

Effects of the smoking process on odour characteristics of smoked herring (Clupea harengus) and relationships with phenolic compound content

The relationship between smoking parameters and odour characteristics, evaluated by a trained sensory panel, were studied on smoked herring. In addition, a possible correlation between the content of 10 phenolic compounds and sensory perceptions was investigated. Five smoking techniques were applied, combining smoke production conditions, performed by pyrolysis of beech wood sawdust or by friction of beech wood log, with smoke deposition, either in a controlled kiln (traditional smoking) or by an electrostatic process. In the fifth smoking technique, a purified condensate of beech smokes was vaporised on fish fillets in the smokehouse. The time of smoking was 3 h for traditional smoking and the liquid smoke atomisation process and 12 min for the electrostatic method. The effects of three smoking temperatures (16, 24, 32 °C) were tested for both the traditional and the liquid smoke atomisation processes, as well as the effect of the position of the exhaust valve in the smokehouse in the case of the traditional method. Two different voltages were applied for the electrostatic process, 37 and 42 kV.