Effects of platen temperature on moisture state in poplar lumber during hot-press drying

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.