Introduction Basic terms of mixing

The structure of mixing equipment is relatively simple.
A wide variety of purposes can be achieved by how suitably combine components of various types, sizes, arrangements, and so on.
And, it is not too much to say that the quality of the final product is decided by the quality of mixing operation and equipment.
In this session, we introduce the general structure of mixing equipment and basic terms of mixing, which are necessary to understand the basics of mixing.

Is the most important component of mixing equipment.
The impeller converts the rotational energy from the motor into two conflicting actions: the discharge action, which forms a circulation flow throughout the vessel, and the shear action, which generates shear force locally.
Therefore, when you select the impeller, though it is roughly decided by the viscosity of the raw material, it is extremely important to consider which action to give priority and how they are to be balanced to achieve your intended mixing.
There are a wide variety of impellers in the industry, but the major ones are as follows:

In general, two to eight flat plates or cylindrical pipes are equally positioned at inner wall of a mixing vessel as baffles.
Without baffles condition, rotating an impeller generates only a lateral circulation flow (called “co-rotation”) as shown in Figure 1.
With baffles condition, however, also a vertical flow is generated, disturbing the lateral flow, as shown in Figure 2.
Therefore, baffles are regarded as one of the most simple way to enhance the mixing performance. Especially when mixing a low viscosity liquid, usually baffles are applied. Meanwhile, the mixing performance depends on the arrangement, number, and length of baffles, so that it is necessary to optimize them according to the purpose and conditions.

A motor is the power source for rotating mixing impeller.
Although there are various kinds of motors, AC motors are commonly used in mixing devices. In addition, motor ratings have been standardized by JIS C 4210 in Japan.

It is necessary to decide the mixing power required to achieve the purpose of mixing operation at first, and optimum motor power capacity will be decided in consideration of some margin and mechanical power loss caused by speed reducer, etc.

However, excessive power margin leads to an increase in initial cost as well as an increase in running cost.
In addition, not only for the motor capacity, environmental condition need to be taken into the account to choose proper specification suitable with, for example, power supply (voltage , frequency), hazardous area classification (explosion-proof structure), insulation class, etc.

A speed reducer is a device to decrease rotational speed using a gear or similar, on the other hand it enables to increase rotating force (torque) larger. For example, in case of the same motor capacity, if you reduce the rotational speed by 50%, the torque will double.
It is necessary to select a proper speed reducer suitable for desired rotation speed for the purpose of mixing, among a variety of speed reducer types supplied by many different manufacturers.
Speed reducers supplied by Sumitomo Heavy Industries, Ltd. are commonly used in agitators. Following shows features of the typical speed reducers.

* Cyclo®Drive and Paramax® Drive are registered trademarks of Sumitomo Heavy Industries, Ltd.

A speed variator is a device that provides variable rotation speed. The speed reducers described above are used to decrease the rotation speed of the motor with a fixed constant ratio (reduction ratio), whereas the speed variator is a device that can adjust the rotation speed.
For agitator application, Electrical or mechanical types are commonly used as speed variator systems. A typical electric transmission is an inverter, and a typical mechanical type is Beier Variator®.

Speed variator system is recommend to apply in the following cases:
(1) Viscosity varies during operation of a mixing device
(2) Multi-products production with a single agitator
(3) Experimental purpose in R&D phases to find optimum mixing speeds

Following table shows comparison of two speed variator systems. In recent years, Variable Frequency Drives (VFD) trend to be selected more often.

A shaft seal is a leakage control device that prevents liquids and gases leakage out from a mixing vessel along the rotating shaft.
The shaft seal requires both high airtightness to seal the interior of the vessel and functionality to stably transmit rotation. So, it's very important component to maintain the overall performance, safety and economic efficiency of agitators.
The shaft seal type should be selected based on the operating conditions such as pressure, temperature, properties of liquids to be used, safety hazards, corrosiveness, rotation speed, presence or absence of solids and others.
Although many types of mechanical seals are available in the industry, those commonly used in mixing devices are shown below.

In general, an agitator is a rotating machine having a long agitator shaft supported at one side only (overhanging support condition).
So, it is required to provide stable mechanical strength enough for the operating condition.
On the mechanical design to decide the shaft diameter, all loading conditions acting the impeller and shaft should be taken into considerations, not only the torque to rotate the impeller, but also radial force due to unstable liquid flow in the tank, counter axial force due to impeller thrust force.
In addition, it's very important to avoid a risk of the resonance phenomenon, which will be induced in the condition that a natural frequency of the agitator shaft and the rotating speed of the agitator is close. Because the resonance phenomenon causes a dangerous condition with excessive vibration and may make a leaking from mechanical seals and critical damage to the shaft.
However, excessive large shaft diameter makes the agitator cost higher. So, Foot Bearing, which supports the bottom end of the shaft, can be provided to make the smaller shaft diameter.
In any way, the suitable design of the agitator shaft is important in the total viewpoint of the agitator design including the reliability and lifetime.

The shape of the mixing tank (sometimes, it is called as reactor, vessel, tank, autoclave…) is expressed in the ratio of LH (Liquid Height) / D (Tank Diameter). And it's said that the suitable figure will be LH/D=1.0 to 1.5.
In the case of a larger LH/D ratio, that means slender proportion, the uniformity in the tank tends to get worse for the mixing of middle-high viscosity range liquid. On the other hand, in the case of a smaller LH/D ratio, means fat proportion, the tank wall thickness needs to be thicker to withstand the retaining pressure.
So, for scale-up design, the tank shape needs to be investigated to make suitable conditions for all related factors like mixing, heat transfer, gas velocity, etc.
As for the tank bottom, it is common to apply 2:1 ellipsoidal head shape from the viewpoints not only the strength as a pressure retainer but to make a smooth mixing flow pattern that can avoid the stagnant region.
And, tank internal parts, such as coils, nozzles, baffles, etc they will be provided depends on the purpose. When installing such parts, it is required to make reliable mechanical strength without obstructing the mixing flow pattern.