Preliminary Study On Process Of Vacuum Drying For Lumber Of Plantation Larch

In the paper,lumber of plantation larch (width:50mm) was studied used with continuous process of vacuum drying for convection heating,in order to provide the theory basis for the formulation of drying schedule.constant pressure of vacuum drying experiment and fluctuating pressure of vacuum drying experiment and intermittent vacuum drying experiment were carried out,in order to probe into influence and regularity about the parameters of pressure and medium temperature,and the constant or intermittent process on drying rate and moisture content deviation in thickness and surface check and honey check.The main conclusions obtained from the research were as follows:(1)Regularity of vacuum drying of lumber of plantation larch under two kinds of constant pressure, corresponding to three different initial temperature of drying medium was as follows: Initial temperature of drying medium had more obvious influence on the average drying rate than pressure,but in this experimental conditions,both comprehensive effect should be considered on the influence of drying rate; the drying rate was tending to increase with the rising of initial temperature of drying medium under constant pressure;throughout entire process, the lower the pressure and the drying medium temperature was, the lower moisture content deviation in thickness of samples;windward side deviation was minimum.(2)Regularity of three groups of vacuum experiment under different pressure,choosing P=200 mmHg,T_初=66℃as an example,was as follows: At the same temperature level,pressure had great influence on the whole average drying rate.the lower the pressure was,the faster the average drying rate was,especially when the moisture content of samples was above the fibre saturation point. With the drying process continuing,drying rate of material was decreased by gradually,among the results,the decrease of P=200 mmHg was most obvious,and the decrease trend of P=4000-200 mmHg and P=600-200 mmHg was close to linear and lower.Among the three vacuum drying experiment,moisture content deviation in thickness of samples was rising with the pressure rising,and the lower the pressure was,the easier to achieve first-class quality.In this experimental conditions,drying rate was fastest when pressure was 200 mmHg and initial temperature of drying medium was 66℃.(3)When choosing continuous process of vacuum drying,whether the pressure was constant or fluctuant,none sample all samples did not happen internal check.(4)Regularity of surface check happening was as follows: all the surface check happened on surface A,but there was almost no surface check on surface C during the continuous process of vacuum drying. The amount and total length of surface check was increased,and width of surface check had deepen trend in the same postion with initial temperature of drying medium rising under constant pressure. Pressure had great influence on the amount and total length of surface check under the same temperature level.With the pressure rising,the amount and total length of surface check was increased,the width of surface check had deepen trend. When opened the champer and checked the samples, serious surface check generated at the early stage of drying had different levels of healing the latter stage of drying;the lower the vacuum drying was,the higher the level of healing was.In this experiment with the drying condition: P= 200mmHg,T_初=61℃,the width of surface check was controlled below 2 mm and its drying quality was best among.(5)When using intermittent precess of vacuum drying, average drying rate was increased obviously,moisture content deviation in thickness of samples was much lower,controlled under 2%,and there was almost no internal check.Although surface evaporation was too big to make more small table check on surface A and surface C,the total length of samples was decreased obviously.Overall,the intermittent process of vacuum drying was better than continuous process of vacuum drying.Giving consideration to drying rate and drying quality,continuous process was carried out at the early stage,intermittent process was adopt in order to improve the heat transfer in the closed container and to prompt the internal moisture to migrate.