基于碳纤维红外板加热的干燥装备设计与试验

摘要

为探索基于碳纤维红外板加热的真空脉动干燥特性，该文将碳纤维红外干燥技术和真空脉动干燥技术相结合，设计了基于碳纤维红外板的真空脉动干燥装备。该装备由干燥室、真空系统、单层干燥单元、控制系统组成。为便于分析，将实际真空脉动过程划分为4个阶段：抽真空阶段、真空保持阶段、破空阶段、常压保持阶段。设计了基于MODBUS协议的控制系统，以触摸屏为主机，单片机为从机，组成控制器网络。基于干燥室内真空度的监测，采用时序控制，实现干燥室内“真空—常压”的连续转换。基于对碳纤维红外板温度的监测，结合物料内部温度的反馈，实现对干燥温度的有效调控。并以20 mm×20 mm×5 mm的苹果块为试验原料进行试验验证。结果表明：1）该干燥装备设计方案和控制方案可靠，可实现“真空—常压”的连续脉动，并有效干燥物料；2）碳纤维红外板功率1.1 kW/m2，发热面距离料盘上表面3 cm情况下，干燥效果较佳；3）当碳纤维红外板表面温度为65℃时，在真空保持阶段，苹果块内部温度约为31℃，常压阶段，会迅速上升到约37℃。干燥后期，碳纤维红外板表面温度有波动下降趋势，适当降低其温度有助于干燥进行。相比红外热风干燥，苹果块干燥时间缩短30%；4）两种干燥方式下干燥的苹果块色泽存在明显差异，真空脉动红外干燥较优。该文研究的干燥装备和研究结果可应用于苹果块等果蔬物料的干燥，并可为红外干燥技术、真空脉动干燥技术的联合应用提供理论依据。%The carbon fiber infrared plate has been used in fruits and vegetables drying as an innovative heating source, which has the advantages of higher heating rate, compact radiation distance, and simple structure. Apple is one of the most common drying materials. Whereas, the traditional natural open sun drying of apple cubes takes almost two days. In addition, during long drying time, products are very sensitive to microbial spoilage. Infrared radiation heating has several advantages such as high efficiency, low energy consumption, and high quality of dried products, and has been widely used in food industry. Many previous investigations indicate that vacuum pulsed drying can extensively enhance the drying rate and keep good quality of the dried products, such as bright color, high content of the heat sensitivity nutrients. In this work,based on carbon fiber infrared heating technology, a vacuum pulsed drying equipment was designed in order to combine the advantages of infrared heating and vacuum pulsed drying technology. The equipment was composed of a drying chamber, vacuum system, single drying unit and control system. For the convenience of analysis, the actual vacuum pulsed process was divided into four stages: The vacuum phase, the vacuum holding stage, the breaking stage, and the normal pressure stage. The automatic control scheme was realized based on MODBUS control protocol with the flow-process framework of the control system containing the touch screen and single chip microcomputer system. Sequential control system was realized for the continuous conversion of drying chamber from vacuum to atmospheric pressure. Combined with the feedback of the material internal temperature, drying temperature can be controlled effectively by monitoring temperature of the carbon fiber plate. The 20 mm×20 mm×5 mm apple cubes were dried to test this equipment. Result indicated that the equipment design and control system of the drying equipment were reliable, which can realize the continuous pulsing from vacuum to atmospheric pressure. When the internal temperature of the samples was kept at 31℃ , its drying time took about just 380 min, which was decreased about 30%compared with infrared hot air drying at the same drying temperature. Our results also showed that the most suitable power density of carbon fiber infrared heating plate was 1.1 kW/m2 and the distance between heating surface and the upper surface of material plate was only 3 cm. In addition, when the surface temperature of the carbon fiber infrared plate was 65℃ , the internal temperature of apple cubes was 31℃ at the vacuum holding stage, and it would rise to 37℃ quickly at the normal pressure stage. The energy supply of carbon fiber infrared heating plate was excessive at the late stage of drying, and the surface temperature had a trend of fluctuating downward. The findings indicated that it was helpful to reduce the temperature of infrared plate, especially in the late drying stages. Moreover, the color attributes of the products dried under vacuum pulsed drying based on infrared heating were better that of the infrared hot air dried samples. The findings of the current work provide theoretical basis and technology references for the design of vacuum pulsed drying equipment based on carbon fiber infrared heating and its practical application in agricultural products’ drying.