薄层热风干燥过程含水率在线测量装置研制与试验

针对薄层热风干燥过程自动称量精度受温度波动、气流扰动及机械振动等因素影响的问题,该研究研制了薄层热风干燥装置及其控制系统,以实现干燥过程物料质量及含水率的自动获取、及时查看和数据存储。系统改进了传统称量结构,采用悬吊称量的方式,将称量传感器置于干燥装置外,与干燥环境相隔离以提高称量传感器使用寿命;采用变频器输出频率表征风速的方式,有效避免了热式风速传感器在变温环境中的测量误差;通过称量传感器温度-质量的归一化偏差校正方法及“停风-稳定-称量-恢复”准静态的自动称量流程,开展了温度波动、气流扰动误差特性及装置验证试验,保证了干燥过程中物料含水率的准确获取。试验表明：恒载标定时,采用温度-质量归一化偏差校正方法校正后的质量与恒载质量最大偏差值为0.368 g,与实测质量相比,平均绝对百分比误差降低了84.4%,测量不确定度降低了72.2%;采用停风检测方案后,实测质量与恒载质量的最大偏差值由37.1 g降至0.31 g;物料加载时,在干燥温度35、45及55℃条件下,以手动称量方式下获取的质量及含水率为标准值,自动称量方式下的质量绝对误差分别为0.337、0.415和0.472 g,含水率...

Development and experiments of an online moisture content measuring device in thin layer hot-air drying process

Abstract: Thin layer hot-air drying is one of the most important forms during drying, particularly with a wide range of applications. Among them, the moisture content is a significant quality index of drying products. Automatic weighing can be normally used to obtain the mass during drying, due to better universality, high accuracy, and simple architecture. However, temperature fluctuation, airflow disturbance, and mechanical vibration can posed the significant impacts on the weighing accuracy. This study aims to mitigate these impacts on weighing accuracy. A new online measuring device and the control system were developed for the moisture content during thin layer hot-air drying. Some features were automatically collected to timely view the data storage of material mass and moisture content in the drying process. The weighting measurement range was 0-2 000 g in the control system. The inverter, axial fan, and thermal air velocity sensor were also used to accurate control and detect of air velocity in the drying air velocity control system. The strain gauge weighing sensor AT8518 and weighing indicator XSB5 were then applied in the hardware of automatic weighing system. The Pt100 type temperature sensor was adopted to detect the temperature of the strain gauge weighing sensor AT8518. The weighing sensor was placed outside the drying chamber in the isolation from the drying environment, in order to increase the life span of the weighing sensor and the reliability of the drying device. As such, the direct interference of the drying environment was avoided to the weighing sensor. The frequency of the inverter was used to characterize the air velocity, in order to effectively avoid the measurement error of the thermal air velocity sensor in the variable temperature environment. The correlation coefficient of the fitting curve and the 95% confidence band indicated that the air velocity values were accurately converted from the current frequency of the inverter. Furthermore, the temperature fluctuation error of weighing sensor was calculated to establish the linear relationship between the normalized deviation of temperature change and weighing error. The measured mass was dynamically compensated by numerical correction as well. The constant load (987.16 g) calibration test showed that the maximum deviation between the corrected mass and the constant load mass was 0.368 g, and the mean average percentage error was reduced by 84.4%, the guide to the uncertainty in measurement was reduced by 72.2%, compared with the measured mass. Among the calibration values, the numerical range of 987.1-987.2 g was accounted for 77.8% of the total number of tests. A stop-air detection strategy was proposed to eliminate the airflow disturbance. Air velocity was calibrated with frequency of the inverter. The instant frequency of the inverter was recorded while weighing, and then the inverter was stopped for the complete clearance of the air velocity and mechanical vibration of the drying device. The values of mass were then continuously weighed to obtain the average value of the measured mass. At the same time, the numerical correction was conducted to obtain the corresponding moisture content. In the case of loading material of soybeans at 35, 45 and 55 °C, compared with the manual weighing, the absolute mass errors of automatic weighing were 0.337, 0.415, and 0.472 g, respectively, the absolute errors of moisture content were 0.141%, 0.304%, and 0.252%, respectively, and the root mean square error was less than 0.065, which fully met the requirements of online detection of material mass and moisture content in the drying process. The online moisture content measuring device for the thin layer hot-air drying was of great significance to evaluate and regulate the drying process for the agricultural materials. The finding can provide a strong reference for the high product quality and the low labor intensity.