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1、 395THE RESULTS OF EXPERIMENTAL RESEARCH COEFFICIENT AND MODEL HEAT TRANSFER OF THE ROTATING CYLINDER Dragia TOLMA Slavica PRVULOVI Ljiljana RADOVANOVI Abstract: This paper covers results of experimental and theoretical researches referring to the technique of contact drying. There are given the tes
2、ts of the system parameters of the cylinder dryer with the layer of the dried material and without it, on the surface of the rotating cylinder of the contact dryer in the exploitation conditions. Application of the contact dryers is represented especially in food industry in plants for industrial pr
3、ocessing of grains.On the basics of the tests, the temperature field, the criteria equations were given which characterize heat transfer in the dynamic conditions of dryer roll rotation, and the other process relevant parameters are determined. Key words: rotating cylinder, heat transfer, temperatur
4、e and velocity field 1. INTRODUCTION Roll dryers are the equipment used for drying of colloid solutions, suspensions, viscous liquids and pastes. Relatively simple construction and low specific energy consumption make these dryers very attractive for application in chemical, food and textile industr
5、y. Intensive exchange of heat and substance in this system of drying is accomplished thanks to the drying principle based on the direct contact of heated roll and wet material. Heat transfer systems of such of kind and likewise are introduced in literature Aihara et al. (1990), Yong and Minkowycz (1
6、989), Song et al. (1991), Tolmac (1997), Kiwan and Zeitoin (2008), Prvulovic , et al. (2007), Raudensk, M. (1993). There are very few quality and quantity data for these dryers which could enable the calculation including the coefficients of heat transfer. In the literature one can find data referri
7、ng to the coefficient of heat transfer in very wide limits from 105 to 345 (W/m2K), which does not enable the system designers to make precise calculation. Therefore, further research is necessary in this area as well as finding out of new models which could be as appropriate as possible to the actu
8、al criteria. 2. DESCRIPTION OF EXPERIMENTAL PLANT The tests are done on the industrial plant of the cylinder dryer, with the cylinder diameter d = 1220 mm and the length L = 3048 mm, that is heated inside by steam vapor. The scheme of the industrial plant is on the Fig. 1. Fig.1. The technological s
9、cheme of the cylinder dryer plant and the experimental apparatus; 1 cylinder; 2 bringing cylinders; 3 the scattering cylinder; 4 knife; 5 the pipeline for the wet material transporting; 6 the worm conveyor; 7 steam pipeline 8 the scheme of the measuring places: (a) with the dried material layer on t
10、he surface of the rotating cylinder, (b) without the dried material layer on the surface of the rotating cylinder396 When the cylinder is heated and when the constant working pressure of pp = 4 bar is released, the necessary experimental measurements in the stationary conditions are being done. Unde
11、r the stationary conditions is meant the stationeries during a great number of rotations (when no stationeries that appear in every cylinder rotation separately are excluded). The tests are done under the next conditions: 1) The atmospheric pressure pa=1 bar 2) The water vapor pressure pp=4 bar 3) T
12、he water vapor temperature Tp=140 0C 4) The number of cylinder rotations n=7,5 min-1 5) Thickness of cylinder envelope 1= 35 mm 6) The thickness of the dried material lay2=0.25 mm 7) The cylinder surface A=11,5m2 8) The dried material moisture - in the beginning of the drying it was w1=65% - at the
13、ending of the drying it was w2=5% 9) Water vapor consumption mp=268 (kg/h) 10) The dried material is 35% mass solution of starch and water. 3. DEFINING OF TOTAL COEFFICIENT OF HEAT TRANSFER Total heat flux from vapor onto surrounding air, can be given in the next form: ()2 t/qmWTThopt= (1) For big c
14、ylinder diameters in relation to envelope thickness, we can with the great punctuality use the term for the coefficient of heat transfer as for flat wall the equation (2). So, for example, for cylinder diameter d=1220mm and cylinder wall thickness 1=35mm, if we define the heat transfer coefficient f
15、or flat wall, the mistake is 1.66% in relation to the variable of the heat transfer coefficient for cylinder body. Because of that a simpler form for total coefficient of heat transfer, given by the relation (2) will be applied. When in the cylinder surface is raised the level of drying material, the coefficient of heat transfer is defined according to the next equation: mWhkkhhmmtm222111/111+=(2) Influential parameter on the mechanism of heat transfer is the coefficient of heat transfer (hm). The value of Nussles number is defined out of the equation. (3) On the b
