Comparative study on direct/bypass blowing of tunnel dryers

The most common way to make dehydrated vegetables in China now is tunnel drying. In this process, the hot air blower is set on top of or beside the tunnel. A study on the hot air flow pattern (blower positioned on top) indicated that the wind speed distribution was uneven and there were two relatively static air zones on the top of the inside of the tunnel, resulting in uneven drying and in extreme mildewing. Authors of this paper developed a new structure in which the blower is set in front of the tunnel to improve air flow pattern. Results of drying carrot granules showed: a) moisture contents of the granule were more even (11%～13%) than that of the bypass model (11%～20%); b) 2.25 MJ energy consumption was reduced for 1 kg of water dehydrated; c) drying time was shortened by 13%. However, the new structure takes up 19% more land than that of positioning the blower on the top of the tunnel.