Box type hot air dryer has been widely used for agriculture products because of its simple structure and convenient operation. How to shorten drying time, reduce energy consumption and improve products’ quality becomes one of the important directions of research. As to the material which easily crusts in the surface, it is expected to use high relative humidity pretreatment and then reduce the relative humidity to enhance drying rate. In this circumstance, the temperature and moisture gradient between sample and drying air become consistent and high efficient drying process can be achieved. However, it is difficult to confirm the magnitude of relative humidity and its keeping time in each step during drying process. It is expected that material’s temperature and relative humidity of hot air are the main 2 factors that have a significant influence on the drying process. High temperature of hot air drying leads to high temperature of material, so that drying time can be reduced. Material’s internal moisture movement towards surface can be accelerated because of material temperature increment. Besides, the vapor partial pressure difference between material’s surface and drying medium can be increased by reducing drying relative humidity, which can improve drying rate. In addition, material’s temperature and relative humidity of drying hot air are the 2 aspects which reflect the mass transfer and heat transfer of the drying procedure. High relative humidity can increase material’s temperature but decrease drying rate. Conversely, low relative humidity may increase drying rate but hinder increment of material’s temperature. The drying procedure can be divided to initial preheating period, medium-term temperature increment period and late drying period based on material’s temperature curve change. So according to material’s temperature, drying period can be specified and then relative humidity can be controlled so that drying process can be optimized. In this paper, the control of relative humidity was designed for internal recycling hot air drying technology. In initial preheating period, relative humidity was set to a high value. And the material’s temperature was raised rapidly. When the slowly increasing trend of material’s temperature was detected by the automatic control program, the drying process transferred to medium-term drying procedure. In this period, material’s temperature was kept at a specific value with the moisture removing fan open. When the material’s temperature began to increase, the moisture removing fan was closed and the material was heated. When material’s temperature approached drying hot air temperature or material’s temperature couldn’t rise rapidly, drying process transferred to late drying period. Carrot was adopted to test the performance of the automatic control program of relative humidity. The experiment result showed that in the way of self adaptive control of relative humidity, the maximum deviation of relative humidity was 1% in the preheating period, and the drying temperature of material kept under dehumidifying increased gradually. The maximum error between set temperature and achieved temperature was 0.8℃. In the last drying period, the judge on the drying end time by the moisture content in 15 min was 9 min later than the judge by weighing method. Drying time was reduced by 19.7% compared to the drying condition of 50% relative humidity in the initial period and 20% in the late period as well as the drying condition of 50% relative humidity pretreatment and removing moisture continuously afterwards. The present work comes up with a new way of controlling relative humidity in internal recycling hot air drying technology, which is useful for enhancing drying rate.%针对热风干燥中，表面易结壳农产品物料阶段降湿干燥中各阶段高湿和低湿保持时间较难确定的问题，该文提出了在干燥介质温度和风速一定时，基于监测物料温度的热风干燥相对湿度控制方式。该控制方式在前期预热阶段保持较高恒定的相对湿度值，使物料迅速升温；中期干燥阶段物料温度保持特定值进行排湿干燥，物料温度有上升趋势时停止排湿使之升温；后期降速干燥阶段，物料保持较高温度值进行排湿干燥。胡萝卜的热风干燥验证试验研究结果表明，预热阶段，相对湿度控制最大偏差为1.0%；中期干燥阶段，物料排湿干燥物料温度保持值逐渐升高，物料温度上升至保持温度的最大误差为0.8℃；在后期干燥阶段，检测湿含量之差小于0.5 g/kg，判定干燥结束相对于称量判定干燥结束终点时间延迟为9 min。该干燥时间相比于前期相对湿度50%后期连续排湿和前期相对湿度50%后期相对湿度20%缩短了19.7%。该文提出了一种基于监测物料温度的热风干燥相对湿度调控策略，控制精度高，延迟时间短，相比于前期高湿后期低湿的干燥工艺能显著缩短干燥时间，提高干燥效率。
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