36 Drying of Wood

oards by conduction whereas the vacuum levelis used to enhance the internal mass transfer.. Discontinuously operating kilns, with twoperiods of about 1 h alternate: a heating periodat atmospheric pressure and a drying period atreduced pressure.. Finally, the most recent ‘‘high-vac’’ kilns use aslightly higher pressure level (more than 100mbar), together a very high linear air velocity(10 m s 1 or more), to compensate for the loss ofthermal capacity of the air; this method hasproved to be very effective.In the latter method, uniformity of the air distributionthrough the load is important to ensure evenness ofdrying, with regions of low velocities resulting inhigher final moisture contents (Ledig and Militzer,1999). The positions of fans and heating coils havean important bearing on the temperature and on thefinal moisture content of the load (Hedlund, 1996).Vacuum dryers with overhead fans provide a fairlycontrolled airflow path through the load, but otherfan locations can result in ill-defined pressure andsuction sides. An overhead-fan dryer, however, wasfound to yield a systematic variation in temperaturebetween the door and the other end of the dryer,which might have been reduced by dividing the unitinto separate temperature-control zones. Techniquesto overcome the inherent poor heat transfer invacuum dryers include the use of heated plates betweenthe boards or intermittent heating with superheatedsteam. Another suggested technique employs aheating cycle at atmospheric pressure, when the heattransfer is better, followed by a vacuum-drying cycle(Guilmain et al., 1996). Tests on drying oakwood atpilot and industrial scales showed that the discontinuousprocess was faster, with less susceptibility tomechanical damage of the wood, but the thermalconsumption was higher than under continuousvacuum conditions.Behnke and Militzer (1996) have produced avacuum-dryer model for design and process-controlpurposes based on a characteristic drying curve for thewood’sdryingbehavior.Hilderbrand(1989)claimsthatcommercialdryingtimeinvacuumkilnsvariesbetweenone half and one third of those found in conventionalconvective kilns under atmospheric pressure.36.3.5 .2 Dehum idifi er Kiln sDrying at low temperatures, which is a feature ofseasoning refractory timbers, is energy-inefficient.A dehumidifier kiln reduces the thermal-energy consumptionby incorporating an air-conditioning unitthat recovers heat by cooling the kiln air below itsdew point and, in effect, recycling the latent heat ofcondensation. As moisture is removed as condensedliquid rather than vapor in warm discharged air, theassociated thermal loss is avoided. However, a smallamount of venting is needed for humidity-controlpurposes. Volatile organic chemicals normally removedwith the vented moist air now appear in thecondensate stream, which potentially could be sent toa separate unit for chemical recovery.Figure 36.48 shows the layout of a heat-pumpdehumidifying kiln. Moist air is drawn over the evaporatorand condenser consecutively in a Rankine cycleheat pump. Besides these basic elements such as anevaporator, a condenser with its associated compressor,and expansion valve, there is an accumulator thatprevents the refrigerant from entering and damagingthe compressor and a subcooling heat exchanger toenhance the effectiveness of the heat pump.The performance of dehumidifier kilns is normallyexpressed in terms of the specific moisture-extractionrate (SMER), which is the amount of moistureextracted per unit energy input. Two such ratiosmay be defined: one representing how effectively thedehumidifier extracts moisture from the air as condensateand the other (the kiln SMER) representinghow efficiently the kiln removes moisture from thelumber including the condensate and venting. Thekiln SMER for convective kilns is limited to about0.8 to 0.9 kg kWh 1 , compared with values in therange of 1.5 to 2.5 kg kWh 1 for commercial dehumidifierkilns (Davis, 2001). Some of the lower valuesmay reflect the poor insulation of the tested kilnsrather than a defect in the process.Dehumidifying kilns are limited in operating temperatureby the working limits of the compressor(