Mathematical model for a heat pump dryer for aromatic plant

Abstract

A mathematical model was developed to evaluate the performance of heat pump dryer for drying of aromatic plants. The model consists of three sub-models; namely, drying model, heat pump model, and performance model. Drying model was developed based on mass balance, heat balance, heat transfer and drying rate equations. Heat pump sub-model consists of some theoretical and empirical equations for estimating the parameters of evaporator, compressor, condenser and expansion valve. The performance sub-model was the equations for prediction of drying efficiency, COP (coefficient of performance), MER (moisture evaporating rate) and SMER (specific moisture evaporating rate). The model was validated with the experimental data. The experiments was conducted in a fixed bed drying of valerian roots (Valeriana officinalis L.) in cooperation with a agricultural company (Agrargenossenschaft Nöbdenitz e.G., Thüringen) in Thüringen, Germany. Data logger was used to record the temperature, relative humidity, humidity ratio and enthalpy of air at different positions of the dryer equipped with different types of sensors. The average drying air temperature was 36.84°C and relative humidity was about 20%. About 89 hours were required to reduce the moisture content of valerian roots from 89 to 9% (wb). The simulated results (temperature, relative humidity and moisture content) agreed well with the experimental results. The average COP, MER and SMER and drying efficiency were 5.45, 140.03 kg/h, 0.038 kg/kWh, and 78.23%, respectively. This model may be used for design data for heat pump dryer for drying of aromatic plants as well as other heat sensitive crops.