The Design And Experiment Of Hot Air Drying Technology Based On Being Controlled Temperature And Humidity

Hot air drying technology takes advantages of the heat resource to heat air in the drying chamber so that temperature and moisture content gradient will be generated between the drying air and material. The hot air drying method was widely used in verities of agriculture products processing for its simple structure and convenient operation. However, the non-homogeneous of temperature and velocity was the universal problems of box type hot air dryer, which may lead to low efficient drying and non-homogeneous drying effect.The box type hot air dryer with controlled temperature and humidity was designed. Carrot was used to test the performance of the box type hot air dryer with controlled temperature and relative humidity. Drying kenetics and drying quality was investigated under different constant relative humidity and step down relative humidity with constant drying temperature. In addition, based on material internal temperature, atuo control relative humidity strategy was work out. At last, the applicability of the control method was studied.The drying machine mainly consists of internal recycle fan, heating system, humidifying equipment, weighing unit, and automatic control system. Auto control system used industrial control touch screen to program human computer interface. Such automatic control system can show real time temperature and relative humidity of the drying air, material’s temperature, weight changing and moisture content. Internal temperature and relative humidity uniformity can be improved by setting turbulent fan and precise control of relative humidity is achieved by setting wet curtain. Additionally, auto weighting is completed by auto weigh equipment and control system. Carrot was used to test the performance of the box type hot air dryer with controlled temperature and relative humidity. The experiment result showed that under 60℃ and 20% relative humidity, internal temperature and relative humidity were uniform and drying time was consistent in different positions with no significant differences. Compared to switch value control method, integral separated digital PID method can realize the precise control of relative humidity. The max deviation is 0.5%. In the range of 20～70℃, the maximum deviation of auto weighting structure was 0.3 g in the range of 0～300 g. The material temperature curve seemed to be overlapped in the initial drying period with same enthalpy value drying air. On the other hand, drying time was decreased when carrot was dried with high relative humidity and then low relative humidity. A box type hot air dryer was designed, which has uniform air velocity as well as temperature and relative humidity distribution. Additionally, the dryer realized the precise control of relative humidity and show real time samples’ temperature and weight.Carrot slabs were used to explore the drying characteristic under different constant drying RH and under step down drying RH with constant drying temperature and constant air velocity. Result showed that under constant RH, drying time was decreased as reducing RH and drying time under 20% RH was reduced by 27.6% compared to that of 50 %RH. Drying rate would rise up in the initial drying period and then drop afterwards. Additionally, drying rate is more rapidly under low RH. However, material’s temperature would be the highest under 50% RH drying condition. Besides, under step down RH drying condition, when 50%RH was kept 30 min and then was changed to20%RH, the drying time was saved by 18.5% compared to constant 20%RH drying condition. When 50%RH was kept 30 min or more than 30 min, there were two accelerating periods of drying rate. The material’s temperature would decrease as RH decreased because of moisture evaporates. Weibull model can fit the drying curves under different conditions accurately. The scale parameter ranged from 1.864 to 3.635 and shape parameter ranged from 1.296 to 1.713. So there is a lag period according to shape parameter value. Based on Weibull model, the moisture effective diffusivity were calculated in the range of 1.17×10-9-2.92×10-9 m2/s. Taking rehydration ratio, color value, drying time and energy consuming into account,50%RH keeping for 30 min and then reduced to 20%RH was proposed as the favorable condition for drying carrot slabs.It is difficult to confirm the magnitude of the relative humidity of each step and its keeping time in step down drying process. It is expected that material’s temperature and relative humidity of the hot air were the main two factors that have a significant influence on the drying process. According to material’s temperature, drying period can be specified and then relative humidity can be controlled so that drying process can be optimized. The control of relative humidity was designed for internal recycle hot air drying technology. In initial pre-heating period, relative humidity was set at a high value. And the material’s temperature was raised rapidly. When the material’s temperature slowly increment trend was detected by the auto control program, the drying process transit 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, moisture removing fan was closed to heat material. When material’s temperature approached to drying hot air temperature or material’s temperature can’t rise rapidly, drying process transited to late drying period. Carrot was adopted to test the performance of the auto control relative humidity program. The experiment result showed that in the way of self adaptive control of relative humidity, drying time was reduced by 19.7% compared to the drying conditions 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. Material’s temperature curve increment rapidly in the initial period, stepped up gradually in the medium drying period and tend to be consistent with hot air temperature. The moisture content in drying chamber as well as relative humidity rise up in the preheating period and then oscillatory went down. On the other side, in the way of auto control relative humidity, the dried product has a high rehydration ratio of 4.11 and low energy consumption.Longan and yam slices were used to study the applicability of hot air dirying technology based on controlled temperature and humidity. With continuously removing humidity and auto control humidity drying condition, longan’s drying time was 22.3 and 26.4 h, respectively. And when the yam slices thickness was 6 mm, the continuous removing humidity drying process was more efficient than auto control drying condition. But when thickness was thicker than 6 mm, the conclusion was converse. With continuously removing drying condition, if the Weibull model’s shape parameter of current drying curve was lower than 1,it was suggested that such material was not suitable for auto control humidity drying condition. When yam slices thickness was 6 mm, Bi value ranged form 0.093 to 0.108, which indicated that inside and outside temperature of yam slices tended to be consistent. Additionally, when thickness was thicker than 6 mm, Bi changed from 0.124 to 0.144, the yam slices’s temperature was affected by internal thermal conductivity and external convective heat transfer. In conclusion, auto control humidity drying condition was not suitable to the material which had hardened crusting. As to the materials which easily crust during drying, auto control humidity drying condition can be used when Bi was more than 1. And when Bi was little than 1,it is suggested to drying with continuously removing humidity condition.