Drying of Hemp for Long Fibre Production

Drying of hemp stems is an important stage in the production of this fibre crop. Strategies for harvesting and managing the cut crop are needed that maximise the stem drying rate so that periods of good weather can be used to enable a consistent high-quality fibre to be produced. The equilibrium relative humidity for a range of stem moisture contents from 7 to 35% wet basis was determined for hemp stems between 5 and 40 °C and expressed using the modified Halsey equation. Drying behaviour of stems exposed to excess air showed that stems stripped of leaves and heads immediately before cutting dried significantly faster than unstripped, control stems. Retted stems dried at least four times faster than unretted controls. Drying of swaths similar to those produced by a commercial cutter, showed that stripped stems dried significantly faster than unstripped under good weather conditions. Thus, stripping was confirmed as having potential to accelerate swath drying. When chopped stems, in a normal width of swath and spread to twice the width were compared with unchopped controls, the chopped material dried but also wetted faster. Because spreading exposed the stems more to solar radiation, wind and rainfall, both drying and wetting were enhanced. Turning a partially dried swath, particularly of chopped material, was effective in promoting drying where atmospheric conditions were favourable. Sufficient data are presented to allow modelling of drying of the materials in swath.