Shandong Tianli Drying Technology Co.

Jinan, China

Shandong Tianli Drying Technology Co.

Jinan, China

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Wu J.,Shandong University | Wu J.,Shandong Tianli Drying Technology Co. | Li X.,Shandong Tianli Drying Technology Co. | Chen B.,Shandong Jianzhu University | And 4 more authors.
Nongye Gongcheng Xuebao/Transactions of the Chinese Society of Agricultural Engineering | Year: 2014

Heat transfer coefficient is one of the most crucial parameters in thermal calculation and design for an externally heated rotary kiln. Suitably designed kiln dimensions, structure and operating parameters rely on the accuracy of the employed heat transfer coefficient. For an externally heated kiln, heat transfers from an outside source to inside particles through a wall. Generally, the filling ratio in an externally heated rotary kiln is low. So, the heat transfer mechanism for large particles with a low filling ratio in an externally heated rotary kiln is quite different from that in an internally heated rotary kiln, whose filling ratio is usually more than 15 percent. Despite the existence of some achievements in particles motion behavior and heat transfer mechanisms in an internally heated rotary kiln, so far, there is no reliable heat transfer model to describe the heat transfer process between the kiln's surface and particles in an externally heated rotary kiln with low filling large particles. As a result, the main approach of heat transfer coefficient determination is still an experimental test. On the basis of heat transfer mechanism analysis, this paper regards the heat transfer process between the kiln's surface and large particles as consisting of heat conduction between the kiln's surface and gas film, heat convection between the gas film and particles, and heat radiation between the kiln's surface and particles. Finally, a mathematical model is created for the prediction of the heat transfer coefficient between the kiln's surface and large particles. To validate the developed model, a series of experimental tests are performed. Alumina spherical grains with a diameter of 6 mm are used as testing particles. When the filling ratio is 5 percent, the heat transfer coefficients are measured in the range of 220°C-420°C at 20°C surface temperature intervals, corresponding to the rotary speeds of 1r/min, 2r/min, and 3r/min, respectively. The tests find that the heat transfer coefficient only slightly increases with rotary speed increase. However, the coefficient increases intensely when the kiln's surface temperature increases. Comparisons of the experimental results and predictions show that the maximum relative error (emax) is about 9.8 percent, and the average error (eave) is 5.86 percent. According to the engineering design heat transfer coefficient model experience, the model is able to well match the engineering requirement that it can refer to thermal calculation. The results also show that, for the testing material in this paper, the fraction of radiation heat transferred from kiln's surface to particles is more than 75 percent of the total heat transfer when the surface temperature is higher than 220°C. If the surface temperature is beyond 320°C, a more intense increasing percentage of cure will appear. The error analysis shows that the prediction values are all larger than testing results, which could be caused by the assumptions of both particle distribution and radiation heat transfer between the kiln's surface and particles. To obtain a more accurate heat transfer coefficient model for large particles with low filling ratio in an externally heated rotary kiln, it is necessary to carry out further investigate into the performance of the motion behavior of particles. The achievement in this paper is helpful for further investigation of heat transfer mechanism in an externally heated rotary kiln.


Wu J.,Shandong University | Wu J.,Shandong Tianli Drying Technology Co. | Li X.,Shandong Academy of Sciences | Chen B.,Shandong University | And 3 more authors.
Nongye Gongcheng Xuebao/Transactions of the Chinese Society of Agricultural Engineering | Year: 2013

Heat transfer coefficient is one of the most crucial parameters in thermal calculation and design for a tube rotary dryer. The dimension, structure and operating parameters of a suitably designed dryer rely on the accuracy of the employed heat transfer coefficient. Because of the existence of tubes, particles' motion behavior and heat transfer mechanism in a tube rotary dryer are more complicated than in a conventional rotary dryer. So far, there is no reliable heat transfer model to describe the heat transfer process between the tubes' surface and particles in a tube rotary dryer. As a result, the main approach of heat transfer coefficient determination is still an experimental test. The main reason is the insufficiency of understanding on the mechanism of heat transfer between heating tube's surface and particles. Our experimental investigation showed that heat transfer between tubes' surface and particles obeyed different mechanisms in different material cases of fine powder, grain and block. This paper aims at the material case of grain. In this case, the main influence factor on heat transfer was the gas film on the surface of tubes. Based on the analysis of heat transfer mechanism, this paper redeemed that heat transfer between tubes surface and particles consisted of heat convection between tubes and gas film, heat conduction between gas film and particles, and, heat radiation between tubes surface and particles. By experimenting on traces of particle layer expansion in the dryer, the influence of particle on the gas boundary layer on tube surface was also investigated. Finally, a mathematical model was carried out for the prediction of heat transfer coefficient between tubes surface and particles. In order to validate the developed model, a series of experimental tests were performed. Ceramic spherical grains with a diameter of 2mm were used as testing particles. 6 heat transfer coefficients corresponding to 6 rotational speeds were carried out. Comparison of the experimental results and predictions showed that the maximum relative error (emax) was -12.14%, while the minimum error (emin) was -9.78%. According to the engineering design experience, the model was able to well meet engineering requirements, and offer guidance for drying process calculation. The results also showed that the fraction of radiation heat transferred from tubes' surface to particles was nearly as high as 8% of the total heat transfer. While, in case of this experiment, the temperature of heating tubes' surface was only in the range of 75~85°C. As a result, the heat radiation transferred to particles should be taken into consideration of the model, because in practice, the tubes' surface temperature can be at a relative high level (generally 150-300°C). The error analysis showed that, disregarded insufficient study of the thickness determination of gas boundary layer on the tube surface, the model still brought a fixed error at a level of about 10%. However, as our investigation went on, more understanding on performances of boundary layer and motion behavior of particles and gas media were to be obtained and, a more accurate heat transfer coefficient model for tube rotary dryer would be hopefully carried out.


Wu J.,Shandong University | Wu J.,Shandong Tianli Drying Technology Co. | Wang H.,Shandong Tianli Drying Technology Co. | Shi Y.,Shandong University | And 3 more authors.
Advanced Materials Research | Year: 2012

Drying is a crucial step in the process of copper powder flash smelting for the direct influences on the quality of smelting. This paper proposes a unique system to match the special requirements of drying raw copper powder, such as large mass rate, small diameter dust and abrasion. On the bases of drying features and energy conservation, optimization design of drying system and equipment was also carried out. In site tests showed the satisfaction on working stability, energy thrift and quality of final product.

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