VACUUM-PRESS DRYING OF THICK SOFTWOOD LUMBERS

In vacuum-press drying of softwood species, wood lemperaiure exhibited a low temperature gradient, and plateau temperature of core lasied during all stages of drying. The drying curves were close to “linear”. The drying rates of the short lumbers were higher for red pine and western hemlock, lower for white pine and similar for larch lumber compared to the long ones. Transverse and longitudinal moisture gradients were small for western hemlock and red pine, and great for larch and white pine lumber. Energy consumption curves were split into three sections: increasing moderately, fairly constant and increasing rapidly. Casehardening stress of dried lumber occurred very slightly. Dried lumbers exhibited strong tendency for fine end checking, slight surface checking and no internal checking. Shrinkage appeared to be low.     

Impact of conventional drying and thermal post-treatment on the residual stresses and shape deformations of larch lumber

Quality evaluation and grading of thermally treated wood products are of fundamental importance to their commercial utilization. The combined impacts of conventional drying, thermal post-treatment, and transverse dimensions of lumber over the residual stresses and shape deformations of larch (Larix gmelinii) wood were examined. Larch specimens with two different thicknesses (25 and 40?mm) and three different widths (100, 150, and 200?mm) were dried using conventional technology and thereafter thermally treated at 200°C for 1?h at atmospheric superheated steam conditions. Drying and residual stresses and shape deformations of both kiln-dried and thermally post-treated lumbers were measured and statistically analyzed. The influential mechanisms of lumber thickness and width over the drying stresses and shape deformations of thermally post-treated lumbers were revealed. The drying and residual stress measurement based on the prong test was recommended as a potential quality evaluation strategy for the thermally post-treated larch lumbers. As the lumber width increased from 100 to 150?mm, and then to 200?mm, the cupping deformation of thermally post-treated larch lumber increased substantially. These results provided some practical foundations for the quality evaluation of thermally treated wood.     

Drying Characteristics of Thick Lumber in a Laboratory Radio-Frequeocy/Vacuum Dryer

ABSTRACT

A series of forty two drying runs of two wood species and two cross-sectional dimensions of wood squares were carried out in a laboratory radio-frequency/vacuum (RF/V) dryer. The experimental temperature, pressure and moisture content levels as functions of space and time are presented. The results showed that western red cedar and western hemlock can be dried to a final moisture content of 15% in about 24 and 32 hours, respectively. The quality of the dried specimens was exceptional- Detailed analysis revealed the absence of internal drying stresses, internal and external checking and surface discoloration. Furthermore, evaluation of moisture content distribution in the longitudinal and transverse direction showed minimum variation compared to conventional kiln drying. The experiments also revealed that RF/V drying rates are directly affected by the level of the electrode plate voltage. Drying rates decreased with time when the voltage remained constant throughout the drying cycle thus resulting in long drying times. That was because of the changing dielectric properties of wood due to moisture content reduction during drying. Raising the voltages with time though, resulted in constant drying rates and shorter diying times.     

Simulation of Drying Using a Kiln Model

A mathematical model was developed for simulating a convective batch lumber drying process. The model incorporates mass and heat transfer relationships within the lumber stack, as well as thermodynamic properties of the wood and drying air. It takes into account the change of air properties along the stack and its effect on the mass and heat transfer parameters. The model relies on a drying rate function that is an empirical correlation based on single-board tests. A drying rate function for western hemlock (Tsuga heterophylla) lumber was developed. The drying rate function was obtained based on experiment results from 500 small boards dried over a range of conditions used in commercial practice. The model was first validated against data available in the literature and then against large batches of hemlock dried in a laboratory kiln. In both cases, the model output was in good agreement with the average moisture content, the drying rates, and the temperatures measured in the larger batches.     

HIGH TEMPERATURE DRYING FROM THE THREE PRIMARY ORTHOGONAL SURFACES OF LOBLOLLY PINE

While several permeability studies have been performed, little information is available on the flow of water in wood along its three orthogonal axes during high temperature drying. Much southern yellow pine construction lumber is dried at temperatures above 100°C, thus how water moves in wood is of concern. In this experiment individual Loblolly pine (Pinus taeda) sapwood cubes were coated on two of their three orthogonal faces (longitudinal, radial, and tangential) and the effect of this sealing on the drying rate was determined. From these cubes, drying rates from each of the three primary planes were determined. The free water movement/evaporation between the radial and tangential faces was not statistically different, while the longitudinal rate was approximately three times faster. In commercial practice, the faster longitudinal rate is negated by the very small portion of transverse surface area relative to total lumber surface area. This partially explains why end checking and moisture variation in high temperature dried southern yellow pine lumber are not significant causes of degrade. There was no statistically significant difference in permeability across the seasons tested.     

THE EFFECT OF DRYING TEMPERATURE ON THE MECHANO-SORPTIVE BEHAVIOR OF RED OAK LUMBER

ABSTRACT

Matched units of 3.2 cm thick red oak lumber were dried simultaneously in a steam heated and dehumidification kiln. The two pairs of runs are designated Set I and Set II. The objective was to compare the mechano-sorptive behavior and board shrinkages while using the recommended U.S. Forest Products Laboratory schedule with the steam kiln and comparatively low temperature drying in the dehumidification kiln.

Drying rates in Set I and Set II were comparable for the two kilns up to approximately 450 hours, which illustrated the dependence of the drying rate on the relative humidity of the kiln air rather than its temperature. Subsequently, the stepwise increases in dry bulb temperature for the steam kiln were accompanied by accelerated drying.

Less compression set developed in the interior mechano-sorptive slices for the dehumidification kiln runs. On an average, the maximum compression set for the core slices from the dehumidification kiln was about S0% of that for core slices from the steam heated kiln. Simultaneously the surface slices from the dehumidification kiln developed more tension set than those from the steam heated kiln. Board width shrinkage at the end of drying, at the same average moisture content, was greatest for the steam kiln. The greater shrinkage is attributed to greater compression set due to the higher drying temperatures. These results support McMillen)s explanation for the effect of drying temperatures upon sets and the shrinkage of red oak lumber.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

HIGH TEMPERATURE DRYING FROM THE THREE PRIMARY ORTHOGONAL SURFACES OF LOBLOLLY PINE

While several permeability studies have been performed, little information is available on the flow of water in wood along its three orthogonal axes during high temperature drying. Much southern yellow pine construction lumber is dried at temperatures above 100°C, thus how water moves in wood is of concern. In this experiment individual Loblolly pine (Pinus taeda) sapwood cubes were coated on two of their three orthogonal faces (longitudinal, radial, and tangential) and the effect of this sealing on the drying rate was determined. From these cubes, drying rates from each of the three primary planes were determined. The free water movement/evaporation between the radial and tangential faces was not statistically different, while the longitudinal rate was approximately three times faster. In commercial practice, the faster longitudinal rate is negated by the very small portion of transverse surface area relative to total lumber surface area. This partially explains why end checking and moisture variation in high temperature dried southern yellow pine lumber are not significant causes of degrade. There was no statistically significant difference in permeability across the seasons tested.     

MODELLING OF STRESS DEVELOPMENT DURING DRYING AND RELIEF DURING STEAMING IN PINUS RADIATA LUMBER

ABSTRACT

A one-dimensional stress model was proposed for drying of radiata pine lumber, which has considered wood moisture shrinkage, instantaneous stress-strain relationships, mechano-sorptive creep, time-induced creep and temperature effects. In addition, wood hardening behaviour in the plastic region and differences between stress increase and decrease have been taken into account. The proposed Stress model can predict stress development and relief in a drying cycle once the required wood mechanical and Theological properties have been quantified.

Drying experiments were performed to dry Pinus radiata sap wood boards of 100×40×590 mm in a tunnel dryer. In the experiment, wood temperature, moisture content gradient and residual stress through board thickness were measured. The drying cycle included HT drying, cooling and final steam conditioning. The measured stress patterns were in agreement with the model predictions. However, more accurate calculations will be made once the detailed experimental data for radiata pine wood mechanical and rheological properties are available.     

ENERGY QUANTIFICATION AND MECHANO-SORPTIVE BEHAVIOR IN THE KILN DRYING OF 2.5 cm THICK RED OAK LUMBER

ABSTRACT

Energy consumption was quantified in the drying of 2.5 cm thick red oak lumber in a 1.2 m³ steam heated experimental dry kiln. The aluminum pre-fab kiln is located indoors and was wrapped with 5 cm thick extruded polystyrene (R=10) for the experimental drying. Transmission loss accounted for about 80% of the energy/kilogram of water removed from the wood. The maximum value was for the 60°C DBT due to comparatively high transmission loss combined with maximum venting loss. Batch venting reduced average energy consumption per kiln run by over 6% compared to the conventional kiln schedule.

Average board shrinkage was over 2% less for drying with a low temperature schedule compared to drying by the conventional schedule for red oak. This was due to less compression set developing in the board interior at low wood temperatures, accompanied by more tension set in the surface layers. The mechano-sorptive results were in full agreement with McMillen's conclusions from his research on red oak.     

爆破处理对赤桉板材干燥速度的影响

对赤桉板材在压力为0.8MPa,温度为95℃的条件下进行爆破预处理,并与未处理板材的干燥速度作对比研究。爆破处理后赤桉板材含水率有所下降,与未爆破的赤桉板材相比,渗透性和干燥速度有明显的提高。当含水率在50%～30%时,爆破处理过的赤桉板材和未爆破处理过的赤桉板材的含水率降低速度最快;当含水率在纤维饱和点以下时,爆破处理对赤桉板材干燥速度的改善效果不显著。同时在该试验压力下,赤桉板材的力学强度没有降低,说明爆破预处理没有影响赤桉板材的宏观结构,具有一定的可行性。     

Knot,Heartwood, and Sapwood Extractives Related to VOCs from Drying Southern Pine Lumber

Abstract

The presence of knots or heartwood influences the amount and composition of volatile organic compound (VOC) emissions associated with drying of southern pine lumber. Experimental kiln charges of lumber containing 0 to 5% of knot volume gave VOC emissions ranging from 2.86 to 4.25 lb of carbon/dry ton of wood. Studies of emissions from sapwood and knots showed that knots contain about ten times the amount of volatile terpenes found in sapwood. Consequently half of the total volatile emissions would come from 5% of knot volume in sapwood. However, correlation coefficients of knot frequency or volume with VOC emissions obtained from experimental kiln studies were only 0.57 and 0.51, respectively. Southern pine heartwood contains about five times the amount of volatile terpenes found in sapwood and more of the volatile compounds present in heartwood are emitted than from knots. Therefore, small variations in the amount of heartwood result in large changes in VOC emissions in drying southern pine lumber.     

Ambrosia Beetle Host Selection Among Logs of Douglas Fir, Western Hemlock, and Western Red Cedar with Different Ethanol and α-Pinene Concentrations

Logs from Douglas fir, Pseudotsuga menziesii; western hemlock, Tsuga heterophylla; and western red cedar, Thuja plicata, were left in the forest through winter. In April, segments from these logs were removed and randomly positioned adjacent to one another allowing ambrosia beetles to select their preferred host. In early June the tissues of Douglas fir and western hemlock logs contained significantly higher ethanol concentrations and ambrosia beetle (Trypodendron lineatum and Gnathotrichus spp.) densities than logs of western red cedar. Low beetle attack densities in western red cedar were probably a consequence of low ethanol concentrations. Although Douglas fir tissues produced significantly higher ethanol concentrations than western hemlock, the beetles did not effectively discriminate between these two conifer species. Ethanol and α-pinene were significant covariates for the ambrosia beetle densities. α-Pinene concentrations were highest in the phloem of western red cedar, intermediate in Douglas fir, and nearly absent in western hemlock. α-Pinene did not synergize the beetle's response to ethanol or to ethanol + pheromone during host selection, and it may have functioned as a deterrent.     

Evaluating the Use of Humidification Systems During Heat Treatment of MPB Lumber

Lumber produced from lodgepole pine logs attacked by the mountain pine beetle (MPB) infestation in British Columbia, Canada, exhibits very low initial moisture content (MC). Depending on the time elapsed since attack, the initial MC can be significantly lower than the fiber saturation point (FSP ～30%). Lumber exhibiting 19% MC or less is considered ready for the dimension lumber market and does not necessarily need to be kiln dried. However, phytosanitary regulations require that lumber products be heat treated before delivery to customers. For lumber already quite dry, the Canadian Food Inspection Agency (CFIA) kiln drying/heat treatment schedules may be too long and can result in overdrying. For the significant volume of MPB lumber with an initial MC below 19%, the use of low-pressure steam or cold water spray could allow this lumber to be heat treated without causing further drying. This approach should result in improved lumber quality because overdrying would certainly be minimized and probably eliminated.

In this study, two sorts (dry and wet sort) of MPB lumber were dried in a pilot laboratory kiln humidified with low-pressure steam or cold water spray. Twelve drying runs were carried out. The experimental results indicated that the times to reach the temperature of 56°C in the core of the lumber were shortened and warp for the dry-sort lumber group was reduced when the low-pressure steam or cold water spray system was used. Thus, for MPB lumber with a low initial MC, the utilization of lower pressure steam or cold water spray during heat treatment represents an attractive alternative to reduce kiln residence time, minimize or eliminate overdrying, and improve lumber quality.     

Kiln-drying optimization for quality pacific coast hemlock timber

The pacific coast hemlock or “hem-fir” is a dominant species mix in British Columbia that is difficult to kiln-dry; hence, many mills are quite conservative with their drying schedules. Furthermore, mills tend to dry hem-fir with no green moisture content presorting, thus promoting high moisture differences within and between dried timbers. In this study, application of green chain moisture-based sorting, coupled with a modified drying schedule, was considered to be a potential way to improve drying times, moisture content spreads, and lumber quality. Modified schedules coupled to three-group green moisture content presorting, i.e., mixed, low, and high, were compared to a standard industrial one. To evaluate the process and product quality, final moisture content variation between and within lumbers, drying rates, warp, surface and internal checks, shrinkage, and casehardening were assessed. Data analysis revealed that there was no significant difference between the drying runs in terms of final moisture content variation, except in the high initial moisture content group. In regard to the sorting, high initial moisture content sorting helped to reduce the final moisture content variation. In particular, the modified schedule, when there was a high initial moisture content sorting, did improve the uniformity of final moisture content in comparison to the industrial one.     

Drying Characteristics of Thick Lumber in a Laboratory Radio-Frequeocy/Vacuum Dryer

A series of forty two drying runs of two wood species and two cross-sectional dimensions of wood squares were carried out in a laboratory radio-frequency/vacuum (RF/V) dryer. The experimental temperature, pressure and moisture content levels as functions of space and time are presented. The results showed that western red cedar and western hemlock can be dried to a final moisture content of 15% in about 24 and 32 hours, respectively. The quality of the dried specimens was exceptional- Detailed analysis revealed the absence of internal drying stresses, internal and external checking and surface discoloration. Furthermore, evaluation of moisture content distribution in the longitudinal and transverse direction showed minimum variation compared to conventional kiln drying. The experiments also revealed that RF/V drying rates are directly affected by the level of the electrode plate voltage. Drying rates decreased with time when the voltage remained constant throughout the drying cycle thus resulting in long drying times. That was because of the changing dielectric properties of wood due to moisture content reduction during drying. Raising the voltages with time though, resulted in constant drying rates and shorter diying times.     

TEMPERATURE,DISTRIBUTIONS AND HEAT TRANSFER DURING THE DRYING OF LUMBER

ABSTRACT

This paper describes techniques that have been developed for accurately measuring the surface temperature of drying lumber using a radiation thermometer, and interior temperatures at various depths using 30 gauge thermocouples. Methods for calculating heat transfer coefficients during the drying of lumber are also described. Experimental results, showing surface and interior temperatures, and moisture content, as functions of drying time are presented.     

Design of Solar–Dehumidification Wood Drying Kiln Characterized by High Capacity and Temperature

ABSTRACT

Solar–;dehumidification wood drying kilns attract more and more interest due to their energy saving characteristics. However. such dryers are only available for small effective drying volume capacities. less than 30 m³, and for low drying temperature under 65 °C, for Chinese domestic products. In fact, wood drying kilns over 60 m³and high drying temperatures upto 95 °C or even higher are commonly desired from economic and process technology points o f view. For instance, if the drying temperature is below 60 °C. a long drying period will result in high operational costs and may also cause mould as well colour changes on the lumber surface. It is also known that the anti- decay ability of the seasoned lumber is lhus reduced. As such. the design of the solar–dehumidification wood drying kiln with an effective drying volume capacity of 60 m³ and a drying temperature upto 95 ⁰C was made. Two compression processes for dehumidification and heat pump systems were adopted. The refrigerant of R1⁽²⁾. which has low RODP and RGE, was utilized as the actuating medium. The values of COP. PER and SMER were 4.42. 1.19 and 3.08 kg ( H20) /( kWh) for the dehumidification system. The valucs of COP and PER were 3.05 and 0.824 for the heat pump (heat supply) system. The averaged total COP and PER over the both were 3.74 and 1.01 respectively.     

Visual Method to Assess Lumber Sorting Before Drying

This article explores the possibility of using a simplified but intuitive method to quickly assess the potential benefits of sorting lumber before industrial kiln drying. The method consists of using scatter plots to visualize the probability of obtaining a certain drying result, such as final moisture content, as a function of a property of the green lumber that can be measured in practice. The method was first validated with four drying runs of 116 mm × 52 mm hemlock lumber: one run contained unsorted lumber and the others contained the same type of lumber but sorted into low, medium, and high groups depending on the electrical capacitance of the green wood. After validation, the scatter plots were used to assess the benefits of two typical industrial sorting strategies, namely, sorting by electric capacitance and sorting by weight. It was found that both methods have the potential to increase lumber production and reduce over dried lumber in approximately the same magnitude. For a typical industrial schedule, sorting into three groups reduced the drying time by approximately 10% and over dried lumber to practically zero.