Design and experiment of vacuum pulsed drying equipment based on carbon fiber infrared heating plate

Abstract: 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.