Effects of platen temperature on moisture state in poplar lumber during hot-press drying
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摘要:目的对热压干燥过程中杨木锯材芯层温度和压力进行测试，探究热压板温度对热压干燥过程中杨木锯材芯层温度和压力等参数及水分状态的影响，为热压干燥机理研究提供依据。方法采用集成探针同步测量并记录热压干燥过程中杨木锯材芯层温度和压力，通过对杨木锯材芯层压力测量值与测量温度对应的饱和蒸汽压力值(压力理论值)进行对比分析，进而推测热压板温度对热压干燥过程中杨木锯材水分状态的影响。结果当热压板温度从120℃升高到140℃时，杨木锯材芯层压力峰值从146.4kPa增大到213.1kPa，相应温度峰值从102.8℃升高到123.7℃，温度和压力同时达到峰值，到达峰值时间从17.5min缩短到11.6min。当热压板温度为120和130℃时，含水率高于纤维饱和点的杨木锯材芯层水分为过压的未饱和水，热压干燥后杨木锯材芯层终含水率(48.55%和49.88%)高于纤维饱和点；当热压板温度升高到140℃时，杨木锯材芯层自由水受热汽化形成水蒸气，并随着蒸汽温度的升高由饱和状态转化为过热状态，热压干燥后杨木锯材芯层终含水率(27.70%)低于纤维饱和点。结论热压干燥过程中热压板温度越高，杨木锯材芯层温度和压力达到的峰值越高，峰值持续时间越短。热压干燥过程中含水率高于纤维饱和点的杨木锯材水分状态根据热压板温度不同，可为液态水(过压的未饱和水)、饱和水蒸气或过热蒸汽状态。Abstract:ObjectiveIn order to provide basis for analyzing hot-press drying mechanism, the temperature and pressure in poplar lumber (Populus tomentosa) were monitored during hot-press drying, and the effects of platen temperature on temperature, pressure and moisture state in the core layer of poplar lumber were further investigated.MethodTemperature and pressure in the core layer of poplar lumber were measured with the integrated probe and real-time recorded with a data logger at the same time in hot-press drying, and the effects of platen temperature on moisture state were further analyzed in accordance with the comparison of measured pressure and saturated pressure (theoretical pressure) of vapor calculated with measured temperature in poplar lumber.ResultWith the increase of platen temperature from 120 to 140℃, the peak value of pressure increased from 146.4 to 213.1kPa, whereas that of temperature increased from 102.8 to 123.7℃. The temperature and pressure reached peak values at the same time, and the time to peak values decreased from 17.5 to 11.6min. The moisture in the core layer of poplar lumber with moisture content (MC) beyond fiber saturation point (FSP) was unsaturated water under overpressure condition with the platen temperatures of 120℃ and 130℃, the final MC (48.55% and 49.88%) of core layer was greater than FSP. The heat was transferred from platens to the core layer and resulted in the vaporization of free water in poplar lumber when the platen temperature increased to 140℃. State of the steam in core layer of poplar lumber changed from the saturation state to superheated state with the further increase of steam temperature, and the final core MC (27.70%) was lower than FSP.ConclusionIt was concluded that the higher the platen temperature was, the higher the peak temperature and pressure reached, and the shorter the duration time for keeping peak pressure was in hot-press drying. When the poplar lumber's MC was greater than FSP, the moisture state in poplar lumber may be liquid state (unsaturated water under overpressure condition), saturated steam or superheated steam states depends on different platen temperature levels in hot-press drying.
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Keywords:
- poplar lumber /
- hot-press drying /
- temperature /
- pressure /
- moisture state
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图 1 杨木锯材热压干燥试验示意图
Figure 1. Schematic diagram of poplar lumber in hot-press drying test
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图 3 不同热压板温度条件下杨木锯材芯层温度变化曲线
Figure 3. Curves of core layer temperature in poplar lumber under different platen temperatures
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图 4 不同热压板温度条件下杨木锯材芯层压力变化曲线
Figure 4. Curves of core layer pressure in poplar lumber under different platen temperatures
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图 5 不同热压板温度条件下杨木锯材芯层压力测量值(PM)与蒸汽压力理论值(PT)的对比
Figure 5. Comparison of the measured pressure (PM) and theoretical pressure (PT) in core layer of poplar lumber under different platen temperatures
表 1 杨木锯材热压干燥前后的含水率分布
Table 1 Moisture content (MC) distribution in poplar lumber before and after hot-press drying
热压板温度
Platen temperature/℃初含水率
Initial MC/%终含水率
Final MC/%表层终含水率
Final MC of surface layer/%芯层终含水率
Final MC of core layer/%120 51.76 39.68 23.09 48.55 130 47.42 35.33 28.66 49.88 140 38.08 16.64 12.35 27.70 
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