过程装备与控制工程专业英语翻译 20.doc

Reading material 20 Basic Stirred Tank DesignThe dimensions of the liquid content of a vessel and the dimensions and arrangement of impellers, baffles and other internals are factors that influence the amount of the energy required for achieving a needed amount of agitation or quality of mixing. The internal arrangements depend on the objectives of the operation: whether it is to maintain homogeneity of a reacting mixture or to keep a solid suspended or a gas dispersed or to enhance heat or mass transfer. A basic range of design factors, however, can be defined to cover the majority of the cases, for example as in Fig.4.4 (a).The Vessel A dished bottom requires less power than a flat one. When a single impeller is to be used, a liquid level equal to the diameter is optimum, with the impeller located at the center for an all-liquid system. Economic and manufacturing considerations, however, often dictate higher ratios of depth to diameter.Baffles Except at very high Reynolds numbers, baffles are needed to prevent vortexing and rotation of the liquid mass as a whole. When solids are present or when a heat transfer jacket is used, the baffles are offset from the wall a distance equal to one-sixth the baffle width which is about one-twelfth the tank diameter. Four radial baffles at equal spacing are standard; six are only slightly more effective, and three appreciably less so. When the mixer needed, particularly at low viscosity.Draft Tubes A draft tube is a cylindrical housing around and slightly larger in diameter than the impeller. Its height may be little more than the diameter of the impeller or it may extend the full depth of the liquid, depending on the flow pattern that is required. Usually draft tubes are used with axial impellers to direct suction and discharge streams. An impeller-draft tube system behaves as an axial flow pump of somewhat low efficiency. Its top to bottom circulation behavior is of particular valve in deep tanks for suspension of solids and for dispersion of gases.Impeller Size This depends on the kind of impeller and operating conditions described by the Roynolds, Froude, and Power numbers as well as individual characteristics whose effects have been correlated. For the popular turbine impeller, the ratio of diameters of impeller and vessel falls in the range, d/, the lower values at high rpm, in gas dispersion, for example.Impeller Speed With commercially available motors and speed reducers, standard speeds are 37, 45, 56, 68, 84, 100, 125, 155, 190, and 320 rpm. Power requirements usually are not great enough to justify the use of continuously adjustable steam turbine drives. Two-speed drives may be required when starting torques are high, as with a settled slurry.Impeller Location As a first approximation, the impeller can be placed at 1/6 the liquid level off the bottom. In some cases there is provision for changing the position of the impeller on the shaft. For off-bottom suspension of solids, an impeller location of 1/3 the impeller diameter off the bottom may be satisfactory.Kinds of Impellers A rotating impeller in a fluid imparts flow and shear to it, the shear resulting from the flow of one portion of the fluid past another. Limiting cases of flow are in the axial or radial directions so that impellers are classified conveniently according to which of these flows is dominant. By reason of reflections from vessel surfaces and obstruction by baffles and other internals, however, flow patterns in most cases are mixed.Because the performance of a particular shape of impeller usually cannot be predicted quantitatively, impeller design is largely an exercise of judgment so a considerable variety has been put forth by various manufacturers. A few common types are illustrated on Fig. 4. 4 (b)(i) and are described as follows:b. The three-bladed mixing propeller is modeled on the marine propeller but has a pitch selected for maximum turbulence. They are used at relatively high speeds (up to 1800 rpm ) with low viscosity fluids, up to about 4000 cP. The stabilizing ring shown in the illustration sometimes is included to minimize shaft flutter and vibration particularly at low liquid level.c. The turbine with flat vertical blades extending to the shaft is suited to the vast majority of mixing duties up to 100000 cP or so at high pumping capacity.d. The horizontal plate to which the impeller blades of this turbine are attached has a stabilizing effect. Backward curved blades may be used for the same reason as for type e.e. Turbine with blades are inclined (usual,ly). Constructions with two to eight blades arde used, six being most common. Combined axial and radial flow are achieved. Especially effective for heat exchange with vessel walls or internal coils.f. Curved blade turbines effectively disperse fibrous materials without fouling. The swept back blades have a lower starting torque than straight ones, which is important when starting up settled slurries.g. Shrouded turbines consisting of a rotor and a stator ensure a high degree of radial flow and shearing action, and are well adapted to emulsification and dispersion.h. Anchor paddles fit the contour of the container, prevent sticking of pasty materials, and promote good heat transfer with the wall.i.Gate paddles are used in wide, shallow tanks and for materials of high viscosity when low shear is adequate. Shaft speeds are low.阅读材料20基本搅拌槽设计容器的液体容量、叶轮、挡板和其他内部构件的尺寸和安装是影响振动次数和搅拌质量的因素。内部构件的安装取决于操作的目的：是为了保持反应混合物的均匀或是固体悬浮物或是气体的分散或是为了提高传热系数。设计的基本因素，可以包括大多数情况，如图4.4（a）所示：容器 下凹的底部和平底的比起来需要更少的能量。若仅需一个搅拌器，则对于全液体系统来说，液面高度和直径相等、将搅拌器安装于中心是最好的。若从经济性和制造的角度考虑，则要求更深的深度。挡板 除非雷诺数很高，否则都需要用挡板来防止涡流和液体的整体旋转产生的洞。若出现固体或使用传热套，需在离壁面距离为六分之一叶轮宽度或容器十二分之一直径地方装支管。等距安装四个辐射状的挡板是标准的安装模式。六个只能小幅提高效率，三个就明显不行了。若搅拌器的轴偏离中心，搅拌就不会产生明显的漩涡，挡板也可以不用，尤其是在低黏度情况下。循环管 循环管程圆柱状，直径比叶轮稍大。高度比叶轮直径小些，也可以和液体深度等高，依据流动形式的要求决定。通常循环管用在轴向叶轮处用于引导吸入或排流。叶轮-循环管系统用作轴向流动泵，只是效率有些低。它从顶部至底部的循环对于深容器的固体悬浮物和气体分散物来说具有特殊的作用。叶轮的尺寸 取决于由雷诺数、佛罗德数和动力数值描述的叶轮的种类及操作条件，这些条件同样和液体性质有关联。对于常用的叶轮机叶轮来说，叶轮直径和压力容器下陷的比率范围在0.30.6之间，高转速时比值更低，在气体分离时用到。叶轮的转速 商用电动机和减速器的标准转速是37,45,56,68,84,100,125,155,190和320转/秒。动力要求对于决定是否使用可调蒸气叶轮机来说通常不够。当起始转矩很高时就需要两级速度来固