Air speed effects during drying of harvested pasture material

Laboratory and field investigations indicated effects of air speed on the drying rates of harvested clover and ryegrass. In laboratory tests, the drying rates of material of high moisture content increased with increasing air speed to 0.4 m sec-1 past single units or through bulked material, and to 4 m sec-1 over bulked material. These critical air speeds decreased as drying progressed. Critical through-speeds, initially unattainable even with high speeds over bulked material, could then be attained at moderate over-speeds. The effects were confirmed during field tests involving material bulked into swaths and windrows such as are employed in the field drying of mown pasture for conservation as hay. Effective external (surface) resistances of material in swaths and windrows were estimated to be 8–12 times those of plant units exposed singly. Evidence is presented that "turning" of material during field drying, to release entrapped water vapour, is necessary even on windy days in the early stages of drying, and on still days in the later stages. Under still but otherwise favourable conditions, turning appears necessary also as a substitute for the continuous supply of warm air to spaces within the bulk.