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Figure 54. Load cell output example for trial BBT11 – E. pilularis 4.3.3 Airflow An example of measured airflow results from the anemometers for trial MES1, is shown in Figure 55. As the airflow was reversed every 30 minutes, the airflow measurements are represented as markers. This allows us to observe the predominant airflow measurements as coloured bands. For each anemometer, we can observe two bands: the upper band represents air entering the anemometer side of the stack and the lower represents air exiting the opposite side. As the kiln only consists of one fan, anemometer 1 was consistently placed on the fan side of the stack and anemometer 2 on the non–fan side. A common trend between trials was lower airflow measured on the air outlet side of the stack than that measured on the air inlet side. This common phenomenon occurs due to a drop of pressure across the stack, friction effects and the removal of water from the material, and thus necessitates frequent fan reversals to obtain even drying across the stack. The obvious difference between the airflows measured in these trials compared to conventional drying is the magnitude. Transferring the necessary heat of evaporation to the stack is the bottleneck in convective vacuum drying of wood. Therefore, higher gas/steam velocities are required to compensate for the lower gas density and to obtain similar heat and mass transfer characteristics as under normal pressure. According to Malmquist and Noack (1960), the required velocity of the drying medium to achieve an acceptable heat transfer in a superheated steam atmosphere has to be about four times higher than normal when reducing the pressure conditions from 1033 mbar to 200 mbar. The prescribed airflow to dry these species conventionally is between 1 and 2 m/s. The maximum airflows recorded during the vacuum trials range between approximately 4.5 and 9 m/s. 96 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final reportPDF Image | Evaluation of super-heated steam vacuum drying
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