Agitator type

According to different material systems and different stirring purposes, there are many kinds of agitator structures. Table 4- 1 lists several commonly used agitator structures.

According to the working principle, the mixers listed in Table 4- 1 can be divided into two categories. One is propeller type, which works in the same principle as the impeller of axial flow pump, with smaller diameter than the container, but higher rotation speed. Generally, the peripheral velocity of blade tip is 5 ~ 15m/s, which is suitable for low viscosity (η < 10n s) and has the characteristics of large flow rate and low pressure head. When the blade rotates, the liquid moves axially and tangentially in the propeller. Therefore, the liquid moves spirally after leaving the propeller. The axial velocity makes the liquid flow along the shaft (usually downward), and when it reaches the tank bottom, it returns to the propeller inlet along the wall, forming the circulating mainstream as shown in Figure 4-2. The main stream turbulence caused by propeller agitator is not high, but the circulation is large. Therefore, it is suitable for the stirring process with the purpose of macro-leveling, especially for occasions where the container is required to be uniform up and down, such as the preparation of solid suspension.

Table 4- 1 Common agitator types and main data

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The other type is represented by turbine, whose working principle is similar to that of centrifugal pump impeller, and the diameter of impeller is generally 0.3 ~ 0.5 times that of container. The rotating speed is relatively high, and the tangential speed at the end is generally 3 ~ 8m/s, which is suitable for mixing liquids with low viscosity or medium viscosity (η < 50n s/m2). Compared with propeller type, it has the characteristics of smaller flow and higher pressure head. In the turbine agitator, the liquid moves tangentially and radially and rushes out of the outlet at a high absolute speed. The radial velocity of the outlet liquid makes the liquid flow to the wall, and then flows into the agitator in two forms, up and down, forming the main circulating flow. As shown in Figure 4-3. Compared with propeller type, the main flow loop caused by turbine agitator is more tortuous, and the absolute velocity of outlet is very high, which causes violent vortex motion and great shear force near the outer edge of blade, and can disperse liquid micelle more finely. Therefore, the turbine agitator is more suitable for the mixing process that requires small-scale uniformity. However, the tank of turbine agitator has two loops, which is not suitable for materials that are easy to be layered (such as suspension containing heavy solid particles).

The working principle of a straight propeller agitator is similar to that of a turbine agitator. Its blades are long, usually two, and its rotation speed is slow, the radial velocity of liquid is small, and the head is low. When the liquid layer is shallow or the liquid needs to be discharged without stopping stirring, the flat paddle impeller is the most suitable. The working principle of paddle agitator is similar to that of propeller agitator, which can produce axial liquid flow. The flow range is small, but the radial stirring range is large, which can be used to stir liquids with high viscosity.

Anchor agitator and frame agitator are actually variants of paddle agitator. They have larger rotating radius (only slightly smaller than the inner diameter of the tank), lower rotating speed and smaller pressure head, but the range of paddle agitation is larger, so they can be used for stirring high-viscosity liquids.

The working principle of belt agitator is similar to that of propeller agitator, and the liquid flows axially in the agitator. This kind of agitator also has the characteristics of large rotating radius, wide mixing range, slow rotating speed and low pressure head, and is suitable for mixing high viscosity liquids. When it rotates, it will produce the axial flow of liquid, so the mixing effect is better.

In addition to mechanical stirring, other methods can be used to realize the stirring operation, such as airflow stirring, jet stirring and static mixer.