Design, development and validation of laboratory test rigs for performing high-precision drying experiments

A high-precision laboratory dryer (HPD TF1) was developed to achieve controlled and stable climatic conditions during the drying of light bulk materials. The HPD TF1 high-precision laboratory dryer can be used to generate drying curves with a defined temperature, relative humidity and air velocity. The development of most laboratory dryers often involves a trade-off between measurement accuracy and the sufficient production of dried material required for product quality analysis. Therefore, another laboratory dryer (HPD TF3+) was developed, consisting of three high-precision individual drying columns, each capable of holding 600 g of sample mass, and a flatbed dryer that can be loaded with up to 20 kg of fresh product. Drying curves can be recorded simultaneously with electronic balances in the three precision dryers and also in the flatbed dryer. To produce a high-quality dried product, a homogeneous airflow through the bulk material must be ensured. This requires knowledge of the airflow resistance. The airflow resistance of medicinal and aromatic plants has not yet been sufficiently investigated. Since the airflow resistance of plant materials changes during drying due to shrinkage and self-compaction, the test rig must also be able to keep the air temperature constant at adjustable levels and record the drying progress based on mass loss. For this purpose, a test stand (DPA) was developed to measure the pressure drop at different heights of a bulk of plant material. The airstream flows through the bulk at adjustable velocity and temperature.