Re-circulating heat pump assisted continuous bed drying and energy analysis

摘要

Specialty crops such as ginseng, herbs and echinacea need to be dried at low temperatures (30-35 deg C) for product quality optimization. A drying system that is both energy efficient and preserves product quality is desired. A re-circulating heat pump continuous bed dryer system was designed, constructed and field-tested for this purpose. The heat pump dryer system was experimentally evaluated using several potential herbal and medicinal crops such as alfalfa, catnip, wormwood, red clover, portulaca, dandelion and ginseng. These crops were dried either in chopped, sliced or whole form, depending on the part of the plant (aerial or root). The specific moisture extraction rates (SMER) obtained for various crops were in the range of 0.06-0.61 kgkWh"1. It took 5 days and 190 kWh of energy to reduce the average moisture content of ginseng roots below 10 percent (wb), while commercial dryers currently in use would take on an average 14 days and 244 kWh of energy at comparable loading rates. The re-circulating nature of the heat pump dryer made it 22 percent more energy efficient and resulted in 65 percent reduced drying time compared to conventional dryers incorporating electric coil heaters.