Abstract
The supplied energy for grain dryer is finally converted into the energy of concentration of grain moisture, and stored in grain. This energy is released as heat of wetting when the moisture absorption occurs. In this study, the method of exergy evaluation for grain dryer in consideration of this energy of moisture concentration was proposed and the concrete examples were shown.
1. The exergy of the moist air and moist grain in consideration of moisture concentration difference from surrounding condition were expressed respectively in equations (3-9, 15). The calculated values were illustrated in Fig. 3-3-6.
2. Energy and exergy flow of actual heated air dryers of rice were illustrated in Fig. 3-7, 8. The conversion efficiency of fuel energy to moisture concentration energy for heated air dryers of rice was 2-4%. The conversion efficiency of fuel exergy to moisture concentration exergy was extremely low: about 0.5-1%. The exergy loss due to combustion in heated air producer and irreversible heat transfer to grain and irreversible removal and evaporation of grain moisture in drying chamber occupied the great part. The exergy loss due to heat quantity loss such as wall loss and exhaust loss was small quantity.
3. The changes of exergy of flow air and grain (Fig. 3-9, 10) and exergy flow (Fig. 3-11) in grain drying system with heat pump using atmospheric and exhaust heat as heat source were obtained.
4. The exergy efficiency of heated air producer of the heat pump using exhaust heat as heat source was 18%, and the conversion efficiency of exergy of heated air to grain moisture concentration exergy was 13%. Finally, the conversion efficiency of supplied exergy to grain moisture concentration exergy for this system was 2.3%, and this was 4 times of the exergy efficiency of ordinary heated air dryers of rice.
