Research On Technology Of Radio Frequency/vacuum Drying For Larch Boxed-heart Square Timber

In this paper, a series of Radio Frequency/Vacuum drying tests were conducted on boxed-heart square timber of plantation Dahurian Larch (size: 1800 mm×100 mm×100 mm), the drying characteristic for the larch was known well and the optimized drying schedule was gained from the continuous adjustment of drying parameters. The effect of kurfed and superheated steam pretreatment on the drying quality and rate were investigated on the basis of the drying schedule. Then the two pretreatment were combined and the optimized combining treatment was given out. The main conclusions obtained from the research are as follows:(1) In the process of drying test, temperature differences existed in different position of samples, it means sample temperature at the position closing to anode panel is higher than that closing to cathode panel. In these tests, control temperature was measured at the position of 1/3 of sample closing to cathode panel, and sample temperature was measured at the position of sample core. It is usually that the sample temperature is always 10～20℃higher than control temperature. Radio frequency oscillator was running while the control temperature was below the setting value, otherwise it was out of running. Kiln temperature was measured the air in the tank, it always kept at nearly 35℃in these tests.(2) IP was very high during the early stage, which was easy to reach to the setting point, while EL was very low, which usually kept below the setting point. As the drying process went on, dielectric constant decreased significantly for the reducing of moisture content, then IP decreased while EL increased gradually, that is the why IP curve dropped and EL curve went up slowly. There was always at consistent vacuum in kiln during the whole drying process.(3) The optimized schedule gained from these test of Radio Frequency/Vacuum drying was that IP was 0.4A, EL was 2kV, radio frequency heating time was 3min and interval time was 1min, the control temperature was 50℃, vacuum pressure was set on 13.33kPa(100mmHg) during the early stage and 6.67kPa(50mmHg) during the last stage.(4) Before radio frequency/vacuum drying, kerf treatment made along the longitudinal direction of boxed-heart lumber of larch could reduce surface checks apparently. The kerf depth try to reach to pith and do not exceed the center of cross section, the requirement for kerf width was not more than 3mm. When sample moisture content was above the fiber saturation point, kerf treatment had obvious effect on increasing drying rate, but during the whole drying process, that efficiency was not very obvious. In the drying process, the kerf width was broadened for the losing of drying stress, which caused drying shrinkage value increase. Actually the maximum value was not more than 4%, which was the sum of shrinkage and shape changing. The moisture content at kerf position was lower than that at other position of samples.(5) Superheated steam pretreatment could reduce surface check effectually during the radio frequency vacuum drying. The samples treated with superheated steam had more uniform distribution of final moisture content, but the total drying cycle had a little bit extending. Combining of the kerf and superheated steam treatment had the better efficacy of reducing surface check than any single one of the treatment. In this study, the optimized steam temperature was 105℃in the combining treatment, and the total drying time was about 80 hours, which was not longer than that of no treatment.(6) Sample made combining treatment with superheated steam at105℃and kurfing had the less drying shrinkage on side A than that treated with single method and combining treatment with superheated team at 110℃, there was no obvious difference on side B. In the early drying stage of 10～20 hours, the radio frequency heating efficiency was above 60% and the samples moisture content were above 20%. As the drying process continued, sample moisture content reduced as well as the heating efficiency. When sample final moisture content was about 12%, the heating efficiency was 20%.