Re-use of the Exhaust Air Energy of Drier (III)

Performance and Energy Flow of Grain Drier with Heat-pump Using Atmospheric and Exhaust Heat as Heat Source

Abstract

The drying system (Fig. 1) with heat pump using atmospheric heat and exhaust heat as heat source is used in experiment of rice drying; the changes of states of flow, air, rough rice and refrigerant, the energy flow and efficiency are clearly determined.
1. The experimental conditions and the main results are presented in Tab. 1. With 600kg of rough rice, the evaporating rate is 6.36kg/h, the drying rate is 1.33%d.b./h (0.92%w.b./h) and the energy consumption per 1kg of removed water is 300kcal/kg·H₂O. The energy consumption is well improved and becomes about 1/4 of a common heated air rice drier.
2. The operating condition of heat pump using exhaust air as heat source is approximately fitted with the design value, the heat rejected at condenser is 7070kcal/h, the heat recovered by evaporator is 5280kcal/h and the consumed electric power is 2.2kW.
The influence of the change of temperature of exhaust on the heat rejected at condenser decreases due to the refrigerant flow control characteristic of expansion valve. Accordingly, the degree of superheat is high in comparison with the general refrigerating cycle except for the starting period. The coefficient of performance during drying period is 3.95, the ratio of rejected heat by consumed electric power is 3.72 and the satisfactory results are obtained due to the utilization of high enthalpy exhaust air as a heat source.
3. The change of state of flow air is shown in Fig. 3. The inlet air (1) is heated to the desired temperature of 48°C by using only heat pump (atmospheric temperature+27°C), the temperature of heated air (2) (3) is stable due to the self-regulations of the heat pump and the system. The exhaust air (4) which passes through the layer of rough rice is humidified and cooled, is blown into the condenser, cooled to the temperature which is lower than the inlet air by 5°C and then discharged. Subsequently, the inlet flow of sensible heat of atmospheric air is larger than the outlet flow of sensible heat of exhaust air by 1300kcal/h. In case of common heated air drier, this relation is inversely different.
4. The change of state of rough rice is illustrated on the enthalpy-concentration diagram(Fig. 7). With the progress of drying, the moist grain increases its enthalpy and approaches the state of equilibrium with heated air marked by _??_.
5. The energy flow of this drying system is expressed in Fig. 10. Among the heat flow 7070kcal/h to heat the fresh air by the heat pump, the recovered heat occupies 75% and the power of compressor is 25%. On the other hand, among the heat flow into rough rice from heated air, sensible heat is 1000kcal/h in total and the latent heat of grain moisture evaporation including the moisture concentration energy is about 4000kcal/h. The enthalpy increment of rough rice before and after drying is about 540kcal/h, among this, the moisture concentration energy which is accumulated in rough rice due to the removal of moisture is 244kcal/h. Accordingly, the conversion efficiency of the air heat quantity to moisture concentration energy is 3.5%. Moreover, the conversion efficiency of the supplied energy (electric power consumption of compressor) to moisture concentration energy is 13% which is about 4 times of ordinary combustion heating.